We present measurements of the Cabibbo-suppressed decays $\Lambda_{c}^+\ra \lam K^+$ and $\Lambda_{c}^+\ra \si K^+$ (both first observations), $\Lambda_{c}^+\ra \Sigma^+ K^+\pi^-$ (seen with large statistics for the first time), $\Lambda_{c}^+ \ra p K^+ K^-$ and $\Lambda_{c}^+\ra p \phi$ (measured with improved accuracy). Improved branching ratio measurements for the decays $\Lambda_{c}^+ \ra \sig K^+ K^-$ and $\Lambda_{c}^+\ra \sig \phi$, which are attributed to W-exchange diagrams, are shown. We also present the first evidence for $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ and set an upper limit on the non-resonant decay $\Lambda_{c}^+ \ra \sig K^+ K^-$. This analysis was performed using 32.6~fb$^{-1}$ of data collected by the Belle detector at the asymmetric $e^+ e^-$ collider KEKB.
lamc_preprint.tex
\documentclass[a4paper,12pt]{article}
\usepackage{graphicx,epsfig,geometry}
%\pagestyle{empty}
\geometry{hmargin=1.5cm}
\geometry{tmargin=-.25cm}
\geometry{bmargin=3.2cm}

\renewcommand{\baselinestretch}{1.2}

%\documentclass{elsart}
%\usepackage{graphicx}
%\usepackage{amssymb}
%\documentstyle[aps,prl,preprint,floats,epsfig]{revtex} 
   
\newcommand{\ra}{\rightarrow}
\newcommand{\lam}{\Lambda^0}
\newcommand{\om}{\Omega^-}
\newcommand{\sig}{\Sigma^+}
\newcommand{\si}{\Sigma^0}
\newcommand{\xx}{\Xi^-}
 
\begin{document}
%\begin{frontmatter}

\epsfysize3cm
\epsfbox{belle.eps}    % BELLE-logo (you need file belle.eps)
%\begin{flushright}
\vskip -3cm
\noindent
\hspace*{4.5in}BELLE Preprint 2001-17 \\
\hspace*{4.5in}KEK Preprint 2001-137 \\
%\end{flushright}

%\preprint{\tighten\vbox{\hbox{\hfil 18 September 2001~~~~~~~~~~~~~~~~~~~~
%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~BELLE-NOTE-428-2}}}

\vspace*{3cm}

\begin{center}
{\Large \bf 
Observation of Cabibbo-suppressed
and W-exchange\\[10pt]
 {\boldmath $\Lambda_{c}^+$} baryon decays\footnote{
submitted to Phys. Lett. B.}
}
\end{center}

\vspace*{1cm}

\begin{center}
{\large The Belle Collaboration}\\

\vspace*{1cm}

  K.~Abe$^{9}$,               % KEK
  K.~Abe$^{42}$,              % TohokuGakuin
  R.~Abe$^{32}$,              % Niigata
  T.~Abe$^{43}$,              % Tohoku
  I.~Adachi$^{9}$,            % KEK
  Byoung~Sup~Ahn$^{17}$,      % Korea
  H.~Aihara$^{44}$,           % Tokyo
  M.~Akatsu$^{25}$,           % Nagoya
% K.~Asai$^{26}$,             % Nara
% M.~Asai$^{10}$,             % Hiroshima
  Y.~Asano$^{49}$,            % Tsukuba
  T.~Aso$^{48}$,              % Toyama
  V.~Aulchenko$^{2}$,         % BINP
  T.~Aushev$^{14}$,           % ITEP
  A.~M.~Bakich$^{40}$,        % Sydney
  Y.~Ban$^{36}$,              % Peking
  E.~Banas$^{30}$,            % Krakow
  S.~Behari$^{9}$,            % KEK
  P.~K.~Behera$^{50}$,        % Utkal
% D.~Beiline$^{2}$,           % BINP
  A.~Bondar$^{2}$,            % BINP
  A.~Bozek$^{30}$,            % Krakow
  T.~E.~Browder$^{8}$,        % Hawaii
  B.~C.~K.~Casey$^{8}$,       % Hawaii
  P.~Chang$^{29}$,            % Taiwan
  Y.~Chao$^{29}$,             % Taiwan
  B.~G.~Cheon$^{39}$,         % Sungkyunkwan
  R.~Chistov$^{14}$,          % ITEP
  S.-K.~Choi$^{7}$,           % Gyeongsang
  Y.~Choi$^{39}$,             % Sungkyunkwan
  L.~Y.~Dong$^{12}$,          % IHEP
% J.~Dragic$^{23}$,           % Melbourne
  A.~Drutskoy$^{14}$,         % ITEP
  S.~Eidelman$^{2}$,          % BINP
  V.~Eiges$^{14}$,            % ITEP
  Y.~Enari$^{25}$,            % Nagoya
% C.~W.~Everton$^{23}$,       % Melbourne
  F.~Fang$^{8}$,              % Hawaii
  H.~Fujii$^{9}$,             % KEK
  C.~Fukunaga$^{46}$,         % TMU
  M.~Fukushima$^{11}$,        % ICRR
  N.~Gabyshev$^{9}$,          % KEK
  A.~Garmash$^{2,9}$,         % BINP+KEK
  T.~Gershon$^{9}$,           % KEK
  A.~Gordon$^{23}$,           % Melbourne
% K.~Gotow$^{51}$,            % VPI
% H.~Guler$^{8}$,             % Hawaii
  R.~Guo$^{27}$,              % Kaohsiung
  J.~Haba$^{9}$,              % KEK
  H.~Hamasaki$^{9}$,          % KEK
% K.~Hanagaki$^{37}$,         % Princeton
  F.~Handa$^{43}$,            % Tohoku
  K.~Hara$^{34}$,             % Osaka
  T.~Hara$^{34}$,             % Osaka
  N.~C.~Hastings$^{23}$,      % Melbourne
  H.~Hayashii$^{26}$,         % Nara
  M.~Hazumi$^{8}$,            % KEK
  E.~M.~Heenan$^{23}$,        % Melbourne
% Y.~Higasino$^{25}$,         % Nagoya
  I.~Higuchi$^{43}$,          % Tohoku
  T.~Higuchi$^{44}$,          % Tokyo
% T.~Hirai$^{45}$,            % TIT
  T.~Hojo$^{34}$,             % Osaka
  T.~Hokuue$^{25}$,           % Nagoya
  Y.~Hoshi$^{42}$,            % TohokuGakuin
  K.~Hoshina$^{47}$,          % TUAT
  S.~R.~Hou$^{29}$,           % Taiwan
  W.-S.~Hou$^{29}$,           % Taiwan
  S.-C.~Hsu$^{29}$,           % Taiwan
  H.-C.~Huang$^{29}$,         % Taiwan
  Y.~Igarashi$^{9}$,          % KEK
  T.~Iijima$^{9}$,            % KEK
  H.~Ikeda$^{9}$,             % KEK
% K.~Ikeda$^{26}$,            % Nara
  K.~Inami$^{25}$,            % Nagoya
  A.~Ishikawa$^{25}$,         % Nagoya
  H.~Ishino$^{45}$,           % TIT
  R.~Itoh$^{9}$,              % KEK
  H.~Iwasaki$^{9}$,           % KEK
  Y.~Iwasaki$^{9}$,           % KEK
  D.~J.~Jackson$^{34}$,       % Osaka
  P.~Jalocha$^{30}$,          % Krakow
  H.~K.~Jang$^{38}$,          % Seoul
% M.~Jones$^{8}$,             % Hawaii
  R.~Kagan$^{14}$,            % ITEP
% H.~Kakuno$^{45}$,           % TIT
% J.~Kaneko$^{45}$,           % TIT
  J.~H.~Kang$^{53}$,          % Yonsei
  J.~S.~Kang$^{17}$,          % Korea
  P.~Kapusta$^{30}$,          % Krakow
  N.~Katayama$^{9}$,          % KEK
  H.~Kawai$^{3}$,             % Chiba
  H.~Kawai$^{44}$,            % Tokyo
% Y.~Kawakami$^{25},          % Nagoya
  N.~Kawamura$^{1}$,          % Aomori
  T.~Kawasaki$^{32}$,         % Niigata
  H.~Kichimi$^{9}$,           % KEK
  D.~W.~Kim$^{39}$,           % Sungkyunkwan
  Heejong~Kim$^{53}$,         % Yonsei
  H.~J.~Kim$^{53}$,           % Yonsei
  H.~O.~Kim$^{39}$,           % Sungkyunkwan
  Hyunwoo~Kim$^{17}$,         % Korea
  S.~K.~Kim$^{38}$,           % Seoul
  T.~H.~Kim$^{53}$,           % Yonsei
  K.~Kinoshita$^{5}$,         % Cincinnati
% S.~Kobayashi$^{38}$,        % Saga
% S.~Koishi$^{45}$,           % TIT
  H.~Konishi$^{47}$,          % TUAT
% K.~Korotushenko$^{37}$,     % Princeton
  S.~Korpar$^{22,15}$,        % Ljubljana
  P.~Kri\v zan$^{21,15}$,     % Ljubljana
  P.~Krokovny$^{2}$,          % BINP
  R.~Kulasiri$^{5}$,          % Cincinnati
  S.~Kumar$^{35}$,            % Panjab
% T.~Kuniya$^{38}$,           % Saga
% E.~Kurihara$^{3}$,          % Chiba
  A.~Kuzmin$^{2}$,            % BINP
  Y.-J.~Kwon$^{53}$,          % Yonsei
  J.~S.~Lange$^{6}$,          % Frankfurt
  G.~Leder$^{13}$,            % Vienna
  S.~H.~Lee$^{38}$,           % Seoul
  D.~Liventsev$^{14}$,        % ITEP
  R.-S.~Lu$^{29}$,            % Taiwan
  J.~MacNaughton$^{13}$,      % Vienna
% D.~Marlow$^{37}$,           % Princeton
  T.~Matsubara$^{44}$,        % Tokyo
% S.~Matsui$^{25}$,           % Nagoya
  S.~Matsumoto$^{4}$,         % Chuo
  T.~Matsumoto$^{25}$,        % Nagoya
  Y.~Mikami$^{43}$,           % Tohoku
  K.~Miyabayashi$^{26}$,      % Nara
  H.~Miyake$^{34}$,           % Osaka
  H.~Miyata$^{32}$,           % Niigata
% L.~C.~Moffitt$^{23}$,       % Melbourne
  G.~R.~Moloney$^{23}$,       % Melbourne
% G.~F.~Moorhead$^{23}$,      % Melbourne
  S.~Mori$^{49}$,             % Tsukuba
  T.~Mori$^{4}$,              % Chuo
% A.~Murakami$^{38}$,         % Saga
  T.~Nagamine$^{43}$,         % Tohoku
  Y.~Nagasaka$^{10}$,         % Hiroshima
  Y.~Nagashima$^{34}$,        % Osaka
  T.~Nakadaira$^{44}$,        % Tokyo
% T.~Nakamura$^{45}$,         % TIT
  E.~Nakano$^{33}$,           % OsakaCity
  M.~Nakao$^{9}$,             % KEK
% H.~Nakazawa$^{4}$,          % Chuo
  J.~W.~Nam$^{39}$,           % Sungkyunkwan
% S.~Narita$^{43}$,           % Tohoku
  Z.~Natkaniec$^{30}$,        % Krakow
  K.~Neichi$^{42}$,           % TohokuGakuin
  S.~Nishida$^{18}$,          % Kyoto
  O.~Nitoh$^{47}$,            % TUAT
  S.~Noguchi$^{26}$,          % Nara
% T.~Nozaki$^{9}$,            % KEK
  S.~Ogawa$^{41}$,            % Toho
  T.~Ohshima$^{25}$,          % Nagoya
% Y.~Ohshima$^{45}$,          % TIT
  T.~Okabe$^{25}$,            % Nagoya
% T.~Okazaki$^{26}$,          % Nara
  S.~Okuno$^{16}$,            % Kanagawa
  S.~L.~Olsen$^{8}$,          % Hawaii
  W.~Ostrowicz$^{30}$,        % Krakow
  H.~Ozaki$^{9}$,             % KEK
  P.~Pakhlov$^{14}$,          % ITEP
  H.~Palka$^{30}$,            % Krakow
  C.~S.~Park$^{38}$,          % Seoul
  C.~W.~Park$^{17}$,          % Korea
  H.~Park$^{19}$,             % Kyungpook
  K.~S.~Park$^{39}$,          % Sungkyunkwan
  L.~S.~Peak$^{40}$,          % Sydney
  J.-P.~Perroud$^{20}$,       % Lausanne
  M.~Peters$^{8}$,            % Hawaii
  L.~E.~Piilonen$^{51}$,      % VPI
% E.~Prebys$^{37}$,           % Princeton
% J.~L.~Rodriguez$^{8}$,      % Hawaii
  N.~Root$^{2}$,              % BINP
  M.~Rozanska$^{30}$,         % Krakow
  K.~Rybicki$^{30}$,          % Krakow
  J.~Ryuko$^{34}$,            % Osaka
  H.~Sagawa$^{9}$,            % KEK
  Y.~Sakai$^{9}$,             % KEK
  H.~Sakamoto$^{18}$,         % Kyoto
% H.~Sakaue$^{33}$,           % OsakaCity
  M.~Satapathy$^{50}$,        % Utkal
% A.~Satpathy$^{9,5}$,        % KEK+Cincinnati
  O.~Schneider$^{20}$,        % Lausanne
  S.~Schrenk$^{5}$,           % Cincinnati
  S.~Semenov$^{14}$,          % ITEP
  K.~Senyo$^{25}$,            % Nagoya
% Y.~Settai$^{4}$,            % Chuo
  M.~E.~Sevior$^{23}$,        % Melbourne
  H.~Shibuya$^{41}$,          % Toho
  B.~Shwartz$^{2}$,           % BINP
% A.~Sidorov$^{2}$,           % BINP
  J.~B.~Singh$^{35}$,         % Panjab
  S.~Stani\v c$^{49}$,        % Tsukuba
  A.~Sugi$^{25}$,             % Nagoya
  A.~Sugiyama$^{25}$,         % Nagoya
  K.~Sumisawa$^{9}$,          % KEK
  T.~Sumiyoshi$^{9}$,         % KEK
% J.-I.~Suzuki$^{9}$,         % KEK
  K.~Suzuki$^{3}$,            % Chiba
  S.~Suzuki$^{52}$,           % Yokkaichi
  S.~Y.~Suzuki$^{9}$,         % KEK
  S.~K.~Swain$^{8}$,          % Hawaii
% H.~Tajima$^{44}$,           % Tokyo
  T.~Takahashi$^{33}$,        % OsakaCity
  F.~Takasaki$^{9}$,          % KEK
  M.~Takita$^{34}$,           % Osaka
  K.~Tamai$^{9}$,             % KEK
  N.~Tamura$^{32}$,           % Niigata
  J.~Tanaka$^{44}$,           % Tokyo
  M.~Tanaka$^{9}$,            % KEK
  Y.~Tanaka$^{24}$,           % Nagasaki
  G.~N.~Taylor$^{23}$,        % Melbourne
  Y.~Teramoto$^{33}$,         % OsakaCity
  M.~Tomoto$^{9}$,            % KEK
  T.~Tomura$^{44}$,           % Tokyo
  S.~N.~Tovey$^{23}$,         % Melbourne
% K.~Trabelsi$^{8}$,          % Hawaii
% W.~Trischuk$^{37,\dagger}$, % Princeton
  T.~Tsuboyama$^{9}$,         % KEK
  T.~Tsukamoto$^{9}$,         % KEK
  S.~Uehara$^{9}$,            % KEK
  K.~Ueno$^{29}$,             % Taiwan
  Y.~Unno$^{3}$,              % Chiba
  S.~Uno$^{9}$,               % KEK
  Y.~Ushiroda$^{9}$,          % KEK
  S.~E.~Vahsen$^{37}$,        % Princeton
  K.~E.~Varvell$^{40}$,       % Sydney
  C.~C.~Wang$^{29}$,          % Taiwan
  C.~H.~Wang$^{28}$,          % Lien-Ho
  J.~G.~Wang$^{51}$,          % VPI
  M.-Z.~Wang$^{29}$,          % Taiwan
  Y.~Watanabe$^{45}$,         % TIT
  E.~Won$^{38}$,              % Seoul
  B.~D.~Yabsley$^{9}$,        % KEK
  Y.~Yamada$^{9}$,            % KEK
  M.~Yamaga$^{43}$,           % Tohoku
  A.~Yamaguchi$^{43}$,        % Tohoku
% H.~Yamamoto$^{43}$,         % Tohoku
% T.~Yamanaka$^{34}$,         % Osaka
  Y.~Yamashita$^{31}$,        % NihonDental
  M.~Yamauchi$^{9}$,          % KEK
  S.~Yanaka$^{45}$,           % TIT
  J.~Yashima$^{9}$,           % KEK
  M.~Yokoyama$^{44}$,         % Tokyo
  K.~Yoshida$^{25}$,          % Nagoya
  Y.~Yuan$^{12}$,             % IHEP
  Y.~Yusa$^{43}$,             % Tohoku
% H.~Yuta$^{1}$,              % Aomori
  C.~C.~Zhang$^{12}$,         % IHEP
  J.~Zhang$^{49}$,            % Tsukuba
  H.~W.~Zhao$^{9}$,           % KEK
  Y.~Zheng$^{8}$,             % Hawaii
  V.~Zhilich$^{2}$,           % BINP
and
  D.~\v Zontar$^{49}$         % Tsukuba
\end{center}

\small
\begin{center}
$^{1}${Aomori University, Aomori}\\
$^{2}${Budker Institute of Nuclear Physics, Novosibirsk}\\
$^{3}${Chiba University, Chiba}\\
$^{4}${Chuo University, Tokyo}\\
$^{5}${University of Cincinnati, Cincinnati OH}\\
$^{6}${University of Frankfurt, Frankfurt}\\
$^{7}${Gyeongsang National University, Chinju}\\
$^{8}${University of Hawaii, Honolulu HI}\\
$^{9}${High Energy Accelerator Research Organization (KEK), Tsukuba}\\
$^{10}${Hiroshima Institute of Technology, Hiroshima}\\
$^{11}${Institute for Cosmic Ray Research, University of Tokyo, Tokyo}\\
$^{12}${Institute of High Energy Physics, Chinese Academy of Sciences, 
Beijing}\\
$^{13}${Institute of High Energy Physics, Vienna}\\
$^{14}${Institute for Theoretical and Experimental Physics, Moscow}\\
$^{15}${J. Stefan Institute, Ljubljana}\\
$^{16}${Kanagawa University, Yokohama}\\
$^{17}${Korea University, Seoul}\\
$^{18}${Kyoto University, Kyoto}\\
$^{19}${Kyungpook National University, Taegu}\\
$^{20}${IPHE, University of Lausanne, Lausanne}\\
$^{21}${University of Ljubljana, Ljubljana}\\
$^{22}${University of Maribor, Maribor}\\
$^{23}${University of Melbourne, Victoria}\\
$^{24}${Nagasaki Institute of Applied Science, Nagasaki}\\
$^{25}${Nagoya University, Nagoya}\\
$^{26}${Nara Women's University, Nara}\\
$^{27}${National Kaohsiung Normal University, Kaohsiung}\\
$^{28}${National Lien-Ho Institute of Technology, Miao Li}\\
$^{29}${National Taiwan University, Taipei}\\
$^{30}${H. Niewodniczanski Institute of Nuclear Physics, Krakow}\\
$^{31}${Nihon Dental College, Niigata}\\
$^{32}${Niigata University, Niigata}\\
$^{33}${Osaka City University, Osaka}\\
$^{34}${Osaka University, Osaka}\\
$^{35}${Panjab University, Chandigarh}\\
$^{36}${Peking University, Beijing}\\
$^{37}${Princeton University, Princeton NJ}\\
%%%$^{38}${Saga University, Saga}\\
$^{38}${Seoul National University, Seoul}\\
$^{39}${Sungkyunkwan University, Suwon}\\
$^{40}${University of Sydney, Sydney NSW}\\
$^{41}${Toho University, Funabashi}\\
$^{42}${Tohoku Gakuin University, Tagajo}\\
$^{43}${Tohoku University, Sendai}\\
$^{44}${University of Tokyo, Tokyo}\\
$^{45}${Tokyo Institute of Technology, Tokyo}\\
$^{46}${Tokyo Metropolitan University, Tokyo}\\
$^{47}${Tokyo University of Agriculture and Technology, Tokyo}\\
$^{48}${Toyama National College of Maritime Technology, Toyama}\\
$^{49}${University of Tsukuba, Tsukuba}\\
$^{50}${Utkal University, Bhubaneswer}\\
$^{51}${Virginia Polytechnic Institute and State University, Blacksburg VA}\\
$^{52}${Yokkaichi University, Yokkaichi}\\
$^{53}${Yonsei University, Seoul}\\
%%%$^{\dagger}${on leave from University of Toronto, Toronto ON}
\end{center}

\normalsize



\normalsize

%\tighten

%*******************************************************************************

\newpage

\begin{abstract}

We present measurements of the Cabibbo-suppressed decays  
$\Lambda_{\rm c}^+\ra \lam K^+$ and $\Lambda_{c}^+\ra \si K^+$
(both first observations), $\Lambda_{c}^+\ra \Sigma^+ K^+\pi^-$ 
(seen with large statistics for the first time), 
$\Lambda_{c}^+ \ra p K^+ K^-$ and $\Lambda_{c}^+\ra p \phi$
(measured with improved accuracy).
Improved branching ratio measurements for the decays
$\Lambda_{c}^+ \ra \sig K^+ K^-$ and $\Lambda_{c}^+\ra \sig \phi$,
which are attributed to W-exchange diagrams, are shown. 
We also present the first evidence for $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ 
and set an upper limit on the non-resonant decay $\Lambda_{c}^+ \ra \sig K^+ K^-$.
This analysis was performed using 32.6~fb$^{-1}$ of data collected 
by the Belle detector at the asymmetric $e^+ e^-$ collider KEKB. 

\end{abstract}

%*******************************************************************************

\section{Introduction}

Decays of charmed baryons, unlike charmed mesons, 
are not colour or helicity suppressed, allowing us to investigate the
contribution of W-exchange diagrams. 
There are also possible interference effects due to the presence of identical
quarks. This makes the study of these decays a useful tool to test 
theoretical models that predict exclusive decay rates~\cite{theory}.

During the past several years there has been significant progress in the
experimental study of hadronic decays of charmed baryons.
New results on masses, widths, lifetimes and decay asymmetry 
parameters have been published by various experiments~\cite{PDG}.
However the accuracy of branching ratio measurements does not exceed 30\% for
many Cabibbo-favoured modes: for Cabibbo-suppressed and W-exchange dominated
decays, the experimental accuracy is even worse.
As a result, we are not yet able to conclusively distinguish between the decay
rate predictions made by different theoretical models.

In this paper we present a study of $\Lambda_{c}^+$ baryons produced in the
$e^+ e^- \ra q {\bar q}$ continuum at Belle, relying on the excellent particle
identification capability of the detector to measure decays with 
kaons in the final state.
We report the first observation of the Cabibbo-suppressed decays
$\Lambda_{c}^+ \ra \lam K^+$ and $\Lambda_{c}^+ \ra \si K^+$, and the
first observation of $\Lambda_{c}^+ \ra \sig K^+ \pi^-$ with large statistics.
(Here and throughout this paper, the inclusion of charge-conjugate states is
implied.)
We present improved measurements of the Cabibbo-suppressed decays
$\Lambda_{c}^+ \ra p K^+ K^-$ and $\Lambda_{c}^+\ra p \phi$,
and the W-exchange decays  $\Lambda_{c}^+ \ra \sig K^+ K^-$ and  
$\Lambda_{c}^+\ra \sig \phi$;
we also report the first evidence for $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$,
and set an upper limit on non-resonant $\Lambda_{c}^+ \ra \sig K^+ K^-$ decay.

%*******************************************************************************

\section{Data and Selection Criteria}
\label{section-selection}

The data used for this analysis were taken on the $\Upsilon(4S)$ 
resonance and in the nearby continuum
using the Belle detector at the asymmetric $e^+ e^-$ collider KEKB. 
The integrated luminosity of the data sample is equal to 32.6~fb$^{-1}$. 

Belle is a general purpose detector based on a 1.5 T superconducting solenoid;
a detailed description can be found elsewhere~\cite{BELLE_DETECTOR}.
Tracking is performed with a silicon vertex detector (SVD) composed of three
concentric layers of double-sided silicon strip detectors, and a 50 layer
drift chamber.
Particle identification for charged hadrons, important for the measurement of
final states with kaons and/or protons,  is based on the combination of  
energy loss measurements $(dE/dx)$ in the drift chamber,
time of flight measurements and aerogel {\v C}erenkov counter information. 
For each charged track, measurements from these three subdetectors are 
combined to form $K/\pi$ and $p/K$ likelihood ratios in the range from 0 to 1,
%\\[-3pt]
%\begin{equation}
$${\rm P}(K/\pi) = \mathcal{L}(K)/(\mathcal{L}(K) + \mathcal{L}(\pi)),~~ 
{\rm P}(p/K) = \mathcal{L}(p)/(\mathcal{L}(p) + \mathcal{L}(K)),$$
%\end{equation}
where $\mathcal{L}(p)$, $\mathcal{L}(K)$ and $\mathcal{L}(\pi)$ are the likelihood values
assigned to each identification hypothesis for a given track.

For the analyses presented here, we require P($K/\pi) < 0.9$ for pions, 
P($K/\pi) > 0.6$ for kaons, and P($p/K) > 0.9$ for protons, unless
stated otherwise.
Candidate $\pi^0$'s are reconstructed from pairs of photons detected in the 
CsI calorimeter, with a minimum energy of 50~MeV per photon. The interaction
point (IP) coordinates in the $r-\phi$ plane are determined from beam profile 
measurements. Other particles are identified as follows:
\begin{itemize}
  \item	$\lam$ are reconstructed in the decay mode $\lam \rightarrow p \pi^-$,
	fitting the $p$ and $\pi^-$ tracks to a common vertex and  
	requiring an invariant mass in a $\pm 3$~MeV$/c^2$ ($\approx \pm 3\sigma$)
	interval around the nominal value.
	The likelihood ratio cut on the proton is relaxed to P$(p/K) > 0.4$.
	We then make the following cuts on the $\lam$ decay vertex: 
	\begin{itemize}
	  \item	the closest distance of approach along the beam direction
                between the proton and pion tracks must be less than 1~cm;
	  \item	the distance between the decay vertex and the interaction point
		in the $r-\phi$ plane must be greater than 1~mm;
	  \item	the cosine of the angle in the $r-\phi$ plane
                between the $\lam$ momentum vector and the
		vector pointing from the IP to the decay vertex
		must be greater than 0.995.
	\end{itemize}
  \item	$K^0_{\rm S}$ are reconstructed in the decay mode $K^0_{\rm S} \rightarrow \pi^+ \pi^-$,
	fitting the $\pi^+$ and $\pi^-$ tracks to a common vertex and  
	requiring an invariant mass in a $\pm 7$~MeV$/c^2$ ($\approx \pm 3\sigma$)
	interval around the nominal value.
	We then make the same vertex cuts as in the $\lam$ case. 
  \item	$\sig$ are reconstructed in the decay mode $\sig \rightarrow p \pi^0$,
	requiring an invariant mass within $\pm 10\, \mathrm{MeV}/c^2$
	($\approx \pm 2 \sigma$) of the nominal value.
%	Since the $\sig$ mass resolution is dominated by the uncertainty in the
%	$\pi^0$ momentum, rather than the choice of $\sig$ decay vertex, we
%	use the IP to estimate the vertex for the $\pi^0$ fit; this
%	approximation has been checked in the Monte Carlo.
        We require the proton to have at least one hit in the SVD, to improve its
        impact parameter resolution with respect to the IP; 
%       furthermore, this impact
%       parameter in the $r-\phi$ plane is required to be $> 200\,\mu\mathrm{m}$, 
	we then require the impact parameter in the $r-\phi$ plane
	to be greater than $200\,\mu\mathrm{m}$,
        to make sure the $\sig$ vertex is displaced from the IP.
%	The displaced $\sig$ vertex is taken into account using
%	the impact parameter of the decay proton in the $r-\phi$ plane
%	with respect to the IP, $d_{r\phi}$.
%	We require the proton to have at least one hit in the SVD,
%	to improve the impact parameter resolution, and then make a cut
%	$d_{r\phi} > 200\,\mu\mathrm{m}$.
  \item	$\si \rightarrow \lam \gamma$ decays are formed using identified $\lam$
	and photons with calorimeter cluster energies above $0.1$ GeV;
	we accept candidates with invariant masses within $\pm 6$~MeV$/c^2$
	($\approx \pm 1.5\sigma$) of the nominal value.
\end{itemize}

To suppress combinatorial and $B\overline{B}$ backgrounds,
we require $\Lambda_{c}^+$ candidates to have scaled momentum
$x_p=p^\ast/p^\ast_{\rm max} > 0.5$; here 
$p^\ast$ is the reconstructed momentum of the $\Lambda_c^+$ candidate
in the $e^+e^-$ center of mass, 
and
$p^\ast_{\rm max} = \sqrt{s/4-M^2}$,
%$x_p=p^\ast(\Lambda_{c}^+)/\sqrt{E_{CMS}^2/4-M^2} >0.5$, where 
%$p^\ast(\Lambda_{c}^+)$ and $E_{CMS}$ are the candidate momentum and total
%$e^+e^-$ beam energy in the center of mass frame, respectively, 
where $\sqrt{s}$ is the total center of mass energy
and $M$ is the
reconstructed mass of the $\Lambda_{c}^+$ candidate.
In modes where there are two or more charged tracks at the $\Lambda_{c}^+$
vertex, we perform a vertex fit and require $\chi^2/\mathrm{n.d.f.} < 9$.

%When doing the fit with a Gaussian or Breit-Wigner function, the central value
%is always allowed to float. The width is also floating, unless stated otherwise.
In the various mass fits described below, the central value and width of
the signal peaks are always allowed to float, unless stated otherwise. 
Wherever the final state includes a hyperon, we improve the invariant mass resolution
by plotting the corrected mass difference, \emph{e.g.}\ 
$M(\sig K^+ K^-) - M(\sig) + M_{\sig}^{\rm PDG}$
instead of $M(\sig K^+ K^-)$.

%*******************************************************************************

\section{Observation of the decays
	{\boldmath $\Lambda_{\lowercase {c}}^+ \rightarrow \lam K^+$} and
	{\boldmath $\Lambda_{\lowercase {c}}^+\ra \Sigma^0 K^+$}}

The Cabibbo-suppressed decay $\Lambda_{\lowercase {c}}^+\ra \lam K^+$ has not
been previously observed. Reconstructing $\lam K^+$ combinations as described
in Section~\ref{section-selection}, we see a clear signal at the $\Lambda_{c}^+$
mass, as shown in Fig.~\ref{fig_lamc_lam0k_1}.

To study backgrounds due to Cabibbo-allowed decays, we select a second
sample with a reversed identification requirement P$(K/\pi)<0.1$ applied to the
``kaon''. In the mass spectrum of this sample, where the kaon mass hypothesis is still
used, we see a broad structure centered around $2.4\, \mathrm{GeV}/c^2$,
%using the MC simulation we find that this is 
produced by
$\Lambda_{c}^+\ra \lam \pi^+$ and $\Lambda_{c}^+\ra \Sigma^0 \pi^+$ decays. 
%The background due to $\Lambda_{c}^+ \ra \Sigma^0 \pi^+$ is dangerous, since the
%mass shifts due to the undetected $\gamma$ (from $\Sigma^0\ra \lam \gamma$)
%and $\pi \rightarrow K$ misidentification partially cancel, leaving a peak close
%to the $\Lambda_{c}^+$ mass.  
%Feed-downs from other Cabibbo-allowed $\Lambda_{\rm c}$ decays,
%$\Lambda_{c}^+ \ra \lam \pi^+\pi^0$,
%$\Lambda_{c}^+ \ra \lam \pi^+\pi^+\pi^-$ and
%$\Lambda_{c}^+ \ra \Sigma^0 \pi^+ \pi^0$,
%produce broad reflections due to the missing particle(s), and no prominent
%structures are seen. 
\begin{figure}[h]
\centering
\begin{picture}(550,200)
\put(80,40){\rotatebox{90}{\large \bf Entries/(5 MeV/{\boldmath $c^2$})}}
%\put(-50,250){\large $\frac{N}{5.0~{\rm MeV}/c^2}$} 
\put(145,0){\boldmath $M(\lam K^+)-M(\lam)+1.116,~~~{\rm [GeV}/c^2]$} 
\put(110,10){\includegraphics[width=0.6\textwidth]{lamc_lam0k_kid06_repro_32fb_new.eps}}
\end{picture}
\caption{$\Lambda_{\lowercase {c}}^+\ra \lam K^+$: invariant mass spectrum of
	the selected $\lam K^+$ combinations.
The broad structure to the right of the signal peak, due to 
$\Lambda_c^+ \ra \lam \pi^+$ and $\Lambda_c^+ \ra \si \pi^+$ decays, is included
in the fit.
}
\label{fig_lamc_lam0k_1}
\end{figure}
We fit this distribution
using two Gaussians 
%with floating central values and widths 
(to model the $\Lambda_{c}^+\ra \lam \pi^+$ and $\Lambda_{c}^+\ra \Sigma^0 \pi^+$
contributions), and a second order polynomial (to model the broad reflections
and the remaining background).
%the result of the fit is superimposed on
%Fig.~\ref{fig_lamc_lam0k_1}(b).
The shape of this function is then used to model the $\Lambda_c^+ \ra \lam \pi^+ (\si \pi^+)$
background in the main sample.
%(Fig.~\ref{fig_lamc_lam0k_1}(a)).
The remaining combinatorial background in Fig.~\ref{fig_lamc_lam0k_1} is 
represented using a second order polynomial, and the 
$\Lambda_{c}^+\ra \lam K^+$ signal is described by a Gaussian 
with width $\sigma=5.4\, \mathrm{MeV}/c^2$ (fixed from Monte Carlo);
the result of the fit is shown by the superimposed curve.
We find a yield of $265 \pm 35$ $\Lambda_{c}^+\ra \lam K^+$ decays,
the first observation of this decay mode. 
%\begin{figure}[h]
%\centering
%\begin{picture}(550,280)
%\put(-25,230){\large $\frac{N}{5.0~{\rm MeV}/c^2}$} 
%\put(150,20){$M(\lam \pi^+)-M(\lam)+1.116,~{\rm GeV}/c^2$} 
%\put(45,30){\includegraphics[width=0.85\textwidth]{lamc_lam0pi_repro_32fb_new.eps}}
%\end{picture}
%\caption{The invariant mass spectrum for the normalization mode
%	$\Lambda_{c}^+\ra \lam\pi^+$. The selection requirements and fit 
%	are described in the text.
%	The broad feature below the $\Lambda_{c}^+$ mass is due to
%	$\Lambda_{c}^+\ra \Sigma^0 \pi^+$ decays.}
%\label{fig_lamc_lam0k_11}
%\end{figure}

For normalization, we use the decay $\Lambda_{c}^+\ra \lam\pi^+$.
The $\Lambda_{c}^+\ra \lam\pi^+$ mass
distribution is 
%shown in Fig.~\ref{fig_lamc_lam0k_11}. A 
fitted with a Gaussian for the signal and 
a second order polynomial for the background.
%(restricted to the region above the $\Lambda_{c}^+ \ra \si \pi^+$ reflection)
We find $4550 \pm 111$ events. 
%the fitted signal width $\sigma=6.6\pm 0.2$~MeV$/c^2$ is consistent with the MC
%prediction of 6.7~MeV$/c^2$. 
The relative reconstruction efficiency was determined using, Monte Carlo simulation (MC),
to be
$\epsilon(\Lambda_{c}^+\ra \lam K^+)/\epsilon(\Lambda_{c}^+\ra \lam\pi^+) 
 = 0.79$. Using this value, we extract 
%the branching ratio
$$
   \frac{\mathcal{B}(\Lambda_{c}^+\ra \lam K^+)}
  {\mathcal{B}(\Lambda_{c}^+\ra \lam\pi^+)} = 0.074\pm 0.010 \pm 0.012;
$$ 
the first error is statistical, and the second is systematic. We provide a detailed description of the 
sources of systematic error for this and other measured decay modes in Section~\ref{errors}.

The Cabibbo-suppressed decay $\Lambda_{c}^+\ra \Sigma^0 K^+$ is reconstructed in
a similar way, with the scaled momentum cut tightened to $x_p>0.6$ to suppress
the large background due to soft photons. The invariant mass distribution of
the selected $\si K^+$ candidates is shown in Fig.~\ref{fig_lamc_sig0k_1}:
a peak is seen at the $\Lambda_{c}^+$ mass, and a reflection due to misidentified
two-body Cabibbo-allowed $\Lambda_{c}^+$ decays is seen at higher masses. 
The superimposed curve shows the result of a fit following the 
method described for $\lam K^+$, with the exception that in this case the width of the
signal Gaussian is fixed from the MC to 
$\sigma=5.0$~MeV$/c^2$.
We find $75\pm 18$ $\Lambda_{c}^+\ra \si K^+$ events, the first observation of
this decay mode. 
%\begin{figure}[p]
%\centering
%\begin{picture}(550,200)
%\put(30,190){\large $\frac{N}{5.0~{\rm MeV}/c^2}$} 
%\put(165,20){$M(\Sigma^0 \pi^+)-M(\Sigma^0)+1.192,~{\rm GeV}/c^2$} 
%\put(90,25){\includegraphics[width=0.7\textwidth]{lamc_sig0pi_repro_32fb_new.eps}}
%\end{picture}
%\caption{The invariant mass spectrum for the normalization mode
%	$\Lambda_{c}^+ \ra \Sigma^0 \pi^+$. The selection requirements and fit 
%	are described in the text.
%	The broad enhancement around the $\Lambda_{c}^+$ mass region is due to
%	$\Lambda_{c}^+\ra \Lambda^0 \pi^+$ decays.}
%\label{fig_lamc_sig0k_11}
%\end{figure}
For normalization, we use the decay $\Lambda_{c}^+\ra \si \pi^+$.
%shown in Fig.~4. 
\begin{figure}[h]
\centering
\begin{picture}(550,200)
\put(80,40){\rotatebox{90}{\large \bf Entries/(5 MeV/{\boldmath $c^2$})}}
%\put(-50,250){\large $\frac{N}{5.0~{\rm MeV}/c^2}$} 
\put(145,0){\boldmath $M(\Sigma^0 K^+)-M(\Sigma^0)+1.192,~~~[{\rm GeV}/c^2]$} 
\put(110,10){\includegraphics[width=0.6\textwidth]{lamc_sig0k_kid06_repro_32fb_new.eps}}
\end{picture}
\caption{$\Lambda_{c}^+ \ra \Sigma^0 K^+$: invariant mass spectrum of the
	selected $\Sigma^0 K^+$ combinations.
The broad structure to the right of the signal peak, due to 
$\Lambda_c^+ \ra \lam \pi^+$ and $\Lambda_c^+ \ra \si \pi^+$ decays, is included
in the fit.
}
\label{fig_lamc_sig0k_1}
\end{figure}
We fit the distribution with a Gaussian for the signal,
a second Gaussian to describe the broad enhancement due to $\Lambda_{c}^+\ra \lam \pi^+$
(with the addition of a random $\gamma$), and a second order polynomial for the
remaining background. 
%The fitted width of the signal $\sigma=6.0$~MeV$/c^2$ 
%is consistent with the MC prediction $\sigma=6.1$~MeV$/c^2$.
The fit gives $1597 \pm 67$ $\Lambda_{c}^+\ra \si \pi^+$ decays. 
The relative reconstruction efficiency found to be  
$\epsilon(\Lambda_{c}^+\ra \si K^+)/\epsilon(\Lambda_{c}^+\ra \si\pi^+) 
 = 0.84$ in the MC: we then calculate
%extract the branching ratio
$$
   \frac{\mathcal{B}(\Lambda_{c}^+\ra \si K^+)}
  {\mathcal{B}(\Lambda_{c}^+\ra \si\pi^+)} = 0.056\pm 0.014 \pm 0.008.
$$ 

%*******************************************************************************

\section{Observation of the {\boldmath $\Lambda_{\lowercase {c}}^+ \ra \sig K^+ \pi^-$} decay}

The first evidence for the Cabibbo-suppressed decay $\Lambda_{c}^+ \ra \sig K^+ \pi^-$
was published by the NA32 collaboration in 1992~\cite{NA32_lamc_sigkpi}:
they found 2 events in the signal region. 
Reconstructing $\sig K^+ \pi^-$ combinations with the cuts of
Section~\ref{section-selection} tightened to require
$x_p > 0.6$, we see a clear signal peak at the $\Lambda_{c}^+$ mass,
as shown in Fig.~\ref{fig_lamc_sigkpi}.
The tighter cut is used to suppress the large combinatorial background.
We also form $\sig K^+ \pi^-$ combinations using ``$\sig$'' candidates
from mass sidebands (two 10~MeV/$c^2$ intervals centered $20$~MeV/$c^2$
below and above the nominal $\sig$ mass~\cite{PDG}), shown 
with the shaded histogram: no enhancement is seen near the $\Lambda_{c}^+$ mass.

The mass distribution is fitted with a Gaussian 
for the signal (with width fixed to $3.6$~MeV$/c^2$ from the MC) and a second
order polynomial for the background:
we find $105 \pm 24$ $\Lambda_{c}^+ \ra \sig K^+ \pi^-$ events. 
For normalization we reconstruct $\Lambda_{c}^+ \ra \sig \pi^+ \pi^-$ decays
with the same cuts, finding $2368\pm 89$ events. 
The relative efficiency of the $\Lambda_{c}^+ \ra \sig K^+ \pi^-$ channel
reconstruction with respect to $\Lambda_{c}^+ \ra \sig \pi^+ \pi^-$ 
is found to be $0.94$ in the MC. Using this value,
we obtain 
%extract the branching ratio
$$ 
   \frac{\mathcal{B}(\Lambda_{c}^+ \ra \sig K^+ \pi^-)}
  {\mathcal{B}(\Lambda_{c}^+ \ra \sig \pi^+ \pi^-)} = 0.047 \pm 0.011 \pm 0.008.
$$
\begin{figure}[h]
\centering
\begin{picture}(550,200)
\put(80,40){\rotatebox{90}{\large \bf Entries/(3 MeV/{\boldmath $c^2$})}}
%\put(-50,250){\large $\frac{N}{3.0~{\rm MeV}/c^2}$} 
\put(135,0){\boldmath $M(\sig K^+ \pi^-)-M(\sig)+1.189,~~~[{\rm GeV}/c^2]$} 
\put(110,10){\includegraphics[width=0.6\textwidth]{lamc_sigkpi_repro_32fb_new.eps}}
\end{picture}
\caption{$\Lambda_{c}^+ \ra \sig K^+ \pi^-$: invariant mass spectrum of the
	selected $\sig K^+ \pi^-$  combinations. The shaded histogram
	shows the equivalent spectrum for the $\sig$ sidebands. The mass difference
        for the sidebands is corrected using the central value of the corresponding
        sideband interval.}
\label{fig_lamc_sigkpi}
\end{figure}
%************* Sig + K+ K- *************************************************************

\section{Measurement of the {\boldmath $\Lambda_{\lowercase {c}}^+ \ra \sig K^+ K^-$} and 
{\boldmath $\sig \phi$} decays}
\label{section-sigkk}

The decays $\Lambda_{c}^+ \ra \sig K^+ K^-$ and $\Lambda_{c}^+ \ra \sig \phi$
proceed dominantly via W-exchange diagrams, and were observed by CLEO in
1993~\cite{CLEO_lamc_sigkk}.
% with branching ratios
%$\mathcal{B}(\Lambda_{c}^+ \ra \sig K^+ K^-)/
%\mathcal{B}(\Lambda_{c}^+ \ra p K^- \pi^+) = 0.070 \pm 0.012 \pm 0.011$ and
%$\mathcal{B}(\Lambda_{c}^+ \ra \sig \phi)/\mathcal{B}(\Lambda_{c}^+ \ra p K^- \pi^+) = 0.069 \pm 0.023 \pm 0.016$.
Here we measure these decay channels with improved accuracy 
and provide the first evidence
for the $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ decay.

Figure~\ref{fig_lamc_sigkk} shows the invariant mass spectrum for
$\Lambda_{c}^+ \ra \sig K^+ K^-$ combinations selected according to
Section~\ref{section-selection}.
\begin{figure}[h]
\centering
\begin{picture}(550,200)
\put(80,40){\rotatebox{90}{\large \bf Entries/(2 MeV/{\boldmath $c^2$})}}
%\put(-50,250){\large $\frac{N}{2.0~{\rm MeV}/c^2}$} 
\put(135,0){\boldmath $M(\sig K^+ K^-)-M(\sig)+1.189,~~~[{\rm GeV}/c^2]$} 
\put(110,10){\includegraphics[width=0.6\textwidth]{lamc_sigkk_repro_32fb_new.eps}}
\end{picture}
\caption{$\Lambda_{c}^+ \ra \sig K^+ K^-$: invariant mass spectrum of the selected
	$\sig K^+ K^-$  combinations.}  
\label{fig_lamc_sigkk}
\end{figure}
A clear peak is seen at the $\Lambda_{c}^+$ mass, over a low background.
We fit the distribution using a Gaussian (with width fixed to 2.2~MeV$/c^2$
from the MC) plus a second order polynomial: the fit yields $246\pm 20$
 $\Lambda_{c}^+ \ra \sig K^+ K^-$ decays.
For normalization we reconstruct the $\Lambda_{c}^+ \ra \sig \pi^+ \pi^-$ decay
mode with equivalent cuts, 
%shown in Fig.~\ref{fig_lamc_sigpipi}, 
and fit
the distribution with a Gaussian and a second order polynomial: we find 
$3650\pm 138$ $\Lambda_{c}^+ \ra \sig \pi^+ \pi^-$ events.
The relative efficiency of the $\Lambda_{c}^+ \ra \sig K^+ K^-$ decay 
reconstruction with respect to the $\Lambda_{c}^+ \ra \sig \pi^+ \pi^-$ 
decay is calculated by MC simulation and is found to be $0.89$. 
We thus extract 
%a branching ratio
$$
\frac{\mathcal{B}(\Lambda_{c}^+ \ra \sig K^+ K^-)}{\mathcal{B}(\Lambda_{c}^+ \ra \sig \pi^+ \pi^-)} = 
0.076 \pm 0.007 \pm 0.009.
$$

%\begin{figure}[h]
%\centering
%\begin{picture}(550,220)
%\put(30,190){\large $\frac{N}{2.0~{\rm MeV}/c^2}$} 
%\put(155,20){$M(\sig \pi^+ \pi^-)-M(\sig)+1.189,~{\rm GeV}/c^2$} 
%\put(90,25){\includegraphics[width=0.7\textwidth]{lamc_sigpipi_loose_repro_32fb_new.eps}}
%\end{picture}
%\caption{The invariant mass spectrum for the normalization mode
%	$\Lambda_{c}^+ \ra \sig \pi^+ \pi^-$, using equivalent cuts to the
%	$\Lambda_{c}^+ \ra \sig K^+ K^-$ analysis.}  
%\label{fig_lamc_sigpipi}
%\end{figure}

In order to obtain the $\Lambda_{c}^+ \ra \sig \phi$ signal,
we take $\sig K^+ K^-$ from a $\pm 5$~MeV$/c^2$ window around the fitted
$\Lambda_{c}^+$ mass (2286~MeV$/c^2$), and plot the invariant mass of the 
$K^+ K^-$ combination, as shown in Fig.~\ref{fig_lamc_sigkk_mkk} (points with error bars);
the equivalent distribution is also shown for $\sig K^+ K^-$ in 5~MeV/$c^2$ 
sidebands
\begin{figure}[h]
\centering
\begin{picture}(550,200)
\put(80,40){\rotatebox{90}{\large \bf Entries/(2 MeV/{\boldmath $c^2$})}}
%\put(-50,250){\large $\frac{N}{2.0~{\rm MeV}/c^2}$} 
\put(195,0){\boldmath $M(K^+ K^-),~~~[{\rm GeV}/c^2]$} 
\put(110,10){\includegraphics[width=0.6\textwidth]{lamc_sigkk_mkk_repro_32fb_new.eps}}
\end{picture}
\caption{Fitting for the $\Lambda_{c}^+ \ra \sig \phi$ component:
	the invariant mass spectra of $K^+ K^-$ combinations from
	the $\Lambda_{c}^+ \ra \sig K^+ K^-$ signal area (points with error bars)
	and $\Lambda_{c}^+$ sidebands (shaded histogram) are shown.}  
\label{fig_lamc_sigkk_mkk}
\end{figure}
centered 12.5~MeV$/c^2$ below and above the fitted $\Lambda_{c}^+$ mass
(shaded histogram).
The distributions are fitted with a Breit-Wigner function (describing the $\phi$ signal)
convolved with a Gaussian of fixed width
(representing the detector mass resolution)
plus a second order polynomial multiplied by a square root threshold factor.
%(to account for the mass threshold).
The intrinsic width of the $\phi$ Breit-Wigner function is fixed to its
nominal value~\cite{PDG}, and the width of the Gaussian resolution is
fixed to 1.0~MeV$/c^2$ based on the MC simulation.
The fit yields $153 \pm 15$ events for the $\phi$ signal in the $\Lambda_{c}^+$
region and $27 \pm 7$ in the $\Lambda_{c}^+$ sidebands. 
To extract the $\Lambda_{c}^+ \ra \sig \phi$ contribution we subtract the $\phi$
yield in the sidebands from the yield in the $\Lambda_{c}^+$ signal region,
correcting for the phase space factor obtained from the $\sig K^+ K^-$
background fitting function.
After making a further correction for the missing signal outside the
$\Lambda_{c}^+$ mass interval,
we obtain $129 \pm 17$ $\Lambda_{c}^+ \ra \sig \phi$ decays.

The relative efficiency of the $\Lambda_{c}^+ \ra \sig \phi$ reconstruction
with respect to $\Lambda_{c}^+ \ra \sig \pi^+ \pi^-$ 
is calculated using the MC and found to be $0.84$. 
Taking into account the $\phi$ branching fraction
$\mathcal{B}(\phi \ra K^+ K^-) = (49.4 \pm 0.7)\%$~\cite{PDG},
we calculate 
%a branching ratio
$$
   \frac{\mathcal{B}(\Lambda_{c}^+ \ra \sig \phi)}
  {\mathcal{B}(\Lambda_{c}^+ \ra \sig \pi^+ \pi^-)}
  = 0.085 \pm 0.012 \pm 0.012.
$$

We also search for resonant structure in the $\sig K^-$ system in these
decays. Figure~\ref{fig_lamc_sigkk_sigk} shows the $\sig K^-$ invariant mass spectrum for $\sig K^+ K^-$
combinations in a $\pm 5$~MeV$/c^2$ interval around the fitted $\Lambda_{c}^+$
mass (data points):
we also require $| M(K^+ K^-) - m_\phi | > 10\, \mathrm{MeV}/c^2$ to suppress
$\phi \ra K^+ K^-$.
\begin{figure}[h]
\centering
\begin{picture}(550,200)
\put(80,40){\rotatebox{90}{\large \bf Entries/(4 MeV/{\boldmath $c^2$})}}
%\put(-50,250){\large $\frac{N}{4.0~{\rm MeV}/c^2}$} 
\put(145,0){\boldmath $M(\sig K^-)-M(\sig)+1.189,~~~[{\rm GeV}/c^2]$} 
\put(110,10){\includegraphics[width=0.6\textwidth]{lamc_sigkk_sigk_repro_32fb_new_wside.eps}}
\end{picture}
\caption{Fitting for the $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ component:
	the invariant mass spectrum of $\sig K^-$ combinations from the
	$\Lambda_{c}^+ \ra \sig K^+ K^-$ signal area (points with error bars) 
	and $\Lambda_{c}^+$ sidebands (shaded histogram) are shown, with
	the $\phi \ra K^+ K^-$ signal region excluded in both cases.}  
\label{fig_lamc_sigkk_sigk}
\end{figure}
Also shown is the $\sig K^-$ invariant mass spectrum from $\sig K^+ K^-$ combinations
selected inside $5\, \mathrm{MeV}/c^2$ sideband intervals centered 12.5~MeV$/c^2$
below and above the fitted $\Lambda_{c}^+$ mass (shaded histogram).
The $\sig K^-$ mass distribution shows evidence for the $\Xi(1690)^0$
resonant state. In order to extract this resonant contribution the histograms
are fitted with a relativistic Breit-Wigner function (describing the $\Xi(1690)^0$ signal)
plus a $(M_{\rm max} - M)^\alpha$ function multiplied by a square root threshold factor
(here $M_{\rm max}$ is the maximal allowed value of the $\sig K^-$ invariant mass).
The fit yields $82 \pm 15$ events for the $\Xi(1690)^0$ signal in the
$\Lambda_{c}^+$ region, with a fitted mass ($1688 \pm 2$)~MeV$/c^2$
and width ($11 \pm 4$)~MeV in good agreement with
previous measurements of the $\Xi(1690)^0$ parameters~\cite{PDG}.
To fit the sidebands, the function parameters are fixed to the central values
obtained from the signal fit, and both the signal 
and background normalizations are floated. A yield of $9 \pm 4$ events is found.

The $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ contribution is obtained by subtracting
the $\Xi(1690)^0$ yield in the sidebands from the yield in the $\Lambda_{c}^+$
signal region, correcting the sideband contribution using the 
phase space factor obtained from the $\sig K^+ K^-$ background fitting function.
After a further correction for the missing signal outside the
$\Lambda_{c}^+$ mass interval, we obtain
$75 \pm 16$ $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ decays.
%Assuming the relative efficiency for reconstruction of the
%$\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ channel with respect to  
%$\Lambda_{c}^+ \ra \sig \pi^+ \pi^-$ to be the same as for the inclusive
%$\sig K^+ K^-$ mode, 
We then find
$$
   \frac{\mathcal{B}(\Lambda_{c}^+ \ra \Xi(1690)^0 K^+)}
 	{\mathcal{B}(\Lambda_{c}^+  \ra \sig \pi^+ \pi^-)}
   \times
   \mathcal{B}(\Xi(1690)^0 \ra \sig K^-)
   = 0.023 \pm 0.005 \pm 0.005;
$$
the possible effects due to interference with 
$\Lambda_{c}^+ \ra \sig \phi$ are included in the systematic error
(see the discussion in Section~\ref{errors}).

Finally, the non-resonant $\Lambda_{c}^+ \ra \sig K^+ K^-$ contribution is
estimated by making invariant mass cuts
$| M(K^+ K^-) - m_\phi |	> 10\, \mathrm{MeV}/c^2$ and
$| M(\sig K^-) - M_{\Xi(1690)^0}| > 20\, \mathrm{MeV}/c^2$ to suppress the
$\phi$ and $\Xi(1690)^0$ contributions (here, $M_{\Xi(1690)^0}$ is the fitted
$\Xi(1690)^0$ mass). The resulting $\sig K^+ K^-$ mass spectrum
is 
%shown in Fig.~\ref{fig_lamc_sigkk_nonres}. A 
fitted with a Gaussian (with width fixed to 2.2~MeV$/c^2$ from the MC) plus a
second order polynomial. 
%shown on the figure, 
The fit yields $34 \pm 9$ events.
Integrating the $\phi$ Breit-Wigner function over the allowed $M(K^+ K^-)$
region, we find that $14\%$ of the total $\Lambda_{c}^+ \ra \sig \phi$ signal
contributes to this sample: $18\pm 3$ events.
The contribution of the $\Xi(1690)^0$ mass tails is estimated to be
approximately 12$\%$ of the fitted $\Xi(1690)^0$ signal: $9 \pm 2$ events.
Subtracting these contributions, $7 \pm 10$ non-resonant events remain.
The phase space correction factor to account for the missing region around the
$\phi$ and $\Xi(1690)^0$ masses is found to be 1.63 by MC simulation of the
non-resonant $M(K^+ K^-)$  spectrum. Applying this correction we obtain
$11 \pm 16$ $\Lambda_{c}^+ \ra \sig K^+ K^-$ non-resonant decays.
%The relative efficiency for the reconstruction of the $\Lambda_{c}^+ \ra \sig K^+ K^-$ (non-resonant) 
%channel with respect to the 
%$\Lambda_{c}^+ \ra \sig \pi^+ \pi^-$ channel was taken to be the same as for the inclusive $\sig K^+ K^-$ mode.
Taking into account the systematic error, we obtain an upper limit 
%on the branching ratio
$$
   \frac{\mathcal{B}(\Lambda_{c}^+ \ra \sig K^+ K^-)_{\mbox{\scriptsize \rm non-res}}}
  {\mathcal{B}(\Lambda_{c}^+  \ra \sig \pi^+ \pi^-)}
  < 0.018
$$
at the 90\% confidence level, 
including the possible effects due to interference with 
$\Lambda_{c}^+ \ra \sig \phi$ in the systematic errors
(see Section~\ref{errors}).
%\begin{figure}[t!]
%\centering
%\begin{picture}(550,220)
%\put(30,190){\large $\frac{N}{2.0~{\rm MeV}/c^2}$} 
%\put(150,20){$M(\sig K^+ K^-)-M(\sig)+1.189,~{\rm GeV}/c^2$} 
%\put(90,25){\includegraphics[width=0.7\textwidth]{lamc_sigkk_nonres_repro_32fb_new.eps}}
%\end{picture}
%\caption{The $\sig K^+ K^-$ invariant mass spectrum, after suppressing
%	$\sig \phi$ and  $\Xi(1690)^0 K^+$ contributions:
%	the selection requirements and fit are described in the text.}  
%\label{fig_lamc_sigkk_nonres}
%\end{figure}

%*******************************************************************************

\section{Evidence for the {\boldmath $\Xi(1690)^0$} 
%resonance
in
{\boldmath $\Lambda_{\lowercase {c}}^+ \ra \Lambda^0 {\bar K^0} K^+$} decays}
\label{lamkk}

Another possible decay mode of the $\Xi(1690)^0$ resonant state is $\Xi(1690)^0 \ra \lam {\bar K^0}$.
Hence we have searched for the decay $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ by reconstructing 
$\Lambda_{c}^+ \ra \Lambda^0 K^0_{\rm S} K^+$ decays and looking at the $\Lambda^0 K^0_{\rm S}$ invariant
mass distribution. Reconstructing $\Lambda^0 K^0_{\rm S} K^+$ combinations with the cuts of
Section~\ref{section-selection} we obtain an invariant mass spectrum,
which 
%shown in
%Fig.~\ref{fig_lamc_lamkk}. The distribution 
is fitted with a Gaussian for the signal and 
a second order polynomial for the background: we find $363 \pm 26$ 
$\Lambda_{c}^+ \ra \Lambda^0 K^0_{\rm S} K^+$ events. In order to obtain the $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$
signal, we 
take $\lam K^0_{\rm S} K^+$ from a $\pm 10$~MeV$/c^2$ window ($\approx 2.5 \sigma$) 
around the fitted $\Lambda_{c}^+$ mass
(2287~MeV$/c^2$), and plot the invariant mass of the $\Lambda^0 K^0_{\rm S}$ combination,
as shown in Fig.~\ref{fig_lamc_lamk_lamkk}
(points with error bars); the equivalent distribution is also shown for $\lam K^0_{\rm S} K^+$ from
$10$~MeV$/c^2$ sideband intervals centered $20$~MeV$/c^2$ below and above the fitted $\Lambda_{c}^+$ mass
(shaded histogram). A peak at the expected position is clearly seen.
%
\begin{figure}[h]
\centering
\begin{picture}(550,200)
\put(80,40){\rotatebox{90}{\large \bf Entries/(5 MeV/{\boldmath $c^2$})}}
%\put(-50,250){\large $\frac{N}{5.0~{\rm MeV}/c^2}$} 
%\put(-5,210){\large $\Xi_c^0 \rightarrow \Sigma^+ K^-$} 
\put(145,0){\boldmath $M(\lam K^0_{\rm S})-M(\lam)+1.116,~~~[{\rm GeV}/c^2]$} 
\put(110,10){\includegraphics[width=0.6\textwidth]{lamc_lamk0k_lam0k0_repro_32fb_new_bfunc.eps}}
\end{picture}
\caption{Fitting for the $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ component:
	the invariant mass spectrum of $\lam K^0_{\rm S}$ combinations from the
	$\Lambda_{c}^+ \ra \lam K^0_{\rm S} K^+$ signal area (points with error bars) 
	and $\Lambda_{c}^+$ sidebands (shaded histogram) are shown.
        The dashed curve represents the background function.}
\label{fig_lamc_lamk_lamkk}
\end{figure}
We use a fitting procedure similar to that described in Section~\ref{section-sigkk} for the $\Lambda_{c}^+ \ra 
\sig \phi$ analysis. After subtraction of the sideband contribution and corrections we
obtain $93 \pm 26$ $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ decays.
%The distributions are fitted with a Breit-Wigner function (describing the signal)
%plus a third order polynomial multiplied by a square root threshold factor. The fit yields 
%$96 \pm 24$ for the $\Xi(1690)^0$ signal in the $\Lambda_{c}^+$ region, with a fitted mass ($1685 \pm 2$~MeV$/c^2$)
%and width ($15 \pm 4$~MeV) in good agreement with the measurements using $\Xi(1690)^0 \ra \sig K^-$
%decay mode. To fit the sidebands, the function parameters were fixed to the central values
%obtained from the signal fit, and the normalization was floated: a yield of $4 \pm 10$ events is found.
%The $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ contribution was obtained by subtracting
%the $\Xi(1690)^0$ yield in the sidebands from the yield in the $\Lambda_{c}^+$
%signal region, correcting the sideband contribution using the 
%phase space factor obtained from the $\lam K^0_{\rm S} K^+$ background fitting function.
%After a further correction for the missing signal outside the
%$\Lambda_{c}^+$ mass interval, we obtain
%$93 \pm 26$ $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ decays.
This confirms our observation of the $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ decay:
significant signals are seen for both $\Xi(1690)^0 \ra \sig K^-$ and
$\Xi(1690)^0 \ra \lam {\bar K^0}$.
%This result supports the evidence for $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ decay
%using the $\Xi(1690)^0 \ra \sig K^-$ decay mode.

Using the normalization to the inclusive decay mode $\Lambda_{c}^+ \ra \lam {\bar K^0} K^+$
and the measured values for the $\mathcal{B}(\Lambda_c^+ \ra \sig \pi^+ \pi^-)$ and
$\mathcal{B}(\Lambda_c^+ \ra \lam {\bar K^0} K^+)$~\cite{PDG} we find
$$
   \frac{\mathcal{B}(\Lambda_{c}^+ \ra \Xi(1690)^0 K^+)}
 	{\mathcal{B}(\Lambda_{c}^+  \ra \lam {\bar K^0} K^+)}
   \times \mathcal{B}(\Xi(1690)^0 \ra \lam {\bar K^0})
   = 0.26 \pm 0.08 \pm 0.03.
$$
Using the value of the $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+,~\Xi(1690)^0 \ra \sig K^-$ combined
branching ratio, obtained in Section~\ref{section-sigkk}, 
and the ratio of the normalization decay rates~\cite{PDG},
we find the following ratio of
$\Xi(1690)^0$ decay rates:
$$
   \frac{\mathcal{B}(\Xi(1690)^0 \ra \sig K^-)}
 	{\mathcal{B}(\Xi(1690)^0  \ra \lam {\bar K^0})} = 0.50 \pm 0.26.
$$

The corresponding ratio of the $\Xi(1690)^0$ decay rates quoted by~\cite{PDG} ($1.8\pm 0.6$
after isospin correction) is based on a single measurement reported in~\cite{xi1690}.
In order to check for possible interference effects, we studied the corresponding $\lam K^+$
invariant mass distribution and did not find any structure above a smooth background.
%due to its valence quark content ($uds,~u{\bar s}$) we expect any resonant contribution to the
%$\lam K^+$ final state to be small.
We have also searched for the $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$ decay in the 
$\Lambda_{c}^+ \ra (\Xi^- \pi^+) K^+$ decay mode, but did not find  any $\Xi(1690)^0$ signal in the
$\Xi^- \pi^+$ invariant mass spectrum, in agreement with the $\mathcal{B}(\Xi(1690)^0 \ra \Xi^- \pi^+)$
upper limit value from~\cite{xi1690}.
%\begin{figure}[p]
%\centering
%\begin{picture}(550,220)
%\put(30,190){\large $\frac{N}{2.0~{\rm MeV}/c^2}$} 
%\put(-5,210){\large $\Xi_c^0 \rightarrow \Sigma^+ K^-$} 
%\put(155,20){$M(\lam K^0_{\rm S} K^+)-M(\lam)+1.116,~{\rm GeV}/c^2$} 
%\put(90,25){\includegraphics[width=0.7\textwidth]{lamc_lamk0k_repro_32fb_new.eps}}
%\end{picture}
%\caption{$\Lambda_{c}^+ \ra \lam K^0_{\rm S} K^+$: invariant mass spectrum of the
%	selected $\lam K^0_{\rm S} K^+$  combinations.} 
%\label{fig_lamc_lamkk}
%\end{figure}

%*********************** p K K **********************************************************


\section{Measurement of the {\boldmath $\Lambda_{\lowercase {c}}^+ \ra {\lowercase {p}} K^+ K^-$}
and {\boldmath $\Lambda_{\lowercase {c}}^+ \ra {\lowercase {p}} \phi$} decays}

The first evidence for the $\Lambda_{c}^+ \ra p \phi$ decay was reported by NA32 in 1990, who claimed
a signal of $2.8 \pm 1.9$ events~\cite{NA32_lamc_pphi}.
The decay $\Lambda_{c}^+ \ra p K^+ K^-$ was observed for the first time by E687
in 1993, who also obtained an upper limit for the branching ratio of
$\Lambda_{c}^+ \ra p \phi$~\cite{E687_lamc_pkk}.
The most recent statistically significant resonant analysis was published by
CLEO in 1996, 
who found the following branching ratios:
$\mathcal{B}(\Lambda_{c}^+ \ra p K^+ K^-)/\mathcal{B}(\Lambda_{c}^+ \ra p K^- \pi^+) = 0.039 \pm 0.009 \pm 0.007$
and $\mathcal{B}(\Lambda_{c}^+ \ra p \phi)/\mathcal{B}(\Lambda_{c}^+ \ra p K^- \pi^+) = 
0.024 \pm 0.006 \pm 0.003$~\cite{CLEO_lamc_pkk}.

\begin{figure}[h]
\centering
\begin{picture}(550,200)
\put(80,40){\rotatebox{90}{\large \bf Entries/(2 MeV/{\boldmath $c^2$})}}
%\put(-50,250){\large $\frac{N}{2.0~{\rm MeV}/c^2}$} 
\put(190,0){\boldmath $M(p K^+ K^-),~~~[{\rm GeV}/c^2]$} 
\put(110,10){\includegraphics[width=0.6\textwidth]{lamc_pkk_repro_32fb_new.eps}}
\end{picture}
\caption{$\Lambda_{c}^+ \ra p K^+ K^-$: invariant mass spectrum of the selected
	$p K^+ K^-$ combinations.} 
\label{fig_lamc_pkk}
\end{figure}
Reconstructing $\Lambda_{c}^+ \ra p K^+ K^-$ candidates according to
the procedure described in Section~\ref{section-selection},
we see a clear peak at the $\Lambda_{c}^+$ mass, as shown
in Fig.~\ref{fig_lamc_pkk}. 
We fit the distribution with a Gaussian
(with width fixed to $2.8\,\mathrm{MeV}/c^2$ from the MC) plus a second order
polynomial, and find $676\pm 89$ $\Lambda_{c}^+ \ra p K^+ K^-$ events.
For normalization we reconstruct the $\Lambda_{c}^+ \ra p K^- \pi^+$ decay
mode with equivalent cuts 
%(shown in Fig.~\ref{fig_lamc_pkpi}) 
and fit the
distribution with a double Gaussian for the large signal peak, and a second 
order polynomial, finding $51680\pm 650$ events.
The relative efficiency of the $\Lambda_{c}^+ \ra p K^- K^+$ decay reconstruction
with respect to $\Lambda_{c}^+ \ra p K^- \pi^+$ 
is found to be $0.93$ in the MC: using this value, we extract 
%the branching ratio 
$$
   \frac{\mathcal{B}(\Lambda_{c}^+ \ra p K^+ K^-)}
  {\mathcal{B}(\Lambda_{c}^+ \ra p K^- \pi^+)}
  = 0.014 \pm 0.002 \pm 0.002.
$$

%\begin{figure}[p!]
%\centering
%\begin{picture}(550,220)
%\put(30,190){\large $\frac{N}{2.0~{\rm MeV}/c^2}$} 
%\put(200,20){$M(p K^- \pi^+),~{\rm GeV}/c^2$} 
%\put(90,25){\includegraphics[width=0.7\textwidth]{lamc_pkpi_repro_32fb_new.eps}}
%\end{picture}
%\caption{The invariant mass spectrum for the normalization mode
%	$\Lambda_{c}^+ \ra p K^- \pi^+$, using equivalent cuts to the
%	$\Lambda_{c}^+ \ra p K^+ K^-$ analysis.}
%\label{fig_lamc_pkpi}
%\end{figure}

In order to obtain the $\Lambda_{c}^+ \ra p \phi$ signal we take $p K^+ K^-$
from a $\pm 6\, \mathrm{MeV}/c^2$ window
around the fitted $\Lambda_{c}^+$ mass (2286~MeV$/c^2$),
and plot the invariant mass of the $K^+K^-$ combination, as shown in
Fig.~\ref{fig_lamc_pkk_mkk} (points with error bars); 
the equivalent distribution is also shown for $p K^+ K^-$ from
$6\, \mathrm{MeV}/c^2$ sideband intervals centered $13\, \mathrm{MeV}/c^2$ below and
above the fitted $\Lambda_{c}^+$ mass (shaded histogram).
\begin{figure}[h]
\centering
\begin{picture}(550,200)
\put(80,40){\rotatebox{90}{\large \bf Entries/(2 MeV/{\boldmath $c^2$})}}
%\put(-50,250){\large $\frac{N}{2.0~{\rm MeV}/c^2}$} 
\put(195,0){\boldmath $M(K^+ K^-),~~~[{\rm GeV}/c^2]$} 
\put(110,10){\includegraphics[width=0.6\textwidth]{lamc_pkk_mkk_repro_32fb_new.eps}}
\end{picture}
\caption{Fitting for the $\Lambda_{c}^+ \ra p \phi$ component: 
	the invariant mass spectra of $K^+ K^-$ combinations from the
	$\Lambda_{c}^+ \ra p K^+ K^-$ signal area (points with error bars)
	and sidebands (shaded histogram).}
\label{fig_lamc_pkk_mkk}
\end{figure}
The distributions are 
fitted using a method similar to that used for the $\Lambda_{c}^+ \ra \sig \phi$ analysis
(Section~\ref{section-sigkk}).
%fitted in the same way as it was done in Section~\ref{section-sigkk}
%for $\Lambda_{c}^+ \ra \sig \phi$ analysis.
%The distributions are fitted with a Breit-Wigner function
%(describing the $\phi$ signal)
%convolved with a Gaussian of a fixed width
%(representing the detector mass resolution)
%plus a third order polynomial multiplied by a square root threshold factor.
%The width of the $\phi$ Breit-Wigner function was fixed to its nominal value~\cite{PDG},
%and the width of the Gaussian was fixed to $1.0\, \mathrm{MeV}/c^2$ based on MC simulation.
%The fit yields $661 \pm 33$ events for the $\phi$ signal in the $\Lambda_{c}^+$
%region and $327 \pm 25$ in the sidebands. 
%To extract the $\Lambda_{c}^+ \ra p \phi$ contribution we subtract
%the $\phi$ yield in the sidebands from the yields in the $\Lambda_{c}^+$ signal
%region, correcting for the phase space factor obtained from the $p K^+ K^-$
%background fitting function.
After making a sideband subtraction and correction for the signal outside the $\Lambda_{c}^+$ mass
interval we obtain $345 \pm 43$ $\Lambda_{c}^+ \ra p \phi$ decays.

The reconstruction efficiency of the $\Lambda_{c}^+ \ra p \phi$ decay 
relative to $\Lambda_{c}^+ \ra p K^- \pi^+$ was calculated using the MC 
and found to be $0.89$. 
%Taking into account the $\phi$ branching fraction
%$\mathcal{B}(\phi \ra K^+ K^-) = 49.2\%$~\cite{PDG},
%we calculate a branching ratio 
Using this value, we extract
$$
   \frac{\mathcal{B}(\Lambda_{c}^+ \ra p \phi)}
  {\mathcal{B}(\Lambda_{c}^+ \ra p K^- \pi^+)}
  = 0.015 \pm 0.002 \pm 0.002.
$$

%\begin{figure}[ht!]
%\centering
%\begin{picture}(550,220)
%\put(30,190){\large $\frac{N}{2.0~{\rm MeV}/c^2}$} 
%\put(200,20){$M(p K^+ K^-),~{\rm GeV}/c^2$} 
%\put(90,25){\includegraphics[width=0.7\textwidth]{lamc_pkk_nonphi_repro_32fb_new.eps}}
%\end{picture}
%\caption{The $\Lambda_{c}^+ \ra p K^+ K^-$ invariant mass spectrum,
%	after suppressing the $p \phi$ contribution:
%	the selection requirements and fit are described in the text.}  
%\label{fig_lamc_pkk_nonphi}
%\end{figure}
The non-$\phi$ $\Lambda_{c}^+ \ra p K^+ K^-$ signal is estimated by making an
invariant mass cut $| M(K^+ K^-) - m_\phi | > 10\, \mathrm{MeV}/c^2$ to suppress
the $\phi \ra K^+ K^-$ contribution. 
%The resulting $p K^+ K^-$ mass spectrum is 
%shown in Fig~\ref{fig_lamc_pkk_nonphi}. A 
%fitted with a Gaussian (with width fixed to $2.8\, \mathrm{MeV}/c^2$ from the 
%MC) and a second order polynomial, shown on the figure,
%yields $368 \pm 75$ events. 
%Integrating the $\phi$ Breit-Wigner function over the allowed $M(K^+K^-)$
%region, we find that $14\%$ of the total $\Lambda_{c}^+ \ra p \phi$ signal
%($48\pm 6$ events) contributes: subtracting these, we have
%$320 \pm 75$ events.
%The phase space correction factor accounting for the missing region around the
%$\phi$ mass is found to be 1.075 by MC simulation of the non-$\phi$
%$M(K^+K^-)$  spectrum: applying this correction 
%
After fitting the resulting $p K^+ K^-$ mass spectrum and applying corrections
accounting for the $\phi$ tails and the missing phase space region around the
$\phi$ mass we obtain
$344 \pm 81$ $\Lambda_{c}^+ \ra p K^+ K^-$ non-$\phi$ decays .
%Assuming the relative efficiency for reconstruction of
%$\Lambda_{c}^+ \ra p K^+ K^-$ (non-$\phi$) decays with respect to
%$\Lambda_{c}^+ \ra p K^- \pi^+$ to be the same as for inclusive $p K^+ K^-$,
This corresponds to 
%branching ratio is
$$
   \frac{\mathcal{B}(\Lambda_{c}^+ \ra p K^+ K^-)_{\mbox{\scriptsize \rm non-$\phi$}}}
  {\mathcal{B}(\Lambda_{c}^+  \ra p K^- \pi^+)}
  = 0.007 \pm 0.002 \pm 0.002.
$$

% ************* Systematic errors *********************************************

\section{Systematic errors}
\label{errors}

%We have considered several sources for the systematic errors of our measurements.
%First is the uncertainty in kaon identification efficiency, which enters all the
%ratios of the signal branchings to the reference ones. As has been checked on data with
%$D^{*+}$-tagged $D^0 \ra K^- \pi^+$ decays, there is no significant
%difference between the MC prediction and kaon efficiency measured on data for momenta 
%range below 3.0 GeV$/c$. Consistency has also been checked over the whole
%interval of polar angles. For the $\Lambda_{c}^+$ decay modes presented in our
%analyses the kaon momenta do not exceed 3.0 GeV$/c$, apart from distribution tails
%representing a few percent of the events. The 
%efficiency uncertainty accounting for the kaon and pion-ID cuts is taken to
%be $6\%$ per single $K/\pi$ ratio. 
%For the fits where the Gaussian width was fixed to the MC prediction, we have redone the fit
%with floating width, and the resulting change in the fitted number of events
%was taken as systematic uncertainty. 
%The double Gaussian fits performed on the large statistics sample of 
%$\Lambda_{c}^+ \ra p K^- \pi^+ (\lam \pi^+)$ decays show that 6$\%$ (10$\%$)
%of the signal events lie in non-Gaussian tails. Therefore a systematic uncertainty
%of 6$\%$ (10$\%$) has been added on corresponding Cabibbo-suppressed channels
%(where the fits were performed with a single Gaussian) to take into account
%possible non-Gaussian tails.
%The shape of the background function was varied by changing the order
%of the polynomial function.

We have considered several possible sources for the systematic errors in our measurements.
The most important is the uncertainty in the pion and kaon identification efficiencies,
which affects all ratios of signal and reference branchings. 
Based on a study of kaons and pions
from $D^{*+}$-tagged $D^0 \ra K^- \pi^+$ decays, we assign
% no significant difference
%between data and MC efficiencies for momenta below 3.0 GeV$/c$ (in the 
%$\Lambda_{c}^+$ decay modes presented here, kaon momenta do not exceed 3.0 GeV$/c$,
%except for distribution tails representing a few percent of the events). 
%Data and MC
%efficiencies are also found to be consistent over the full range of polar angles.
%We assign 
a systematic uncertainty of $6\%$ per $K/\pi$ ratio
(\emph{e.g.}\ $6\%$ for $\lam K^+/\lam \pi^+$,  $12\%$ for $\sig K^+ K^-/\sig \pi^+ \pi^-$).
%based on the statistical error of the $D^{*+}$-tagged samples.

Possible biases due to fitting procedure have also been studied.
In each fit, the shape of the background function has been varied by changing
the order of the polynomial function, with any change in the signal yield being
taken as a systematic uncertainty. For each fit where the width of the signal Gaussian
was fixed to the MC prediction, we have redone the fit with a floating width,
and taken the resulting change in the yield as a systematic uncertainty.
For the $\Lambda_c^+ \ra p K^+ K^-$ and $\lam K^+$ analyses, we have assigned additional
uncertainties of $6\%$ and  $10\%$ respectively on the signal yields, based on the
fractions of signal events found in non-Gaussian tails for the normalization modes $p K^- \pi^+$
and $\lam \pi^+$ (the $p K^+ K^-$ and $\lam K^+$ samples are too small to fit for the 
presence of non-Gaussian tails).
 
For the Breit-Wigner fit of the $\phi$ signal we have varied the function by letting the
width of the convolved Gaussian float and varying the shape of the background
parameterization. We have also included the $1.4\%$ uncertainty
of $\mathcal{B}(\phi \ra K^+ K^-)$ and varied the $\phi$ nominal width within
its error~\cite{PDG}.
In the case of $\Lambda_{c}^+ \ra \sig \phi$ and $p \phi$ 
decays there is an additional source of systematic error due
to the difference in kinematics between the signal and normalization modes.
This has been estimated to be $6\%$ for $\sig \phi$ and $4\%$ for $p \phi$,
based on the difference between the MC predictions for the 
efficiency in resonant and non-resonant cases. 

In the $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+,~\Xi(1690)^0 \ra \sig K^-$ resonant analysis we neglected the
possible interference between $\Xi(1690)^0$ and $\phi$ contributions. MC studies show
that this leads to an uncertainty of less than $5\%$,
due to phase space limitations in the interference region.
%For the $\Lambda_{c}^+ \ra p K^- \pi^+$ reference decay mode a $6\%$ systematic uncertainty
%was included due to the fitting of non-Gaussian tails of the signal.
%For the $\Lambda_{c}^+ \ra \lam \pi^+$ reference signal the non-Gaussian contribution
%is estimated to be 10$\%$.

%*******************************************************************************

\section{Conclusions}

In summary, we report the first observation of
the Cabibbo-suppressed decays $\Lambda_{c}^+\ra \lam K^+$ and
$\Lambda_{c}^+\ra \si K^+$,
and the first observation of $\Lambda_{c}^+\ra \Sigma^+ K^+\pi^-$ with
large statistics.
The decays $\Lambda_{c}^+ \ra p K^+ K^-$, 
$\Lambda_{c}^+\ra p \phi$ and $\Lambda_{c}^+\ra (p K^+ K^-)_{\mbox{\scriptsize \rm non-$\phi$}}$,  
and the W-exchange decays $\Lambda_{c}^+ \ra \sig K^+ K^-$ and
$\Lambda_{c}^+ \ra \sig \phi$ have been measured with the best accuracy to date.
We have also observed evidence for the decay $\Lambda_{c}^+ \ra \Xi(1690)^0 K^+$
and set an upper limit on the
non-resonant decay mode $\Lambda_{c}^+ \ra \sig K^+ K^-$.
The results for these decay modes are listed in Table 1.

\section{Acknowledgment}


We wish to thank the KEKB accelerator group for the excellent
operation of the KEKB accelerator. We acknowledge support from the
Ministry of Education, Culture, Sports, Science, and Technology of Japan
and the Japan Society for the Promotion of Science; the Australian
Research
Council and the Australian Department of Industry, Science and
Resources; the Department of Science and Technology of India; the BK21
program of the Ministry of Education of Korea and the CHEP SRC
program of the Korea Science and Engineering Foundation; the Polish
State Committee for Scientific Research under contract No.2P03B 17017;
the Ministry of Science and Technology of Russian Federation; the
National Science Council and the Ministry of Education of Taiwan; and
the U.S. Department of Energy.

% Most recent acknowledgements, included 26.10.2001

%*******************************************************************************

\begin{thebibliography}{99}

\bibitem{theory}
Y. Kohara, Nuovo Cim. {\bf A} 111 (1998) 67; \\
M. A. Ivanov et al., Phys. Rev. {\bf D} 57 (1998) 5632;\\
K. K. Sharma and R. C. Verma, Phys. Rev. {\bf D} 55 (1997) 7067; \\
L. Chau et al., Phys. Rev. {\bf D} 54 (1996) 2132;\\
A. Datta, {hep-ph/9504428};\\
T. Uppal et al., Phys. Rev. {\bf D} 49 (1994) 3417;\\ 
P. Zenczykowski, Phys. Rev. {\bf D} 50 (1994) 402;\\ 
J. K\"orner and M. Kr\"amer, Z. Phys. {\bf C} 55 (1992) 659.

\bibitem{PDG}
D. E. Groom et al.,
EPJ {\bf C} 15 (2000) 1 and 2001 off-year partial update 

\bibitem{BELLE_DETECTOR} 
A. Abashian et al. (Belle Collaboration), KEK Progress Report 2000--4 (2000),\\
to be published in Nucl. Instr. Meth. {\bf A}. 

\bibitem{NA32_lamc_sigkpi}
S. Barlag et al. (NA32 Collaboration),
Phys. Lett. {\bf B} 283 (1992) 465. 

\bibitem{CLEO_lamc_sigkk}
P. Avery et al. (CLEO Collaboration),
Phys. Rev. Lett. 71 (1993) 2391. 

\bibitem{xi1690}
C. Dionisi et al.,
Phys. Lett. {\bf B} 80 (1978) 145. 

\bibitem{NA32_lamc_pphi}
S. Barlag et al. (NA32 Collaboration),
Z. Phys. {\bf C} 48 (1990) 29. 

\bibitem{E687_lamc_pkk}
P. L. Frabetti et al. (E687 Collaboration),
Phys. Lett. {\bf B} 314 (1993) 477. 

\bibitem{CLEO_lamc_pkk}
J. P. Alexander et al. (CLEO Collaboration),
Phys. Rev. {\bf D} 53 (1996) R1013. 

\bibitem{CLEO_lamc_sigpipi}
Y. Kubota et al. (CLEO Collaboration),
Phys. Rev. Lett. 71 (1993) 3255. 

\end{thebibliography}

%\begin{table}[p]
%\caption{Summary of the results obtained in this paper. The last column shows the corresponding results
%	from previous experiments, where they exist.}   
%\medskip
%\normalsize
%\tiny
{\rotatebox{-90}{
\begin{tabular}{lcccccc}
 & & & & & & \\
$\Lambda_c^+$ signal mode & 
Signal & 
$\Lambda_c^+$ reference & 
Reference &
Relative &
$\mathcal{B}_{\rm signal}/\mathcal{B}_{\rm reference}$ & 
Other measurements  \\[-5pt]
 & 
yield & 
mode & 
yield &
efficiency &
 & 
 \\ 
\hline
$\lam K^+$ & 
$265 \pm 35$ & 
$\lam \pi^+$ &
$4550 \pm 111$ &
0.79 &
$0.074\pm 0.010\pm 0.012$ & 
$-$ \\
$\si K^+$ &
$75 \pm 18$ & 
$\si \pi^+$ &
$1597 \pm 67$ &
0.84 &
$0.056\pm 0.014\pm 0.008$ & 
$-$ \\
$ \Sigma^+ K^+ \pi^-$  & 
$105 \pm 24$ &
$ \Sigma^+ \pi^+ \pi^-$ &
$2368 \pm 89$ &
0.94 &
$0.047 \pm 0.011 \pm 0.008$ &  
$0.24^{+0.24}_{-0.16}$~\cite{NA32_lamc_sigkpi} \\
$ \Sigma^+ K^+ K^-$ &
$246 \pm 20$ & 
$ \Sigma^+ \pi^+ \pi^-$ &
$3650 \pm 138$ &
0.89 &
$0.076 \pm 0.007 \pm 0.009$ & 
$0.094 \pm 0.017 \pm 0.019$~\cite{CLEO_lamc_sigkk,CLEO_lamc_sigpipi} \\
$\Sigma^+ \phi$ & 
$129 \pm 17$ &
$ \Sigma^+ \pi^+ \pi^-$ &
$3650 \pm 138$ &
0.84 &
$0.085 \pm 0.012 \pm 0.012$ & 
$0.094 \pm 0.033 \pm 0.025$~\cite{CLEO_lamc_sigkk,CLEO_lamc_sigpipi} \\
%$  \Xi(1690)^0 K^+$ &  &  & \\ 
$ \Xi(1690)^0 K^+,~\Xi(1690)^0 \ra  \sig K^-$ &
$75 \pm 16$ & 
$\Sigma^+ \pi^+ \pi^-$ &
$3650 \pm 138$ &
0.89 &
$0.023 \pm 0.005 \pm 0.005$ & 
$-$ \\
$ \Xi(1690)^0 K^+,~\Xi(1690)^0 \ra  \lam {\bar K^0}$ &
$93 \pm 26$ &
$\lam {\bar K^0} K^+$ &
$363 \pm 26$ &
1.00 & 
$0.26 \pm 0.08 \pm 0.03$ & 
$-$ \\
${ \Sigma^+ K^+ K^-}$ (\rm non-res) & 
$-$ &
$ \Sigma^+ \pi^+ \pi^-$ & 
$3650 \pm 138$ &
0.89 &
$< 0.018$ @ 90\% CL & 
$-$ \\
$ p K^+ K^-$ &
$676 \pm 89$ & 
$ p K^- \pi^+$ &
$51680 \pm 650$ &
0.93 & 
$0.014 \pm 0.002 \pm 0.002$ & 
$0.039 \pm 0.009 \pm 0.007$~\cite{CLEO_lamc_pkk} \\
$ p \phi$ & 
$345 \pm 43$ &
$ p K^- \pi^+$ &
$51680 \pm 650$ &
0.89 & 
$0.015 \pm 0.002 \pm 0.002$ & 
$0.024 \pm 0.006 \pm 0.003$~\cite{CLEO_lamc_pkk} \\
${ p K^+ K^-}$ (non-$\phi$) &
$344 \pm 81$ & 
$ p K^- \pi^+$ &
$51680 \pm 650$ &
0.93 & 
$0.007 \pm 0.002 \pm 0.002$ & 
$-$ \\
\end{tabular}
}}
{\rotatebox{-90}{
The last column shows the most accurate previous measurement
of each decay mode, where applicable.}}
{\rotatebox{-90}{
Table 1. Summary of the results obtained in this paper. }}
%\end{table}

\end{document}

hep-ex0111032.extracted_bib
\begin{thebibliography}{99}

\bibitem{theory}
Y. Kohara, Nuovo Cim. {\bf A} 111 (1998) 67; \\
M. A. Ivanov et al., Phys. Rev. {\bf D} 57 (1998) 5632;\\
K. K. Sharma and R. C. Verma, Phys. Rev. {\bf D} 55 (1997) 7067; \\
L. Chau et al., Phys. Rev. {\bf D} 54 (1996) 2132;\\
A. Datta, {hep-ph/9504428};\\
T. Uppal et al., Phys. Rev. {\bf D} 49 (1994) 3417;\\ 
P. Zenczykowski, Phys. Rev. {\bf D} 50 (1994) 402;\\ 
J. K\"orner and M. Kr\"amer, Z. Phys. {\bf C} 55 (1992) 659.

\bibitem{PDG}
D. E. Groom et al.,
EPJ {\bf C} 15 (2000) 1 and 2001 off-year partial update 

\bibitem{BELLE_DETECTOR} 
A. Abashian et al. (Belle Collaboration), KEK Progress Report 2000--4 (2000),\\
to be published in Nucl. Instr. Meth. {\bf A}. 

\bibitem{NA32_lamc_sigkpi}
S. Barlag et al. (NA32 Collaboration),
Phys. Lett. {\bf B} 283 (1992) 465. 

\bibitem{CLEO_lamc_sigkk}
P. Avery et al. (CLEO Collaboration),
Phys. Rev. Lett. 71 (1993) 2391. 

\bibitem{xi1690}
C. Dionisi et al.,
Phys. Lett. {\bf B} 80 (1978) 145. 

\bibitem{NA32_lamc_pphi}
S. Barlag et al. (NA32 Collaboration),
Z. Phys. {\bf C} 48 (1990) 29. 

\bibitem{E687_lamc_pkk}
P. L. Frabetti et al. (E687 Collaboration),
Phys. Lett. {\bf B} 314 (1993) 477. 

\bibitem{CLEO_lamc_pkk}
J. P. Alexander et al. (CLEO Collaboration),
Phys. Rev. {\bf D} 53 (1996) R1013. 

\bibitem{CLEO_lamc_sigpipi}
Y. Kubota et al. (CLEO Collaboration),
Phys. Rev. Lett. 71 (1993) 3255. 

\end{thebibliography}