Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Search for the decay $K^+\to \pi^+ \nu \bar\nu$ in the momentum region
  $P_\pi < 195 {\rm ~MeV/c}$   We have searched for the decay $K^+ \to \pi^+ \nu \bar\nu$ in the kinematic
region with pion momentum below the $K^+ \to \pi^+ \pi^0$ peak. One event was
observed, consistent with the background estimate of $0.73\pm 0.18$. This
implies an upper limit on $B(K^+ \to \pi^+ \nu \bar\nu)< 4.2\times 10^{-9}$
(90% C.L.), consistent with the recently measured branching ratio of
$(1.57^{+1.75}_{-0.82}) \times 10^{-10}$, obtained using the standard model
spectrum and the kinematic region above the $K^+ \to \pi^+ \pi^0$ peak. The
same data were used to search for $K^+ \to \pi^+ X^0$, where $X^0$ is a weakly
interacting neutral particle or system of particles with $150 < M_{X^0} < 250
{\rm ~MeV/c^2}$.

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The categories of this paper are: math-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: q-bio
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The categories of this paper are: cond-mat
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The categories of this paper are: math
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The categories of this paper are: dg-ga
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The categories of this paper are: nucl-ex
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The categories of this paper are: econ
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The categories of this paper are: acc-phys
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The categories of this paper are: quant-ph
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The categories of this paper are: funct-an
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The categories of this paper are: stat
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The categories of this paper are: adap-org
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The categories of this paper are: atom-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: cs