document: the physics potential of forward proton tagging at the lhc has attracted a great deal of attention in recent years @xcite . 
   a main focus of interest is the central exclusive production ( cep ) process , @xmath7 , in which the protons remain intact and the central system @xmath8 is separated from the outgoing protons by a large rapidity gap . to a very good approximation 
   , @xmath8 is constrained to be in a colour singlet , @xmath9 , state . 
   observation of any particle , such as a standard model higgs boson , in the central exclusive channel would therefore provide a direct observation of its quantum numbers . 
   furthermore , by detecting the outgoing protons and measuring their energy loss accurately , it is possible to measure the mass of the centrally produced particle regardless of its decay products @xcite . 
   because of these unique properties , it has been proposed that forward proton detectors should be installed either side of the interaction points of atlas and/or cms . 
   the fp420 collaboration has proposed to install detectors in the region 420 m from the interaction points @xcite . 
   these detectors would allow the detection of central systems in the approximate mass range 70  gev  @xmath10  gev . 
   proposals also exist to upgrade the capabilities of atlas and cms to detect protons in the 220 m region @xcite . 
   these detectors , when used in conjunction with 420 m detectors , would extend the accessible mass range well beyond @xmath11 gev .    in this paper , we focus on the central exclusive production of the standard model ( sm ) higgs boson and a supersymmetric ( mssm ) higgs boson , with @xmath12 gev . the cep process is shown schematically in figure [ centralexclusive ] . 
   for this mass region , the dominant decay channel of the higgs boson is to @xmath13 , which is very difficult to observe in conventional higgs searches at the lhc because of the large qcd background . 
   this is not the case in central exclusive production due to the @xmath9 selection rule , which suppresses the leading order central exclusive @xmath13 background by a factor of @xmath14 , where @xmath15 is the mass of the @xmath13 di - jet system . as we shall see , this renders the @xmath13 decay channel observable at the lhc in certain scenarios if appropriate proton tagging detectors are installed . 
   the structure of the paper is as follows . 
   firstly , we give a brief overview of the proposed forward detector upgrades at 220 m and 420 m at atlas and cms , including a simulation of the acceptance of the detectors . 
   we then discuss the predicted signal cross sections and survey the background processes . taking a 120 gev standard model higgs boson as the benchmark 
   , we perform a simulated analysis including an estimation of the detector acceptance and smearing effects and level 1 trigger strategies . 
   the analysis is then extended to the mssm for a particular choice of parameters .    , 
   there are two properties of the proposed forward detector systems that are critical to this analysis ; the acceptance of the detectors in the mass range of interest and the ability of the forward detectors to correctly associate the detected outgoing protons with a higgs boson candidate event measured in the central detector . 
   this matching is critical at high luminosities , where the large number of proton - proton collisions per bunch crossing ( often referred to as pile - up ) leads to a high probability that forward protons from single diffractive or double pomeron ( dpe ) collisions not associated with a higgs candidate event will enter the forward detectors during the same bunch crossing . 
   the proposed forward detectors aim to associate particular protons with the higgs candidate event by making a measurement of the outgoing proton time - of - flight ( tof ) from the interaction vertex to the detectors . 
   the difference in the arrival times of the protons on opposite sides of the central detector , @xmath16tof , allows a vertex measurement to be made under the assumption that the detected pair of protons originate from a single hard interaction . 
   this vertex can then be matched with the vertex of a candidate higgs event reconstructed using the central detector alone . 
   the current design goal of the forward detectors is to achieve a 10ps accuracy in the tof measurement @xcite , which translates into a vertex measurement accurate to 2.1 mm . 
   the use of fast - timing measurements is discussed in more detail in section [ olap ] . 
   it has been suggested that the central detector could also be used provide a third timing measurement @xcite , which could allow for an improved rejection of pile - up events . 
   we discuss the effect of this possibility in section [ sec : results ] . 
   the acceptance of the forward detectors is governed by the distance of the active edge of the detector from the beam , which determines the smallest measurable energy loss of the outgoing protons , and the aperture of the lhc beam elements between the interaction point and the forward detectors ; protons that lose too much momentum will be absorbed by beam elements , imposing an upper limit on the measurable momentum loss of the protons . 
   the distance of the active edge of the detector from the beam depends primarily on the beam size at each detector location . 
   previous estimates @xcite have assumed that the closest distance , @xmath17 , is given by @xmath18 where @xmath19 is the gaussian beam size at the detector location and 0.5 mm is a constant term that accounts for the distance from the sensitive edge of the detector to the bottom edge of the window . 
   the beam size @xmath20 is approximately 250 @xmath21 m at 420 m and 100 @xmath21 m at 220 m , leading to a distance of closest approach of 3 mm for detectors at 420 m and 1.5 mm for detectors at 220 m . 
   it is likely that the detectors will begin operation at a larger distance from the beam , at least until the detectors and machine background conditions are well understood @xcite . 
   figure [ forwardaccept ] ( a ) shows the acceptance for events in which both outgoing protons are detected at 420 m around ip1 ( atlas ) , as a function of the mass of the central system for different detector distances from the beam . 
   the protons are generated using the exhume monte carlo @xcite and tracked through the lhc beam lattice using the fptrack program with version 6.500 of the lhc optics @xcite . 
   the central system mass , @xmath15 , is calculated from the forward proton momenta , @xmath22 where the @xmath23 are the fractional momentum losses of the protons and @xmath24 is the centre - of - mass energy of the collision . 
   the acceptance for a 120 gev higgs boson is independent of the distance of approach of the detectors from the beam up to approximately 7 mm in the case where both protons are detected at 420 m . 
   this is consistent with the findings of @xcite . 
   the situation is very different for events in which one proton is tagged at 220 m and the other at 420 m , as shown in figure [ forwardaccept ] ( b ) . in this case , the acceptance is increased when either detector is moved closer to the beam . for a 120 gev higgs boson , with 420 m detectors at 5 mm and 220 m detectors at 2 mm , 
   the acceptance is 28% if both protons are tagged at 420 m ( symmetric tag ) with an additional 16% acceptance if one proton is tagged at 220 m and one at 420 m ( asymmetric ) . 
   moving the 420 m detectors inwards to 3 mm and the 220 m detectors to 1.5 mm increases the asymmetric acceptance by up to a factor of three , as shown in figure [ forwardaccept ] ( b ) . 
   figures [ forwardaccept ] ( a ) and ( b ) also demonstrate the increasing importance of 220 m detectors as the mass of the central system increases . at ip5 ( cms ) 
   the symmetric acceptance is identical to that at ip1 . 
   for the asymmetric tags , however , the acceptance is worse by a factor of @xmath25 across the mass range of interest . 
   this is caused by the horizontal ( rather than vertical ) plane of the crossing angle of the beams at ip5 @xcite . in this paper 
   we concentrate on the measurement around ip1 ( atlas ) . 
   central exclusive signal and background events are simulated with parton showering and hadronisation effects using exhume v1.3.4 @xcite . 
   exhume contains a direct implementation of the khoze , martin and ryskin ( kmr ) calculation of the central exclusive production process @xcite . 
   the cross section for the cep of a standard model higgs boson decaying to @xmath27 as a function of the higgs mass is shown in figure [ sigmamh ] . 
   the rapid decrease in cross section at @xmath28 gev occurs because the primary decay channel changes from @xmath13 to @xmath29 . for masses above @xmath30 gev 
   , it is expected that the higgs boson should be observed in the @xmath29 channel with a luminosity of 30 fb@xmath4 using forward proton tagging @xcite . 
   the primary uncertainties in the predicted cross section come from two sources ; the parton distribution functions ( pdf ) and the soft survival ( often termed rapidity gap survival ) factor . 
   the cep cross sections are relatively sensitive to the pdfs because the derivative of the gluon density enters to the fourth power . 
   figure [ sigmamh ] shows the cross section prediction for three different pdf choices , cteq6 m , mrst2002nlo and cteq6l . the cross section for the cep of a standard model higgs boson of mass @xmath31 gev decaying into b - quarks varies from 1.86  fb ( mrst2002nlo ) to 7.38  fb(cteq6l1 ) . 
   the spread of a factor of @xmath32 is consistent with the findings of @xcite . 
   for the purposes of this analysis , we chose cteq6 m as our default pdf as it lies between the two extremes . 
   there is some justification for choosing an nlo pdf because the kmr calculation contains a nlo qcd k - factor ( 1.5 ) for sm higgs boson production . 
   the soft survival factor , @xmath33 , is the probability that there are no additional hard scatters in a single @xmath34 collision and is expected to vary from process to process . for cep processes , we take the exhume default of @xmath35 at lhc energies . until very recently 
   , there was a consensus that @xmath33 should be between @xmath36 and @xmath37 for the cep of a higgs boson at the lhc @xcite . 
   two very recent studies have predicted a lower value @xcite and it remains to be seen whether a new theoretical consensus can be reached . in any case 
   , @xmath33 will be measurable in early lhc data . 
   , for three different proton parton distribution functions.[sigmamh],scaledwidth=50.0% ]    the mssm contains three neutral higgs bosons - two scalar and one pseudo - scalar . 
   the @xmath38 , parity - even selection rules strongly suppress cep of the pseudo - scalar . 
   this can be advantageous in areas of mssm parameter space where the pseudo - scalar is almost degenerate in mass to one ( or both ) of the scalar higgs bosons , since cep would provide a clean and complimentary measurement of the mass of the scalar only @xcite , and allow nearly - degenerate higgs bosons to be distinguished @xcite . 
   furthermore , at large values of tan@xmath39 , the cross section for the cep of the scalar higgs bosons can be strongly enhanced relative to the cep of a sm higgs boson of the same mass @xcite . 
   we choose a point in parameter space defined by the @xmath40 scenario @xcite with @xmath41 gev and tan@xmath42 , resulting in the scalar higgs boson having a mass of 119.5 gev and a decay width of 3.3 gev . with this choice of parameters 
   , the lightest scalar higgs boson has an enhanced cep cross section and is almost degenerate in mass with the pseudo - scalar . 
   the cep cross section for the lightest higgs boson decaying to b - quarks in this scenario is predicted to be @xmath43 fb . the uncertainty on this prediction 
   is the same as that for the sm higgs boson . 
   the backgrounds to central exclusive higgs production can be broken down into three categories ; central exclusive di - jet production , double pomeron exchange and overlap . 
   the most difficult backgrounds to deal with are the central exclusive ( cep ) di - jet backgrounds . 
   central exclusive @xmath13 production is suppressed at leading order by the @xmath45 selection rule , but is still present and forms an irreducible continuum background beneath the higgs mass peak . 
   central exclusive glue - glue production has a much larger cross section than @xmath13 production since it is not suppressed . 
   it contributes to the background when the gluon jets are mis - identified as b - jets ( 1.3% probability for each mis - tag at atlas - see section [ detector ] ) . 
   central exclusive @xmath46 events do not contribute to the background for two reasons . 
   firstly , @xmath46 production is suppressed with respect to the @xmath13 process by @xmath47 . 
   secondly , the @xmath46 background is further suppressed by a factor of @xmath48 relative to the @xmath13 cross section assuming that the probability of mis - identifying a c - quark is approximately 0.1 for a b - tag efficiency of 0.6 @xcite . 
   higher order cep di - jet backgrounds such as the 3-jet final state @xmath49 process have been studied in @xcite . 
   it is expected that these backgrounds will be lower than the lo @xmath50 and @xmath27 backgrounds after experimental cuts , but they should be implemented into the exhume monte carlo and a full study performed before definitive conclusions can be drawn . 
   double pomeron exchange ( dpe ) is defined as the process @xmath51 , where @xmath52 is a central system produced by pomeron - pomeron fusion ( @xmath53 ) . 
   the pomeron is assigned a partonic structure and so there are always pomeron remnants accompanying the hard scatter as shown in figure [ dpepomeron ] . 
   the relevant background processes are separated into @xmath13 and @xmath54 , where @xmath55 represents light - quark and gluons jets . 
   the dpe events are simulated using the pomwig v2.0 event generator @xcite , which implements the diffractive parton distribution functions measured by the h1 collaboration @xcite . 
   we use h1 2006 fit b , although we discuss the effect of using different diffractive pdfs . 
   we choose the pomwig option to treat the valence partons in the pomeron as gluons . 
   pomwig is normalised to the h1 data , and therefore does not account for the soft survival factor . for hard double pomeron events , we take @xmath35 . 
   pomwig is also capable of generating scatters involving sub - leading ( non - diffractive ) exchanges ( @xmath56 ) . 
   we do not include this contribution because the cross section for @xmath57 @xmath13 events is @xmath58 0.034 fb for @xmath59 , @xmath60 gev and @xmath61-quark @xmath62 gev . 
   this is negligible compared to the cross section for @xmath63 @xmath13 , which is @xmath5865 fb in the same kinematic region . 
   system via double pomeron exchange ( dpe ) . 
   [ dpepomeron],scaledwidth=50.0% ]      overlap events are defined as a coincidence between an event that produces a central system of interest , @xmath52 , and one or more diffractive events ( single diffractive @xmath64 or dpe ) which produce protons in the acceptance range of a forward detector in the same bunch crossing . on average 
   there will be 3.5 interactions per bunch crossing at low instantaneous luminosity ( 10@xmath65 @xmath3 s@xmath4 ) and 35 interactions at high instantaneous luminosity ( 10@xmath66 @xmath3 s@xmath4 ) . 
   we investigate three types of overlap event ; [ p][x][p ] , [ pp][x ] and [ px][p ] . 
   the square brackets specify the interaction to which a part of the overlap event belongs - in this notation both the cep and dpe events would be [ pxp ] 
   .    the cross section , @xmath67 , for the overlap background may be estimated by @xmath68 } \ , \left [ \ , \sum_{n=3}^{\infty } \frac{\lambda^{n } e^{-\lambda}}{n ! } \ ,   p_{2[p]}\left(n-1\right ) \ , + \ , \sum_{n=2}^{\infty }   \frac{\lambda^{n } e^{-\lambda}}{n ! } \ ,   p_{[pp ] } \left(n-1\right )   \right ] \nonumber \\ & & + \ , \ , \sigma_{[px ] } \ , \sum_{n=2}^{\infty } \frac{\lambda^{n } e^{-\lambda}}{n ! } \ 
   ,   p_{[p ] } \left(n-1\right ) , \end{aligned}\ ] ] where @xmath69}$ ] is the inclusive ( @xmath70 ) di - jet cross section , @xmath71 is the average number of @xmath34 interactions per bunch crossing and @xmath72 is the actual number of interactions in a specific bunch crossing . because the actual number of interactions is not fixed , we sum over all possible numbers and weight each configuration by a poisson distribution . in the first term , @xmath73}\left(n\right)$ ] is the probability that , given @xmath74 interactions , there are at least two events that produce a forward proton ( one on each side of the interaction point ) by any mechanism . 
   this is dominated by soft single diffractive events @xmath64 . 
   @xmath73}\left(n\right)$ ] is given by a trinomial distribution , i.e. @xmath75}\left(n\right ) = \sum_{r+q=2}^{n } \sum_{q=1}^{r+q-1 } \frac{n!}{\left(n-\left[r+q\right]\right ) ! 
   \ , r ! \ , q ! } \ 
   , \left ( f_{[p]}^{+}\right)^{r } \ , \left(f_{[p]}^{-}\right)^{q } \ , \left ( 1 - f_{[p]}^{+ } - f_{[p]}^{-}\right ) ^{n - r - q}\ ] ] where , for example , @xmath76}^{+}$ ] is the fraction of events at the lhc that produce a forward proton in the @xmath77 direction and within the forward detector acceptance .    in the second term , @xmath78}(n)$ ] is defined as the probability that there is at least one event that contains an outgoing proton on each side of the interaction point within the acceptance of the forward detectors ( dominated by the soft double pomeron process @xmath79 ) . 
   @xmath78}(n)$ ] is a binomial distribution that utilises the event fraction @xmath80}$ ] . 
   note that double pomeron events can also contribute to the first term in equation [ olapxs ] , in the case where only one of the outgoing protons falls within the detector acceptance .    the third term deals with a two - fold coincidence between a single diffractive di - jet event ( @xmath64 ) , which produces a proton within the forward detector acceptance and a hard central diffractive system @xmath52 that mimics the signal , and an overlap event that produces a proton on the opposite side . 
   @xmath81}$ ] is the total single diffractive di - jet cross section ( @xmath64 ) , i.e the outgoing proton can be on either side of the interaction point . 
   @xmath82}(n)$ ] is defined as the probability that there is at least one event with a forward proton in the forward detector acceptance on the opposite side of the ip to the single diffractive proton from the hard event . 
   this is defined in a similar way to @xmath78}(n)$ ] , but using the event fraction @xmath76}$ ] .    for small proton momentum losses , 
   the cross section for events at the lhc that contain a forward proton is dominated by the single diffractive cross section , @xmath83 @xcite ; @xmath84 where @xmath85 is the total cross section at the lhc , @xmath86 is the soft - survival factor for soft single diffraction ( 0.087 ) , @xmath87 is the pomeron trajectory , @xmath88 is the squared 4-momentum transfer at the proton vertex , @xmath89 is the pomeron nucleon coupling and @xmath90 is the triple pomeron vertex . 
   the value of @xmath76}$ ] is calculated by integrating equation [ sdxs ] using standard monte carlo techniques , for a specific acceptance range in @xmath91 and @xmath88 . 
   the cross section for the [ p][x][p ] di - jet events , for parton @xmath92gev and @xmath93 , @xmath94 is shown in figure [ olaplumidep ] ( a ) . 
   the cross section increases by two orders of magnitude from low to high luminosity .    at larger values of @xmath91 
   , there will be an additional contribution from non - diffractive ( i.e. reggeon exchange ) events . 
   this contribution is estimated using the pythia @xcite and phojet @xcite event generators . 
   the predictions for the diffractive and non - diffractive contributions to @xmath76}$ ] are compared to equation [ sdxs ] in table [ fractions ] . 
   as expected , the diffractive contribution dominates at small @xmath91 . 
   the non - diffractive contribution becomes increasingly important at higher @xmath91 , but remains smaller than the diffractive contribution for @xmath95 . 
   phojet predicts a higher fraction of non - diffractive events than pythia and also predicts that a large fraction of forward protons are due to dpe events . in this analysis 
   we use the prediction for single diffraction given by equation [ sdxs ] for @xmath96 ( which would give hits only at 420 m ) , and take the non - diffractive contribution to be negligible . for @xmath97 , we add the pythia prediction for the non - diffractive component , since this agrees more closely with theoretical expectations that the non - diffractive fraction of @xmath34 collisions at the lhc in this kinematic range should be between 1.0% and 1.7% @xcite . 
   .the fraction of events at the lhc that produce a forward proton on one side of the interaction point in a specific kinematic range . 
   the pythia and phojet event generators are compared to the single diffractive cross section given in equation [ sdxs ] . 
   sd labels the outgoing proton from single diffractive scatters and nd labels the protons produced from non - diffractive scatters . 
   dpe labels double pomeron exchange events . 
   [ fractions ] [ cols="^,^,^,^,^,^ " , ] 
   we have shown that the central exclusive production of the lightest scalar higgs boson , for the choice of mssm parameter space described in section [ scenarios ] , can be observed with a significance of at least @xmath1 in the @xmath13 decay channel within 3 years of data taking at the lhc , if suitable proton tagging detectors are installed around atlas and cms and the current predictions for the cep cross section are correct . 
   we have evaluated the most important backgrounds and shown that they can be rejected with high efficiency using a set of exclusivity variables . 
   we have also shown that a fraction of b - jet events can be retained by the currently forseen level 1 trigger hardware , if the trigger strategies we outline are adopted . whilst we have only considered a particular choice of parameters in detail 
   , the general conclusions should hold for a wide range of scenarios . as a general rule , 
   if the cep cross section for the production of the higgs boson is greater than @xmath98 fb , and the higgs boson decays predominantly to b - quarks , then the analysis presented here should apply if the decay width is not too large . 
   the analysis will also apply to any new particle that decays predominantly to b - quarks . 
   it is worth speculating what the future experimental strategy might be if a higgs sector such as the @xmath0 scenario of the mssm is discovered at the lhc or tevatron , and the cep channel proves to be observable with the forward detector configurations currently proposed . 
   the largest loss of cep signal events comes from the limited acceptance of the proton detectors ( between @xmath99 and @xmath100 depending on detector configuration ) and the l1 trigger efficiency , which is at best around @xmath101 at high luminosity for the strategies we consider in this paper . 
   if a hardware upgrade of the l1 trigger systems of atlas and cms were to increase the trigger latency such that the 420 m detector signals could be included , then a trigger efficiency of close to @xmath102 could be achieved . 
   figure [ upgrade ] ( a ) shows a typical mass fit , with 300 fb@xmath4 of data taken at high luminosity using 420 m detectors alone ( i.e. symmetric events ) , assuming 100% trigger efficiency . 
   the significance of this signal is approximately @xmath103 . if it is assumed that , in addition , fast timing detector improvements can be made as described in section [ mssm ] 
   , then the significance rises to nearly @xmath104 , as shown in figure [ upgrade ] ( b ) . in this case , a measurement could be made within 100 fb@xmath4 . 
   combining the symmetric and asymmetric analyses increases the significance such that a 5@xmath105 measurement could be achieved within 30 fb@xmath4 . 
   the mass of the lightest scalar higgs can be measured with an accuracy of better than @xmath106 gev . for the @xmath0 scenario considered here , 
   the width of the lightest higgs is not much larger than the mass resolution of the proton detectors , and therefore the extraction of the higgs width from the fits is marginal . in other scenarios with widths in excess of @xmath107 gev , however , a direct measurement of the width would also be possible . 
   we remind the reader that observation of any particle in the cep channel would provide a direct measurement of the quantum numbers of the particle . furthermore 
   , if the pseudo - scalar higgs is close in mass to the lightest scalar higgs , then cep would provide an unambiguous separation of the two states since the pseudo - scalar can not be produced . 
   finally , with such a trigger strategy in addition to improved fast timing to reduce the overlap backgrounds , the sm higgs may be obervable in the b - jet channel . 
   we would like to thank mike albrow , michele arneodo , andrew brandt , albert deroeck , jeff forshaw , valery khoze , henri kowalski , paul newman , will plano , misha ryskin , marek tasevsky and chris tevlin for interesting discussions and suggestions throughout this project . 
   this work was funded in the uk by stfc and the royal society . 
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summary: a detailed study is presented of the search for higgs bosons in the b - decay channel in the central exclusive production process at the lhc . 
   we present results for proton tagging detectors at both 220 m and 420 m around atlas or cms . 
   we consider two benchmark scenarios ; a standard model ( sm ) higgs boson and the @xmath0 scenario of the minimal supersymmetric standard model ( mssm ) . 
   detector acceptance , smearing and event trigger strategies are considered . 
   we find that the sm higgs will be challenging to observe in the b - jet channel without improvements to the currently proposed experimental configuration , but a neutral scalar mssm higgs boson could be observable in the b - jet channel with a significance of @xmath1 or greater within three years of data taking at all luminosities between @xmath2  @xmath3  s@xmath4 and @xmath5  @xmath3  s@xmath4 , and at @xmath6 or greater after three years in certain scenarios .