The recently proposed COMPASS experiment at CERN attempts a measurement of the gluon polarisation with a precision of delta(Delta g/g) = 0.1. The experiment uses open charm muoproduction to tag the photon-gluon fusion process

Mallot, G K

arXiv

The recently proposed COMPASS experiment at CERN attempts a measurement of the gluon polarisation with a precision of delta(Delta g/g) = 0.1. The experiment uses open charm muoproduction to tag the photon-gluon fusion process.The recently proposed COMPASS experiment at CERN attempts a measurement of the gluon polarisation with a precision of delta(Delta g/g) = 0.1. The experiment uses open charm muoproduction to tag the photon-gluon fusion process

Mallot, G.K.

CERN Document Server

hep-ex/9611016Spin Physics with COMPASSG.K. MallotInstitut fur Kernphysik der Universitat Mainz, Becherweg 45, D-55099 Mainz, GermanyOn Behalf of the COMPASS CollaborationThe recently proposed COMPASS experiment at CERN attempts a measure-ment of the gluon polarisation with a precision of g=g ' 0:1. The experi-ment uses open charm muoproduction to tag the photon-gluon fusion process.One of the most urgent questions in understanding the nucleon's spin structure isthe polarisation of gluons, g=g. A large value of g could explain the smallnessof the contribution of the quark spins to the nucleon spin,  [1, 2]. Hints for alarge value of g in the order of 2{3 h at Q2= 10 GeV2come from a recent QCDanalysis [3] of existing g1data. However, an unambiguous determination of g, canonly be obtained from a process involving the gluon distribution in leading order.A particularly clean such process is open charm production via the photon-gluonfusion process, g ! cc, shown in Fig. 1. Contributions from quark distributionscan be neglected because there is no or only a small intrinsic charm quark contentin the nucleon. The scale is set by the charm quark mass, 4mc' 10 GeV2.pµccqγ *µkgFigure 1: The photon-gluon fusion diagram.The main goal of the COMPASSexperiment is the determination ofg=g from the cross section asymme-try for polarised open charm muopro-duction from a xed polarised target.The tagging of charm events is basedon the identication of D0mesonsvia their D ! K decay channels.The experiment will also provide highstatistics data for g1, semi-inclusivemuon scattering, and the transversitystructure function, h1. The layoutof the apparatus is similar to that ofthe SMC experiment and parts of thissetup will be used.For real or quasi-real photons the charm-production cross section via the photon-gluon fusion process can be written asg!cc= (s^) + g(s^); (1)Presented at SPIN '96|The 12th. International Symposium on High-Energy Spin Physics(Amsterdam, Sept. 1996).1050100(nb)(GeV2)sˆσ∆σ1 10 10200.511.5210-210-11xgx∆gxFigure 2: The photon-gluon cross sections and  as a function of s^ (top) and xgand xg, as a function of x = xg(bottom).10 210 3σ(ν)∆σ(ν)(nb)00.10.20.30.40 50 100 150 200AγcNcν (GeV)Figure 3: The photon-nucleon cross sec-tions,  and , (top) and the AccN, (bot-tom) as a function of .where s^ = (q + k)2is the energy squared and ;gare the helicities in the photon-gluon c.m. system. The spin-averaged and the spin-dependent part, (s^) and (s^),are known to next-to-leading and leading order [4, 5], respectively. Both terms, and , rise sharply at the threshold, s^ = 4m2c, and  changes sign at about fourtimes the threshold (Fig. 2). The photon-nucleon cross-section asymmetry, AccN, forthe process N ! cc is shown in Fig. 3. It is obtained by integrating the crosssections over the kinematically allowed range xming xg 1AccN() =N!ccXN!ccX=R2M4m2cds^(s^)g(xg; s^)R2M4m2cds^ (s^) g(xg; s^): (2)Here xg= s^=2M denotes the nucleon-momentum fraction carried by the gluon,which in this process is dierent from the kinematic variable, x = Q2=2M.From coherence arguments [6] it is expected that the gluon polarisation behaveslike g(x)=g(x) / x for x! 0. Such a behaviour is used in most parametrisationsof the polarised gluon distribution function. Therefore, g=g is expected to be largeonly at rather large values of x. For xg 0:1 the gluon polarisation can exceed 0.5.This value of xgcorresponds to a photon energy of order 50 GeV for which the s^and xgaxes in Fig. 2 correspond to each other. For the rate estimates we use thepolarised gluon distribution of Ref. [7] (set B) shown in Fig. 2.COMPASS is a xed-target experiment similar to that of the SMC and usesa \Common Muon and Proton Apparatus for Structure and Spectroscopy". Apartfrom the spin physics programme discussed here in part the proposal [8] also containsa spectroscopy programme with hadron beams. The experiment is statistics limited2even with the compared to the SMC experiment ve times higher muon intensity of2108muons per spill of 2.4 s every 14.4 s. The necessity to detect pions and kaonsfrom the D decays in a wide angular range requires a two stage magnetic spectro-meter with particle identcation in both spectrometer stages. Downstream of thepolarised target the new hadron stage of the spectrometer covers hadron angles inthe range 200 mrad. Its large-aperture dipole magnet provides a bending power of1 Tm. In the second stage of the spectrometer the scattered muon and fast hadronsare measured. It uses the present SMC spectrometer magnet. Particle identica-tion will be performed by ring-imaging Cherenkov counters and by electromagneticand hadronic calorimeters in each of the two spectrometer stages. The upstreamand downstream RICH provide pion-kaon separation in the momentum range 3{65 GeV/c and 30{120 GeV/c, respectively. Existing lead-glass arrays will be usedfor the electromagnetic calorimeters. The hadronic calorimeters will in the muonprogramme mainly serve trigger purposes and tag deep-inelastic events. Both spec-trometer stages end with a muon wall consisting of a hadron absorber followed bytracking chambers and trigger hodoscopes. The tracking in the beam region will beperformed by scintillating bre detectors. The large angles of the produced hadronswith respect to the incoming beam also require a new solenoid for polarised targetwith an opening of about 200 mrad matching that of the rst spectrometer stage.As target materials lithium deuteride,6LiD, for the deuteron and ammonia, NH3,for the proton are foreseen, polarised to 50 % and 85 %, respectively. The nuclearstructure of6LiD is well described by the \alpha + deuteron" picture, which resultsin the favourable dilution factor of f = 0:50 compared to f = 0:16 for ammonia.The diameter of the two oppositely polarised 60 cm long target cells will be reducedto 3 cm in order to minimise the amount of material traversed. The nominal lumi-nosity amounts to L = 51032cm 2s 1. The minimum xgvalue one wants to accessdetermines the maximum photon energy, , needed. The muon energy should onlybe slightly larger than , in order to obtain a large average depolarisation factor,D. A muon energy of 100 GeV appears to be the optimal choice for xg' 0:1. Themuon energy can be increased up to 200 GeV to explore smaller values of xg.For the kinematics of the COMPASS experiment we nd in average 1.2 D0mesonsper initial cc pair including D mesons from D?decays. We concentrate on the two-body decays D0(cu)! K +and c:c: with branching ratios of 4 %. A major concernis the combinatorial background of kaon-pion pairs within the D mass window. Themass of the D meson will be recontructed with a resolution of MD' 10 MeV. Inthe range 25 <  < 85 GeV the open charm production cross section amounts to2 nb compared to 500 nb for ordinary photoproduction. Often the distance betweenthe production and the decay vertex is used in charm experiments to clean up thekaon-pion sample. This technique cannot be applied in the COMPASS experiment,because this distance of a few mm cannot be resolved due to multiple scattering inthe target. Kaons emitted at large angles, K, in the D's rest frame with respect tothe D's direction of ight in the laboratory frame have large transverse momenta.On the other hand, kaons from ordinary fragmentation have small transverse mo-3                                                                                                                                                     Figure 4: Monte-Carlo simulation: a) D ! K and b) combinatorial background versuszDand j cos ?Kj. The lines indicates the cuts used in the rate estimates.menta and thus dominantly mimic decays with small K. The background rejectionwas studied in Monte Carlo simulations using the AROMA [9] and JETSET eventgenerators for the photon-gluon fusion process and the background, respectively(Fig. 4). The best result is obtained with the requirements j cos Kj  0:5 andzD= ED=  0:25, which improve the background-to-signal ratio by a factor 1750to about NB=Ncc' 3:8 on the expense of loosing 65 % of the D mesons. Fora running time of 2 1/2 years with 150 days/year and assuming a combined e-ciency of 0.25 for the muon beam and the experimental apparatus the statisticalerror of the measured asymmetry is AccN= 0:076. Due to the higher gure ofmerit of the6LiD target a similar precision will already be reached after the rst 11/2 years. The result can be improved using D?tagging by the soft pion from thedecay D?+! D0+s! (K +)+s: Considering only the soft pions with momentalarger than 1 GeV/c and taking possible re-interaction in the target into accountthe statistical error of the asymmetry reduces toAccN= 0:05 corresponding to gg= 0:14: (3)As in the inclusive case the muon-nucleon asymmetry is reduced from the virtual-photon asymmetry by the depolarisation factor, AccN= DAccN(Fig. 5). The sensitiv-ity to g=g peaks at xg= 0:14 and covers the range 0:07  xg 0:4 (Fig. 6). Apartfrom the c.m. energy, s^, the asymmetry for the elementary photon-gluon fusion pro-cess, (s^;^)=(s^), also depends on on the c.m. angle,^, between the photon-gluonaxis and the cc axis. The sensitivity is larger for small angles,^, corresponding inthe laboratory frame to small transverse momenta, pT. Rejecting D mesons with400.250.5CAB AγcNc00.20.2 0.4 0.6 0.8CABAµcNcyFigure 5: Photon-nucleon asymmetry (top),and muon-nucleon asymmetry (bottom), asa function of y = =Efor the threegluon distributions of Ref. [7]. The datapoints indicate the projected precision ofthe COMPASS measurement.010002000300040005000600070000 0.2 0.4 0.6 0.8 1xgNo of eventsFigure 6: Number of accepted photon-gluonfusion events as a function of xgfor therange 35    85 GeV.pT> 1 GeV=c thus yields a larger analysing power leading toAccN= 0:04 corresponding to gg= 0:11: (4)The three and four-body decay channels may further improve the precision, in par-ticular if the D?tagging can be applied. This is still under investigation.Apart from the measurement of g=g COMPASS oers a rich spin-physics pro-gramme at high Q2with a high luminosity including the transversity structurefunction h1, spin-avour decomposition of the structure functions, and lambda po-larisation in both the target and current fragentation regions. The SPSLC has rec-ommended the COMPASS experiment for approval. After commissioning in 1999data taking could start in the year 2000.I like to thank the COMPASS Collaboration and in particular my Mainz col-leagues, A. Bravar, D. von Harrach, E. Kabu, and J. Pretz for the help in preparingthis manuscript.1. The SM Collaboration, D. Adams et al., Phys. Lett. B 357 (1995) 248.2. The E-143 Collaboration, K. Abe et al., Phys. Rev. Lett. 75 (1995) 25.3. R. D. Ball, S. Forte, and G. Ridol, Phys. Lett. B378 (1996) 255.4. A. D. Watson, Z. Phys. C12 (1982) 123.5. M. Gluck and E. Reya, Z. Phys. C39 (1988) 569.6. S. J. Brodsky, M. Burkardt, and I. Schmidt, Nucl. Phys. B 441 (1994) 197.7. T. Gehrmann and W. J. Stirling, Z. Phys. C65 (1994) 461.8. The COMPASS Collaboration, COMPASS proposal, CERN/SPSLC 96-14,SPSC/P297 (Geneva, March 1996); CERN/SPSLC 96-30 (Geneva, May 1996).59. G. Ingelman, J. Rathsman, and G. Schuler, DESY preprint, AROMA version 2.1Manual.6

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