/home/www/ftp/data/hep-ex/dir_0209049/0209049.dvi 1 Latest measurements of beauty quark production at HERA V. Chiochia aDESY]DESY - Deutsches Elektronen-Synchrotron, Notkestrasse 85, 22607 Hamburg, Germany a[ The latest results of beauty quark production measurements at HERA are presented. New measurements have been obtained both in the photoproduction and the deep inelastic scattering regimes. The results were compared with the NLO QCD calculations. 1. Introduction Production of heavy quarks is a fascinating tool to understand the nucleon structure as well as the underlying QCD parton dynamics. In addition production of b quarks is a source of background for many searches for new physics at existing and future colliders. The high b mass (Mb � ΛQCD) should provide a solid basis for a perturbative calculation. Nevertheless, so far discrepancies between the experimental cross section measure- ments and the NLO QCD calculation have been found. The experimental procedure often relies on the measurement of the transverse momentum prelT of the muon produced in the semi-leptonic decay with respect to the axis of the closest jet. This spectrum is harder for b quarks than for c quarks and, therefore, allows a statistical separation of the signal from the background. Additional infor- mation such as lifetime measurements provided by silicon detectors can also be used. An alterna- tive tag is given by the coincidence of D∗ mesons and muons which provide sensitivity to the re- gion of low transverse b quark momenta and is less affected by background. The recent experimental results obtained at the HERA collider are reviewed in both the photo- production (Q2 ∼ 0 GeV2) and the Deep In- elastic Scattering (DIS: Q2 > 1 GeV2) kinematic regimes. 2. Open Beauty in Photoproduction When the exchanged photon has small virtual- ity (Q2) the time-scale of the interaction is such that its hadronic structure can be revealed. Pho- toproduction at HERA can therefore be similar to the processes at hadron colliders and supplies complementary information. Both ZEUS and H1 already published results on b photoproduction [1–3]. The ZEUS experiment has now measured the differential cross sections of beauty photopro- duction using events with at least two jets and a muon in the final state [4]. The luminosity used is almost three times larger than in the previous measurements. The fraction of events from b de- cays has been extracted using the prelT method. The kinematic region is defined by Q2 < 1 GeV2, 0.2 < y < 0.8, pJet1(2)T > 7(6) GeV, |ηJet1(2)| < 2.5, pµT > 2.5 GeV and −1.6 < ηµ < 2.3 1. Fig- ure 1 shows a comparison between the measured differential cross section and a NLO QCD calcu- lation for different regions of the muon pseudora- pidity. The QCD prediction was calculated using the program FMNR [5]. The hadronization is mod- eled by a Peterson function and the spectrum of the semi-leptonic muon momentum was extracted from PYTHIA [6]. The bands around the NLO pre- diction show the results obtained by varying the b quark mass as well as the renormalization and factorization scales. The measured cross sections are a factor 1.4 larger than the central NLO pre- diction but compatible with it within the experi- mental and theoretical uncertainties. In addition, a dijet cross section (ep → bbX → jet jet X) has been determined using PYTHIA to extrapolate to the unmeasured part 1η = − ln(tan θ/2) is the pseudorapidity, where θ is the polar angle measured with respect to the proton beam direction. 2 ZEUS 0 5 10 15 20 25 30 35 40 -1.5 -1 -0.5 0 0.5 1 1.5 2 ηµ d σ /d η µ (p b ) dσ  dηµ (ep→bb – →e jj µ X) Q2<1 GeV2 0.27,6 GeV |ηj|<2.5pT pT µ>2.5 GeV ZEUS (prel.) 96-00 NLO QCD x hadr. NLO QCD Figure 1. Differential cross section of beauty pho- toproduction as a function of the muon pseudo- rapidity compared with NLO QCD calculations. of the muon kinematics and to correct for the branching ratio. For this measurement a different data sample has been used with looser cuts on the transverse momentum of the muon at large pseu- dorapidities. The result is σdijet = 733±61±104 pb while the NLO QCD prediction is 381+117−78 pb, corresponding to a data excess of a factor of two. 3. Open Beauty in DIS First results in the DIS region have been already released by the H1 collaboration [7]. Thanks to the high luminosity in the new ZEUS measurement [8] differential distributions have been measured for the first time in DIS. Events were selected by requiring the presence of at least one muon in the final state and at least one jet in the Breit frame2. A total visible cross sec- tion of σvis = 38.7 ± 7.7+6.1−5.0 pb was measured for the reaction ep → ebbX → e jet µ X in the kinematic region defined by: Q2 > 2 GeV2, 2In the Breit frame, defined by ~γ + 2x ~P = ~0, where ~γ is the momentum of the exchanged photon, x is th Bjorken scaling variable and ~P is the proton momentum, a purely space-like photon and a proton collide head-on. 0.05 < y < 0.7, pµ > 2 GeV, 30◦ < θµ < 160◦ and one jet in the Breit frame with EBreitT > 6 GeV and −2 < ηLab < 2.5. Also for this measure- ment the prelT method was applied. This result has been compared with a NLO QCD calculation im- plemented in the HVQDIS program [9], after fold- ing the b quark momentum spectrum with a Pe- terson fragmentation function and subsequently with a spectrum of the semi-leptonic muon mo- mentum extracted from RAPGAP [10]. The NLO QCD prediction is 28.1+5.3−3.5 pb which agrees with the measured value within the errors. The differ- ential cross section as a function of Q2 compared to the NLO calculation is shown in Fig. 2. The 10 -3 10 -2 10 -1 1 10 10 2 10 3 ZEUS Q2 (GeV2) d σ /d Q 2 ( p b /G e V 2 ) ZEUS (prel.) 99-00 NLO QCD (HVQDIS) 4.5 < mb < 5.0 GeV 1/4(Q2+4mb 2) < µ2 < 4(Q2+4mb 2) σ(e+p → e+ bb - X → e+ µ± Jet X) 0.05 < y < 0.7 Pµ > 2 GeV, 30 o < θµ < 160 o E t,Jet Breit > 6 GeV, -2 < η Jet Lab < 2.5 Figure 2. Differential beauty cross section as a function of Q2 compared with the NLO QCD cal- culations. prediction of the Monte Carlo program CASCADE [11], which implements a calculation based on the CCFM evolution equations [12] and uses a kT - dependent gluon density, is 35 pb which is in very good agreement with the measurement. In addi- tion, the simulation gives a good description of the measured differential cross sections. 4. D∗ − µ correlations The separation of charm and beauty contri- butions to the signal can also be performed by 3 exploiting the charge and angle correlations of the D∗ meson and of the muon in the reaction ep → ebbX → eD∗µX. Of particular interest is the configuration in which the muon and the D∗ originate from the same parent B-meson yielding unlike charge sign D∗ − µ pairs produced in the same hemisphere. Using this strategy and performing a likeli- hood fit on the kinematic distributions H1 has extracted the cross sections of beauty and charm production in the kinematic region defined by pD ∗ T > 1.5 GeV, |ηD ∗| < 1.5, pµT > 1 GeV, |ηµ| < 1.74 and 0.05 < y < 0.75 [13]. The measured values, which confirm previous results, are respectively σbvis = 380 ± 120 ± 130 pb and σcvis = 720 ± 115 ± 245 pb which are well above LO+parton shower Monte Carlo expectations. A similar analysis has been conducted by ZEUS [14] whose selection has been optimized for decays of b quarks. The beauty cross section, measured in a slighty different phase space (pD ∗ T > 1.9 GeV, |ηD∗| < 1.5, pµT > 1.4 GeV, −1.75 < ηµ < 1.3), is σbvis = 214 ± 52+96−84 pb. The result is in good agreement with the the H1 measurement once the same cuts are applied. In order to compare the measured cross section with NLO QCD pre- dictions a photoproduction-enriched sample has been selected by applying the cuts Q2 < 1 GeV2 and 0.05 < y < 0.85. Furthermore, the mea- surement is restricted in a b quark rapidity range ζb < 1 where the distributions of the Monte Carlo program used to extrapolate agree with the re- spective FMNR spectra within ±15%. The result for the extrapolated cross section is σ γp→b(b)X = 15.1±3.9+3.8−4.7 nb while the NLO prediction of FMNR for this reaction is only 5.0+1.7−1.1 nb. 5. Conclusions and outlook The understanding of the b quark production mechanism is an outstanding puzzle in QCD. A set of new visible cross sections of beauty pro- duction, defined close to the detector acceptance, have been measured at HERA. The results are about a factor 1.4 higher than the NLO QCD predictions but consistent within the experimen- tal and theoretical uncertainties both in the pho- toproduction and the DIS regions. An excess of measured cross sections over the NLO QCD pre- diction is observed when attempts are made to extrapolate cross sections to regions which are not directly measured by the detectors. The HERA collider is now starting a new phase of operation at higher luminosities. Together with the enhanced b-tagging capabilities of the new H1 and ZEUS vertex detector and track- ing triggers more precise and differential measure- ments can be expected within the next five years. Acknowledgments I would like to thank my collegues of the H1 and ZEUS collaborations for their work and their sug- gestions to this talk. REFERENCES 1. C. Adloff et al. [H1 Collaboration], Phys. Lett. B 467 (1999) 156 [Erratum-ibid. B 518 (2001) 331]. 2. H1 Coll., contrib. paper no. 979/982 to ICHEP2000, Osaka 2000 3. J. Breitweg et al. [ZEUS Collaboration], Eur. Phys. J. C 18 (2001) 625. 4. ZEUS Coll., contrib. paper no. 785. 5. S. Frixione, M. L. Mangano, P. Nason and G. Ridolfi, Phys. Lett. B 348 (1995) 633. 6. T. Sjostrand, Comput. Phys. Commun. 82 (1994) 74; E. Norrbin and T. Sjostrand, Eur. Phys. J. C 17 (2000) 137. 7. H1 Coll., contrib. paper no. 1013. 8. ZEUS Coll., contrib. paper no. 783. 9. B. W. Harris and J. Smith, Phys. Rev. D 57 (1998) 2806. 10. H. Jung, Comput. Phys. Commun. 86 (1995) 147. 11. H. Jung and G. P. Salam, Eur. Phys. J. C 19 (2001) 351; H. Jung, Comput. Phys. Com- mun. 143 (2002) 100. 12. M. Ciafaloni, Nucl. Phys. B 296 (1988) 49; S. Catani, F. Fiorani and G. Marchesini, Phys. Lett. B 234 (1990) 339; G. Marchesini, Nucl. Phys. B 445 (1995) 49. 13. H1 Coll., contrib. paper no. 1016. 14. ZEUS Coll., contrib. paper no. 784.