Manchester, HEP2007 International Europhysics Conference on High Energy Physics

1. Beauty production at HERA using the ZEUS experiment A. Yagües, Humboldt Universität zu Berlin on behalf of the ZEUS Collaboration • OUTLINE: • HERA Collider and ZEUS detector • Heavy quark production at HERA • Beauty PHP : dijets + muon • Beauty PHP: dijets + electron • Beauty contribution to the proton structure function: F2bb (DIS) • Summary and Conclusions Manchester, HEP2007 International Europhysics Conference on High Energy Physics

2. The HERA ep collider: proton accelerator electron accelerator Delivered luminosity: 190 pb-1 92-00 (HERA I) 560 pb-1 02-07 (HERA II) Ep = 920 GeV Ee = 27.5 GeV √s ~ 318 GeV

3. The ZEUS detector: • Multi-purpose detector: • covers almost full solid angle •  inner tracking detector in 1.5 T magnetic field (superconducting solenoid) •  uranium-scintillator calorimeter (H1:liquid Argon CAL) •  muon chambers •  … BCAL CTD FMUON FCAL Detector upgraded for HERA II: Micro Vertex Detector (MVD) sec.vertices PV

4. Heavy quark production at HERA Dominant process:boson-gluon fusion • Kinematic Variables: • Q2 = -q2 =(k-k’)2Neg. squared momentum transfer • (virtuality ofexchanged boson) • s =(k+p)2 ≈ 4EeEpCM Energy, at HERA √s = 318 GeV • x = Q2/2p•qBjorken scaling variable: momentum fraction of parton interacting with lepton in infinite momentum frame (QPM) • y = p•q/p•kInelasticity: lepton momentum fraction transferred to boson in proton rest frame e(k’) (k)  • Kinematic Regimes: • Deep Inelastic Scattering (DIS):Q2 > 1 GeV2 • Photoproduction (PHP): Q2<1GeV2 (Q2~10-3, quasi-real ) Why study heavy quark production at HERA?: • Test of perturbative QCD due to the hard scale given by the heavy quark mass. • Study ofmulti-scale problem: often mass scales compete with other hard scales (pT,Q2…) , additional theoretical uncertainties enter. • Better understanding of structure of the proton

5. Massive scheme/FO: → mb b massive neglects [αsln(Q2/mb2 )]n… reliable for pT ~ mQ FMNR (PHP) and HVQDIS (DIS) Massless scheme/NLL: → Q2, pT b massless resums [αsln(Q2/mb2 )]n … reliable for pT >> mQ pQCD approximations:assume one dominant hard scale γ γ γ √αs g g Variable schemes(Variable Flavour Number Scheme): • At small Q2  massive, at large Q2 massless • MRST04, CTEQ6HQ…

6. Tagging Beauty with μ+jets (I) - Semileptonic decay: ep→e’bX→μjjX - Separate b from c and uds: ● Large b mass large muon PTrel ●Large b lifetime large muon impact param.  μ Jets → Simultaneous 2-dim PTrel and  fit (enhanced statistics and reduced syst.errors)

7. Tagging Beauty with μ+jets (II) Q2<1GeV2 0.2<y<0.8 pTμ>2.5 GeV -1.6<μ<2.3 pTjet>7(6) GeV |jet|<2.5 - Compatible with previous measurements - NLO calculations provide reasonable good description of data

8. Tagging Beauty with e+jets (I) - Semileptonic decay: ep→e’bX→e’jj+eX - b-fraction extracted from likelihood fit using variables sensitive to e- identification and semileptonic decays

9. Tagging Beauty with e+jets (II) - Measurements in good agreement within errors with theoretical predictions

10. Summary of PHP measurements Cross sections for b production extrapolated using NLO calculations General good agreement with NLO QCD predictions

11. Measurement of F2bb neglected (small y) - F2bb is related to double differential cross section: d2σbb/(dx•dQ2)=(2πα2/Q4x) {[1+ (1-y)2]F2bb(x,Q2)-y2FLbb(x,Q2)) - F2bb can be calculated from the ratio of measured and theory cross sections: F2bb(x,Q2)=[σmeasured/σNLO]∙F2bbNLO(x,Q2) for process (ep→ebbX→eμjX) - PTrel method used to extract b-signal Q2>4GeV2 (DIS), 0.05<y<0.7 pTμ>1.5 GeV, -1.6>μ ETjet>5 GeV, -2.<jet<2.5 Data sample ~ 39 pb-1 (10% data) • First measurements F2bb(x,Q2) at ZEUS • ZEUS data lie above H1 data but compatible within errors • At high x and Q2, still statistically limited • MRST04 and CTEQ6HQ differ up to factor two! • First NNLO calculations from Robert Thorne • Within current experimental errors, theoretical differences can not be yet resolved

12. Conclusions • Heavy Flavour production in ep collisions is: - good testing ground for perturbative QCD - better understanding of multi-scale problem • Good description achieved with NLO calculations in all measurements • First measurement of F2bb at ZEUS - good agreement between both experiments and with theory (NLO, NNLO) • Most of HERA-II data not analyzed yet: - expect improved results soon! - new analyses possible

13. BACKUP SLIDES

15. Beauty Contribution to F2 Effect of parity violation Cross section: d2σ/(dx•dQ2)= (2πα2/Q4x) {[1+ (1-y)2] F2(x,Q2)- y2 FL(x,Q2) + …xF3} Long. structure function Dominant term to cross section Significant at high y Significant at high Q2 The Structure of the Proton - In the QPM,F2 can be expressed as: F2= F2em+ [Q2/(Q2+Mz2)] F2γ/Z+ [Q2/(Q2+Mz2)]2 F2Z is dominated by F2em contribution at low Q2

16. Theoretical Calculations I • Fixed Order (F0): - heavy quarks produced at perturbative level - reliable for pT ~ mQ • Frixione et.al. (FMNR) used for charm and beauty in photoproduction • Harris and Smith (HVQDIS) used for charm and beauty in deep inelastic scattering

17. Theoretical Calculations II • Next-to-leading log (NLL): -„massless“ heavy quarks: active constituents of p and g - masses only used for final state kinematics - reliable for pT >> mQ • Cacciari et. Al. and Kniehl et al. used for charm and beauty in photoproduction (only inclusive production, can not do e.g. dijets) • No program for DIS • Combined (FONLL): - matched calculations of both schemes - FO at low pT, NLL at large pT

18. What do we do after work?

19. F2bb measurements (II): - Reasonable good description of the data by theoretical predictions. In bins of Q2