Production of beauty quarks in deep inelastic scattering at HERA

1. Vincenzo Chiochia Deutsches Elektronen Synchrotron on behalf of the ZEUS Collaboration Production of beauty quarks in deep inelastic scattering at HERA XI International Workshop on Deep Inelastic Scattering St. Petersburg, 23-27 April 2003

2. Outline • Event topology • Event selection • Signal extraction and cross section calculation • QCD predictions and related uncertainties • Results: total and differential cross sections • Comparison with other measurements • Summary and outlook Production of beauty quarks in deep inelastic scattering at HERA

3. Barrel and rear calorimeter electron proton Process topology in the ZEUS experiment laboratory frame (forward) (rear) Inner muon chambers Central tracking detector Outer muon chambers Production of beauty quarks in deep inelastic scattering at HERA

4. Increase acceptance on signal • Increase background rejection • Kinematic region • Background rejection and • good electron reconstruction • Probability cut: remove fake • muons • Momentum cut: from inner and • outer muon chamber requirement • Polar angle: good associated • track Summary of the event selection Production of beauty quarks in deep inelastic scattering at HERA

5. Introduce an observable to distinguish, on statistical basis, between the different processes: pTrel jet axis pTrel muon e+ p • Jet and and muon are associated using the jet algorithm Extraction of the s.l. b-decays (1) • After the selection cuts the data sample is a mixture of several processes: • Fake muons: Punch-through, Sail-through, pions and kaons in-flight decays (cross checked with real data) • Semi-leptonic decays of charmed hadrons • Semi-leptonic decays of beauty hadrons Production of beauty quarks in deep inelastic scattering at HERA

6. Extraction of the s.l. b-decays (2) • 836 events after all cuts • Charm and light-quark ptrel distributions are added together according to the respective cross sections • The b-fraction is extracted with the Binned maximum likelihood method fb = (25.3±4.8)% • The algorithm takes in account the statistical errors of the data and MC samples in each bin ~210 beauty events Production of beauty quarks in deep inelastic scattering at HERA

7. ~25% of events from b-decays Comparison with the simulation muon variables Good description of the event kinematics with the extracted fractions jet variables Production of beauty quarks in deep inelastic scattering at HERA

8. Muon momentum spectrum extracted from MC pQCD Experiment Peterson function with e=0.002 O(as) corrections Lepton b quark Fragmentation Neutrino Charmed hadron QCD predictions: NLO calculations The calculation of the NLO QCD predictions proceeds in three steps: • HVQDIS (B.Harris, J.Smith, hep-ph/9503484): g*gbb, g*gbbg, g*qbbq, etc. • Fragmentation of the b-quark into a B-meson • Semileptonic decay of the B-meson Production of beauty quarks in deep inelastic scattering at HERA

9. ep  e b b X  e m Jet X Kinematic region: Q2> 2 GeV2, 0.05<y<0.7 At least one jet in the Breit frame with: ETBreit> 6 GeV, -2<hLab<2.5 At least one muon with: pm>2 GeV, 30<qm<160 Results and QCD predictions Measured cross section: svis = 38.7±7.7(stat.)+6.1(syst.)-5.0(syst.) pb NLO QCD (HVQDIS): sNLO = 28 + 5.3 - 3.5 pb mR,F2 = Q2+4mb2 mb=4.75 GeV Peterson frag. function Proton parton distribution:CTEQ5F4 Data/NLO ~ 1.4 but in agreement within exp. and theoretical uncertainties mR,F2 = pT2+4mb2 Production of beauty quarks in deep inelastic scattering at HERA

10. MC NLO ds/dQ2 measured at HERA for the first time! Production of beauty quarks in deep inelastic scattering at HERA

11. MC NLO ds/dlog(xbj) Production of beauty quarks in deep inelastic scattering at HERA

12. Factorization and renormalization scales  b-quark mass: 10-15% CTEQ5F4/ZEUS NLO FIT Gluon uncertainty from the ZEUS NLO QCD fit:~5% NLO QCD uncertainties more sources: Hadronization (jet): ~ 10% Fragmentation (muon): 5-10% Production of beauty quarks in deep inelastic scattering at HERA

13. H1 DIS: jets extrapolated to the full phase space H1 Photoproduction: jets extrapolated to the full phase space ZEUS D*-muon: extrapolated to the parton level This analysis visible cross section jets in the Breit frame ZEUS Photoproduction muon extrapolated to the full phase space The visible cross section agrees with the NLO! A tricky business... red: ICHEP02 results Production of beauty quarks in deep inelastic scattering at HERA

14. Summary and conclusions • We have measured a visible cross-section of b-quark production in DIS and single differential cross-sections (Q2, xbj) for the first time at HERA • The cross section is measured with events containing the scattered positron, a hard jet in the Breit frame and a muon traversing the inner and outer muon chambers. • Detailed analysis of the theoretical uncertaities has been performed • The description of the b-quark production is still an open question. Cannot draw a unique conclusion from the existing results. • The measured cross-section is a factor ~1.4 above the NLO QCD predictions but in agreement within the experimental and theoretical uncertainties • Similar result in the ZEUS measurement in photoproduction Production of beauty quarks in deep inelastic scattering at HERA

15. SPARE SLIDES Production of beauty quarks in deep inelastic scattering at HERA

16. Acceptance, efficiency, purity ME+PS CDM ME+PS (e-p) ME*+CCFM Production of beauty quarks in deep inelastic scattering at HERA

17. b-quark fragmentation [hep-ph/0204025] Production of beauty quarks in deep inelastic scattering at HERA

18. Increasing emission angle Increasing virtuality Event simulation: Monte Carlo • Parton shower with • CCFM-like evolution • CASCADE • Matrix Element with incoming off-shell gluon • Angular ordering in the gluon ladder • Direct (pointlike) component only • Gluon density unintegrated in kT and extracted from a fit to the HERA structure function F2 • Lund string model Parton shower with DGLAP Evolution • Matrix Element with incoming on-shell gluon • KT ordering in the gluon ladder • RAPGAP: • Lund string model • Color dipole model and ME+PS ...more samples generated to understand the model dependance Light quarks: ~120·106 events (200 pb-1) Charm quarks: ~7.5·106 events (94 pb-1) Beauty quarks: ~430·103 events (540 pb-1) Production of beauty quarks in deep inelastic scattering at HERA

19. Visible vs extrapolated measurements • Visible cross section: the kinematic region is close to the detector acceptance • Example: ep  bbx  m Jet Jet X • Model assumptions are not absorbed in the experimental result but in the corresponding QCD predictions → published measurements are then safe • Extrapolated cross sections: • Extrapolation to an unmeasured kinematic region: the unmeasured phase space is taken from a Monte Carlo simulation (for example: extrapolation to the jet or muon full phase space) • It is nice to quote more general and inclusive measurements, but... • Is the h and ET spectrum of the extrapolated region the same in the NLO as in the Monte Carlo? • How large are the extrapolation factors? Are they reported in the published results? • Is the extrapolation considered in the estimation of the systematic uncertainties? • Extrapolation from detector to parton/hadron level: the corresponding correction factors are extracted from a Monte Carlo simulation Production of beauty quarks in deep inelastic scattering at HERA