Study of Beauty Photoproduction with the ZEUS Experiment at the Electron-Proton Collider HERA

1. Disputation Ana Gloria Yagües Molina Humboldt Universität zu Berlin Study of Beauty Photoproductionwith the ZEUS Experiment at theElectron-Proton Collider HERA

2. Contents: Importance of heavy quark production Kinematics of beauty at ep scattering Models and theoretical predictions HERA and ZEUS The analysis in a nutshell: Event selection Main distributions Beauty extraction Cross sections/Results Summary and Conclusions

3. Motivation I: Why beauty production? • Understandig QCD in as much detail as possible: • Theoretical understanding:Heavy flavour production expected to be well described by pQCD (hard scale given by mQ) • Study ofmulti-scale problem: often mass scales compete with other hard scales (pT,Q2…). Perturbative expansion can not be optimized for all scales → additional theoretical uncertainties. • Understanding of proton structure:Measurement of structure function F2bb Dominant diagram at HERA Boson-Gluon Fusion Beauty PHP - Disputation - Ana Yagües

4. Motivation II: why THIS study? • Previous and ongoing analysis at ZEUS on beauty production use muon or electron identification associated to jets (Only semileptonic decays) • Goal of this analysis: • Exploit full MVD potential for b-tagging by reconstructing the decay vertices (sec.vtx.) → Allows hadronic b-decays to be included in analysis (inclusive analysis) → Cross checks with previous analysis → Improved/new measurements • How? • Using sensitive variables like decay length and invariant mass of particle associated to the secondary vertices to discriminate beauty signal from background PTrel μ + δ - sec.vtx Beauty PHP - Disputation - Ana Yagües

5. Kinematics of b-production at HERA • Kinematic Variables: • Q2 = -q2 =(k-k’)2 Neg. squared momentum transfer • (virtuality ofexchanged boson) • s =(k+p)2 ≈ 4EeEpCM Energy, √s = 318 GeV • Y = p•q/p•k Inelasticity: lepton momentum fraction transferred to boson in proton rest frame γ • Kinematic Regimes: • Deep Inelastic Scattering (DIS): Q2 >1 GeV2 • Signature: e-/e+ scattered at large angle → visible in detector • Photoproduction (PHP): Q2<1 GeV2 • Signature:e-/e+ scattered at small angle → escapes into beam pipe Beauty PHP - Disputation - Ana Yagües

6. Models and Theoretical Predictions: PYTHIA: • LO + (DGLAP) PS • Hadronization: Lund String Model • NLO QCD predictions: • Massive scheme/FO: →mb • b massive, produced only in the hard scattering • neglects [αsln(Q2/mb2 )]n • reliable for pT ~ mQ • FMNR Beauty PHP - Disputation - Ana Yagües

7. The HERA ep collider: In 2004-data analysis:L = 35 pb-1 In 2005-data analysis:L = 133 pb-1 Ep = 920 GeV Ee = 27.5 GeV √s ~ 318 GeV Beauty PHP - Disputation - Ana Yagües

8. The ZEUS detector Multi-purpose detector: covers almost full solid angle  inner tracking detector in 1.4 T magnetic field (superconducting solenoid)  uranium-scintillator calorimeter (CAL)  cylindrical drift chamber (CTD)  Micro Vertex Detector Beauty PHP - Disputation - Ana Yagües

9. The Idea of Secondary Vertex b-Tagging Tracks from decay of long-lived particles have mostly positive Lwhile tracks coming directly from PV are equally likely to be positive or negative Beauty PHP - Disputation - Ana Yagües

10. Strategy of the Analysis 1 Event selection & sec.vtx. reconstruction 2 Decay length & invariant mass determination 3 Statistical extraction of b-fraction 4 Cross section calculation 5 Compare with other results/theoretical predictions Strong limitations of 2004 data analysis due to misalignments/calibrations problems, not accurate enough understanding of tracking/vertexing procedures…. 6 WHAT DID WE LEARN? 7 DO IT BETTER! (2005 data) Beauty PHP - Disputation - Ana Yagües

11. Event Selection I: • Trigger: • TLT HFL5 (2 Jets with ET>4.5, η <2.5, Pz/E<0.95 and E-Pz<100) • PHP sample: • Removing event if 1 e- with P(e)≥0.9 && E’e≥5 GeV && yel≤ 0.9 • 0.2 < yJB < 0.8 • Jet selection (kt-jets A): • 2 Jets with PT 1, > 7 GeV and PT ,2 > 6 GeV • |ηjet| < 2.5 • Track selection: • ZTT tracks (tracks containing MVD info) • PT > 0.5 GeV • χ2 < 100 • # SL in CTD > 3 • # hits inside MVD ≥4 • Vertex: • -50<Zvertex<50 cm Yel = 1 – (E’e/2Ee)(1-cosθ’e) yJB = (E-Pz)/2Ee Beauty PHP - Disputation - Ana Yagües

12. Event Selection II: • track-jet association: • R2 = [(ηtrk-ηjet)2+(Φtrk-Φjet)2] MC Beauty sample Each track is associated to closest jet in (η,Φ) plane: requiring R<1 Beauty PHP - Disputation - Ana Yagües

13. A beauty(-ful) event Beauty PHP - Disputation - Ana Yagües

14. Control plots (2005): Tracks Beauty PHP - Disputation - Ana Yagües

15. Control plots (2005): Jets Beauty PHP - Disputation - Ana Yagües

16. Decay Length Distributions Msv>2 GeV/c2 Incl. light flavour Beauty Light flavour Charm Lxy Almost pure beauty content Lxy/σxy Beauty PHP - Disputation - Ana Yagües

17. Data distribution is wider than the MC - dedicated smearing procedure developed for decay lengths analysis→ smear reconstructed MVD cluster w/o smearing Smearing 20 microm Smearing 30 microm Smearing 40 microm Smearing 50 microm Smearing 60 microm Beauty PHP - Disputation - Ana Yagües

18. (S+-S-) distribution Light flavour is symmetric except for fluctuations • In order to cancel syst. errors and to get rid of background (use symmetry of light flavour)  build up the difference S+-S- Beauty signal is enhanced for higher values of the decay length Beauty PHP - Disputation - Ana Yagües

19. Invariant Mass Distribution • Invariant mass at sec. vertex: MSV Mass generated by c-quark limited by D-meson (~1.8GeV/c2) (Incorrect track attachment to SV causes higher mass values) Charm dominant Beauty dominant Beauty PHP - Disputation - Ana Yagües

20. Invariant mass and (S+-S-) in bins of the invariant mass Beauty PHP - Disputation - Ana Yagües

21. Extraction of Beauty Signal 1.4 < Msv< 2 GeV/c2 Msv> 2 GeV/c2 Beauty PHP - Disputation - Ana Yagües

22. In Bins of pT: 16 GeV <pT 6<pT<11 GeV 11<pT<16 GeV 1.4 < Msv< 2 GeV/c2 Msv> 2 GeV/c2 Beauty PHP - Disputation - Ana Yagües

23. Cross Section / Results: A is the acceptance A= NrecMC/NgenMC , L: luminosity and thenumber of measured b-jets Nbmeas. = NbrecMC• fb where fb is the scale factor from fit * A b-jet is defined as a jet containing a B (or anti-B) hadron Systematics not yet fully calculated Beauty PHP - Disputation - Ana Yagües

24. Differential b-quarkcross section dσ/dpbT New measurements possible! Very good agreement with previous results and theoretical predictions Beauty PHP - Disputation - Ana Yagües

25. To reach this point,a long hard way behind us….2004 data Analysis • Not accurate MVD alignment • Decay length significance (L/σ) could NOT be used. Only simple L • much less statistics: → L = 38 pb-1(2004 data)instead L=132 pb-1 (2005 data) → big statistical errors • understanding of tracking and vertexing techniques was still ongoing Beauty PHP - Disputation - Ana Yagües

26. To have an idea… • Invariant mass was not properly described • → correction needed • region below Msv<0.7 had to be rejected • Only decay length (not significance) used • Small statistics Beauty PHP - Disputation - Ana Yagües

27. Summary: • b-jet total cross section and differential cross sections in bins of pT and η has been calculated for 35 pb-1 data sample (2004) and for 132pb-1 data sample (2005) (preliminary) • No lepton required in final state inclusive analysis (had+lep) exploiting MVD info. First measurement of this kind in ZEUS!! • Cross check with previous result (obtained extrapolating cross section for dijets event + μ): good agreement within errors • Measured differential cross sections compared to MC model PYTHIA (LO+PS) and NLO QCD predictions: reasonable agreement within still large syst. and stat. errors Despite the strong limitations, the goal to establish a new beauty tagging algorithm was reached. The feasibility of the method was confirmed Systematics of the 2005 data analysis is ongoing IT ALREADY SHOWS VERY PROMISING RESULTS Beauty PHP - Disputation - Ana Yagües

28. BACKUP SLIDES

29. Systematic Uncertainties • The total syst. error is calculated as the sum in quadrature of all individuals uncertainties • This analysis is dominated by statistical errors → other syst.uncertainties are considered small Beauty PHP - Disputation - Ana Yagües

30. Helix parametrization Pattern recognition (hits combined for first estimation) Track fitting Kalman filtering procedure (MVD hits added, multiple scattering…) Beauty PHP - Disputation - Ana Yagües

31. MC studies I:Revertexing tracks associated to a jet - All selected tracks are revertexed with VXU routine (G.Hartner) - Remove all vertices at more than 1cm in x-y plane and 30 cm in z from nominal interaction point (sec.interactions) - If 2 opp. charged tracks come from same sec.vtx with mass compatible with K0s and λ, those tracks are removed from the sec.vtx FWHM~260 μm (σ~110 μm) MC beauty sample Beauty PHP - Disputation - Ana Yagües

32. Micro Vertex Detector 125 mm 64 mm Two half modules are then glued together to form a full module Five full modules are disposed over a carbon fibre support 1) Two single sensors are glued next to each other with perpendicular strips on the same side (half module) 2) Two symmetric assemblies are placed on top of each other (full module) 3) Five full modules fixed side by side on a carbon fibre structure (ladder) 4) FMVD: back to back layers of Si sensors of same type but with trapezoidal shape (wheels) Beauty PHP - Disputation - Ana Yagües

33. Beauty PHP - Disputation - Ana Yagües

34. Beauty PHP - Disputation - Ana Yagües

35. Fit results (2005 data Analysis) - Χ2 becomes drastically worse - Beauty scale factor barely change within errors Beauty PHP - Disputation - Ana Yagües

36. before after Fits results 2005 Beauty PHP - Disputation - Ana Yagües

37. Checks on shift No dependence on φ observed Beauty PHP - Disputation - Ana Yagües

38. Checks on shift Beauty PHP - Disputation - Ana Yagües

39. In bins of  (2005) Beauty PHP - Disputation - Ana Yagües

40. First hints on SHIFT (Verena’s Studies) Studies point out that the shift is related to the ZH variable Very good agreement after setting ZH and cotθH to cero as well as the cov.matrix elements depending on them Source of problem found. Working on the solution Beauty PHP - Disputation - Ana Yagües

41. Previous Measurement: DESY 03-212 PTrel • pTrel is the transverse momentum of the muon candidate relative to the jet axis • Large b-mass → hard pTrel spectrum • Statistical separation μ • semileptonic b-decay → μ associated to jet in final state δ Beauty PHP - Disputation - Ana Yagües

42. dedicated smearing study : MVD hits smearing Beauty PHP - Disputation - Ana Yagües

43. Previous Measurement: DESY 03-212 (cont’d) μ-jet cross sections corrected to b-jets using PYTHIA and accounts for the b→μ branching ratio Beauty PHP - Disputation - Ana Yagües

44. Control Plots I: Jets • Highest-PT jet with ≥ 2 trks • MC normalized to data • Resolved and direct samples mixed by reweighting each to 1/Lsample Good agreement between data and MC • Data LF+cc+bb Beauty PHP - Disputation - Ana Yagües

45. Control Plots II: Tracks Good agreement between data and MC Beauty PHP - Disputation - Ana Yagües

46. 2a Decay Length Distributions • Data distribution wider than MC (dead material, detector resolution…) → MC Smearing applied MC Smearing based on Impact Parameter studies → nevertheless remarkable improvement in the agreement MC-data Beauty PHP - Disputation - Ana Yagües

47. In Bins of η(2004): -1.6<η<-0.6 -0.6<η<0.4 0.4<η<1.4 1.4 < Msv< 2 GeV/c2 Msv> 2 GeV/c2 Beauty PHP - Disputation - Ana Yagües

48. 5 Differential b-quark cross section dσ/dpbT 6 WHAT DID WE LEARN? • A new b-tagging algorithm was established. First measurement of this kind in ZEUS • The experimental data is in good agreement with theoretical predictions (mb ~ pT) • Need to reduce syst. and extend analysis toward higher pT values 2005 data analysis • General good agreement with NLO QCD predictions • And with previous measurements Beauty PHP - Disputation - Ana Yagües