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

Study of Beauty Photoproductionwith the ZEUS Experiment at theElectron-Proton Collider HERA

Ana Gloria Yages MolinaHumboldt Universitt zu Berlin Disputation

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 ConclusionsContents:

22/02/2008Beauty PHP - Disputation - Ana Yages* 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 of multi-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

Boson-Gluon FusionDominant diagram at HERA

Beauty PHP - Disputation - Ana Yages

22/02/2008Beauty PHP - Disputation - Ana Yages* 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 backgroundMotivation II: why THIS study?PTrel+-sec.vtx


22/02/2008Beauty PHP - Disputation - Ana Yages* Kinematics of b-production at HERA Kinematic Variables:Q2 = -q2 =(k-k)2 Neg. squared momentum transfer (virtuality of exchanged boson) s =(k+p)2 4EeEp CM Energy, s = 318 GeVY = pq/pk Inelasticity: lepton momentum fraction transferred to boson in proton rest frame Kinematic Regimes: Deep Inelastic Scattering (DIS): Q2 >1 GeV2 Signature: e-/e+ scattered at large angle visible in detector Photoproduction (PHP): Q2

22/02/2008Beauty PHP - Disputation - Ana Yages* Models and Theoretical Predictions: PYTHIA: LO + (DGLAP) PS Hadronization: Lund String ModelNLO QCD predictions:Massive scheme/FO: mbb massive, produced only in the hard scatteringneglects [sln(Q2/mb2 )]n reliable for pT ~ mQFMNR


22/02/2008Beauty PHP - Disputation - Ana Yages* Ep = 920 GeVEe = 27.5 GeVs ~ 318 GeVThe HERA ep collider:In 2004-data analysis: L = 35 pb-1In 2005-data analysis: L = 133 pb-1


22/02/2008Beauty PHP - Disputation - Ana Yages* The ZEUS detectorMulti-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


22/02/2008Beauty PHP - Disputation - Ana Yages* The Idea of Secondary Vertex b-Tagging Tracks from decay of long-lived particles have mostly positive L while tracks coming directly from PV are equally likely to be positive or negative


22/02/2008Beauty PHP - Disputation - Ana Yages* 7 DO IT BETTER! (2005 data)4 Cross section calculation1 Event selection & sec.vtx. reconstructionStrategy of the Analysis2 Decay length & invariant mass determination3 Statistical extraction of b-fractionStrong limitations of 2004 data analysis due to misalignments/calibrations problems, not accurate enough understanding of tracking/vertexing procedures.

5 Compare with other results/theoretical predictions6 WHAT DID WE LEARN?


22/02/2008Beauty PHP - Disputation - Ana Yages*

Trigger: TLT HFL5 (2 Jets with ET>4.5, 0.5 GeV 2 < 100 # SL in CTD > 3 # hits inside MVD 4 Vertex: -50

22/02/2008Beauty PHP - Disputation - Ana Yages* Event Selection II:MC Beauty sample track-jet association:R2 = [(trk-jet)2+(trk-jet)2]Each track is associated to closest jet in (,) plane: requiring R

22/02/2008Beauty PHP - Disputation - Ana Yages* A beauty(-ful) event


22/02/2008Beauty PHP - Disputation - Ana Yages* Control plots (2005): Tracks


22/02/2008Beauty PHP - Disputation - Ana Yages* Control plots (2005): Jets


22/02/2008Beauty PHP - Disputation - Ana Yages* Decay Length DistributionsLxyLxy/xyMsv>2 GeV/c2Almost pure beauty contentIncl. light flavourBeautyLight flavourCharm


22/02/2008Beauty PHP - Disputation - Ana Yages* - dedicated smearing procedure developed for decay lengths analysis smear reconstructed MVD cluster w/o smearingSmearing 20 micromSmearing 30 micromSmearing 40 micromSmearing 50 micromSmearing 60 microm Data distribution is wider than the MC


22/02/2008Beauty PHP - Disputation - Ana Yages* In order to cancel syst. errors and to get rid of background (use symmetry of light flavour) build up the difference S+-S-(S+-S-) distributionBeauty signal is enhanced for higher values of the decay lengthLight flavour is symmetric except for fluctuations


22/02/2008Beauty PHP - Disputation - Ana Yages* Invariant mass at sec. vertex: MSVMass generated by c-quark limited by D-meson (~1.8GeV/c2)(Incorrect track attachment to SV causes higher mass values)Invariant Mass DistributionCharm dominantBeauty dominant


22/02/2008Beauty PHP - Disputation - Ana Yages* Invariant mass and (S+-S-) in bins of the invariant mass


22/02/2008Beauty PHP - Disputation - Ana Yages* Extraction of Beauty Signal 1.4 < Msv< 2 GeV/c2Msv> 2 GeV/c2


22/02/2008Beauty PHP - Disputation - Ana Yages* In Bins of pT: 1.4 < Msv< 2 GeV/c2Msv> 2 GeV/c26

22/02/2008Beauty PHP - Disputation - Ana Yages* Cross Section / Results:A is the acceptance A= NrecMC/NgenMC , L: luminosity and the number of measured b-jets Nb meas. = Nb recMC fb where fb is the scale factor from fit* A b-jet is defined as a jet containing a B (or anti-B) hadronSystematics not yet fully calculated


22/02/2008Beauty PHP - Disputation - Ana Yages* New measurements possible!Very good agreement with previous resultsand theoretical predictionsDifferential b-quark cross section d/dpbT


22/02/2008Beauty PHP - Disputation - Ana Yages* Not accurate MVD alignment Decay length significance (L/) 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 ongoingTo reach this point,a long hard way behind us. 2004 data Analysis


22/02/2008Beauty PHP - Disputation - Ana Yages* Invariant mass was not properly described correction needed region below Msv

22/02/2008Beauty PHP - Disputation - Ana Yages* b-jet total cross section and differential cross sections in bins of pT and 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 + ): good agreement within errorsMeasured differential cross sections compared to MC model PYTHIA (LO+PS) and NLO QCD predictions:reasonable agreement within still large syst. and stat. errorsSummary: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


BACKUP SLIDES

22/02/2008Beauty PHP - Disputation - Ana Yages* 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


22/02/2008Beauty PHP - Disputation - Ana Yages* Helix parametrization Pattern recognition(hits combined for first estimation)Track fitting Kalman filtering procedure (MVD hits added, multiple scattering)


22/02/2008Beauty PHP - Disputation - Ana Yages* 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 , those tracks are removed from the sec.vtxFWHM~260 m (~110 m)MC beauty sample


22/02/2008Beauty PHP - Disputation - Ana Yages* Micro Vertex Detector1) Two single sensors are glued next to each other with perpendicular strips on the same side (half module)2) Two symmetric assemblies areplaced 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)


22/02/2008Beauty PHP - Disputation - Ana Yages*


22/02/2008Beauty PHP - Disputation - Ana Yages* Fit results (2005 data Analysis)- 2 becomes drastically worse- Beauty scale factor barely change within errors


22/02/2008Beauty PHP - Disputation - Ana Yages* beforeafterFits results 2005


22/02/2008Beauty PHP - Disputation - Ana Yages* Checks on shiftNo dependence on observed


22/02/2008Beauty PHP - Disputation - Ana Yages* Checks on shift


22/02/2008Beauty PHP - Disputation - Ana Yages* In bins of (2005)


22/02/2008Beauty PHP - Disputation - Ana Yages* Very good agreement after setting ZH and cotH to cero as well as the cov.matrix elements depending on themStudies point out that the shift is related to the ZH variable First hints on SHIFT (Verenas Studies)Source of problem found. Working on the solution


22/02/2008Beauty PHP - Disputation - Ana Yages* Previous Measurement: DESY 03-212PTrel pTrel is the transverse momentum of the muon candidate relative to the jet axisLarge b-mass hard pTrel spectrumStatistical separation semileptonic b-decay associated to jet in final state


22/02/2008Beauty PHP - Disputation - Ana Yages* dedicated smearing study : MVD hits smearing


22/02/2008Beauty PHP - Disputation - Ana Yages* Previous Measurement: DESY 03-212 (contd)-jet cross sections corrected to b-jets using PYTHIA and accounts for the b branching ratio


22/02/2008Beauty PHP - Disputation - Ana Yages* Control Plots I: Jets Highest-PT jet with 2 trks Data LF+cc+bb MC normalized to data Resolved and direct samples mixed by reweighting each to 1/LsampleGood agreement between data and MC


22/02/2008Beauty PHP - Disputation - Ana Yages* Control Plots II: TracksGood agreement between data and MC


22/02/2008Beauty PHP - Disputation - Ana Yages* 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-data2a


22/02/2008Beauty PHP - Disputation - Ana Yages* In Bins of (2004): 1.4 < Msv< 2 GeV/c2Msv> 2 GeV/c2-1.6

22/02/2008Beauty PHP - Disputation - Ana Yages* Differential b-quark cross section d/dpbT General good agreement with NLO QCD predictions And with previous measurements56WHAT DID WE LEARN?2005 data analysis A new b-tagging algorithm wasestablished. First measurement of this kind in ZEUS The experimental data is ingood agreement with theoreticalpredictions (mb ~ pT) Need to reduce syst. and extend analysis toward higher pTvalues


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

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