Presentation on theme: "Open beauty production in pp collisions at 7 TeV with CMS Kajari Mazumdar Tata Institute of Fundamental Research Mumbai, India. on behalf of CMS Collaboration,"— Presentation transcript:
Open beauty production in pp collisions at 7 TeV with CMS Kajari Mazumdar Tata Institute of Fundamental Research Mumbai, India. on behalf of CMS Collaboration, LHC, CERN ICPAQGP, GOA, December 2010
Open Beauty and Heavy Ions Heavy flavours are important probes of QGP in heavy ion collisions. Several issues: Production time scale of heavy flavour shorter than that of the deconfined state. Lifetime of heavy flavours are larger than that of the medium b,c quarks fragment inside the hot medium. Hard partons are likely to loose energy while traversing through the dense medium. Does the energy quench depend on mass of the hadron? -->Study and compare the properties of J/ψ, ϒ, B-mesons. The tussle between the heavy meson dissociation and decay of c/b quark in dense medium may cause significant modification in measured b-cross- section in QGP compare with pp measurement. ICPAQGP, Goa, December
LHC accelerator and CMS experiment CMS recorded about 43 pb -1 integrated luminosity for pp collision at 7 TeV and ~9 b -1 of Pb-Pb collision in LHC instantaneous luminosity during April to October: to /cm -2 / s -1 CMS trigger strategy changed accordingly Results shown today uses only a fraction of the total pp collision data. HI collisions of 2010 just getting over. No result on open beauty measurement in HI data of CMS available as yet. ICPAQGP, Goa, December pp collisions at LHC provides the important normalization for HI to estimate the shadow effect. LHC provides abundant rates for heavy flavour production accurate measurements possible in both pp and HI collision data.
ICPAQGP, Goa, December Open beauty: double b-jet candidate CMS B-physics programme for p-p collisions: 1. Measure differential and total cross-sections for beauty production. 2. Study exclusive B-decays. 3. Search for New Physics in rare decays.
Important subdetectors 1.Charged particle tracking in pixel layer and Si-strip detectors Long lifetime and large mass reconstruction Measurement of transverse momentum (p T ) Primary and secondary vertex reconstruction Muon measurements: 3 different sub-detector types Muon identification Muon p T measurement Semileptonic decays Particle Flow algorithm for jet reconstruction upto 20 GeV/c, with capability for b-jet identification. Minimum bias and inclusive single jet triggers ICPAQGP, Goa, December B-physics studies mainly depend on: Muon trigger: p T > 3GeV/c + impact parameter |d 0 |< 2cm + dedicated quarkonia triggers p T 1 GeV/c =0, | | = 2.5
Tracking Algorithmic Efficiency: p T >1.0GeV/c | |<1.0 o 201.0GeV/c | |<1.0 o 20
Tracking performance ICPAQGP, Goa, December Measure upto p T ~ few hundred MeV/c Good efficiency and purity for p T >1 GeV/c 8
Muon identification ICPAQGP, Goa, December
10 Particle flow technique in CMS Utilise:. good separation of charged particles in large tracker volume.. good resolution for soft particles upto few hundred MeV/c.. very good electromagnetic calorimeter. in multijet events only 10% of energy correspond to neutrals Big improvement in energy resolution (factor of 2 wrt measure- ments using only calorimetric info) and particle identification. Reduces jet scale uncertainty by factor of 2. Correlate charged tracks with energy deposits in the calorimeter, correct For energy loss along trajectory.
ICPAQGP, Goa, December Semileptonic decays, b +X B-hadron kinematics lepton is harder use relative p T of muon wrt jet to identify b-event. Use shape templates from simulation, to get fraction of b,c and light quark contributions. validate with b-enriched data sample: f b = 44±1 % Consider charm decays + in-flight decays from light quarks, gluons Template for misidentified hadron validated with data Consider events with: p T > 6 GeV/c, wrt z-direction, | |< 2.1
ICPAQGP, Goa, December 2010 Estimation of b-quark production cross- section in semileptonic decays Trigger eff= 82% reconstruction=97% -jet association=77% Visible rate Models for fake muon + underlying effect dominate experimental uncertainty ~ 15-20% Larger discrepancy with theory at low p T and central region Uncertainty in luminosity ~ 11% 12
ICPAQGP, Goa, December Measurement of inclusive b-jets Tag/identify b-jets to include higher momentum b-events. utilise hard fragmentation, large mass, large lifetime of b. Different systematics compared to semileptonic decays ! Several algorithms for b-jet identification: use discriminators based on track counting, secondary vertex, semi-leptonic decays, b-jet probability threshold values in each case decide sample purity, identification efficiency. B-tagging efficiency in data, using muon spectrum in b-jets fraction of tagged, untagged events obtained from fit to samples which pass and fail discriminator: Secondary vertex tagging method utilization of atleast 3 tracks to reconstruct secondary vertex best efficiency and high purity. 13
ICPAQGP, Goa, December b-jet candidate selection: discrimination between b and light-flavoured jets Basic track qualities: Number of pixel hits >2 Total number of silicon (pixel+strip) hits>8 Chi 2 /ndof of track fit <5 Transverse momentum >1 GeV/c Unsigned transverse impact parameter d xy <0.2 cm Unsigned longitudinal impact parameter d z < 17 cm Distance of closest approach to the jet axis < 0.07 cm Decay length < 5 cm.
ICPAQGP, Goa, December Sample purity determined from secondary vertex mass fit: ~73% b-tagging in data B-fraction in tagged sample, using MC
ICPAQGP, Goa, December Estimation of cross-section Stat. Uncertainty in cross-section ~ 2% Syst. Uncert. ~ 20% take ratio wrt inclusive jets to reduce systematics due to luminosity, energy resolution, energy scale of jets. Agreement good with Pythia for p T >40 GeV/c Shape difference with for large p T and in forward direction.
ICPAQGP, Goa, December Exclusive B-meson decays in pp collision Reconstructed modes: B + J/ψ K +, Bs J/ψ Several on going analyses: B 0 J/ψ K 0, J/ψ K *0 Bs b J/ψ Fit 3 gaussians for signal + exponential for background CMS DPS Maximum likelihood fit for single gaussian+ first order polynomial
ICPAQGP, Goa, December Conclusion Discussed the capability of CMS experiment for open-b physics with pp collision data: exclusive decays and measurement of b-production cross-section from low to reasonably high transverse momentum, using semileptonic decays and b-tagged jets. Agreement with theoretical predictions (LO & NLO) reasonable, though some differences are significant in different kinematic regions. Measurements with larger statistics being finalised. CMS will carry forward b-physics programme in heavy ion collision data. Expect interesting results in near future. Stay tuned!