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,

Open beauty production in pp collisions at 7 TeV with CMS

Kajari MazumdarTata Institute of Fundamental Research

Mumbai, India.

on behalf ofCMS 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 20102

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 2010 .

• LHC instantaneous luminosity during April to October: 1027 to 1032 /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 2010 3

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.

CMS: precise charged particle tracking, robust lepton identification, finely segmented calorimeter


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 (pT) Primary and secondary vertex reconstruction

• Muon measurements: 3 different sub-detector types Muon identification Muon pT 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


B-physics studies mainly depend on:

Muon trigger: pT> 3GeV/c + impact parameter |d0|< 2cm+ dedicated quarkonia triggers

pT resolution @ 1 GeV/c 0.7% @ =0, 2% @ || = 2.5

Tracking Algorithmic Efficiency:

pT>1.0GeV/c ||<1.0

o 20<N<35• 90<N<110

pT>1.0GeV/c

||<1.0o 20<N<35• 90<N<110

• The CMS Tracker feels at home in high charged particles multiplicity environment.

• It has been designed to tackle thousand of tracks per event.

• It provides good tracking performance also for the Heavy Ion running of LHC (~104 tracks per event).

Fake rate:

Tracking in low and high multiplicity environments of pp collision


Tracking performance


• Measure upto pT ~ few hundred MeV/c• Good efficiency and purity for pT >1 GeV/c

8

Muon identification



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 neutralsBig 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, correctFor energy loss along trajectory.


Semileptonic decays, b +X

B-hadron kinematics lepton is harder use relative pT 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: fb = 44±1 %Consider charm decays + in-flight decays from light quarks, gluonsTemplate for misidentified hadron validated with data

Consider events with:pT > 6 GeV/c, wrt z-direction, ||< 2.1


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 effectdominate experimentaluncertainty ~ 15-20%

Larger discrepancy with theory at low pT

and central region

Uncertainty inluminosity ~ 11%

12


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 vertexbest efficiency and high purity.

13


b-jet candidate selection: discrimination between b and light-flavoured jets

Basic track qualities:Number of pixel hits >2Total number of silicon (pixel+strip) hits>8Chi2/ndof of track fit <5Transverse momentum >1 GeV/cUnsigned transverse impact parameter dxy <0.2 cmUnsigned longitudinal impact parameter dz < 17 cmDistance of closest approach to the jet axis < 0.07 cmDecay length < 5 cm.


Sample purity determined from secondary vertex mass fit: ~73%

b-tagging in data

B-fraction in taggedsample, using MC


Estimation of cross-sectionStat. 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 pT>40 GeV/cShape difference with MC@NLO for large pT and in forward direction.


Exclusive B-meson decays in pp collisionReconstructed modes: B+J/ψ K+ , BsJ/ψ Several on going analyses: B0J/ψ K0 , J/ψ K*0

Bs bJ/ψ

Fit 3 gaussians for signal + exponential for background

CMS DPS-2010-040

Maximum likelihood fit for single gaussian+ first order polynomial


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!


BACKUP


Tagging performance

Jet pt (10,30)

Jet pt (30,50)





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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,