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Vineet Kumar, P. Shukla, R. K. Choudhury, and S. KailasNuclear Physics Division, Bhabha Atomic Research Centre

Measurement of Z boson production in PbPbcollisions at s

NN = 2.76 TeV

Plan of talk

Motivation of Z0 measurement in heavy ion collisions

Expected statistics of Z0 with first year heavy ion run

Z0 measurement in HI collisions

Event selection

Signal extraction

Cuts

Corrections

Results

Summary and outlook

Z0 measurement at LHC

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Huge energy jump from RHIC: factor 14 ! Hottest matter ever created in the laboratory Standard candle of the initial stateShadowing : PDF can be modifieded (suppressed in the LHC x region) in nuclei (~10 to 20%)Energy loss and multiple scattering of the initial partons (~2%) A candle for heavy ion collisionsProbes are modified in the QGP ! baseline to compare to (usually pp and pA or dA)Z boson is unmodified by the presence of the QGP Reference for dimuon measurements in heavy ion collisionsReference for quarkonia production and opposite-side jet in a Z-jet process

Expected statistics of Z0 with first year heavy ion run

We expect nearly 150 Z0 with first Year Pb Pb data assuming participant scaling of hard cross section and no initial state effect.

Z0 measured cross section

NZ

: number of measured Z bosons

Y = 4.8 : rapidity acceptance of the dimuons used to normalizeߡ the measurement in to report a cross section per unit of rapidity

and = acceptance and efficiency corrections N

MB: corresponding minbias event

PbPb

: total innelastic cross-section for Pb+Pb collisions at 2.76

TeV

PbPb

= 7.65 ± 0.76 b based on a standard Glabur approach.

Event Selection

Minimum bias collisionsL1 BscMinBiasThreshold1 (L1a36)OR L1 HcalHfCoincidencePm (L1a44) Default offline HI PAG selectionCoincidence of 3 HF towers above thresholdSelection of at least a two-track fitted vertexCluster-shape filter re-run offline Reject BSC beam halo L1 technical bits Dimuon triggerHLT HIL1DoubleMuOpen Core (until run 151020) only 1ZHLT HIL2DoubleMu3 Core Statistics eq no of MB 51,016,587/0.97 (eff) = 52,594,419 L = 6.88 b -1 ( using Glauber calculation )

Global muon quality cuts

keep as much signal as possibleLooking at Z embedded in HYDJET Keep 97.6 % MC signal.

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First ever Z0 Candidate in HI collisions

m = 92.67 GeV/c2

pT= 14.64 GeV/c y = –1.28φ = – 0.93

pT1 = 29.7φ1 = 0.71η1= –2.28pT2 = 33.8φ2 = –1.98η2 = –0.38

Raw Invariant Mass distribution

Pat dimuons with trigger

matchNo muon quality cut

applied 39 Z candidates

counting in [60-120]

GeV/c2 over background Signal RBW convoluted

with a gaussian.Background with first

order polinomial.

Pat dimuons with trigger

match 38 Z bosons counting in

[60-120] GeV/c2 (no like-sign

events in this mass range)Fit in [60-120] GeV/c2 with a

RBW convoluted with a

gaussian just to estimate the

mass (90.3 0.6 GeV/c2) and the width ( gauss = 1.8 0.8)

Invariant Mass distribution

Corrections

x corrections are derived from a Monte Carlo Z sample embedded in real

data, counting events as if they were data. The sample was generated flat in y, pt and embedded at wrt centrality but reweighted by realistic distributions.

We obtain the following integrated xcorrection :simulated Z embedded in data x = 47.6 1.1%simulated Z embedded in HYDJET x = 48.2 0.6%

DiMuon acceptance x reconstruction efficiency

Acc x eff is found flat in pT.Variation in rapidity is mostly due to

acceptance Slightly smaller for central bins as expected.

DiMuon acceptance x reconstruction efficiency

We used different generator functions to

weight the input distributionThis is done to see weather variations are

large.Variations are found small and included as a systematic error in acc x eff values.

Uncertainties

Biggest error is statstical estat = 1/N1/2 = 17 % Signal counting ecounting = - 4 % N

MB from MB efficiency eMB = 1%

eTotal 10 % e acceptance

= 3 %

Results

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d 2 /dy dpT

vs pT

RAA

for Z0

Summary

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Z0 is measured in Heavy ion collisions. Inclusive cross section is measured. p

T , rapidity and N

part dependence of cross section is measured.

Within the statstical uncertanity no modification of Z0 prodution is obseved with N

coll.

Back Up

Thanks to all colleagues in CMS collaboration

Thanks to the organizers Nuclear physics symposium

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Back Up

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float NCollArray[50]={1747.49,1566.92,1393.97,1237.02,1095.03,979.836,863.228,765.968,677.894,594.481, 522.453,456.049,399.178,347.174,299.925,258.411,221.374,188.676,158.896,135.117, 112.481,93.5697,77.9192,63.2538,52.0938,42.3553,33.7461,27.3213,21.8348,17.1722, 13.5661,10.6604,8.31383,6.37662,5.12347,3.73576,3.07268,2.41358,2.10707,1.76851, };

Binary Collisions in different centrality bins

The charged Particle Multiplicity

Predictions for PbPbN.Armesto: http://arxiv.org/pdf/0903.1330v2

PP at 7 TeV

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