Post on 14-Mar-2022
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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 %
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.
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