Indian J. Phys. 85 (1) 205-209 (2011)

© 2011 IACS*Corresponding Author

B-meson measurement via secondary J/� production in

Pb-Pb collisions at NNs = 5.5 TeV in the CMS

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Abstract : The production of heavy quarks in heavy-ion collisions is important because it is expected to giveinformation about high-density QCD matter. One interesting channel of heavy quark measurements at LHC is thedecay of B to J��. The J�� mesons from this channel can be separated from the primary J�� (produced atcollision vertex) by using the secondary vertex which is efficiently reconstructed with the CMS tracker. In thiswork, the generator level results for the primary and secondary J�� obtained through dimuon pairs are presentedfor pp and PbPb collisions at NNs = 5.5 TeV.

Keywords : Heavy ion collision, heavy quark production

PACS Nos. : 25.75.-q, 25.75.Cj

1. Introduction

Collisions of heavy ions at the Large Hadron Collider (LHC) at energies NNs = 5.5

TeV will create strongly interacting matter at very high temperature where a phasetransition to quark gluon plasma (QGP) is expected. The CMS (Compact MuonSolenoid) experiment [1] has extensive heavy ion physics programs besides p+pphysics programs. Quarks lose energy in the medium by gluon bremsstrahlung. Suchgluon radiation is suppressed at angle smaller than the ratio between their mass andenergy. Thus, the heavy quarks are predicted to lose less energy as compared to lightquarks. However at RHIC it was found that c quarks also lose substantial energy asmuch as light quarks [2]. The yield of charm and bottom quarks at LHC would be atleast one order of magnitude larger than those at RHIC [3] enabling more precisesystematic studies of the energy loss phenomena of heavy flavours for the first time.

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One interesting channel at LHC is secondary J�� coming from the decays of B-mesons with branching ratio 1.16%. The secondary J�� subsequently decays to twomuons with branching ratio 5.93%. On the other hand, the J�� particle produced earlyin the collision is considered to give crucial information on many body aspects of QCDmatter and QGP formation. This J�� produced at collision vertex is referred in thepresent study as ‘primary’. The secondary J�� coming from B decays can beseparated from the primary using the secondary vertex information, as the B flies toa measurable distance before decaying. We present secondary vertex reconstructionefficiency for the CMS detector.

The CMS detector is shown in Figure 1. The � and � coverage of the CMSmuon system is given by � < 2.4 and 0 < � < 2�. The muon system consists of DT(Drift Tube Chamber), CSC (Cathod Strip Chamber) and RPC (Resistive Plate Chamber).The large muon acceptance and very good dimuon resolution resulting from the highgranularity of the of the CMS detector are well suited for heavy quark physics requiringhigh statistics and good signal to background ratio.

Figure 1. The CMS detector.

2. Results and discussion

We produce both primary and secondary J�� events at NNs = 5.5 TeV for pp

collisions using PYTHIA [4] event generator Version 6.409. This PYTHIA version is partof the CMS software CMSSW_1_6_6. The produced BB pairs are forced to decayto J�� which then decays to muon pairs. It is convenient to quote all the numbersfor one month (106 s) of LHC running. The luminosity for PbPb collisions integrated over

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for two kinematic cuts applied on both of the muons from J�� decays. The 8th and9th rows give the accepted number of muon pairs in those cuts. The ratio ofsecondary J�� to primary J�� in all phase space is 0.34 and in the CMS acceptance(|�| < 2.4, pT > 5.0 GeV�c) it is 0.05. The numbers in the table are calculated withthe generated particles and do not include the tracking and particle identificationefficiency. The � and pT acceptance of the CMS detector is different for primary andsecondary J�� events. A total of 11300 B’s coming through J�� are expected in thecut |�| < 2.4 and pT > 5.0 GeV�c. In an earlier study [5] a total of 13000 B’s wereobtained in the same cut. A total of 230000 primary J�� are expected in the cut |�|< 2.4 and pT > 3.5 GeV�c.

Figure 2 shows the � distribution of accepted B mesons with a kinematic cut

pT� > 5 GeV�c and |��| < 2.4 produced in minimum bias Pb+Pb collisions at NNs

= 5.5 TeV for one month of LHC running. Figure 3 shows the pT distribution of theseB mesons.

Table 1. Comparison of direct J�� and B ��J�� for PbPb collisions at 5.5 TeV for onemonth of LHC running.

J�� � �+�– B ��J���� �+�–

Cross section 506 mb (J��) 7355 mb ( BB )

No. of BB – 3.7 × 109

No. of J�� 2.5 × 108 8.6 × 107

No. of �+�– pairs 1.5 × 107 5.15 × 106

Acceptance cuts |��| < 2.4 0.5386 0.6169

Acceptance cuts |��| < 2.4 and p�T > 5.0 0.016 0.0022

Acceptance cuts |��| < 2.4 and p�T > 3.5 0.030 0.011

Accepted �+�– for |��| < 2.4 and p�T > 5.0 2.3 × 105 11300

Accepted �+�– for |��| < 2.4 and p�T > 3.5 4.4 × 105 57000

one month is about 0.5 nb–1. The BB and J�� cross sections for pp collisions aretaken from CERN yellow report [3] and extrapolated to PbPb collisions. The J��production cross section per nucleon pair is 11.7 �b for PbPb minimum bias collisionsat 5.5 TeV. This is calculated using parton distribution function (pdf) MRST HO with cquark mass at 1.2 GeV�c2. The BBbar production cross section per nucleon pair isused as 0.17 mb obtained using pdf CTEQ5M1 and b quark mass at 4.75 GeV�c2.The above values of cross section include nuclear shadowing using EKS98parameterizations. The inclusive B � J�� branching ratio is 1.16% and the J�� �

�+�– branching ratio is 5.93%.

Table 1 summarizes the number of produced muon pairs before and afterapplying acceptance cuts of the CMS detector for the two processes J�� � �+�– andB � J��. The 6th and 7th rows of the Table give the acceptance of the CMS detector

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The reconstruction capabilities of the CMS Silicon Tracking System for chargedparticles with pT > 0.8 GeV�c have been studied in detail using a full detectorsimulation. Figure 4 shows the track transverse impact parameter resolution for heavy

Figure 5. Transverse distance, �r, distribution of �+�–

pairs from secondary (solid histogram) and primarydashed histogram) J�� decays [5].

Figure 4. The track transverse impact parameterresolution achieved in heavy ion events (with dNch�

dy|y=0 = 3200) in the barrel (full symbols) and in theforward (open sysmbols) regions [5].

Figure 3. pT distribution of B mesons with p �T > 5

GeV�c and |��| < 2.4, produced in one month ofminimum bias Pb+Pb collisions (0.5 nb–1) at NNs =5.5 TeV.

Figure 2. � distribution of B mesons with p �T > 5 GeV�

c and |��| < 2.4, produced in one month of minimumbias Pb+Pb collisions (0.5 nb–1) at NNs = 5.5 TeV.

pT[GeV/c]

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ions (with dNch�dy|y=0 = 3200) in the barrel and in the forward regions. The impactparameter resolution is as low as 50 µm for high pT tracks good enough to measuresecondary displaced vertices from heavy-quark meson decays [5]. The muons fromprimary J�� mesons come from the primary vertex while those decayed from B mesondecays appear at some distance from the interaction point. The path length betweenthe primary and secondary vertices depends on the meson lifetime and its momentum(Lorentz boost). An efficient way to select the dimuons from B decays is to requirea minimum transverse distance, �r. If Pmin is the track point with minimal distance tothe beam axis, z, then �r is the distance in the x-y plane between the points P1,min

and P2,min, of two different muon tracks. Figure 5 shows the transverse distance �rdistribution of �+�– pairs from secondary and primary J�� decays. Here the spatialresolution for muon track finding has been taken from Pb-Pb simulations. Muon pairsfrom bb decays show a rather flat distribution while those from primary J�� decaysare concentrated at small �r values, vanishing at �r ~ 70 µm. Fast simulation studiesincluding the track and vertex resolutions indicate that a cut �r > 50 µm suppressesthe primary J�� rate by two orders of magnitude while the rate for J�� coming frombb is only reduced by 30% [5].

3. SummaryA comparative study of primary J�� and secondary J�� from B decays has beenmade. A total of 11300 B’s coming through J�� are expected in the cut |�| < 2.4 andpT > 5.0 GeV�c. The ratio of secondary J�� to primary J�� in all phase space is 0.34and with the CMS acceptance, |�| < 2.4 and pT > 5.0 GeV�c, it becomes 0.05. Thisratio is very important in finding out the effect of QGP formation as both processes areaffected by different final state effects in the presence of QGP. Studies show thatsecondary J�� carries energy loss information of the b quarks. The secondary vertexcan be efficiently reconstructed in the CMS and can be used to separate primary andsecondary J��.

AcknowledgmentsThe authors are thankful to D d’Enterria and O Kodolova for the fruitful discussions.

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