Evidence of b-jet quenching in PbPb collisions at √ s NN = 2.76 TeV

: The production of jets associated to bottom quarks is measured for the first time in PbPb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair. Jet spectra are reported in the transverse momentum (pt) range of 80-250 GeV, and within pseudorapidity abs(eta < 2). The nuclear modification factor (R[AA]) calculated from these spectra shows a strong suppression in the b-jet yield in PbPb collisions relative to the yield observed in pp collisions at the same energy. The suppression persists to the largest values of pt studied, and is centrality dependent. The R[AA] is about 0.4 in the most central events, similar to previous observations for inclusive jets. This implies that jet quenching does not have a strong dependence on parton mass and flavor in the jet pt range studied. Abstract The production of jets associated to bottom quarks is measured for the ﬁrst time in PbPb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair. Jet spectra are reported in the transverse momentum ( p T ) range of 80–250 GeV/ c , and within pseudorapidity | η | < 2. The nuclear modiﬁcation factor ( R AA ) calculated from these spectra shows a strong suppression in the b-jet yield in PbPb collisions relative to the yield observed in pp collisions at the same energy. The suppression persists to the largest values of p T studied, and is centrality dependent. The R AA is about 0.4 in the most central events, similar to previous observations for inclusive jets. This implies that jet quenching does not have a strong dependence on parton mass and ﬂavor in the jet p T range studied.


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By colliding heavy nuclei at the Large Hadron Collider (LHC), one expects to reach sufficiently large energy densities to form a quark-gluon plasma (QGP), a state which is characterized by effective deconfinement of the color degrees of freedom [1,2]. Hard-scattered partons are expected to suffer energy loss as they traverse the QGP via elastic and inelastic interactions [3]. This is commonly thought to be the mechanism responsible for the observed suppression of high transverse momentum (p T ) hadrons and jets, or "jet quenching", in nuclear collisions [4][5][6][7][8][9][10][11][12][13][14]. Measurements of parton energy loss are expected to reveal the fundamental thermodynamic and transport properties of this phase of matter (see Refs. [15,16] for recent reviews).
The quenching of jets in heavy-ion collisions is expected to depend upon the flavor of the fragmenting parton [17]. For example, under the assumption that radiative energy loss is the dominant mechanism, gluon jets are expected to quench more strongly than quark jets, owing to the larger color factor for gluon emission from gluons than from quarks. Moreover, jets from heavy quarks may radiate less strongly than those from light flavor due to the so-called "deadcone" effect [18], particularly when the parton p T is comparable to its mass. The relevance of this mechanism in heavy-ion collisions, however, is debated [19].
Until now, identification of reconstructed b jets has not been performed in heavy-ion collisions. Recent data on single-particle production of B mesons (via non-prompt J/ψ) [20] suggest a mass dependence of the suppression, compared to D mesons [21] and non-identified charged particles [22,23]. Compared to B mesons, b jets provide a more direct connection to the b-quark energy loss, albeit typically in a different range of p T . Through comparisons with the existing measurements of inclusive jet production [24], b-quark jet (b jet) measurements can be used to study the flavor dependence of jet quenching, which in turn provides insight on the dynamics of parton energy loss.
The Compact Muon Solenoid (CMS) detector has excellent capabilities to perform b-jet identification (b tagging) measurements as demonstrated in Ref. [25]. Measurements of the b-jet cross section [26] and b-jet angular correlations [27] have been performed in pp collisions at 7 TeV. This Letter presents the first measurements of b-jet production in heavy-ion collisions using a dataset corresponding to an integrated luminosity of 150 µb − parton flavor is assigned to reconstructed jets by matching them in ∆R = ∆η 2 + ∆φ 2 to generator-level partons (φ is the azimuthal angle measured in radians in the plane transverse to the beams). If a bottom quark is found within ∆R < 0.3 then the jet is considered to be a b jet, irrespective of any other partons in the cone. This definition includes b quarks from gluon splitting (g → bb), even if the splitting occurs late in the parton shower (i.e., at low virtuality), consistent with the theoretical treatment of heavy-flavor production in Ref. [38]. We note that b jets from gluon splitting comprise about 60% of the total b-jet cross section (according to PYTHIA simulations) and are expected to interact differently with the QGP than those from primary b quarks [39]. To compare with PbPb data, PYTHIA events are embedded into PbPb events produced by the HYDJET generator (version 1.8) [40], which is tuned to reproduce event properties, such as charged-hadron multiplicity, p T spectra, and elliptic flow.
Identification of b jets is based on kinematic variables related to the relatively long lifetime and large mass of b hadrons. Charged tracks of p T > 1 GeV/c that are associated to jets are used to reconstruct secondary vertices (SV) from b hadrons and/or subsequent c-hadron decays from the b → c cascade. The contribution of b jets is enhanced by requiring that SVs are far enough from the primary vertex, using a selection on the significance of the three-dimensional flight distance. This selection is chosen to give a misidentification rate of roughly 1% for light jets and 10% on charm-quark jets (c jets), based on simulation. The corresponding b-tagging efficiency is about 65% for pp and 45% for PbPb collisions.
The SV invariant mass is calculated from the constituent tracks. An example SV mass distribution, for jets with 80 < p T < 90 GeV/c, is shown in Fig. 1. For each jet p T bin, the b-jet purity ( f b ), i.e., the ratio of the number of b jets to that of inclusive jets in the tagged sample, is extracted by means of a template fit. The shapes of the light-quark, c and b contributions are determined from simulation, while their normalizations are allowed to float. After tagging, the three contributions are of comparable magnitude, as shown in the figure, but the b-quark contribution dominates above the c-quark mass threshold near 2 GeV/c 2 , which allows for an accurate determination of the b-jet contribution.
For the systematic studies described below, an alternative b-tagging strategy is employed, which uses the jet probability (JP) algorithm [25]. In contrast to direct reconstruction of SVs, the JP tagger is based on an estimate of the compatibility of tracks with the primary vertex, using their three-dimensional impact parameter significance. A probability density for this compatibility is obtained directly from data using tracks with negative impact parameter, i.e., tracks that appear to come from the side of the primary vertex opposite the jet direction. Such tracks are unlikely to be associated with heavy-flavor decays. The impact parameter (IP) has the same sign as the scalar product of the vector pointing from the primary vertex to the point of closest approach with the jet direction. Tracks originating from the decay of particles traveling along the jet axis will tend to have positive IP values.
The b-jet yield, in a given p T bin, is obtained as tagged jets and is the b-tagging efficiency. The efficiency is determined from simulation and cross-checked using the so-called reference lifetime tagger method, which uses the JP tagger to determine the efficiency of the SV tagger directly from data, taking advantage of the dataderived calibration of the JP tagger [25]. The simulation reproduces the estimate of from data to within 5%.
The unfolded b-jet p T spectra in PbPb collisions are shown in Fig. 2 for several centrality selections. The PbPb data are divided by T AA , computed from a Glauber model (for a review, see Ref. [41]), to scale to the expectation for pp collisions in the absence of nuclear effects. The value of the T AA is the number of nucleon-nucleon (NN) collisions divided by the total inelastic NN cross section and may be interpreted as the NN equivalent luminosity per PbPb collision. Also shown is the measured b-jet cross section in pp collisions. The cross section is compared to PYTHIA simulations, which agree well with the data, as is the case at √ s = 7 TeV for the p T range covered by the present study [26]. The points are placed along the abscissa at the centers of the bins.
The systematic uncertainties fall into two general categories: b tagging and jet reconstruction. The b-tagging uncertainty on b-jet yields varies from about 12 to 18%, depending on jet p T and collision system. The uncertainty is evaluated via the following systematic variations of the tagging procedure, which influence the extracted b-tagging purity and efficiency values: • varying the SV flight distance selection such that differs by about 10%, • using from the reference lifetime tagger method [25], rather than from simulation, • fixing the c jet to light-quark jet normalization, rather than allowing them to float independently in the template fits, • using a non-b-jet template produced from jets with small JP in data, • varying the gluon-splitting contribution in the b-jet and c-jet templates by 50%.
The uncertainty on the spectra due to the jet reconstruction is 10-12% for pp and 15-17% for PbPb, and is comprised of the following sources: • a 10% uncertainty in the jet energy resolution [42], • a 2% uncertainty in the jet energy scale (JES) [42], • an additional, centrality-dependent, 1-2% uncertainty in the JES in PbPb collisions due to the underlying event, evaluated from random-cone and embedding studies, • an uncertainty in the unfolding procedure evaluated by varying the number of iterations and the presumed prior spectrum.
The pp luminosity has an uncertainty of 3.6%, while the uncertainty in T AA varies from about 4% for centrality 0-10% to 15% for 50-100% [14]. Figure 3 shows the centrality-integrated b-jet nuclear modification factor (R AA ), which is the ratio of the T AA -normalized PbPb yield and the measured pp cross section in Fig. 2, as a function of p T . A significant suppression of the yield with respect to the pp expectation is observed in b jets, which is indicative of the parton energy loss in the hot medium. No strong trend is observed as a function of p T , although the data hint a modest rise at higher p T . The jet and b-tagging systematic uncertainties in R AA are obtained by varying the pp and PbPb data simultaneously. This results in partial cancellation, giving a systematic uncertainty of 16-21%, which is dominated by the b-tagging uncertainty. all collision centralities, b jets are found to be suppressed over the 80-250 GeV/c p T range explored in this study. For the 80-110 GeV/c p T range, R AA is found to decrease with collision centrality. At larger p T , the trend is less evident due to the reduced statistical precision. The b-jet suppression is found to be qualitatively consistent with that of inclusive jets [24]. The absence of a strong dependence of the jet suppression on the mass of the fragmenting parton would favor a perturbative model in which mass effects are expected to be small at large p T , as in Ref. [39], when compared against a model based on strong coupling (via the AdS/CFT correspondence) [17], in which mass effects could persist to large p T . A weaker mass dependence, such as the one predicted in Ref. [43], cannot be ruled out with the present uncertainties.
We congratulate our colleagues in the CERN accelerator departments for the excellent performance of the LHC and thank the technical and administrative staffs at CERN and at other CMS institutes for their contributions to the success of the CMS effort. In addition, we gratefully acknowledge the computing centres and personnel of the Worldwide LHC Computing Grid for delivering so effectively the computing infrastructure essential to our analyses. Finally, we acknowledge the enduring support for the construction and operation of the LHC and the CMS detector provided by the following funding agencies: [9] STAR Collaboration, "Distributions of charged hadrons associated with high transverse momentum particles in pp and Au + Au collisions at √ s NN = 200-GeV", Phys. Rev. Lett.  [34] G. D'Agostini, "A multidimensional unfolding method based on Bayes' theorem", Nucl. Instrum. Meth. A 362 (1995) 487, doi:10.1016/0168-9002(95)00274-X.