- Email: [email protected]
Overview of results on jets from the CMS Collaboration
Available online at www.sciencedirect.com
Nuclear Physics A 904–905 (2013) 146c–153c www.elsevier.com/locate/nuclphysa
Overview of results on jets from the CMS Collaboration G´abor I. Veres (for the CMS Collaboration)1 CERN, CH-1211, Gen`eve 23, Switzerland
Abstract This paper reviews recent experimental results on jet production and properties in heavy-ion collisions from the CMS Collaboration. These studies include various energy loss phenomena (di-jet and photon-jet energy imbalance, nuclear modification factors – RAA – for inclusive and b-quark jets); observables related to jet properties like jet shapes and fragmentation functions; and measurements of high-pT charged hadrons (RAA up to 100 GeV/c, azimuthal asymmetry up to 60 GeV/c and two-particle correlations triggered by a high-pT hadron). The presented results utilize the high statistics PbPb data, about 150 μb−1 , collected in 2011 at the LHC. 1. Introduction Strongly interacting matter at extreme temperatures can be studied experimentally by colliding heavy ions at ultra-relativistic energies. The properties of the medium created in these collisions can be analyzed by examining processes with known cross sections in vacuum. These “probes” are often chosen to be hard processes, creating particles or jets at high energy or large mass. They fall into two categories: those that are strongly interacting and are thus expected to be modified, like (charged) hadrons, jets and quarkonium states; and those that remain largely unmodified like high-energy isolated photons, Z0 and W± bosons. The CMS experiment at the LHC is ideally suited to measure these hard processes, due to the large η-coverage of the calorimeters, muon and tracker detectors embedded in a 3.8 T magnetic field [1]. In this paper the most recent results from CMS, related to jet production and properties will be reviewed in PbPb collisions at √ sNN = 2.76 TeV. All of these results make use of the high-statistics data set recorded in 2011, corresponding to 150 μb−1 integrated luminosity. Pairs of di-jets exhibit a more pronounced transverse momentum (pT ) imbalance in central PbPb collisions compared to pp collisions [2], indicating a wide distribution of energy loss of high-pT partons propagating through the dense medium created in the interaction. At the same time, the distribution of the azimuthal angle difference between the two jets remains sharply peaked around 180 degrees, leading to the conclusion that the parton energy loss does not cause a visible angular decorrelation. Recently, these studies have been extended to pT up to 350 GeV/c, and measured as a function of pT and centrality [3], and the pT -imbalance with respect to the pp reference is found to be pT -independent within the systematic uncertainties. The transverse momentum difference between di-jet partners in central PbPb collisions is observed to be carried away mostly by low-pT particles (pT < 4 GeV/c), especially those at large angles with respect 1A
list of members of the CMS Collaboration and acknowledgments can be found at the end of this issue. © CERN for the benefit of the CMS Collaboration. Open access under CC BY-NC-ND license.
0375-9474/ © 2013 CERN Published by Elsevier B.V. http://dx.doi.org/10.1016/j.nuclphysa.2013.01.056
Open access under CC BY-NC-ND license.
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to the subleading (away side) jet axis, thereby restoring momentum conservation [2]. Within the sizeable systematic and statistical uncertainties achievable with the data taken in 2010, it was also shown that the hard part of the jet fragmentation functions, defined in this case as the distribution of ξ = ln(pjet /ptrack ), where ptrack is the projection of the track momentum onto the jet axis (for || || track pT > 4 GeV/c), is similar between PbPb and pp collisions [4]. In the following, updates and extensions of these measurements will be discussed for observables using: a) fully reconstructed and background-subtracted jets; b) correlations of jets and charged tracks; and c) high-pT tracks that originate from jet fragmentation. 2. Energy loss studies with fully reconstructed jets While jet imbalance studies are robust against uncertainties of jet energy scale corrections, they require two reconstructed jets and cannot account for subdominant jets below the selection threshold. In order to measure inclusive single-jet pT -spectra in PbPb and pp collisions, and form their ratios (nuclear modification factors, RAA ) that are scaled by the appropriate nuclear overlap functions, T AA , for each centrality bin, careful treatment of the jet energy resolutions and corrections, as well as the application of statistical unfolding methods are necessary. The first preliminary results on jet nuclear modification factors between 100 and 300 GeV/c of jet pT are presented in Fig. 1, in six centrality bins, 0–5% representing the 5% most central PbPb collisions. A suppression of the jet yield reconstructed from the PbPb data is observed, which is independent of pT in the presented range, and which increases with centrality, reaching about a factor of two in the case of the most central events [5]. Such a reduction of the jet yield can
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originate from the combination of the steeply falling pT -spectrum and finite amount of energy loss, as well as from a decrease of the number of jets created or detected at a given pT . Jet nuclear modification factors can be compared to those of single particles (photons [6], W± [7] and Z0 bosons [8], charged particles [9] and b-quarks inferred from secondary J/ψ-s [10]), as shown in Fig. 2. On the linear pT -scale used in Fig. 2 the similarity of RAA factors obtained for charged particles in the 50–100 GeV range, and for jets in the 100–200 GeV range is clearly visible. It will be demonstrated in Section 3 that the hard part (ξ < 1) of the jet fragmentation functions in pp and PbPb collisions are very similar. Taking into account that the leading charged particle typically originates from the fragmentation of jets with about two times larger transverse momentum, this agreement between RAA factors also reflects the consistency between the charged hadron and jet RAA measurements and the fragmentation function studies. High-energy photons are not modified by the strongly interacting medium and may serve as an energy tag for the jet partner in γ-jet events. These collisions produce a jet and a photon approximately back-to-back in azimuth, with similar (but not precisely the same) transverse mojet menta, and the ratio of those is denoted by x Jγ = pT /pγT . The distribution of x Jγ in central PbPb collisions is shifted to lower values with respect to the distribution in pp collisions [11]. The mean value of this ratio, x Jγ , is presented on the left panel of Fig. 3 as a function of the number of nucleons participating in the PbPb collision, Npart , used for characterizing the collision centrality. The filled square indicates the result obtained from the pp data taken at 2.76 TeV, limited by statistical uncertainties, but in agreement with pp events simulated by the PYTHIA [12] Monte Carlo generator, embedded in PbPb events simulated by HYDJET [13]. While the pp reference shows almost no dependence of x Jγ on centrality, the γ-jet PbPb events exhibit a significantly lower ratio. At the same time, γ-jet events also lack angular decorrelation; the middle panel of Fig. 3 demonstrates that there is a good agreement between the widths of the distributions of the azimuthal angle difference, Δφ Jγ , between the photon and jet, in the PbPb data and the pp reference. The left panel on Fig. 3 shows the average fraction of those isolated photons that are associated with a partner jet above 30 GeV/c, R Jγ , as a function of centrality. In central
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PbPb events, a significant fraction of jet partners get their energy diminished below the 30 GeV/c threshold, while the pp reference does not show any visible dependence on Npart . Therefore, a sizeable jet (parton) energy loss is observed in the γ-jet channel—similar to what was seen in the di-jet studies—although a direct model-independent comparison would be a complicated task. Inclusive jets are dominated by gluon- and light quark-jets. The parton energy loss is expected to be different for light and heavy flavours, based on the dead cone effect [14], arousing interest in the separation of bottom-quark jets. These b-quark jets may be experimentally recognized (tagged) by the presence of a B-hadron, creating a secondary, high-mass vertex at its decay point. They have been tagged based on high values of flight distance significance. The efficiency of the secondary vertex tagging can be estimated in a data-driven way. The b-jet tagging purity is determined from template fits to the secondary vertex invariant mass distributions. This way, the fraction of b-quark jets among inclusive jets can be measured as a function of transverse momentum. The analysis was completed for pp (231 nb−1 ) and PbPb (150 μb−1 ) collisions at 2.76 TeV, and the first results on the b-quark fraction [15] are plotted in Fig. 4. The fractions of b-jets in pp and PbPb collisions are comparable, with no pT -dependence in the 80–200 GeV/c range. The combination of the RAA factors for inclusive jets and the above ratio leads to an RAA value for b-quark jets of about 0.5, with sizeable systematic uncertainties [15]. Still, this is the first time that a significant quenching of fully reconstructed and identified b-quark jets in heavy-ion collisions has been demonstrated. 3. Correlations between charged tracks and reconstructed jets As discussed earlier, a significant amount of jet quenching was observed in di-jet, γ-jet and bquark jet events. These observations raise a question related to the parton energy loss mechanism; do partons first lose energy in the nuclear medium and subsequently fragment as they would do in vacuum, or does the energy loss modify the fragmentation process itself? The first observations based on the data collected in 2010 have shown that the hard part of the jet fragmentation is similar between pp and PbPb collisions (at least for tracks with pT above 4 GeV/c), and that the pT missing from the reconstructed subleading jets are carried by low momentum particles mostly at large angles with respect to the subleading jet axis. Taking advantage of the 20 times larger data sample collected in 2011, it was possible to extend the analysis of jet fragmentation functions, not only with increased statistics for jets, but also including tracks with pT down to
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Figure 4: Left panel: the b-jet to inclusive jet ratio in pp collisions as a function of jet pT (symbols) compared to PYTHIA [12] events (histogram) at 2.76 TeV center-of-mass energy. Right panel: The b-jet to inclusive jet ratio in minimum bias PbPb collisions as a function of jet pT (symbols) compared to pp events generated by PYTHIA [12] and embedded in PbPb events generated by the HYDJET [13] model. The error bars represent statistical, the (yellow) bands represent systematic uncertainties.
1 GeV/c [16]. Measurements of jet fragmentation properties provide an experimental constraint which is complementary to measurements of jet production, such as jet RAA . These can also be used to connect jet observables to measurements of high-pT particle production. For the present study, jets with at least 100 GeV/c transverse momentum were selected, after reconstruction with the Particle Flow algorithm that optimally combines tracking and calorimetric information [17], within the |η| < 2 range, using a radius parameter of R = 0.3 and the anti-kT clustering algorithm. Tracks that are close to the jet axis, in the Δη2 + Δφ2 < 0.3 range, are included in the correlation studies. The soft part of the fragmentation function is contaminated, at low-pT even dominated by the background from the underlying event, which is subtracted by collecting the background contribution from a “jet” cone reflected around η = 0 (this method was cross-checked with the event mixing method). The resulting, background-subtracted fragmentation functions in PbPb collisions are compared to the pp reference histograms in four centrality bins on the top panels of Fig. 5, extended to larger ξ values. The bottom panels show the ratio of the fragmentation functions between PbPb and pp collisions [16]. These results are consistent with the findings using the data collected in 2010, but they have much better statistical and systematic precision and the data extend to larger ξ. We can conclude that there is no significant modification of the jet fragmentation at low ξ (the region of the leading hadrons) in PbPb collisions with respect to the pp reference, while there is a slight depletion in the mid-ξ region, accompanied by an excess at high ξ (low track pT ). After the examination of the longitudinal fragmentation pattern, one can also characterize the jets and their modification in the transverse direction by the measurement of jet shapes: the transverse momentum fraction, ρ(r), carried by charged particles as a function of their distance r = Δη2 + Δφ2 from the jet axis. Only those particles are included that belong to the jet, which is achieved by statistical subtraction of the background using the η-reflection method. The jet shape results are shown on the top panels of Fig. 6 in four centrality classes for PbPb collisions (solid symbols) compared to the pp reference (open symbols), the latter being constructed from results on pp collisions taking into account the difference of the jet energy resolution and of the
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Figure 5: Top row: fragmentation function in PbPb collisions (symbols) and for the pp reference (histogram) in four centrality classes, for jets with pT > 100 GeV/c, and tracks with pT > 1 GeV/c. The bottom row shows the ratio of each PbPb fragmentation function and its pp reference. Error bars represent statistical, and (yellow) boxes represent systematic uncertainties. The definition of the z variable is: z = ptrack /pjet , where ptrack is the projection of the track || || momentum onto the jet axis.
jet pT -spectrum between the two different colliding systems [16]. The bottom panels show the ratio between the jet shapes measured in PbPb collisions and the pp reference. The observed enhancement at large radii in the PbPb case is consistent with the excess of the low-pT part of the jet fragmentation functions, since low-pT particles are known to extend to larger radii within jets, compared to the high-pT jet core. Still, the jet shape and fragmentation results provide complementary information and cannot be derived from each other. 4. Correlations between charged particles at high pT It is well known that the azimuthal angle distribution of low-pT charged particles in PbPb collisions is not uniform, leading to a non-zero second Fourier-coefficient, v2 , reflecting the collective flow of the created medium originating from pressure gradients building up in the elongated nuclear overlap zone. At high pT , such non-uniformity can be caused by path-length dependent parton energy loss and/or jet fragmentation, not by hydrodynamic phenomena any more. Recent measurements of the v2 coefficient from the CMS experiment have shown that v2 is indeed significantly non-zero up to pT of 40–60 GeV/c[18]. A good knowledge of the single-particle azimuthal asymmetries is essential for the deeper analysis of jet modifications. Correlations between a high-pT charged particle that originates trig from jet fragmentation (with pT ) and other charged particles (with passoc ) inherit contributions T from these single-particle asymmetries which have to be subtracted in order to study the remaining correlation structure characteristic of the near-side and away-side jets. After subtraction, the ratio of the near (away) side particle yields between PbPb and pp data, IAA , is presented in the top (bottom) row of Fig. 7 [19]. As expected from jet fragmentation function results, there is an enhancement at low associated particle pT on the near side in PbPb collisions with respect to pp collisions, while there is no significant difference at high pT . On the away side, however,
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Figure 7: Near-side (top row) and away-side (bottom row) IAA ratios derived from the vn -subtraction method (solid trig circles) at |Δη| < 1 and from the long-range Δη-subtraction method (black squares) shown for four different pT ranges √ assoc as a function of pT for PbPb collisions at sNN = 2.76 TeV, for the 10% most central events. The error bars represent statistical uncertainties while the brackets represent the systematic uncertainties.
the associated particle yield is suppressed at high pT (in accordance with the jet quenching phenomenon), while at low pT the enhancement is in agreement with the observation that the energy lost by the away-side jet reappears in form of low-pT particles.
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5. Summary The CMS experiment has shown that the di-jet energy imbalance and the jet nuclear modification factors do not depend strongly on jet pT . The first photon-jet correlations support the jet quenching picture, and b-quark jets have also been found to be quenched. Jet shapes and fragmentation functions indicate an excess at low pT (large radii) but no modification is observed at high pT (small radii), supported by two-particle correlation results triggered by a highpT particle. Positive v2 coefficients were found to persist to very high pT , reflecting path-length dependent energy loss. In summary, the jet quenching picture painted by the CMS results has become more precise, detailed and quantitative, challenging theoretical descriptions and providing a new insight into the behaviour of the high density medium formed in heavy-ion collisions. References [1] S. Chatrchyan, et al. (CMS Collaboration), The CMS experiment at the CERN LHC, JINST 3 (2008) S08004. [2] S. Chatrchyan, et al. (CMS Collaboration), Observation and studies of jet quenching in PbPb collisions at nucleonnucleon center-of-mass energy = 2.76 TeV, Phys.Rev. C84 (2011) 024906. [3] S. Chatrchyan, et al. (CMS Collaboration), Jet momentum dependence of jet quenching in PbPb collisions at √ sNN = 2.76 TeV, Phys.Lett. B712 (2012) 176–197. [4] S. Chatrchyan, et al. (CMS Collaboration), Measurement of jet fragmentation into charged particles in pp and PbPb √ collisions at sNN = 2.76 TeV, JHEP 2012 (2012) 087. [5] S. Chatrchyan, et al. (CMS Collaboration), Nuclear modification factor of high transverse momentum jets in √ PbPb collisions at sNN = 2.76 TeV, CMS-PAS-HIN-12-004, 2012. URL: http://cdsweb.cern.ch/record/ 1472722. [6] S. Chatrchyan, et al. (CMS Collaboration), Measurement of isolated photon production in pp and PbPb collisions √ at sNN = 2.76 TeV, Phys.Lett. B710 (2012) 256–277. √ [7] S. Chatrchyan, et al. (CMS Collaboration), Study of W boson production in PbPb and pp collisions at sNN = 2.76 TeV, Phys.Lett. B715 (2012) 66–87. [8] S. Chatrchyan, et al. (CMS Collaboration), Z boson production with the 2011 data in PbPb collisions, CMS-PASHIN-12-008, 2012. URL: http://cdsweb.cern.ch/record/1472723. [9] S. Chatrchyan, et al. (CMS Collaboration), Study of high-pT charged particle suppression in PbPb compared to pp √ collisions at sNN = 2.76 TeV, Eur.Phys.J. C72 (2012) 1945. [10] S. Chatrchyan, et al. (CMS Collaboration), J/ψ results from CMS in PbPb collisions, with 150 μb−1 data, CMSPAS-HIN-12-014, 2012. URL: http://cdsweb.cern.ch/record/1472735. [11] S. Chatrchyan, et al. (CMS Collaboration), Studies of jet quenching using isolated-photon+jet correlations in PbPb √ and pp collisions at sNN = 2.76 TeV, 2012. arXiv:1205.0206. [12] T. Sjostrand, S. Mrenna, P. Z. Skands, PYTHIA 6.4 Physics and Manual, JHEP 0605 (2006) 026. [13] I. Lokhtin, A. Snigirev, A Model of jet quenching in ultrarelativistic heavy ion collisions and high-pT hadron spectra at RHIC, Eur.Phys.J. C45 (2006) 211–217. [14] N. Armesto, C. A. Salgado, U. A. Wiedemann, Medium induced gluon radiation off massive quarks fills the dead cone, Phys.Rev. D69 (2004) 114003. [15] S. Chatrchyan, et al. (CMS Collaboration), Measurement of the b-jet to inclusive jet ratio in PbPb and pp collisions at 2.76 TeV with the CMS detector, CMS-PAS-HIN-12-003, 2012. URL: http://cdsweb.cern.ch/record/ 1472721. [16] S. Chatrchyan, et al. (CMS Collaboration), Detailed characterization of jets in heavy ion collisions using jet shapes and jet fragmentation functions, CMS-PAS-HIN-12-013, 2012. URL: http://cdsweb.cern.ch/ record/1472734. [17] S. Chatrchyan, et al. (CMS Collaboration), Commissioning of the Particle-Flow reconstruction in minimum-bias and jet events from pp collisions at 7 TeV, CMS-PAS-PFT-10-002, 2010. URL: http://cdsweb.cern.ch/ record/1279341. [18] S. Chatrchyan, et al. (CMS Collaboration), Azimuthal anisotropy of charged particles at high transverse momenta √ in PbPb collisions at sNN = 2.76 TeV, Phys.Rev.Lett. 109 (2012) 022301. [19] S. Chatrchyan, et al. (CMS Collaboration), Very high-pT triggered dihadron correlations in PbPb and pp collisions √ at sNN = 2.76 TeV, CMS-PAS-HIN-12-010, 2012. URL: http://cdsweb.cern.ch/record/1472744.
Nuclear Physics A 904–905 (2013) 146c–153c www.elsevier.com/locate/nuclphysa
Overview of results on jets from the CMS Collaboration G´abor I. Veres (for the CMS Collaboration)1 CERN, CH-1211, Gen`eve 23, Switzerland
Abstract This paper reviews recent experimental results on jet production and properties in heavy-ion collisions from the CMS Collaboration. These studies include various energy loss phenomena (di-jet and photon-jet energy imbalance, nuclear modification factors – RAA – for inclusive and b-quark jets); observables related to jet properties like jet shapes and fragmentation functions; and measurements of high-pT charged hadrons (RAA up to 100 GeV/c, azimuthal asymmetry up to 60 GeV/c and two-particle correlations triggered by a high-pT hadron). The presented results utilize the high statistics PbPb data, about 150 μb−1 , collected in 2011 at the LHC. 1. Introduction Strongly interacting matter at extreme temperatures can be studied experimentally by colliding heavy ions at ultra-relativistic energies. The properties of the medium created in these collisions can be analyzed by examining processes with known cross sections in vacuum. These “probes” are often chosen to be hard processes, creating particles or jets at high energy or large mass. They fall into two categories: those that are strongly interacting and are thus expected to be modified, like (charged) hadrons, jets and quarkonium states; and those that remain largely unmodified like high-energy isolated photons, Z0 and W± bosons. The CMS experiment at the LHC is ideally suited to measure these hard processes, due to the large η-coverage of the calorimeters, muon and tracker detectors embedded in a 3.8 T magnetic field [1]. In this paper the most recent results from CMS, related to jet production and properties will be reviewed in PbPb collisions at √ sNN = 2.76 TeV. All of these results make use of the high-statistics data set recorded in 2011, corresponding to 150 μb−1 integrated luminosity. Pairs of di-jets exhibit a more pronounced transverse momentum (pT ) imbalance in central PbPb collisions compared to pp collisions [2], indicating a wide distribution of energy loss of high-pT partons propagating through the dense medium created in the interaction. At the same time, the distribution of the azimuthal angle difference between the two jets remains sharply peaked around 180 degrees, leading to the conclusion that the parton energy loss does not cause a visible angular decorrelation. Recently, these studies have been extended to pT up to 350 GeV/c, and measured as a function of pT and centrality [3], and the pT -imbalance with respect to the pp reference is found to be pT -independent within the systematic uncertainties. The transverse momentum difference between di-jet partners in central PbPb collisions is observed to be carried away mostly by low-pT particles (pT < 4 GeV/c), especially those at large angles with respect 1A
list of members of the CMS Collaboration and acknowledgments can be found at the end of this issue. © CERN for the benefit of the CMS Collaboration. Open access under CC BY-NC-ND license.
0375-9474/ © 2013 CERN Published by Elsevier B.V. http://dx.doi.org/10.1016/j.nuclphysa.2013.01.056
Open access under CC BY-NC-ND license.
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to the subleading (away side) jet axis, thereby restoring momentum conservation [2]. Within the sizeable systematic and statistical uncertainties achievable with the data taken in 2010, it was also shown that the hard part of the jet fragmentation functions, defined in this case as the distribution of ξ = ln(pjet /ptrack ), where ptrack is the projection of the track momentum onto the jet axis (for || || track pT > 4 GeV/c), is similar between PbPb and pp collisions [4]. In the following, updates and extensions of these measurements will be discussed for observables using: a) fully reconstructed and background-subtracted jets; b) correlations of jets and charged tracks; and c) high-pT tracks that originate from jet fragmentation. 2. Energy loss studies with fully reconstructed jets While jet imbalance studies are robust against uncertainties of jet energy scale corrections, they require two reconstructed jets and cannot account for subdominant jets below the selection threshold. In order to measure inclusive single-jet pT -spectra in PbPb and pp collisions, and form their ratios (nuclear modification factors, RAA ) that are scaled by the appropriate nuclear overlap functions, T AA , for each centrality bin, careful treatment of the jet energy resolutions and corrections, as well as the application of statistical unfolding methods are necessary. The first preliminary results on jet nuclear modification factors between 100 and 300 GeV/c of jet pT are presented in Fig. 1, in six centrality bins, 0–5% representing the 5% most central PbPb collisions. A suppression of the jet yield reconstructed from the PbPb data is observed, which is independent of pT in the presented range, and which increases with centrality, reaching about a factor of two in the case of the most central events [5]. Such a reduction of the jet yield can
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originate from the combination of the steeply falling pT -spectrum and finite amount of energy loss, as well as from a decrease of the number of jets created or detected at a given pT . Jet nuclear modification factors can be compared to those of single particles (photons [6], W± [7] and Z0 bosons [8], charged particles [9] and b-quarks inferred from secondary J/ψ-s [10]), as shown in Fig. 2. On the linear pT -scale used in Fig. 2 the similarity of RAA factors obtained for charged particles in the 50–100 GeV range, and for jets in the 100–200 GeV range is clearly visible. It will be demonstrated in Section 3 that the hard part (ξ < 1) of the jet fragmentation functions in pp and PbPb collisions are very similar. Taking into account that the leading charged particle typically originates from the fragmentation of jets with about two times larger transverse momentum, this agreement between RAA factors also reflects the consistency between the charged hadron and jet RAA measurements and the fragmentation function studies. High-energy photons are not modified by the strongly interacting medium and may serve as an energy tag for the jet partner in γ-jet events. These collisions produce a jet and a photon approximately back-to-back in azimuth, with similar (but not precisely the same) transverse mojet menta, and the ratio of those is denoted by x Jγ = pT /pγT . The distribution of x Jγ in central PbPb collisions is shifted to lower values with respect to the distribution in pp collisions [11]. The mean value of this ratio, x Jγ , is presented on the left panel of Fig. 3 as a function of the number of nucleons participating in the PbPb collision, Npart , used for characterizing the collision centrality. The filled square indicates the result obtained from the pp data taken at 2.76 TeV, limited by statistical uncertainties, but in agreement with pp events simulated by the PYTHIA [12] Monte Carlo generator, embedded in PbPb events simulated by HYDJET [13]. While the pp reference shows almost no dependence of x Jγ on centrality, the γ-jet PbPb events exhibit a significantly lower ratio. At the same time, γ-jet events also lack angular decorrelation; the middle panel of Fig. 3 demonstrates that there is a good agreement between the widths of the distributions of the azimuthal angle difference, Δφ Jγ , between the photon and jet, in the PbPb data and the pp reference. The left panel on Fig. 3 shows the average fraction of those isolated photons that are associated with a partner jet above 30 GeV/c, R Jγ , as a function of centrality. In central
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PbPb events, a significant fraction of jet partners get their energy diminished below the 30 GeV/c threshold, while the pp reference does not show any visible dependence on Npart . Therefore, a sizeable jet (parton) energy loss is observed in the γ-jet channel—similar to what was seen in the di-jet studies—although a direct model-independent comparison would be a complicated task. Inclusive jets are dominated by gluon- and light quark-jets. The parton energy loss is expected to be different for light and heavy flavours, based on the dead cone effect [14], arousing interest in the separation of bottom-quark jets. These b-quark jets may be experimentally recognized (tagged) by the presence of a B-hadron, creating a secondary, high-mass vertex at its decay point. They have been tagged based on high values of flight distance significance. The efficiency of the secondary vertex tagging can be estimated in a data-driven way. The b-jet tagging purity is determined from template fits to the secondary vertex invariant mass distributions. This way, the fraction of b-quark jets among inclusive jets can be measured as a function of transverse momentum. The analysis was completed for pp (231 nb−1 ) and PbPb (150 μb−1 ) collisions at 2.76 TeV, and the first results on the b-quark fraction [15] are plotted in Fig. 4. The fractions of b-jets in pp and PbPb collisions are comparable, with no pT -dependence in the 80–200 GeV/c range. The combination of the RAA factors for inclusive jets and the above ratio leads to an RAA value for b-quark jets of about 0.5, with sizeable systematic uncertainties [15]. Still, this is the first time that a significant quenching of fully reconstructed and identified b-quark jets in heavy-ion collisions has been demonstrated. 3. Correlations between charged tracks and reconstructed jets As discussed earlier, a significant amount of jet quenching was observed in di-jet, γ-jet and bquark jet events. These observations raise a question related to the parton energy loss mechanism; do partons first lose energy in the nuclear medium and subsequently fragment as they would do in vacuum, or does the energy loss modify the fragmentation process itself? The first observations based on the data collected in 2010 have shown that the hard part of the jet fragmentation is similar between pp and PbPb collisions (at least for tracks with pT above 4 GeV/c), and that the pT missing from the reconstructed subleading jets are carried by low momentum particles mostly at large angles with respect to the subleading jet axis. Taking advantage of the 20 times larger data sample collected in 2011, it was possible to extend the analysis of jet fragmentation functions, not only with increased statistics for jets, but also including tracks with pT down to
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1 GeV/c [16]. Measurements of jet fragmentation properties provide an experimental constraint which is complementary to measurements of jet production, such as jet RAA . These can also be used to connect jet observables to measurements of high-pT particle production. For the present study, jets with at least 100 GeV/c transverse momentum were selected, after reconstruction with the Particle Flow algorithm that optimally combines tracking and calorimetric information [17], within the |η| < 2 range, using a radius parameter of R = 0.3 and the anti-kT clustering algorithm. Tracks that are close to the jet axis, in the Δη2 + Δφ2 < 0.3 range, are included in the correlation studies. The soft part of the fragmentation function is contaminated, at low-pT even dominated by the background from the underlying event, which is subtracted by collecting the background contribution from a “jet” cone reflected around η = 0 (this method was cross-checked with the event mixing method). The resulting, background-subtracted fragmentation functions in PbPb collisions are compared to the pp reference histograms in four centrality bins on the top panels of Fig. 5, extended to larger ξ values. The bottom panels show the ratio of the fragmentation functions between PbPb and pp collisions [16]. These results are consistent with the findings using the data collected in 2010, but they have much better statistical and systematic precision and the data extend to larger ξ. We can conclude that there is no significant modification of the jet fragmentation at low ξ (the region of the leading hadrons) in PbPb collisions with respect to the pp reference, while there is a slight depletion in the mid-ξ region, accompanied by an excess at high ξ (low track pT ). After the examination of the longitudinal fragmentation pattern, one can also characterize the jets and their modification in the transverse direction by the measurement of jet shapes: the transverse momentum fraction, ρ(r), carried by charged particles as a function of their distance r = Δη2 + Δφ2 from the jet axis. Only those particles are included that belong to the jet, which is achieved by statistical subtraction of the background using the η-reflection method. The jet shape results are shown on the top panels of Fig. 6 in four centrality classes for PbPb collisions (solid symbols) compared to the pp reference (open symbols), the latter being constructed from results on pp collisions taking into account the difference of the jet energy resolution and of the
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jet pT -spectrum between the two different colliding systems [16]. The bottom panels show the ratio between the jet shapes measured in PbPb collisions and the pp reference. The observed enhancement at large radii in the PbPb case is consistent with the excess of the low-pT part of the jet fragmentation functions, since low-pT particles are known to extend to larger radii within jets, compared to the high-pT jet core. Still, the jet shape and fragmentation results provide complementary information and cannot be derived from each other. 4. Correlations between charged particles at high pT It is well known that the azimuthal angle distribution of low-pT charged particles in PbPb collisions is not uniform, leading to a non-zero second Fourier-coefficient, v2 , reflecting the collective flow of the created medium originating from pressure gradients building up in the elongated nuclear overlap zone. At high pT , such non-uniformity can be caused by path-length dependent parton energy loss and/or jet fragmentation, not by hydrodynamic phenomena any more. Recent measurements of the v2 coefficient from the CMS experiment have shown that v2 is indeed significantly non-zero up to pT of 40–60 GeV/c[18]. A good knowledge of the single-particle azimuthal asymmetries is essential for the deeper analysis of jet modifications. Correlations between a high-pT charged particle that originates trig from jet fragmentation (with pT ) and other charged particles (with passoc ) inherit contributions T from these single-particle asymmetries which have to be subtracted in order to study the remaining correlation structure characteristic of the near-side and away-side jets. After subtraction, the ratio of the near (away) side particle yields between PbPb and pp data, IAA , is presented in the top (bottom) row of Fig. 7 [19]. As expected from jet fragmentation function results, there is an enhancement at low associated particle pT on the near side in PbPb collisions with respect to pp collisions, while there is no significant difference at high pT . On the away side, however,
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Figure 7: Near-side (top row) and away-side (bottom row) IAA ratios derived from the vn -subtraction method (solid trig circles) at |Δη| < 1 and from the long-range Δη-subtraction method (black squares) shown for four different pT ranges √ assoc as a function of pT for PbPb collisions at sNN = 2.76 TeV, for the 10% most central events. The error bars represent statistical uncertainties while the brackets represent the systematic uncertainties.
the associated particle yield is suppressed at high pT (in accordance with the jet quenching phenomenon), while at low pT the enhancement is in agreement with the observation that the energy lost by the away-side jet reappears in form of low-pT particles.
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5. Summary The CMS experiment has shown that the di-jet energy imbalance and the jet nuclear modification factors do not depend strongly on jet pT . The first photon-jet correlations support the jet quenching picture, and b-quark jets have also been found to be quenched. Jet shapes and fragmentation functions indicate an excess at low pT (large radii) but no modification is observed at high pT (small radii), supported by two-particle correlation results triggered by a highpT particle. Positive v2 coefficients were found to persist to very high pT , reflecting path-length dependent energy loss. In summary, the jet quenching picture painted by the CMS results has become more precise, detailed and quantitative, challenging theoretical descriptions and providing a new insight into the behaviour of the high density medium formed in heavy-ion collisions. References [1] S. Chatrchyan, et al. (CMS Collaboration), The CMS experiment at the CERN LHC, JINST 3 (2008) S08004. [2] S. Chatrchyan, et al. (CMS Collaboration), Observation and studies of jet quenching in PbPb collisions at nucleonnucleon center-of-mass energy = 2.76 TeV, Phys.Rev. C84 (2011) 024906. [3] S. Chatrchyan, et al. (CMS Collaboration), Jet momentum dependence of jet quenching in PbPb collisions at √ sNN = 2.76 TeV, Phys.Lett. B712 (2012) 176–197. [4] S. Chatrchyan, et al. (CMS Collaboration), Measurement of jet fragmentation into charged particles in pp and PbPb √ collisions at sNN = 2.76 TeV, JHEP 2012 (2012) 087. [5] S. Chatrchyan, et al. (CMS Collaboration), Nuclear modification factor of high transverse momentum jets in √ PbPb collisions at sNN = 2.76 TeV, CMS-PAS-HIN-12-004, 2012. URL: http://cdsweb.cern.ch/record/ 1472722. [6] S. Chatrchyan, et al. (CMS Collaboration), Measurement of isolated photon production in pp and PbPb collisions √ at sNN = 2.76 TeV, Phys.Lett. B710 (2012) 256–277. √ [7] S. Chatrchyan, et al. (CMS Collaboration), Study of W boson production in PbPb and pp collisions at sNN = 2.76 TeV, Phys.Lett. B715 (2012) 66–87. [8] S. Chatrchyan, et al. (CMS Collaboration), Z boson production with the 2011 data in PbPb collisions, CMS-PASHIN-12-008, 2012. URL: http://cdsweb.cern.ch/record/1472723. [9] S. Chatrchyan, et al. (CMS Collaboration), Study of high-pT charged particle suppression in PbPb compared to pp √ collisions at sNN = 2.76 TeV, Eur.Phys.J. C72 (2012) 1945. [10] S. Chatrchyan, et al. (CMS Collaboration), J/ψ results from CMS in PbPb collisions, with 150 μb−1 data, CMSPAS-HIN-12-014, 2012. URL: http://cdsweb.cern.ch/record/1472735. [11] S. Chatrchyan, et al. (CMS Collaboration), Studies of jet quenching using isolated-photon+jet correlations in PbPb √ and pp collisions at sNN = 2.76 TeV, 2012. arXiv:1205.0206. [12] T. Sjostrand, S. Mrenna, P. Z. Skands, PYTHIA 6.4 Physics and Manual, JHEP 0605 (2006) 026. [13] I. Lokhtin, A. Snigirev, A Model of jet quenching in ultrarelativistic heavy ion collisions and high-pT hadron spectra at RHIC, Eur.Phys.J. C45 (2006) 211–217. [14] N. Armesto, C. A. Salgado, U. A. Wiedemann, Medium induced gluon radiation off massive quarks fills the dead cone, Phys.Rev. D69 (2004) 114003. [15] S. Chatrchyan, et al. (CMS Collaboration), Measurement of the b-jet to inclusive jet ratio in PbPb and pp collisions at 2.76 TeV with the CMS detector, CMS-PAS-HIN-12-003, 2012. URL: http://cdsweb.cern.ch/record/ 1472721. [16] S. Chatrchyan, et al. (CMS Collaboration), Detailed characterization of jets in heavy ion collisions using jet shapes and jet fragmentation functions, CMS-PAS-HIN-12-013, 2012. URL: http://cdsweb.cern.ch/ record/1472734. [17] S. Chatrchyan, et al. (CMS Collaboration), Commissioning of the Particle-Flow reconstruction in minimum-bias and jet events from pp collisions at 7 TeV, CMS-PAS-PFT-10-002, 2010. URL: http://cdsweb.cern.ch/ record/1279341. [18] S. Chatrchyan, et al. (CMS Collaboration), Azimuthal anisotropy of charged particles at high transverse momenta √ in PbPb collisions at sNN = 2.76 TeV, Phys.Rev.Lett. 109 (2012) 022301. [19] S. Chatrchyan, et al. (CMS Collaboration), Very high-pT triggered dihadron correlations in PbPb and pp collisions √ at sNN = 2.76 TeV, CMS-PAS-HIN-12-010, 2012. URL: http://cdsweb.cern.ch/record/1472744.