Jets In PbPb Collisions Research Articles

First measurements of N-subjettiness in central Pb-Pb collisions at sqrt{s_{mathrm{NN}}} = 2.76 TeV

The ALICE Collaboration reports the first fully-corrected measurements of the N-subjettiness observable for track-based jets in heavy-ion collisions. This study is performed using data recorded in pp and Pb-Pb collisions at centre-of-mass energies of sqrt{s} = 7 TeV and sqrt{s_{mathrm{NN}}} = 2.76 TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness, τ2/τ1, which is sensitive to the rate of two-pronged jet substructure, is presented. Energy loss of jets traversing the strongly interacting medium in heavy-ion collisions is expected to change the rate of two-pronged substructure relative to vacuum. The results are presented for jets with a resolution parameter of R = 0.4 and charged jet transverse momentum of 40 ≤ pT,jet ≤ 60 GeV/c, which constitute a larger jet resolution and lower jet transverse momentum interval than previous measurements in heavy-ion collisions. This has been achieved by utilising a semi-inclusive hadron-jet coincidence technique to suppress the larger jet combinatorial background in this kinematic region. No significant modification of the τ2/τ1 observable for track-based jets in Pb-Pb collisions is observed relative to vacuum PYTHIA6 and PYTHIA8 references at the same collision energy. The measurements of τ2/τ1, together with the splitting aperture angle ∆R, are also performed in pp collisions at sqrt{s} = 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at sqrt{s} = 7 TeV, in order to validate the model as a vacuum reference for the Pb-Pb centre-of-mass energy. The PYTHIA references for τ2/τ1 are shifted to larger values compared to the measurement in pp collisions. This hints at a reduction in the rate of two-pronged jets in Pb-Pb collisions compared to pp collisions.

The effect of jet-induced medium excitation on Λ/k0s in jet in Pb-Pb collisions

We use CoLBT-hydro model for simultaneous event-by-event simulations of jet propagation and hydrodynamics evolution of the bulk medium including jet-induced medium excitation. The final reconstructed jet in heavy-ion collision include not only hadrons from medium response for the deposited energy and momentum of hard partons, but also hadrons from the fragmentation and recombination process of in-medium hard partons. We carry out the study with CoLBT-hydro of medium modification of the Λ/k0s in jet in Pb-Pb collisions at sNN=2.76TeV. We qualitatively describe the enhancement of the Λ/k0s ratio in jet in the intermediate pT region due to the contribution from jet-induced medium flow.

Understanding partonic energy loss from measured light charged particles and jets in PbPb collisions at LHC energies

We perform a comprehensive study of partonic energy loss reflected in the nuclear modification factors of charged particles and jets measured in PbPb collisions at = 2.76 and 5.02 TeV in wide transverse momentum (p T) and centrality range. The p T distributions in pp collisions are fitted with a modified power law and the nuclear modification factor in PbPb collisions can be obtained using effective shift (Δp T) in the spectrum measured at different centralities. Driven by physics consideration, the functional form of energy loss given by Δp T can be assumed as power law with different power indices in three different p T regions. The power indices and the boundaries of three p T regions are obtained by fitting the measured nuclear modification factor as a function of p T in all collision centralities simultaneously. The energy loss in different collisions centralities are described in terms of fractional power of number of participants. It is demanded that the power law functions in three p T regions and their derivatives are continuous at the p T boundaries. The Δp T for light charged particles is found to increase linearly with p T in low p T region below ∼5–6 GeV c−1 and approaches a constant value in high p T region above ∼22–29 GeV c−1 with an intermediate power law connecting the two regions. The method is also used for jets and it is found that for jets, the Δp T increases approximately linearly even at very high p T.

Jet properties in PbPb and pp collisions at sqrt{s_{mathrm{N};mathrm{N}}}=5.02 TeV

Modifications of the properties of jets in PbPb collisions, relative to those in pp collisions, are studied at a nucleon-nucleon center-of-mass energy of sqrt{s_{mathrm{N};mathrm{N}}}=5.02 TeV via correlations of charged particles with the jet axis in relative pseudorapidity (Δη), relative azimuth (Δϕ), and relative angular distance from the jet axis varDelta mathrm{r}=sqrt{{left(varDelta eta right)}^2+{left(varDelta phi right)}^2} . This analysis uses data collected with the CMS detector at the LHC, corresponding to integrated luminosities of 404 μb−1 and 27.4 pb−1 for PbPb and pp collisions, respectively. Charged particle number densities, jet fragmentation functions, and jet shapes are presented as a function of PbPb collision centrality and charged-particle track transverse momentum, providing a differential description of jet modifications due to interactions with the quark-gluon plasma.

Photon-Jet Correlations in pp and PbPb collisions at 5.02 TeV with CMS

Electromagnetic probes such as photons do not participate in the strong interaction, and thus provide a clean measurement of the initial state in nuclear collisions. Correlations of photons balancing with jets in PbPb collisions constitute the golden channel to study parton energy loss in strongly interacting matter, since the photon not only determines the initial transverse momentum of the balancing parton, but also preferentially selects quark jets. We will present new results from pp and PbPb collisions at 5.02 TeV collision energy, using the high statistics data collected with the CMS detector in the 2015 LHC run. The results include detailed studies of azimuthal and momentum correlations of isolated photons and associated jets, as well as jet IAA, as a function of photon pT and collision centrality.

Heavy Flavored Jet Modification at CMS

The energy loss of jets in heavy-ion collisions is expected to depend on the flavor of the fragmenting parton. Thus, measurements of jet quenching as a function of flavor place powerful constraints on the thermodynamical and transport properties of the hot and dense medium. Specifically, pPb measurements provide crucial insights into the behavior of the cold nuclear matter effect, which is required to fully understand the hot and dense medium effects on jets in PbPb collisions. In these proceedings, we present the heavy flavor jet spectra and measurements of the nuclear modification factors of b jets in both PbPb and pPb as a function of transverse momentum and pseudorapidity, at sNN=2.76 and 5.02 TeV, respectively. In addition, we present the first ever measurement of charm-tagged jets in a heavy-ion environment, including cross-sections and comparisons to PYTHIA in both pPb and pp.

Correlations and flavors in jets in ALICE

We report on the measurement of hadron composition in charged jets in pp at √s = 7 TeV and show the first data on particle type dependent jet fragmentation at the LHC. Further, we present (˄ + ˄)/2K0s ratios measured in charged jets in Pb-Pb collisions at √sNN = 2.76 TeV and in p-Pb collisions at √sNN = 5.02 TeV. While the ratio of the inclusive pT spectra of ˄ and KSS exhibits centrality dependent enhancement both in the Pb-Pb and p-Pb system, the (˄ + ˄)/2K0s ratio measured in charged jets reveals that jet fragmentation does not contribute to the observed baryon anomaly.Finally, we discuss the measurement of semi-inclusive pT spectra of charged jets that recoil from a high-pT hadron trigger in Pb-Pb and pp collisions at √sNN = 2.76 TeV and √s= 7 TeV, respectively. The jet yield uncorrelated with the trigger hadron is removed at the event-ensemble level without introducing a bias on the jet population, which is therefore infrared and collinear safe. The recoil jet yield in central Pb-Pb is found to be suppressed w.r.t. that from pp PYTHIA reference. On the other hand, there is no sign of intra-jet broadening even for anti-kT jets with a resolution parameter as large as R = 0.5.

Production of strange particles in charged jets in Pb–Pb and p–Pb collisions measured with ALICE

Measurements of spectra of identified particles produced in jets in heavy-ion collisions can provide further important insights into the interplay of various hadronisation processes which participate in the particle production in the hot and dense medium. In this contribution, we present the measurements of the pT spectra of Λ baryons and K0S mesons produced in association with charged jets in Pb-Pbcollisions at = 2.76 TeV and in p-Pb collisions at = 5.02 TeV. The results are obtained with the ALICE detector at the LHC, exploiting the excellent particle identification capabilities of this experiment. Baryon-to-meson ratios of the spectra of strange particles associated with jets are studied in central collisions and are compared to the ratios obtained for inclusive particles and for particles coming from the underlying event.

Jet structure in pp and Pb-Pb collisions with the ALICE experiment

We report results from the ALICE collaboration on jet production and jet properties in pp and Pb-Pb collisions at the LHC. The charged particle jet production cross-section and measurements of jet fragmentation and jet shape in pp collisions at = 7 TeV are presented. The results are confronted with simulations from Monte Carlo event generators. Measurements of charged particle jets in Pb-Pb collisions at = 2.76 TeV with resolution parameter R = 0.2 and R = 0.3 show a strong momentum dependent suppression in central relative to peripheral collisions. The analysis of the semi-inclusive distribution of charged particle jets recoiling from a high-pT trigger hadron allows an unbiased measurement of the jet structure for larger cone radii. No significant redistribution of the energy flow inside the jet cone is observed. Di-hadron correlations allow to assess jet structure at very low particle momenta. For 2 < pT,trig < 3 GeV/c and 1 < PT,assoc < 2 GeV/c, a strongly asymmetric near-side correlation, broader in Δη than Δϕ, is observed.

Measurement of charged jet suppression in Pb-Pb collisions at $ \sqrt{{{s_{\mathrm{NN}}}}} $ = 2.76 TeV

A measurement of the transverse momentum spectra of jets in Pb-Pb collisions at $ \sqrt{{{s_{\mathrm{NN}}}}} $ = 2.76TeV is reported. Jets are reconstructed from charged particles using the anti-k T jet algorithm with jet resolution parameters R of 0.2 and 0.3 in pseudo-rapidity |η| < 0.5. The transverse momentum p T of charged particles is measured down to 0.15 GeV/c which gives access to the low p T fragments of the jet. Jets found in heavy-ion collisions are corrected event-by-event for average background density and on an inclusive basis (via unfolding) for residual background fluctuations and detector effects. A strong suppression of jet production in central events with respect to peripheral events is observed. The suppression is found to be similar to the suppression of charged hadrons, which suggests that substantial energy is radiated at angles larger than the jet resolution parameter R = 0.3 considered in the analysis. The fragmentation bias introduced by selecting jets with a high p T leading particle, which rejects jets with a soft fragmentation pattern, has a similar effect on the jet yield for central and peripheral events. The ratio of jet spectra with R = 0.2 and R = 0.3 is found to be similar in Pb-Pb and simulated PYTHIA pp events, indicating no strong broadening of the radial jet structure in the reconstructed jets with R < 0.3.

Results on Jet Spectra and Structure from ALICE

Full jet reconstruction in ALICE uses the combined information from charged and neutral particles. Essentially all jet constituents can be measured with large efficiency down to very low transverse momenta (pT > 150 MeV/c). This has the advantage to introduce a minimum bias on the jet fragmentation, in particular for low jet momenta and in the presence of quenching. In this article, we present preliminary results from reconstruction of charged jets in Pb-Pb collisions at sqrt(s_NN) = 2.76 TeV. The inclusive charged jet spectrum, the jet nuclear modification factors (R_AA, R_CP), the ratio of spectra measured with different resolution parameters and hadron-jet correlations are discussed. For pp data at the same center of mass energy, the inclusive spectrum of fully reconstructed jets and its resolution parameter dependence are reported.

Prompt photon in association with a heavy-quark jet in Pb-Pb collisions at the LHC

Abstract We present a phenomenological study of the associated production of a prompt photon and a heavy quark jet (charm or bottom) in Pb-Pb collisions at the LHC. This channel allows for estimating the amount of energy loss experienced by the charm and bottom quarks propagating in the dense QCD medium produced in those collisions. Calculations are carried out at next-to-leading order (NLO) accuracy using the BDMPS-Z heavy-quark quenching weights. The quenching of the single heavy-quark jet spectrum reflects fairly the hierarchy in the heavy quark energy loss assumed in the perturbative calculation. On the contrary, the single photon spectrum in heavy-ion collisions is only modified at low momenta, for which less heavy-quark jets pass the kinematic cuts. On top of single particle spectra, the two-particle final state provides a range of observables (photon-jet pair momentum, jet asymmetry, among others) which are studied in detail. The comparison of the photon-jet pair momentum, from p-p to Pb-Pb collisions, is sensitive to the amount of energy lost by the heavy-quarks and could therefore be used in order to better understand parton energy loss processes in the heavy quark sector.

Measurement of jet fragmentation into charged particles in pp and PbPb collisions at $ \sqrt{{{s_{\mathrm{NN}}}}} = 2.76 $ TeV

Abstract Jet fragmentation in pp and PbPb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair was studied using data collected with the CMS detector at the LHC. Fragmentation functions are constructed using charged-particle tracks with transverse momenta p T &gt; 4 GeV/c for dijet events with a leading jet of p T &gt; 100 GeV/c. The fragmentation functions in PbPb events are compared to those in pp data as a function of collision centrality, as well as dijet-p T imbalance. Special emphasis is placed on the most central PbPb events including dijets with unbalanced momentum, indicative of energy loss of the hard scattered parent partons. The fragmentation patterns for both the leading and subleading jets in PbPb collisions agree with those seen in pp data at 2.76 TeV. The results provide evidence that, despite the large parton energy loss observed in PbPb collisions, the partition of the remaining momentum within the jet cone into high-p T particles is not strongly modified in comparison to that observed for jets in vacuum.