Suppression In Pb Research Articles

Measurement of substructure-dependent jet suppression in Pb+Pb collisions at 5.02 TeV with the ATLAS detector

The ATLAS detector at the Large Hadron Collider has been used to measure jet substructure modification and suppression in Pb+Pb collisions at a nucleon-nucleon center-of-mass energy $\sqrt{s_{_\mathrm{NN}}}=5.02~\mathrm{TeV}$ in comparison with $pp$ collisions at $\sqrt{s}=5.02~\mathrm{TeV}$. The Pb+Pb data, collected in 2018, have an integrated luminosity of $1.72~\mathrm{nb^{-1}}$, while the $pp$ data, collected in 2017, have an integrated luminosity of $260~\mathrm{pb}^{-1}$. Jets used in this analysis are clustered using the anti-$k_{t}$ algorithm with a radius parameter $R=0.4$. The jet constituents, defined by both tracking and calorimeter information, are used to determine the angular scale $r_\mathrm{g}$ of the first hard splitting inside the jet by reclustering them using the Cambridge-Aachen algorithm and employing the soft-drop grooming technique. The nuclear modification factor, $R_\mathrm{AA}$, used to characterize jet suppression in Pb+Pb collisions, is presented differentially in $r_\mathrm{g}$, jet transverse momentum, and in intervals of collision centrality. The $R_\mathrm{AA}$ value is observed to depend significantly on jet $r_\mathrm{g}$. Jets produced with the largest measured $r_\mathrm{g}$ are found to be twice as suppressed as those with the smallest $r_\mathrm{g}$ in central Pb+Pb collisions. The $R_\mathrm{AA}$ values do not exhibit a strong variation with jet $p_\mathrm{T}$ in any of the $r_\mathrm{g}$ intervals. The $r_\mathrm{g}$ and $p_\mathrm{T}$ dependence of jet $R_\mathrm{AA}$ is qualitatively consistent with a picture of jet quenching arising from coherence and provides the most direct evidence in support of this approach.

γ -hadron spectra in p+Pb collisions at sNN=5.02 TeV

Under the assumption that a quark-gluon plasma droplet is produced and its evolution can be described by hydrodynamics in p + A collisions, $\gamma$-triggered hadron spectra are studied within a next-to-leading-order perturbative QCD parton model with the medium-modified parton fragmentation functions. The initial conditions and space-time evolution of the small QGP droplet are provided by the superSONIC hydrodynamic model simulations and parton energy loss in such a medium is described by the high-twist (HT) approach. The scaled jet transport coefficient $\hat{q}/T^3$ in this HT approach is extracted from single hadron suppression in central A + A collisions at the same colliding energy. Numerical results for this scenario show that $\gamma$-hadron spectra at $p_{\rm T}^\gamma=12-40$ GeV/$c$ are suppressed by 5\% $\sim$ 10\% in the most central 0 - 10\% p + Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. The suppression becomes weaker at higher transverse momentum of the $\gamma$ trigger. As a comparison, $\gamma$-hadron suppression in Pb + Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ and 5.02 TeV is also predicted.

Interplaying mechanisms behind single inclusive jet suppression in heavy-ion collisions

The suppression factor for single inclusive jets in Pb+Pb collisions at the Large Hadron Collider (LHC) has a weak dependence on the transverse momentum $p_T$ and remains almost the same at two colliding energies, $\sqrt{s}=2.76$ and 5.02 TeV, though the central rapidity density of bulk hadrons increases by about 20\%. This phenomenon is investigated within the Linear Boltzmann Transport (LBT) model, which includes elastic and inelastic processes based on perturbative QCD for both jet shower and recoil medium partons as they propagate through a quark-gluon plasma (QGP). With the dynamic evolution of the QGP given by the 3+1D CLVisc hydrodynamic model with event-by-event fully fluctuating initial conditions, single inclusive jet suppression in Pb+Pb collisions from LBT agrees well with experimental data. The weak $\sqrt{s}$ and $p_T$-dependence of the jet suppression factor at LHC are found to result directly from the $\sqrt{s}$-dependence of the initial jet $p_T$ spectra and slow $p_T$-dependence of the jet energy loss. Contributions from jet-induced medium response, influence of radial expansion, both of which depend on jet-cone size, and jet flavor composition all conjoin to give a slow $p_T$-dependence of jet energy loss and the single jet suppression factor $R_{\rm AA}$, their dependence on $\sqrt{s}$ and jet-cone size. Single inclusive jet suppression at $\sqrt{s}=200$ GeV is also predicted that actually decreases slightly with $p_T$ in the $p_T<50$ GeV/$c$ range because of the steeper initial jet spectra though the $p_T$-dependence of the jet energy loss is weaker than that at LHC.

Centrality and transverse momentum dependent suppression of Upsilon (1S) and Upsilon (2S) in p\u2013Pb and Pb\u2013Pb collisions at the CERN Large Hadron Collider

Deconfined QCD matter in heavy-ion collisions has been a topic of paramount interest for many years. Quarkonia suppression in heavy-ion collisions at the relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) experiments indicate the quark-gluon plasma (QGP) formation in such collisions. Recent experiments at LHC have given indications of hot matter effect in asymmetric p–Pb nuclear collisions. Here, we employ a theoretical model to investigate the bottomonium suppression in Pb–Pb at sqrt{s_{NN}}=2.76, 5.02 TeV, and in p–Pb at sqrt{s_{NN}}=5.02 TeV center-of-mass energies under a QGP formation scenario. Our present formulation is based on an unified model consisting of suppression due to color screening, gluonic dissociation along with the collisional damping. Regeneration due to correlated Qbar{Q} pairs has also been taken into account in the current work. We obtain here the net bottomonium suppression in terms of survival probability under the combined effect of suppression plus regeneration in the deconfined QGP medium. We mainly concentrate here on the centrality, N_text {part} and transverse momentum, p_{T} dependence of Upsilon (1S) and Upsilon (2S) states suppression in Pb–Pb and p–Pb collisions at mid-rapidity. We compare our model predictions for Upsilon (1S) and Upsilon (2S) suppression with the corresponding experimental data obtained at the LHC energies. We find that the experimental observations on p_t and N_text {part} dependent suppression agree reasonably well with our model predictions.

Charged-hadron suppression in Pb+Pb and Xe+Xe collisions measured with the ATLAS detector

The ATLAS detector at the LHC recorded 0.49 nb−1 of Pb+Pb collisions and 25 pb−1 of pp collisions, both at the center-of-mass energy 5.02 TeV per nucleon pair. Recently, ATLAS also recorded 3μb−1 of Xe+Xe collisions at the center-of-mass energy 5.44 TeV, which provides a new opportunity to study the system-size dependence of the charged-hadron production in heavy-ion collisions. The large acceptance of the ATLAS detector allows to measure the spectra of charged hadrons in a wide range of pseudorapidity and transverse momentum. The nuclear modification factors RAA are constructed as a ratio of the spectra measured in Pb+Pb or Xe+Xe collisions to that measured in pp collisions. The RAA obtained in the two systems are presented for different centrality intervals and the results are discussed.

Effect of medium recoil and pT broadening on single inclusive jet suppression in high-energy heavy-ion collisions in the high-twist approach

Jet energy loss and single inclusive jet suppression in high-energy heavy-ion collisions are studied within a pQCD parton model that includes both elastic and radiative interactions between jet shower and medium partons as they propagate through the quark-gluon plasma. The collisional energy loss of jets with a given cone-size is found to be relatively small comparing with the radiative energy loss. However the effect of transverse momentum broadening due to elastic scattering is significant in the calculation of radiative energy loss within the higher-twist formalism. The nuclear modification factors for single inclusive jets with different cone-sizes are calculated and compared to experimental data as measured by ALICE and ATLAS experiments in Pb+Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. Results on jet suppression in Pb+Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV are also presented.

Bottomonium suppression atsNN=2.76TeV using a model based on color screening and gluonic dissociation with collisional damping

We present a model to explain the bottomonium suppression in Pb$+$Pb collisions at midrapidity obtained from Large Hadron Collider (LHC) energy, $\sqrt{{s}_{NN}}=2.76$ TeV. The model consists of two decoupled mechanisms, namely, color screening during bottomonium production followed by gluon induced dissociation along with collisional damping. The quasiparticle model (QPM) is used as equation of state (EOS) for the quark-gluon plasma (QGP) medium. The feed-down from higher $\ensuremath{\Upsilon}$ states, such as $\ensuremath{\Upsilon}(1P)$, $\ensuremath{\Upsilon}(2S)$, and $\ensuremath{\Upsilon}(2P)$, dilated formation times for bottomonium states, and viscous effect of the QGP medium are other ingredients included in the current formulation. We further assume that the QGP is expanding according to ($1+1$)-dimensional Bjorken's boost invariant scaling law. The net suppression (in terms of ${p}_{T}$ integrated survival probability) for bottomonium states at midrapidity is obtained as a function of centrality, and the result is then compared both quantitatively and qualitatively with the recent LHC experimental data in the midrapidity region recently published by the CMS Collaboration. We find that the current model, based on Debye color screening plus gluonic dissociation along with collisional damping, better describes the centrality dependence of bottomonium suppression at LHC energy as compared to the color screening model alone.

Measurements of Jet Suppression with ATLAS

Abstract Measurements of inclusive jet suppression in Pb + Pb collions at s N N = 2.76 TeV with the ATLAS detector are reported. The centrality, p T and jet size dependence of the central-to-peripheral ratio, R CP , is presented. The results indicate that jets are suppressed by approximately a factor of two in the most central collisions, and a significant dependence on the jet size is observed. The path length dependence of jet quenching is studied through measurements of inclusive jet suppression as a function of angle with respect to the event plane. The azimuthal modulation of the jet yield is quantified by its second Fourier coefficent, ν 2 , which is significant in all p T and centrality ranges presented.

J/ψsuppression in a dense baryonic medium

We have examined the available latest SPS data on $J/\psi$ suppression in Pb+Pb and In+In collisions at 158 A GeV. Our employed model, with parameters fixed by $p$-$p$ and $p$-nucleus collisions, gives excellent description of NA50 and NA60 data on centrality dependence of $J/\psi$ suppression. The model is then applied to predict the centrality dependence of $J/\psi$ production in Au+Au collisions at FAIR energy domain. A much larger suppression of $J/\psi$ is predicted. In addition the possible effects of a baryon rich medium on $J/\psi$ production is also investigated.

Suppression of high [formula omitted] hadrons in Pb + Pb collisions at LHC

Hadron production and their suppression in Pb+Pb collisions at LHC at a center-of-mass energy of sNN=2.76 TeV are studied within a multiphase transport (AMPT) model whose initial conditions are obtained from the recently updated HIJING 2.0 model. The centrality dependence of charged hadron multiplicity dNch/dη at midrapidity was found quite sensitive to the largely uncertain gluon shadowing parameter sg that determines the nuclear modification of the gluon distribution. We find final-state parton scatterings reduce considerably hadron yield at midrapidity and enforces a smaller gluon shadowing to be consistent with dNch/dη data at LHC. With such a constrained parton shadowing, charged hadron and neutral pion production over a wide transverse momenta range are investigated in AMPT. Relative to nucleon–nucleon collisions, the particle yield in central heavy ion collisions is suppressed due to parton energy loss. While the calculated magnitude and pattern of suppression is found consistent with that measured in Au+Au collisions at sNN=0.2 TeV at RHIC, at the LHC energy the suppression is overpredicted which may imply the medium formed at LHC is less opaque than expected from simple RHIC extrapolations. Reduction of the QCD coupling constant αs by ∼30% in the higher temperature plasma formed at LHC as compared to that at RHIC was found to reproduce the measured suppression at LHC.

STRONGLY INTERACTING QGP AND QUARKONIUM SUPPRESSION AT RHIC AND LHC ENERGIES

We have developed an equation of state for strongly interacting quark–gluon plasma (QGP) in the framework of strongly coupled plasma with appropriate modifications to take account of color and flavor degrees of freedom and the interactions among themselves. For this purpose we used the effective potential to improve the plasma parameter (Γ) by correcting the full Cornell potential with a dielectric function embodying the effects of the deconfined medium and not its Coulomb part alone and obtain the equation of state in terms of Γ. Our results on thermodynamic observables viz. pressure, energy density, speed of sound etc. nicely fit to the results of lattice equation of state for gluon, massless as well massive flavored plasma. We have then employed our equation of state to estimate the quarkonium suppression in an expanding QGP produced in the relativistic heavy-ion collisions. We have found that our predictions matches with the recent PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions at BNL RHIC within the limit of other uncertainties. We have also predicted for the ϒ suppression in Pb+Pb collisions at LHC energy which could be tested in the ALICE experiments at CERN LHC.

Quarkonia measurements by the CMS experiment in pp and Pb–Pb collisions

Quarkonia have been studied in different collision systems and energy in order to understand the effects of the hot and dense medium created in heavy-ion collisions. CMS is well suited to measure quarkonia decays to muons given the muon identification and charged particle tracking capability. We report here prompt, non-prompt and ϒ production measured by the CMS experiment in pp collisions at TeV. In addition, the and ϒ production in Pb–Pb at TeV and pp collisions at the same per nucleon energy are measured and compared. Prompt and non-prompt contributions are separated for the first time in heavy-ion collisions, as is the ground from the excited states in the ϒ family. Suppression in Pb–Pb at TeV is quantified for prompt , and ϒ(1S), as well as the relative suppression of ϒ(2S+3S) compared to ϒ(1S).

Sequential melting of charmonium states in an expanding quark–gluon plasma and J/ψ suppression at RHIC and LHC energy collisions

We have developed a hydrodynamic model to study sequential melting of charmonium states in an expanding quark–gluon plasma (QGP) medium. According to the initial fluid temperature profile, J/ψ's are randomly distributed in the transverse plane. As the fluid evolves in time, the free streaming J/ψ's are suppressed if the local fluid temperature exceeds a critical temperature. PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions at mid-rapidity are explained by sequential melting of the charmonium states, χc, ψ′ and J/ψ, in the expanding medium. The critical temperatures TJ/ψ ≈ 2.09Tc and agree with lattice-motivated calculations. The feed-down fraction F depends on whether the cold nuclear matter effect is included or not. It changes from F = 0.3 with cold nuclear matter effect included to F = 0.5 when the effect is neglected. The model fails to reproduce the PHENIX data on the centrality dependence of J/ψ suppression in Cu+Cu collisions at mid-rapidity, indicating that the mechanism of J/ψ suppression is different in Au+Au and Cu+Cu collisions. We also use the model to predict for the centrality dependence of J/ψ suppression in Pb+Pb collisions at LHC energy, = 5500 GeV. In LHC energy, J/ψ's are more suppressed in mid central collisions than in Au+Au collisions at RHIC energy.

J/ψ suppression in the threshold model at RHIC and LHC energy

In the QGP-based threshold model (Blaizot et al 2000 Phys. Rev. Lett. 85 4012, Blaizot and Ollitrault 1996 Phys. Rev. Lett. 77 1703), in addition to the normal nuclear absorption, J/ψ's are subjected to an ‘anomalous’ suppression such that above a threshold density nJ/ψ, all the J/ψ's are melted. In the threshold model we have analyzed the recent PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions at RHIC. Feedback from decay of the state χ is accounted for. J/ψ's are anomalously suppressed above a threshold density, nJ/ψ = 3.57 ± 0.17 fm−2. Threshold density for anomalous suppression of the state χ is uncertain to a large extent, nχ = 0.32 ± 0.32 fm−2. The fraction F of the state χ cannot be determined unambiguously, depending on the nuclear absorption, it can vary from 20% to 40%. We have also predicted for the suppression in Pb+Pb collisions at LHC energy. In central Pb+Pb collisions, J/ψ's are suppressed by a factor of 3–4. The suppression pattern is rather similar to that in Au+Au collisions, if not slighty less in central collisions. Using the PHENIX data on the participant number dependence of the Bjorken energy density, we have also estimated the QGP formation time. For critical temperature Tc = 192 MeV, the estimated QGP formation time ranges between 0.07 and 0.09 fm c−1.

Effects of multiple scattering in cold nuclear matter on J/ψ suppression and 〈pT2〉 in heavy ion collisions

Coherent multiple scatterings of c c ¯ quark pairs in the environment of heavy ion collisions have been used in a previous work by Qiu et al. [J. Qiu, J.P. Vary, X. Zhang, Phys. Rev. Lett. 88 (2002) 232301; J. Qiu, J.P. Vary, X. Zhang, Nucl. Phys. A 698 (2002) 571, nucl-th/0106040] to study J / ψ suppression. That model suggests that heavy quark re-scatterings in a cold nuclear medium can completely explain the centrality dependence of the observed J / ψ suppression in Pb + Pb collisions at the SPS [M.C. Abreu, et al., NA50 Collaboration, Phys. Lett. B 521 (2001) 195]. Their calculations also revealed significant differences under the assumptions of a color singlet or color octet production mechanism. A more recent analytic calculation [H. Fujii, Phys. Rev. C 67 (2003) 031901], which includes incoherent final-state re-scatterings with explicit momentum transfer fluctuations in three dimensions, indicates much less suppression and little sensitivity to the production mechanism. In this Letter, we study simultaneously both the J / ψ suppression and p T modifications, at SPS and RHIC energies. We mainly focus on incoherent momentum transfer fluctuations in two dimensions, which is more appropriate for the heavy-ion collision kinematics. Our analytic and Monte Carlo calculations reinforce the analytic results in [H. Fujii, Phys. Rev. C 67 (2003) 031901]. Additionally, we find that the experimental J / ψ suppression and 〈 p T 2 〉 from nucleus–nucleus collisions at the SPS or RHIC cannot simultaneously be described in this incoherent multiple scattering framework for any value of the fluctuation strength parameter 〈 k T 2 〉 .

Can J/ψ suppression and pT broadening signal the deconfinement transition at RHIC?

We have analysed the latest NA50 data on J/ψ suppression in Pb+Pb collisions at CERN SPS. It is shown that a QCD-based nuclear absorption model, where J/ψs are absorbed in a nuclear medium, could explain the latest NA50 data on the centrality dependence of the J/ψ over Drell–Yan ratio. The model also explains the NA50 data on the J/ψ over minimum bias ratio and the pT broadening of J/ψs. A QGP-based threshold model, where all J/ψs are suppressed above a threshold density, also explains the data sets with smeared threshold density. Even at RHIC energy, centrality dependence of J/ψ suppression or pT broadening could not distinguish between the two models.

Charm quenching in heavy-ion collisions at the LHC

D-meson suppression in Pb–Pb collisions at the LHC due to charm quark in-medium energy loss is estimated within a model that describes the available quenching measurements at RHIC. The result is compared to that previously published by the author. The expected sensitivity of the ALICE experiment for studying charm energy loss via fully reconstructed $D0$-meson decays is also presented.

Simple Models of the Contribution of Intermediate Stage Gluons to J/ψ Suppression at the CERN SPS

We construct three simple scenarios of the time dependence of density of intermediate stage gluons in nuclear collisions in the CERN SPS energy range. Gluons with energy of about 0.6–1.0 GeV are assumed to be produced in nucleon-nucleon collisions in a Glauber type model. The rate of gluon production is given by the parameter ng/nn equal to the average number of gluons produced per nucleon-nucleon collision. The value of this parameter determines the behaviour of the gas of gluons. The number of gluons increases due to gluon branching and processes like g+g → g+g+g and decreases due to the hadronization. Gluons are assumed to be able to dissociate J/ψ in g+J/ψ collisions, the dissociation cross-section σgγ is taken as a free parameter. In the first scenario, the energy density of the gas of gluons never reaches the critical energy density ec ≈ 0.7 GeV/fm3 and gluons rapidly hadronize. In the second scenario, the critical energy density is reached but the system of gluons is unable to reach thermalization. In the third scenario gluons reach thermalization and the thermalized system suppresses J/ψ by the Matsui-Satz mechanism. The third scenario under the assumption of a small value of σgψ is able to describe qualitatively the data on J/ψ suppression in Pb−Pb interactions obtained by the NA50 Collaboration. Other scenarios have problems with getting the rather abrupt onset of J/ψ suppression.

AnomalousJ/ψsuppression and charmonium dissociation cross sections

We study $J/\psi$ suppression in Pb+Pb collisions at CERN-SPS energies in hadronic matter with energy- and temperature-dependent charmonium dissociation cross sections calculated in the quark-interchange model of Barnes and Swanson. We find that the variation of J/$\psi$ survival probability from peripheral to central collisions can be explained as induced by hadronic matter absorption in central collisions.

Nuclear absorption andJ/ψsuppression in Pb+Pb collisions

We have analyzed the NA58 data on $J/\ensuremath{\psi}$ suppression in Pb+Pb collisions. $J/\ensuremath{\psi}$ production is assumed to be a two step process, (i) formation of $c\overline{c}$ pair, which is accurately calculable in QCD and (ii) formation of $J/\ensuremath{\psi}$ meson from the $c\overline{c}$ pair, which can be conveniently parametrized. In a $pA/AA$ collision, a $c\overline{c}$ pair gains relative square momentum as it passes through the nuclear medium and some of the $c\overline{c}$ pairs can gain enough momentum to cross the threshold to become an open charm meson, leading to suppression in $pA/AA$ collisions. A new prescription is proposed for the gain in momentum square, consistent with Krammer process. The model could explain the ${E}_{T}$ dependence of $J/\ensuremath{\psi}$ over Drell-Yan ratio.