Nuclear Modification Factor Research Articles

Effects of the formation time of parton shower on jet quenching in heavy-ion collisions* *Supported by the National Natural Science Foundation of China (12175122, 2021-867, 11890710, 11890713, 14-547 )

Jet quenching has successfully served as a hard probe to study the properties of Quark-Gluon Plasma (QGP). As a multi-particle system, jets require time to develop from a highly virtual parton to a group of partons close to mass shells. In this study, we present a systematical analysis on the effects of this formation time on jet quenching in relativistic nuclear collisions. Jets from initial hard scatterings were simulated with Pythia, and their interactions with QGP were described using a Linear Boltzmann Transport (LBT) model that incorporates both elastic and inelastic scatterings between jet partons and the thermal medium. Three different estimations of the jet formation time were implemented and compared, including instantaneous formation, formation from single splitting, and formation from sequential splittings, before which no jet-medium interaction was assumed. We found that deferring the jet-medium interaction with a longer formation time not only affects the overall magnitude of the nuclear modification factor of jets but also its dependence on the jet transverse momentum.

Recent Jet Measurements in ALICE

The precise measurement of jets in pp collisions plays a critical role in the study of quantum chromodynamics (QCD). Measurements of jets in relativistic heavy-ion collisions are additionally useful for probing QCD matter in the regime of high densities and temperatures since the partons in the jet evolution interact strongly with the medium. This phenomenon is called jet quenching and is characterized by both jet energy loss and modification of the internal structure of the jet. The ALICE experiment is well suited for studies of jets and their corresponding internal structure due to the high precision tracking system. These proceedings report on an overview of recent ALICE jet results in both pp and Pb–Pb collisions. Results in pp collisions include measurements of generalized angularities of groomed and inclusive jets, a measurement of the Lund jet plane, and a measurement of jet axis differences. Results from Pb–Pb collisions include measurements of jet splittings, studies of subjet fragmentation, as well as a measurement of the jet nuclear modification factor made with machine learning techniques. These new results, when combined with the corresponding theoretical comparisons, provide information regarding the QCD processes of the parton shower and hadronization, as well as the underlying physics of jet quenching.

Recent quarkonium measurements with ALICE

Quarkonium production is one of the essential probes for studying the properties of the quark-gluon plasma (QGP) created in relativistic heavy-ion collisions. The suppression of J/ψ due to color screening in the medium was initially proposed as direct evidence of QGP formation. The J/ψ in heavy-ion collisions is affected by the (re-)generation of uncorrelated charm-anticharm (cc‾) quark pairs in the medium as well as the underlying cold nuclear matter effects. The heavier quarkonium states ϒ, the re-(generated) effect is found significantly smaller than J/ψ since the density of uncorrelated beauty-antibeauty (bb‾) quark pairs in the medium is much smaller than cc‾ pairs. Both the J/ψ and ϒ can be used as the probes to study the quark mass dependent (re-)generation effect. In this contribution, the recent measurements of J/ψ and ϒ production in various collision systems performed by the ALICE are shown. The measured quarkonium production cross-section, the nuclear modification factor RpA, RAA, and the flow coefficient v2 are discussed. Results are compared with the model calculations.

Elucidating the jet cross-section in pp and pPb collisions at sNN = 5.02TeV

In this paper, we present the charged particles jet cross-section study in [Formula: see text] and [Formula: see text] collisions at [Formula: see text],TeV using PYTHIA8 (PYTHIA8.186 and PYTHIA8.306) and HIJING 2.0 models, in kinematic range of [Formula: see text],GeV/c for [Formula: see text] collisions and [Formula: see text],GeV/c for [Formula: see text] collisions. The jets are reconstructed for [Formula: see text], 0.3, 0.4 and 0.6 for the pseudorapidity interval of [Formula: see text] 0.9-R. The jets algorithms: cell jet and slow jet inside PYTHIA8.186 and anti-[Formula: see text] algorithm using HIJING 2.0 have been used for Monte Carlo simulations. The simulation results are then compared with measurements of the ALICE experiment. It is observed that all models’ predictions are lower than experimental data up to [Formula: see text],GeV/c; after that, the Monte Carlo study of Cell and slow jet satisfies the experimental data. The CR tunes could not explain the ALICE data at high [Formula: see text] values due to nonperturbative QCD effects. We have also calculated the nuclear modification factor [Formula: see text], and no clear suppression has been seen in the case of [Formula: see text] collisions. The results from the models indicate that the QGP is not formed and that the cold nuclear matter effect exists in pPb collisions.

Extracting the jet transport coefficient from hadron suppressions by confronting current NLO parton fragmentation functions

Nuclear modification factors of single hadrons and dihadrons at large transverse momentum ( $$p_{\mathrm{T}}$$ ) in high-energy heavy-ion collisions are studied in a next-to-leading-order (NLO) perturbative QCD parton model. Parton fragmentation functions (FFs) in $$A+A$$ collisions are modified due to jet energy loss which is proportional to the jet transport coefficient $$\hat{q}$$ characterizing the interaction between the parton jet and the produced medium. By confronting 6 current sets of NLO parton FFs for large $$p_{\mathrm{T}}$$ hadron productions, we extract $$\hat{q}$$ quantitatively via a global fit to data for both single hadron and dihadron suppressions and obtain $$\hat{q}/T^3 = 4.74 - 6.72$$ at $$T = 370$$ MeV in central $$Au+Au$$ collisions at $$\sqrt{s_{\mathrm{NN}}}=200$$ GeV, and $$\hat{q}/T^3 = 3.07 - 3.98$$ at $$T = 480$$ MeV in central $$Pb+Pb$$ collisions at $$\sqrt{s_{\mathrm{NN}}}=2.76$$ TeV. The numerical results show that the uncertainties for $$\hat{q}$$ extraction are brought by the different contributions of gluon-to-hadron in the six sets of FFs due to gluon energy loss being 9/4 times of quark energy loss.

Production of K*(892)0 and ϕ(1020) in pp and Pb−Pb collisions at sNN=5.02 TeV

The production of K$^{*}(892)^{0}$ and $\phi(1020)$ mesons in proton-proton (pp) and lead-lead (Pb-Pb) collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV has been measured using the ALICE detector at the Large Hadron Collider (LHC). The transverse momentum ($p_{\mathrm{T}}$) distributions of K$^{*}(892)^{0}$ and $\phi(1020)$ mesons have been measured at midrapidity $(|y|<0.5)$ up to $p_{\mathrm{T}} = 20$ GeV$/c$ in inelastic pp collisions and for several Pb-Pb collision centralities. The collision centrality and collision energy dependence of the average transverse momenta agree with the radial flow scenario observed with stable hadrons, showing that the effect is stronger for more central collisions and higher collision energies. The $\mathrm{K^{*0}/K}$ ratio is found to be suppressed in Pb-Pb collisions relative to pp collisions: this indicates a loss of the measured K$^{*}(892)^{0}$ signal due to rescattering of its decay products in the hadronic phase. In contrast, for the longer-lived $\phi(1020)$ mesons, no such suppression is observed. The nuclear modification factors ($R_{\rm AA}$) of K$^{*}(892)^{0}$ and $\phi(1020)$ mesons are calculated using pp reference spectra at the same collision energy. In central Pb-Pb collisions for $p_{\rm T} > 8$ GeV$/c$, the $R_{\rm AA}$ values of K$^{*}(892)^{0}$ and $\phi(1020)$ are below unity and observed to be similar to those of pions, kaons, and (anti)protons. The $R_{\rm AA}$ values at high $p_{\mathrm T}$ ($>$~8 GeV$/c$) for K$^{*}(892)^{0}$ and $\phi(1020)$ mesons are in agreement within uncertainties for $\sqrt{s_\mathrm{NN}} = 5.02$ and 2.76 TeV.

Investigating color screening in proton-nucleus collisions with complex potentials* *Supported by the National Natural Science Foundation of China (NSFC) (12175165, 11705125); S.S. acknowledges support from U.S. Department of Energy, Office of Science, Office of Nuclear Physics, (DE-FG88ER40388); X.D acknowledges support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through the CRC-TR 211 'Strong-interaction matter under extreme conditions’– project number 315477589

Color screening and parton inelastic scattering modify the heavy-quark antiquark potential in mediums consisting of particles from quantum chromodynamics (QCD), leading to the suppression of quarkonium production in relativistic heavy-ion collisions. Owing to the small charm/anti-charm () pair production number in proton-nucleus (pA) collisions, the correlation between different pairs is negligible, which makes the Schrödinger equation viable for tracking the evolution of only one pair. We employ the time-dependent Schrödinger equation with an in-medium potential to study the evolution of charmonium wave functions in a hydrodynamic-like QCD medium produced in pA collisions. We explore different parametrizations of real and imaginary parts of the potential and calculate the nuclear modification factors () of and in TeV energy p-Pb collisions at the Large Hadron Collider (LHC). Comparing strong and weak screening scenarios with experimental data in this approach, we arrive at the conclusion that color screening is weak at temperatures close to the deconfined phase transition. Moreover, the imaginary part of the potential is crucial in describing the experimental data, which is consistent with widely studied semi-classical approaches, where dissociation rates are essential.

[formula omitted] formation from random charm and anti-bottom quarks in the quark-gluon plasma

We study the Bc+ production in Pb-Pb collisions at sNN=5.02 TeV. In the quark-gluon plasma (QGP) produced in heavy-ion collisions, heavy quarks make random motions with the energy loss. We employ the Langevin equations to study the non-equilibrium distributions of heavy quarks and the Instantaneous Coalescence Model (ICM) to study the hadronization process. Due to abundant charm and bottom quarks in the QGP, their coalescence probability is significantly enhanced compared with the situations in proton-proton collisions. We find that the final production of Bc+ is increased by the coalescence process, which makes the nuclear modification factor (RAA) of Bc+ larger than unit. Our model explains the experimental data well at semi-central and central collisions. The observation of RAA(Bc+)>1 is regarded as an evident and strong signal of the existence of the deconfined medium generated in heavy-ion collisions.

Medium Effects for Hadron-Tagged Jets in Proton–Proton Collisions

We study the medium modification factor {{I}_{{pp}}} within the light-cone path integral approach to induced gluon emission. We use parametrization of the running coupling {{alpha }_{s}}(Q,T) which has a plateau around Q sim kappa T. We calculate {{I}_{{pp}}} with no free parameters using κ fitted to the LHC data on the nuclear modification factor {{R}_{{AA}}}. We find that the theoretical multiplicity dependence of the ratio {{I}_{{pp}}}{text{/}}langle {{I}_{{pp}}}rangle for 5.02 TeV pp collisions agrees reasonably with the recent preliminary ALICE data [1].

Memory effects on energy loss and diffusion of heavy quarks in the quark-gluon plasma

We study the dynamics of heavy quarks in a thermalized quark-gluon plasma with a time-correlated thermal noise, $\ensuremath{\eta}$. In this case, it is said that $\ensuremath{\eta}$ has memory. We use an integro-differential Langevin equation in which the memory enters via the thermal noise and the dissipative force. We assume that the time correlations of the noise decay exponentially on a timescale, $\ensuremath{\tau}$, which we treat as a free parameter. We compute the effects of $\ensuremath{\tau}\ensuremath{\ne}0$ on the thermalization time of the heavy quarks, on their momentum broadening, and on the nuclear modification factor. We find that overall memory slows down the momentum evolution of heavy quarks: In fact, transverse momentum broadening and the formation of ${R}_{AA}$ are slowed down by memory and the thermalization time of the heavy quarks become larger. The potential impact on other observables is discussed briefly.

Nuclear modification of Y states in pPb collisions at [formula omitted

Production cross sections of ▪, ▪, and ▪ states decaying into ▪ in proton-lead (pPb) collisions are reported using data collected by the CMS experiment at sNN=5.02TeV. A comparison is made with corresponding cross sections obtained with pp data measured at the same collision energy and scaled by the Pb nucleus mass number. The nuclear modification factor for ▪ is found to be ▪. Similar results for the excited states indicate a sequential suppression pattern, such that ▪. The suppression of all states is much less pronounced in pPb than in PbPb collisions, and independent of transverse momentum ▪ and center-of-mass rapidity ▪ of the individual ▪ state in the studied range ▪ and ▪. Models that incorporate final-state effects of bottomonia in pPb collisions are in better agreement with the data than those which only assume initial-state modifications.

Particle production at large p T in Xe+Xe collisions with jet quenching using the higher twist approach* *Supported by the Guangdong Major Project of Basic and Applied Basic Research (2020B0301030008) and the Natural Science Foundation of China with Project (11935007, 11805167)

The production of , η, and ϕ in the most central (0%–5%) Xe+Xe collisions at = 5.44 TeV is investigated in the framework of the perturbative QCD (pQCD) improved parton model at an accuracy of next-to-leading order (NLO). The jet quenching effect is effectively incorporated by medium-modified fragmentation functions via the higher-twist approach. Predictions of the nuclear modification factors of , η, and ϕ as functions of the transverse momentum are made with the jet transport parameter , which is extracted from the available experimental data of charged hadrons provided by ALICE and CMS. The particle ratios as functions of in Xe+Xe collisions at = 5.44 TeV as well as in 0%–5% Pb + Pb collisions at = 5.02 TeV are also presented. The numerical simulations of the scaled ratios of charged hadron production in the Xe+Xe 5.44 TeV system over those in the Pb+Pb 5.02 TeV system give a good description of the CMS data, and the scaled ratios of , η, and ϕ production coincide with the curve of charged hadron production.

Measurements of heavy-flavor production as a function of multiplicity with ALICE at the LHC

In this contribution, the production of heavy-flavor hadrons as a function of multiplicity, via the study of the \mathbf{D}𝐃-meson and heavy-flavor hadron decay leptons self-normalized yields in pp collisions at the center of mass energy \bm{\sqrt{s} = 13}𝐬=13 TeV is discussed. Comparisons are made with similar measurements of J/\bm{\psi}𝛙 at \bm{\sqrt{s} = 13}𝐬=13 TeV and various model calculations. The \mathbf{\Lambda_{c}^{+}/D^{0}}𝚲𝐜+/𝐃0 and \mathbf{D^{+}_{s}/D^{0}}𝐃𝐬+/𝐃0 yield ratios in different multiplicity intervals in pp collisions at \bm{\sqrt{s} = 13}𝐬=13 TeV are also reported. In addition, the ALICE measurement of \mathbf{\Lambda_{c}^{+}}𝚲𝐜+ production in p–Pb collisions at \bm{\sqrt{s_{{NN}}}}𝐬𝐍𝐍 = 5.02 TeV down to transverse momentum (\bm{p_{T}}𝐩𝐓) = 0 GeV/\bm{c}𝐜 is presented. Finally, the nuclear modification factor is shown for open charm hadrons at \bm{\sqrt{s} = 5.02}𝐬=5.02 TeV in p–Pb collisions. Finally, measurements of the elliptic flow of heavy-flavor hadron decay leptons in p–Pb systems are presented, which hint towards a possible collective behaviour in high multiplicity p–Pb collisions.

Energy dependence of intrinsic charm production: Determining the best energy for observation

Background: A nonperturbative charm production contribution, known as intrinsic charm, was predicted in the early 1980s. Recent results have provided new evidence for its existence but further confirmation is needed. Purpose: $J/\psi$ and $\bar D$ meson production are calculated with a combination of perturbative QCD and intrinsic charm to determine the best energy range to study intrinsic charm production. Methods: $J/\psi$ and $\bar D$ meson production are calculated in perturbative QCD to next-to-leading order in the cross section. Cold nuclear matter effects, including nuclear modification of the parton densities and $p_T$ broadening by multiple scattering are taken into account in the production of both; absorption by nucleons is also included for the $J/\psi$. Contributions from intrinsic charm are calculated assuming production from a $|uud c \bar c \rangle$ Fock state. Results: The $J/\psi$ and $\bar D$ meson rapidity and $p_T$ distributions are calculated as a function of rapidity and transverse momentum $p_T$ over a wide range of center-of-mass energies with and without intrinsic charm in $p+p$ collisions. The nuclear modification factor, $R_{pA}$, is also calculated for $p+{\rm Pb}$ interactions at appropriate energies. Previous fixed-target data as a function of Feynman $x$, $x_F$, are also compared to calculations within the approach. Good agreement with the data is found when intrinsic charm is included. Conclusions: The intrinsic charm signal may be largest at midrapidity for future low energy fixed target experiments such as the proposed NA60+.

ϕ meson production in p+Al, p+Au, d+Au, and He3+Au collisions at sNN=200GeV

Small nuclear collisions are mainly sensitive to cold-nuclear-matter effects; however, the collective behavior observed in these collisions shows a hint of hot-nuclear-matter effects. The identified-particle spectra, especially the $\phi$ mesons which contain strange and antistrange quarks and have a relatively small hadronic-interaction cross section, are a good tool to study these effects. The PHENIX experiment has measured $\phi$ mesons in a specific set of small collision systems $p$$+$Al, $p$$+$Au, and $^3$He$+$Au, as well as $d$$+$Au [Phys. Rev. C {\bf 83}, 024909 (2011)], at $\sqrt{s_{_{NN}}}=200$ GeV. The transverse-momentum spectra and nuclear-modification factors are presented and compared to theoretical-model predictions. The comparisons with different calculations suggest that quark-gluon plasma may be formed in these small collision systems at $\sqrt{s_{_{NN}}}=200$ GeV. However, the volume and the lifetime of the produced medium may be insufficient for observing strangeness-enhancement and jet-quenching effects. Comparison with calculations suggests that the main production mechanisms of $\phi$ mesons at midrapidity may be different in $p$$+$Al versus $p/d/$$^3$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. While thermal quark recombination seems to dominate in $p/d/$$^3$He$+$Au collisions, fragmentation seems to be the main production mechanism in $p$$+$Al collisions.

Measuring the light meson nuclear modification factor in p-Pb collisions over an unprecedented $p_{\text{T}}$ range with ALICE

Differential invariant cross sections of light neutral mesons in p-Pb collisions at \sqrt{s_{NN}}sNN = 8.16 TeV and in pp collisions at \sqrt{s}s = 8 TeV have been measured up to very high transverse momentum (p_TpT). By combining independent reconstruction techniques available in ALICE using the EMCal and PHOS calorimeters as well as the central barrel tracking detectors, the combined spectra cover almost two orders of magnitude in p_TpT~for the \pi^0π0~meson. The nuclear modification factor R_{pPb}RpPb~has been measured for the \pi^0π0~and \etaη~mesons and is found to be consistent with NLO pQCD, CGC and energy loss calculations. Comparisons to the R_{pPb}RpPb~of \pi^0π0~measured in \sqrt{s_{NN}}sNN = 5.02 TeV hint at a stronger suppression at low p_TpT~with increasing collision energy.

Measurement of ψ(2S) nuclear modification at backward and forward rapidity in p+p, p+Al , and p+Au collisions at sNN=200 GeV

Suppression of the J/ψ nuclear-modification factor has been seen as a trademark signature of final-state effects in large collision systems for decades. In small systems, the nuclear modification was attributed to cold-nuclear-matter effects until the observation of strong differential suppression of the ψ(2S) state in p+A and d+A collisions suggested the presence of final-state effects. Results of J/ψ and ψ(2S) measurements in the dimuon decay channel are presented here for p+p, p+Al, and p+Au collision systems at sNN=200GeV. The results are predominantly shown in the form of the nuclear-modification factor, RpA, the ratio of the ψ(2S) invariant yield per nucleon-nucleon collision in collisions of proton on target nucleus to that in p+p collisions. Measurements of the J/ψ and ψ(2S) nuclear-modification factor are compared with shadowing and transport-model predictions, as well as to complementary measurements at Large Hadron Collider energies.2 MoreReceived 9 February 2022Accepted 10 May 2022DOI:https://doi.org/10.1103/PhysRevC.105.064912©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLeptonic, semileptonic & radiative decaysRelativistic heavy-ion collisionsPhysical SystemsCharmed mesonsHadron collidersMuonsQuarkoniaNuclear Physics

Observation of the B_{c}^{+} Meson in Pb-Pb and pp Collisions at sqrt[s_{NN}]=5.02 TeV and Measurement of its Nuclear Modification Factor.

The B_{c}^{+} meson is observed for the first time in heavy ion collisions. Data from the CMS detector are used to study the production of the B_{c}^{+} meson in lead-lead (Pb-Pb) and proton-proton (pp) collisions at a center-of-mass energy per nucleon pair of sqrt[s_{NN}]=5.02 TeV, via the B_{c}^{+}→(J/ψ→μ^{+}μ^{-})μ^{+}ν_{μ} decay. The B_{c}^{+} nuclear modification factor, derived from the Pb-Pb-to-pp ratio of production cross sections, is measured in two bins of the trimuon transverse momentum and of the Pb-Pb collision centrality. The B_{c}^{+} meson is shown to be less suppressed than quarkonia and most of the open heavy-flavor mesons, suggesting that effects of the hot and dense nuclear matter created in heavy ion collisions contribute to its production. This measurement sets forth a promising new probe of the interplay of suppression and enhancement mechanisms in the production of heavy-flavor mesons in the quark-gluon plasma.

Charm and beauty isolation from heavy flavor decay electrons in p+p and Pb+Pb collisions at [formula omitted] = 5.02 TeV at LHC

We present an analysis on the heavy flavor hadron decay electrons with charm and beauty contributions decomposed via a data driven method in p+p and Pb+Pb collisions at sNN = 5.02 TeV at LHC. The transverse momentum pT spectra, nuclear modification factor RAA and azimuthal anisotropic flow v2 distributions of electrons from charm and beauty decays are obtained. We find that the electron RAA from charm (RAAc→e) and beauty (RAAb→e) decays are suppressed at pT>2.0 GeV/c in Pb+Pb collisions, which indicates that charm and beauty interact with and lose their energy in the hot-dense medium. A less suppression of RAAb→e than RAAc→e at 2.0<pT<8.0 GeV/c is observed, which is consistent with the mass-dependent partonic energy loss scenario. A non-zero electron v2 from beauty decays (v2b→e) is observed and in good agreement with ALICE measurement. At low pT from 1.0 to 3.0 GeV/c, a discrepancy between RHIC and LHC results is observed with an 85% confidence level, which might be a possible hint of an energy-dependent behavior of beauty quarks reacting with the medium created in heavy-ion collisions. At 3.0<pT<7.0 GeV/c, v2b→e deviates from a number-of-constituent-quark (NCQ) scaling hypothesis, which favors that beauty quark is unlikely thermalized in heavy-ion collisions at LHC energy.

Systematic study of nuclear effects in p+Al, p+Au, d+Au , and He3+Au collisions at sNN=200 GeV using π0 production

The PHENIX Collaboration presents a systematic study of inclusive π0 production from p+p, p+Al, p+Au, d+Au, and He3+Au collisions at sNN=200GeV. Measurements were performed with different centrality selections as well as the total inelastic, 0–100%, selection for all collision systems. For 0–100% collisions, the nuclear-modification factors, RxA, are consistent with unity for pT above 8GeV/c, but exhibit an enhancement in peripheral collisions and a suppression in central collisions. The enhancement and suppression characteristics are similar for all systems for the same centrality class. It is shown that for high-pT−π0 production, the nucleons in the d and He3 interact mostly independently with the Au nucleus and that the counterintuitive centrality dependence is likely due to a physical correlation between multiplicity and the presence of a hard scattering process. These observations disfavor models where parton energy loss has a significant contribution to nuclear modifications in small systems. Nuclear modifications at lower pT resemble the Cronin effect—an increase followed by a peak in central or inelastic collisions and a plateau in peripheral collisions. The peak height has a characteristic ordering by system size as p+Au>d+Au>He3+Au>p+Al. For collisions with Au ions, current calculations based on initial-state cold nuclear matter effects result in the opposite order, suggesting the presence of other contributions to nuclear modifications, in particular at lower pT.6 MoreReceived 11 November 2021Accepted 3 May 2022DOI:https://doi.org/10.1103/PhysRevC.105.064902©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHadronic decaysLeptonic, semileptonic & radiative decaysParticle & resonance productionParticle decaysRelativistic heavy-ion collisionsPhysical SystemsMesonsTechniquesHadron collidersNuclear Physics