Nuclear Modification Research Articles

Measurement of the Prompt D^{0} Nuclear Modification Factor in p-Pb Collisions at sqrt[s_{NN}]=8.16 TeV.

The production of prompt D^{0} mesons in proton-lead collisions in both the forward and backward rapidity regions at a center-of-mass energy per nucleon pair of sqrt[s_{NN}]=8.16 TeV is measured by the LHCb experiment. The nuclear modification factor of prompt D^{0} mesons is determined as a function of the transverse momentum p_{T}, and the rapidity in the nucleon-nucleon center-of-mass frame y^{*}. In the forward rapidity region, significantly suppressed production with respect to pp collisions is measured, which provides significant constraints on models of nuclear parton distributions and hadron production down to the very low Bjorken-x region of ∼10^{-5}. In the backward rapidity region, a suppression with a significance of 2.0-3.8 standard deviations compared to parton distribution functions in a nuclear environment expectations is found in the kinematic region of p_{T}>6 GeV/c and -3.25<y^{*}<-2.5, corresponding to x∼0.01.

Recent ALICE results on quarkonium production in nuclear collisions

Quarkonium production has long been regarded as a potential probe of deconfinement in nucleus-nucleus collisions. Recently, the production of J/ψ via regeneration within the quark-gluon plasma (QGP) or at the phase boundary has been identified as an important ingredient for the interpretation of quarkonium production results from lead-lead collisions at the Large Hadron Collider (LHC). Quarkonium polarization could also be used to investigate the properties of the hot and dense medium created at LHC energies, as well as the initial stages of the heavy-ion collision. In this contribution, the latest ALICE quarkonium results are presented and discussed. These include, among others, the nuclear modification of (prompt, non-prompt and inclusive) J/ψ, the ψ(2S) production, and the J/ψ polarization, all measured in lead-lead collisions at the LHC. The results are compared with available theoretical model calculations.

Predictions for exclusive varUpsilon photoproduction in ultraperipheral {textrm{Pb}}+{textrm{Pb}} collisions at the LHC at next-to-leading order in perturbative QCD

We present predictions for the rapidity-differential cross sections of exclusive varUpsilon photoproduction in ultraperipheral collisions (UPCs) of lead ions at the Large Hadron Collider (LHC). We work in the framework of collinear factorization at next-to-leading order (NLO) in perturbative QCD, modeling the generalized parton distributions (GPDs) through the Shuvaev transform of nuclear parton distribution functions (nPDFs). Our direct NLO predictions depend significantly on nPDF uncertainties and on the choices of the factorization and renormalization scales, but to a much lesser degree on GPD modeling. To tame the scale dependence and to account for the fact that the NLO calculations generally underpredict the photoproduction measurements on protons, we also present alternative, data-driven predictions. In this approach the underlying photoproduction cross sections on lead are found by combining their nuclear modifications calculated at NLO with the measured photoproduction cross sections on protons. The data-driven strategy reduces the uncertainties associated with the scale choices, and essentially eliminates the effects of GPD modeling thereby leaving the cross sections sensitive mainly to the input nPDFs. Our estimates indicate that the process is measurable in {textrm{Pb}}+{textrm{Pb}} collisions at the LHC.

Comparison of inclusive and photon-tagged jet suppression in 5.02 TeV Pb+Pb collisions with ATLAS

Parton energy loss in the quark–gluon plasma (QGP) is studied with a measurement of photon-tagged jet production in 1.7 nb−1 of Pb+Pb data and 260 pb−1 of pp data, both at sNN=5.02 TeV, with the ATLAS detector. The process pp →γ+jet+X and its analogue in Pb+Pb collisions is measured in events containing an isolated photon with transverse momentum (pT) above 50 GeV and reported as a function of jet pT. This selection results in a sample of jets with a steeply falling pT distribution that are mostly initiated by the showering of quarks. The pp and Pb+Pb measurements are used to report the nuclear modification factor, RAA, and the fractional energy loss, Sloss, for photon-tagged jets. In addition, the results are compared with the analogous ones for inclusive jets, which have a significantly smaller quark-initiated fraction. The RAA and Sloss values are found to be significantly different between those for photon-tagged jets and inclusive jets, demonstrating that energy loss in the QGP is sensitive to the colour-charge of the initiating parton. The results are also compared with a variety of theoretical models of colour-charge-dependent energy loss.

Distributions of the nuclear modification factor of pions, kaons and protons in the most central Pb–Pb collisions at $$\sqrt{{s}_{NN}}$$ = 2.76 TeV

Distributions of the nuclear modification factor of pions, kaons and protons in the most central Pb–Pb collisions at $$\sqrt{{s}_{NN}}$$ = 2.76 TeV

Inclusive and multiplicity dependent production of electrons from heavy-flavour hadron decays in pp and p-Pb collisions

Measurements of the production of electrons from heavy-flavour hadron decays in pp collisions at sqrt{s} = 13 TeV at midrapidity with the ALICE detector are presented down to a transverse momentum (pT) of 0.2 GeV/c and up to pT = 35 GeV/c, which is the largest momentum range probed for inclusive electron measurements in ALICE. In p-Pb collisions, the production cross section and the nuclear modification factor of electrons from heavy-flavour hadron decays are measured in the pT range 0.5 < pT< 26 GeV/c at sqrt{s_{textrm{NN}}} = 8.16 TeV. The nuclear modification factor is found to be consistent with unity within the statistical and systematic uncertainties. In both collision systems, first measurements of the yields of electrons from heavy-flavour hadron decays in different multiplicity intervals normalised to the multiplicity-integrated yield (self-normalised yield) at midrapidity are reported as a function of the self-normalised charged-particle multiplicity estimated at midrapidity. The self-normalised yields in pp and p-Pb collisions grow faster than linear with the self-normalised multiplicity. A strong pT dependence is observed in pp collisions, where the yield of high-pT electrons increases faster as a function of multiplicity than the one of low-pT electrons. The measurement in p-Pb collisions shows no pT dependence within uncertainties. The self-normalised yields in pp and p-Pb collisions are compared with measurements of other heavy-flavour, light-flavour, and strange particles, and with Monte Carlo simulations.

Unraveling gluon jet quenching through [formula omitted] production in heavy-ion collisions

Jet quenching has long been regarded as one of the key signatures for the formation of quark-gluon plasma in heavy-ion collisions. Despite significant efforts, the separate identification of quark and gluon jet quenching has remained as a challenge. Here we show that J/ψ in high transverse momentum (pT) region provides a uniquely sensitive probe of in-medium gluon energy loss since its production at high pT is particularly dominated by gluon fragmentation. Such gluon-dominance is first demonstrated for the baseline of proton-proton collisions within the framework of leading power non-relativistic quantum chromodynamics factorization formalism. We then use the linear Boltzmann transport model combined with hydrodynamics for the simulation of jet-medium interaction in nucleus-nucleus collisions. The satisfactory description of experimental data on both nuclear modification factor RAA and elliptic flow v2 reveals, for the first time, that the gluon jet quenching is the driving force for high pTJ/ψ suppression. This novel finding is further confirmed by the data-driven Bayesian analyses of relevant experimental measurements, from which we also obtain the first quantitative extraction of the gluon energy loss distribution in the quark-gluon plasma.

Nuclear Modification Factor of Neutral Pions in the Forward and Backward Regions in p-Pb Collisions.

The nuclear modification factor of neutral pions is measured in proton-lead collisions collected at a center-of-mass energy per nucleon of 8.16TeV with the LHCb detector. The π^{0} production cross section is measured differentially in transverse momentum (p_{T}) for 1.5<p_{T}<10.0 GeV and in center-of-mass pseudorapidity (η_{c.m.}) regions 2.5<η_{c.m.}<3.5 (forward) and -4.0<η_{c.m.}<-3.0 (backward) defined relative to the proton beam direction. The forward measurement shows a sizable suppression of π^{0} production, while the backward measurement shows the first evidence of π^{0} enhancement in proton-lead collisions at the LHC. Together, these measurements provide precise constraints on models of nuclear structure and particle production in high-energy nuclear collisions.

J/ψ production at midrapidity in p-Pb collisions at sqrt{s_{textrm{NN}}} = 8.16 TeV

The production of inclusive, prompt and non-prompt J/ψ was studied for the first time at midrapidity (−1.37 < ycms< 0.43) in p-Pb collisions at sqrt{s_{textrm{NN}}} = 8.16 TeV with the ALICE detector at the LHC. The inclusive J/ψ mesons were reconstructed in the dielectron decay channel in the transverse momentum (pT) interval 0 < pT< 14 GeV/c and the prompt and non-prompt contributions were separated on a statistical basis for pT> 2 GeV/c. The study of the J/ψ mesons in the dielectron channel used for the first time in ALICE online single-electron triggers from the Transition Radiation Detector, providing a data sample corresponding to an integrated luminosity of 689 ± 13 μb−1. The proton-proton reference cross section for inclusive J/ψ was obtained based on interpolations of measured data at different centre-of-mass energies and a universal function describing the pT-differential J/ψ production cross sections. The pT-differential nuclear modification factors RpPb of inclusive, prompt, and non-prompt J/ψ are consistent with unity and described by theoretical models implementing only nuclear shadowing.

Charged-hadron production in pp, p+Pb, Pb+Pb, and Xe+Xe collisions at sqrt{s_{textrm{NN}}} = 5 TeV with the ATLAS detector at the LHC

This paper presents measurements of charged-hadron spectra obtained in pp, p+Pb, and Pb+Pb collisions at sqrt{s} or sqrt{s_{textrm{NN}}} = 5.02 TeV, and in Xe+Xe collisions at sqrt{s_{textrm{NN}}} = 5.44 TeV. The data recorded by the ATLAS detector at the LHC have total integrated luminosities of 25 pb−1, 28 nb−1, 0.50 nb−1, and 3 μb−1, respectively. The nuclear modification factors RpPb and RAA are obtained by comparing the spectra in heavy-ion and pp collisions in a wide range of charged-particle transverse momenta and pseudorapidity. The nuclear modification factor RpPb shows a moderate enhancement above unity with a maximum at pT ≈ 3 GeV; the enhancement is stronger in the Pb-going direction. The nuclear modification factors in both Pb+Pb and Xe+Xe collisions feature a significant, centrality-dependent suppression. They show a similar distinct pT-dependence with a local maximum at pT ≈ 2 GeV and a local minimum at pT ≈ 7 GeV. This dependence is more distinguishable in more central collisions. No significant |η|-dependence is found. A comprehensive comparison with several theoretical predictions is also provided. They typically describe RAA better in central collisions and in the pT range from about 10 to 100 GeV.

Measurement of Υ production in pp collisions at sqrt{s} = 5 TeV

The production cross-sections of Υ mesons, namely Υ(1S), Υ(2S) and Υ(3S), in pp collisions at sqrt{s} = 5 TeV are measured with a data sample corresponding to an integrated luminosity of 9.13 ± 0.18 pb−1 collected by the LHCb detector. The Υ mesons are reconstructed in the decay mode Υ → μ+μ−. Double differential cross-sections times branching fractions, as functions of the transverse momentum pT and the rapidity y of the Υ mesons, are measured in the range pT< 20 GeV/c and 2.0 < y < 4.5. The results integrated over these pT and y ranges areσΥ1S×BΥ1S→μ+μ−=2101±33±83pb,σΥ2S×BΥ2S→μ+μ−=526±20±21pb,σΥ3S×BΥ3S→μ+μ−=242±16±10pb,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sigma \\left(\ extrm{Y}(1S)\\right)\ imes \\mathcal{B}\\left(\ extrm{Y}(1S)\ o {\\mu}^{+}{\\mu}^{-}\\right)=2101\\pm 33\\pm 83\\ \ extrm{pb}, $$$$ \\sigma \\left(\ extrm{Y}(2S)\\right)\ imes \\mathcal{B}\\left(\ extrm{Y}(2S)\ o {\\mu}^{+}{\\mu}^{-}\\right)=526\\pm 20\\pm 21\\ \ extrm{pb}, $$$$ \\sigma \\left(\ extrm{Y}(3S)\\right)\ imes \\mathcal{B}\\left(\ extrm{Y}(3S)\ o {\\mu}^{+}{\\mu}^{-}\\right)=242\\pm 16\\pm 10\\ \ extrm{pb}, $$\\end{document}where the first uncertainties are statistical and the second are systematic. The ratios of cross-sections between measurements of two different Υ states and between measurements at different centre-of-mass energies are determined. The nuclear modification factor of Υ(1S) at sqrt{s} = 5 TeV is updated as well using the directly measured cross-section results from this analysis.

Multiplicity and rapidity dependence of textrm{K}^{*}(mathrm {{textbf {892}}})^{0} and mathrm {phi ({textbf {1020}})} production in p–Pb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV

The transverse-momentum (p_{textrm{T}}) spectra of K^{*}(892)^{0}~and mathrm {phi (1020)}~measured with the ALICE detector up to p_{textrm{T}} = 16 GeV/c in the rapidity range -1.2< y < 0.3, in p–Pb collisions at the center-of-mass energy per nucleon–nucleon collision sqrt{s_{textrm{NN}}} = 5.02 TeV are presented as a function of charged particle multiplicity and rapidity. The measured p_{textrm{T}} distributions show a dependence on both multiplicity and rapidity at low p_{textrm{T}} whereas no significant dependence is observed at high p_{textrm{T}} . A rapidity dependence is observed in the p_{textrm{T}} -integrated yield (dN/dy), whereas the mean transverse momentum left( langle p_{textrm{T}} rangle right) shows a flat behavior as a function of rapidity. The rapidity asymmetry (Y_{textrm{asym}}) at low p_{textrm{T}} (< 5 GeV/c) is more significant for higher multiplicity classes. At high p_{textrm{T}} , no significant rapidity asymmetry is observed in any of the multiplicity classes. Both K^{*}(892)^{0}~and mathrm {phi (1020)}~show similar Y_{textrm{asym}}. The nuclear modification factor (Q_{textrm{CP}}) as a function of p_{textrm{T}} shows a Cronin-like enhancement at intermediate p_{textrm{T}} , which is more prominent at higher rapidities (Pb-going direction) and in higher multiplicity classes. At high p_{textrm{T}} (> 5 GeV/c), the Q_{textrm{CP}} values are greater than unity and no significant rapidity dependence is observed.

First measurement of Λc+ production down to pT=0 in pp and p -Pb collisions at sNN=5.02 TeV

The production of prompt $\mathrm {\Lambda_{c}^{+}}$ baryons has been measured at midrapidity in the transverse momentum interval $0<p_{\rm T}<1$ GeV/$c$ for the first time, in pp and p-Pb collisions at a centre-of-mass energy per nucleon-nucleon collision $\sqrt{s_\mathrm{NN}} = 5.02$ TeV. The measurement was performed in the decay channel ${\rm \Lambda_{c}^{+}\to p K^{0}_{S}}$ by applying new decay reconstruction techniques using a Kalman-Filter vertexing algorithm and adopting a machine-learning approach for the candidate selection. The $p_{\rm T}$-integrated $\mathrm {\Lambda_{c}^{+}}$ production cross sections in both collision systems were determined and used along with the measured yields in Pb-Pb collisions to compute the $p_{\rm T}$-integrated nuclear modification factors $R_{\rm pPb}$ and $R_\mathrm{AA}$ of $\mathrm{\Lambda_{c}^{+}}$ baryons, which are compared to model calculations that consider nuclear modification of the parton distribution functions. The $\mathrm{\Lambda_{c}^{+}/D^0}$ baryon-to-meson yield ratio is reported for pp and p-Pb collisions. Comparisons with models that include modified hadronisation processes are presented, and the implications of the results on the understanding of charm hadronisation in hadronic collisions are discussed. A significant ($3.7\sigma$) modification of the mean transverse momentum of $\mathrm {\Lambda_{c}^{+}}$ baryons is seen in p-Pb collisions with respect to pp collisions, while the $p_{\rm T}$-integrated $\mathrm{\Lambda_{c}^{+}/D^0}$ yield ratio was found to be consistent between the two collision systems within the uncertainties.

Measurement of the Z boson production cross-section in proton-lead collisions at sqrt{s_{textrm{NN}}} = 8.16 TeV

This article presents the first measurement of the differential Z-boson production cross-section in the forward region using proton-lead collisions with the LHCb detector. The dataset was collected at a nucleon-nucleon centre-of-mass energy of sqrt{s_{textrm{NN}}} = 8.16 TeV in 2016, corresponding to an integrated luminosity of 30.8 nb−1. The forward-backward ratio and the nuclear modification factors are measured together with the differential cross-section as functions of the Z boson rapidity in the centre-of-mass frame, the transverse momentum of the Z boson and a geometric variable ϕ*. The results are in good agreement with the predictions from nuclear parton distribution functions, providing strong constraining power at small Bjorken-x.

Searching for QGP droplets with high- pT hadrons and heavy flavor

The search for the smallest quark-gluon plasma (QGP) droplets in nature has motivated recent small collisions system programs at RHIC and LHC. Unambiguous identification of jet quenching due to final-state interactions is key to confirming QGP formation in these reactions. We compute the nuclear modification factors $R_{AA}$ and $R_{p(d)A}$ of charged hadrons and heavy flavor mesons in large (Au-Au, Xe-Xe, Pb-Pb) and small ($d$-Au, $p$-Pb, O-O) colliding systems, respectively. Our results include the Cronin effect and initial-state parton energy loss in cold nuclear matter. In the final state, hard partons undergo collisional energy loss and branching that was recently derived using Soft-Collinear-Effective-Theory with Glauber Gluon (SCET$_{\rm G}$). In large colliding systems, medium-modified QCD evolution of the fragmentation functions dominates the nuclear correction. As the system size decreases, we find that cold nuclear matter effects, collisional energy loss, and QGP-induced radiations can become equally important. A systematic scan over the medium size and mass/flavor dependence of $R_{AA}$ provides the opportunity to separate these individual contributions and identify QGP signatures in small systems. Predictions for $R_{AA}^{h}$, $R_{AA}^{D}$, $R_{AA}^{B}$ in O-O collisions at $\sqrt{s}=7$ TeV are presented with and without the formation of a QGP and contrasted with the corresponding $R_{p(d)A}$ calculations. Upcoming single-hadron measurements at the LHC will not only test the O-O predictions for both light and heavy flavor production, but will shed light on the possibly very different dynamics of $p$-A and A-A reactions at similar soft particle production multiplicities.

Systematic study of hadrons and their quark-component nuclear modification factors

We have systematically studied the connection (correspondence) between hadrons and their quark-component nuclear modification factors and the flavor (mass) ordering at both parton and hadron levels in nucleus-nucleus collisions at energies available at the CERN Large Hadron Collider, using the paciae model. It turns out that the correspondence and the mass ordering generally hold, irrespective of the rapidity, centrality, reaction energy, and collision system size. The nuclear modification factors of hadrons in the final hadronic state show clear mass ordering, which should be studied further both theoretically and experimentally.

Measurement of the nuclear modification factor of b-jets in 5.02 TeV Pb+Pb collisions with the ATLAS detector

This paper presents a measurement of b-jet production in Pb+Pb and pp collisions at sqrt{s_{_text {NN}}}=5.02 TeV with the ATLAS detector at the LHC. The measurement uses 260 text {pb}^{-1} of pp collisions collected in 2017 and 1.4 text {nb}^{-1} of Pb+Pb collisions collected in 2018. In both collision systems, jets are reconstructed via the anti-k_{t} algorithm. The b-jets are identified from a sample of jets containing muons from the semileptonic decay of b-quarks using template fits of the muon momentum relative to the jet axis. In pp collisions, b-jets are reconstructed for radius parameters R= 0.2 and R= 0.4, and only R= 0.2 jets are used in Pb+Pb collisions. For comparison, inclusive R= 0.2 jets are also measured using 1.7 text {nb}^{-1} of Pb+Pb collisions collected in 2018 and the same pp collision data as the b-jet measurement. The nuclear modification factor, R_text {AA}, is calculated for both b-jets and inclusive jets with R= 0.2 over the transverse momentum range of 80–290 GeV. The nuclear modification factor for b-jets decreases from peripheral to central collisions. The ratio of the b-jet R_text {AA} to inclusive jet R_text {AA} is also presented and suggests that the R_text {AA} for b-jets is larger than that for inclusive jets in central Pb+Pb collisions. The measurements are compared with theoretical calculations and suggest a role for mass and colour-charge effects in partonic energy loss in heavy-ion collisions.

New microscopic model for J/ψ production in heavy ion collisions

We present a new model for the creation of $J/\ensuremath{\psi}$ mesons in ultrarelativistic heavy ion collisions, which allows one to follow the individual heavy quarks from their creation until the detector through the quark-gluon plasma, which is formed in these collisions and described by the EPOS2 event generator. The $c$ and $\overline{c}$ quarks interact via a potential, based on results of lattice gauge calculations. The annihilation and creation of $J/\ensuremath{\psi}$ is described by a density matrix approach whose time evolution is studied in the expanding system. The comparison with PbPb data at $\sqrt{s}=5.02$ TeV shows that this model can describe simultaneously the nuclear modification factor ${R}_{AA}$ and the elliptic flow ${v}_{2}$ of the $J/\ensuremath{\psi}$ at low transverse momentum. Perspectives for further improvement are discussed.

Production of Υ(nS) mesons in Pb+Pb and pp collisions at 5.02 TeV

A measurement of the production of vector bottomonium states, $\varUpsilon(\textrm{1S})$, $\varUpsilon(\textrm{2S})$, and $\varUpsilon(\textrm{3S})$, in Pb+Pb and $pp$ collisions at a center-of-mass energy per nucleon pair of 5.02 TeV is presented. The data correspond to integrated luminosities of 1.38~$\textrm{nb}^{-1}$ of Pb+Pb data collected in 2018, 0.44 $\textrm{nb}^{-1}$ of Pb+Pb data collected in 2015, and 0.26 $\textrm{fb}^{-1}$ of $pp$ data collected in 2017 by the ATLAS detector at the Large Hadron Collider. The measurements are performed in the dimuon decay channel for transverse momentum $p_\textrm{T}^{\mu\mu} < 30$~ GeV, absolute rapidity $|y^{\mu\mu}|<1.5$, and Pb+Pb event centrality 0-80%. The production rates of the three bottomonium states in Pb+Pb collisions are compared with those in $pp$ collisions to extract the nuclear modification factors as functions of event centrality, $p_\textrm{T}^{\mu\mu}$, and $|y^{\mu\mu}|$. In addition, the suppression of the excited states relative to the ground state is studied. The results are compared with theoretical model calculations.

Jet Quenching for Heavy Flavors in AA and pp Collisions

We perform a global analysis of experimental data on jet quenching for heavy flavors for scenarios with and without quark-gluon plasma formation in pp collisions. We find that the theoretical predictions for the nuclear modification factor RAA for heavy flavors at the LHC energies are very similar for these scenarios, and the results for RAA and v2 agree reasonably with the LHC data. The agreement with data at the RHIC top energy becomes somewhat better for the intermediate scenario, in which the quark-gluon plasma formation in pp collisions occurs only at the LHC energies. Our fits to heavy flavor RAA show that description of jet quenching for heavy flavors requires somewhat bigger αs than data on jet quenching for light hadrons.