Proton-lead Collisions Research Articles

Medium-assisted enhancement of χc1(3872) production from small to large colliding systems

Studies of exotic hadrons such as the χc1(3872) state provide crucial insights into the fundamental force governing the strong interaction dynamics, with an emerging frontier to investigate their production in high energy collisions where a partonic medium is present. The latest experimental measurements from the Large Hadron Collider show an intriguing evolution pattern of the χc1(3872)-to-ψ(2S) yield ratio from proton-proton collisions with increasing multiplicities toward proton-lead and lead-lead collisions. Here we propose a mechanism of medium-assisted enhancement for the χc1(3872) production, which competes with the more conventional absorption-induced suppression and results in a nonmonotonic trend from small to large colliding systems. Realistic simulations from this model offer a quantitative description of all available data. Predictions are made for the centrality dependence of this observable in PbPb collisions as well as for its system-size dependence from OO and ArAr to XeXe and PbPb collisions. In both cases, a nonmonotonic behavior emerges as the imprint of the competition between enhancement and suppression and can be readily tested by future data. Published by the American Physical Society 2024

Nuclear Parton Distribution Functions After the First Decade of LHC Data

We present a review of the conceptual basis, current knowledge, and recent progress regarding global analysis of nuclear parton distribution functions (PDFs). After introducing the theoretical foundations and methodological approaches for the extraction of nuclear PDFs from experimental data, we discuss how different measurements in fixed-target and collider experiments provide increasingly precise constraints on various aspects of nuclear PDFs, including shadowing, antishadowing, the EMC effect, Fermi motion, flavor separation, deuteron binding, and target-mass and other higher-twist effects. Particular emphasis is given to measurements carried out in proton–lead collisions at the Large Hadron Collider, which have revolutionized the global analysis during the past decade. These measurements include electroweak boson, jet, light hadron, and heavy flavor observables. Finally, we outline the expected impact of the future Electron Ion Collider and discuss the role and interplay of nuclear PDFs with other branches of nuclear, particle, and astroparticle physics.

Measurement of Ξc+ production in pPb collisions at sNN=8.16 TeV at LHCb

A study of prompt Ξc+ production in proton-lead collisions is performed with the LHCb experiment at a center-of-mass energy per nucleon pair of 8.16 TeV in 2016 in pPb and Pbp collisions with an estimated integrated luminosity of approximately 12.5 and 17.4 nb−1, respectively. The Ξc+ production cross section, as well as the Ξc+ to Λc+ production cross-section ratio, are measured as a function of the transverse momentum and rapidity and compared to the latest theory predictions. The forward-backward asymmetry is also measured as a function of the Ξc+ transverse momentum. The results provide strong constraints on theoretical calculation and are a unique input for hadronization studies in different collision systems. ©2024 CERN, for the LHCb Collaboration 2024 CERN

The dynamic hadronization of charm quarks in heavy-ion collisions

The Pythia8/Angantyr model for heavy ion collisions was recently updated with a mechanism for global colour reconnection. The colour reconnection model used is QCD colour algebra inspired and enhances baryon production due to the formation of string junctions. In this paper, we present updates to the junction formation and string fragmentation mechanisms, connected to heavy quark fragmentation. This allows for the simulation of heavy quark fragmentation, using junction formation, in heavy ion collisions. The framework is validated for proton collisions, and we show results for charm baryon production in proton-lead collisions.

Production of η and η′ mesons in pp and pPb collisions

The production of η and η′ mesons is studied in proton-proton and proton-lead collisions collected with the LHCb detector. Proton-proton collisions are studied at center-of-mass energies of 5.02 and 13TeV and proton-lead collisions are studied at a center-of-mass energy per nucleon of 8.16TeV. The studies are performed in center-of-mass (c.m.) rapidity regions 2.5<yc.m.<3.5 (forward rapidity) and −4.0<yc.m.<−3.0 (backward rapidity) defined relative to the proton beam direction. The η and η′ production cross sections are measured differentially as a function of transverse momentum for 1.5<pT<10GeV and 3<pT<10GeV, respectively. The differential cross sections are used to calculate nuclear modification factors. The nuclear modification factors for η and η′ mesons agree at both forward and backward rapidity, showing no significant evidence of mass dependence. The differential cross sections of η mesons are also used to calculate η/π0 cross-section ratios, which show evidence of a deviation from the world average. These studies offer new constraints on mass-dependent nuclear effects in heavy-ion collisions, as well as η and η′ meson fragmentation. ©2024 CERN, for the LHCb Collaboration 2024 CERN

Probing initial state effects in nuclear collisions via jet and top-quark measurements with the ATLAS detector

Proton-lead collisions at the Large Hadron Collider energies offer a unique possibility to investigate initial state effects in nuclear collisions. Thanks to its wide acceptance, ATLAS can measure several probes that can help characterise these effects over a wide kinematic range. By analyzing the centrality dependence of dijet production, it is possible to investigate the suppression of dijet events in central collisions compared to peripheral ones and correlate it to the kinematic of the parton-parton scattering, accessible via the measurement of both jets in the final state. Also, the first measurement of the inclusive crosssection for top-quark pair production in dilepton and lepton+jet decay modes is reported. This process is sensitive to effects at high Bjorken-x values, which are hard to access experimentally using other available probes, and thus provides complementary information on the behavior of nuclear parton densities. In 2016, the ATLAS experiment collected 165 nb−1 of proton-lead collisions at a center-of-mass energy of 8.16 TeV per nucleon pair. In this article, new measurements of the centrality dependence of dijet production and the observation of top-quark pair production using this dataset are presented.

Overview of recent CMS results

Recent results from the CMS Collaboration are presented. These measurements include a full physics program using ultraperipheral collisions such as photon-photon and photon-ion interactions, small collision systems including proton-proton and proton-lead collisions, and many measurements of hadronic ion-ion collisions. The properties of the quark-gluon plasma produced in ion-ion collisions are studied in detail. The measurements examine the number of degrees of freedom of the medium, the strength of jet quenching effects in the medium, the role of heavy flavor in hadronization processes, and more.

Establishing the Range of Applicability of Hydrodynamics in High-Energy Collisions

We simulate the space-time dynamics of high-energy collisions based on a microscopic kinetic description, in order to determine the range of applicability of an effective description in relativistic viscous hydrodynamics. We find that hydrodynamics provides a quantitatively accurate description of collective flow when the average inverse Reynolds number Re−1 is sufficiently small and the early pre-equilibrium stage is properly accounted for. By determining the breakdown of hydrodynamics as a function of system size and energy, we find that it is quantitatively accurate in central lead-lead collisions at LHC energies, but should not be used in typical proton-lead or proton-proton collisions, where the development of collective flow cannot accurately be described within hydrodynamics.

Measurements of charm quark production and hadronization at CMS

The study of charm quark hadrons is an important probe into the hadronization of heavy quarks. More specifically, we present results on the production of Λc baryon, the nuclear modification factors (RAA), and the Λc/D0 yield ratios at √SNN = 5.02 TeV in proton-proton (pp) collisions and in different centrality regions in lead-lead (PbPb) collisions, using data recorded with the CMS detector in 2017 and 2018, respectively. The reported RAA for Λc provides useful information regarding the energy loss mechanism and the hadronization processes of charm quark in the quark-gluon plasma. The transverse momentum (pT) dependence of the RAA is similar to that of other charm and beauty hadrons but with its minimum shifted towards higher pT. Comparing the Λc/D0 production ratio in pp and PbPb collisions suggests that coalescence as a hadronization process is not significant for pT > 10 GeV/c. The ratio becomes comparable to the measurements in e+e− collisions for pT > 30 GeV/c. We also present results of the Λc baryon and D0 meson production and their ratios in proton-lead (pPb) collisions at √SNN = 8.16 TeV as a function of pT and final-state multiplicity using the data recorded by the CMS experiment in 2016. We do not observe significant multiplicity dependence for the baryon over meson ratio for charm hadrons. Based on a previous study, the difference between the results from charm quarks and those from light quarks suggests coalescence processes for heavy quarks do not increase further with multiplicity, unlike light quarks.

Studies of the relative suppression of excited quarkonium states with CMS

One of the findings of the LHC heavy ion program is the observation of stronger suppression of the excited quarkonium states compared to the ground states in lead-lead (PbPb) and proton-lead (pPb) collisions. Such differences among the states may imply dissociation effects occurring at late stages, after the evolution of heavy quark pairs into well-defined physical states. The variety of binding energies within the quarkonia families offers an experimental tool to characterize the phenomena at play. Measuring the excited states is crucial as they represent significant feed-down contributions to the production of the ground states and must be accounted for in the interpretation of the data. We present studies of the relative suppression of quarkonia in pPb and PbPb collisions performed by CMS. For the first time, the nuclear modification factor has been extended to the strongly suppressed Υ(3S) state. Moreover, nuclear modification factors as well as excited-to-ground state cross section ratios are measured as a function of particle transverse momentum and rapidity, and event activity. The results are compared with several model calculations incorporating initial- and final-state effects.

Transverse momentum spectra of f0(980) from coalescence model

We use a coalescence model to generate f0(980) particles for four configurations: ss¯ meson, uu¯ss¯ tetraquark, KK¯ molecule, and uu¯ p-wave state. The phase-space information of the coalescing constituents is taken from a multi-phase transport (AMPT) simulation of proton-proton (pp) and proton-lead (pPb) collisions at the LHC. It is shown that the transverse momentum spectra and production yields of f0(980) differ significantly among the configurations. It is suggested that the pT spectra of the f0(980) compared to those of other hadrons (such as pion) and the ratio of the f0(980)pT spectra in pPb over pp collisions can be exploited to tell the configuration of the f0(980).

Large rapidity gaps in proton–nucleus interaction

We analyse the cross-section of events with Large Rapidity Gaps observed in proton–lead collisions by the CMS Collaboration (arXiv:2301.07630). The role of the transverse size of elementary pN amplitude is discussed. We emphasize that the cross-section of incoming proton dissociation caused by the photon radiated off the lead ion is close to the value of dσ/dΔηF\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$d\\sigma /d\\Delta \\eta ^F$$\\end{document} measured by the CMS, and it is not clear why there is no room in the data for the Pomeron-induced contribution

Forward gamma +jet production in proton-proton and proton-lead collisions at LHC within the FoCal calorimeter acceptance

Using the small-x improved transverse momentum dependent factorization (ITMD), which, for a general two-to-two massless scattering can be proved within the color glass condensate (CGC) theory for transverse momenta of particles greater than the saturation scale, we provide predictions for isolated forward photon and jet production in proton-proton and proton-nucleus collisions within the planned ALICE FoCal detector acceptance. We study azimuthal correlations, p_T spectra, as well as normalized ratios of proton-proton cross sections for different energies. The only TMD distribution needed for that process is the “dipole” TMD gluon distribution, which in our computations is based on HERA data and undergoes momentum space BK evolution equation with DGLAP corrections and Sudakov resummation. We conclude, that the process provides an excellent probe of the dipole TMD gluon distribution in saturation regime.

Study of the Bose-Einstein correlations of same-sign pions in proton-lead collisions

Correlations of same-sign charged pions are analysed using proton-lead collision data collected by the LHCb experiment at a nucleon-nucleon centre-of-mass energy of 5.02 TeV, corresponding to an integrated luminosity of 1.06 nb−1. Bose-Einstein correlations are observed in the form of an enhancement of pair production for same-sign charged pions with a small four-momentum difference. The dependence of the correlation radius and the intercept parameter on the reconstructed charged-particle multiplicity is investigated. The measured correlation radii scale linearly with the cube root of the reconstructed charged-particle multiplicity, being compatible with predictions of hydrodynamic models on the collision system evolution.

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.

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.

Two-particle azimuthal correlations in γp interactions using pPb collisions at [formula omitted

The first measurements of the Fourier coefficients (VnΔ) of the azimuthal distributions of charged hadrons emitted from photon-proton (γp) interactions are presented. The data are extracted from 68.8 nb−1 of ultra-peripheral proton-lead (pPb) collisions at sNN=8.16TeV using the CMS detector. The high energy lead ions produce a flux of photons that can interact with the oncoming proton. This γp system provides a set of unique initial conditions with multiplicity lower than in photon-lead collisions but comparable to recent electron-positron and electron-proton data. The VnΔ coefficients are presented in ranges of event multiplicity and transverse momentum (pT) and are compared to corresponding hadronic minimum bias pPb results. For a given multiplicity range, the mean pT of charged particles is smaller in γp than in pPb collisions. For both the γp and pPb samples, V1Δ is negative, V2Δ is positive, and V3Δ consistent with 0. For each multiplicity and pT range, V2Δ is larger for γp events. The γp data are consistent with model predictions that have no collective effects.

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.

Strange hadron collectivity in pPb and PbPb collisions

The collective behavior of {textrm{K}}_{textrm{S}}^0 and Lambda /overline{Lambda} strange hadrons is studied by measuring the elliptic azimuthal anisotropy (v2) using the scalar-product and multiparticle correlation methods. Proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy sqrt{s_{textrm{NN}}} = 8.16 TeV and lead-lead (PbPb) collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV collected by the CMS experiment at the LHC are investigated. Nonflow effects in the pPb collisions are studied by using a subevent cumulant analysis and by excluding events where a jet with transverse momentum greater than 20 GeV is present. The strange hadron v2 values extracted in pPb collisions via the four- and six-particle correlation method are found to be nearly identical, suggesting the collective behavior. Comparisons of the pPb and PbPb results for both strange hadrons and charged particles illustrate how event-by-event flow fluctuations depend on the system size.

Two-photon Fusion Production of \(e^{+}e^{-}\) in Proton–Lead Collision

Two-photon Fusion Production of \(e^{+}e^{-}\) in Proton–Lead Collision