Global constraint on the magnitude of anomalous chiral effects in heavy-ion collisions

When searching for anomalous chiral effects in heavy-ion collisions, one of the most crucial points is the relationship between the signal and the background. In this letter, we present a simulation in a modified blast wave model at LHC energy, which can simultaneously characterize the majority of measurable quantities, in particular, the chiral magnetic effect (CME) and the chiral magnetic wave (CMW) observables. Such a universal description, for the first time, naturally and quantitatively unifies the CME and the CMW studies and brings to light the connection with the local charge conservation (LCC) background. Moreover, a simple phenomenological approach is performed to introduce the signals, aiming at quantifying the maximum allowable strength of the signals within experimental precision. Such a constraint provides a novel perspective to understand the experimental data and sheds new light on the study of anomalous chiral effects as well as charge dependent correlations.Comment: 8 pages, 5 figure

Exploring the hadronic phase of relativistic heavy-ion collisions with resonances in ALICE

Short-lived resonances are a good tool to study the hadronic phase that characterizes the late-stage evolution of heavy-ion collisions. Regeneration and rescattering processes taking part for resonances in the hadronic phase modify their measured yields. This can be studied by measuring resonance to stable particle yield ratios as a function of system size and comparing them to model predictions with and without hadronic interactions. With the excellent tracking and particle identification capabilities that ALICE has been endowed with, a comprehensive set of both mesonic and baryonic resonances have been measured. Recent results on resonance production in pp, p$-$Pb, Xe$-$Xe and Pb$-$Pb collisions at various centre of mass energies are presented. Recent results on K*$^{\pm}$(892), $\Sigma$*$^{\pm}$(1385) and $\Xi$*$^{0}$(1820), are presented. The results are further compared to lower energy measurements and different model predictions wherever available

Search for the Chiral Magnetic Wave Using the ALICE Detector in Pb-Pb Collisions at sNN\sqrt{ {s}_{NN}} = 5.02 TeV

In a non-central heavy-ion collision, a strong magnetic field is created which is theorised to give rise to collective excitation in the hot and dense medium (QGP). As a result of this non-trivial chiral currents, the elliptic flow of produced particles show charge dependence which is called the Chiral Magnetic Wave (CMW). Here, we present systematic studies of charge dependent Fourier coefficients (v$_{n}$) of azimuthal distribution of particles for Pb-Pb collisions at $\sqrt{ {s}_{NN}}$ = 5.02 TeV. These $v_{n}$ measurements are performed for charged particles (pions) in the pseudorapidity ($\eta$) and transverse momentum ($p_\mathrm{{T}}$) ranges $\left| \eta \right| < 0.8$ and $0.2< p_{\textrm{T}} < 1.0$ (0.5) GeV/c. The third-order Fourier coefficient (v$_{3}$) shows a similar behaviour with a similar magnitude of the normalised slope as seen for v$_{2}$. The similarity of normalised slopes for v$_{2}$ and v$_{3}$ in Pb-Pb collisions suggests that the effect of CMW, on the charge dependent splitting of v$_{2}$ at LHC energy, is consistent with zero

Probing rescattering effect in heavy-ion collisions with ALICE at the LHC

Hadronic resonances are short-lived particles which decay via strong interaction. During the hadron gas phase that follows the hadronisation of the medium created in heavyion collisions and spans from the chemical to the kinetic freeze-out, resonances having lifetimes comparable to the duration of the hadronic phase take part in two processes, called regeneration and rescattering. These effects lead to the modification of their yields, which depend on their lifetime, the hadronic cross section of their decay products, and the hadronic phase lifetime. Rescatterings with other hadrons in the medium alter the momentum of the resonance decay products and prevent the reconstruction of the resonance with an invariant-mass analysis. In turn, pseudoelastic scattering could also regenerate the resonance leading to an enhancement of their yields. Both these competing effects determine the final yield of the resonances at kinetic freeze out. These processes can be studied from the ratios of the yields of resonances to stable particles having same quark content, as a function of system size and comparing with model predictions with and without hadronic interactions

J/ψ production as a function of charged-particle multiplicity in p–Pb collisions at √sNN = 8.16 TeV

Inclusive J/ψ yields and average transverse momenta in p-Pb collisions at a center-of-mass energy per nucleon pair sNN−−−√ = 8.16 TeV are measured as a function of the charged-particle pseudorapidity density with ALICE. The J/ψ mesons are reconstructed at forward (2.03<ycms<3.53) and backward (−4.46<ycms<−2.96) center-of-mass rapidity in their dimuon decay channel while the charged-particle pseudorapidity density is measured around midrapidity. The J/ψ yields at forward and backward rapidity normalized to their respective average values increase with the normalized charged-particle pseudorapidity density, the former showing a weaker increase than the latter. The normalized average transverse momenta at forward and backward rapidity manifest a steady increase from low to high charged-particle pseudorapidity density with a saturation beyond the average value

Production of Λ and K0S in jets in p–Pb collisions at √sNN = 5.02 TeV and pp collisions at √s = 7 TeV

The production of Λ baryons and K0S mesons (V0 particles) was measured in p-Pb collisions at sNN−−−√=5.02 TeV and pp collisions at s√=7 TeV with ALICE at the LHC. The production of these strange particles is studied separately for particles associated with hard scatterings and the underlying event to shed light on the baryon-to-meson ratio enhancement observed at intermediate transverse momentum (pT) in high multiplicity pp and p-Pb collisions. Hard scatterings are selected on an event-by-event basis with jets reconstructed with the anti-kT algorithm using charged particles. The production of strange particles associated with jets pchT,jet>10 and pchT,jet>20 GeV/c in p-Pb collisions, and with jet pchT,jet>10 GeV/c in pp collisions is reported as a function of pT. Its dependence on angular distance from the jet axis, R(V0,jet), for jets with pchT,jet>10 GeV/c in p-Pb collisions is reported as well. The pT-differential production spectra of strange particles associated with jets are found to be harder compared to that in the underlying event and both differ from the inclusive measurements. In events containing a jet, the density of the V0 particles in the underlying event is found to be larger than the density in the minimum bias events. The Λ/K0S ratio associated with jets in p-Pb collisions is consistent with the ratio in pp collisions and follows the expectation of jets fragmenting in vacuum. On the other hand, this ratio within jets is consistently lower than the one obtained in the underlying event and it does not show the characteristic enhancement of baryons at intermediate pT often referred to as "baryon anomaly" in the inclusive measurements

Direct observation of the dead-cone effect in quantum chromodynamics

In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD). These partons subsequently emit further partons in a process that can be described as a parton shower which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass mQ and energy E, within a cone of angular size mQ/E around the emitter. Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics

Energy dependence of coherent photonuclear production of J/ψ mesons in ultra-peripheral Pb–Pb collisions at √sNN = 5.02 TeV

The cross section for coherent photonuclear production of J/ψ is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of sNN−−−√=5.02 TeV. Cross sections are presented in five different J/ψ rapidity ranges within |y|<4, with the J/ψ reconstructed via its dilepton decay channels. In some events the J/ψ is not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17<WγPb,n<920 GeV, where WγPb,n is the centre-of-mass energy per nucleon of the γPb system. This range corresponds to a Bjorken-x interval spanning about three orders of magnitude: 1.1×10−5<x<3.3×10−2. In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies

Measurement of inclusive and leading subjet fragmentation in pp and Pb–Pb collisions at √sNN = 5.02 TeV

This article presents new measurements of the fragmentation properties of jets in both proton-proton (pp) and heavy-ion collisions with the ALICE experiment at the LHC. We report distributions of the fraction zr of transverse momentum carried by subjets of radius r within jets of radius R. Charged-particle jets are reconstructed at midrapidity using the anti-kT algorithm with jet radius R=0.4, and subjets are reconstructed by reclustering the jet constituents using the anti-kT algorithm with radii r=0.1 and r=0.2. In pp collisions, we measure both the inclusive and leading subjet distributions. We compare these measurements to perturbative calculations at next-to-leading logarithmic accuracy, which suggest a large impact of threshold resummation and hadronization effects on the zr distribution. In heavy-ion collisions, we measure the leading subjet distributions, which allow access to a region of harder jet fragmentation than has been probed by previous measurements of jet quenching via hadron fragmentation distributions. The zr distributions enable extraction of the parton-to-subjet fragmentation function and allow for tests of the universality of jet fragmentation functions in the quark-gluon plasma (QGP). We find no significant modification of zr distributions in Pb-Pb compared to pp collisions. However, the distributions are also consistent with a hardening trend for zr<0.95, as predicted by several jet quenching models. As zr→1 our results indicate that any such hardening effects cease, exposing qualitatively new possibilities to disentangle competing jet quenching mechanisms. By comparing our results to theoretical calculations based on an independent extraction of the parton-to-jet fragmentation function, we find consistency with the universality of jet fragmentation and no indication of factorization breaking in the QGP

Dielectron production at midrapidity at low transverse momentum in peripheral and semi-peripheral Pb–Pb collisions at √sNN = 5.02 TeV

The first measurement of the e+e− pair production at low lepton pair transverse momentum (pT,ee) and low invariant mass (mee) in non-central Pb−Pb collisions at sNN−−−√=5.02 TeV at the LHC is presented. The dielectron production is studied with the ALICE detector at midrapidity (|ηe|<0.8) as a function of invariant mass (0.4≤mee<2.7 GeV/c2) in the 50−70% and 70−90% centrality classes for pT,ee<0.1 GeV/c, and as a function of pT,ee in three mee intervals in the most peripheral Pb−Pb collisions. Below a pT,ee of 0.1 GeV/c, a clear excess of e+e− pairs is found compared to the expectations from known hadronic sources and predictions of thermal radiation from the medium. The mee excess spectra are reproduced, within uncertainties, by different predictions of the photon−photon production of dielectrons, where the photons originate from the extremely strong electromagnetic fields generated by the highly Lorentz-contracted Pb nuclei. Lowest-order quantum electrodynamic (QED) calculations, as well as a model that takes into account the impact-parameter dependence of the average transverse momentum of the photons, also provide a good description of the pT,ee spectra. The measured ⟨p2T,ee⟩−−−−−√ of the excess pT,ee spectrum in peripheral Pb−Pb collisions is found to be comparable to the values observed previously at RHIC in a similar phase-space region