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

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

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

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

Production of muons from heavy-flavour hadron decays at high transverse momentum in Pb–Pb collisions at sNN\sqrt{s_{\rm NN}} = 5.02 and 2.76 TeV

Measurements of the production of muons from heavy-flavour hadron decays in Pb–Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 and 2.76 TeV using the ALICE detector at the LHC are reported. The nuclear modification factor $R_{AA}$ at $\sqrt{s_{\rm NN}}$ = 5.02 TeV is measured at forward rapidity (2.5 7 GeV/c). The $R_{AA}$ shows an increase of the suppression of the yields of muons from heavy-flavour hadron decays with increasing centrality. A suppression by a factor of about three is observed in the 10% most central collisions. The $R_{AA}$ at $\sqrt{s_{\rm NN}}$ = 5.02 TeV is similar to that at $\sqrt{s_{\rm NN}}$ = 2.76 TeV. The precise $R_{AA}$ measurements have the potential to distinguish between model predictions implementing different mechanisms of parton energy loss in the high-density medium formed in heavy-ion collisions. They place important constraints for the understanding of the heavy-quark interaction with the hot and dense QCD medium

Elliptic flow of electrons from beauty-hadron decays in Pb–Pb collisions at √sNN = 5.02 TeV

The elliptic flow of electrons from beauty hadron decays at midrapidity (|y| < 0.8) is measured in Pb-Pb collisions at sNN−−−√ = 5.02 TeV with the ALICE detector at the LHC. The azimuthal distribution of the particles produced in the collisions can be parameterized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, v2. The v2 coefficient is measured for the first time in transverse momentum (pT) range 1.3-6 GeV/c in the centrality class 30-50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length cτ≈ 500 μm compared to that of charm hadrons and most of the other background sources. The v2 of electrons from beauty hadron decays at midrapidity is found to be positive with a significance of 3.75σ. The results provide insights on the degree of thermalization of beauty quarks in the medium. A model assuming full thermalization of beauty quarks is strongly disfavoured by the measurement at high pT, but is in agreement with the results at low pT. Transport models including substantial interactions of beauty quarks with an expanding strongly-interacting medium describe the measurement

Higher harmonic non-linear flow modes of charged hadrons in Pb–Pb collisions at √sNN =5.02 TeV

Anisotropic flow coefficients, vn, non-linear flow mode coefficients, χn,mk, and correlations among different symmetry planes, ρn,mk are measured in Pb-Pb collisions at sNN−−−√=5.02 TeV. Results obtained with multi-particle correlations are reported for the transverse momentum interval 0.2<pT<5.0 GeV/c within the pseudorapidity interval 0.4<|η|<0.8 as a function of collision centrality. The vn coefficients and χn,mk and ρn,mk are presented up to the ninth and seventh harmonic order, respectively. Calculations suggest that the correlations measured in different symmetry planes and the non-linear flow mode coefficients are dependent on the shear and bulk viscosity to entropy ratios of the medium created in heavy-ion collisions. The comparison between these measurements and those at lower energies and calculations from hydrodynamic models places strong constraints on the initial conditions and transport properties of the system