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

Enhanced deuteron coalescence probability in jets

The transverse-momentum (pT) spectra and coalescence parameters B2 of (anti)deuterons are measured in pp collisions at s√=13 TeV for the first time in and out of jets. In this measurement, the direction of the leading particle with the highest pT in the event (pleadT>5 GeV/c) is used as an approximation for the jet axis. The event is consequently divided into three azimuthal regions and the jet signal is obtained as the difference between the Toward region, that contains jet fragmentation products in addition to the underlying event (UE), and the Transverse region, which is dominated by the UE. The coalescence parameter in the jet is found to be approximately a factor of 10 larger than that in the underlying event. This experimental observation is consistent with the coalescence picture and can be attributed to the smaller average phase-space distance between nucleons inside the jet cone as compared to the underlying event. The results presented in this Letter are compared to predictions from a simple nucleon coalescence model, where the phase space distributions of nucleons are generated using PYTHIA 8 with the Monash 2013 tuning, and to predictions from a deuteron production model based on ordinary nuclear reactions with parametrized energy-dependent cross sections tuned on data. The latter model is implemented in PYTHIA 8.3. Both models reproduce the observed large difference between in-jet and out-of-jet coalescence parameters, although the almost flat trend of the BJet2 is not reproduced by the models, which instead give a decreasing trend

Pseudorapidity distributions of charged particles as a function of mid- and forward rapidity multiplicities in pp collisions at s\sqrt{s} = 5.02, 7 and 13 TeV

The multiplicity dependence of the pseudorapidity density of charged particles in proton–proton (pp) collisions at centre-of-mass energies $\sqrt{s}~=~5.02$, 7 and 13 TeV measured by ALICE is reported. The analysis relies on track segments measured in the midrapidity range ($|\eta | < 1.5$). Results are presented for inelastic events having at least one charged particle produced in the pseudorapidity interval $|\eta |<1$. The multiplicity dependence of the pseudorapidity density of charged particles is measured with mid- and forward rapidity multiplicity estimators, the latter being less affected by autocorrelations. A detailed comparison with predictions from the PYTHIA 8 and EPOS LHC event generators is also presented. The results can be used to constrain models for particle production as a function of multiplicity in pp collisions

Measurement of the low-energy antideuteron inelastic cross section

In this Letter, we report the first measurement of the inelastic cross section for antideuteron-nucleus interactions at low particle momenta, covering a range of 0.3 ≤ p < 4 GeV/c. The measurement is carried out using p-Pb collisions at a center-of-mass energy per nucleon–nucleon pair of $\sqrt{s_{\rm NN}}$ = 5.02 TeV, recorded with the ALICE detector at the CERN LHC and utilizing the detector material as an absorber for antideuterons and antiprotons. The extracted raw primary antiparticle-to-particle ratios are compared to the results from detailed ALICE simulations based on the geant4 toolkit for the propagation of (anti)particles through the detector material. The analysis of the raw primary (anti)proton spectra serves as a benchmark for this study, since their hadronic interaction cross sections are well constrained experimentally. The first measurement of the inelastic cross section for antideuteron-nucleus interactions averaged over the ALICE detector material with atomic mass numbers ⟨A⟩ = 17.4 and 31.8 is obtained. The measured inelastic cross section points to a possible excess with respect to the Glauber model parametrization used in geant4 in the lowest momentum interval of 0.3 ≤ p < 0.47 GeV/c up to a factor 2.1. This result is relevant for the understanding of antimatter propagation and the contributions to antinuclei production from cosmic ray interactions within the interstellar medium. In addition, the momentum range covered by this measurement is of particular importance to evaluate signal predictions for indirect dark-matter searches

Jet fragmentation transverse momentum distributions in pp and p-Pb collisions at s \sqrt{s} , sNN \sqrt{s_{\mathrm{NN}}} = 5.02 TeV

Jet fragmentation transverse momentum (j$_{T}$) distributions are measured in proton-proton (pp) and proton-lead (p-Pb) collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV with the ALICE experiment at the LHC. Jets are reconstructed with the ALICE tracking detectors and electromagnetic calorimeter using the anti-k$_{T}$ algorithm with resolution parameter R = 0.4 in the pseudorapidity range |η| < 0.25. The j$_{T}$ values are calculated for charged particles inside a fixed cone with a radius R = 0.4 around the reconstructed jet axis. The measured j$_{T}$ distributions are compared with a variety of parton-shower models. Herwig and Pythia 8 based models describe the data well for the higher j$_{T}$ region, while they underestimate the lower j$_{T}$ region. The j$_{T}$ distributions are further characterised by fitting them with a function composed of an inverse gamma function for higher j$_{T}$ values (called the “wide component”), related to the perturbative component of the fragmentation process, and with a Gaussian for lower j$_{T}$ values (called the “narrow component”), predominantly connected to the hadronisation process. The width of the Gaussian has only a weak dependence on jet transverse momentum, while that of the inverse gamma function increases with increasing jet transverse momentum. For the narrow component, the measured trends are successfully described by all models except for Herwig. For the wide component, Herwig and PYTHIA 8 based models slightly underestimate the data for the higher jet transverse momentum region. These measurements set constraints on models of jet fragmentation and hadronisation

Measurement of nuclear effects on ψ(2S)\psi\rm{(2S)} production in p-Pb collisions at sNN=8.16\sqrt{\textit{s}_{\rm NN}} = 8.16 TeV

Inclusive ψ(2S) production is measured in p-Pb collisions at the centre-of-mass energy per nucleon-nucleon pair $\sqrt{s_{\mathrm{NN}}}$ = 8.16 TeV, using the ALICE detector at the CERN LHC. The production of ψ(2S) is studied at forward (2.03 < y$_{cms}$< 3.53) and backward (−4.46 < y$_{cms}$< −2.96) centre-of-mass rapidity and for transverse momentum p$_{T}$< 12 GeV/c via the decay to muon pairs. In this paper, we report the integrated as well as the y$_{cms}$- and p$_{T}$-differential inclusive production cross sections. Nuclear effects on ψ(2S) production are studied via the determination of the nuclear modification factor that shows a strong suppression at both forward and backward centre-of-mass rapidities. Comparisons with corresponding results for inclusive J/ψ show a similar suppression for the two states at forward rapidity (p-going direction), but a stronger suppression for ψ(2S) at backward rapidity (Pb-going direction). As a function of p$_{T}$, no clear dependence of the nuclear modification factor is found. The relative size of nuclear effects on ψ(2S) production compared to J/ψ is also studied via the double ratio of production cross sections [σ$_{ψ(2S)}$/σ$_{J/ψ}$]pPb/[σ$_{ψ(2S)}$/σ$_{J/ψ}$]$_{pp}$ between p-Pb and pp collisions. The results are compared with theoretical models that include various effects related to the initial and final state of the collision system and also with previous measurements at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV.[graphic not available: see fulltext]Inclusive $\psi$(2S) production is measured in p-Pb collisions at the centre-of-mass energy per nucleon-nucleon pair $\sqrt{s_{\rm{NN}}}=8.16$ TeV, using the ALICE detector at the CERN LHC. The production of $\psi$(2S) is studied at forward ($2.03 < y_{\rm cms} < 3.53$) and backward ($-4.46 < y_{\rm cms} < -2.96$) centre-of-mass rapidity and for transverse momentum $p_{\rm{T}}$ $<$ 12 GeV/$c$ via the decay to muon pairs. In this paper, we report the integrated as well as the $y_{\rm cms}$- and $p_{\rm T}$-differential inclusive production cross sections. Nuclear effects on $\psi$(2S) production are studied via the determination of the nuclear modification factor that shows a strong suppression at both forward and backward centre-of-mass rapidities. Comparisons with corresponding results for inclusive J/$\psi$ show a similar suppression for the two states at forward rapidity (p-going direction), but a stronger suppression for $\psi$(2S) at backward rapidity (Pb-going direction). As a function of $p_{\rm T}$, no clear dependence of the nuclear modification factor is found. The relative size of nuclear effects on $\psi$(2S) production compared to J/$\psi$ is also studied via the double ratio of production cross sections $[\sigma_{\psi(2S)}/\sigma_{\rm{J/}\psi}]_{\rm pPb}/[\sigma_{\psi(2S)}/\sigma_{\rm{J/}\psi}]_{\rm pp}$ between p-Pb and pp collisions. The results are compared with theoretical models that include various effects related to the initial and final state of the collision system and also with previous measurements at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV

J/ψ\psi elliptic and triangular flow in Pb-Pb collisions at sNN\sqrt{s_{\rm NN}} = 5.02 TeV

The inclusive J/ψ elliptic (v$_{2}$) and triangular (v$_{3}$) flow coefficients measured at forward rapidity (2.5 < y < 4) and the v$_{2}$ measured at midrapidity (|y| < 0.9) in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV using the ALICE detector at the LHC are reported. The entire Pb-Pb data sample collected during Run 2 is employed, amounting to an integrated luminosity of 750 μb$^{−1}$ at forward rapidity and 93 μb$^{−1}$ at midrapidity. The results are obtained using the scalar product method and are reported as a function of transverse momentum p$_{T}$ and collision centrality. At midrapidity, the J/ψ v$_{2}$ is in agreement with the forward rapidity measurement. The centrality averaged results indicate a positive J/ψ v$_{3}$ with a significance of more than 5σ at forward rapidity in the p$_{T}$ range 2 < p$_{T}$< 5 GeV/c. The forward rapidity v$_{2}$, v$_{3}$, and v$_{3}$/v$_{2}$ results at low and intermediate p$_{T}$ (p$_{T}$ ≲ 8 GeV/c) exhibit a mass hierarchy when compared to pions and D mesons, while converging into a species-independent curve at higher p$_{T}$. At low and intermediate p$_{T}$, the results could be interpreted in terms of a later thermalization of charm quarks compared to light quarks, while at high p$_{T}$, path-length dependent effects seem to dominate. The J/ψ v$_{2}$ measurements are further compared to a microscopic transport model calculation. Using a simplified extension of the quark scaling approach involving both light and charm quark flow components, it is shown that the D-meson v$_{n}$ measurements can be described based on those for charged pions and J/ψ flow