Probing the chiral magnetic wave with charge-dependent flow measurements in Pb-Pb collisions at the LHC

The Chiral MagneticWave (CMW) phenomenon is essential to provide insights into the strong interaction in QCD, the properties of the quark-gluon plasma, and the topological characteristics of the early universe, offering a deeper understanding of fundamental physics in high-energy collisions. Measurements of the charge-dependent anisotropic flow coefficients are studied in Pb-Pb collisions at center-of-mass energy per nucleon-nucleon collision v sNN = 5.02TeV to probe the CMW. In particular, the slope of the normalized difference in elliptic (v2) and triangular (v3) flow coefficients of positively and negatively charged particles as a function of their event-wise normalized number difference, is reported for inclusive and identified particles. The slope rNorm 3 is found to be larger than zero and to have a magnitude similar to rNorm 2, thus pointing to a large background contribution for these measurements. Furthermore, rNorm 2 can be described by a blast wave model calculation that incorporates local charge conservation. In addition, using the event shape engineering technique yields a fraction of CMW (fCMW) contribution to this measurement which is compatible with zero. This measurement provides the very first upper limit for fCMW, and in the 10-60% centrality interval it is found to be 26% (38%) at 95% (99.7%) confidence level

First Measurement of the |t| Dependence of Incoherent J/psi Photonuclear Production

The first measurement of the cross section for incoherent photonuclear production of J/ψ vector mesons as a function of the Mandelstam |t| variable is presented. The measurement was carried out with the ALICE detector at midrapidity, |y|<0.8, using ultraperipheral collisions of Pb nuclei at a center-of-mass energy per nucleon pair of sqrt[s_{NN}]=5.02 TeV. This rapidity interval corresponds to a Bjorken-x range (0.3-1.4)×10^{-3}. Cross sections are given in five |t| intervals in the range 0.04<|t|<1 GeV^{2} and compared to the predictions by different models. Models that ignore quantum fluctuations of the gluon density in the colliding hadron predict a |t| dependence of the cross section much steeper than in data. The inclusion of such fluctuations in the same models provides a better description of the data

Measurement of inclusive charged-particle jet production in pp and p-Pb collisions at sqrt(s_NN) = 5.02 TeV

Measurements of inclusive charged-particle jet production in pp and p-Pb col- lisions at center-of-mass energy per nucleon-nucleon collision √sNN = 5.02 TeV and the corresponding nuclear modification factor Rch jet pPb are presented, using data collected with the ALICE detector at the LHC. Jets are reconstructed in the central rapidity region |ηjet| < 0.5 from charged particles using the anti-kT algorithm with resolution parameters R = 0.2, 0.3, and 0.4. The pT-differential inclusive production cross section of charged-particle jets, as well as the corresponding cross section ratios, are reported for pp and p-Pb collisions in the trans- verse momentum range 10 < pch T,jet < 140 GeV/c and 10 < pch T,jet < 160 GeV/c, respectively, together with the nuclear modification factor Rch jet pPb in the range 10 < pch T,jet < 140 GeV/c. The analysis extends the pT range of the previously-reported charged-particle jet measure- ments by the ALICE Collaboration. The nuclear modification factor is found to be consistent with one and independent of the jet resolution parameter with the improved precision of this study, indicating that the possible influence of cold nuclear matter effects on the production cross section of charged-particle jets in p-Pb collisions at √sNN = 5.02 TeV is smaller than the current precision. The obtained results are in agreement with other minimum bias jet measurements available for RHIC and LHC energies, and are well reproduced by the NLO perturbative QCD Powheg calculations with parton shower provided by Pythia8 as well as by Jetscape simulations

Measurements of Chemical Potentials in Pb-Pb Collisions at sNN=5.02 TeV

: This Letter presents the most precise measurement to date of the matter-antimatter imbalance at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair sqrt[s_{NN}]=5.02 TeV. Using the Statistical Hadronization framework, it is possible to obtain the value of the electric charge and baryon chemical potentials, μ_{Q}=-0.18±0.90 MeV and μ_{B}=0.71±0.45 MeV, with unprecedented precision. A centrality-differential study of the antiparticle-to-particle yield ratios of charged pions, protons, Ω baryons, and light (hyper)nuclei is performed. These results indicate that the system created in Pb-Pb collisions at the LHC is on average baryon-free and electrically neutral at midrapidity

Measurement of the radius dependence of charged-particle jet suppression in Pb–Pb collisions at sqrt(s_NN) = 5.02 TeV

The ALICE Collaboration reports a differential measurement of inclusive jet suppression using pp and Pb-Pb collision data at a center -of -mass energy per nucleon-nucleon collision root SNN = 5.02 TeV. Charged -particle jets are reconstructed using the anti-kT algorithm with resolution parameters R = 0.2, 0.3, 0.4, 0.5, and 0.6 in pp collisions and R = 0.2, 0.4, 0.6 in central (0-10%), semi -central (30-50%), and peripheral (60-80%) Pb-Pb collisions. A novel approach based on machine learning is employed to mitigate the influence of jet background. This enables measurements of inclusive jet suppression in new regions of phase space, including down to the lowest jet pT >= 40 GeV/c at R = 0.6 in central Pb-Pb collisions. This is an important step for discriminating different models of jet quenching in the quark-gluon plasma. The transverse momentum spectra, nuclear modification factors, derived cross section, and nuclear modification factor ratios for different jet resolution parameters of charged -particle jets are presented and compared to model predictions. A mild dependence of the nuclear modification factor ratios on collision centrality and resolution parameter is observed. The results are compared to a variety of jet -quenching models with varying levels of agreement

Multiplicity and event-scale dependent flow and jet fragmentation in pp collisions at s \sqrt{s} = 13 TeV and in p–Pb collisions at sNN \sqrt{s_{\textrm{NN}}} = 5.02 TeV

Abstract Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at $$\sqrt{s}$$ s = 13 TeV and p–Pb collisions at $$\sqrt{s_{\textrm{NN}}}$$ s NN = 5.02 TeV. The correlation functions are measured as a function of relative azimuthal angle ∆φ and pseudorapidity separation ∆η for pairs of primary charged particles within the pseudorapidity interval |η| &lt; 0.9 and the transverse-momentum interval 1 &lt; pT&lt; 4 GeV/c. Flow coefficients are extracted for the long-range correlations (1.6 &lt; |∆η| &lt; 1.8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.</jats:p

Photoproduction of K+ K- Pairs in Ultraperipheral Collisions

K^{+}K^{-} pairs may be produced in photonuclear collisions, either from the decays of photoproduced φ(1020) mesons or directly as nonresonant K^{+}K^{-} pairs. Measurements of K^{+}K^{-} photoproduction probe the couplings between the φ(1020) and charged kaons with photons and nuclear targets. The kaon-proton scattering occurs at energies far above those available elsewhere. We present the first measurement of coherent photoproduction of K^{+}K^{-} pairs on lead ions in ultraperipheral collisions using the ALICE detector, including the first investigation of direct K^{+}K^{-} production. There is significant K^{+}K^{-} production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range 1.1<1.4 GeV/c^{2} above the φ(1020) resonance, for rapidity |y_{KK}|<0.8 and p_{T,KK}<0.1 GeV/c, the measured coherent photoproduction cross section is dσ/dy=3.37±0.61(stat)±0.15(syst) mb. The center-of-mass energy per nucleon of the photon-nucleus (Pb) system W_{γPb,n} ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for φ(1020) photoproduction alone. The mass spectrum is fit to a cocktail consisting of φ(1020) decays, direct K^{+}K^{-} photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct K^{+}K^{-} photoproduction are presented

Measurement of the fraction of jet longitudinal momentum carried by <math display="inline"><msubsup><mi mathvariant="normal">Λ</mi><mi>c</mi><mo>+</mo></msubsup></math> baryons in <math display="inline"><mi>p</mi><mi>p</mi></math> collisions

International audienceRecent measurements of charm-baryon production in hadronic collisions have questioned the universality of charm-quark fragmentation across different collision systems. In this work the fragmentation of charm quarks into charm baryons is probed, by presenting the first measurement of the longitudinal jet momentum fraction carried by Λc+ baryons, z∥ch, in hadronic collisions. The results are obtained in proton-proton (pp) collisions at s=13  TeV at the LHC, with Λc+ baryons and charged (track-based) jets reconstructed in the transverse momentum intervals of 3≤pTΛc+&lt;15  GeV/c and 7≤pTjet ch&lt;15  GeV/c, respectively. The z∥ch distribution is compared to a measurement of D0-tagged charged jets in pp collisions as well as to pythia 8 simulations. The data hints that the fragmentation of charm quarks into charm baryons is softer with respect to charm mesons, in the measured kinematic interval, as predicted by hadronization models which include color correlations beyond leading-color in the string formation