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

Charm production and fragmentation fractions at midrapidity in pp collisions at s s \sqrt{\textrm{s}} = 13 TeV

Abstract Measurements of the production cross sections of prompt D0, D+, D*+, D s + $${\textrm{D}}_{\textrm{s}}^{+}$$ , Λ c + $${\Lambda}_{\textrm{c}}^{+}$$ , and Ξ c + $${\Xi}_{\textrm{c}}^{+}$$ charm hadrons at midrapidity in proton-proton collisions at s $$\sqrt{s}$$ = 13 TeV with the ALICE detector are presented. The D-meson cross sections as a function of transverse momentum (p T) are provided with improved precision and granularity. The ratios of p T-differential meson production cross sections based on this publication and on measurements at different rapidity and collision energy provide a constraint on gluon parton distribution functions at low values of Bjorken-x (10 −5–10 −4). The measurements of Λ c + $${\Lambda}_{\textrm{c}}^{+}$$ ( Ξ c + $${\Xi}_{\textrm{c}}^{+}$$ ) baryon production extend the measured p T intervals down to p T = 0(3) GeV/c. These measurements are used to determine the charm-quark fragmentation fractions and the c c ¯ $$\textrm{c}\overline{\textrm{c}}$$ production cross section at midrapidity (|y| < 0.5) based on the sum of the cross sections of the weakly-decaying ground-state charm hadrons D0, D+, D s + $${\textrm{D}}_{\textrm{s}}^{+}$$ , Λ c + $${\Lambda}_{\textrm{c}}^{+}$$ , Ξ c 0 $${\Xi}_{\textrm{c}}^0$$ and, for the first time, Ξ c + $${\Xi}_{\textrm{c}}^{+}$$ , and of the strongly-decaying J/ψ mesons. The first measurements of Ξ c + $${\Xi}_{\textrm{c}}^{+}$$ and Σ c 0 , + + $${\Sigma}_{\textrm{c}}^{0,++}$$ fragmentation fractions at midrapidity are also reported. A significantly larger fraction of charm quarks hadronising to baryons is found compared to e+e − and ep collisions. The c c ¯ $$\textrm{c}\overline{\textrm{c}}$$ production cross section at midrapidity is found to be at the upper bound of state-of-the-art perturbative QCD calculations

Charged-particle production as a function of the relative transverse activity classifier in pp, p–Pb, and Pb–Pb collisions at the LHC

Measurements of charged-particle production in pp, p–Pb, and Pb–Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defined event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum pTtrig in the range 8 < 15 GeV/c. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and final-state radiation. The relative transverse activity classifier, RT=NchT/NchT, is used to group events according to their UE activity, where NchT is the charged-particle multiplicity per event in the transverse region and NchT is the mean value over the whole analysed sample. The energy dependence of the RT distributions in pp collisions at s = 2.76, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle pT spectra as a function of RT in the three azimuthal regions in pp, p–Pb, and Pb–Pb collisions at sNN = 5.02 TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like effects in small collision systems (pp and p–Pb)

System-size dependence of the hadronic rescattering effect at energies available at the CERN Large Hadron Collider

The first measurements of K∗(892)0 resonance production as a function of charged-particle multiplicity in Xe-Xe collisions at sNN=5.44 TeV and pp collisions ats=5.02 TeV using the ALICE detector are presented. The resonance is reconstructed at midrapidity (|y| < 0.5) using the hadronic decay channel K∗0 →K±π∓. Measurements of transverse-momentum integrated yield, mean transverse-momentum, nuclear modification factor of K∗0, and yield ratios of resonance to stable hadron (K∗0/K) are compared across different collision systems (pp, p-Pb, Xe-Xe, and Pb-Pb) at similar collision energies to investigate how the production of K∗0 resonances depends on the size of the system formed in these collisions. The hadronic rescattering effect is found to be independent of the size of colliding systems and mainly driven by the produced charged-particle multiplicity, which is a proxy of the volume of produced matter at the chemical freeze-out. In addition, the production yields of K∗0 in Xe-Xe collisions are utilized to constrain the dependence of the kinetic freeze-out temperature on the system size using the hadron resonance gas-partial chemical equilibrium model

K*(892)± resonance production in Pb-Pb collisions at sNN =5.02 TeV

The production of K∗(892)± meson resonance is measured at midrapidity (|y|8 GeV/c, consistent with measurements for other light-flavored hadrons. The smallest values are observed in most central collisions, indicating larger energy loss of partons traversing the dense medium

Femtoscopic correlations of identical charged pions and kaons in pp collisions at s =13 TeV with event-shape selection

Collective behavior has been observed in high-energy heavy-ion collisions for several decades. Collectivity is driven by the high particle multiplicities that are produced in these collisions. At the CERN Large Hadron Collider (LHC), features of collectivity have also been seen in high-multiplicity proton-proton collisions that can attain particle multiplicities comparable to peripheral Pb-Pb collisions. One of the possible signatures of collective behavior is the decrease of femtoscopic radii extracted from pion and kaon pairs emitted from high-multiplicity collisions with increasing pair transverse momentum. This decrease can be described in terms of an approximate transverse mass scaling. In the present work, femtoscopic analyses are carried out by the ALICE Collaboration on charged pion and kaon pairs produced in pp collisions at s=13TeV from the LHC to study possible collectivity in pp collisions. The event-shape analysis method based on transverse sphericity is used to select for spherical versus jetlike events, and the effects of this selection on the femtoscopic radii for both charged pion and kaon pairs are studied. This is the first time this selection method has been applied to charged kaon pairs. An approximate transverse-mass scaling of the radii is found in all multiplicity ranges studied when the difference in the Lorentz boost for pions and kaons is taken into account. This observation does not support the hypothesis of collective expansion of hot and dense matter that should only occur in high-multiplicity events. A possible alternate explanation of the present results is based on a scenario of common emission conditions for pions and kaons in pp collisions for the multiplicity ranges studied

Prompt and non-prompt J/ψ production at midrapidity in Pb–Pb collisions at sNN \sqrt{s_{\textrm{NN}}} = 5.02 TeV

Abstract The transverse momentum (pT) and centrality dependence of the nuclear modification factor RAA of prompt and non-prompt J/ψ, the latter originating from the weak decays of beauty hadrons, have been measured by the ALICE collaboration in Pb–Pb collisions at $$\sqrt{s_{\textrm{NN}}}$$ s NN = 5.02 TeV. The measurements are carried out through the e+e− decay channel at midrapidity (|y| &lt; 0.9) in the transverse momentum region 1.5 &lt; pT &lt; 10 GeV/c. Both prompt and non-prompt J/ψ measurements indicate a significant suppression for pT &gt; 5 GeV/c, which becomes stronger with increasing collision centrality. The results are consistent with similar LHC measurements in the overlapping pT intervals, and cover the kinematic region down to pT = 1.5 GeV/c at midrapidity, not accessible by other LHC experiments. The suppression of prompt J/ψ in central and semicentral collisions exhibits a decreasing trend towards lower transverse momentum, described within uncertainties by models implementing J/ψ production from recombination of c and $$\overline{\textrm{c}}$$ c ¯ quarks produced independently in different partonic scatterings. At high transverse momentum, transport models including quarkonium dissociation are able to describe the suppression for prompt J/ψ. For non-prompt J/ψ, the suppression predicted by models including both collisional and radiative processes for the computation of the beauty-quark energy loss inside the quark-gluon plasma is consistent with measurements within uncertainties.</jats:p

Light-flavor particle production in high-multiplicity pp collisions at s s \sqrt{\textrm{s}} = 13 TeV as a function of transverse spherocity

Abstract Results on the transverse spherocity dependence of light-flavor particle production (π, K, p, ϕ, K*0, K S 0 $${\textrm{K}}_{\textrm{S}}^0$$ , Λ, Ξ) at midrapidity in high-multiplicity pp collisions at s $$\sqrt{s}$$ = 13 TeV were obtained with the ALICE apparatus. The transverse spherocity estimator S O p T = 1 $$\left({S}_{\textrm{O}}^{p_{\textrm{T}}=1}\right)$$ categorizes events by their azimuthal topology. Utilizing narrow selections on S O p T = 1 $${S}_{\textrm{O}}^{p_{\textrm{T}}=1}$$ , it is possible to contrast particle production in collisions dominated by many soft initial interactions with that observed in collisions dominated by one or more hard scatterings. Results are reported for two multiplicity estimators covering different pseudorapidity regions. The S O p T = 1 $${S}_{\textrm{O}}^{p_{\textrm{T}}=1}$$ estimator is found to effectively constrain the hardness of the events when the midrapidity (|η| < 0.8) estimator is used. The production rates of strange particles are found to be slightly higher for soft isotropic topologies, and severely suppressed in hard jet-like topologies. These effects are more pronounced for hadrons with larger mass and strangeness content, and observed when the topological selection is done within a narrow multiplicity interval. This demonstrates that an important aspect of the universal scaling of strangeness enhancement with final-state multiplicity is that high-multiplicity collisions are dominated by soft, isotropic processes. On the contrary, strangeness production in events with jet-like processes is significantly reduced. The results presented in this article are compared with several QCD-inspired Monte Carlo event generators. Models that incorporate a two-component phenomenology, either through mechanisms accounting for string density, or thermal production, are able to describe the observed strangeness enhancement as a function of S O p T = 1 $${S}_{\textrm{O}}^{p_{\textrm{T}}=1}$$

Measurements of long-range two-particle correlation over a wide pseudorapidity range in p–Pb collisions at sNN \sqrt{s_{\textrm{NN}}} = 5.02 TeV

Abstract Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the “ridge” phenomenon, were discovered in heavy-ion collisions, and later found in pp and p–Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small collision systems. In this Letter, measurements of the long-range correlations in p–Pb collisions at $$\sqrt{s_{\textrm{NN}}}$$ s NN = 5.02 TeV are extended to a pseudorapidity gap of ∆η ~ 8 between particles using the ALICE forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of ∆η ~ 8 for the first time in p–Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small collision systems such as p–Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient, v2(η), is extracted from the long-range correlations. The v2(η) results are presented for a wide pseudorapidity range of –3.1 &lt; η &lt; 4.8 in various centrality classes in p–Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small collision systems, the v2(η) measurements are compared with hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small collision systems.</jats:p

Charged-particle production as a function of the relative transverse activity classifier in pp, p–Pb, and Pb–Pb collisions at the LHC

Abstract Measurements of charged-particle production in pp, p–Pb, and Pb–Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defined event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum $$\left({p}_{\textrm{T}}^{\textrm{trig}}\right)$$ p T trig in the range 8 &lt;$${p}_{\textrm{T}}^{\textrm{trig}}$$ p T trig &lt; 15 GeV/c. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and final-state radiation. The relative transverse activity classifier, $${R}_{\textrm{T}}={N}_{\textrm{ch}}^{\textrm{T}}/\left\langle {N}_{\textrm{ch}}^{\textrm{T}}\right\rangle$$ R T = N ch T / N ch T , is used to group events according to their UE activity, where $${N}_{\textrm{ch}}^{\textrm{T}}$$ N ch T is the charged-particle multiplicity per event in the transverse region and $$\left\langle {N}_{\textrm{ch}}^{\textrm{T}}\right\rangle$$ N ch T is the mean value over the whole analysed sample. The energy dependence of the RT distributions in pp collisions at $$\sqrt{s}$$ s = 2.76, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle pT spectra as a function of RT in the three azimuthal regions in pp, p–Pb, and Pb–Pb collisions at $$\sqrt{s_{\textrm{NN}}}$$ s NN = 5.02 TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like effects in small collision systems (pp and p–Pb).</jats:p