Search for: All records

Abstract Quantum Chromodynamics predicts a phase transition from hadronic matter to quark–gluon plasma (QGP) at high temperatures and energy densities, where quarks and gluons (partons) are no longer confined within hadrons. The QGP forms in ultrarelativistic heavy-ion collisions. Anisotropic flow coefficients, quantifying the azimuthal expansion of produced matter, probe QGP properties. Flow measurements in high-energy heavy-ion collisions show a distinctive grouping of anisotropic flow for baryons and mesons at intermediate transverse momentum – a feature associated with flow imparted at the quark level, confirming QGP existence. The observation of QGP-like features in proton–proton and proton–ion collisions has sparked debate about QGP formation in smaller systems. For the first time, we demonstrate the distinctive grouping of anisotropic flow for baryons and mesons in high-multiplicity proton–lead and proton–proton collisions at the Large Hadron Collider (LHC). These results are described by a model including hydrodynamic flow followed by hadron formation via quark coalescence, consistent with the formation of partonic flowing systems in these collisions. 

A<sc>bstract</sc> The measurement of three-dimensional femtoscopic correlations between identical charged kaons (K^±K^±) produced in p–Pb collisions at center-of-mass energy per nucleon pair$$\sqrt{{s}_{\text{NN}}}=5.02$$TeV with ALICE at the LHC is presented for the first time. This measurement, supplementary to those in pp and Pb–Pb collisions, allows understanding the particle-production mechanisms at different charged-particle multiplicities and provides information on the dynamics of the source of particles created in p–Pb collisions, for which a general consensus does not yet exist. It is shown that the measured source sizes increase with charged-particle multiplicity and decrease with increasing pair transverse momentum. These trends for K^±K^±are similar to the ones observed earlier in identical charged-pion and$${\text{K}}_{\text{s}}^{0}{\text{K}}_{\text{s}}^{0}$$correlations in Pb–Pb collisions at various energies and inπ^±π^±correlations in p–Pb collisions at$$\sqrt{{s}_{\text{NN}}}=5.02$$TeV. At comparable multiplicity, the source sizes measured in p–Pb collisions agree within uncertainties with those observed in pp collisions, and there is an indication that they are smaller than those observed in Pb–Pb collisions. The obtained results are also compared with predictions from the hadronic interaction model EPOS 3, which tends to underestimate the source size for the most central collisions and agrees with the data for semicentral and peripheral events. Furthermore, the time of maximal emission for kaons is extracted. It turns out to be comparable with the value obtained in highly peripheral Pb–Pb collisions at the same energy, indicating that the kaon emission evolution is similar to that in p–Pb collisions. 

This paper presents the first measurement of the angle between different jet axes (denoted as $\mathrm{\Delta }{R}_{\mathrm{axis}}$) in $\text{Pb-Pb}$collisions. The measurement is carried out in the 0–10 % most-central events at $\sqrt{s_{NN}}=5.02$TeV. Jets are assembled by clustering charged particles at midrapidity using the $\text{anti-}{k}_{\mathrm{T}}$algorithm with resolution parameters $R=0.2$and 0.4 and transverse momenta in the intervals $40<p_{\mathrm{T}}^{\mathrm{ch}\mathrm{jet}}<140\mathrm{GeV}/c$and $80<p_{\mathrm{T}}^{\mathrm{ch}\mathrm{jet}}<140\mathrm{GeV}/c$, respectively. Measurements at these low transverse momenta enhance the sensitivity to quark-gluon plasma (QGP) effects. A comparison to models implementing various mechanisms of jet energy loss in the QGP suggests that the observed narrowing of the $\text{Pb-Pb}$distribution relative to $\mathit{pp}$can be explained if quark-initiated jets are more likely to emerge from the medium than gluon-initiated jets. These new measurements disfavor intrajet $p_{\mathrm{T}}$broadening as implemented in a simple model calculation with the Baier-Dokshitzer-Mueller-Peigne-Schiff formalism for energy loss in the QGP. The comparison of $\text{Pb-Pb}$and $\mathit{pp}$collisions shows sensitivity to the angular scale at which the QGP can resolve two independent splittings, favoring mechanisms that incorporate incoherent energy loss. 

Abstract The transverse momentum spectra and integrated yields of anti-$$\Sigma $$ $\Sigma$hyperons ($$\overline{\Sigma }^{\pm } $$ ${\overline{\Sigma }}^{\pm }$) have been measured in$$\text {pp}$$ $\text{pp}$and$$\text {p}{-}\text {Pb}$$ $\text{p}-\text{Pb}$collisions at$$\sqrt{s_{\textrm{NN}}}=5.02$$ $\sqrt{s_{\text{NN}}}=5.02$TeV with the ALICE experiment. Measurements are performed via the newly accessed decay channel$$\overline{\Sigma }^{\pm } \!\!\rightarrow \mathrm{\overline{n}} \pi ^{\pm }$$ ${\overline{\Sigma }}^{\pm }\to \overline{n}{\pi }^{\pm }$. A new method of antineutron reconstruction with the PHOS electromagnetic spectrometer is developed and applied to this analysis. The$$p_{\textrm{T}}$$ $p_{\text{T}}$spectra of$$\overline{\Sigma }^{\pm } $$ ${\overline{\Sigma }}^{\pm }$are measured in the range$$0.5<3$$ $0.5<p_{\text{T}}<3$GeV/$$c$$ $c$and compared to predictions of the PYTHIA 8, DPMJET, PHOJET, EPOS LHC and EPOS4 models. The EPOS LHC and EPOS4 models provide the best descriptions of the measured spectra both in$$\text {pp}$$ $\text{pp}$and$$\text {p}{-}\text {Pb}$$ $\text{p}-\text{Pb}$collisions, while models which do not account for multiparton interactions provide a considerably worse description at high$$p_{\textrm{T}}$$ $p_{\text{T}}$. The total yields of$$\overline{\Sigma }^{\pm } $$ ${\overline{\Sigma }}^{\pm }$in both$$\text {pp}$$ $\text{pp}$and$$\text {p}{-}\text {Pb}$$ $\text{p}-\text{Pb}$collisions are compared to predictions of the Thermal-FIST model and dynamical models PYTHIA 8, DPMJET, PHOJET, EPOS LHC and EPOS4. All models reproduce the total yields in both colliding systems within uncertainties. The nuclear modification factors$$R_\textrm{pPb}$$ $R_{\text{pPb}}$for both$$\overline{\Sigma }^{+} $$ ${\overline{\Sigma }}^{+}$and$$\overline{\Sigma }^{-} $$ ${\overline{\Sigma }}^{-}$are evaluated and compared to those of protons,$$\Lambda $$ $\Lambda$and$$\Xi $$ $\Xi$hyperons, and predictions of EPOS LHC and EPOS4 models. No deviations of$$R_\textrm{pPb}$$ $R_{\text{pPb}}$for$$\overline{\Sigma }^{\pm } $$ ${\overline{\Sigma }}^{\pm }$from the model predictions or measurements for other hadrons are found within uncertainties.