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  1. Photoproduction of K + K Pairs in Ultraperipheral Collisions

    >more » K+K−ϕ(1020)K+K−K+K−ϕ(1020)K+K−K+K−K+K−1.1<MKK<1.4  GeV/c2ϕ(1020)|yKK|<0.8pT,KK<0.1  GeV/cdσ/dy=3.37±0.61(stat)±0.15(syst)  mbWγPb,nϕ(1020)ϕ(1020)K+K−K+K−« less
  2. Measurement of the fraction of jet longitudinal momentum carried by Λ c + baryons in p p collisions

    Recenmore » t 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 Λ c + baryons, z ch , in hadronic collisions. The results are obtained in proton-proton ( p p ) collisions at s = 13 TeV at the LHC, with Λ c + baryons and charged (track-based) jets reconstructed in the transverse momentum intervals of 3 p T Λ c + < 15 GeV / c and 7 p T jet ch < 15 GeV / c , respectively. The z ch distribution is compared to a measurement of D 0 -tagged charged jets in p p collisions as well as to 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. © 2024 CERN, for the ALICE Collaboration 2024 CERN« less
  3. ALICE luminosity determination for Pb–Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV

    Luminosity determination within the ALICE experiment is based on the measurement, in van der Meer scans, of the cross sections for visible processes involving one or more detectors (visible cross sections). In 2015 and 2018, the Large Hadron Collider provided Pb–Pb collisions at a centre-of-mass energy per nucleon pair ofmore » $$\sqrt{s_{NN}}$$ = 5.02 TeV. Two visible cross sections, associated with particle detection in the Zero Degree Calorimeter (ZDC) and in the V0 detector, were measured in a van der Meer scan. This article describes the experimental set-up and the analysis procedure, and presents the measurement results. The analysis involves a comprehensive study of beam-related effects and an improved fitting procedure, compared to previous ALICE studies, for the extraction of the visible cross section. The resulting uncertainty of both the ZDC-based and the V0-based luminosity measurement for the full sample is 2.5%. The inelastic cross section for hadronic interactions in Pb–Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV, obtained by efficiency correction of the V0-based visible cross section, was measured to be 7.67 ± 0.25 b, in agreement with predictions using the Glauber model.« less
  4. Skewness and kurtosis of mean transverse momentum fluctuations at the LHC energies

    The first measurements of skewness and kurtosis of mean transverse momentum (more » $$\langle$$$$p_T\rangle$$) fluctuations are reported in Pb–Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV, Xe–Xe collisions at $$\sqrt{s_{NN}}$$ = 5.44 TeV and pp collisions at $$\sqrt{s}$$ = 5.02 TeV using the ALICE detector. The measurements are carried out as a function of system size $$\langle$$dNch/dη$$\rangle$$$$^{1/3}_{|η|<0.5}$$, using charged particles with transverse momentum ($$p_T$$) and pseudorapidity (η), in the range 0.2<$$p_T$$<3.0 GeV/c and |η|<0.8, respectively. In Pb–Pb and Xe–Xe collisions, positive skewness is observed in the fluctuations of $$\langle$$$$p_T\rangle$$ for all centralities, which is significantly larger than what would be expected in the scenario of independent particle emission. This positive skewness is considered a crucial consequence of the hydrodynamic evolution of the hot and dense nuclear matter created in heavy-ion collisions. Furthermore, similar observations of positive skewness for minimum bias pp collisions are also reported here. Kurtosis of $$\langle$$$$p_T\rangle$$ fluctuations is found to be in good agreement with the kurtosis of Gaussian distribution, for most central Pb–Pb collisions. Hydrodynamic model calculations with MUSIC using Monte Carlo Glauber initial conditions are able to explain the measurements of both skewness and kurtosis qualitatively from semicentral to central collisions in Pb–Pb system. Color reconnection mechanism in PYTHIA8 model seems to play a pivotal role in capturing the qualitative behavior of the same measurements in pp collisions.« less
  5. Femtoscopic correlations of identical charged pions and kaons in $pp$ collisions at $$\sqrt{s}$$ = 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 atmore » $$\sqrt{s}$$=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.« less
  6. Prompt and non-prompt J/$$ψ$$ production at midrapidity in Pb–Pb collisions at $$\sqrt{s_{\textrm{NN}}}$$ = 5.02 TeV

    The transverse momentum (pT) and centrality dependence of the nuclear modif cation 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 atmore » $$\sqrt{s_{\textrm{NN}}}$$ = 5.02 TeV. The measurements are carried out through the e+e- decay channel at midrapidity (|y| < 0.9) in the transverse momentum region 1.5 < pT < 10 GeV/c. Both prompt and non-prompt J/ψ measurements indicate a significant suppression for pT > 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 $$\overline{c}$$ 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.« less
  7. 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 atmore » $$\sqrt{s_{NN}}$$= 5.44 TeV and pp collisions at $$\sqrt{s}$$ = 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.« less
  8. ψ ( 2 S ) Suppression in Pb-Pb Collisions at the LHC

    The production of themore » $ψ(2S)$ charmonium state was measured with ALICE in Pb-Pb collisions at $$\sqrt{^sNN}$$ = $5.02$ $TeV$, in the dimuon decay channel. A significant signal was observed for the first time at LHC energies down to zero transverse momentum, at forward rapidity ($2.5 < y < 4$). The measurement of the ratio of the inclusive production cross sections of the $ψ(2S)$ and $J/ψ$ resonances is reported as a function of the centrality of the collisions and of transverse momentum, in the region p$$T$$ < 12 $GeV/c$. The results are compared with the corresponding measurements in $pp$ collisions, by forming the double ratio [σψ(2S)J/ψ]Pb-Pb/[σψ(2S))/σJ/ψ]$pp$. It is found that in Pb-Pb collisions the $ψ(2S)$ is suppressed by a factor of ~2 with respect to the $J/ψ$. The $ψ(2S)$ nuclear modification factor $$R$$AA was also obtained as a function of both centrality and p$$T$$. The results show that the $ψ(2S)$ resonance yield is strongly suppressed in Pb-Pb collisions, by a factor of up to ~3 with respect to $pp$. Comparisons of cross section ratios with previous Super Proton Synchrotron findings by the NA50 experiment and of $$R$$$AA$ with higher-p$$T$$ results at LHC energy are also reported. These results and the corresponding comparisons with calculations of transport and statistical models address questions on the presence and properties of charmonium states in the quark-gluon plasma formed in nuclear collisions at the LHC.« less
  9. Pseudorapidity dependence of anisotropic flow and its decorrelations using long-range multiparticle correlations in Pb–Pb and Xe–Xe collisions

    The pseudorapidity dependence of elliptic (v2), triangular (v3), and quadrangular (v4) flow coefficients of charged particles measured in Pb–Pb collisions at a centre-of-mass energy per nucleon pair ofmore » $$\sqrt{s_{NN}}=$$ 5.02 TeV and in Xe–Xe collisions at $$\sqrt{s_{NN}}=$$ 5.44 TeV with ALICE at the LHC are presented. The measurements are performed in the pseudorapidity range –3.5 < η < 5 for various centrality intervals using two- and multi-particle cumulants with the subevent method. The flow probability density function (p.d.f.) is studied with the ratio of flow coefficient v2 calculated with four- and two-particle cumulant, and suggests that the variance of flow p.d.f. is independent of pseudorapidity. The decorrelation of the flow vector in the longitudinal direction is probed using two-particle correlations. The results measured with respect to different reference regions in pseudorapidity exhibit differences, argued to be a result of saturating decorrelation effect above a certain pseudorapidity separation, in contrast to previous publications which assign this observation to non-flow effects. The results are compared to 3 + 1 dimensional hydrodynamic and the AMPT transport model calculations. Neither of the models is able to simultaneously describe the pseudorapidity dependence of measurements of anisotropic flow and its fluctuations. The results presented in this work highlight shortcomings in our current understanding of initial conditions and subsequent system expansion in the longitudinal direction. Therefore, they provide input for its improvement.« less
  10. 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 momentummore » $$(p^{trig}_T)$$ in the range 8 < $$p^{trig}_T$$< 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, $$R_T = N^T_{ch}/\langle N^T_{ch}\rangle$$, is used to group events according to their UE activity, where $$N^T_{ch}$$ is the charged-particle multiplicity per event in the transverse region and $$\langle N^T_{ch}\rangle$$ is the mean value over the whole analysed sample. The energy dependence of the RT distributions in pp collisions at $$\sqrt{s_{NN}}$$ = 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 $$\sqrt{s_{NN}}$$ = 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).« less
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