Search for: All records

A<sc>bstract</sc> The yields of prompt and non-prompt J/ψand the fraction of non-prompt J/ψare measured at midrapidity (|y| < 0.9) via the dielectron decay channel as a function of the midrapidity charged-particle multiplicity (|η| < 0.9) in pp collisions at$$\sqrt{s}=13$$TeV. The J/ψyields and the multiplicity are normalized by their average value in inelastic collisions. The multiplicity-dependent yield ratio between prompt J/ψand D⁰is reported. The multiplicity is further divided into three azimuthal regions with respect to the J/ψmomentum: toward the J/ψemission direction, transverse, or opposite to it. A stronger-than-linear increase of the self-normalized yields is observed for both prompt and non-prompt J/ψproduction, with similar trends. This behaviour is also observed in the toward region, while a weaker increase is observed in the transverse and away regions. 

Abstract Two-particle angular correlations probe particle production mechanisms and the underlying event-wide phenomena present in hadronic collisions. The correlations are examined as a function of rapidity and azimuthal-angle differences ($$\Delta y, \Delta \varphi $$ $\Delta y,\Delta \phi$) for pairs of like- and unlike-sign pions, kaons, and (anti-)protons produced in pp collisions at$$\sqrt{s}$$ $\sqrt{s}$= 13 TeV, measured by the ALICE experiment. Two-particle correlation functions are provided together with$$\Delta y$$ $\Delta y$and$$\Delta \varphi $$ $\Delta \phi$projections and compared to Monte Carlo (MC) model predictions. For the first time, the measurement is performed as a function of the event’s charged-particle density. Previous studies conducted for pp collisions at$$\sqrt{s}$$ $\sqrt{s}$= 7 TeV at ALICE revealed a near-side anticorrelation for baryon–baryon and antibaryon–antibaryon pairs, whose origin remains unresolved. Here, an additional approach is introduced to study the multiplicity dependence and the expected inverse multiplicity scaling of the correlation function. This method highlights qualitative differences in the underlying sources of correlations, such as quantum-statistics effects, final-state interactions, and resonance decays. The puzzling near-side anticorrelation in baryon baryon measurements is observed across all multiplicity classes and continues to challenge current particle-production models. Furthermore, the multiplicity dependence of the correlations between mesons provides an independent probe of the sensitivity of current MC models to soft-QCD effects and hadronization dynamics. The presented measurements, together with the baryon results, enrich the experimental picture of two-particle correlations in pp collisions and serve as valuable input for ongoing theoretical developments. 

A<sc>bstract</sc> Measurements of transverse momentum (p_T) and pseudorapidity (η) dependent flow vector fluctuations in p–Pb collisions at$$\sqrt{{s}_{NN}}=5.02$$TeV at the CERN Large Hadron Collider are presented. By studying long-range two-particle correlations with a template fit method, potential biases from non-flow effects such as jets and resonance decays are effectively suppressed. Significantp_T- andη-dependent fluctuations of the second-harmonic flow vector are observed with more than 5σconfidence in p–Pb collisions, similar to the observations in Pb–Pb collisions. The influence of residual non-flow effects has been evaluated and cannot account for the observed fluctuations, thereby confirming the observation of flow vector fluctuations in small collision systems at the LHC. Comparisons to model calculations from 3DGlauber+MUSIC+UrQMD and the parton transport model from AMPT are also presented. The measurements provide constraints on the theoretical modelling of the three-dimensional initial geometry and its event-by-event fluctuations, offering critical insights into the origin of collective flow in small collision systems at the LHC. 

A<sc>bstract</sc> The probability to observe a specific number of strange and multi-strange hadrons (n_S), denoted as P(n_S), is measured by ALICE at midrapidity (|y|<0.5) in$$\sqrt{s}=5.02$$TeV proton-proton (pp) collisions, dividing events into several multiplicity-density classes. Exploiting, for the first time, a technique based on counting the number of strange-particle candidates event-by-event, this measurement allows one to extend the study of strangeness production beyond the mean of the distribution. This constitutes a new test bench for production mechanisms, probing events with a large imbalance between strange and non-strange content. The analysis of a large-statistics data sample makes it possible to extract P(n_S) up to a maximumn_Sof 7 for$${\text{K}}_{\text{S}}^{0}$$, 5 for Λ and$$\overline{\Lambda }$$, 4 for Ξ⁻and$${\overline{\Xi } }^{+}$$, and 2 for Ω⁻and$${\overline{\Omega } }^{+}$$. From this, the probability of producing strange hadron multiplets per event is calculated, thereby enabling the extension of the study of strangeness enhancement to extreme situations where several strange quarks hadronize in a single event at midrapidity. Moreover, comparing hadron combinations with differentuanddquark compositions and equal overallsquark content, the contribution to the enhancement pattern coming from non-strangeness related mechanisms is isolated. The results are compared with state-of-the-art phenomenological models implemented in commonly used Monte Carlo event generators, including PYTHIA 8 Monash 2013, PYTHIA 8 with QCD-based Color Reconnection and Rope Hadronization (QCD-CR + Ropes), and EPOS LHC, which incorporates both partonic interactions and hydrodynamic evolution. These comparisons show that the new approach dramatically enhances the sensitivity to the different underlying physics mechanisms modeled by each generator. 

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. 

Abstract The measurement of$$\Sigma ^{+}$$ ${\Sigma }^{+}$production in pp collisions at$$\sqrt{s}=13$$ $\sqrt{s}=13$ TeV is presented. The measurement is performed at midrapidity in both minimum-bias and high-multiplicity pp collisions at$$\sqrt{s} =13$$ $\sqrt{s}=13$ TeV. The$$\Sigma ^{+}$$ ${\Sigma }^{+}$is reconstructed via its weak-decay topology in the decay channel$$\Sigma ^{+} \rightarrow \mathrm{{p}} + \pi ^{0}$$ ${\Sigma }^{+}\to p+{\pi }^0$with$$\pi ^{0} \rightarrow \gamma + \gamma .$$ ${\pi }^0\to \gamma +\gamma .$In a novel approach, the neutral pion is reconstructed by combining photons that convert in the detector material with photons measured in the calorimeters. The transverse-momentum$$(p_{\textrm{T}})$$ $\left(p_{\text{T}}\right)$distributions of the$$\Sigma ^{+}$$ ${\Sigma }^{+}$and its rapidity densities$${\textrm{d}}N$$ $\text{d}N$/$${\textrm{d}}y$$ $\text{d}y$in both event classes are reported. The$$p_{\textrm{T}}$$ $p_{\text{T}}$spectrum in minimum-bias collisions is compared to QCD-inspired event generators. The ratio of$$\Sigma ^{+}$$ ${\Sigma }^{+}$to previously measured$$\Lambda $$ $\Lambda$baryons is in good agreement with calculations from the Statistical Hadronization Model. The high efficiency and purity of the novel reconstruction method for$$\Sigma ^{+}$$ ${\Sigma }^{+}$presented here will enable future studies of the interaction of$$\Sigma ^{+}$$ ${\Sigma }^{+}$with protons in the context of femtoscopic measurements, which could be crucial for understanding the equation of state of neutron stars.