Two-particle angular correlations explore particle production mechanisms and underlying event-wide phenomena present in the systems created in hadronic collisions. These correlations are examined as a function of rapidity and azimuthal-angle differences ($Δy, Δφ$) for pairs of like- and unlike-sign pions, kaons, and (anti-)protons produced in pp collisions at $\sqrt{s}$ = 13 TeV, measured by the ALICE experiment. Two-particle correlation functions are provided, along with $Δy$ and $Δφ$ projections, and are 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. The shapes of the correlation functions are studied in detail for each particle pair. Previous studies conducted for pp collisions at $\sqrt{s}$ = 7 TeV at ALICE have revealed an anticorrelation at small relative angles for baryon-baryon and antibaryon-antibaryon pairs, whose origin remains an open question. In this work, an additional approach is introduced to study the multiplicity dependence of the correlation functions in more detail and reveal qualitative differences in the underlying sources of correlations, such as quantum statistics, final-state interactions, and resonance decays. The puzzling near-side anticorrelation in baryon-baryon measurements is observed across all multiplicity classes and remains a challenge for models of particle production in pp collisions. Furthermore, the multiplicity dependence of the correlations between mesons provides an independent means to explore 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.
2D (Δy,Δφ) rescaled two-particle correlation functions (C_R) for π⁺π⁺ + π⁻π⁻ in multiplicity class 0-0.17%, corresponding to panel a.
2D (Δy,Δφ) rescaled two-particle correlation functions (C_R) for π⁺π⁺ + π⁻π⁻ in multiplicity class 0-1%, corresponding to panel -.
2D (Δy,Δφ) rescaled two-particle correlation functions (C_R) for π⁺π⁺ + π⁻π⁻ in multiplicity class 1-5%, corresponding to panel -.
In this letter, the first measurement of the femtoscopic correlation of protons and $Σ^+$ hyperons is presented and used to study the p$-Σ^+$ interaction. The measurement is performed with the ALICE detector in high-multiplicity triggered pp collisions at $\sqrt{s} = 13$ TeV. The $Σ^+$ hyperons are reconstructed using a missing-mass approach in the decay channel to $\textrm{p} + π^0$ with $π^0\rightarrowγγ$, while both $Σ^+$ and protons are identified using a machine learning approach. These techniques result in a high reconstruction efficiency and purity, which allows the measurement of the p$-Σ^+$ correlation function for the first time. Thanks to the high significance achieved in the p$-Σ^+$ correlation signal, it is possible to discriminate between the predictions of different models of the N$-Σ$ interaction and to accomplish a first determination of the p$-Σ^+$ scattering parameters.
Raw correlation function $C(k^{*})$ of $\textrm{p}-\Sigma^+$ and $\bar{\textrm{p}}-\bar{\Sigma}^-$ in high-multiplicity triggered pp collisions at $\sqrt{{s}}=13$ TeV. The data points are shifted to the center of gravity of the mixed-event distribution.
Corrected (genuine) correlation function $C(k^{*})$ of $\textrm{p}-\Sigma^+$ and $\bar{\textrm{p}}-\bar{\Sigma}^-$ in high-multiplicity triggered pp collisions at $\sqrt{{s}}=13$ TeV. The data points are corrected by the genuine $\lambda$ parameter, assuming that the non-genuine correlation function is unity (flat). No momentum unfolding is applied to the data points. The data points are shifted to the center of gravity of the mixed-event distribution.
Meson-baryon systems with strangeness content provide a unique laboratory for investigating the strong interaction and testing theoretical models of hadron structure and dynamics. In this work, the measured correlation functions for oppositely charged $Ξ-{\rm K}$ and $Ξ-π$ pairs obtained in high-multiplicity pp collisions at $\sqrt{s} = 13$ TeV at the LHC are presented. For the first time, high-precision data on the $Ξ-{\rm K}$ interaction are delivered at small relative momenta. The scattering lengths, extracted via the Lednický-Lyuboshits expression of the pair wavefunction, indicate a repulsive and a shallow attractive strong interaction for the $Ξ-{\rm K}$ and $Ξ-π$ systems, respectively. The $Ξ(1620)$ and $Ξ(1690)$ states are observed in the $Ξ-π$ correlation function and their properties, mass and width, are determined. These measurements are in agreement with other available results. Such high-precision data can help refine the understanding of these resonant states, provide stronger constraints for chirally motivated potentials, and address the key challenge of describing the coupled-channel dynamics that may give rise to molecular configurations.
The $\Xi$K correlation function.
The genuine $\Xi$K correlation function.
The $\Xi\pi$ correlation function.
The measurement of $\Sigma^{+}$ production in pp collisions at $\sqrt{s} = 13$ TeV is presented. The measurement is performed at midrapidity in both minimum-bias and high-multiplicity pp collisions at $\sqrt{s} = 13$ TeV. The $\Sigma^{+}$ is reconstructed via its weak-decay topology in the decay channel $\Sigma^{+} \rightarrow {p} + \pi^{0}$ with $\pi^{0} \rightarrow \gamma + \gamma$. 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 ($p_{T}$) distributions of the $\Sigma^{+}$ and its rapidity densities d$N/$dy in both event classes are reported. The $p_{T}$ spectrum in minimum-bias collisions is compared to QCD-inspired event generators. The ratio of $\Sigma^{+}$ to previously measured $\Lambda$ baryons is in good agreement with calculations from the Statistical Hadronization Model. The high efficiency and purity of the novel reconstruction method for $\Sigma^{+}$ presented here will enable future studies of the interaction of $\Sigma^{+}$ with protons in the context of femtoscopic measurements, which could be crucial for understanding the equation of state of neutron stars.
$p_\mathrm{T}$-differential production yield of $\Sigma^+$ and $\bar{\Sigma}^-$ baryons in the high-multiplicity triggered pp collisions at $\sqrt{{s}}=13~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.8$.
$p_\mathrm{T}$-differential production yield of $\Sigma^+$ and $\bar{\Sigma}^-$ baryons in the minimum-bias triggered pp collisions at $\sqrt{{s}}=13~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.8$.
The first measurement at midrapidity ($|y| < 0.5$) of the production yield of the strange-charm baryons $Ξ_c^+$ and $Ξ_c^0$ as a function of transverse momentum ($p_{\rm T}$) in different charged-particle multiplicity classes in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ALICE experiment at the LHC is reported. The $Ξ_c^+$ baryon is reconstructed via the $Ξ_c^+ \rightarrow Ξ^-π^+π^+$ decay channel in the range $4 < p_{\rm T} < 12$ GeV/$c$, while the $Ξ_c^0$ baryon is reconstructed via both the $Ξ_c^0 \rightarrow Ξ^-π^+$ and $Ξ_c^0 \rightarrow Ξ^-e^+ν_e$ decay channels in the range $2 < p_{\rm T} < 12$ GeV/$c$. The baryon-to-meson ($Ξ_c^{0,+}/{\rm D}^0$) and the baryon-to-baryon ($Ξ_c^{0,+}/Λ_{\rm c}^+$) production yield ratios show no significant dependence on multiplicity. In addition, the observed yield ratios are not described by theoretical predictions that model charm-quark fragmentation based on measurements at $e^+e^-$ and $e^-$p colliders, indicating differences in the charm-baryon production mechanism in pp collisions. A comparison with different event generators and tunings, including different modelling of the hadronisation process, is also discussed. Moreover, the branching-fraction ratio of BR($Ξ_c^0 \rightarrow Ξ^-e^+ν_e$)/BR($Ξ_c^0 \rightarrow Ξ^-π^+$) is measured as 0.825 $\pm$ 0.094 (stat.) $\pm$ 0.081 (syst.). This value supersedes the previous ALICE measurement, improving the statistical precision by a factor of 1.6.
$p_{\rm T}$-differential per-event yield of prompt $\Xi_c^0$ baryons measured in the different multiplicity classes.
$p_{\rm T}$-differential per-event yield of prompt $\Xi_c^+$ baryons measured in the different multiplicity classes.
Ratio between the prompt $\Xi_c^0$ baryons in a multiplicity class to the multiplicity-integrated (INEL $>$ 0) class.
The dependence of f$_0$(980) production on the final-state charged-particle multiplicity is reported for proton-proton (pp) collisions at the centre-of-mass energy, $\sqrt{s}= 13$ TeV. The production of f$_0$(980) is measured with the ALICE detector via the f$_0(980) \rightarrow π^{+}π^{-}$ decay channel in a midrapidity region of $|y| < 0.5$. The evolution of the integrated yields and mean transverse momentum of f$_{0}$(980) as a function of charged-particle multiplicity measured in pp at $\sqrt{s} = 13$ TeV follows the trends observed in pp at $\sqrt{s} = 5.02$ TeV and in proton-lead (p-Pb) collisions at $\sqrt{s_{\rm{NN}}} = 5.02$ TeV. Particle yield ratios of f$_{0}$(980) to $π^{\pm}$ and K$^{*}$(892)$^{0}$ are found to decrease with increasing charged-particle multiplicity. These particle ratios are compared with calculations from the canonical statistical thermal model as a function of charged-particle multiplicity. The thermal model calculations provide a better description of the decreasing trend of particle ratios when no strange or antistrange quark composition for f$_{0}$(980) is assumed, which suggests that the tetraquark interpretation of the f$_{0}$(980) is disfavored.
Transverse momentum spectra in different multiplicity classes. Each spectrum is corrected for the branching ratio of (46 $\pm$ 6)% based on [Phys. Rev. Lett. 111 no. 6, (2013) 062001].
The ratio of transverse momentum spectrum to the INEL > 0 spectrum
Transverse momentum integrated f0(980) yield in pp collisions
The first results of K$^*$(892)$^{\pm}$ production at midrapidity ($|y| < 0.5$) in pp collisions at $\sqrt{s} = 13$ TeV as a function of the event multiplicity are presented. The K$^*$(892)$^{\pm}$ has been reconstructed via its hadronic decay channel K$^*$(892)$^{\pm} \rightarrow π^{\pm} + K_{\rm S}^0$ using the ALICE detector at the LHC. For each multiplicity class the differential transverse momentum ($p_{\rm T}$) spectrum, the mean transverse momentum $\langle p_{\rm T} \rangle$, the $p_{\rm T}$-integrated yield (d$N$/d$y$), and the ratio of the K$^*$(892)$^{\pm}$ to $K_{\rm S}^0$ yields are reported. These are consistent with previous K$^*$(892)$^0$ resonance results with a higher level of precision. Comparisons with phenomenological models such as PYTHIA6, PYTHIA8, EPOS-LHC, and DIPSY are also discussed. A first evidence of a significant K$^*$(892)$^{\pm}$/$K_{\rm S}^0$ suppression in pp collisions is observed at a 7$σ$ level passing from low to high multiplicity events. The ratios of the $p_{\rm T}$-differential yields of K$^*$(892)$^{\pm}$ and $K_{\rm S}^0$ in high and low multiplicity events are also presented along with their double ratio. For $p_{\rm T} \lesssim 2$ GeV/$c$ this double ratio persists below unity by more than $3σ$ suggesting that the suppression affects mainly low $p_{\rm T}$ resonances. The measured decreasing trend of the K$^*$(892)$^{\pm}$/$K_{\rm S}^0$ ratio with increasing multiplicity, which in heavy-ion collisions is typically attributed to the rescattering of decay particles of the short-lived resonances, is reproduced by the EPOS-LHC model without the use of hadronic afterburners.
K$^{*\pm}$ transverse momentum spectrum - V0M multiplicity class I, average of particle and antiparticle
K$^{*\pm}$ transverse momentum spectrum - V0M multiplicity class II, average of particle and antiparticle
K$^{*\pm}$ transverse momentum spectrum - V0M multiplicity class III, average of particle and antiparticle
The transverse momentum spectra and integrated yields of $\overlineΣ^{\pm}$ have been measured in pp and p-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV with the ALICE experiment. Measurements are performed via the newly accessed decay channel $\overlineΣ^{\pm} \rightarrow {\rm\overline{n}}π^{\pm}$. A new method of antineutron reconstruction with the PHOS electromagnetic spectrometer is developed and applied to this analysis. The $p_{\rm T}$ spectra of $\overlineΣ^{\pm}$ are measured in the range $0.5 < p_{\rm T} < 3$ GeV/$c$ 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 pp and p-Pb collisions, while models which do not account for multiparton interactions provide a considerably worse description at high $p_{\rm T}$. The total yields of $\overlineΣ^{\pm}$ in both pp and p-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 $R_{\rm pPb}$ for both $\overlineΣ^{+}$ and $\overlineΣ^{-}$ are evaluated and compared to those of protons, $Λ$ and $Ξ$ hyperons, and predictions of EPOS LHC and EPOS4 models. No deviations of $R_{\rm pPb}$ for $\overlineΣ^{\pm}$ from the model predictions or measurements for other hadrons are found within uncertainties.
$p_\mathrm{{T}}$-differential production yield of $\overline{\Sigma}^{+}$ in INEL pp collisions at $\sqrt{s}=5.02~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.5$.
$p_\mathrm{{T}}$-differential production yield of $\overline{\Sigma}^{-}$ in INEL pp collisions at $\sqrt{s}=5.02~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.5$.
$p_\mathrm{{T}}$-differential production yield of $\overline{\Sigma}^{+}$ in NSD p-Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02~\mathrm{{TeV}}$ in the rapidity interval $|y_\mathrm{CMS}|<0.5$.
The azimuthal-correlation distributions between electrons from the decays of heavy-flavor hadrons and associated charged particles in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV are reported for the 0-10% and 30-50% centrality classes. This is the first measurement to provide access to the azimuthal-correlation observables in the heavy-flavor sector in Pb-Pb collisions. The analysis is performed for trigger electrons from heavy-flavor hadron decays with transverse momentum $4 < p_{\rm T}^{\rm e} < 16$ GeV/$c$, considering associated particles within the transverse-momentum range $1 < p_{\rm T}^{\rm assoc} < 7$ GeV/$c$, and a pseudorapidity difference of $|Δη| < 1$ between the trigger electron and associated particles. The per-trigger nuclear modification factor ($I_{\rm AA}$) is calculated to compare the near- and away-side peak yields to those in pp collisions at $\sqrt{s} = 5.02$ TeV. In 0-10% central collisions, the $I_{\rm AA}$ indicates a hint of enhancement of associated-particle yields with $p_{\rm T} < 3$ GeV/$c$ on the near side, and a suppression of yields with $p_{\rm T} > 4$ GeV/$c$ on the away side. The $I_{\rm AA}$ for electron triggers from heavy-flavor hadron decays is compared with that for light-flavor and strange-particle triggers to investigate the dependence on different fragmentation processes and parton-medium dynamics, and is found to be the same within uncertainties.
$\Delta\varphi$ correlation distribution between electrons from heavy-flavor hadron decays and associated charged particles for $4 < p_\mathrm{T}^\mathrm{e} < 12$ GeV/$c$ and $1 < p_\mathrm{T}^\mathrm{assoc} < 7$ GeV/c in Pb--Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV in the 0--10% centrality class. The publication shows the correlation distribution for $p_\mathrm{T}^\mathrm{assoc}$ intervals $1 < p_\mathrm{T}^\mathrm{assoc} < 2$, $2 < p_\mathrm{T}^\mathrm{assoc} < 3$, and $5 < p_\mathrm{T}^\mathrm{assoc}< 7$ GeV/$c$. Appendix Figure 1 shows all $p_\mathrm{T}^\mathrm{assoc}$ intervals.
$\Delta\varphi$ correlation distribution between electrons from heavy-flavor hadron decays and associated charged particles for $4 < p_\mathrm{T}^\mathrm{e} < 12$ GeV/$c$ and $1 < p_\mathrm{T}^\mathrm{assoc} < 7$ GeV/c in Pb--Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV in the 30--50% centrality class. The publication shows the correlation distribution for $p_\mathrm{T}^\mathrm{assoc}$ intervals $1 < p_\mathrm{T}^\mathrm{assoc} < 2$, $2 < p_\mathrm{T}^\mathrm{assoc} < 3$, and $5 < p_\mathrm{T}^\mathrm{assoc}< 7$ GeV/$c$. Appendix Figure 2 shows all $p_\mathrm{T}^\mathrm{assoc}$ intervals.
Baseline of the $\Delta\varphi$ correlation distribution between electrons from heavy-flavor hadron decays and associated charged particles for $4 < p_\mathrm{T}^\mathrm{e} < 12$ GeV/$c$ in Pb--Pb collisions in the 0--10% and 30--50% centrality classes at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV. The publication shows the baseline for $p_\mathrm{T}^\mathrm{assoc}$ intervals $1 < p_\mathrm{T}^\mathrm{assoc} < 2$, $2 < p_\mathrm{T}^\mathrm{assoc} < 3$, and $5 < p_\mathrm{T}^\mathrm{assoc}< 7$ GeV/$c$.
Ultrarelativistic heavy-ion collisions produce a state of hot and dense strongly interacting QCD matter called quark--gluon plasma (QGP). On an event-by-event basis, the volume of the QGP in ultracentral collisions is mostly constant, while its total entropy can vary significantly due to quantum fluctuations, leading to variations in the temperature of the system. Exploiting this unique feature of ultracentral collisions allows for the interpretation of the correlation of the mean transverse momentum of produced charged hadrons and the number of charged hadrons as a measure for the speed of sound. It is determined by fitting the relative increase in transverse momentum with respect to the relative change in the average charged-particle density measured at midrapidity. This study reports the event-average transverse momentum of charged particles as well as the self-normalized variance, skewness, and kurtosis of the event-by-event transverse momentum distribution in ultracentral Pb-Pb collisions at a center-of-mass energy of 5.02 TeV per nucleon pair using the ALICE detector. Different centrality estimators based on charged-particle multiplicity or the transverse energy of the event are used to select ultracentral collisions. By ensuring a pseudorapidity gap between the region used to define the centrality and the region used to perform the measurement, the influence of biases on the rise of the mean transverse momentum is tested. The measured values are found to strongly depend on the exploited centrality estimator. The variance shows a steep decrease towards ultracentral collisions, while the skewness variables show a maximum, followed by a fast decrease. These non-Gaussian features are understood in terms of the vanishing of the impact-parameter fluctuations contributing to the event-to-event transverse momentum distribution.
Average number of participating nucleons ($\langle N_{\mathrm{part}} \rangle$) as a function of centrality percentile in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimators based on $N_{\mathrm{ch}}$, ${N_{\mathrm{tracklets}}}$, and $E_{\mathrm{T}}$ within $|\eta|\leq 0.8$.
Average number of participating nucleons ($\langle N_{\mathrm{part}} \rangle$) as a function of centrality percentile in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimator based on $N_{\mathrm{ch}} \in$ $-3.7<\eta<-1.7$ and $2.8 < \eta <5.1$.
Normalized $p_{\mathrm{T}}$-spectrum ratio as a function as a function of centrality in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimator based on $N_{\mathrm{ch}} \in$ $0.5 \leq |\eta|\leq 0.8$.
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