117 Search Results

The first results of K*(892)^± 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)^± has been reconstructed via its hadronic decay channel K*(892)^± → π^± + K$$^0_{\textrm{S}}$$ using the ALICE detector at the LHC. For each multiplicity class, the differential transverse momentum (p_T) spectrum, the mean transverse momentum ⟨p_T⟩, the p_T-integrated yield (dN/dy), and the ratio of the K*(892)^± to K$$^0_{\textrm{S}}$$ 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,more » PYTHIA8, EPOS-LHC, and DIPSY are also discussed. For the first time, a significant K*(892)^±/K$$^0_{\textrm{S}}$$ suppression in pp collisions is observed at a 7σ level passing from low to high multiplicity events. The ratios of the p_T-differential yields of K*(892)^± and K$$^0_{\textrm{S}}$$ in high and low multiplicity events are also presented along with their double ratio. For p_T ≲ 2 GeV/c this double ratio persists below unity by more than 3σ suggesting that the suppression affects mainly low p_T resonances. The measured decreasing trend of the K*(892)^±/K$$^0_{\textrm{S}}$$ 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.« less

This Letter presents measurements of long-range transverse-momentum correlations using a new observable, 𝑣_0⁡(𝑝_T), serving as a probe of event-by-event radial-flow fluctuations, the underlying radial expansion, and the medium’s properties in heavy-ion collisions. Results are reported for inclusive charged particles, pions, kaons, and protons across various centrality intervals in Pb-Pb collisions at $$\sqrt{𝑠_{\textrm{NN}}}$$ = 5.02 TeV, recorded by the ALICE detector. A pseudorapidity-gap technique, similar to that used in anisotropic-flow studies, is employed to suppress short-range correlations. At low 𝑝_T, a characteristic mass ordering consistent with hydrodynamic collective flow is observed. At higher 𝑝_T (>3 GeV/𝑐), protons exhibit larger 𝑣₀⁡(𝑝_T) thanmore » pions and kaons, in agreement with expectations from quark-recombination models. Comparisons to viscous hydrodynamic calculations with varying bulk viscosity and equation of state demonstrate the sensitivity of the 𝑣₀⁡(𝑝_T) observable to these key medium properties. The findings establish 𝑣₀⁡(𝑝_T) as a valuable addition to the set of observables used in Bayesian analyses for extracting the transport properties and constraining the equation of state of strongly interacting matter, while also helping to systematically explore its sensitivity and impact within such global studies.« less

Femtoscopy of nonidentical particle pairs has been instrumental for precision measurements of both two-particle sources and the final-state interactions in high-energy elementary and heavy-ion collisions. The majority of measurements assessing the source properties are based on identical particle pairs, providing direct access to the characteristics of the single-particle source. The work in this paper demonstrates, via femtoscopy measurements of charged pion-deuteron pairs in Pb-Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV, the feasibility of accessing the characteristics of the single-particle femtoscopic source by using particle pairs with large mass differences such as pions and deuterons. The first experimental results of themore » measurement of deuteron source sizes in ultrarelativistic heavy-ion collisions are presented. The results show good agreement with the trend derived from other charged hadrons such as pions, kaons, and protons as a function of transverse mass, indicating similar source properties.« less

This paper presents the measurement of the isolated prompt photon inclusive production cross section in pp and p–Pb collisions by the ALICE Collaboration at the LHC. The measurement is performed in p–Pb collisions at centre-of-mass energies per nucleon pair of $$\sqrt{s_{NN}}$$ = 5.02 Te and 8.16 TeV, as well as in pp collisions at $$\sqrt{s}$$ = 5.02 TeV and 8 TeV. The cross section is obtained at midrapidity (|γ| < 0.7) using a charged-track based isolation momentum p$$^{iso,~ch}_{T}$$ < 1.5 GeV/c in a cone with radius R = 0.4. The data for both collision systems are well reproduced by perturbativemore » QCD (pQCD) calculations at next-to-leading order (NLO) using recent parton distribution functions for free (PDF) and bound (nPDF) nucleons. Furthermore, the nuclear modification factor R_pA for both collision energies is consistent with unity for p_T > 20 GeV/c. However, deviations from unity (R_pA < 1) of up to 20% are observed for p_T < 20 GeV/c with limited significance, indicating the possible presence of nuclear effects in the initial state of the collision.« less

High-energy hadronic collisions generate environments characterized by temperatures above 100 MeV (refs.), about 100,000 times hotter than the centre of the Sun. At present, it is therefore unclear how light (anti)nuclei with mass number A of a few units, such as the deuteron, ³He or ⁴He, each bound by only a few MeV, can emerge from these collisions. Here, the ALICE Collaboration reports that deuteron–pion momentum correlations in proton–proton (pp) collisions provide model-independent evidence that about 90% of the observed (anti)deuterons are produced in nuclear reactions following the decay of short-lived resonances, such as the Δ(1232). These findings, obtained by themore » ALICE Collaboration at the Large Hadron Collider, resolve a gap in our understanding of nucleosynthesis in ultrarelativistic hadronic collisions. Apart from offering insights on how (anti)nuclei are formed in hadronic collisions, the results can be used in the modelling of the production of light and heavy nuclei in cosmic rays and dark-matter decays.« less

Heavy-flavor quarks produced in proton-proton (pp) collisions provide a unique opportunity to investigate the evolution of quark-initiated parton showers from initial hard scatterings to final-state hadrons. By examining jets that contain heavy-flavor hadrons, this study explores the effects of both perturbative and nonperturbative QCD on jet formation and structure. The angular differences between various jet axes, Δ⁢𝑅_axis, offer insight into the radiation patterns and fragmentation of charm quarks. The first measurement of 𝐷⁰-tagged jet axes differences in pp collisions at $$\sqrt{s}$$ =5.02  TeV by the ALICE experiment at the LHC is presented for jets with transverse momentum 𝑝$$^{ch jet}_{T}$$ ≥ 10 GeV/𝑐more » and 𝐷⁰ mesons with 𝑝$$^{D^0}_{T}$$ ≥ 5  GeV/𝑐. In this 𝐷⁰-meson-tagged jet measurement, three jet axis definitions, each with different sensitivities to soft, wide-angle radiation, are used: the standard axis, soft drop groomed axis, and winner-takes-all axis. Measurements of the radial distributions of 𝐷⁰ mesons with respect to the jet axes, Δ⁢𝑅_axis−D⁰, are reported, along with the angle, Δ⁢𝑅_axis, between the three jet axes. The 𝐷⁰ meson emerges as the leading particle in these jets, closely aligning with the winner-takes-all axis and diverging from the standard jet axis. The results also examine how varying the sensitivity to soft radiation with grooming influences the orientation of the soft drop jet axis and uncover that charm-jet structure is more likely to survive grooming when the soft drop axis is further from the 𝐷⁰ direction, providing further evidence of the dead-cone effect recently measured by ALICE.« less

The first measurement of the 𝑒⁺⁢𝑒⁻ pair production at midrapidity and low invariant mass in central Pb-Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV at the Large Hadron Collider is presented. The yield of 𝑒⁺⁢𝑒⁻ pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass (𝑚𝑒⁢𝑒) and pair transverse momentum (𝑝_{T,𝑒⁢𝑒}) ranges 𝑚_𝑒⁢𝑒 < 3.5 GeV/𝑐² and 𝑝_{T,𝑒⁢𝑒} < 8 GeV/𝑐. For 0.18 < 𝑚_𝑒⁢𝑒 < 0.5 GeV/𝑐² the ratio of data to the cocktail of hadronic contributions amounts to 1.40 ± 0.11⁢(stat.) ± 0.23 ⁢(syst.) ± 0.16⁢ (cocktail) and 1.42 ± 0.11⁢ (stat.) ± 0.23more » (syst.)$$^{+0.24}_{−0.29}$$ ⁢(cocktail), including or not including medium effects in the estimation of the heavy-flavor background, respectively. It is consistent with predictions from two different models for an additional contribution of thermal 𝑒+⁢𝑒− pairs from the hadronic and partonic phases. In the intermediate-mass range (1.2 < 𝑚_𝑒⁢𝑒 < 2.6 GeV/𝑐²), the pair transverse impact parameter of the 𝑒⁺⁢𝑒⁻ pairs (DCA𝑒⁢𝑒, where “DCA” denotes “distance of closest approach”) is used for the first time in Pb-Pb collisions to separate displaced dielectrons from heavy-flavor hadron decays from a possible (thermal) contribution produced at the interaction point. The data are consistent with a suppression of 𝑒⁺⁢𝑒⁻ pairs from $$𝑐\bar{𝑐}$$ and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb-Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV is reported via the study of virtual direct photons in the transverse momentum range 1 < 𝑝_T < 5 GeV/𝑐. A model including prompt photons, as well as photons from the preequilibrium and fluid-dynamic phases, can reproduce the result, while being at the upper edge of the data uncertainties.« less

The intense photon fluxes from relativistic nuclei provide an opportunity to study photonuclear interactions in ultraperipheral collisions. In particular, it allows for the investigations of excited, light-flavour vector mesons. The measurement of coherently photoproduced 𝜋⁺𝜋⁻𝜋⁺𝜋⁻ final states in ultraperipheral Pb–Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV is presented for the first time. The cross section, d𝜎/d𝑦, times the branching ratio (𝜌 → 𝜋⁺𝜋⁺𝜋⁻𝜋⁻) is found to be 47.8 ± 2.3 (stat.) ± 7.7 (syst.) mb in the rapidity interval |𝑦| < 0.5. The invariant mass distribution is not well described with a single Breit-Wigner resonance without an interference term. Includingmore » in- terference with a non-resonant contribution results in the mass and width values being too far from those reported in PDG, while the production of two interfering resonances, 𝜌(1450) and 𝜌(1700), also provides a good description of the data. The values of the masses (𝑚) and widths (Γ) of the resonances extracted from the fit assuming two interfering resonances are 𝑚₁ = 1385 ± 14 (stat.) ± 3 (syst.) MeV/𝑐², Γ₁ = 431 ± 36 (stat.) ± 82 (syst.) MeV/𝑐², 𝑚₂ = 1663 ± 13 (stat.) ± 22 (syst.) MeV/𝑐² and Γ₂ = 357 ± 31 (stat.) ± 49 (syst.) MeV/𝑐², respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions.« less

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 ($$\langle$$p_T$$\rangle$$) of produced charged hadrons and the number of charged hadrons as a measure for the speed of sound, c_s. This speed is related to the rate atmore » which compression waves travel in the QGP and is determined by fitting the relative increase in $$\langle$$p_T$$\rangle$$ with respect to the relative change in the average charged-particle density ($$\langle$$dN_ch/dη$$\rangle$$) measured at mid-rapidity. This study reports the event-average $$\langle$$p_T$$\rangle$$ of charged particles as well as the variance, skewness, and kurtosis of the event-by-event transverse momentum per charged particle ([p_T]) 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 and their potential effects on the rise of the mean transverse momentum is tested. The measured c$$^{2}_{s}$$ is found to strongly depend on the exploited centrality estimator and ranges between 0.1146±0.0028 (stat.)±0.0065 (syst.) and 0.4374±0.0006 (stat.)±0.0184 (syst.) in natural units. The self-normalized variance shows a steep decrease towards ultracentral collisions, while the self-normalized 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 [p_T] distribution.« less

A signal consistent with the Ω⁡(2012) baryon has been observed with a significance of 15⁢𝜎 in pp collisions at $$\sqrt{𝑠}$$ = 13 TeV at the LHC. In this paper, the analysis technique is described and measurements of the mass and width of the Ω⁡(2012) are reported, along with the first measurement of its transverse-momentum spectrum and yield. This paper corroborates the observation by the Belle Collaboration of this excited Ω state and the observation that the Ω⁡(2012) has a rather narrow width for a strongly decaying resonance. The yield measurement is combined with a statistical thermal model calculation of strangemore » baryon yield ratios to obtain estimates of the Ω⁢(2012)⁻ → $$Ξ⁢\bar{\textrm{K}}$$ branching ratios. These results will improve our understanding of the internal structure and mass spectrum of excited baryon states and serve as a baseline for searches regarding modifications of these properties in high-temperature media.« less