9 Search Results

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

The coherent J∕𝜓 photoproduction cross section is measured for the first time at midrapidity in peripheral to semicentral Pb − Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV. The centrality differential cross section d𝜎∕d𝑦 is reported for the centrality range 40–90%, together with the doubly-differential cross section d²𝜎∕d𝑦d𝑝_T, extracted in two peripheral centrality classes. The J∕𝜓 mesons are reconstructed in the dielectron channel, in the rapidity interval |𝑦| < 0.9 using the ALICE central barrel detectors. The J∕𝜓 cross section at midrapidity is statistically compatible to the earlier ALICE measurement at forward rapidity and at the same centre-of-mass energy, and showsmore » only a mild centrality dependence over the covered range. Several sets of theoretical calculations taking into account the hadronic overlap in the collisions but ignoring possible final-state effects from a hot expanding medium are found to give a fairly good description of the current measurements within uncertainties.« less

In this letter, we present the first measurement of direct photons at the transverse momentum of 1 < 𝑝_T < 6 GeV/𝑐 at midrapidity 𝜂 < 0.8 in inelastic and high-multiplicity proton–proton collisions at a centre-of-mass energy of $$\sqrt{s}$$ = 13 TeV. The fraction of virtual direct photons in the inclusive virtual photon spectrum is obtained from a fit to the dielectron invariant mass spectrum. In the limit of zero invariant mass, this fraction is equal to the relative contribution of real direct photons in the inclusive real photon spectrum. Contributions from decays of light- flavour neutral mesons are estimatedmore » using independent measurements in proton–proton collisions at the same energy and the same event class. For the first time at the LHC energies, a direct-photon signal is observed at low 𝑝T in both inelastic and high-multiplicity event classes, with a significance of 3.2𝜎 and 1.9𝜎 in terms of standard deviations, correspondingly. The yield of direct photons in inelastic pp collisions is compared to perturbative QCD calculations. The integrated photon yield is studied as a function of charged-particle multiplicity and is compared to the results from other experiments and theoretical calculations. The results show a significant increase of direct- photon yield with charged-particle multiplicity.« less

This study presents the first measurement of the f₁(1285) resonance using the ALICE detector in inelastic proton–proton collisions at a center-of-mass energy of 13 TeV. The resonance is reconstructed at midrapidity (|y| < 0.5) through the hadronic decay channel f₁(1285)→ K$$^0_S$$K^±π^±. Key measurements include the determination of its mass, transverse-momentum integrated yield, and average transverse momentum. Additionally, the ratio of the transverse-momentum integrated yield of f₁(1285) to pion is compared with calculations from the canonical statistical hadronization model. The model calculation, assuming a zero total strangeness content for f₁(1285), reproduces the data within 1σ deviation, shedding light on the quarkmore » composition of f₁(1285).« less

ALICE is a large experiment at the CERN Large Hadron Collider. Located 52 meters underground, its detectors are suitable to measure muons produced by cosmic-ray interactions in the atmosphere. In this paper, the studies of the cosmic muons registered by ALICE during Run 2 (2015–2018) are described. The analysis is limited to multimuon events defined as events with more than four detected muons (N_μ > 4) and in the zenith angle range 0° < θ < 50°. The results are compared with Monte Carlo simulations using three of the main hadronic interaction models describing the air shower development in themore » atmosphere: QGSJET-II-04, EPOS-LHC, and SIBYLL 2.3d. The interval of the primary cosmic-ray energy involved in the measured muon multiplicity distribution is about 4 × 10¹⁵ < E_prim < 6 × 10¹⁶ eV. In this interval none of the three models is able to describe precisely the trend of the composition of cosmic rays as the energy increases. However, QGSJET-II-04 is found to be the only model capable of reproducing reasonably well the muon multiplicity distribution, assuming a heavy composition of the primary cosmic rays over the whole energy range, while SIBYLL 2.3d and EPOS-LHC underpredict the number of muons in a large interval of multiplicity by more than 20% and 30%, respectively. The rate of high muon multiplicity events (N_μ > 100) obtained with QGSJET-II-04 and SIBYLL 2.3d is compatible with the data, while EPOS-LHC produces a significantly lower rate (55% of the measured rate). For both QGSJET-II-04 and SIBYLL 2.3d, the rate is close to the data when the composition is assumed to be dominated by heavy elements, an outcome compatible with the average energy E_prim ∼ 10¹⁷ eV of these events. This result places significant constraints on more exotic production mechanisms.« less

The ALICE Collaboration presents a new suite of jet substructure measurements in Pb–Pb and pp collisions at a center-of-mass energy per nucleon pair $$\sqrt{s_{NN}}$$ = 5.02 TeV. These measurements provide access to the internal structure of jets via the momentum and angle of their constituents, probing how the quark–gluon plasma modifies jets, an effect known as jet quenching. Jet grooming additionally removes soft wide-angle radiation to enhance perturbative accuracy and reduce experimental uncertainties. We report the groomed and ungroomed jet mass m_jet and jet angularities λ$$^{κ}_{α}$$ using κ = 1 and α > 0. Charged-particle jets are reconstructed at midrapiditymore » using the anti-k_T algorithm with resolution parameter R = 0.2. A narrowing of the jet mass and angularity distributions in Pb–Pb collisions with respect to pp is observed and is enhanced for groomed results, confirming modification of the jet core. By using consistent jet definitions and kinematic cuts between the mass and angularities for the first time, previous inconsistencies in the interpretation of quenching measurements are resolved, rectifying a hurdle for understanding how jet quenching arises from first principles and highlighting the importance of a well-controlled baseline. These results are compared with a variety of theoretical models of jet quenching, providing constraints on jet energy-loss mechanisms in the quark–gluon plasma.« less

The first measurement of ³_ΛH and $$^{3}_{\overline{Λ}}$$$$\overline{H}$$ differential production with respect to transverse momentum and centrality in Pb–Pb collisions at √s_NN = 5.02 TeV is presented. The ³_ΛH has been reconstructed via its two-charged-body decay channel, i.e., ³_ΛH → ³He → π^-. A Blast-Wave model fit of the p_T-differential spectra of all nuclear species measured by the ALICE collaboration suggests that the kinetic freeze-out surface is consistent with that of other nuclei. The ratio between the integrated yields of ³_ΛH and ³He is compared to predictions from the statistical hadronisation model and the coalescence model, with the latter being favouredmore » by the presented measurements.« less

The production yields of antideuterons and antiprotons are measured in pp collisions at a center-of-mass energy of $$\sqrt{s}$$ = 13 TeV, as a function of transverse momentum (p_T) and rapidity (y), for the first time rapidity-differentially up to |y| = 0.7. The measured spectra are used to study the p_T and rapidity dependence of the coalescence parameter B₂, which quantifies the coalescence probability of antideuterons. The p_T and rapidity dependence of the obtained B₂ is extrapolated for p_T > 1.7 GeV/c and |y| = 0.7 using the phenomenological antideuteron production model implemented in Pythia 8.3 as well as a baryonmore » coalescence afterburner model based on EPOS 3. Such measurements are of interest to the astrophysics community, since they can be used for the calculation of the flux of antinuclei from cosmic rays, in combination with coalescence models.« less

This Letter presents the first measurement of the impact-parameter dependent angular anisotropy in the decay of coherently photoproduced $$ρ$$⁰ mesons. The $$ρ$$⁰ mesons are reconstructed through their decay into pion pairs. The measured anisotropy corresponds to the amplitude of the cos(2φ) modulation, where φ is the angle between the two vectors formed by the sum and the difference of the transverse momenta of the pions, respectively. The measurement was performed by the ALICE Collaboration at the LHC using data from ultraperipheral Pb–Pb collisions at a center-of-mass energy of $$\sqrt{s_{NN}}$$ = 5.02 TeV per nucleon pair. Different impact-parameter regions are selectedmore » by classifying the events in nuclear-breakup classes. The amplitude of the cos(2φ) modulation is found to increase by about one order of magnitude from large to small impact parameters. Theoretical calculations describe the measured cos(2φ) anisotropy and its impact-parameter dependence as the result of a quantum interference effect at the femtometer scale, arising from the ambiguity regarding which of the nuclei is the photon source in the interaction.« less