94 Search Results

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

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

The ALICE Collaboration reports measurements of the large relative transverse momentum (𝑘_𝑇) component of jet substructure in 𝑝⁢𝑝 and Pb-Pb collisions at center-of-mass energy per nucleon pair $$\sqrt{s_{NN}}$$ = 5.02  TeV. Enhancement in the yield of such large-𝑘_T emissions in head-on Pb-Pb collisions is predicted to arise from partonic scattering with quasiparticles of the quark-gluon plasma. The analysis utilizes charged-particle jets reconstructed by the anti-𝑘_T algorithm with resolution parameter 𝑅 = 0.2 in the transverse-momentum interval 60 < 𝑝_T,ch,jet < 80  GeV/𝑐. The soft drop and dynamical grooming algorithms are used to identify high transverse momentum splittings in the jet shower. Comparisonmore » of measurements in Pb-Pb and 𝑝⁢𝑝 collisions shows medium-induced narrowing, corresponding to yield suppression of high-𝑘_𝑇 splittings, in contrast to the expectation of yield enhancement due to quasiparticle scattering. The measurements are compared to theoretical model calculations incorporating jet modification due to jet-medium interactions (“jet quenching”), both with and without quasiparticle scattering effects. These measurements provide new insight into the underlying mechanisms and theoretical modeling of jet quenching.« less

We report on the measurement of inclusive, non-prompt, and prompt J/ψ-hadron correlations by the ALICE Collaboration at the CERN Large Hadron Collider in pp collisions at a center-of-mass energy of 13 TeV. The correlations are studied at midrapidity (|y| < 0.9) in the transverse momentum ranges p_T < 40 GeV/c for the J/ψ and 0.15 < p_T < 10 GeV/c and |η| < 0.9 for the associated hadrons. The measurement is based on minimum bias and high multiplicity data samples corresponding to integrated luminosities of L_int = 34 nb⁻¹ and L_int = 6.9 pb⁻¹, respectively. In addition, two more datamore » samples are employed, requiring, on top of the minimum bias condition, a threshold on the tower energy of E = 4 and 9 GeV in the ALICE electromagnetic calorimeters, which correspond to integrated luminosities of L_int = 0.9 pb⁻¹ and L_int = 8.4 pb⁻¹, respectively. The azimuthally integrated near and away side yields of associated charged hadrons per J/ψ trigger are presented as a function of the J/ψ and associated hadron transverse momentum. The measurements are discussed in comparison to PYTHIA calculations.« less

The correlations between event-by-event fluctuations of symmetry planes are measured in Pb-Pb collisions at a center-of-mass energy per nucleon pair $$\sqrt{s_{NN}}$$ = 5.02 TeV recorded by the ALICE detector at the Large Hadron Collider. This analysis is conducted using the Gaussian estimator technique, which is insensitive to biases from correlations between different flow amplitudes. The study presents, for the first time, the centrality dependence of correlations involving up to five different symmetry planes. The correlation strength varies depending on the harmonic order of the symmetry plane and the collision centrality. Comparisons with measurements from lower energies indicate no significant differencesmore » within uncertainties. Additionally, the results are compared with hydrodynamic model calculations. Although the model predictions provide a qualitative explanation of the experimental results, they overestimate the data for some observables. This is particularly true for correlators that are sensitive to the nonlinear response of the medium to initial-state anisotropies in the collision system. As these new correlators provide unique information—independent of flow amplitudes—their usage in future model developments can further constrain the properties of the strongly interacting matter created in ultrarelativistic heavy-ion collisions.« less

The production yields of the orbitally excited charm-strange mesons D_s⁢1⁡(1⁺)⁢(2536)+ and 𝐷$$^{*}_{𝑠⁢2}$$⁡(2⁺)⁢(2573)⁺ were measured for the first time in proton-proton (pp) collisions at a center-of-mass energy of $$\sqrt{s}$$ =13 TeV with the ALICE experiment at the LHC. The D$$^{+}_{s⁢1}$$ and D$$^{*+}_{s⁢2}$$ mesons were measured at midrapidity (|𝑦| < 0.5) in minimum-bias and high-multiplicity pp collisions in the transverse-momentum interval 2< 𝑝_T < 24 GeV/𝑐. Their production yields relative to the D$$^{+}_{s}$$ ground-state yield were found to be compatible between minimum-bias and high-multiplicity collisions, as well as with previous measurements in e^±⁢p and e⁺⁢e⁻ collisions. The measured D$$^{+}_{s⁢1}$$/D$$^{+}_{s}$$ and D$$^{*+}_{s⁢2}$$/D$$^{+}_{s}$$ yieldmore » ratios are described by statistical hadronization models and can be used to tune the parameters governing the production of excited charm-strange hadrons in Monte Carlo generators, such as pythia 8.« 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

In this Letter, the first evidence of the $$^4_{\bar{Λ}}$$$$\overline{He}$$ antihypernucleus is presented, along with the first measurement at the LHC of the production of (anti)hypernuclei with mass number 𝐴 = 4, specifically (anti)⁢$$^4_Λ$$H and (anti)⁢$$^4_Λ$$He. In addition, the antiparticle-to-particle ratios for both hypernuclei ($$^4_{\bar{Λ}}$$$$\bar{H}$$⁡/$$^4_Λ$$H and $$^4_{\bar{Λ}}$$$$\overline{He}$$/$$^4_Λ$$He) are shown, which are sensitive to the baryochemical potential of the strongly interacting matter created in heavy-ion collisions. The results are obtained from a data sample of central Pb-Pb collisions, collected during the 2018 LHC data taking at a center-of-mass energy per nucleon pair of $$\sqrt{𝑠_{NN}}$$ = 5.02 TeV. The yields measured for themore » average of the charge-conjugated states are found to be [0.78 ± 0.19⁢(stat) ± 0.17⁢(syst)] × 10⁻⁶ for the (anti)⁢$$^4_Λ$$H and [1.08 ± 0.34⁢(stat)± 0.20⁢(syst)] × 10⁻⁶ for the (anti)⁢$$^4_Λ$$He, and the measured antiparticle-to-particle ratios are in agreement with unity. The presence of (anti)⁢$$^4_Λ$$H and (anti)⁢$$^4_Λ$$He excited states is expected to strongly enhance the production yield of these hypernuclei. The yield values exhibit a combined deviation of 3.3⁢𝜎 from the theoretical ground-state-only expectation, while the inclusion of the excited states in the calculations leads to an agreement within 0.6⁢𝜎 with the present measurements. Additionally, the measured (anti)$$^4_Λ$$H and (anti)⁢$$^4_Λ$$He masses are compatible with the world-average values within the uncertainties.« less

The first measurement of the polarisation of coherently photoproduced J/ψ mesons in ultra-peripheral Pb–Pb collisions, using data at $$\sqrt{s_{NN}}$$ = 5.02 TeV, is presented. The J/ψ meson is measured via its dimuon decay channel in the forward rapidity interval –4.0 < y < –2.5 using the ALICE detector at the CERN LHC. An event sample corresponding to an integrated luminosity of 750 μb^–1 ± 5% (syst) is analysed. Hadronic activity is highly suppressed since the interaction is mediated by a photon. The polar and azimuthal angle distributions of the decay muons are measured, and the polarisation parameters λ_θ, λ_φ, λ_θφ,more » are extracted. The analysis is carried out in the helicity frame. The results are found to be consistent with a transversely polarised J/ψ. These values are compared with previous measurements by the H1 and ZEUS experiments. The polarisation parameters of coherent J/ψ photoproduction in Pb–Pb collisions are found to be consistent with the s-channel helicity conservation hypothesis.« less