17 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

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 p + π⁰ with π⁰ → $γγ$, 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 tomore » 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.« less

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 $$Ξ$$-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 $$Ξ$$-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 $$Ξ$$-K and $$Ξ$$ − 𝜋 systems, respectively.more » 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 .« 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

This work reports femtoscopic correlations of p – – p ($$\overline{p}$$ – – $$\overline{p}$$) and p – – d ($$\overline{p}$$ – – $$\overline{d}$$) pairs measured in Pb–Pb collisions at center-of-mass energy per nucleon $$\sqrt{s_{NN}}$$ = 5.02 TeV in the ALICE Collaboration. A fit to the measured proton-proton correlation functions allows one to extract the dependence of the nucleon femtoscopic radius of the particle-emitting source on the pair transverse mass (m_T) and on the average charge particle multiplicity $$\langle$$dN_ch/dη$$\rangle$$^1/3 for three centrality intervals (0–10%, 10 – – 30 %, 30 – – 50 %). In both cases, the expected power-law andmore » linear scalings are observed, respectively. The measured p–d correlations can be described by both two- and three-body calculations, indicating that the femtoscopy observable is not sensitive to the short-distance features of the dynamics of the p-(p-n) system, due to the large inter-particle distances in Pb–Pb collisions at the LHC. Indeed, in this study, the minimum measured femtoscopic source sizes for protons and deuterons have a minimum value at 2.73$$^{+0.05}_{–0.05}$$ and 3.10$$^{+1.04}_{–0.86}$$ fm, respectively, for the 30–50% centrality collisions. Moreover, the m_T-scaling obtained for the p–p and p–d systems is compatible within 1σ of the uncertainties. These findings provide new input for fundamental studies on the production of light (anti)nuclei under extreme conditions.« 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

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

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