109 Search Results

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

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

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

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

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

We present the measurement of the 𝑝_T-differential production cross section of 𝜔 mesons in pp and 𝑝-Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV at midrapidity by ALICE. In addition, the first measurement of the nuclear modification factor 𝑅_𝑝⁢Pb for 𝜔 mesons at LHC energies is presented, complementing the existing measurements of lighter neutral mesons such as the 𝜋⁰ and 𝜂. Within the measured 𝑝_T range, the 𝑅_𝑝⁢Pb of 𝜔 mesons is compatible with no cold nuclear matter effects within the uncertainties, consistent with previous measurements at lower energies. The 𝜔/𝜋⁰ ratio is presented for both collision systems, showing no collisionmore » system dependence within the uncertainties. The comparison to previously published 𝜔/𝜋⁰ ratios at lower and higher collision energies in pp collisions suggests a decreasing trend of the ratio above _𝑝T = 4 GeV/𝑐 with increasing collision energy. The data in both collision systems are compared to predictions from PYTHIA 8, EPOS LHC, and DPMJET event generators, revealing significant shortcomings in these models’ ability to describe the production of 𝜔 mesons.« less

This paper presents the primary charged-particle multiplicity distributions in proton–lead collisions at a centre-of-mass energy per nucleon–nucleon collision of $$\sqrt{s_{NN}}$$ = 5.02 TeV. The distributions are reported for non-single diffractive collisions in different pseudorapidity ranges. The measurements are performed using the combined information from the Silicon Pixel Detector and the Forward Multiplicity Detector of ALICE. The multiplicity distributions are parametrised with a double negative binomial distribution function which provides satisfactory descriptions of the distributions for all the studied pseudorapidity intervals. The data are compared to models and analyzed quantitatively, evaluating the first four moments (mean, standard deviation, skewness, and kurtosis).more » The shape evolution of the measured multiplicity distributions is studied in terms of KNO variables and it is found that none of the considered models reproduces the measurements. This paper also reports on the average charged-particle multiplicity, normalised by the average number of participating nucleon pairs, as a function of the collision energy. The multiplicity results are then compared to measurements made in proton–proton and nucleus–nucleus collisions across a wide range of collision energies.« less

The interaction between pions and nucleons plays a crucial role in hadron physics. It represents a fundamental building block of the low-energy QCD dynamics and is subject to several resonance excitations. This work studies the p--π^±dynamics using femtoscopic correlations in high-multiplicity pp collisions at $$\sqrt{s}$$ = 13 TeV measured by ALICE at the LHC. As the final-state interaction between protons and pions is well constrained by scattering experiments and the study of pionic hydrogen, the results give access to information on the particle-emitting source in pp collisions using the femtoscopy methods. The scaling of the source size of primordial protonsmore » and pions against their pair transverse mass is extracted. The results are compared with the source sizes studied with p–p, p--K⁺, and π^±–π^± pairs by ALICE in the same collision system and are found to be in agreement for the different particle pairs. This reinforces recent findings by ALICE of a common emission source for all hadron-pairs in pp collisions at LHC energies. Furthermore, the p--p--π^± systems are studied using three-particle femtoscopy in pp collisions at TeV. The presence of three-body effects is analyzed utilizing the cumulant expansion method. In this formalism, the known two-body interactions are subtracted in order to isolate the three-body effects. For both, p--p--π⁺ and p--p--π^–, a non-zero cumulant is found, indicating effects beyond pairwise interactions. These results give information on the coupling of the pion to multiple nucleons.« less