16 Search Results

The nuclear modification factor R AA of charm jets, identified by the presence of a D0 meson among the jet constituents, has been measured for the first time in Pb–Pb collisions at a centre-of-mass energy per nucleon pair s NN = 5.02 TeV with the ALICE detector at the LHC. The D0 mesons and their charge conjugates are reconstructed from the hadronic decay D 0 → K − π + . Jets are reconstructed from D0-meson candidates and charged particles using the anti-k T algorithm with jet resolution parameter R = 0.3 , in the jet transverse momentum (p T)more » range 5 < p T ch jet < 50 GeV/c and pseudorapidity |η ch jet| < 0.6. A hint of reduced suppression in the charm-jet R AA is observed in comparison to inclusive jets in central Pb–Pb collisions with a significance of about 2σ in 20 < p T ch jet < 50 GeV/c, suggesting the in-medium energy loss to depend on both the difference between quark and gluon coupling strength (Casimir colour-charge effect) and quark mass (dead-cone effect). The data are compared with model calculations that include mass effects in the in-medium energy loss. Several state-of-the-art models are consistent with the data, with the LIDO model providing the best description of the data in the common kinematic range of inclusive and D0-tagged jets, highlighting the role of mass effects in interpreting the results.« less

Quantum Chromodynamics predicts a phase transition from hadronic matter to quark–gluon plasma (QGP) at high temperatures and energy densities, where quarks and gluons (partons) are no longer confined within hadrons. The QGP forms in ultrarelativistic heavy-ion collisions. Anisotropic flow coefficients, quantifying the azimuthal expansion of produced matter, probe QGP properties. Flow measurements in high-energy heavy-ion collisions show a distinctive grouping of anisotropic flow for baryons and mesons at intermediate transverse momentum – a feature associated with flow imparted at the quark level, confirming QGP existence. The observation of QGP-like features in proton–proton and proton–ion collisions has sparked debate about QGPmore » formation in smaller systems. For the first time, we demonstrate the distinctive grouping of anisotropic flow for baryons and mesons in high-multiplicity proton–lead and proton–proton collisions at the Large Hadron Collider (LHC). These results are described by a model including hydrodynamic flow followed by hadron formation via quark coalescence, consistent with the formation of partonic flowing systems in these collisions.« less

The distribution of angular correlations between prompt charm hadrons and primary charged particles in pp collisions is sensitive to the charm-quark hadronisation process. In this letter, charm-baryon correlations are measured for the first time by studying the azimuthal-angle difference between charged particles and prompt Λ c + baryons produced in pp collisions at a centre-of-mass energy of s = 13 TeV, with the ALICE detector. Λ c + baryons are reconstructed at midrapidity ( | y | < 0.5 ) in the transverse-momentum interval 3 < p T < 16 GeV/c, and correlated with charged particles with p T >more » 0.3 GeV/c and pseudorapidity | η | < 0.8 . For 3 < p T Λ c + , D < 5 GeV/c, the comparison with published measurements of D-meson and charged-particle correlations in the same collision system hints at a larger number of low-momentum particles associated with Λ c + -baryon triggers than with D-meson triggers, both in the collinear and opposite directions with respect to the trigger particle. These differences can be quantified by the comparison of the properties of the near- and away-side correlation peaks, and are not reproduced by predictions of various Monte Carlo event generators, generally underpredicting the associated particle yields at p T assoc < 1 GeV/c. This tension between Λ c + -baryon and D-meson associated peak yields could suggest a modified fragmentation of the charm quark, or a different hadronisation process, when a charm baryon is produced in the final state.« 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 first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton–proton (pp) collisions is reported. The mean charged particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at center-of-mass energy $$\sqrt{s}$$ = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region (|η| < 0.9) using the sequential recombination anti-k_T algorithm with jet resolution parameters R = 0.2, 0.3, and 0.4 for the trans verse momentum (p_T) interval 5–110 GeV/c. The high multiplicity events are selected by the forward V0 scintilla tor detectors.more » The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet p_T in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation function variables z^ch and ξ^ch are measured for different jet-p_T intervals. Jet-p_T independent fragmentation of leading jets is observed for wider jets except at high- and low-z^ch values. The observed “hump-backed plateau” structure in the ξ^ch distribution indicates suppression of low-p_T particles. In high-multiplicity events, an enhancement of the fragmen tation probability of low-z^ch particles accompanied by a suppression of high-zch particles is observed compared to minimum-bias events. This behavior becomes more promi nent for low-p_T jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet p_T. These measurements provide important constraints to models of jet fragmentation.« less

The coherent photoproduction of J/ψ and ψ' mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy $$\sqrt{s_{\mathrm {NN}}}~=~5.02$$ TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The J/ψ is reconstructed using the dilepton (l⁺l^-) and proton–antiproton decay channels, while for the ψ' the dilepton and the l⁺l^-π⁺π^- decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233 μb^-1. The results are compared with theoretical models for coherent J/ψ and ψ' photoproduction. The coherent cross sectionmore » is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.64 at a Bjorken-x of around 6×10^-4, such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent J/ψ cross section including ALICE measurements at forward rapidity. The ratio of ψ' to J/ψ coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.« less

The production of \(\pi ^{\pm }\), \(\mathrm{K}^{\pm }\), \(\mathrm{K}^{0}_{S}\), \(\mathrm{K}^{*}(892)^{0}\), \(\mathrm{p}\), \(\phi (1020)\), \(\Lambda \), \(\Xi ^{-}\), \(\Omega ^{-}\), and their antiparticles was measured in inelastic proton–proton (pp) collisions at a center-of-mass energy of \(\sqrt{s}\) = 13 TeV at midrapidity (\(|y|<0.5\)) as a function of transverse momentum (\(p_{\mathrm{T}}\)) using the ALICE detector at the CERN LHC. Furthermore, the single-particle \(p_{\mathrm{T}}\) distributions of \(\mathrm{K}^{0}_{S}\), \(\Lambda \), and \(\overline{\Lambda }\) in inelastic pp collisions at \(\sqrt{s} = 7\) TeV are reported here for the first time. The \(p_{\mathrm{T}}\) distributions are studied at midrapidity within the transverse momentum range \(0\le p_{\mathrm{T}}\le 20\) GeV/c,more » depending on the particle species. The \(p_{\mathrm{T}}\) spectra, integrated yields, and particle yield ratios are discussed as a function of collision energy and compared with measurements at lower \(\sqrt{s}\) and with results from various general-purpose QCD-inspired Monte Carlo models. A hardening of the spectra at high \(p_{\mathrm{T}}\) with increasing collision energy is observed, which is similar for all particle species under study. The transverse mass and \(x_{\mathrm{T}}\equiv 2p_{\mathrm{T}}/\sqrt{s}\) scaling properties of hadron production are also studied. As the collision energy increases from \(\sqrt{s}\) = 7–13 TeV, the yields of non- and single-strange hadrons normalized to the pion yields remain approximately constant as a function of \(\sqrt{s}\), while ratios for multi-strange hadrons indicate enhancements. The \(p_\mathrm{{T}}\)-differential cross sections of \(\pi ^{\pm }\), \(\mathrm {K}^{\pm }\) and \(\mathrm {p}\) (\(\overline{\mathrm{p}}\)) are compared with next-to-leading order perturbative QCD calculations, which are found to overestimate the cross sections for \(\pi ^{\pm }\) and \(\mathrm{p}\) (\(\overline{\mathrm{p}}\)) at high \(p_\mathrm{{T}}\).« less

Here, we present a study of the inclusive charged-particle transverse momentum (p_T) spectra as a function of charged-particle multiplicity density at mid-pseudorapidity, dN_ch/dη, in pp collisions at $$ \sqrt{s}$$ = 5.02 and 13 TeV covering the kinematic range |η| < 0.8 and 0.15 < p_T < 20 GeV/c. The results are presented for events with at least one charged particle in |η| < 1 (INEL>0). The p_T spectra are reported for two multiplicity estimators covering different pseudorapidity regions. The p_T spectra normalized to that for INEL>0 show little energy dependence. Moreover, the high-p_T yields of charged particles increase faster than themore » charged-particle multiplicity density. The average p_T as a function of multiplicity and transverse spherocity is reported for pp collisions at $$ \sqrt{s}$$=13 TeV. For low- (high-) spherocity events, corresponding to jet-like (isotropic) events, the average p_T is higher (smaller) than that measured in INEL>0 pp collisions. Within uncertainties, the functional form of ⟨p_T⟩ (N_ch) is not affected by the spherocity selection. While EPOS LHC gives a good description of many features of data, PYTHIA overestimates the average p_T in jet-like events.« less

The procedure for the energy calibration of the high granularity electromagnetic calorimeter PHOS of the ALICE experiment is presented. The methods used to perform the relative gain calibration, to evaluate the geometrical alignment and the corresponding correction of the absolute energy scale, to obtain the nonlinearity correction coefficients and finally, to calculate the time-dependent calibration corrections, are discussed and illustrated by the PHOS performance in proton-proton (pp) collisions at √s = 13 TeV. After applying all corrections, the achieved mass resolutions for π⁰ and η mesons for p_T > 1.7 GeV/c are σ_m^π0 = 4.56 ± 0.03 MeV/c² and σ_m^ηmore » = 15.3 ± 1.0 MeV/c², respectively.« less

The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of J / ψ vector mesons off proton targets in ultra–peripheral p–Pb collisions at a centre-of-mass energy per nucleon pair sNN=5.02 TeV. The e + e - and μ + μ - decay channels are used to measure the cross section as a function of the rapidity of the J / ψ in the range - 2.5 < y< 2.7 , corresponding to an energy in the γp centre-of-mass in the interval 40 < W γp < 550 GeV. The measurements, which are consistent with a power law dependencemore » of the exclusive J / ψ photoproduction cross section, are compared to previous results from HERA and the LHC and to several theoretical models. They are found to be compatible with previous measurements.« less