Measurement of the Groomed Jet Radius and Momentum Splitting Fraction in pp and Pb-Pb Collisions at root S-NN=5.02 TeV

This article presents groomed jet substructure measurements in pp and Pb-Pb collisions at root s(NN )= 5.02 TeV with the ALICE detector. The soft drop grooming algorithm provides access to the hard parton splittings inside a jet by removing soft wide-angle radiation. We report the groomed jet momentum splitting fraction, z(g), and the (scaled) groomed jet radius, theta(g). Charged-particle jets are reconstructed at midrapidity using the anti-k(T) algorithm with resolution parameters R = 0.2 and R = 0.4. In heavy-ion collisions, the large underlying event poses a challenge for the reconstruction of groomed jet observables, since fluctuations in the background can cause groomed parton splittings to be misidentified. By using strong grooming conditions to reduce this background, we report these observables fully corrected for detector effects and background fluctuations for the first time. A narrowing of the theta(g) distribution in Pb-Pb collisions compared to pp collisions is seen, which provides direct evidence of the modification of the angular structure of jets in the quark-gluon plasma. No significant modification of the z(g) distribution in Pb-Pb collisions compared to pp collisions is observed. These results are compared with a variety of theoretical models of jet quenching, and provide constraints on jet energy-loss mechanisms and coherence effects in the quark-gluon plasma.Peer reviewe

K-S(0)- and (anti-)Lambda-hadron correlations in pp collisions at root s=13 TeV

Two-particle Azimuthal correlations are measured with the ALICE apparatus in pp collisions at root s = 13 TeV to explore strangeness- and multiplicity-related effects in the fragmentation of jets and the transition regime between bulk and hard production, probed with the condition that a strange meson (K-S(0)) or baryon (Lambda) with transverse momentum p(T) > 3 GeV/c is produced. Azimuthal correlations between kaons or Lambda hyperons with other hadrons are presented at midrapidity for a broad range of the trigger (3 < p(T)(trigg) < 20 GeV/c) and associated particle p(T) (1 GeV/c < p(T)(assoc) < p(T)(trigg)), for minimum-bias events and as a function of the event multiplicity. The near- and away-side peak yields are compared for the case of either K-S(0) or Lambda((Lambda) over bar) being the trigger particle with that of inclusive hadrons (a sample dominated by pions). In addition, the measurements are compared with predictions from PYTHIA 8 and EPOS LHC event generators.Peer reviewe

Inclusive J/psi production at midrapidity in pp collisions at root s=13 Tev

We report on the inclusive J/psi production cross section measured at the CERN Large Hadron Collider in proton-proton collisions at a center-of-mass energy root s = 13 TeV. The J/psi mesons are reconstructed in the e(+)e(-) decay channel and the measurements are performed at midrapidity (vertical bar y vertical bar < 0.9) in the transverse-momentum interval 0 < p(T) < 40 GeV/c, using a minimum-bias data sample corresponding to an integrated luminosity L-i(nt) = 32.2 nb(-1) and an Electromagnetic Calorimeter triggered data sample with L-i(nt) = 8.3 pb(-1). The p(T)-integrated J/psi production cross section at midrapidity, computed using the minimumbias data sample, is d sigma/dy vertical bar(y) =0 = 8.97 +/- 0.24 (stat) 0.48 (syst) +/- 0.15 (lumi) mu b. An approximate logarithmic dependence with the collision energy is suggested by these results and available world data, in agreement with model predictions. The integrated and p(T)-differential measurements are compared with measurements in pp collisions at lower energies and with several recent phenomenological calculations based on the non-relativistic QCD and Color Evaporation models.Peer reviewe

Anisotropic flow of identified hadrons in Xe-Xe collisions at root s(NN)=5.44 TeV

Measurements of elliptic (v(2)) and triangular (v(3)) flow coefficients of pi(+/-), K-+/-, p+(p) over bar, K-S(0), and Lambda+(Lambda) over bar obtained with the scalar product method in Xe-Xe collisions at root sNN = 5.44TeV are presented. The results are obtained in the rapidity range |y| < 0.5 and reported as a function of transverse momentum, p(T), for several collision centrality classes. The flow coefficients exhibit a particle mass dependence for pT < 3 GeV/c, while a grouping according to particle type (i.e., meson and baryon) is found at intermediate transverse momenta (3 < p(T) < 8 GeV/c). The magnitude of the baryon v(2) is larger than that of mesons up to pT = 6 GeV/c. The centrality dependence of the shape evolution of the p(T)-differential v(2) is studied for the various hadron species. The v(2) coefficients of pi(+/-), K-+/-, and p+(p) over bar are reproduced by MUSIC hydrodynamic calculations coupled to a hadronic cascade model (UrQMD) for p(T) < 1 GeV/c. A comparison with vn measurements in the corresponding centrality intervals in Pb-Pb collisions at root sNN = 5.02TeV yields an enhanced v(2) in central collisions and diminished value in semicentral collisions.Peer reviewe

Prompt and non-prompt J/psi production cross sections at midrapidity in proton-proton collisions at root s=5.02 and 13 TeV

The production of J/psi is measured at midrapidity (vertical bar y vertical bar < 0.9) in proton-proton collisions at root s = 5.02 and 13 TeV, through the dielectron decay channel, using the ALICE detector at the Large Hadron Collider. The data sets used for the analyses correspond to integrated luminosities of L-int = 19.4 +/- 0.4 nb(-1 )and L-int = 32.2 +/- 0.5 nb(-1) at root s = 5.02 and 13 TeV, respectively. The fraction of non-prompt J/psi mesons, i.e. those originating from the decay of beauty hadrons, is measured down to a transverse momentum p(T) = 2 GeV/c (1GeV/c) at root s = 5.02 TeV (13 TeV). The p(T) and rapidity (y) differential cross sections, as well as the corresponding values integrated over p(T) and y, are carried out separately for prompt and non-prompt J/psi mesons. The results are compared with measurements from other experiments and theoretical calculations based on quantum chromodynamics (QCD). The shapes of the p(T) and y distributions of beauty quarks predicted by state-of-the-art perturbative QCD models are used to extrapolate an estimate of the b (b) over bar pair cross section at midrapidity and in the total phase space. The total b (b) over bar cross sections are found to be sigma(b (b) over bar) = 541 +/- 45 (stat.) +/- 69 (syst.)(-12)(+10) (extr.) mu b and sigma(b (b) over bar) = 218 +/- 37 (stat.)+/- 31 (syst.)(-9.1)(+8.2) (extr.) mu b at root s = 13 and 5.02 TeV, respectively. The value obtained from the combination of ALICE and LHCb measurements in pp collisions at root s = 13 TeV is also provided.Peer reviewe

Polarization of Lambda and (Lambda)over-bar Hyperons along the Beam Direction in Pb-Pb Collisions at root s(NN)=5.02 TeV

The polarization of the Lambda and (Lambda) over bar hyperons along the beam (z) direction, P-z, has been measured in Pb-Pb collisions at root s(NN) = 5.02 TeV recorded with ALICE at the Large Hadron Collider (LHC). The main contribution to P-z comes from elliptic flow-induced vorticity and can be characterized by the second Fourier sine coefficient P-z,P-s2 = , where phi is the hyperon azimuthal emission angle and Psi(2) is the elliptic flow plane angle. We report the measurement of P-z,P-s2 for different collision centralities and in the 30%-50% centrality interval as a function of the hyperon transverse momentum and rapidity. The P-z,P-s2 is positive similarly as measured by the STAR Collaboration in Au-Au collisions at root s(NN) = 200 GeV, with somewhat smaller amplitude in the semicentral collisions. This is the first experimental evidence of a nonzero hyperon P-z in Pb-Pb collisions at the LHC. The comparison of the measured P-z,P-s2 with the hydrodynamic model calculations shows sensitivity to the competing contributions from thermal and the recently found shear-induced vorticity, as well as to whether the polarization is acquired at the quark-gluon plasma or the hadronic phase.Peer reviewe

Hypertriton Production in p-Pb Collisions at root S-NN =5.02 TeV

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Study of very forward energy and its correlation with particle production at midrapidity in pp and p-Pb collisions at the LHC

The energy deposited at very forward rapidities (very forward energy) is a powerful tool for characterising proton fragmentation in pp and p-Pb collisions. The correlation of very forward energy with particle production at midrapidity provides direct insights into the initial stages and the subsequent evolution of the collision. Furthermore, the correlation with the production of particles with large transverse momenta at midrapidity provides information complementary to the measurements of the underlying event, which are usually interpreted in the framework of models implementing centrality-dependent multiple parton interactions. Results about very forward energy, measured by the ALICE zero degree calorimeters (ZDCs), and its dependence on the activity measured at midrapidity in pp collisions at root s = 13 TeV and in p-Pb collisions at root s(NN) = 8.16 TeV are discussed. The measurements performed in pp collisions are compared with the expectations of three hadronic interaction event generators: PYTHIA 6 (Perugia 2011 tune), PYTHIA 8 (Monash tune), and EPOS LHC. These results provide new constraints on the validity of models in describing the beam remnants at very forward rapidities, where perturbative QCD cannot be used.Peer reviewe

Antihelium-3 fluxes near Earth using data-driven 2 estimates for annihilation cross section

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Measurements of the groomed and ungroomed jet angularities in pp collisions at root s=5.02 TeV

The jet angularities are a class of jet substructure observables which characterize the angular and momentum distribution of particles within jets. These observables are sensitive to momentum scales ranging from perturbative hard scatterings to nonperturbative fragmentation into final-state hadrons. We report measurements of several groomed and ungroomed jet angularities in pp collisions at root s = 5.02 TeV with the ALICE detector. Jets are reconstructed using charged particle tracks at midrapidity (vertical bar eta vertical bar< 0.9). The anti-k(T) algorithm is used with jet resolution parameters R = 0.2 and R = 0.4 for several transverse momentum p(T)(ch jet) intervals in the 20-100 GeV/c range. Using the jet grooming algorithm Soft Drop, the sensitivity to softer, wide-angle processes, as well as the underlying event, can be reduced in a way which is well-controlled in theoretical calculations. We report the ungroomed jet angularities, lambda(alpha), and groomed jet angularities, lambda(alpha,g), to investigate the interplay between perturbative and nonperturbative effects at low jet momenta. Various angular exponent parameters alpha = 1, 1.5, 2, and 3 are used to systematically vary the sensitivity of the observable to collinear and soft radiation. Results are compared to analytical predictions at next-to-leading-logarithmic accuracy, which provide a generally good description of the data in the perturbative regime but exhibit discrepancies in the nonperturbative regime. Moreover, these measurements serve as a baseline for future ones in heavy-ion collisions by providing new insight into the interplay between perturbative and nonperturbative effects in the angular and momentum substructure of jets. They supply crucial guidance on the selection of jet resolution parameter, jet transverse momentum, and angular scaling variable for jet quenching studies.Peer reviewe