Two-pion femtoscopy in p-Pb collisions at root(NN)-N-S=5.02 TeV

We report the results of the femtoscopic analysis of pairs of identical pions measured in p-Pb collisions at root(NN)-N-S = 5.02 TeV. Femtoscopic radii are determined as a function of event multiplicity and pair momentum in three spatial dimensions. As in the pp collision system, the analysis is complicated by the presence of sizable background correlation structures in addition to the femtoscopic signal. The radii increase with event multiplicity and decrease with pair transverse momentum. When taken at comparable multiplicity, the radii measured in p-Pb collisions, at high multiplicity and low pair transverse momentum, are 10%-20% higher than those observed in pp collisions but below those observed in A-A collisions. The results are compared to hydrodynamic predictions at large event multiplicity as well as discussed in the context of calculations based on gluon saturation.Peer reviewe

Rapidity and transverse-momentum dependence of the inclusive J/psi nuclear modification factor in p-Pb collisions at root s(NN)=5.02 TeV

We have studied the transverse-momentum (p(T)) dependence of the inclusive J/psi production in p-Pb collisions at root s(NN) = 5.02 TeV, in three center-of-mass rapidity (y(cms)) regions, down to zero p(T). Results in the forward and backward rapidity ranges (2.03 <y(cms) <3.53 and -4.46 <y(cms) <-2.96) are obtained by studying the J/psi decay to mu(+)mu(-), while the mid-rapidity region (-1.37 <y(cms) <0.43) is investigated by measuring the e(+)e(-) decay channel. The p(T) dependence of the J/psi production cross section and nuclear modification factor are presented for each of the rapidity intervals, as well as the J/psi mean p(T) values. Forward and mid-rapidity results show a suppression of the J/psi yield, with respect to pp collisions, which decreases with increasing p(T). At backward rapidity no significant J/psi suppression is observed. Theoretical models including a combination of cold nuclear matter effects such as shadowing and partonic energy loss, are in fair agreement with the data, except at forward rapidity and low transverse momentum. The implications of the p-Pb results for the evaluation of cold nuclear matter effects on J/psi production in Pb-Pb collisions are also discussed.Peer reviewe

Centrality dependence of inclusive J/psi production in p-Pb collisions at root S-NN=5.02TeV

We present a measurement of inclusive J/psi production in p-Pb collisions at root S-NN = 5.02 TeV as a function of the centrality of the collision, as estimated from the energy deposited in the Zero Degree Calorimeters. The measurement is performed with the ALICE detector down to zero transverse momentum, p(T), in the backward (-4.46 <y(cms) <-2.96) and forward (2.03 <y(cms) <3.53) rapidity intervals in the dimuon decay channel and in the mid-rapidity region (-1.37 <y(cms) <0.43) in the dielectron decay channel. The backward and forward rapidity intervals correspond to the Pb-going and p-going direction, respectively. The p(T)-differential J/psi production cross section at backward and forward rapidity is measured for several centrality classes, together with the corresponding average p(T) and p(T)(2) values. The nuclear modification factor is presented as a function of centrality for the three rapidity intervals, and as a function of p(T) for several centrality classes at backward and forward rapidity. At mid-and forward rapidity, the J/psi yield is suppressed up to 40% compared to that in pp interactions scaled by the number of binary collisions. The degree of suppression increases towards central p-Pb collisions at forward rapidity, and with decreasing p(T) of the J/psi. At backward rapidity, the nuclear modification factor is compatible with unity within the total uncertainties, with an increasing trend from peripheral to central p-Pb collisions.Peer reviewe

Centrality dependence of pion freeze-out radii in Pb-Pb collisions at root s(NN)=2.76 TeV

We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb-Pb collisions at root s(NN) = 2.76 TeV as a function of collision centrality and the average transverse momentum of the pair k(T). Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with k(T), following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with (1/3). This behavior is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller root s(NN), a decrease in the ratio R-out/R-side is seen, which is in qualitative agreement with a specific prediction from hydrodynamic models: a change from inside-out to outside-in freeze-out configuration. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the CERN Large Hadron Collider.Peer reviewe

Probing the Effects of Strong Electromagnetic Fields with Charge-Dependent Directed Flow in Pb-Pb Collisions at the LHC

The first measurement at the LHC of charge-dependent directed flow (v(1)) relative to the spectator plane is presented for Pb-Pb collisions at root s(NN) = 5.02 TeV. Results are reported for charged hadrons and D-0 mesons for the transverse momentum intervals p(T) > 0.2 GeV/c and 3 <p(T) <6 GeV/c in the 5%-40% and 10%-40% centrality classes, respectively. The difference between the positively and negatively charged hadron v(1) has a positive slope as a function of pseudorapidity eta, d Delta v(1)/d eta = [1.68 +/- 0.49(stat) +/- 0.41(syst)] x 10(-4). The same measurement for D-0 and (D) over bar (0) mesons yields a positive value d Delta v(1)/d eta = [4.9 +/- 1.7(stat)] +/- 0.6(syst)] x 10(-1), which is about 3 orders of magnitude larger than the one of the charged hadrons. These measurements can provide new insights into the effects of the strong electromagnetic field and the initial tilt of matter created in noncentral heavy ion collisions on the dynamics of light (u, d, and s) and heavy (c) quarks. The large difference between the observed Delta v(1) of charged hadrons and D-0 mesons may reflect different sensitivity of the charm and light quarks to the early time dynamics of a heavy ion collision. These observations challenge some recent theoretical calculations, which predicted a negative and an order of magnitude smaller value of d Delta v(1)/d eta for both light flavor and charmed hadrons.Peer reviewe

Investigation of the p-Sigma(0) interaction via femtoscopy in pp collisions

This Letter presents the first direct investigation of the p-Sigma(0) interaction, using the femtoscopy technique in high-multiplicity pp collisions at root s = 13 TeV measured by the ALICE detector. The Sigma(0) is reconstructed via the decay channel to Lambda gamma, and the subsequent decay of Lambda to p pi(-). The photon is detected via the conversion in material to e(+)e(-) pairs exploiting the capability of the ALICE detector to measure electrons at low transverse momenta. The measured p-Sigma(0) correlation indicates a shallow strong interaction. The comparison of the data to several theoretical predictions obtained employing the Correlation Analysis Tool using the Schrodinger Equation (CATS) and the Lednicky-Lyuboshits approach shows that the current experimental precision does not yet allow to discriminate between different models, as it is the case for the available scattering and hypernuclei data. Nevertheless, the p-Sigma(0) correlation function is found to be sensitive to the strong interaction, and driven by the interplay of the different spin and isospin channels. This pioneering study demonstrates the feasibility of a femtoscopic measurement in the p-Sigma(0) channel and with the expected larger data samples in LHC Run 3 and Run 4, the p-Sigma(0) interaction will be constrained with high precision. (C) 2020 European Organization for Nuclear Research. Published by Elsevier B.V.Peer reviewe

Two-particle differential transverse momentum and number density correlations in p-Pb collisions at 5.02 TeV and Pb-Pb collisions at 2.76 TeV at the CERN Large Hadron Collider

We present measurements of two-particle differential number correlation functions R-2 and transverse momentum correlation functions P-2, obtained from p-Pb collisions at 5.02 TeV and Pb-Pb collisions at 2.76 TeV. The results are obtained by using charged particles in the pseudorapidity range vertical bar eta vertical bar <1.0 and transverse momentum range 0.2 <p(T) <2.0 GeV/c as a function of pair separation in pseudorapidity, vertical bar Delta eta vertical bar, azimuthal angle Delta phi, and for several charged-particle multiplicity classes. Measurements are carried out for like-sign and unlike-sign charged-particle pairs separately and combined to obtain charge-independent and charge-dependent correlation functions. We study the evolution of the width of the near-side peak of these correlation functions with collision centrality. Additionally, we study Fourier decompositions of the correlators in Delta phi as a function of pair separation vertical bar Delta eta vertical bar. Significant differences in the dependence of their harmonic coefficients on multiplicity classes are found. These differences can be exploited, in theoretical models, to obtain further insight into charged-particle production and transport in heavy-ion collisions. Moreover, an upper limit of nonflow contributions to flow coefficients nu(n) measured in Pb-Pb collisions based on the relative strength of Fourier coefficients measured in p-Pb interactions is estimated.Peer reviewe

One-dimensional charged kaon femtoscopy in p-Pb collisions at root s(NN)=5.02 TeV

The correlations of identical charged kaons were measured in p-Pb collisions at root s(NN) = 5.02 TeV by the ALICE experiment at the LHC. The femtoscopic invariant radii and correlation strengths were extracted from one-dimensional kaon correlation functions and were compared with those obtained in pp and Pb-Pb collisions at root s = 7 TeV and root s(NN) = 2.76 TeV, respectively. The presented results also complement the identical-pion femtoscopic data published by the ALICE collaboration. The extracted radii increase with increasing charged-particle multiplicity and decrease with increasing pair transverse momentum. At comparable multiplicities, the radii measured in p-Pb collisions are found to be close to those observed in pp collisions. The obtained femtoscopic parameters are reproduced by the EPOS 3 hadronic interaction model and disfavor models with large initial size or strong collective expansion at low multiplicities.Peer reviewe