Global baryon number conservation encoded in net-proton fluctuations measured in Pb–Pb collisions at √sNN = 2.76 TeV

Experimental results are presented on event-by-event net-proton fluctuation measurements in Pb–Pb collisions at √SNN=2.76 TeV, recorded by the ALICE detector at the CERN LHC. These measurements have as their ultimate goal an experimental test of Lattice QCD (LQCD) predictions on second and higher order cumulants of net-baryon distributions to search for critical behavior near the QCD phase boundary. Before confronting them with LQCD predictions, account has to be taken of correlations stemming from baryon number conservation as well as fluctuations of participating nucleons. Both effects influence the experimental measurements and are usually not considered in theoretical calculations. For the first time, it is shown that event-by-event baryon number conservation leads to subtle long-range correlations arising from very early interactions in the collisions.publishedVersio

Measurement of jet radial profiles in Pb\u2013Pb collisions at 1asNN = 2.76 TeV

The jet radial structure and particle transverse momentum (pT) composition within jets are presented in centrality-selected Pb\u2013Pb collisions at 1asNN = 2.76 TeV. Track-based jets, which are also called charged jets, were reconstructed with a resolution parameter of R = 0.3 at midrapidity |\u3b7ch jet| < 0.6 for transverse momenta pT, ch jet = 30\u2013120 GeV/c. Jet\u2013hadron correlations in relative azimuth and pseudorapidity space (\u3c6,\u3b7) are measured to study the distribution of the associated particles around the jet axis for different pT,assoc-ranges between 1 and 20 GeV/c. The data in Pb\u2013Pb collisions are compared to reference distributions for pp collisions, obtained using embedded PYTHIA simulations. The number of high-pT associate particles (4 < pT,assoc < 20 GeV/c) in Pb\u2013Pb collisions is found to be suppressed compared to the reference by 30 to 10%, depending on centrality. The radial particle distribution relative to the jet axis shows a moderate modification in Pb\u2013Pb collisions with respect to PYTHIA. High-pT associate particles are slightly more collimated in Pb\u2013Pb collisions compared to the reference, while low-pT associate particles tend to be broadened. The results, which are presented for the first time down to pT, ch jet = 30 GeV/c in Pb\u2013Pb collisions, are compatible with both previous jet\u2013hadron-related measurements from the CMS Collaboration and jet shape measurements from the ALICE Collaboration at higher pT, and add further support for the established picture of in-medium parton energy loss

Evidence of Spin-Orbital Angular Momentum Interactions in Relativistic Heavy-Ion Collisions

The first evidence of spin alignment of vector mesons (K^{*0} and ϕ) in heavy-ion collisions at the Large Hadron Collider (LHC) is reported. The spin density matrix element ρ_{00} is measured at midrapidity (|y|<0.5) in Pb-Pb collisions at a center-of-mass energy (sqrt[s_{NN}]) of 2.76 TeV with the ALICE detector. ρ_{00} values are found to be less than 1/3 (1/3 implies no spin alignment) at low transverse momentum (p_{T}<2  GeV/c) for K^{*0} and ϕ at a level of 3σ and 2σ, respectively. No significant spin alignment is observed for the K_{S}^{0} meson (spin=0) in Pb-Pb collisions and for the vector mesons in pp collisions. The measured spin alignment is unexpectedly large but qualitatively consistent with the expectation from models which attribute it to a polarization of quarks in the presence of angular momentum in heavy-ion collisions and a subsequent hadronization by the process of recombination

Study of the \u39b\u2013\u39b interaction with femtoscopy correlations in pp and p\u2013Pb collisions at the LHC

This work presents new constraints on the existence and the binding energy of a possible \u39b\u2013\u39b bound state, the H-dibaryon, derived from \u39b\u2013\u39b femtoscopic measurements by the ALICE collaboration. The results are obtained from a new measurement using the femtoscopy technique in (Figure presented.) collisions at s=13 TeV and p\u2013Pb collisions at sNN=5.02 TeV, combined with previously published results from (Figure presented.) collisions at s=7 TeV. The \u39b\u2013\u39b scattering parameter space, spanned by the inverse scattering length f0 121 and the effective range d0, is constrained by comparing the measured \u39b\u2013\u39b correlation function with calculations obtained within the Lednick\ufd model. The data are compatible with hypernuclei results and lattice computations, both predicting a shallow attractive interaction, and permit to test different theoretical approaches describing the \u39b\u2013\u39b interaction. The region in the (f0 121,d0) plane which would accommodate a \u39b\u2013\u39b bound state is substantially restricted compared to previous studies. The binding energy of the possible \u39b\u2013\u39b bound state is estimated within an effective-range expansion approach and is found to be B\u39b\u39b=3.2 122.4+1.6(stat) 121.0+1.8(syst) MeV

ϒ production in p–Pb collisions at sNN=8.16 TeV

ϒproduction in p–Pbinteractions is studied at the centre-of-mass energy per nucleon–nucleon collision √sNN=8.16TeV with the ALICE detector at the CERN LHC. The measurement is performed reconstructing bottomonium resonances via their dimuon decay channel, in the centre-of-mass rapidity intervals 2.03 &lt;3.53and −4.46 &lt;−2.96, down to zero transverse momentum. In this work, results on the ϒ(1S) production cross section as a function of rapidity and transverse momentum are presented. The corresponding nuclear modification factor shows a suppression of the ϒ(1S) yields with respect to ppcollisions, both at forward and backward rapidity. This suppression is stronger in the low transverse momentum region and shows no significant dependence on the centrality of the interactions. Furthermore, the ϒ(2S) nuclear modification factor is evaluated, suggesting a suppression similar to that of the ϒ(1S). A first measurement of the ϒ(3S) has also been performed. Finally, results are compared with previous ALICE measurements in p–Pbcollisions at √sNN=5.02TeV and with theoretical calculations

Search for a common baryon source in high-multiplicity pp collisions at the LHC

We report on the measurement of the size of the particle-emitting source from two-baryon correlations with ALICE in high-multiplicity pp collisions at s=13 TeV. The source radius is studied with low relative momentum p–p, p‾–p‾, p–Λ, and p‾–Λ‾ pairs as a function of the pair transverse mass mT considering for the first time in a quantitative way the effect of strong resonance decays. After correcting for this effect, the radii extracted for pairs of different particle species agree. This indicates that protons, antiprotons, Λ s, and Λ‾ s originate from the same source. Within the measured mT range (1.1–2.2) GeV/c2the invariant radius of this common source varies between 1.3 and 0.85 fm. These results provide a precise reference for studies of the strong hadron–hadron interactions and for the investigation of collective properties in small colliding systems. © 2020 CERN for the benefit of the ALICE CollaborationPeer reviewe

First measurement of the |t|-dependence of coherent J/ψ photonuclear production

The first measurement of the cross section for coherent J/ψ photoproduction as a function of |t|, the square of the momentum transferred between the incoming and outgoing target nucleus, is presented. The data were measured with the ALICE detector in ultra-peripheral Pb–Pb collisions at a centre-of-mass energy per nucleon pair sNN=5.02TeV with the J/ψ produced in the central rapidity region |y|<0.8, which corresponds to the small Bjorken-x range (0.3−1.4)×10−3. The measured |t|-dependence is not described by computations based only on the Pb nuclear form factor, while the photonuclear cross section is better reproduced by models including shadowing according to the leading-twist approximation, or gluon-saturation effects from the impact-parameter dependent Balitsky–Kovchegov equation. These new results are therefore a valid tool to constrain the relevant model parameters and to investigate the transverse gluonic structure at very low Bjorken-x.publishedVersio

Search for collectivity with azimuthal J/psi-hadron correlations in high multiplicity p-Pb collisions at ,root s(NN)=5.02 and 8.16 TeV

We present a measurement of azimuthal correlations between inclusive J/psi and charged hadrons in p-Pb collisions recorded with the ALICE detector at the CERN LHC. The J/psi are reconstructed at forward (p-going, 2.03 <y <3.53) and backward (Pb-going, -4.46 <y <-2.96) rapidity via their mu(+)mu(-) decay channel, while the charged hadrons are reconstructed at mid-rapidity (vertical bar eta vertical bar <1.8). The correlations are expressed in terms of associated charged-hadron yields per W . trigger. A rapidity gap of at least 1.5 units is required between the trigger J/psi and the associated charged hadrons. Possible correlations due to collective effects are assessed by subtracting the associated per-trigger yields in the low-multiplicity collisions from those in the high-multiplicity collisions. After the subtraction, we observe a strong indication of remaining symmetric structures at Delta phi approximate to 0 and Delta phi approximate to pi), similar to those previously found in two-particle correlations at middle and forward rapidity. The corresponding second-order Fourier coefficient (v(2) ) in the transverse momentum interval between 3 and 6 GeV/c is found to be positive with a significance of about 5 sigma. The obtained results are similar to the J/psi v(2) coefficients measured in Pb-Pb collisions at root s(NN) = 5.02 TeV, suggesting a common mechanism at the origin of the J/psi v(2) . (C) 2018 The Author. Published by Elsevier B.V.Peer reviewe