Enhanced production of multi-strange hadrons in high-multiplicity proton-proton collisions

At sufficiently high temperature and energy density, nuclear matter undergoes a transition to a phase in which quarks and gluons are not confined: the quark-gluon plasma (QGP)(1). Such an exotic state of strongly interacting quantum chromodynamics matter is produced in the laboratory in heavy nuclei high-energy collisions, where an enhanced production of strange hadrons is observed(2-6). Strangeness enhancement, originally proposed as a signature of QGP formation in nuclear collisions(7), is more pronounced for multi-strange baryons. Several effects typical of heavy-ion phenomenology have been observed in high-multiplicity proton-proton (pp) collisions(8,9), but the enhanced production of multi-strange particles has not been reported so far. Here we present the first observation of strangeness enhancement in high-multiplicity proton-proton collisions. We find that the integrated yields of strange and multi-strange particles, relative to pions, increases significantly with the event charged-particle multiplicity. The measurements are in remarkable agreement with the p-Pb collision results(10,11), indicating that the phenomenon is related to the final system created in the collision. In high-multiplicity events strangeness production reaches values similar to those observed in Pb-Pb collisions, where a QGP is formed.Peer reviewe

D-Meson Azimuthal Anisotropy in Midcentral Pb-Pb Collisions root S-NN=5.02 TeV

The azimuthal anisotropy coefficient v(2) of prompt D-0, D+, D*+, and D-s(+) mesons was measured in midcentral (30%-50% centrality class) Pb-Pb collisions at a center-of-mass energy per nucleon pair root s(NN)=5.02 TeV, with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decays at midrapidity, |y| < 0.8, in the transverse momentum interval 1 < p(T) < 24 GeV/c. The measured D-meson v(2) has similar values as that of charged pions. The D-s(+) v(2), measured for the first time, is found to be compatible with that of nonstrange D mesons. The measurements are compared with theoretical calculations of charm-quark transport in a hydrodynamically expanding medium and have the potential to constrain medium parameters.Peer reviewe

Production of deuterons, tritons, He-3 nuclei, and their antinuclei in pp collisions at root s=0.9, 2.76, and 7 TeV

Invariant differential yields of deuterons and antideuterons in pp collisions at root s = 0.9, 2.76 and 7 TeV and the yields of tritons, He-3 nuclei, and their antinuclei at root s = 7 TeV have been measured with the ALICE detector at the CERN Large Hadron Collider. The measurements cover a wide transverse momentum (p(T)) range in the rapidity interval vertical bar y vertical bar <0.5, extending both the energy and the pT reach of previous measurements up to 3 GeV/c for A = 2 and 6 GeV/c for A = 3. The coalescence parameters of (anti) deuterons and 3 He nuclei exhibit an increasing trend with pT and are found to be compatible with measurements in pA collisions at low p(T) and lower energies. The integrated yields decrease by a factor of about 1000 for each increase of the mass number with one (anti) nucleon. Furthermore, the deuteron-to-proton ratio is reported as a function of the average charged particle multiplicity at different center-of-mass energies.Peer reviewe

Linear and non-linear flow mode in Pb-Pb collisions at root sNN=2.76 TeV

The second and the third order anisotropic flow, V-2 and V-3, are mostly determined by the corresponding initial spatial anisotropy coefficients, epsilon(2) and epsilon(3), in the initial density distribution. In addition to their dependence on the same order initial anisotropy coefficient, higher order anisotropic flow, Vn(n > 3), can also have a significant contribution from lower order initial anisotropy coefficients, which leads to mode-coupling effects. In this Letter we investigate the linear and non-linear modes in higher order anisotropic flow V-n for n = 4, 5, 6 with the ALICE detector at the Large Hadron Collider. The measurements are done for particles in the pseudorapidity range |eta| <0.8 and the transverse momentum range 0.2 <p(T)<5.0 GeV/c as a function of collision centrality. The results are compared with theoretical calculations and provide important constraints on the initial conditions, including initial spatial geometry and its fluctuations, as well as the ratio of the shear viscosity to entropy density of the produced system. (C) 2017 The Author(s). Published by Elsevier B.V.Peer reviewe

Production of muons from heavy-flavour hadron decays in p-Pb collisions at root s(NN)=5.02 TeV

The production of muons from heavy-flavour hadron decays in p-Pb collisions at root s(NN) = 5.02 TeV was studied for 2 <p(T) <16 GeV/c with the ALICE detector at the CERN LHC. The measurement was performed at forward (p-going direction) and backward (Pb-going direction) rapidity, in the ranges of rapidity in the centre-of-mass system (cms) 2.03 <y(cms) <3.53 and -4.46 <y(cms) <-2.96, respectively. The production cross sections and nuclear modification factors are presented as a function of transverse momentum (P-T). At forward rapidity, the nuclear modification factor is compatible with unity while at backward rapidity, in the interval 2.5 <p(T) <3.5 GeV/c, it is above unity by more than 2 sigma. The ratio of the forward -to -backward production cross sections is also measured in the overlapping interval 2.96 <|y(cms)| <3.53 and is smaller than unity by 3.7 sigma in 2.5 <p(T) <3.5 GeV/c. The data are described by model calculations including cold nuclear matter effects. (C) 2017 The Author(s). Published by Elsevier B.V.Peer reviewe

Measurement of deuteron spectra and elliptic flow in Pb-Pb collisions at root s(NN)=2.76 TeV at the LHC

The transverse momentum (pT) spectra and elliptic flow coefficient (v2) of deuterons and anti-deuterons at mid-rapidity (| y| 1.8 GeV/ c within the experimental uncertainties. The measurement of the coalescence parameter B2is performed, showing a pT dependence in contrast with the simplest coalescence model, which fails to reproduce also the measured v2 coefficient. In addition, the coalescence parameter B2 and the elliptic flow coefficient in the 20–40% centrality interval are compared with the AMPT model which is able, in its version without string melting, to reproduce the measured v2(pT) and the B2(pT) trend

Kaon femtoscopy in Pb-Pb collisions at root s(NN)=2.76 TeV

We present the results of three-dimensional femtoscopic analyses for charged and neutral kaons recorded by ALICE in Pb-Pb collisions at root s(NN) = 2.76 TeV. Femtoscopy is used to measure the space-time characteristics of particle production from the effects of quantum statistics and final-state interactions in two-particle correlations. Kaon femtoscopy is an important supplement to that of pions because it allows one to distinguish between different model scenarios working equally well for pions. In particular, we compare the measured three-dimensional kaon radii with a purely hydrodynamical calculation and a model where the hydrodynamic phase is followed by a hadronic rescattering stage. The former predicts an approximate transverse mass (m(T)) scaling of source radii obtained from pion and kaon correlations. This m(T) scaling appears to be broken in our data, which indicates the importance of the hadronic rescattering phase at LHC energies. A k(T) scaling of pion and kaon source radii is observed instead. The time of maximal emission of the system is estimated by using the three-dimensional femtoscopic analysis for kaons. The measured emission time is larger than that of pions. Our observation is well supported by the hydrokinetic model predictions

Searches for transverse momentum dependent flow vector fluctuations in Pb-Pb and p-Pb collisions at the LHC

The measurement of azimuthal correlations of charged particles is presented for Pb-Pb collisions at sNN=2.76 TeV and p-Pb collisions at sNN=5.02 TeV with the ALICE detector at the CERN Large Hadron Collider. These correlations are measured for the second, third and fourth order flow vector in the pseudorapidity region |η| < 0.8 as a function of centrality and transverse momentum pT using two observables, to search for evidence of pT-dependent flow vector fluctuations. For Pb-Pb collisions at 2.76 TeV, the measurements indicate that pT-dependent fluctuations are only present for the second order flow vector. Similar results have been found for p-Pb collisions at 5.02 TeV. These measurements are compared to hydrodynamic model calculations with event-by-event geometry fluctuations in the initial state to constrain the initial conditions and transport properties of the matter created in Pb–Pb and p–Pb collisions

Measuring (KSK +/-)-K-0 interactions using Pb-Pb collisions at root S-NN=2.76 TeV

We present the first ever measurements of femtoscopic correlations between the KS0 and K± particles. The analysis was performed on the data from Pb–Pb collisions at sNN=2.76 TeV measured by the ALICE experiment. The observed femtoscopic correlations are consistent with final-state interactions proceeding via the a0(980) resonance. The extracted kaon source radius and correlation strength parameters for KS0K− are found to be equal within the experimental uncertainties to those for KS0K+. Comparing the results of the present study with those from published identical-kaon femtoscopic studies by ALICE, mass and coupling parameters for the a0 resonance are tested. Our results are also compatible with the interpretation of the a0 having a tetraquark structure instead of that of a diquark

Evolution of the longitudinal and azimuthal structure of the near-side jet peak in Pb-Pb collisions at sNN =2.76 TeV

In two-particle angular correlation measurements, jets give rise to a near-side peak, formed by particles associated to a higher-pT trigger particle. Measurements of these correlations as a function of pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the centrality and pT dependence of the shape of the near-side peak in the pT range 1<pT<8 GeV/c in Pb-Pb and pp collisions at sNN = 2.76 TeV. A combined fit of the near-side peak and long-range correlations is applied to the data and the peak shape is quantified by the variance of the distributions. While the width of the peak in the Δφ direction is almost independent of centrality, a significant broadening in the Δη direction is found from peripheral to central collisions. This feature is prominent for the low-pT region and vanishes above 4 GeV/c. The widths measured in peripheral collisions are equal to those in pp collisions in the Δφ direction and above 3 GeV/c in the Δη direction. Furthermore, for the 10% most central collisions and 1<pT,assoc< 2 GeV/c, 1<pT,trig< 3 GeV/c, a departure from a Gaussian shape is found: a depletion develops around the center of the peak. The results are compared to A Multi-Phase Transport (AMPT) model simulation as well as other theoretical calculations indicating that the broadening and the development of the depletion are connected to the strength of radial and longitudinal flow