Search for weakly decaying Λn@? and ΛΛ exotic bound states in central Pb-Pb collisions at s_NN=2.76 TeV

North-Holland Pub. Co 2016 Physics letters. Section B Vol.752 No.-

<P>We present results of a search for two hypothetical strange dibaryon states, i.e. the H-dibaryon and the possible (Lambda n) over bar bound state. The search is performed with the ALICE detector in central (0-10%) Pb-Pb collisions at root S-NN = 2.76 TeV, by invariant mass analysis in the decay modes (Lambda n) over bar (d) over bar pi(+) and H-dibaryon -> Lambda p pi(-). No evidence for these bound states is observed. Upper limits are determined at 99% confidence level for a wide range of lifetimes and for the full range of branching ratios. The results are compared to thermal, coalescence and hybrid UrQMD model expectations, which describe correctly the production of other loosely bound states, like the deuteron and the hypertriton. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.</P>

HΛ3 and H@?Λ@?3 production in Pb-Pb collisions at s_NN=2.76 TeV

North-Holland Pub. Co 2016 Physics letters. Section B Vol.754 No.-

<P>The production of the hypertriton nuclei H-3(Lambda) and (3)((Lambda) over bar)(H) over bar has been measured for the first time in Pb-Pb collisions at root s(NN) = 2.76 TeVwith the ALICE experiment at LHC. The p(T)-integrated H-3(Lambda) yield in one unity of rapidity, dN/dy x B.R.((3 Lambda H -> 3He,pi-)) = (3.86 +/- 0.77(stat.) +/- 0.68(syst.)) x 10(-5) in the 0-10% most central collisions, is consistent with the predictions from a statistical thermal model using the same temperature as for the light hadrons. The coalescence parameter B-3 shows a dependence on the transverse momentum, similar to the B-2 of deuterons and the B-3 of He-3 nuclei. The ratio of yields S-3 = H-3(Lambda)/(He-3 x Lambda/p) was measured to be S-3 = 0.60 +/- 0.13(stat.) +/- 0.21(syst.) in 0-10% centrality events; this value is compared to different theoretical models. The measured S-3 is compatible with thermal model predictions. The measured H-3(Lambda) lifetime, tau = 181(-39)(+54)(stat.) +/- 33(syst.) psis in agreement within 1s with the world average value. (C) 2016 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.</P>

Direct photon production in Pb-Pb collisions at s_NN=2.76 TeV

North-Holland Pub. Co 2016 Physics letters: B Vol.754 No.-

<P>Direct photon production at mid-rapidity in Pb-Pb collisions at root s(NN) = 2.76 TeV was studied in the transverse momentum range 0.9 < p(T) < 14 GeV/c. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the ALICE detector material with the e(+)e(-) pair reconstructed in the central tracking system. The results of the two methods were combined and direct photon spectra were measured for the 0-20%, 20-40%, and 40-80% centrality classes. For all three classes, agreement was found with perturbative QCD calculations for p(T) greater than or similar to 5 GeV/c. Direct photon spectra down to p(T) approximate to 1 GeV/c could be extracted for the 20-40% and 0-20% centrality classes. The significance of the direct photon signal for 0.9 < p(T) < 2.1 GeV/c is 2.6 sigma for the 0-20% class. The spectrum in this p(T) range and centrality class can be described by an exponential with an inverse slope parameter of (297 +/- 12(stat) +/- 41(syst)) MeV. State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties. (C) 2016 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.</P>

Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb-Pb collisions at s_NN=2.76 TeV

North-Holland Pub. Co 2016 Physics letters. Section B Vol.754 No.-

<P>The centrality dependence of the charged-particle pseudorapidity density measured with ALICE in Pb-Pb collisions at root s(NN) = 2.76 TeV over a broad pseudorapidity range is presented. This Letter extends the previous results reported by ALICE to more peripheral collisions. No strong change of the overall shape of charged-particle pseudorapidity density distributions with centrality is observed, and when normalised to the number of participating nucleons in the collisions, the evolution over pseudorapidity with centrality is likewise small. The broad pseudorapidity range (-3.5 < eta < 5) allows precise estimates of the total number of produced charged particles which we find to range from 162 +/- 22(syst.) to 17170 +/- 770(syst.) in 80-90% and 0-5% central collisions, respectively. The total charged-particle multiplicity is seen to approximately scale with the number of participating nucleons in the collision. This suggests that hard contributions to the charged-particle multiplicity are limited. The results are compared to models which describe dN(ch)/d(eta) at mid-rapidity in the most central Pb-Pb collisions and it is found that these models do not capture all features of the distributions. (C) 2016 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.</P>

Measurement of electrons from heavy-flavour hadron decays in p-Pb collisions at s_NN=5.02TeV

North-Holland Pub. Co 2016 Physics letters. Section B Vol.754 No.-

<P>The production of electrons from heavy-flavour hadron decays was measured as a function of transverse momentum (p(T)) in minimum-bias p-Pb collisions at root s(NN) = 5.02 TeV using the ALICE detector at the LHC. The measurement covers the p(T) interval 0.5 < p(T) < 12 GeV/c and the rapidity range -1.065 < y(cms) < 0.135 in the centre-of-mass reference frame. The contribution of electrons from background sources was subtracted using an invariant mass approach. The nuclear modification factor R-pPb was calculated by comparing the p(T)-differential invariant cross section in p-Pb collisions to a pp reference at the same centre-of-mass energy, which was obtained by interpolating measurements at root s = 2.76 TeV and root s= 7 TeV. The R-pPb is consistent with unity within uncertainties of about 25%, which become larger for p(T) below 1 GeV/c. The measurement shows that heavy-flavour production is consistent with binary scaling, so that a suppression in the high-p(T) yield in Pb-Pb collisions has to be attributed to effects induced by the hot medium produced in the final state. The data in p-Pb collisions are described by recent model calculations that include cold nuclear matter effects. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.</P>

Centrality dependence of the pseudorapidity density distribution for charged particles in Pb-Pb collisions at s_NN=5.02 TeV

North-Holland Pub. Co 2017 Physics letters. Section B Vol.772 No.-

We present the charged-particle pseudorapidity density in Pb-Pb collisions at s<SUB>NN</SUB>=5.02 TeV in centrality classes measured by ALICE. The measurement covers a wide pseudorapidity range from -3.5 to 5, which is sufficient for reliable estimates of the total number of charged particles produced in the collisions. For the most central (0-5%) collisions we find 21400+/-1300, while for the most peripheral (80-90%) we find 230+/-38. This corresponds to an increase of (27+/-4)% over the results at s<SUB>NN</SUB>=2.76 TeV previously reported by ALICE. The energy dependence of the total number of charged particles produced in heavy-ion collisions is found to obey a modified power-law like behaviour. The charged-particle pseudorapidity density of the most central collisions is compared to model calculations - none of which fully describes the measured distribution. We also present an estimate of the rapidity density of charged particles. The width of that distribution is found to exhibit a remarkable proportionality to the beam rapidity, independent of the collision energy from the top SPS to LHC energies.

Neutral pion and η meson production in proton-proton collisions at s=0.9 TeV and s=7 TeV

ALICE Collaboration,Abelev, B.,Abrahantes Quintana, A.,Adamova, D.,Adare, A.M.,Aggarwal, M.M.,Aglieri Rinella, G.,Agocs, A.G.,Agostinelli, A.,Aguilar Salazar, S.,Ahammed, Z.,Ahmad, N.,Ahmad Masoodi, A North-Holland Pub. Co 2012 Physics letters. Section B Vol.717 No.1

The first measurements of the invariant differential cross sections of inclusive π<SUP>0</SUP> and η meson production at mid-rapidity in proton-proton collisions at s=0.9 TeV and s=7 TeV are reported. The π<SUP>0</SUP> measurement covers the ranges 0.4<p<SUB>T</SUB><7 GeV/c and 0.3<p<SUB>T</SUB><25 GeV/c for these two energies, respectively. The production of η mesons was measured at s=7 TeV in the range 0.4<p<SUB>T</SUB><15 GeV/c. Next-to-Leading Order perturbative QCD calculations, which are consistent with the π<SUP>0</SUP> spectrum at s=0.9 TeV, overestimate those of π<SUP>0</SUP> and η mesons at s=7 TeV, but agree with the measured η/π<SUP>0</SUP> ratio at s=7 TeV.

Two-pion Bose-Einstein correlations in central Pb-Pb collisions at s_NN=2.76 TeV

ALICE Collaboration,Aamodt, K.,Abrahantes Quintana, A.,Adamova, D.,Adare, A.M.,Aggarwal, M.M.,Aglieri Rinella, G.,Agocs, A.G.,Aguilar Salazar, S.,Ahammed, Z.,Ahmad, N.,Ahmad Masoodi, A.,Ahn, S.U.,Akin North-Holland Pub. Co 2011 Physics letters: B Vol.696 No.4

The first measurement of two-pion Bose-Einstein correlations in central Pb-Pb collisions at s<SUB>NN</SUB>=2.76 TeV at the Large Hadron Collider is presented. We observe a growing trend with energy now not only for the longitudinal and the outward but also for the sideward pion source radius. The pion homogeneity volume and the decoupling time are significantly larger than those measured at RHIC.

Production of charged pions, kaons and protons at large transverse momenta in pp and Pb-Pb collisions at s_NN=2.76 TeV

North-Holland Pub. Co 2014 Physics letters. Section B Vol.736 No.-

Transverse momentum spectra of π<SUP>+/-</SUP>, K<SUP>+/-</SUP> and p(p@?) up to p<SUB>T</SUB>=20 GeV/c at mid-rapidity in pp, peripheral (60-80%) and central (0-5%) Pb-Pb collisions at s<SUB>NN</SUB>=2.76 TeV have been measured using the ALICE detector at the Large Hadron Collider. The proton-to-pion and the kaon-to-pion ratios both show a distinct peak at p<SUB>T</SUB>~3 GeV/c in central Pb-Pb collisions. Below the peak, p<SUB>T</SUB><3 GeV/c, both ratios are in good agreement with hydrodynamical calculations, suggesting that the peak itself is dominantly the result of radial flow rather than anomalous hadronization processes. For p<SUB>T</SUB>>10 GeV/c particle ratios in pp and Pb-Pb collisions are in agreement and the nuclear modification factors for π<SUP>+/-</SUP>, K<SUP>+/-</SUP> and p(p@?) indicate that, within the systematic and statistical uncertainties, the suppression is the same. This suggests that the chemical composition of leading particles from jets in the medium is similar to that of vacuum jets.

Long-range angular correlations on the near and away side in p-Pb collisions at s_NN=5.02 TeV

ALICE Collaboration,Abelev, B.,Adam, J.,Adamova, D.,Adare, A.M.,Aggarwal, M.,Aglieri Rinella, G.,Agnello, M.,Agocs, A.G.,Agostinelli, A.,Ahammed, Z.,Ahmad, N.,Ahmad, A.,Ahn, S.A.,Ahn, S.U.,Ajaz, M.,Ak North-Holland Pub. Co 2013 Physics letters: B Vol.719 No.1

Angular correlations between charged trigger and associated particles are measured by the ALICE detector in p-Pb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV for transverse momentum ranges within 0.5<p<SUB>T,assoc</SUB><p<SUB>T,trig</SUB><4 GeV/c. The correlations are measured over two units of pseudorapidity and full azimuthal angle in different intervals of event multiplicity, and expressed as associated yield per trigger particle. Two long-range ridge-like structures, one on the near side and one on the away side, are observed when the per-trigger yield obtained in low-multiplicity events is subtracted from the one in high-multiplicity events. The excess on the near-side is qualitatively similar to that recently reported by the CMS Collaboration, while the excess on the away-side is reported for the first time. The two-ridge structure projected onto azimuthal angle is quantified with the second and third Fourier coefficients as well as by near-side and away-side yields and widths. The yields on the near side and on the away side are equal within the uncertainties for all studied event multiplicity and p<SUB>T</SUB> bins, and the widths show no significant evolution with event multiplicity or p<SUB>T</SUB>. These findings suggest that the near-side ridge is accompanied by an essentially identical away-side ridge.