Energy and system dependence of nuclear modification factors of inclusive charged particles and identified light hadrons measured in p--Pb, Xe--Xe and Pb--Pb collisions with ALICE

We report recent ALICE results on primary charged particle and neutral meson production in pp, p--Pb, Pb--Pb and Xe--Xe collisions at LHC energies. In this article, measurements of the nuclear modification factors $R_{\rm AA}$ of primary charged particles and of light neutral mesons in Pb--Pb, in Xe--Xe and in p--Pb collisions in a wide $p_{\rm T}$ range and different centrality classes are discussed. We compare the nuclear modification factors obtained for different collision systems as a function of transverse momentum, collision centrality as well as charged particle multiplicity (${\rm d}N_{\rm ch} / {\rm d}\eta$). We also present comparison of experimental results to model calculations.Comment: 4 pages, 6 figures, Proceedings of the XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018), 13-19 May 201

核子対あたり重心系エネルギー5.02TeV 陽子-陽子及び鉛-鉛原子核衝突における中性中間子と直接光子測定

広島大学(Hiroshima University)博士(理学)Doctor of Sciencedoctora

Azimuthal anisotropy of charged jet production in root s(NN)=2.76 TeV Pb-Pb collisions

We present measurements of the azimuthal dependence of charged jet production in central and semi-central root s(NN) = 2.76 TeV Pb-Pb collisions with respect to the second harmonic event plane, quantified as nu(ch)(2) (jet). Jet finding is performed employing the anti-k(T) algorithm with a resolution parameter R = 0.2 using charged tracks from the ALICE tracking system. The contribution of the azimuthal anisotropy of the underlying event is taken into account event-by-event. The remaining (statistical) region-to-region fluctuations are removed on an ensemble basis by unfolding the jet spectra for different event plane orientations independently. Significant non-zero nu(ch)(2) (jet) is observed in semi-central collisions (30-50% centrality) for 20 <p(T)(ch) (jet) <90 GeV/c. The azimuthal dependence of the charged jet production is similar to the dependence observed for jets comprising both charged and neutral fragments, and compatible with measurements of the nu(2) of single charged particles at high p(T). Good agreement between the data and predictions from JEWEL, an event generator simulating parton shower evolution in the presence of a dense QCD medium, is found in semi-central collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Forward-central two-particle correlations in p-Pb collisions at root s(NN)=5.02 TeV

Two-particle angular correlations between trigger particles in the forward pseudorapidity range (2.5 2GeV/c. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B. V.Peer reviewe

Elliptic flow of muons from heavy-flavour hadron decays at forward rapidity in Pb-Pb collisions at root s(NN)=2.76TeV

The elliptic flow, v(2), of muons from heavy-flavour hadron decays at forward rapidity (2.5 <y <4) is measured in Pb-Pb collisions at root s(NN)= 2.76TeVwith the ALICE detector at the LHC. The scalar product, two- and four-particle Q cumulants and Lee-Yang zeros methods are used. The dependence of the v(2) of muons from heavy-flavour hadron decays on the collision centrality, in the range 0-40%, and on transverse momentum, p(T), is studied in the interval 3 <p(T)<10 GeV/c. A positive v(2) is observed with the scalar product and two-particle Q cumulants in semi-central collisions (10-20% and 20-40% centrality classes) for the p(T) interval from 3 to about 5GeV/c with a significance larger than 3 sigma, based on the combination of statistical and systematic uncertainties. The v(2) magnitude tends to decrease towards more central collisions and with increasing pT. It becomes compatible with zero in the interval 6 <p(T)<10 GeV/c. The results are compared to models describing the interaction of heavy quarks and open heavy-flavour hadrons with the high-density medium formed in high-energy heavy-ion collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.Peer reviewe

Pseudorapidity and transverse-momentum distributions of charged particles in proton-proton collisions at root s=13 TeV

The pseudorapidity (eta) and transverse-momentum (p(T)) distributions of charged particles produced in proton-proton collisions are measured at the centre-of-mass energy root s = 13 TeV. The pseudorapidity distribution in vertical bar eta vertical bar <1.8 is reported for inelastic events and for events with at least one charged particle in vertical bar eta vertical bar <1. The pseudorapidity density of charged particles produced in the pseudorapidity region vertical bar eta vertical bar <0.5 is 5.31 +/- 0.18 and 6.46 +/- 0.19 for the two event classes, respectively. The transverse-momentum distribution of charged particles is measured in the range 0.15 <p(T) <20 GeV/c and vertical bar eta vertical bar <0.8 for events with at least one charged particle in vertical bar eta vertical bar <1. The evolution of the transverse momentum spectra of charged particles is also investigated as a function of event multiplicity. The results are compared with calculations from PYTHIA and EPOS Monte Carlo generators. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb-Pb collisions at root s(NN)=2.76TeV

Peer reviewe

Measurement of neutral mesons and direct photons in pp and Pb-Pb collisions at sNN\sqrt{s_{\rm NN}} = 5.02 TeV

The new state of matter, called quark-gluon plasma (QGP), created by the high-energy heavy-ion collision has been studied for more than 40 years. Partons originating from initial hard scatterings lose their energy in the hot and dense QCD medium, which results in suppression of hadron production at high transverse momentum ($p_{\rm T}$), compared to pp collisions at the same center-of-mass energy $\sqrt{s_{\rm NN}}$. Light flavor particles are excellent probes to study the suppression in a wide $p_{\rm T}$ range with high precision. Especially, neutral mesons such as $\pi^{0}$ and $\eta$ mesons that decay into two photons can be reconstructed and identified by a fine-segmented electro-magnetic calorimeter in a wide $p_{\rm T}$ range. In this thesis, the suppression of $\pi^{0}$ and $\eta$ mesons in Pb--Pb collisions at the highest energy $\sqrt{s_{\rm NN}} = 5.02$ TeV is reported. By increasing the collision energy, $p_{\rm T}$ spectra of $\pi^{0}$ meson become harder than that at $\sqrt{s_{\rm NN}} = 2.76$ TeV in both pp and Pb-Pb collisions. Nevertheless, the suppression of $\pi^{0}$ meson in Pb-Pb collisions compared to pp collisions is the same level, which is by a factor of up to 8. This indicates the larger energy-loss at the higher collision energy. Comparing light and heavy flavor hadrons, namely $\pi^{0}$ and D mesons, the suppression of D mesons at low $p_{\rm T}$ is weaker than that of $\pi^{0}$ meson. This is interpreted as the smaller energy-loss for charm quarks than for up, down quarks. The suppression pattern of $\eta$ meson seems to be similar to $K^{\pm}$ meson consisting of a strange quark, though uncertainties for the $\eta$ meson measurement is large. Direct photons that are defined as photons not originating from hadron decays are also discussed in this thesis. Direct photons are unique probes to study the space-time evolution of the QGP, since they are not involved in strong interaction and can carry information when they are produced. When focusing on direct photons, $\pi^{0}$ and $\eta$ mesons contribute as huge backgrounds. To subtract decay photon yields, the cocktail simulation where $p_{\rm T}$ spectra of neutral mesons are inputs has been performed. Direct photon spectra or upper limits at the 90% of confidence level have been extracted. Finally, $R_{\rm AA}$ of direct photons has been determined and is consistent with unity at high $p_{\rm T}$ which justifies the measurement. On the other hand, the excess beyond the pQCD calculation is observed at low $p_{\rm T}$ by a factor of up to 4 in central Pb--Pb collisions. This indicates thermal photon emissions from the hot and dense QCD medium. The obtained effective temperature $T_{\rm eff}$ is $345 \pm 222 {\rm (total \ unc.)}$ MeV in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV for centrality 0-10%. This is the first measurement and setting upper limits on direct photons in pp and Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV