Particle identification in ALICE: a Bayesian approach

We present a Bayesian approach to particle identification (PID) within the ALICE experiment. The aim is to more effectively combine the particle identification capabilities of its various detectors. After a brief explanation of the adopted methodology and formalism, the performance of the Bayesian PID approach for charged pions, kaons and protons in the central barrel of ALICE is studied. PID is performed via measurements of specific energy loss ($\mathrm{d}E/\mathrm{d}x$) and time-of-flight. PID efficiencies and misidentification probabilities are extracted and compared with Monte Carlo simulations using high-purity samples of identified particles in the decay channels ${\rm K}^0_S \rightarrow \pi^-\pi^+$, $\phi \rightarrow {\rm K}^-{\rm K}^+$, and $\Lambda \rightarrow {\rm p}\pi^-$ in p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. In order to thoroughly assess the validity of the Bayesian approach, this methodology was used to obtain corrected $p_{\rm T}$ spectra of pions, kaons, protons, and D$^0$ mesons in pp collisions at $\sqrt{s}=7$ TeV. In all cases, the results using Bayesian PID were found to be consistent with previous measurements performed by ALICE using a standard PID approach. For the measurement of D$^0 \rightarrow {\rm K}^-\pi^+$, it was found that a Bayesian PID approach gave a higher signal-to-background ratio and a similar or larger statistical significance when compared with standard PID selections, despite a reduced identification efficiency. Finally, we present an exploratory study of the measurement of $\Lambda_{\rm c}^{+}\rightarrow {\rm p} {\rm K}^-\pi^+$ in pp collisions at $\sqrt{s}=7$ TeV, using the Bayesian approach for the identification of its decay products

Particle identification in ALICE : a Bayesian approach

Peer reviewe

Event-by-Event Identified Particle Ratio Fluctuations in Pb–Pb Collisions with ALICE using the Identity Method

The study of event-by-event fluctuations of identified hadrons may reveal the degrees of freedom of the strongly interacting matter created in heavy-ion collisions and the underlying dynamics of the system. The observable νdyn, which is defined in terms of the moments of identified-particle multiplicity distributions, is used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of particle ratios. The ALICE detector at the LHC is well-suited for the study of νdyn, due to its excellent particle identification capabilities. Particle identification based on the measurement of the specific ionisation energy loss, dE/dx, works well on a statistical basis but suffers from ambiguities when applied on an event-by-event level. A novel experimental technique called the ”Identity Method” is used to overcome such limitations. The first results on identified particle ratio fluctuations in Pb-Pb collisions at √sNN =2.76 TeV in ALICE as a function of centrality are presented. The ALICE results for the most peripheral events indicate an increasing correlation between pions and protons which is not reproduced by the HIJING and AMPT models. On the other hand, for the most central events the ALICE results agree with the extrapolations based on the data at lower energies from CERN-SPS and RHIC

清涼飮料税論

The production of J/\).psi\) and $\psi(2S)$ was measured with the ALICE detector in Pb-Pb collisions at the LHC. The measurement was performed at forward rapidity 2.5 < y < 4 \() down to zero transverse momentum \(p_{\rm T} in the dimuon decay channel. Inclusive J/\).psi\) yields were extracted in different centrality classes and the centrality dependence of the average $p_{\rm T}$ is presented. The J/\).psi\) suppression, quantified with the nuclear modification factor $R_{\rm AA}$ , was studied as a function of centrality, transverse momentum and rapidity. Comparisons with similar measurements at lower collision energy and theoretical models indicate that the J/\).psi\) production is the result of an interplay between color screening and recombination mechanisms in a deconfined partonic medium, or at its hadronization. Results on the $\psi(2S)$ suppression are provided via the ratio of $\psi(2S)$ over J/\).psi\) measured in pp and Pb-Pb collisions

Inclusive quarkonium production at forward rapidity in pp collisions at [Formula: see text]TeV.

We report on the inclusive production cross sections of [Formula: see text], [Formula: see text], [Formula: see text](1S), [Formula: see text](2S) and [Formula: see text](3S), measured at forward rapidity with the ALICE detector in [Formula: see text] collisions at a center-of-mass energy [Formula: see text]&nbsp;TeV. The analysis is based on data collected at the LHC and corresponds to an integrated luminosity of 1.23 pb[Formula: see text]. Quarkonia are reconstructed in the dimuon-decay channel. The differential production cross sections are measured as a function of the transverse momentum [Formula: see text] and rapidity y, over the [Formula: see text] ranges [Formula: see text]&nbsp;GeV/c for [Formula: see text], [Formula: see text]&nbsp;GeV/c for all other resonances, and for [Formula: see text]. The cross sections, integrated over [Formula: see text] and y, and assuming unpolarized quarkonia, are [Formula: see text]&nbsp;[Formula: see text]b, [Formula: see text]&nbsp;[Formula: see text]b, [Formula: see text]&nbsp;nb, [Formula: see text]&nbsp;nb and [Formula: see text]&nbsp;nb, where the first uncertainty is statistical and the second one is systematic. These values agree, within at most [Formula: see text], with measurements performed by the LHCb collaboration in the same rapidity range

Azimuthal anisotropy of charged jet production in √sNN=2.76 TeV Pb–Pb collisions

We present measurements of the azimuthal dependence of charged jet production in central and semi-central sNN=2.76 TeV Pb–Pb collisions with respect to the second harmonic event plane, quantified as v2ch jet. Jet finding is performed employing the anti-kT 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 v2ch jet is observed in semi-central collisions (30–50% centrality) for 2

Centrality dependence of ψ(2S) suppression in p-Pb collisions at √sNN= 5.02 TeV

The inclusive production of the ψ(2S) charmonium state was studied as a function of centrality in p-Pb collisions at the nucleon-nucleon center of mass energy √sNN = 5.02 TeV at the CERN LHC. The measurement was performed with the ALICE detector in the center of mass rapidity ranges −4.46 &lt; ycms&lt; −2.96 and 2.03 &lt; ycms&lt; 3.53, down to zero transverse momentum, by reconstructing the ψ(2S) decay to a muon pair. The ψ(2S) production cross section σψ(2S) is presented as a function of the collision centrality, which is estimated through the energy deposited in forward rapidity calorimeters. The relative strength of nuclear effects on the ψ(2S) and on the corresponding 1S charmonium state J/ψ is then studied by means of the double ratio of cross sections [σψ(2S)/σJ/ψ]pPb/[σψ(2S)/σJ/ψ]pp between p-Pb and pp collisions, and by the values of the nuclear modification factors for the two charmonium states. The results show a large suppression of ψ(2S) production relative to the J/ψ at backward (negative) rapidity, corresponding to the flight direction of the Pb-nucleus, while at forward (positive) rapidity the suppressions of the two states are comparable. Finally, comparisons to results from lower energy experiments and to available theoretical models are presented

Measurement of Ds+ production and nuclear modification factor in Pb-Pb collisions at √sNN = 2.76 TeV

The production of prompt Ds+ mesons was measured for the first time in collisions of heavy nuclei with the ALICE detector at the LHC. The analysis was performed on a data sample of Pb-Pb collisions at a centre-of-mass energy per nucleon pair, (Formula presented.) , of 2.76 TeV in two different centrality classes, namely 0–10% and 20–50%. Ds+ mesons and their antiparticles were reconstructed at mid-rapidity from their hadronic decay channel Ds+ → ϕπ+, with ϕ → K−K+, in the transverse momentum intervals 4 &lt; pT &lt; 12GeV/c and 6 &lt; pT &lt; 12 GeV/c for the 0–10% and 20–50% centrality classes, respectively. The nuclear modification factor RAA was computed by comparing the pT-differential production yields in Pb-Pb collisions to those in proton-proton (pp) collisions at the same energy. This pp reference was obtained using the cross section measured at (Formula presented.) TeV and scaled to (Formula presented.) TeV. The RAA of Ds+ mesons was compared to that of non-strange D mesons in the 10% most central Pb-Pb collisions. At high pT (8 &lt; pT &lt; 12 GeV/c) a suppression of the Ds+-meson yield by a factor of about three, compatible within uncertainties with that of non-strange D mesons, is observed. At lower pT (4 &lt; pT &lt; 8 GeV/c) the values of the Ds+-meson RAA are larger than those of non-strange D mesons, although compatible within uncertainties. The production ratios Ds+/D0 and Ds+/D+ were also measured in Pb-Pb collisions and compared to their values in proton-proton collisions

Anisotropic Flow of Charged Particles in Pb-Pb Collisions at √[s{NN}]=5.02 TeV.

We report the first results of elliptic (v_{2}), triangular (v_{3}), and quadrangular (v_{4}) flow of charged particles in Pb-Pb collisions at a center-of-mass energy per nucleon pair of sqrt[s_{NN}]=5.02  TeV with the ALICE detector at the CERN Large Hadron Collider. The measurements are performed in the central pseudorapidity region |η|&lt;0.8 and for the transverse momentum range 0.