Central Pb Research Articles

Pattern recognition and PID procedure with the ALICE-HMPID

The ALICE apparatus is dedicated to the study of pp, p–Pb and Pb–Pb collisions provided by LHC. ALICE has unique particle identification (PID) capabilities among the LHC experiments exploiting different PID techniques, i.e., energy loss, time-of-flight measurements, Cherenkov and transition radiation detection, calorimetry and topological ID. The ALICE-HMPID is devoted to the identification of charged hadrons. It consists of seven identical RICH counters, with liquid C6F14 as Cherenkov radiator (n≈1.299 at λph=175nm). Photons and charged particles detection is performed by a proportional chamber, coupled with a pad segmented CsI coated photo-cathode. In pp and p–Pb events HMPID provides 3 sigmas separation for pions and kaons up to pT=3GeV/c and for protons up to pT=5GeV/c. PID is performed by means of photon emission angle measurement, a challenging task in the high multiplicity environment of the most central Pb–Pb collisions. A dedicated algorithm has been implemented to evaluate the Cherenkov angle starting from the bi-dimensional ring pattern on the photons detector, it is based on the Hough Transform Method (HTM) to separate signal from background. In this way HMPID is able to contribute to inclusive hadrons spectra measurement as well as to measurements where high purity PID is required, by means of statistical or track-by-track PID. The pattern recognition, the results from angular resolution studies and the PID strategy with HMPID are presented.

Multiplicity fluctuations of identified hadrons in p+p interactions at SPS energies

Study of energy and system size fluctuations of identified hadrons is one of the key goals of NA61/SHINE at the CERN SPS. Results may allow to discover the critical point (CP) of strongly interacting matter as well as to uncover properties of the onset of deconfinement (OD). But fluctuations exhibit numerous other sources starting from most basic ones like volume effects and conservation laws.NA49 seems to observe fluctuations related to CP in collisions of medium size nuclei at top SPS energy. However, this result will remain inconclusive until systematic data on energy and system size dependence will be available. Moreover, fluctuations in p+p as well as in Pb+Pb interactions should be better understood.In this contribution results on multiplicity fluctuations of identified hadrons in p+p interactions at the CERN SPS energies will be presented. The NA61 data will be compared with the corresponding results from central Pb+Pb collisions of NA49 in the common acceptance region of both experiments. Moreover, predictions of models (EPOS, UrQMD and HSD) for both reactions will be tested.

Jet suppression at LHC: theory vs. experiment

Suppression of light and heavy flavor observables is one of the most important probes in studying the properties of QCD matter created at RHIC and LHC experiments. We will here summarize the most up-to-date light and heavy flavor suppression predictions for 2.76 TeV central Pb+Pb collisions at LHC. The predictions are based on our recent improvements in the energy loss calculations that take into account: i) theoretical formalism which includes finite size dynamical QCD medium with finite magnetic mass effects and running coupling, and ii) numerical procedure which includes path-length and multi-gluon fluctuations. Our theoretical predictions, obtained with no free parameters used in model testing, show a very good agreement with the experimental results for all available particle species. Our results show that the developed theoretical formalism is able to robustly explain suppression data at LHC, which strongly suggests that pQCD in Quark-Gluon Plasma can provide a reasonable description of the underlying jet physics in ultra relativistic heavy ion collisions.

Heavy-flavour production in ALICE

We present measurements of heavy-flavour production in Pb–Pb collisions at s NN = 2.76 TeV and in p–Pb collisions at 5.02 TeV by the ALICE Collaboration. A strong suppression of heavy-flavour production at high p T is observed in central Pb–Pb collisions, while non-zero elliptic flow is seen at low p T in semi-central collisions. The cold nuclear matter effects on the measurements in Pb–Pb collisions have been investigated in p–Pb collisions, and no large effects have been observed.

Measurement of negatively charged pion spectra in inelastic p+p interactions at $$p_{\mathbf {lab}}= $$ p lab = 20, 31, 40, 80 and 158 GeV/c

We present experimental results on inclusive spectra and mean multiplicities of negatively charged pions produced in inelastic p+p interactions at incident projectile momenta of 20, 31, 40, 80 and 158 GeV/c ($\sqrt{s} = $ 6.3, 7.7, 8.8, 12.3 and 17.3 GeV, respectively). The measurements were performed using the large acceptance NA61/SHINE hadron spectrometer at the CERN Super Proton Synchrotron. Two-dimensional spectra are determined in terms of rapidity and transverse momentum. Their properties such as the width of rapidity distributions and the inverse slope parameter of transverse mass spectra are extracted and their collision energy dependences are presented. The results on inelastic p+p interactions are compared with the corresponding data on central Pb+Pb collisions measured by the NA49 experiment at the CERN SPS. The results presented in this paper are part of the NA61/SHINE ion program devoted to the study of the properties of the onset of deconfinement and search for the critical point of strongly interacting matter. They are required for interpretation of results on nucleus-nucleus and proton-nucleus collisions.

Medium modifications of jet shapes in Pb + Pb collisions atsNN=2.76TeV within a multiphase transport model

Within a multiphase transport model, medium modifications of differential jet shapes are investigated in Pb+Pb collisions at $\sqrt{s_{_{\rm NN}}}$ = 2.76 TeV. The differential jet shapes are significantly modified by the strong interactions between jets and a partonic medium in Pb+Pb collisions relative to that in p+p collisions. The modifications are slightly weakened by the hadronization of coalescence, but strengthened by resonance decays in hadronic rescatterings. Subleading jets display larger medium modifications than leading jets, especially in central Pb+Pb collisions with large dijet asymmetries. These behaviors of medium modifications of differential jet shapes reflect a dynamical evolution of redistribution of jet energy inside a quenched jet cone in high-energy heavy-ion collisions.

Measurability of , , and mesons via di-electron decays in high-temperature states produced in heavy-ion collisions

We discuss measurability of phi, omega and rho mesons via di-electron decays in high-temperature states produced in heavy-ion collisions, equivalently at different pion multiplicities per heavy-ion collision dN_{pi^{0} + pi^{+-}}/dy = 1000 and 2700 intended for the most central Au+Au collisions at sqrt(s_{NN}) = 200 GeV (RHIC) and the most central Pb+Pb collisions at sqrt(s_{NN}) = 5.5 TeV (LHC), by evaluating the signal-to-background ratios and the statistical significance for the idealized detection system in the numerical simulation. The simulation study provides a guideline to be applicable to a concrete detector design by focusing on only the key experimental issues relevant to the measurement of di-electrons. The results suggest that there are realizable parameter ranges to measure light vector mesons via di-electrons with the reasonable significance level, even in the highest multiplicity case.

Muon Physics in ALICE: The MFT Upgrade Project

The ALICE experiment is dedicated to the study of the quark gluon plasma in heavy-ion collisions at the CERN LHC. The Muon Forward Tracker (MFT) is under consideration by the ALICE experiment to be part of its program of detector upgrades to be installed during the LHC Long Shutdown 2 (LS2) planned for 2018. Designed as a silicon pixel detector added in the Muon Spectrometer acceptance (−4.0 < η < −2.5) upstream of the hadron absorber, the MFT will allow a drastic improvement of the measurements that are presently done with the Muon Spectrometer and, in addition, will give access to new measurements that are not possible with the present Muon Spectrometer setup. Motivations and preliminary results are discussed here, concerning the measurement of prompt and displaced charmonia, open heavy flavors, and low mass dimuons in central Pb–Pb collisions at = 5.5 TeV.

Hadron Formation in Relativistic Nuclear Collisions and the QCD Phase Diagram

We study particle production in ultrarelativistic nuclear collisions at CERN SPS and LHC energies and the conditions of chemical freeze-out. We determine the effect of the inelastic reactions between hadrons occurring after hadronization and before chemical freeze-out employing the ultrarelativistic quantum molecular dynamics hybrid model. The differences between the initial and the final hadronic multiplicities after the rescattering stage resemble the pattern of data deviation from the statistical equilibrium calculations. By taking these differences into account in the statistical model analysis of the data, we are able to reconstruct the original hadrochemical equilibrium points in the (T, μ(B)) plane which significantly differ from chemical freeze-out ones and closely follow the parton-hadron phase boundary recently predicted by lattice quantum chromodynamics.

Universal hadronization condition in heavy ion collisions atsNN=62GeVand atsNN=2.76TeV

We obtain a detailed description of all available hadron multiplicity yields in central Pb--Pb collisions at LHC measured in the rapidity interval $|y|<0.5$. We find that the hadronization of the fireball at LHC occurs at nearly identical intensive physical bulk conditions for all centralities similar to those seen already at RHIC.

P/π Ratio in Di-Hadron Correlations

Particle ratios are important observables used to constrain models of particle production in heavy-ion collisions. In this work we report on a measurement of the p/pi ratio in the transverse momentum range 2.0 < p_{T,assoc} < 4.0 GeV/c, associated with a charged trigger particle of 5.0 < p_{T,trig} < 10.0 GeV/c, in 0-10% central Pb--Pb collisions at sqrt{s_{NN}}=2.76 TeV. The ratio is measured in the jet peak and in a region at large Delta eta separation from the peak (bulk region). The presented results are based on 14M minimum-bias Pb--Pb collisions, recorded by the ALICE detector. It is observed that the p/pi ratio in the bulk region is compatible with the p/pi ratio of an inclusive measurement, and is much larger than the p/pi ratio in the jet peak. The p/pi ratio in the jet peak is compatible with a PYTHIA reference, in which fragmentation in the vacuum is the dominant mechanism of particle production.

Charged particle production in Pb–Pb collisions at the LHC with the ALICE detector

The ALICE collaboration measured charged particle production in $\sqrt{s_{NN}}=2.76$ TeV Pb--Pb collisions at the LHC. We report on results on charged particle multiplicity and transverse momentum spectra. All the results are presented as a function of the centrality of the collision, estimated with a Glauber Monte Carlo fit to multiplicity distributions reconstructed in various detectors. The applicability of the Glauber model at LHC energies, the precision of the centrality determination and the related systematic uncertainties are discussed in detail. Particles are tracked in the pseudorapidity window $|\eta| \lesssim 0.9$\ with the silicon Inner Tracking System (ITS) and the Time Projection Chamber (TPC), over the range $0.15 < \pt \lesssim 50$ GeV/$c$. The low-$p_t$ cut-off is further reduced in the multiplicity measurement using "tracklets", reconstructed in the 2 innermost layers of the ITS. The charged particle multiplicity is measured in $|\eta| < 0.5$ to be $\mathrm{d}N_{ch}/\mathrm{d}\eta = 1601 \pm 60$ in 5% most central Pb--Pb collisions, indicating an energy density a factor $\sim 3$ higher than at RHIC. Its evolution with centrality shows a pattern strikingly similar to the one measured at RHIC. Intermediate ($5 \lesssim \pt \lesssim 15$ GeV/$c$) transverse momentum particles are found to be most strongly suppressed with respect to pp collisions, consistent with a large energy loss of hard-scattered partons in the hot and dense medium. The results are presented in terms of the nuclear modification factor $R_{\mathrm{AA}}$ and compared to theoretical expectations.

Nuclear modification of J/ψ production in Pb–Pb collisions at sNN=2.76TeV

Abstract ALICE is the dedicated heavy-ion experiment at the LHC. Due to the unique particle identification capabilities of the central barrel detectors ( | η | 0.9 ), J / ψ can be measured in the di-electron channel in the very demanding environment of central Pb–Pb collisions at the LHC. In addition J / ψ are measured at forward rapidity ( 2.5 y 4 ) with a dedicated muon spectrometer. ALICE is the only LHC experiment with an acceptance for J / ψ that reaches down to p T = 0 at both, mid- and forward rapidity. Preliminary results on the nuclear modification factor of the inclusive J / ψ production at mid- and forward rapidity in Pb–Pb collisions at s N N = 2.76 TeV are presented.

Dijet asymmetry in Pb+Pb collisions atsNN=2.76TeV within a multiphase transport model

Within a multi-phase transport (AMPT) model, dijet asymmetry is studied in Pb+Pb collisions at $\sqrt{s_{_{\rm NN}}}$ = 2.76 TeV. It is found that a large dijet asymmetry ($A_{J}$) is produced by strong interactions between jets and partonic matter. It is demonstrated that hadronization and final-state hadronic rescatterings have little effects on $A_{J}$. The final $A_{J}$ is found to be driven by both initial $A_{J}$ and partonic jet energy loss, which is consistent with an increasing jet energy loss in a hot and strongly interacting partonic medium in more central Pb+Pb collisions.

Results from the NA61/SHINE and NA49 experiments

Abstract Preliminary results on particle spectra and fluctuations in inelastic p + p interactions measured by NA61/SHINE at the CERN SPS are presented and its future ion program is discussed. We also show results on the excitation functions of event-by-event particle ratio fluctuations in central Pb + Pb collisions from NA49 using a novel approach. We found a dependence of the results on phase space coverage of the apparatus which appears to explain the reported difference between measurements of NA49 and those of STAR in central Au + Au collisions.

Electroweak Boson-Tagged Jet Event Asymmetries at the Large Hadron Collider

Tagged jet measurements provide a promising experimental channel to quantify the similarities and differences in the mechanisms of jet production in proton-proton and nucleus-nucleus collisions. We present the first calculation of the transverse momentum asymmetry of $Z^0/\gamma^*$-tagged jet events and the momentum imbalance of $\gamma$-tagged jet events in $\sqrt{s}=2.76$ TeV reactions at the LHC. Our results combine the ${\cal O}(G_F\alpha_s^2)$, ${\cal O}(G_F\alpha_s^2)$ perturbative cross sections with the radiative and collisional processes that modify parton showers in the presence of dense QCD matter. We find that strong asymmetry momentum and imbalance, respectively, are generated in central Pb+Pb reactions that have little sensitivity to the fluctuations of the underlying soft hadronic background. We present theoretical model predictions for their shape and magnitude.

Measurement of RAA and v2 of electrons from heavy-flavour decays in Pb–Pb collisions at sNN=2.76 TeV with ALICE

Abstract We present measurements of the nuclear modification factor ( R AA ) and azimuthal anisotropy ( v 2 ) of heavy-flavour decay electrons by the ALICE Collaboration at central rapidity in Pb–Pb collisions at s NN = 2.76 TeV . A strong suppression of heavy-flavour decay electron production at high p T is observed in central Pb–Pb collisions, while non-zero v 2 is seen at low p T in semi central collisions.

Study of jet fragmentation with two particle correlations in Pb–Pb collisions at sNN=2.76 TeV by ALICE

Abstract The extension of the published ALICE near side I A A measurement [Aamodt, K. et al. (ALICE collab.), Phys. Rev. Lett. 108, 092301 (2012).] in Pb–Pb collisions at s NN = 2.76 TeV down to associated p T , a s s o c = 0.7 GeV / c is presented. In central Pb–Pb collisions at low trigger ( 2 p T , t r i g 3 GeV / c ) and associated ( 1 p T , a s s o c 2 GeV/c ) transverse momentum the conical Δ ϕ − − Δ η symmetry of the near side jet peak is distorted and the peak is elongated in Δη direction. At high trigger ( 8 p T , t r i g 15 GeV / c ) and associated ( 4 p T , a s s o c 8 GeV/c ) transverse momentum we observe a hint of narrowing of the near side peak along Δη.

Heavy-flavor suppression and azimuthal anisotropy in Pb–Pb collisions at sNN=2.76TeV with the ALICE detector

Heavy-flavor hadron production studies in Pb–Pb collisions at s NN = 2.76 TeV with the 2010 and 2011 data samples are presented. The measurements are performed with the ALICE detector in various decay channels and in a wide kinematic range. Heavy-flavor hadrons exhibit a suppression in the most central Pb–Pb collisions that amounts to a factor 3–5 for p T ∼ 8 – 10 GeV / c . The second harmonic of the azimuthal distribution Fourier decomposition, v 2 , is non-zero for non-central collisions at intermediate p T ( ∼ 3 GeV / c ).

Jet structure in 2.76 TeV Pb–Pb collisions at ALICE

Abstract We present the analysis of the semi-inclusive distribution of reconstructed charged particle jets recoiling from a high p T hadron trigger in central Pb–Pb collisions at s N N = 2.76 TeV measure, subtract and unfold the large underlying event background in such collisions, utilizing a novel technique that does not impose fragmentation bias on the measured jet population. The Pb–Pb measurements are compared to a pp PYTHIA reference distribution generated at the same s . Modification of the jet structure due to quenching is explored by varying the cone radius R ( 0.2 , 0.4 ) and the lower p T cutoff of the charged particle constituents ( 0.15 , 2.0 GeV / c ) .