Charge correlations and collective dynamics in Pb-Pb collisions at sqrt(sNN)=2.76 TeV

The theory of the strong force is called Quantum Chromodynamics (QCD). Within experimental limits it is observed that QCD respects CP-symmetry, although there is no confirmed fundamental explanation for why this is the case. This is known as the strong CP problem. Despite the observed symmetry conservation, it is expected that fluctuations in a larger system may locally result in a violation of CP-symmetry. It was argued that local parity violation, which is driven by the nontrivial topological structure of the QCD vacuum, coupled with a strong magnetic field may result in a separation of charges along the direction of the magnetic field. This phenomenon is called the chiral magnetic effect (CME). Relativistic heavy ion collisions such as at the LHC could meet the conditions for a measurement of the CME. For this, knowledge of the orientation of the magnetic field, which is perpendicular to the reaction plane, is needed. Collision symmetry planes can be estimated with Q-vectors that quantify the preference in the azimuthal direction of measured particles. To correct for detector imperfections in the measurement of the Q-vector, a ROOT-based correction framework was developed, building on corrections proposed by Selyuzhenkov and Voloshin. The functionality is increasingly used as a common tool in the ALICE collaboration. Due to its general applicability to data from heavy ion experiments, the package is publicly available and has already gained interest from researchers in CBM and NA61. With understanding of the orientation of the magnetic field, the CME can be searched for with parity-even two- and multi-particle correlations, such as the charge dependence of two-particle correlations with respect to the reaction plane. However, these observables are also sensitive to the presence of background correlations, notably arising from the anisotropic flow modulation of locally created opposite charged particle pairs (local charge conservation), which obstructed a definite conclusion on the origins of the observed charge dependence. This work presents the measurement of an extended set of mixed harmonic correlations based on 〈cos(φ_α + φ_β − 2Ψ_RP)〉, in Pb-Pb collisions at sqrt(sNN)=2.76 TeV with ALICE, that characterize the facets of the shape of the signal. Additionally, the mentioned correlation is measured with one identified hadron (pion, kaon, or proton), reported as a function of the identified hadron transverse momentum. The measurements of the correlators with different sensitivity to the CME and background effects show varying degrees of charge dependence. The 〈cos(φ_α + φ_β − 2Ψ_RP)〉 correlation, which has maximum sensitivity to the CME but also background effects, shows a charge dependence increasing towards peripheral collisions, as is qualitatively in line with expectations. The second harmonic of this correlator is not sensitive to the charge separation perpendicular to the reaction plane, as associated to the CME, but can be used to quantify correlations from background sources, for example clusters of particles exhibiting charge conservation with angular correlations modulated by anisotropic flow, and is found to be consistent with zero within the systematic uncertainty. The measurement of the CME correlator with pions, kaons and protons reveals a particle type dependence in the charge dependence. More data for higher precision and model comparisons are required to make a better defined and quantitative conclusion about the contributions from background sources in the observation of the charge separation in the various correlations

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

Event-shape engineering for inclusive spectra and elliptic flow in Pb-Pb collisions at root(NN)-N-S=2.76 TeV

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

Charge dependence of identified two-hadron correlation relative to the reaction plane in Pb-Pb collisions measured with ALICE

Recently a non-zero charge dependence of two-particle correlation relative to the reaction plane in relativistic heavy-ion collisions was observed by RHIC and LHC experiments. The interpretation of these results is a hot topic of debate in the heavy-ion community because of its possible implication for our understanding of parity violation in strong interactions. We extend the ALICE measurement of the charge dependent two-particle correlation in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV with one identified hadron (pion, kaon, or proton). Correlations are reported as a function of the identified hadron transverse momentum. These new results are important for disentangling contributions from a number of competing physics effects, such as local charge conservation coupled with strong anisotropic flow, flow fluctuations, and possible contribution from parity violation coupled with strong magnetic fields, the so-called chiral magnetic effect

Charge correlations and collective dynamics in Pb-Pb collisions at sqrt(sNN)=2.76 TeV

The theory of the strong force is called Quantum Chromodynamics (QCD). Within experimental limits it is observed that QCD respects CP-symmetry, although there is no confirmed fundamental explanation for why this is the case. This is known as the strong CP problem. Despite the observed symmetry conservation, it is expected that fluctuations in a larger system may locally result in a violation of CP-symmetry. It was argued that local parity violation, which is driven by the nontrivial topological structure of the QCD vacuum, coupled with a strong magnetic field may result in a separation of charges along the direction of the magnetic field. This phenomenon is called the chiral magnetic effect (CME). Relativistic heavy ion collisions such as at the LHC could meet the conditions for a measurement of the CME. For this, knowledge of the orientation of the magnetic field, which is perpendicular to the reaction plane, is needed. Collision symmetry planes can be estimated with Q-vectors that quantify the preference in the azimuthal direction of measured particles. To correct for detector imperfections in the measurement of the Q-vector, a ROOT-based correction framework was developed, building on corrections proposed by Selyuzhenkov and Voloshin. The functionality is increasingly used as a common tool in the ALICE collaboration. Due to its general applicability to data from heavy ion experiments, the package is publicly available and has already gained interest from researchers in CBM and NA61. With understanding of the orientation of the magnetic field, the CME can be searched for with parity-even two- and multi-particle correlations, such as the charge dependence of two-particle correlations with respect to the reaction plane. However, these observables are also sensitive to the presence of background correlations, notably arising from the anisotropic flow modulation of locally created opposite charged particle pairs (local charge conservation), which obstructed a definite conclusion on the origins of the observed charge dependence. This work presents the measurement of an extended set of mixed harmonic correlations based on 〈cos(φ_α + φ_β − 2Ψ_RP)〉, in Pb-Pb collisions at sqrt(sNN)=2.76 TeV with ALICE, that characterize the facets of the shape of the signal. Additionally, the mentioned correlation is measured with one identified hadron (pion, kaon, or proton), reported as a function of the identified hadron transverse momentum. The measurements of the correlators with different sensitivity to the CME and background effects show varying degrees of charge dependence. The 〈cos(φ_α + φ_β − 2Ψ_RP)〉 correlation, which has maximum sensitivity to the CME but also background effects, shows a charge dependence increasing towards peripheral collisions, as is qualitatively in line with expectations. The second harmonic of this correlator is not sensitive to the charge separation perpendicular to the reaction plane, as associated to the CME, but can be used to quantify correlations from background sources, for example clusters of particles exhibiting charge conservation with angular correlations modulated by anisotropic flow, and is found to be consistent with zero within the systematic uncertainty. The measurement of the CME correlator with pions, kaons and protons reveals a particle type dependence in the charge dependence. More data for higher precision and model comparisons are required to make a better defined and quantitative conclusion about the contributions from background sources in the observation of the charge separation in the various correlations