We report the timing and spatial resolution from the Muon Telescope Detector (MTD) installed in the STAR experiment at RHIC. Cosmic ray muons traversing the STAR detector have an average transverse ...momentum of 6GeV/c. Due to their very small multiple scattering, these cosmic muons provide an ideal tool to calibrate the detectors and measure their timing and spatial resolution. The values obtained were ~100ps and ~1–2cm. These values are comparable to those obtained from cosmic-ray bench tests and test beams.
The Breit-Wheeler process which produces matter and antimatter from photon collisions is experimentally investigated through the observation of 6085 exclusive electron-positron pairs in ...ultraperipheral Au + Au collisions at √sNN = 200 GeV . The measurements reveal a large fourth-order angular modulation of cos 4 Δ ϕ = ( 16.8 ± 2.5 ) % and smooth invariant mass distribution absent of vector mesons ( ϕ , ω , and ρ ) at the experimental limit of ≤ 0.2 % of the observed yields. The differential cross section as a function of e+ e− pair transverse momentum P⊥ peaks at low value with √⟨P⊥2 ⟩ = 38.1 ± 0.9 MeV and displays a significant centrality dependence. These features are consistent with QED calculations for the collision of linearly polarized photons quantized from the extremely strong electromagnetic fields generated by the highly charged Au nuclei at ultrarelativistic speed. The experimental results have implications for vacuum birefringence and for mapping the magnetic field which is important for emergent QCD phenomena.
Nonmonotonic variation with collision energy (sqrts_{NN}) of the moments of the net-baryon number distribution in heavy-ion collisions, related to the correlation length and the susceptibilities of ...the system, is suggested as a signature for the quantum chromodynamics critical point. We report the first evidence of a nonmonotonic variation in the kurtosis times variance of the net-proton number (proxy for net-baryon number) distribution as a function of sqrts_{NN} with 3.1 σ significance for head-on (central) gold-on-gold (Au+Au) collisions measured solenoidal tracker at Relativistic Heavy Ion Collider. Data in noncentral Au+Au collisions and models of heavy-ion collisions without a critical point show a monotonic variation as a function of sqrts_{NN}.
A study of multiplicity and pseudorapidity distributions of inclusive photons measured in pp and p-Pb collisions at a center-of-mass energy per nucleon-nucleon collision of Formula omitted TeV using ...the ALICE detector in the forward pseudorapidity region 2.3 Formula omitted 3.9 is presented. Measurements in p-Pb collisions are reported for two beam configurations in which the directions of the proton and lead ion beam were reversed. The pseudorapidity distributions in p-Pb collisions are obtained for seven centrality classes which are defined based on different event activity estimators, i.e., the charged-particle multiplicity measured at midrapidity as well as the energy deposited in a calorimeter at beam rapidity. The inclusive photon multiplicity distributions for both pp and p-Pb collisions are described by double negative binomial distributions. The pseudorapidity distributions of inclusive photons are compared to those of charged particles at midrapidity in pp collisions and for different centrality classes in p-Pb collisions. The results are compared to predictions from various Monte Carlo event generators. None of the generators considered in this paper reproduces the inclusive photon multiplicity distributions in the reported multiplicity range. The pseudorapidity distributions are, however, better described by the same generators.
A newly developed observable for correlations between symmetry planes, which characterize the direction of the anisotropic emission of produced particles, is measured in Pb-Pb collisions at Formula ...omitted = 2.76 TeV with ALICE. This so-called Gaussian Estimator allows for the first time the study of these quantities without the influence of correlations between different flow amplitudes. The centrality dependence of various correlations between two, three and four symmetry planes is presented. The ordering of magnitude between these symmetry plane correlations is discussed and the results of the Gaussian Estimator are compared with measurements of previously used estimators. The results utilizing the new estimator lead to significantly smaller correlations than reported by studies using the Scalar Product method. Furthermore, the obtained symmetry plane correlations are compared to state-of-the-art hydrodynamic model calculations for the evolution of heavy-ion collisions. While the model predictions provide a qualitative description of the data, quantitative agreement is not always observed, particularly for correlators with significant non-linear response of the medium to initial state anisotropies of the collision system. As these results provide unique and independent information, their usage in future Bayesian analysis can further constrain our knowledge on the properties of the QCD matter produced in ultrarelativistic heavy-ion collisions.
Two-particle correlations with Formula omitted, Formula omitted/ Formula omitted, and charged hadrons as trigger particles in the transverse momentum range Formula omitted GeV/ Formula omitted, and ...associated charged particles within Formula omitted GeV/ Formula omitted, are studied at midrapidity in pp and central Pb-Pb collisions at a centre-of-mass energy per nucleon-nucleon collision Formula omitted TeV with the ALICE detector at the LHC. After subtracting the contributions of the flow background, the per-trigger yields are extracted on both the near and away sides, and the ratio in Pb-Pb collisions with respect to pp collisions ( Formula omitted) is computed. The per-trigger yield in Pb-Pb collisions on the away side is strongly suppressed to the level of Formula omitted Formula omitted for Formula omitted GeV/ Formula omitted as expected from strong in-medium energy loss, while an enhancement develops at low Formula omitted on both the near and away sides, reaching Formula omitted Formula omitted and 2.7 respectively. These findings are in good agreement with previous ALICE measurements from two-particle correlations triggered by neutral pions ( Formula omitted-h) and charged hadrons (h-h) in Pb-Pb collisions at Formula omitted TeV. Moreover, the correlations with Formula omitted mesons and Formula omitted/ Formula omitted baryons as trigger particles are compared to those of inclusive charged hadrons. The results are compared with the predictions of Monte Carlo models.
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