Nuclear Experiment

The p T -differential production cross sections of prompt and non-prompt (produced in beauty-hadron decays) D mesons were measured by the ALICE experiment at midrapidity ( |y|<0.5 ) in proton--proton collisions at s ??=5.02 TeV . The data sample used in the analysis corresponds to an integrated luminosity of (19.3±0.4) n b ?? . D mesons were reconstructed from their decays D 0 ??K ??? + , D + ??K ??? + ? + , and D + s ?��?? + ??K ??K + ? + and their charge conjugates. Compared to previous measurements in the same rapidity region, the cross sections of prompt D + and D + s mesons have an extended p T coverage and total uncertainties reduced by a factor ranging from 1.05 to 1.6, depending on p T , allowing for a more precise determination of their p T -integrated cross sections. The results are well described by perturbative QCD calculations. The fragmentation fraction of heavy quarks to strange mesons divided by the one to non-strange mesons, f s /( f u + f d ) , is compatible for charm and beauty quarks and with previous measurements at different centre-of-mass energies and collision systems. The b b ¯ ¯ ¯ production cross section per rapidity unit at midrapidity, estimated from non-prompt D-meson measurements, is d ? b b ¯ ¯ ¯ /dy | |y|<0.5 =34.5±2.4(stat. ) +4.7 ??.9 (tot.syst.) μb . It is compatible with previous measurements at the same centre-of-mass energy and with the cross section predicted by perturbative QCD calculations.

We report the collision-centrality dependence of cumulants of event-by-event net-proton, net-charge and net-kaon distributions in Au+Au collisions for center-of-mass energy s NN − − − − √ = 54.4 GeV from the STAR experiment. Strong collision-centrality dependence is observed for cumulants ( C n ,n ≤ 4) of the net-particle distributions. The cumulant ratios C 3 / C 2 , C 4 / C 2 and C 6 / C 2 exhibit a weak collision-centrality dependence. The C 6 / C 2 of net-proton and net-charge distributions for most central gold nuclei collisions at s NN − − − − √ = 54.4 GeV show positive sign while they remain negative at 200 GeV for the same collision system. To understand effects of acceptance and baryon number conservation, the measurements are compared to expectations from the UrQMD and HIJING models calculated within the STAR detector acceptance.

We report on the measurement of the beam spin asymmetry in the deeply virtual Compton scattering off 4 He using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab using a 6 GeV longitudinally polarized electron beam incident on a pressurized 4 He gaseous target. We detail the method used to ensure the exclusivity of the measured reactions, in particular the upgrade of CLAS with a radial time projection chamber to detect the low-energy recoiling 4 He nuclei and an inner calorimeter to extend the photon detection acceptance at forward angles. Our results confirm the theoretically predicted enhancement of the coherent ( e 4 He ?? e ??4 He ??γ ??) beam spin asymmetries compared to those observed on the free proton, while the incoherent ( e 4 He ?? e ??p ??γ ??X ??) asymmetries exhibit a 30 % suppression. From the coherent data, we were able to extract, in a model-independent way, the real and imaginary parts of the only 4 He Compton form factor, H A , leading the way toward 3D imaging of the partonic structure of nuclei.

A measurement of vector analyzing powers in elastic deuteron-carbon scattering has been performed at the Cooler Synchrotron COSY of Forschungszentrum Jülich, Germany. Seven kinetic beam energies between 170 and 380 MeV have been used. A vector-polarized beam from a polarized deuteron source was injected, accelerated to the final desired energy and stored in COSY. A thin needle-shaped diamond strip was used as a carbon target, onto which the beam was slowly steered. Elastically scattered deuterons were identified in the forward direction using various layers of scintillators and straw tubes. Where data exist in the literature (at 200 and 270 MeV), excellent agreement of the angular shape was found. The beam polarization of the presented data was deduced by fitting the absolute scale of the analyzing power to these references. Our results extend the world data set and are necessary for polarimetry of future electric dipole moment searches at storage rings. They will as well serve as an input for theoretical description of polarized hadron-hadron scattering.

Differential cross sections for deuteron breakup 1 H(d,pp)n reaction were measured for a large set of 243 geometrical configurations at the beam energy of 80 MeV/nucleon. The cross section data are normalized by the luminosity factor obtained on the basis of simultaneous measurement of elastic scattering channel and the existing cross section data for this process. The results are compared to the theoretical calculations modeling nuclear interaction with and without taking into account the three-nucleon force (3NF) and Coulomb interaction. In the validated region of the phase space both the Coulomb force and 3NF play an important role in a good description of the data. There are also regions, where the improvements of description due to including 3NF are not sufficient.

We report the transverse momentum ( p T ) and centrality dependence of global spin alignment ( ρ 00 ) of K ∗0 vector meson at midrapidity ( |y|<0.5 ) in Au + Au collisions at s NN − − − − √ = 54.4 and 200~GeV with the STAR experiment at RHIC. The K ∗0 results are compared to that of ϕ meson. At low- p T region and midcentral collisions, the K ∗0 ρ 00 is found to be smaller than 1/3 with about 4 σ significance, while that of ϕ meson is observed to be larger than 1/3 with about 3 σ significance. The ρ 00 results are compared between RHIC and LHC energies. The physics implication of our results is also discussed.

The STAR Collaboration at the Relativistic Heavy Ion Collider reports the first measurement of inclusive jet production in peripheral and central Au+Au collisions at \sqrt{s_{NN}}=200 GeV. Jets are reconstructed with the anti-k_{T} algorithm using charged tracks with pseudorapidity |\eta|<1.0 and transverse momentum 0.2<p_{T,jet}^{ch}<30 GeV/c, with jet resolution parameter R=0.2, 0.3, and 0.4. The large background yield uncorrelated with the jet signal is observed to be dominated by statistical phase space, consistent with a previous coincidence measurement. This background is suppressed by requiring a high-transverse-momentum (high-p_T) leading hadron in accepted jet candidates. The bias imposed by this requirement is assessed, and the p_T region in which the bias is small is identified. Inclusive charged-particle jet distributions are reported in peripheral and central Au+Au collisions for 5<p_{T,jet}^{ch}<25 GeV/c and 5<p_{T,jet}^{ch}<30 GeV/c, respectively. The charged-particle jet inclusive yield is suppressed for central Au+Au collisions, compared to both the peripheral Au+Au yield from this measurement and to the pp yield calculated using the PYTHIA event generator. The magnitude of the suppression is consistent with that of inclusive hadron production at high p_T, and that of semi-inclusive recoil jet yield when expressed in terms of energy loss due to medium-induced energy transport. Comparison of inclusive charged-particle jet yields for different values of R exhibits no significant evidence for medium-induced broadening of the transverse jet profile for R<0.4 in central Au+Au collisions. The measured distributions are consistent with theoretical model calculations that incorporate jet quenching.

The STAR Collaboration reports the measurement of inclusive jet production in central (0-10%) and peripheral (60-80%) Au+Au collisions at s NN − − − √ = 200 GeV, using both charged-particle and fully-reconstructed jets. Jet reconstruction is carried out using the anti- k T algorithm with resolution parameters R = 0.2, 0.3 and 0.4. Yield suppression of charged-particle jets is observed for central Au+Au collisions relative to both peripheral Au+Au collisions and a vacuum baseline utilizing PYTHIA 6 simulations. The magnitude of the suppression is similar to that measured at the LHC and can be described by theoretical calculations. No evidence of significant medium-induced jet broadening is observed, based on comparison of jet spectra at varying R . The yield suppression, when expressed as the jet transverse momentum shift corresponding to energy loss, is consistent in magnitude with coincidence measurements at RHIC based on direct-photon and hadron triggers. There is an indication of larger energy loss observed at the LHC.

This paper presents isolated photon-hadron correlations using pp and p-Pb data collected by the ALICE detector at the LHC. For photons with | η | < 0.67 and 12 < p T < 40 GeV/ c , the associated yield of charged particles in the range | η | < 0.80 and 0.5 < p T < 10 GeV/ c is presented. These momenta are much lower than previous measurements at the LHC. No significant difference between pp and p-Pb is observed, with PYTHIA 8.2 describing both data sets within uncertainties. This measurement constrains nuclear effects on the parton fragmentation in p-Pb collisions, and provides a benchmark for future studies of Pb-Pb collisions.

Inclusive ψ (2S) production is measured in p-Pb collisions at the centre-of-mass energy per nucleon-nucleon pair s NN − − − √ =8.16 TeV, using the ALICE detector at the CERN LHC. The production of ψ (2S) is studied at forward ( 2.03< y cms <3.53 ) and backward ( −4.46< y cms <−2.96 ) centre-of-mass rapidity and for transverse momentum p T < 12 GeV/ c via the decay to muon pairs. In this paper, we report the integrated as well as the y cms - and p T -differential inclusive production cross sections. Nuclear effects on ψ (2S) production are studied via the determination of the nuclear modification factor that shows a strong suppression at both forward and backward centre-of-mass rapidities. Comparisons with corresponding results for inclusive J/ ψ show a similar suppression for the two states at forward rapidity (p-going direction), but a stronger suppression for ψ (2S) at backward rapidity (Pb-going direction). As a function of p T , no clear dependence of the nuclear modification factor is found. The relative size of nuclear effects on ψ (2S) production compared to J/ ψ is also studied via the double ratio of production cross sections [ σ ψ(2S) / σ J/ψ ] pPb /[ σ ψ(2S) / σ J/ψ ] pp between p-Pb and pp collisions. The results are compared with theoretical models that include various effects related to the initial and final state of the collision system and also with previous measurements at s NN − − − √ = 5.02 TeV.