We present the first measurement at the LHC of exclusive J/ψ photoproduction off protons, in ultraperipheral proton-lead collisions at sqrts_{NN}=5.02 TeV. Events are selected with a dimuon pair ...produced either in the rapidity interval, in the laboratory frame, 2.5<y<4 (p-Pb) or -3.6<y<-2.6 (Pb-p), and no other particles observed in the ALICE acceptance. The measured cross sections σ(γ+p→J/ψ+p) are 33.2±2.2(stat)±3.2(syst)±0.7(theor) nb in p-Pb and 284±36(stat)_{-32}^{+27}(syst)±26(theor) nb in Pb-p collisions. We measure this process up to about 700 GeV in the γp center of mass, which is a factor of two larger than the highest energy studied at HERA. The data are consistent with a power law dependence of the J/ψ photoproduction cross section in γp energies from about 20 to 700 GeV, or equivalently, from Bjorken x scaling variable between ∼2×10^{-2} and ∼2×10^{-5}, thus indicating no significant change in the gluon density behavior of the proton between HERA and LHC energies.
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Transverse momentum spectra of protons and anti-protons measured in the rapidity range 0<y<3.1 from 0–10% central Au+Au collisions at sNN=62.4 GeV are presented. The rapidity densities, dN/dy, of ...protons, anti-protons and net-protons (Np–Np¯) have been deduced from the spectra over a rapidity range wide enough to observe the expected maximum net-baryon density. From mid-rapidity to y=1 the net-proton yield is roughly constant (dN/dy∼10), but rises to dN/dy∼25 at 2.3<y<3.1. The mean rapidity loss is 2.01±0.14±0.12 units from beam rapidity. The measured rapidity distributions are compared to model predictions. Systematics of net-baryon distributions and rapidity loss vs. collision energy are discussed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Scanning the phases of QCD with BRAHMS Murray, Michael; Collaboration, the BRAHMS
Journal of Physics G: Nuclear and Particle Physics,
08/2004, Volume:
30, Issue:
8
Journal Article, Conference Proceeding
The ALICE measurement of K_{S}^{0} and Λ production at midrapidity in Pb-Pb collisions at sqrts_{NN}=2.76 TeV is presented. The transverse momentum (p_{T}) spectra are shown for several collision ...centrality intervals and in the p_{T} range from 0.4 GeV/c (0.6 GeV/c for Λ) to 12 GeV/c. The p_{T} dependence of the Λ/K_{S}^{0} ratios exhibits maxima in the vicinity of 3 GeV/c, and the positions of the maxima shift towards higher p_{T} with increasing collision centrality. The magnitude of these maxima increases by almost a factor of three between most peripheral and most central Pb-Pb collisions. This baryon excess at intermediate p_{T} is not observed in pp interactions at sqrts=0.9 TeV and at sqrts=7 TeV. Qualitatively, the baryon enhancement in heavy-ion collisions is expected from radial flow. However, the measured p_{T} spectra above 2 GeV/c progressively decouple from hydrodynamical-model calculations. For higher values of p_{T}, models that incorporate the influence of the medium on the fragmentation and hadronization processes describe qualitatively the p_{T} dependence of the Λ/K_{S}^{0} ratio.
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The BRAHMS experiment at RHIC has measured pions, kaons and protons and their antiparticles over a wide range of rapidity and transverse momenta. Au+Au collisions, d+Au and p+p interactions have been ...studied. From the net-proton rapidity distribution a stopping power of 73% can be deduced for central Au+Au collisions. The rapidity distributions of produced particles exhibit Gaussian shapes. At midrapidity the nuclear modification factor for central Au+Au collisions shows a strong pion suppression at intermediate transverse momenta; a suppression which is even stronger at forward rapidities. The nuclear modification factor for d+Au changes from a Cronin-like enhancement of charged hadrons at midrapidity to a suppression at forward rapidities.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The ratio of the yields of antiprotons to protons in pp collisions has been measured by the ALICE experiment at {radical}(s)=0.9 and 7 TeV during the initial running periods of the Large Hadron ...Collider. The measurement covers the transverse momentum interval 0.45<p{sub t}<1.05 GeV/c and rapidity |y|<0.5. The ratio is measured to be R{sub |y|<0.5}=0.957{+-}0.006(stat){+-}0.014(syst) at 0.9 TeV and R{sub |y|<0.5}=0.991{+-}0.005(stat){+-}0.014(syst) at 7 TeV and it is independent of both rapidity and transverse momentum. The results are consistent with the conventional model of baryon-number transport and set stringent limits on any additional contributions to baryon-number transfer over very large rapidity intervals in pp collisions.
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