Bulletin of the American Physical Society
APS April Meeting 2015
Volume 60, Number 4
Saturday–Tuesday, April 11–14, 2015; Baltimore, Maryland
Session E3: Invited Session: Hard Probes in pA Collisions |
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Sponsoring Units: DNP Chair: Anne Sickles, University of Illinois Room: Holiday 2 |
Saturday, April 11, 2015 3:30PM - 4:06PM |
E3.00001: Color fluctuations in pA collisions at collider energies Invited Speaker: Mark Strikman In pA collisions at collider energies a projectile stays in a frozen configuration over the distances which by far exceed the nuclear diameter. As a result proton coherently interacts with nucleons along its impact parameter. In QCD nucleon is build of configurations of different transverse size which are expected to interact with different strength leading to the fluctuations of the global strength of the projectile interaction. Also, configurations of smaller size are expected to have a reduced gluon field leading to a correlation of soft and hard interactions. The shape of the distribution over the strength of interaction is strongly constrained by the diffractive pp data, behavior of the distribution for $\sigma \to $ 0 expected in pQCD, etc [1]. We developed a Monte Carlo procedure for taking into account these effects in soft collisions and collisions with a hard trigger taking into account difference of the transverse scales of hard and soft interactions [2, 3]. We predicted that distribution over the number of wounded nucleons should be broader than in the Glauber model in agreement with the recent LHC data. We argue that a strong violation of the Glauber approximation in the dependence of the rate of forward jet production on centrality observed in pA collisions at the LHC provides the first experimental evidence that parton configurations in the projectile proton containing a parton with large x$_{\mathrm{p}}$ interact with a nuclei with a significantly smaller than average cross section and have smaller than average size. Implementing effects of the interaction strength fluctuations and using the ATLAS analysis of the dependence of the hadron production at backward rapidities on the number of wounded nucleons, we make quantitative predictions for the centrality dependence of the jet production rate as a function of the interaction strength $\sigma $(x$_{\mathrm{p}})$. We find $\sigma $(x$_{\mathrm{p}} = $ 0.6) $\sim$ $\sigma_{\mathrm{tot}}$(pp)/2 [4] which sheds light on the origin of the EMC effect. Future pA dijet studies along these lines would allow to investigate the global 3D structure of the nucleon and in particular trigger on configurations which interact with the strength larger than average. \\[4pt] [1] H. Heiselberg, G. Baym, B. Blaettel, L. L. Frankfurt and M. Strikman, Phys. Rev. Lett. 67, 2946 (1991); Phys. Rev. C 52, 1604 (1995)\\[0pt] [2] M. Alvioli and M. Strikman, Phys. Lett. B 722, 347 (2013) [arXiv:1301.0728 [hep-ph]].\\[0pt] [3] M. Alvioli, L. Frankfurt, V. Guzey and M. Strikman, Phys. Rev. C 90, no. 3, 034914 (2014) [arXiv:1402.2868 [hep-ph]].\\[0pt] [4] M. Alvioli, B. Cole, L. Frankfurt and M. Strikman, arXiv:1409.7381 [hep-ph]. [Preview Abstract] |
Saturday, April 11, 2015 4:06PM - 4:42PM |
E3.00002: Experimental Results on Jets in pA Invited Speaker: Eric Appelt The experimentally observed reduction of jet yields in ultrarelativistic heavy ion (AA) collisions relative to proton-proton (pp) collisions is widely interpreted in terms of energy loss of a hard scattered parton traversing a quark-gluon plasma (QGP) before fragmenting into a jet of hadrons. In order to constrain proposed mechanisms of energy loss, a variety of measurements are needed that quantify both how the jet yields and jet structure are modified in the medium. However, jets may also be modified by differences in the initial state of the nucleus relative to that of the proton. The precise determination of the QGP properties relies on disentangling these additional modifications, collectively termed ``cold nuclear matter'' effects, from energy loss in the QGP. Collisions between heavy ions and protons (pA) provide a potential control environment where cold nuclear matter effects should be present, but QGP formation is generally not expected to occur. In this talk, an overview of recent jet results from proton-lead collisions produced at the LHC will be given. The yield of inclusive jets and distributions of dijet pairs are shown to be compatible with generally accepted theoretical expectations, although significant modification is observed when yields are measured from specific centrality classes of pA collision events. Some measurements of high-$p_T$ charged hadron yields suggest a larger modification in pA collisions relative to pp collisions than for inclusive jet yields. The potential implications of this difference along with other measurements relating to jet structure will be discussed. [Preview Abstract] |
Saturday, April 11, 2015 4:42PM - 5:18PM |
E3.00003: Quarkonia production in p+A collisions - what have we learned? Invited Speaker: Anthony Frawley It has been known for many years that gluon distribution functions in nuclei are modified from those in a proton. The modification should affect the yields of hard probes produced in high energy p+A collisions, since they are produced in hard scattering processes between partons in the proton and the nuclear target. This phenomenon is interesting in itself, and also because it affects the initial state in high energy heavy ion collisions, where a quark gluon plasma (QGP) is formed. Knowledge of the initial state modification in heavy ion collisions is needed before one can try to understand the effects of the QGP on the production of hard probes. The modification of the yield from a hard process that involves a gluon in a nuclear target depends on the fraction of the momentum carried by the gluon, and the momentum transferred in the interaction. For low values of the momentum fraction, the yield is reduced (referred to as shadowing). For large values of the momentum fraction it is enhanced (referred to as anti-shadowing). The modification becomes smaller as the scale of the momentum transfer increases. From simple kinematic considerations, the modification must depend on the rapidity of the detected hard probe and the collision energy. The production of charm and bottom quarks is sensitive to the gluon distributions in the projectile and target. It occurs at a scale (set by the masses of the charm and bottom quarks) that is reasonably low - and so the modification is expected to be significant. Thus a comparison of charmonium or bottomonium production between p+p and p+A collisions should reflect the gluon modification in the target nucleus. There are, however, other processes besides modification of the parton distributions that contribute to quarkonia production in a nuclear target and these need to be understood. They include breakup of the forming quarkonia by collisions with nucleons in the target, and parton energy loss in the cold nucleus. There is also recent evidence of collective flow effects in high energy p+A collisions, suggesting that a small quark gluon plasma may be formed. There is now a large body of data on heavy quarkonia production in p+A collisions at energies ranging from $\sqrt{s_{NN}}$ = 17 GeV to 5 TeV and covering a wide range of rapidities. I will review this data set and discuss what we have learned from it about the issues outlined above. [Preview Abstract] |
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