Tue·Dec 15, 2015Tuesday·December 15, 2015

1. 01*

   Identified charged hadron production in pp and Pb-Pb collisions with ALICE at the LHC

   Maria Vasileiou (for the ALICE Collaboration)🇬🇷

   Nuclear matter under extreme conditions can be investigated in ultra-relativistic heavy-ion collisions. The measurement of transverse momentum distributions and yields of identified particles is a fundamental step in understanding collective and thermal properties of the matter produced in such collisions. The ALICE Experiment results on identified charged hadron production are presented for pp collisions at \sqrt{s} = 0.9, 2.76 and 7 TeV and for Pb-Pb collisions at \sqrt{s_{NN}} = 2.76 TeV. Spectral shapes, production yields and nuclear modification factors are shown and compared to previous experiments and Monte Carlo predictions. The spectral shapes in Pb-Pb collisions indicate a strong increase of the radial flow velocity with respect to RHIC energies, which in hydrodynamic models is expected as a consequence of the increasing particle density. The observed suppression of high transverse momentum particles in central Pb-Pb collisions provides evidence for strong parton energy loss in the hot and dense medium.

   nucl-exhep-exEPJ Web Conf.(2016)·3 citations
2. 02*

   Direct photon measurement in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV with ALICE

   Baldo Sahlmueller (for the ALICE collaboration)🇩🇪

   The ALICE experiment has measured the direct photon spectra in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV for three different centrality selections. The measurement was performed emplying a method utilizing conversion of photons into $e^{+}{e}^{-}$ pairs in the detector material, and a method using the PHOS calorimeter. The two measurements were combined in order to measure direct photons over a broad transverse momentum range of $0.9<p_{\mathrm{T}}<14$ GeV/$c$. A direct photon signal was observed in the thermal region $0.9<p_{\mathrm{T}}<2.1$ GeV/$c$, with a significance of $2.6\sigma$ in the 20\% most central collisions. The corresponding direct photon spectrum can be described by an exponential with an inverse slope parameter of $(304\pm 11^{\mathrm{stat}}\pm 40^{\mathrm{syst}})$ MeV, without subtracting the hard scattering component, which is somewhat larger than the inverse slope parameter of the direct photon spectrum at RHIC energies. Within uncertainties, the data can be described by different models for photon production in heavy-ion collisions. These proceedings provide a summary of the results published in [1] and [2].

   nucl-exNPA(2016)·11 citations
3. 03*

   Rôle of the pion electromagnetic form factor in the $\Delta (1232)\to {\gamma }^{\ast }N$ timelike transition

   G. Ramalho🇧🇷 · M.T Pena🇵🇹 · J. Weil🇩🇪 · H. van Hees🇩🇪 · U. Mosel🇩🇪

   The $\Delta (1232)\to {\gamma }^{\ast }N$ magnetic dipole form factor ($G_M^{\ast }$) is described here within a new covariant model that combines the valence quark core together with the pion cloud contributions. The pion cloud term is parameterized by two terms: one connected to the pion electromagnetic form factor, the other to the photon interaction with intermediate baryon states. The model can be used in studies of pp and heavy ion collisions. In the timelike region this new model improves the results obtained with a constant form factor model fixed at its value at zero momentum transfer. At the same time, and in contrast to the Iachello model, this new model predicts a peak for the transition form factor at the expected position, i.e. at the $\rho$ mass pole. We calculate the decay of the $\Delta \to \gamma N$ transition, the Dalitz decay ($\Delta \to e^{+}{e}^{-}N$), and the $\Delta$ mass distribution function. The impact of the model on dilepton spectra in pp collisions is also discussed.

   ↳ hep-phhep-exhep-latnucl-ex+1PRD(2016)·51 citations
4. 04*

   Nucleon Polarizabilities: from Compton Scattering to Hydrogen Atom

   Franziska Hagelstein (JGU Mainz)🇩🇪 · Rory Miskimen (UMass Amherst)🇺🇸 · Vladimir Pascalutsa (JGU Mainz)🇩🇪

   We review the current state of knowledge of the nucleon polarizabilities and of their role in nucleon Compton scattering and in hydrogen spectrum. We discuss the basic concepts, the recent lattice QCD calculations and advances in chiral effective-field theory. On the experimental side, we review the ongoing programs aimed to measure the nucleon (scalar and spin) polarizabilities via the Compton scattering processes, with real and virtual photons. A great part of the review is devoted to the general constraints based on unitarity, causality, discrete and continuous symmetries, which result in model-independent relations involving nucleon polarizabilities. We (re-)derive a variety of such relations and discuss their empirical value. The proton polarizability effects are presently the major sources of uncertainty in the assessment of the muonic hydrogen Lamb shift and hyperfine structure. Recent calculations of these effects are reviewed here in the context of the "proton-radius puzzle". We conclude with summary plots of the recent results and prospects for the near-future work.

   ↳ nucl-thhep-phnucl-exphysics.atom-phPPNP(2016)·111 citations
5. 05*

   Gluonic Structure of the Constituent Quark

   Nikolai Kochelev🇷🇺 · Hee-Jung Lee🇰🇷 · Baiyang Zhang🇨🇳 · Pengming Zhang🇨🇳

   Based on both the constituent quark picture and the instanton model for QCD vacuum, we calculate the unpolarized and polarized gluon distributions in the constituent quark and in the nucleon. Our approach consists of the two main steps. At the first step, we calculate the gluon distributions inside the constituent quark generated by the perturbative quark-gluon interaction, the non-perturbative quark-gluon interaction, and the non-perturbative quark-gluon-pion anomalous chromomagnetic interaction. The non-perturbative interactions are related to the existence of the instantons, strong topological fluctuations of gluon fields, in the QCD vacuum. At the second step, the convolution model is applied to derive the gluon distributions in the nucleon. A very important role of the pion field in producing the unpolarized and the polarized gluon distributions in the hadrons is discovered. We discuss a possible solution of the proton spin problem.

   ↳ hep-phhep-exnucl-exnucl-thPLB(2016)·11 citations
6. 06*

   Recent results from PHENIX on double helicity asymmetry ($A_{LL}$) measurement at $\sqrt{s}$ = 510 GeV

   H. Guragain🇺🇸

   One of the major objectives of the RHIC spin program at BNL is the measurement of the gluon helicity contribution, $\Delta G$, to the proton spin via measuring the double longitudinal spin asymmetry ($A_{LL}$) in various channels. In PHENIX (Pioneering High Energy Nuclear Interaction eXperiment) the $A_{LL}$ in ${\pi }^0$, $\eta$, $J/\psi$ etc. are measured in wide rapidity range. In this talk, $A_{LL}$ in ${\pi }^0$ production in central rapidity and $J/\psi$ production in forward rapidity will be discussed. The ${\pi }^0$ is reconstructed through its diphoton decay channel within the rapidity range of $\left|\eta \right|$ $<$ 0.35 and azimuthal angle of $180{}^{\circ }$. Similarly, $J/\psi$ is reconstructed via dimuon decay channel within the rapidity range of 1.2 $<$ $\left|\eta \right|$ $<$ 2.2. Preliminary results for $A_{LL}$ in ${\pi }^0$ and $J/\psi$ production from the data collected in the year 2013 at center of mass energy ($\sqrt{s}$) = 510 GeV will be presented. Also, their impact on $\Delta G$ constraint will be discussed. In year 2013, the total integrated luminosity was 150 $p{b}^{-1}$ which is almost ten times the total luminosity recorded in the year 2009 at $\sqrt{s}$ = 200 GeV. Due to increase in the center of mass energy and integrated luminosity, the new measurements can cover the Bjorken x range down to 0.01 for ${\pi }^0$ and 0.002 for $J/\psi$.

   ↳ hep-exnucl-exPoS(2015)·0 citations
7. 07*

   Multiplicity predictions for Pb-Pb collisions at $\sqrt{s}=5.02$ TeV based on the MC Modified Glauber model

   Andrey Seryakov🇷🇺 · Grigory Feofilov🇷🇺

   We present predictions of the mean multiplicity yields for different centrality classes of Pb-Pb collisions at $\sqrt{s}=5.02$ TeV at the LHC, obtained using the Monte-Carlo Modified Glauber model (MGM) [1]. Contrary to generally used standard Glauber calculations, the MGM takes into account the energy conservation in the soft particles production. The only one efficient model parameter was tuned and fixed previously (k=0.35) by fitting the available data on multiplicity for all colliding systems up to RHIC energies. The MGM has shown already a good predictive capability for the yields at $\sqrt{s}=2.76$ TeV, and it is used again within the same MC code for the calculations at $\sqrt{s}=5.02$ TeV

   ↳ nucl-thhep-phnucl-ex1 citation

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.