Bulletin of the American Physical Society
2020 Fall Meeting of the APS Division of Nuclear Physics
Volume 65, Number 12
Thursday–Sunday, October 29–November 1 2020; Time Zone: Central Time, USA
Session SC: Mini-Symposium: Probing nucleons and nuclei in ultra-peripheral collisions and with vector mesons II |
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Chair: Daniel Tapia Takaki, Kansas University |
Sunday, November 1, 2020 10:30AM - 10:42AM |
SC.00001: Probing nucleon substructure in UPCs Heikki Mäntysaari We discuss how simultaneous description of coherent and incoherent vector meson production data from UPCs can be used to constrain the nucleon substructure fluctuations in light and heavy nuclei. We consider subnucleon fluctuations in deuteron, probed in d+Au collisions at RHIC, and in lead, probed in lead-lead collisions at the LHC. We show that the RHIC and LHC data prefers nucleon substructure fluctuations compatible with the HERA data. Additionally, we discuss the most important model uncertainties limiting that currently limit the possibility to apply Color Glass Condensate effective field theory to describe these collisions. In particular, we discuss how to systematically constrain the vector meson wave function which results in a large model uncertainty. References: T. Lappi, H. M, J. Penttala, arXiv:2006.02830 [hep-ph] H.M, B. Schenke, Phys.Rev.C 101 (2020) 1, 015203 H.M, B. Schenke, Phys.Lett.B 772 (2017) 832-838 [Preview Abstract] |
Sunday, November 1, 2020 10:42AM - 10:54AM |
SC.00002: Probing the Nucleus with Linearly Polarized Photons Daniel Brandenburg The intense electromagnetic fields produced by ultra-relativistic heavy nuclei have been proposed as a source of quasi-real photons i.e. in the Weizsacker-Williams equivalent photon method. A photon from one nucleus can fluctuate into a quark antiquark pair and interact directly with the other nucleus to produce a vector meson (e.g. $\rho^0$). It has been demonstrated that the interacting photons are linearly polarized, and therefore that the photon polarization should induce angular modulations in the final state particle distribution. In this talk we present STAR measurements of diffractive photo-production of the $\rho^0$-meson (and direct $\pi^+\pi^-$ pairs) in ultra-peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. By employing a recently proposed technique, we measure the angular distribution of the final state $\pi^+\pi^-$ pairs and observe $\cos 2\Delta\phi$ and $\cos 4\Delta\phi$ modulations. Theoretical predictions suggest that such modulations may provide new insight into nuclear structure and may shed light on the transverse momentum dependent (TMD) distributions of gluons within large nuclei - a topic of great interest both at existing experiments and at a future Electron Ion Collider. [Preview Abstract] |
Sunday, November 1, 2020 10:54AM - 11:06AM |
SC.00003: The meson content of the electromagnetic field of ultra-relativistic nuclei Carlos Bertulani The meson content of the electromagnetic field of a relativistic heavy ion is discussed. The production of vector mesons due to higher electric and magnetic multipole content of the field is shown play a significant role [1,2,3]. References: [1] C.A. Bertulani, to be published. [2] A. Adeluyi, C. A. Bertulani, and M. J. Murray, Phys. Rev. C 86, 047901 (2012). [3] C.A. Bertulani, Phys. Rev. C 79, 047901 (2009). [Preview Abstract] |
Sunday, November 1, 2020 11:06AM - 11:18AM |
SC.00004: Two-particle azimuthal correlations in photo-nuclear ultra-peripheral Pb+Pb collisions at 5.02 TeV with ATLAS Blair Seidlitz This talk presents a measurement of two-particle long-range azimuthal correlations in photo-nuclear collisions using 1.73 nb$^{-1}$ of 5.02 TeV Pb+Pb data collected in 2018 by ATLAS with a dedicated photo-nuclear event trigger. Candidate photo-nuclear events are selected using a combination of the zero-degree calorimeters, forward calorimeters, and reconstructed pseudorapidity gaps constructed from calorimeter clusters and charged-particle tracks. Correlation functions are formed using charged-particle tracks in the event. A template fitting method is employed to subtract the non-flow contribution. Second-order (elliptic) flow coefficients are presented as a function of charged-particle multiplicity and transverse momentum, and significant non-zero values of the flow coefficients are observed. The results are compared to flow coefficients obtained in proton--proton and proton--lead collisions in similar multiplicity ranges. [Preview Abstract] |
Sunday, November 1, 2020 11:18AM - 11:30AM |
SC.00005: Observation of impact parameter dependence of $\mu^{+}\mu^{-}$ acoplanarity in ultra-peripheral PbPb collisions Shuai Yang Photon-photon interactions have been observed in hadronic heavy-ion collisions at very low transverse momentum ($p_T$) regions and the measured $p_T$ and azimuthal angular correlations of lepton pairs via $\gamma\gamma$ scattering in hadronic events exhibit significant broadening compared to that from vacuum production in ultra-peripheral events. There is still no consensus on the origin of the observed broadening, which is mainly from $p_T$ hardening of initial scattered photons as impact parameter decreases toward central hadronic collisions or final-state electromagnetic modifications of lepton pairs in presence of a QGP medium. In this talk, the azimuthal angular correlations and mass spectra of $\mu^{+}\mu^{-}$ pairs via $\gamma\gamma$ scattering will be presented as a function of forward neutron multiplicity and rapidity in ultra-peripheral PbPb collisions with the CMS experiment. The forward neutron multiplicity dependence of $\gamma\gamma \rightarrow \mu^{+}\mu^{-}$ production provides key insight to the origin of observed broadening for photon-photon produced lepton pairs in hadronic collisions while rapidity dependence constrains the relative contributions from leading order and high order photon-photon interactions to measured $\mu^+\mu^-$ pairs. [Preview Abstract] |
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