Fri·Sep 11, 2020Friday·September 11, 2020

1. 01*

   Exclusive $J/\psi$ photoproduction off deuteron in d+Au ultra-peripheral collisions at STAR

   Zhoudunming Tu (for the STAR Collaboration)🇺🇸

   Gluon density and its distributions inside nuclei and the parton modification of bounded nucleons inside a nucleus, are some of the main standing problems in nuclear and particle physics. In recent years, ultra-peripheral collisions (UPC) of heavy ions have provided a new way of probing the gluon density, which is based on coherent diffractive vector-meson productions, e.g., $J/\psi$ meson. For heavy ions, e.g., Pb, the gluon density is found to be significantly suppressed through the UPC $J/\psi$ measurement, suggesting a strong gluon shadowing effect in heavy nuclei. In this analysis, we aim to look at a unique set of data taken by the STAR experiment, where $J/\psi$ mesons are photoproduced off the deuteron target with no other particle produced, except for the deuteron or its breakup products. The Zero Degree Calorimeter response with respect to the deuteron dissociation by detecting a beam-rapidity neutron is also investigated and provides additional information about the underlying physics process. The cross section of $J/\psi$ photoproduction in the photon-deuteron system is measured at the photon-nucleon center-of-mass energy $W\sim 25\mathrm{GeV}$, as well as the momentum transfer $t$ dependence cross section, $d\sigma /dt$. Data suggests a wider gluon density distribution than the Hulthen charge density distribution in deuteron.

   nucl-exhep-exhep-phnucl-thPoS(2021)·3 citations
2. 02*

   Jet substructure in $p$+$p$ and $p$+Au collisions at $\sqrt{s_{\text{NN}}}=200$ GeV at STAR

   Isaac Mooney (for the STAR Collaboration)🇺🇸

   In order to attribute the partonic energy loss experienced by jets (jet quenching) observed in A+A collisions to the traversal of partons through the hot QCD medium, it is necessary to examine the cold nuclear matter (CNM) effects on the corresponding jets. Such an examination has historically been done using $p$+A collisions. We present fully corrected measurements of the jet mass and SoftDrop groomed jet mass in $p$+$p$ and $p$+Au collisions at STAR at $\sqrt{s_{\mathrm{NN}}}=200$ GeV as a function of the event activity (EA) to increase or decrease the magnitude of CNM effects. EA is determined in the backward (Au-going) rapidity ($-5.0<\eta <-3.3$) by the STAR Beam-Beam Counter to minimize auto-correlation with jets measured at mid-rapidity. Comparison of the jet mass distribution in $p$+Au collisions to that in $p$+$p$ collisions allows for isolation of CNM effects in anticipation of an upcoming jet mass measurement in Au+Au collisions.

   nucl-exPoS(2021)·1 citation
3. 03*

   $J/\psi$-$p$ Scattering Length from the Total and Differential Photoproduction Cross Sections

   Lubomir Pentchev (Thomas Jefferson National Accelerator Facility, Newport News, Virginia, USA)🇺🇸 · Igor I. Strakovsky (The George Washington University, Washington, D.C., USA)🇺🇸

   The $J/\psi$-$p$ scattering length, ${\alpha }_{J/\psi p}$, can be extracted from the $J/\psi$ photoproduction cross section near threshold using the Vector Meson Dominance (VMD) model to relate the reaction $\gamma p\to J/\psi p$ to $J/\psi p\to J/\psi p$. Such estimates based on experimental data result in values for $|{\alpha }_{J/\psi p}|$, which are much lower than most of the theoretical predictions. In this work, we study the relations between the different results, depending on the use of the total or the differential cross sections, and the method of extrapolating the data to threshold in the case of a low-statistics data sample, such as the near threshold $J/\psi$ photoproduction dataset. We estimate a range for $|{\alpha }_{J/\psi p}|$ of $0.003$ to $0.025$~fm as extracted from experimental data within the VMD model and discuss possible reasons for such lower values compared to the theoretical results.

   ↳ hep-phhep-exnucl-exnucl-thEPJA(2021)·37 citations
4. 04*

   Early Time Dynamics and the Bulk

   Bin Wu🇨🇭

   Deciphering the origin of collective phenomena in small colliding systems is one of contemporary focuses in heavy-ion physics. It entails penetrating the barrier between two previously separated research topics: thermalization/hydrodynamization and phenomenological studies of collectivity. I first review some recent progress in understanding thermalization/hydrodynamization in large colliding systems, centralized on bottom-up thermalization. Then, using a simple kinetic theory I demonstrate how the investigation of hydrodynamization is intertwined with the study of flow in small colliding systems. Connections of these studies to "hard probes" are also commented where possible.

   ↳ hep-phhep-exnucl-exnucl-thPoS(2021)·1 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.