Wed·Jun 19, 2024Wednesday·June 19, 2024

1. 01*

   Photoproduction of J/$\psi$ and dileptons in Pb-Pb collisions with nuclear overlap

   Nicolas Bizé (for the ALICE collaboration)🇫🇷

   Photon-photon reactions and the production of J/$\psi$ meson through photonuclear reactions have been extensively studied in ultra-peripheral heavy-ion collisions, in which the impact parameter is larger than twice the nuclear radius. In recent years, coherently photoproduced J/$\psi$ and dilepton production via photon-photon interactions have also been observed in nucleus-nucleus (A-A) collisions with nuclear overlap. The former can help to constrain the nuclear gluon distributions at low Bjorken-$x$ and high energy, while the latter could be used to further map the electromagnetic fields produced in heavy-ion collisions. In addition, these measurements can shed light on the theory behind photon-induced reactions in A-A collisions with nuclear overlap, including possible interactions of the measured probes with the formed and fast expanding quark-gluon plasma. Since the produced quarkonium is expected to keep the polarization of the incoming photon due to $s$-channel helicity conservation, the photoproduction origin of the J/$\psi$ yield excess at very low transverse momentum, $p_{\mathrm{T}}$, can be confirmed by the measurement of the J/$\psi$ polarization. The ALICE detector can perform quarkonium production measurements at both mid ($|y|<0.9$) and forward ($2.5<y<4$) rapidities down to $p_{\mathrm{T}}=0$. In the following, the new ALICE measurements of the J/$\psi$ $y$-differential cross section and the first polarization results of coherently photoproduced J/$\psi$ via the dimuon decay channel at forward rapidity in Pb-Pb collisions at $\sqrt{s_{NN}}=$ 5.02 TeV are reported. Additionally, the measurement of an excess with respect to expectations from hadronic production in the dielectron yield, at low mass and $p_{\mathrm{T}}$, at midrapidity in Pb-Pb collisions at $\sqrt{s_{NN}}=$ 5.02~TeV, is presented. The results are compared with available theoretical models.

   nucl-exhep-exPhys.Proc.UPC(2024)·2 citations
2. 02*

   Measurement of Spin-Density Matrix Elements in ${\Delta }^{++}(1232)$ photoproduction

   F. Afzal🇩🇪 · C. S. Akondi🇺🇸 · M. Albrecht🇺🇸 · M. Amaryan🇺🇸 · S. Arrigo🇺🇸 · V. Arroyave🇺🇸 · A. Asaturyan🇦🇲 · A. Austregesilo🇺🇸 · Z. Baldwin🇺🇸 · F. Barbosa🇺🇸 · J. Barlow🇺🇸 · E. Barriga🇺🇸 and 137 other authors

   We measure the spin-density matrix elements (SDMEs) of the ${\Delta }^{++}(1232)$ in the photoproduction reaction $\gamma p\to {\pi }^{-}{\Delta }^{++}(1232)$ with the GlueX experiment in Hall D at Jefferson Lab. The measurement uses a linearly--polarized photon beam with energies from $8.2$ to $8.8$~GeV and the statistical precision of the SDMEs exceeds the previous measurement by three orders of magnitude for the momentum transfer squared region below $1.4$ GeV${}^2$. The data are sensitive to the previously undetermined relative sign between couplings in existing Regge-exchange models. Linear combinations of the extracted SDMEs allow for a decomposition into natural and unnatural--exchange amplitudes. We find that the unnatural exchange plays an important role in the low momentum transfer region.

   nucl-exPLB(2025)·5 citations

* 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.