Wed·Nov 25, 2020Wednesday·November 25, 2020

1. 01*

   Exclusive dimuon production in ultraperipheral Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with ATLAS

   ATLAS Collaboration

   Exclusive dimuon production in ultraperipheral collisions (UPC), resulting from photon-photon interactions in the strong electromagnetic fields of colliding high-energy lead nuclei, $\mathrm{PbPb}(\gamma \gamma )\to {\mu }^{+}{\mu }^{-}({\mathrm{Pb}}^{(\star )}{\mathrm{Pb}}^{(\star )})$, is studied using ${\mathcal{L}}_{\mathrm{int}}=0.48$ nb${}^{-1}$ of $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV lead-lead collision data at the LHC with the ATLAS detector. Dimuon pairs are measured in the fiducial region $p_{\mathrm{T}\mu }>4$ GeV, $|{\eta }_{\mu }|<2.4$, invariant mass $m_{\mu \mu }>10$ GeV, and $p_{\mathrm{T},\mu \mu }<2$ GeV. The primary background from single-dissociative processes is extracted from the data using a template fitting technique. Differential cross sections are presented as a function of $m_{\mu \mu }$, absolute pair rapidity ($|y_{\mu \mu }|$), scattering angle in the dimuon rest frame ($|\cos {\vartheta }_{\mu \mu }^{\star }|$) and the colliding photon energies. The total cross section of the UPC $\gamma \gamma \to {\mu }^{+}{\mu }^{-}$ process in the fiducial volume is measured to be ${\sigma }_{\mathrm{fid}}^{\mu \mu }=34.1\pm 0.3(stat.)\pm 0.7(syst.)$ $\mu \mathrm{b}$. Generally good agreement is found with calculations from STARlight, which incorporate the leading-order Breit-Wheeler process with no final-state effects, albeit differences between the measurements and theoretical expectations are observed. In particular, the measured cross sections at larger $|y_{\mu \mu }|$ are found to be about 10-20% larger in data than in the calculations, suggesting the presence of larger fluxes of photons in the initial state. Modification of the dimuon cross sections in the presence of forward and/or backward neutron production is also studied and is found to be associated with a harder incoming photon spectrum, consistent with expectations.

   nucl-exhep-exPRC(2021)·114 citations
2. 02*

   Performance of photosensors in a high-rate environment for gas Cherenkov detectors

   Chao Peng🇺🇸 · Junqi Xie🇺🇸 · Sylvester Joosten🇺🇸 · Zein-Eddine Meziani🇺🇸 · Alexandre Camsonne🇺🇸 · Mark Jones🇺🇸 · Edward Kaczanowicz🇺🇸 · Melanie Rehfuss🇺🇸 · Nikolaos Sparveris🇺🇸 · Michael Paolone🇺🇸 · Michael Foley🇺🇸 · Michael Minot🇺🇸 · Mark Popecki🇺🇸

   The solenoidal large intensity device (SoLID) at Jefferson Lab will push the boundaries of luminosity for a large-acceptance detector, which necessitates the use of a light-gas threshold Cherenkov counter for online event selection. Due to the high luminosity, the single-photon background rate in this counter can exceed 160 kHz/cm${}^2$ at the photosensors. Therefore, it is essential to validate the high-rate limits of the planned photosensors and readout electronics in order to mitigate the risk of failure. We report on the design and an early set of studies carried out using a small telescopic Cherenkov device in a high-rate environment up to 60 kHz/cm${}^2$, in Hall C at Jefferson Lab. Commercially available multi-anode photomultipliers (MaPMT) and low-cost large-area picosecond photodetectors (LAPPD) were tested using the JLab FADC250 modules for readout. The test beam results show that the MaPMT array and the internal stripline LAPPD can detect and identify single-electron and pair-production events in high-rate environments. Due to its higher quantum efficiency, the MaPMT array provided a better separation between the single-electron and the pair-production events compared to the internal stripline LAPPD. A GEANT4 simulation confirms the experimental performance of our telescopic device.

   ↳ physics.ins-detnucl-exJINST(2022)·3 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.