Speaker
Description
Collisions involving ultraperipheral heavy ions (UPCs) serve as a valuable tool for investigating nuclear parton distribution functions (nPDFs). Specifically, they are instrumental in characterizing nuclear matter at Bjorken-$x<10^{-3}$ and low squared momentum transfer (shadowing/saturation regime). Additionally, UPCs provide an opportunity to explore phenomena beyond the standard model. To maximize the utility of these collisions prior to the advent of the Electron-Ion Collider, the CMS experiment developed dedicated triggers and optimized offline reconstruction for the heavy ion data-taking period in 2023. These triggers relied on using the Zero Degree Calorimeter (ZDC) as a level-1 (L1) trigger detector for the first time, enhancing the selection performance compared to existing UPC triggers and minimum-bias hadronic triggers. This improvement enabled the study of hard processes, such as jets and heavy flavor hadrons, in photon-photon ($\gamma\gamma$) and photon-nucleus ($\gamma\rm{N}$) scatterings. Simultaneously, an alternative approach was implemented to enhance the reconstruction efficiency for low transverse momentum ($p_\rm{T}$) electrons ($p_\rm{T} > 0.2$ GeV), photons ($p_\rm{T} > 0.6$ GeV), and tracks ($p_\rm{T} > 0.05$ GeV). This involved developing modified versions of the standard CMS particle-flow algorithm. In this presentation, we showcase selected results that highlight the performance of the L1 ZDC trigger selection, the efficiency of the L1 trigger algorithms in selecting $\gamma\rm{N}$ dijet events, and the performance of low-$p_\rm{T}$ electrons, photons, tracks, and muons under conditions prevalent in UPC PbPb collisions, as recently recorded in 2023 by the CMS experiment at their record energy of 5.36 TeV
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