Online and Offline Event Selections for UPC Heavy-flavour and Jet Events with CMS
B.C. Kovács · Acta Physica Polonica B Proceedings Supplement · 2025
In this proceeding, we present the experimental techniques developed to select good photo-nuclear events in ultraperipheral heavy-ion collisions (UPCs) with the CMS detector at the Large Hadron Collider (LHC). The CMS Collaboration has recently designed and implemented a new trigger strategy to maximize the number of collected photo-nuclear events in Pb+Pb UPCs. This goal was achieved by integrating the Zero Degree Calorimeters (ZDCs) in the Level-1 trigger system of CMS and by triggering on events based on the presence of forward neutrons in the ZDCs. Therefore, we could select events where only one of the colliding nuclei breaks up while the other one stays intact. A rapidity gap selection, based on the information provided by the hadron forward calorimeters (HFs) was also applied in the direction of the outgoing photon. The presented techniques were used in the first measurement of the D$^0$ production in Pb+Pb UPC collisions at $\sqrt{s_\mathrm{NN}}=5.36\ \mathrm{TeV}$.