UC San Diego

The first observation of the concurrent production of two mesons in proton-nucleus collisions is presented. The analysis is based on a proton-lead ( ) data sample recorded at a nucleon-nucleon center-of-mass energy of 8.16 TeV by the CMS experiment at the CERN LHC and corresponding to an integrated luminosity of . The two mesons are reconstructed in their decay channels with transverse momenta and rapidity . Events where one of the mesons is reconstructed in the dielectron channel are also considered in the search. The process is observed with a significance of 5.3 standard deviations. The measured inclusive fiducial cross section, using the four-muon channel alone, is . A fit of the data to the expected rapidity separation for pairs of mesons produced in single (SPS) and double (DPS) parton scatterings yields and , respectively. This latter result can be transformed into a lower bound on the effective DPS cross section, closely related to the squared average interparton transverse separation in the collision, of at 95% confidence level. © 2024 CERN, for the CMS Collaboration 2024 CERN