2026/07/22 by CMS Collaboration
#nucl-ex #hep-ex
The production of τ lepton pairs via photon-photon fusion, γγ→ ττ, is studied in ultraperipheral lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The dataset, collected by the CMS experiment in 2018, corresponds to an integrated luminosity of 1.70 nb-1. Four different τ+τ- decay final states are analyzed. A simultaneous likelihood fit to the measured lepton transverse momentum (pT) distributions, which incorporates information from both spectral shape and normalization, is used to constrain the anomalous magnetic moment of the τ lepton, aτ, and to extract the cross section of the process. The measured 95% CL interval for aτ is -0.039 \lt aτ \lt 0.032. The fiducial cross section, σγγ→ ττfid = 555+60-14 μb for tau leptons with pTτ \gt 1 GeV and pseudorapidity |ητ| \lt 3, is the most precise measurement for this process at the LHC to date and is in agreement with next-to-leading-order quantum electrodynamics predictions.