Differential measurements of gamma gamma -> tau tau and constraints on tau-lepton electromagnetic moments in Pb+Pb collisions at root s_NN = 5.02 TeV with ATLAS

This paper presents the first differential fiducial measurements of gamma gamma -> tau tau using 1.93 nb-1 of Pb+Pb data at root sNN = 5.02 TeV recorded by the ATLAS detector. Events in which one of the tau -leptons decays into a muon and two neutrinos tau -> nu_tau (nu_mu)bar mu are selected and are categorized into three regions by the presence of an electron or either one or three charged particle track(s) from the second tau-lepton decay. The measurement is performed in events where both Pb ions remain intact and no neutrons are emitted. Differential cross-sections are measured for seven variables in three fiducial regions at particle level. The measurements are compared to theory predictions with different photon flux models and spin correlation effects. For the fiducial region with one muon and one electron in the final state, comparisons to next-to-leading-order electroweak predictions are also made. The transverse momentum (pT) of the decay muon, the pT of the visible decay particles of the other tau-lepton, the total pT, invariant mass, and pseudorapidity of the visible particles from the di-tau system, and the rapidity and acoplanarity of the visible decay particles from either tau-lepton are measured. A maximum-likelihood fit to the muon transverse-momentum distributions in the three regions before unfolding is performed to extract the tau-lepton anomalous magnetic moment a_tau and electric dipole moment d_tau , the latter for the first time in heavy ion collisions. The observed 95% confidence level intervals are -0.057 < a_tau < 0.035 and |d_tau | < 2.7 × 10-16 e cm.