Differential measurements of $\gamma\gamma\to\tau\tau$ and constraints on $\tau$-lepton electromagnetic moments in Pb+Pb collisions at $\sqrt{s_{_\text{NN}}} = 5.02$ TeV with ATLAS

Abstract

This paper presents the first differential fiducial measurements of $\gamma\gamma\to\tau\tau$ using 1.93 nb$^{-1}$ of Pb+Pb data at $\sqrt{s_{_\text{NN}}} = 5.02$ TeV recorded by the ATLAS detector. Events in which one of the $\tau$-leptons decays into a muon and two neutrinos $\tau\to\nu_\tau\bar{\nu}_\mu\mu$ 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 ($p_\text{T}$) of the decay muon, the $p_\text{T}$ of the visible decay particles of the other $\tau$-lepton, the total $p_\text{T}$, 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 $|d_{\tau}|< 2.7 \times 10^{-16}~e\text{cm}$.}