Probing electromagnetic moments of the tau lepton in PbPb collisions at the FCC-hh

TL;DR

This work investigates sensitivity to the tau lepton's electromagnetic moments, $a_\tau$ and $d_\tau$, in ultra-peripheral PbPb collisions at the FCC-hh via the process $PbPb \to Pb\,\gamma\gamma\,Pb \to Pb\,\tau^+\tau^-\,Pb$. By modeling the $\gamma\gamma$ flux with the EPA and parameterizing the tau-photon vertex with form factors $F_1(q^2)$, $F_2(q^2)$, and $F_3(q^2)$ (where $F_2(0)=a_\tau$ and $F_3(0)=-\frac{2m_\tau}{e}d_\tau$), the study evaluates cross sections and backgrounds under realistic kinematic cuts ($p_T>40$ GeV, $|\eta|<2.3$) and exclusivity requirements. The analysis yields 95% CL bounds $|a_\tau|\le 0.0102$ and $|d_\tau|\le 5.75\times 10^{-17}$ e cm, with 3σ and 5σ discovery sensitivities at $|a_\tau|\le 0.0096$, $|d_\tau|\le 5.12\times 10^{-17}$ e cm and $|a_\tau|\le 0.012$, $|d_\tau|\le 7.04\times 10^{-17}$ e cm, respectively. These results are compared to prospective bounds from other future colliders and interpreted within SMEFT, highlighting the complementary role of ultra-peripheral PbPb collisions in probing tau electromagnetic properties. Overall, the FCC-hh PbPb program significantly extends the tau-moment reach and offers a robust, background-controlled channel for BSM tests.

Abstract

A production of a pair of tau leptons in PbPb collisions at the FCC-hh collider is examined. The 95\% C.L. exclusion limits, as well as 3$σ$ and 5$σ$ sensitivity limits on the anomaly magnetic moment of the tau lepton $a_τ$ and its electric dipole moment $d_τ$ are obtained. A comparison with bounds on $a_τ$ and $d_τ$ for other future colliders are given.

Figures (6)

• Figure 1: The differential cross section of the collision $PbPb \rightarrow Pb \,(\gamma\gamma\rightarrow \tau^+\tau^-) \, Pb$ at the FCC-hh versus minimal transverse momenta of the outgoing tau leptons $p_{t,\min}$. The tau lepton electromagnetic moments are taken to be $a_\tau = 0.01$, $d_\tau = 0$.
• Figure 2: The same as in Fig. \ref{['fig:atcsptcut']}, but for the case $a_\tau = 0$, $d_\tau = 5\times10^{-17}$.
• Figure 3: The exclusion significance $SS_{\mathrm{excl}}$ as a function of the anomalous magnetic moment of the tau lepton in PbPb collision at the FCC-hh. The $\tau$ electric dipole moment is fixed and equal to $d_\tau = 0$.
• Figure 4: The exclusion significance $SS_{\mathrm{excl}}$ as a function of the electric dipole moment of the tau lepton in PbPb collision at the FCC-hh. The $\tau$ anomalous magnetic moment is fixed and equal to $a_\tau = 0$.
• Figure 5: The discovery significance $SS_{\mathrm{disc}}$ as a function of the anomalous magnetic moment of the tau lepton in PbPb collision at the FCC-hh. The $\tau$ electric dipole moment is fixed and equal to $d_\tau = 0$.