Feasibility study of tau-lepton anomalous magnetic moment measurements with ultra-peripheral collisions at the LHC

Abstract

Precision measurements of the anomalous electromagnetic moment of leptons (al) may serve as one of the most promising directions in the search for new physics beyond the Standard Model. While the experimental value of the electron magnetic moment agrees with theoretical predictions with up to 11 significant digits, the muon magnetic moment shows deviations from the Standard Model value at the level of 4.2 sigma, indicating the possible occurrence of new physics effects. Although the aτ of the tau lepton with its heavy mass is expected to be ${{m_\tau ^2} \mathord{\left/ {\vphantom {{m_\tau ^2} {m_\mu ^2}}} \right. \kern-\nulldelimiterspace} {m_\mu ^2}} \approx 280$ times more sensitive to new physics effects than aμ, measurements of this quantity are rare. This is because the standard spin precession methods are not suitable for aτ measurements due to the very short tau lifetime. Ultra-peripheral collisions of heavy ions at the LHC may serve as an alternative tool to measure aτ. In ultra-peripheral collisions, hadronic interactions are strongly suppressed and long-distance electromagnetic processes dominate, providing an environment to study the electromagnetic properties of the tau lepton. The di-tau production process PbPb → PbPbγγ → PbPbττ contains two gamma-tau vertices and hence provides enhanced sensitivity to the anomalous magnetic and electric moments. In this contribution we discuss the feasibility of the aτ measurement in ultraperipheral collisions with the ALICE experiment and present projections of the sensitivity of the measurement for the upcoming heavy ion run in 2022 at LHC.