Tau anomalous magnetic moment with ALICE at LHC

The Standard Model (SM) of particle physics is extremely successful in describing physics a subatomic level. However, in spite of its success it can not be considered to be complete. It falls short to explain some of the fundamental characteristics of cosmology, like the existence and nature of Dark Matter and Dark Energy or the observed matter-antimatter asymmetry. In addition there are intrinsic problems of the theory in that it contains many parameters which are not deduced from first principles and in that it is incompatible with general relativity. But although the theory is incomplete there is to date no single confirmed experimental result which is in odd with the SM prediction at a 5-sigma level. Many possible extensions of the theory are proposed to overcome some of its shortcomings. It will however only be by experiment and observation that the validity of these Beyond the Standard Model" (BSM) extensions can be scrutinized. Experimental searches for signs of BSM physics is ongoing in different domains. After the successful confirmation of the existence of the Higgs boson at the LHC quite some effort has been invested in exploring the available data in a manifold of ways to find unexpected BSM results. However, so far none of these measurements does seriously challenge the SM. High-precision measurements of low energy processes provide alternative tools to explore BSM physics and measurements of the anomalous electromagnetic moment (al) of leptons (electron, muon, tau) may serve as one of the most promising directions. While the experimental value of the electron magnetic moment agrees with theoretical predictions with up to 10 significant digits, the muon magnetic moment measured with 10-7 precision shows deviations from the SM at the level of 3.5-sigma indicating possible emergence of BSM effects. Since the contribution of BSM effects to a l are expected to grow proportional with the square of the lepton mass it is the measurement of the tau lepton, the heaviest of the three leptons, which promises to be most sensitive to BSM physics. The experimental data on the tau magnetic moment is scarce since the standard methods used to measure the electron and muon magnetic moments are not applicable due to very short lifetime of tau. Ultra peripheral collisions (UPC) of heavy ions at the LHC may serve as a very promising tool to measure at. UPCs are characterized by impact parameters larger than the sum of ion radii thus leading to the dominance of electromagnetic processes. The electromagnetic fields surrounding heavy ions moving with high speed can be described as a flux of photons. UPCs therefore provide a unique environment to study gamma-gamma interactions at very high energies. In this project we will explore Pb-Pb UPCs with ALICE at LHC to study the electromagnetic properties of the tau lepton in the di-tau production process gamma-gamma to tautau. The goal is to set new limits on the anomalous magnetic moment of the tau lepton.