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.

The Standard Model is an extremely successful theory in particle physics. Although it cannot explain all the fundamental physical phenomena observed in nature, it provides extremely precise calculations of physical parameters within its scope of application. To date, there is no generally accepted experimental result that deviates unequivocally from the predictions of the Standard Model. Various investigations into such deviations are currently underway. This is because an experimentally confirmed deviation could prove that the theory is incomplete and provide clues about the nature of the missing parts of the theory. The exact values of the lepton anomalous electromagnetic moments are believed to be sensitive to "Physics Beyond the Standard Model". The sensitivity is expected to be proportional to the square of the lepton mass. From the three generations of leptons it is hence the tau leptons which are expected to be most susceptible to modifications of the Standard Model. Due to the short life time of the tau the methods applied to investigate the electrons or muons (measurement of the precession frequency in a magnetic field) can not be applied. The idea of measuring the tau anomalous magnetic moment in UPC (Ultra Peripheral Collision) events of heavy ion collisions goes back to a paper by Aguila et at. published in 1991. It is however only now, with the luminosity of the Run3 data at LHC that this approach is promising. With this project, our goal was to measure the electromagnetic properties of the tau lepton using data from the ALICE experiment at the LHC. To do this, we exploit the gamma-gamma-to-tau-tau production process in UPCs and measure its differential cross section in transverse momentum. The value of this cross section depends on the value of the anomalous magnetic moment. During the LHC's long downtime period prior to the start of Run 3, the ALICE experiment underwent a comprehensive upgrade. Some detectors were replaced or upgraded with new components, and the entire software environment was overhauled to handle the expected higher data rates. The procedures developed in previous run periods to process, prepare and analyze the data had to be adapted accordingly. It turned out that this adaption process is laborious and time-consuming. Significant progress was made since the start of Run 3 to understand the system and data fully, but the process had not yet been finalized by the end of the project. Given the preliminary nature of the data, we were able to isolate UPCs from the data stream and extract potential di-tau events. With these we generated a raw distribution of the transverse momentum. A few analysis steps however, like e.g. the proper calibration, still need to be completed to determine the final result.