Charged leptons (electron, muon and tau lepton) have been key to the current understanding of
physics at the shortest length scales. The first evidence for three generations of the Standard
Model (SM) fermions was the discovery of muons in cosmic rays in 1936. The seminal calculation
of the electron magnetic moment within quantum electrodynamics by Schwinger in 1948
was key in establishing quantum field theory and now culminates in the most precise prediction
and measurement of any physical observable. Decays of the two heavier charged leptons,
muon and tau lepton, to a lighter charged lepton, e.g. μ → eγ, provide the most stringent
constraints on possible origins of neutrino masses.
Several experiments with increased sensitivity in charged lepton physics are expected to
deliver exciting results in the next few years. For example, the final result of the Fermilab muon
g − 2 experiment together with an updated theory prediction is expected in 2025. The Mu3e
experiment and similarly the μ − e conversion experiments COMET and Mu2e will soon start
taking data with orders-of-magnitude improvements in sensitivity. Belle II will collect more data
than its predecessors, become more sensitive to rare processes and also provide improved
measurements of precision observables soon.
In light of the upcoming results, we propose a programme on charged lepton physics with
experimental and theoretical participation to discuss the first results and recent developments
as well as to prepare for the upcoming data and to identify new opportunities. Concretely, we
plan to discuss leptonic magnetic and electric dipole moments, charged lepton flavour violation
(cLFV) in different processes, lepton flavour universality (LFU) tests and further topics related
to precision measurements of charged leptons.
Given the numerous experiments active in this area of research as well as advances in studies
of Effective Field Theories (EFTs), analyses of beyond the SM (BSM) theories and the
search for an organising principle in the lepton sector, this proposal is highly timely and a workshop
at MIAPbP in 2026 would be a unique opportunity for exchanging ideas and starting new
collaborations.
Until now, more than 25 colleagues have confirmed their interest in participating in this proposed
MIAPbP programme.