Lepton universality (LU) typically refers to the lepton coupling,
which is considered to be the same for e and mu if the interaction is
electroweak according to the Standard Model, and it is hence a compelling
probe for New Physics.
The same principle of universal electroweak lepton interaction leads
to the expectation that lepton scattering yields are equal for e and mu
under the same kinematic condition. The mere mass difference between
e and mu affects kinematic quantities (such as the relation between
scattering angle and Q^2), and the lepton mass dependence of elastic
cross sections for leptons scattered from structured and pointlike
objects are taken into account.
By comparing e+, e-, mu+, and mu- scattering yields, two-photon
exchange (TPE) effects, universal or not, can be separated from the
general lepton universality test of the e/mu yield ratio.
With its separable mixed beams of e+/mu+ and e-/mu-, respectively, the
MUSE experiment at PSI is not only designed to measure the proton
charge radius with four lepton species, but is also uniquely suited to
probe TPE and LU, while benefitting from partial cancellations of
certain shared systematics.
An overview will be given of the MUSE experiment, the sensitivity, and the present status.
This author is presently supported by NSF grants PHY-1812402 and PHY-2113436.