Constraints from muon related observables in BSM physics

Abstract

The Standard Model is an achievement of modern physics, but is still not without flaws. One of its most important shortcomings is the lack of a dark matter candidate. Some experimental results are also not consistent with the theory either, chief among them the measurements of the anomalous magnetic moment of the muon. In the past few years, beyond the Standard Model scenarios have been proposed to address these issues in tandem. In this work, we present a method to use precision muon-related observables for constraining new physics. The two key quantities are the muon asymmetry parameter, Aµ, and the ratio of hadronic decay rates, Reµ = Re / Rµ. We use Hollik’s on-shell renormalisation scheme to get the one loop electroweak radiative corrections to Z → µ+µ- and e+ e- → µ+µ- . We conclude that if new physics does not alter the theory’s tree-level structure, the one loop matrix elements have Born-like expressions, with redefined left- and right-handed chiral couplings, QED and weak neutral couplings and Weinberg angle. We test our constraint on a model that introduces a vector-like fermion and two scalar singlets of SU(2)L, one of them with a colour charge. We calculate the contributions to the chiral couplings and see that they can lead to a significant deviation to the observables, mainly through the left-handed coupling. This model impacts several flavour-related processes at one loop order. We take advantage of experimental bounds on these processes and on dark matter observables to restric the parameter space of the model by performing a multiparameter scan where we also impose the constraint derived from Aµ and Reµ. The results are in line with previous studies, with only minor alterations from the precision observables. Finally, we employ the same aproach to a different decay channel, Z → τ+ τ-, and compare it with the case of the muon.