Dipole moments of theτlepton as a sensitive probe for physics beyond the standard model

Abstract

CP-violating dipole moments of leptons vanish at least to three loop order and are estimated to be (m l /MeV)ϫ1.6ϫ10 Ϫ40 e cm in the standard model ͑SM͒, where m l is the mass of the lepton. However, they can receive potentially large contributions in some beyond the SM scenarios and this makes them very sensitive probes of new physics. In this article we show that a nonuniversal interaction, involving leptoquarks to the quark-lepton pair of the third generation through helicity-unsuppressed couplings of the order of ordinary gauge couplings, can generate electric and weak dipole moments of the order of 10 Ϫ19 e cm for the lepton. This is greater than pure supersymmetric ͑SUSY͒ and left-right (LR) contributions by almost three orders of magnitude. It is also greater than the mirror fermionic contribution by an order of magnitude. The measurements of d z and d ␥ at CERN LEP, SLC, and TCF are expected to reach sensitivities of 10 Ϫ18 e cm and 10 Ϫ19 e cm, respectively, in the near future. The observation of a nonvanishing dipole moment of at these facilities would, therefore, strongly favor superstring-inspired light leptoquark-mediated interactions, over pure SUSY or LR interactions and perhaps also mirror-generated mixings without some sort of quark-lepton unification as its origin.

CP-violating dipole form factors of the top quark and tau lepton in scalar leptoquark models

“…In this paper we consider the possibility that models with relatively light scalars with complex couplings to a third-generation lepton-quark pair can give rise to EDFF and WDFF of third generation fermions, viz., the top quark and the tau lepton, at a significant level. This possibility for the tau lepton has been considered recently by Mahanta [6] and Bernreuther et al [7]. While we confirm some results for tau of [6] and [7], we also obtain new results for the CP-violating form factors of the top quark, and treat the tau and top cases in a concerted manner.…”

“…This possibility for the tau lepton has been considered recently by Mahanta [6] and Bernreuther et al [7]. While we confirm some results for tau of [6] and [7], we also obtain new results for the CP-violating form factors of the top quark, and treat the tau and top cases in a concerted manner. We have also studied the implications of the existing experimental bounds from LEP on the weak dipole form factor of τ at the Z resonance, and on the masses and couplings of leptoquarks from loop effects contributing to the Z partial widths.…”

“…Finally, we end with a few comments on comparison of our work with other recent work on DFF in third-generation leptoquark models [6,7]. Mahanta [6] has estimated the τ EDFF and has reached the conclusion that it can be as high as 10 −19 e cm for a choice of g φ amd m φ consistent with experimental constraints.…”

Fermion dipole moments in supersymmetric models with explicitly brokenRparity

“…The theoretical predictions for the moments of the heavier fermions like the t, b or the τ , are larger than for first generation particles due to the linear dependence on m f [10]. In models with lepto-quarks, particularly large enhancements of the predicted values of the τ moments by a factor of m t /m τ , are possible [11]. Hence, measurements of the dipole moments of the τ and the t, at current [12] or future e + e − colliders [13] or γγ colliders [14] form a potentially interesting probe of non-standard physics.…”

RETRACTED: Implications of BNL measurement of δa on a class of scalar leptoquark interactions

“…It is a weak isoscalar and has charge 1 3 . The above interaction Lagrangian will generate a one loop correction to the magnetic moment [5] of muon which can be estimated from the following effective…”