Model-independent constraints on leptoquarks from rareμandτlepton processes

Abstract

We perform a model independent analysis so as to constrain the leptoquark (LQ) models from negative searches for µ → eγ, µ → 3e decays (and analogous processes in the τ sector), and coherent µ − e conversion in nuclei. We considerably improve some constraints obtained by analyses known in the literature, analyses which we show have by far underestimated the LQ contributions to the µ → 3e. In particular we find that the coherent µ − e conversion in nuclei mediated by the photon-conversion mechanism and the µ → 3e decay are golden plates where the flavor changing leptoquark couplings, involving the second and third quark generations, can be strongly constrained. This is due to the fact that these processes get the enhancements by large log(m 2 q /m 2 LQ ) terms which are induced by the so-called "photon-penguin" diagrams. These enhancements, which produce a mild GIM suppression in the amplitudes, have not been taken into account in the previous analyses. We show that the µ → eγ decay can set weaker constraints on the LQ models and this is because its amplitude is strongly GIM suppressed by the terms of order O(m 2 q /m 2 LQ ). We also present the results for the corresponding constraints in the τ sector. Finally the prospects of the future muon experiments for the improvement of the present bounds are analyzed and discussed.

Leptoquarks: Neutrino masses and related accelerator signals

“…The most stringent upper bounds for the first generation Yukawa couplings ( R S 1=2 i1 and L S 1 i1 ) are currently found from the upper limit on the lepton flavor violating process Ti ! eTi [11,43]: …”

Searching for leptoquarks at IceCube and the LHC

“…This has been extensively studied in the literature [92,93]. Typically, the tightest constraint arises from μ − e conversion in nuclei that bounds jx 11 x 12 j ≲ 10 −5 for a ∼TeV LQ in model A.…”

Lepton flavor-violating transitions in effective field theory and gluonic operators

“…Assuming the dominance of the dipole operator over all other contributions, one obtains comparable bounds on heavy quark couplings to leptoquarks which are of order 10 −3 (M LQ /1 TeV) 2 for the products of couplings with same chiralities |λ µi LQ λ ei LQ |. Here we only considered quarks of the last two generations i = 2, 3 (either c, t or s, b), LQ = LS 0 , RS 0 , LS 1/2 , RS 1/2 , LV 0 , RV 0 , LV 1/2 and RV 1/2 , with M LQ being the mass of the correspondent scalar or vector LQ [41].…”