Model independent constraints on leptoquarks from rare processes

Abstract

We present model independent constraints on the masses and couplings to fermions of B and L conserving leptoquarks. Such vector or scalar particles could have masses below 100 GeV and be produced at HERA; we list the generation dependent bounds that can be calculated from rare lepton and meson decays, meson-anti-meson mixing and various electroweak tests.

Neutrinoless double-beta decay and physics beyond the standard model

“…LQs which conserve baryon number can be relatively light [60], possibly within reach of accelerator experiments. Also low-energy precision measurements can give limits on LQ properties, for a detailed list on constraints from nonaccelerator searches see, for example [61] and [62]. The mixing of different LQ multiplets by a possible leptoquark-Higgs coupling [11] can lead to a contribution to 0νββ decay, if these couplings violate lepton number [12].…”

Leptoquark signals via ν interactions at neutrino factories

“…But as also discussed in the introduction, strong constraints on the LQ couplings and masses have been obtained in the literature from FCNC processes [7]. In particular, bounds obtained from rare τ decay τ −→ π 0 µ and from µ ←→ e conversion in nuclei would have a direct bearing on the processes considered here.…”

“…It is clear that with the increase in baseline length, the LQ event rate falls off and neutrino oscillations are the main source events examined here. At near-site experiments, on the other hand, the events mainly arise from new interactions and can thus be used to constrain the theory (figures [3][4][5][6][7][8]. In particular one can obtain constraints on LFV couplings between the first and third generation, the bounds on which are generally not available.…”

“…There have been numerous phenomenological studies to put constraints on LQ from low energy flavour changing neutral current (FCNC) processes which are generated by both the scalar and vector LQ interactions, since there is no reason why the quark-lepton couplings with LQ have to be simultaneously diagonal in quark and lepton mass matrices. Direct experimental searches for leptoquarks have also been carried out at the e p collider and bounds obtained [7,8]. In this letter, we compute and analyse the contribution of mediating LFV LQ in ν(ν)-N charged current (CC) interactions, including constraints obtained from low energy phenomenology.…”

Leptoquarks and compositeness scales from a contact interaction analysis of deep inelastic e±p scattering at HERA

“…The strongest bounds for leptoquarks coupling to the first lepton and quark generations arise from atomic parity violation experiments and from universality in leptonic π decays. Davidson et al [12] give M LQ /λ limits in the range of 1.6 to 2.2 TeV for scalar leptoquarks and 2.2 to 3.1 TeV for vector leptoquarks, while Leurer [13] arrives at values up to a factor of ∼ 1.5 either lower or higher, and generally gets larger values for scalar than for vector leptoquarks. Despite of these uncertainties, our most stringent limits for vector leptoquarks are within a factor of two close to the ones extracted from the very low energy experiments.…”