Heavy and light scalar leptoquarks in proton decay

TL;DR

This paper classifies scalar leptoquarks that can cause proton decay, analyzes bounds on their couplings and masses from current experiments, and explores their potential to explain anomalies in top quark asymmetries and muon magnetic moments.

Contribution

It provides a comprehensive list of scalar leptoquarks relevant to proton decay, evaluates experimental bounds, and identifies a candidate leptoquark for explaining certain particle physics anomalies.

Findings

Derived bounds on leptoquark masses from Super Kamiokande data.

Identified a leptoquark that could explain top quark asymmetry and muon g-2.

Highlighted potential proton stability issues with certain leptoquark couplings.

Abstract

We list scalar leptoquarks that mediate proton decay via renormalizable couplings to the Standard Model fermions. We employ a general basis of baryon number violating operators to parameterize contributions of each leptoquark towards proton decay. This then sets the stage for investigation of bounds on the leptoquark couplings to fermions with respect to the most current Super Kamiokande results on proton stability. We quantify if, and when, it is necessary to have leptoquark masses close to a scale of grand unification in the realistic SU(5) and flipped SU(5) frameworks. The most and the least conservative lower bounds on the leptoquark masses are then presented. We furthermore single out a leptoquark without phenomenologically dangerous tree-level exchanges that might explain discrepancy of the forward-backward asymmetries in $t \bar t$ production observed at Tevatron, if relatively light. The same state could also play significant role in explaining muon anomalous magnetic moment. We identify contributions of this leptoquark to dimension-six operators, mediated through a box diagram, and tree-level dimension-nine operators, that would destabilize proton if sizable leptoquark and diquark couplings were to be simultaneously present.