SUSY GUTs with Yukawa unification: a go/no-go study using FCNC processes

TL;DR

This study evaluates the feasibility of exact Yukawa unification in SUSY GUTs using FCNC processes, finding significant challenges unless squark masses are very high, but small deviations from unification can alleviate tensions.

Contribution

It provides a comprehensive global fit analysis showing the constraints on Yukawa unification from FCNC processes and explores the impact of relaxing unification conditions.

Findings

Exact Yukawa unification is difficult under current FCNC constraints.

Relaxing to b-tau unification reduces tension and fits better.

The lightest SUSY particles are predicted to be within reach of LHC searches.

Abstract

We address the viability of exact Yukawa unification in the context of general SUSY GUTs with universal soft-breaking sfermion and gaugino mass terms at the GUT scale. We find that this possibility is challenged, unless the squark spectrum is pushed well above the limits allowed by naturalness. This conclusion is assessed through a global fit using electroweak observables and quark flavour-changing neutral current (FCNC) processes. The problem is mostly the impossibility of accommodating simultaneously the bottom mass and the BR(B --> Xs gamma), after the stringent CDF upper bound on the decay Bs --> mu^+ mu^- is taken into account, and under the basic assumption that the b --> s gamma amplitude have like sign with respect to the Standard Model one, as indicated by the B --> Xs l^+ l^- data. With the same strategy, we also consider the possibility of relaxing Yukawa unification to b - tau Yukawa unification. We find that with small departures from the condition tan beta ~= 50, holding when Yukawa unification is exact, the mentioned tension is substantially relieved. We emphasize that in the region where fits are successful the lightest part of the SUSY spectrum is basically fixed by the requirements of b - tau unification and the applied FCNC constraints. As such, it is easily falsifiable once the LHC turns on.