Lowering the scale of Pati-Salam breaking through seesaw mixing

Abstract

We analyse the experimental limits on the breaking scale of Pati-Salam extensions of the Standard Model. These arise from the experimental limits on rare-meson decay processes mediated at tree-level by the vector leptoquark in the model. This leptoquark ordinarily couples to to both left- and right-handed SM fermions and therefore the meson decays do not experience a helicity suppression. We find that the current limits vary from $\mathcal{O}(80-2500)$ TeV depending on the choice of matrix structure appearing in the relevant three-generational charged-current interactions. We extensively analyse scenarios where additional fermionic degrees of freedom are introduced, transforming as complete Pati-Salam multiplets. These can lower the scales of Pati-Salam breaking through mass-mixing within the charged-lepton and down-quark sectors, leading to a helicity suppression of the meson decay widths which constrain Pati-Salam breaking. We find four multiplets with varying degrees of viability for this purpose: an $SU(2{)}_{L/R}$ bidoublet, a pair of $SU(4)$ decuplets and either a $SU(2{)}_L$ or $SU(2{)}_R$ triplet all of which contain heavy exotic versions of the SM charged leptons. We find that the Pati-Salam limits can be as low as $\mathcal{O}(5-150)$ TeV with the addition of these four multiplets. We also identify an interesting possible connection between the smallness of the neutrino masses and a helicity suppression of the Pati-Salam limits for three of the four multiplets.

arXiv comment92 pages, 12 figures; v2: Matches version to be published by JHEP. Some sections moved from the main text to appendices. Section 5.3 on the fermion decuplet case rewritten to correct an error. Section added commenting on existing literature. Minor changes to Sec. 4.1.3 and the conclusion

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