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 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 $$ \mathcal{O} $$ 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 an 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 $$ \mathcal{O} $$ 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.

Importance of vector leptoquark-scalar box diagrams in Pati-Salam unification with vector-like families

“…Since the mass scales of the meson and LQ are hierarchically different, the RG corrections of the strong coupling constant are included by replacing in Eq. (4.6), [30]…”

“…In recent studies, mainly stimulated by the B meson anomalies, it has been shown that both problems can be resolved by introducing vector-like fermions mixed with the SM chiral fermions [29][30][31]. In this way, imposing relations between PS conserving vector-like masses and those originated from PS breaking, the vector LQ does not couple to a SM lepton and quark simultaneously and thus the leptonic neutral meson decays are absent at the tree-level.…”

Importance of vector leptoquark-scalar box diagrams in Pati-Salam unification with vector-like families

“…Since the LQ is much heavy compared to the meson, the RG corrections of the strong coupling constant are included by replacing in eq. (4.6), [30]…”

“…In recent studies, mainly stimulated by the B meson anomalies, it has been shown that both problems can be resolved by introducing vector-like fermions mixed with the SM chiral fermions [29][30][31]. When specific relations between PS conserving vector-like masses and those originated from PS breaking are imposed, the vector LQ does not couple to a SM lepton and quark simultaneously and thus the serious lepton flavor violating (LFV) processes are absent at the tree-level, thereby allowing the TeV scale LQ.…”

Searches for Baryon Number Violation in Neutrino Experiments: A White Paper

“…In minimal models, the PS breaking scale can be as low as v R 10 3 TeV [250,251], which is coming from experimental limits on the branching ratios for K 0 L → µ ± e ∓ mediated by the new gauge bosons X µ carrying 4/3 charge under B − L. However, in the extended models, where additional fermionic degrees of freedom are introduced, the PS symmetry can even break down close to the TeV scale that has the potential to be directly probed at colliders; for a recent study, see, e.g., Ref. [252]. In light of recent anomalies [253][254][255][256] in the beauty-meson decays, low scale PS models have gained a lot of attention.…”