  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.

High Energy Physics - Phenomenology