Electroweak-flavour and quark-lepton unification: a family non-universal path

Abstract

We present a family-non-universal extension of the Standard Model where the the first two families feature both quark-lepton and electroweak-flavour unification, via the $SU(4)\times Sp(4{)}_L\times Sp(4{)}_R$ gauge group, whereas quark-lepton unification for the third family is realised à la Pati-Salam. Via staggered symmetry breaking steps, this construction offers a natural explanation for the observed hierarchical pattern of fermion masses and mixings, while providing a natural suppression for flavour-changing processes involving the first two generations. The last-but-one step in the symmetry-breaking chain is a non-universal 4321 model, characterised by a vector leptoquark naturally coupled mainly to the third generation. The stability of the Higgs sector points to a 4321$\to$SM symmetry-breaking scale around the TeV, with interesting phenomenological consequences in $B$ physics and collider processes that differ from those of other known 4321 completions.

Comments: 34 pages. Minor modifications of the phenomenological discussion in view of new results on RK(*), which reinforce the consistency of the model