Tri-hypercharge: a separate gauged weak hypercharge for each fermion family as the origin of flavour

TL;DR

This paper introduces a tri-hypercharge extension of the Standard Model, assigning unique hypercharges to each fermion family, which explains fermion mass hierarchies, neutrino mixing, and predicts new TeV-scale gauge bosons with observable phenomenological effects.

Contribution

It proposes a novel tri-hypercharge gauge symmetry framework that naturally accounts for fermion mass hierarchies and neutrino properties, with testable predictions for new gauge bosons.

Findings

A low-scale Z' boson could be as light as a few TeV.

The model provides a mechanism for fermion mass hierarchies and small CKM mixing.

Neutrino masses and mixing can be explained via a natural seesaw mechanism.

Abstract

We propose a tri-hypercharge (TH) embedding of the Standard Model (SM) in which a separate gauged weak hypercharge is associated with each fermion family. In this way, every quark and lepton multiplet carries unique gauge quantum numbers under the extended gauge group, providing the starting point for a theory of flavour. If the Higgs doublets only carry third family hypercharge, then only third family renormalisable Yukawa couplings are allowed. However, non-renormalisable Yukawa couplings may be induced by the high scale Higgs fields (hyperons) which break the three hypercharges down to the SM hypercharge, providing an explanation for fermion mass hierarchies and the smallness of CKM quark mixing. Following a similar methodology, we study the origin of neutrino masses and mixing in this model. Due to the TH gauge symmetry, the implementation of a seesaw mechanism naturally leads to a low scale seesaw, where the right-handed neutrinos in the model may be as light as the TeV scale. We present simple examples of hyperon fields which can reproduce all quark and lepton (including neutrino) masses and mixing. After a preliminary phenomenological study, we conclude that one of the massive $Z'$ bosons can be as light as a few TeV, with implications for flavour-violating observables, LHC physics and electroweak precision observables.