A Rationale for Long-lived Quarks and Leptons at the LHC: Low Energy Flavour Theory

TL;DR

This paper proposes that gauged flavour symmetries naturally predict long-lived, heavy quarks and leptons within the LHC energy range, which are protected by symmetries and could be detected as quasi-stable particles.

Contribution

It introduces a model where flavour symmetries forbid mixing with light fermions, leading to long-lived heavy fermions accessible at the LHC, distinct from previous models.

Findings

Heavy fermions could have masses within LHC reach.

Long-lived particles are consistent with current experimental constraints.

LHC searches are already probing these long-lived states.

Abstract

In the framework of gauged flavour symmetries, new fermions in parity symmetric representations of the standard model are generically needed for the compensation of mixed anomalies. The key point is that their masses are also protected by flavour symmetries and some of them are expected to lie way below the flavour symmetry breaking scale(s), which has to occur many orders of magnitude above the electroweak scale to be compatible with the available data from flavour changing neutral currents and CP violation experiments. We argue that, actually, some of these fermions would plausibly get masses within the LHC range. If they are taken to be heavy quarks and leptons, in (bi)-fundamental representations of the standard model symmetries, their mixings with the light ones are strongly constrained to be very small by electroweak precision data. The alternative chosen here is to exactly forbid such mixings by breaking of flavour symmetries into an exact discrete symmetry, the so-called proton-hexality, primarily suggested to avoid proton decay. As a consequence of the large value needed for the flavour breaking scale, those heavy particles are long-lived and rather appropriate for the current and future searches at the LHC for quasi-stable hadrons and leptons. In fact, the LHC experiments have already started to look for them.