A rationale for long-lived quarks and leptons at the LHC: low energy flavour theory

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.

Exotic particles below the TeV from low scale flavour theories

“…Finally, let us comment on the non-supersymmetric counterpart of these flavour theories [60] with a cutoff lower bounded by neutrino masses and FCNC/CPV restrictions as above. In the simplest case, one needs only one Higgs doublet and one flavon field and, assuming that the Higgs mass can be fixed, the analysis is quite similar to the supersymmetric version, but for the absence of the µ-term and the corresponding higgsino chirality.…”

“…However, gauging these symmetries usually introduce FCNC because of the lighter flavour gauge bosons associated to the lower scales along the same lines also discussed above, and the lowest scale would still be quite high. 11 Finally, let us comment on the non-supersymmetric counterpart of these flavour theories [40] with a cutoff lower bounded by neutrino masses and FCNC/CPV restrictions as above. In the simplest case, one needs only one Higgs doublet and one flavon field and, assuming that the Higgs mass can be fixed, the analysis is quite similar to the supersymmetric version, but for the absence of the µ-term and the corresponding higgsino chirality.…”

Burgeoning the Higgs mass to 125 GeV through messenger-matter interactions in GMSB models

“…Flavor symmetries of this kind can successfully explain observed fermion mass matrices within present experimental uncertainties. Interestingly the anomalous nature of the symmetry can motivate compensatory exotic particles that can be probed at present and future collider experiments [23][24][25], thus providing an handle to probe the existence of these flavor structures. Together with R-parity assignments this will be enough to determine the interaction terms uniquely for most of the scenarios that will be explored in this paper.…”

Can new colored particles illuminate the Higgs?

“…Moverover, these new particles will not mix with the SM fermions, but will couple to the Higgs and the gauge sector respecting the SM symmetry group. One can envisage a new quantum number in connection to an unbroken or nearly unbroken symmetry, exclusive to the new sector, in order to forbid this mixing [52].…”