Abstract
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.
Highlights
Needed to prevent proton decay without forbidding neutrino masses
The key point is that their masses are 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
Of prime importance is the fact that the new states belong to real representations of the SM gauge symmetry, so that their masses mostly arise from their couplings to the flavon(s) rather than to the Higgs boson, and mass mixings with light fermions are forbidden by the discrete symmetry
Summary
For the sake of argument, we take the instance of a single charge abelian flavour symmetry group U(1)X broken by the vacuum expectation value (v.e.v.) of a single complex scalar (φ), the so-called flavon, and assign (different) X-charges to the quarks and leptons of either chirality in the three families so to yield an acceptable description of the observed mass hierarchies and mixings. Most important in our analysis are the chiral charge differences of quarks and leptons, χifj = X(fLi ) − X(fRj ) ≡ fLi − fRj , where fL = q, l, fRi = u, d, e, and i, j = 1, 2, 3 are family indices. The chiral charge differences forbid the couplings of all fermions but the top quark to the Higgs at the renormalizable level, providing the needed chiral protection for their masses. This defines a Froggat-Nielsen effective Lagrangian [11, 12] with a cutoff Λ way above the electroweak scale after integrating out the flavon and the U(1)X gauge boson, denoted Xμ. We basically follow the steps in the supersymmetric version in [9], to which we refer for more details, the presence of scalars there makes the effective theory, as much as its phenomenology, very different from the one discussed below (in particular, the supersymmetric heavy states are short-lived)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
