Flavour physics from an approximate U(2)3 symmetry

Abstract

The quark sector of the Standard Model exhibits an approximate U (2) 3 flavour symmetry. This symmetry, broken in specific directions dictated by minimality, can explain the success of the Cabibbo-Kobayashi-Maskawa picture of flavour mixing and CP violation, confirmed by the data so far, while allowing for observable deviations from it, as expected in most models of ElectroWeak Symmetry Breaking. Building on previous work in the specific context of supersymmetry, we analyze the expected effects and we quantify the current bounds in a general Effective Field Theory framework. As a further relevant example we then show how the U (2) 3 symmetry and its breaking can be implemented in a generic composite Higgs model and we make a first analysis of its peculiar consequences. We also discuss how some partial extension of U (2) 3 to the lepton sector can arise, both in general and in composite Higgs models. An optimistic though conceivable interpretation of the considerations developed in this paper gives reasons to think that new physics searches in the flavour sector may be about to explore an interesting realm of phenomena.

Strong signatures of right-handed compositeness

“…From this it follows that the left-handed partners are often excluded up to 2 TeV and always below 1.5 TeV. This can only be avoided in extensions of MFV where the third generations can be split [19,20]. We emphasize that this is an extremely strong bound that pushes the model into fine tuning territory.…”

“…Here the compositeness of the up quark cannot be small, it being determined by the one of the top. Generalisations allowing to split the third generation can also be considered [19,20].…”

“…We will be interested in the situation where the righthanded up and down quarks are significantly composite. Strictly in MFV models sin φ Ru = sin φ Rt > λ t /g ρ but this can be relaxed in more general constructions based on SU(2) flavor symmetries [19,20]. The SM right-handed quarks can couple to gluon resonances with a trilinear coupling qqρ as large as g ρ sin 2 φ Rq .…”

“…The experimental searches of dijets and tt by CMS and ATLAS imply important bounds on the parameter space of our scenario that we derive in this section. Constraints on spin one resonances from flavor physics are not necessarily negligible, even if MFV is realized, as certain operators (in particular (q L y u y † u q L ) 2 ) are generated at tree level [19,20]. Nevertheless, these bounds are more model dependent (for example they could be avoided in extensions of MFV) and we will not include them here (see however [21]).…”

“…from the first two [19,20]. Those scenarios are attractive phenomenologically as the light generations can be mostly elementary, avoiding compositeness bounds but with the same virtues as MFV for what concerns flavor.…”