Beyond the Standard Model

One of the main goals of the theory group is to study the mechanism of electroweak symmetry breaking and its phenomenological implications. As it is well known the most likely possibility for electroweak breaking is provided by the Higgs mechanism by which a complex (Higgs) boson doublet is introduced: three out of its four components will go after spontaneous symmetry breaking to the longitudinal polarizations of the Z and W gauge bosons which become massive and the fourth one is the real Standard Model Higgs which is expected for detection at the LHC. Among many other virtues the Standard Model Higgs boson can unitarize the longitudinal W and Z elastic and inelastic scattering at all energies provided the Higgs boson mass is somewhat below the TeV scale. However the Standard Model, although it has been confirmed experimentally with great accuracy, is not considered as the fundamental theory of electroweak and strong interactions but better as an effective theory for scales below some cut-off which must not be far from the TeV range. There are a number of reasons for that:

1) The Higgs mass is sensitive to the ultraviolet (UV) physics through the radiative corrections. In particular quantum corrections are quadratically sensitive to the cutoff: this is known as the hierarchy problem. An extra symmetry must then be introduced to solve this problem. In particular supersymmetry (a symmetry between bosons and fermions) is the most motivated candidate and the minimal supersymmetric extension (MSSM) is intensively studied both from the theoretical and from the experimental points of view. In fact supersymmetric searches are (after the Higgs search) among the most motivated searches at LHC concerning physics beyond the Standard Model.

2) The Standard Model suffers from a strong CP problem: its solution by Peccei and Quinn (PQ) involved the introduction of an axion.

3) There is no candidate to Dark Matter in the pure Standard Model. Some candidates are the lightest neutralino in the MSSM and the axion in the PQ model.

4) There is no explanation for the presence of baryons (baryon-to-photon ratio) within the Standard Model. However it has been proved (and partly worked out by our group) that under certain circumstances that will be tested at the LHC the baryon asymmetry can be generated by the MSSM or extensions thereof. This mechanism is called baryogenesis. Another mechanism, leptogenesis, is associated by the violation of lepton number in the neutrino sector.

5) Gravity is not unified (nor quantized) with the rest of interactions. This leads to string theories and in particular to theories with extra dimensions. If the extra dimensions are of sub-millimeter size, only gravity can propagate in its bulk. The presence of very light Kaluza-Klein excitations of the graviton leads to modifications of the Newton-law which are being tested at table-top experiments, or to missing energy (which corresponds to an infinite number of gravitons produced) in collider experiments. If the extra dimensions are much smaller, smaller than 1/TeV, then Kaluza-Klein excitations of the known gauge bosons can be copiously produced in colliders, and in particular at the LHC, in Drell-Yan processes which can be detected as resonances with the same quantum numbers as the Standard Model gauge bosons but with much higher value of its mass.

6) Finally extra dimensions can be warped and provide an alternative solutions to the hierarchy problem. In these models the weak hierarchy is induced simply because the Planck scale is red-shifted to the weak scale by the warp factor. This model was originally introduced by Lisa Randall and Raman Sundrum and it provides a very rich phenomenology as well as the solution to some of well-established typical problems in particle physics, as e.g. the flavor problem. This theory is complemented with the AdS/CFT conjecture of Maldacena in the context of string theories and can provide calculability to non-perturbative theories, as QCD and technicolor, giving new solutions to some longstanding problems.