Service Communautaire d'Information sur la Recherche et le Développement - CORDIS

Final Activity Report Summary - HBPBSM-SPR (Higgs boson physics in the Standard Model and Beyond the Standard Model)

The Standard Model (SM) of strong and electroweak (EW) interactions is the present paradigm of the understanding of particle interactions. Its validity has been tested in the experiments at the per mille level. However, an important particle predicted by the SM still lacks experimental confirmation: the Higgs boson, which is responsible for giving mass to all the other particles. There are reasons to believe that the SM might not be the final theory of particle interactions, and there are several proposals for physics beyond the SM. Nowadays the favourite candidate for physics beyond the SM is Supersymmetry (SUSY).

Two new experiments will search for the Higgs boson, and will explore the possibility of new physics: the Large Hadron Collider (LHC, CERN, Geneva, a proton-proton [pp] collider), and the International Linear Collider (ILC, an electron-positron [e^-e^+] collider). In our work we have performed some phenomenological studies needed by the LHC and the ILC.

Flavour Physics:
Quarks are fundamental particles that come in different kinds (or 'flavours'). The interactions in which a quark changes its flavour but not its electric charge (Flavour Changing Neutral Currents-FCNC) are extremely suppressed in the SM, and the effects of new physics are enhanced. We have computed several observables related to FCNC interactions, in the context of the so-called Non-Minimal Flavour-Violating SUSY (NMFV) models.

We computed the effects of NMFV on the Higgs boson mass and the precision EW observables, and compared the results with current experimental data. We concluded that the presence of NMFV induces a significant shift on the prediction of the EW observables, and that part of the parameter space of NMFV SUSY models is excluded by today's experimental precision of these observables.

We studied the NMFV quantum effects to the production of a Higgs boson, a bottom-quark and a strange-quark at the ILC (e^+e^- ->Hbs). The results show that these production rates are several orders of magnitude larger than in the SM. Unfortunately, the production rates are too small to be detected at the ILC.

Some FCNC processes have already been observed. The most important one being the decay of a bottom-quark into a strange-quark and a photon (b->s gamma). The experimental value agrees with the SM prediction. We studied how this constraint restricts the predictions for the decay width of a Higgs boson into a bottom-quark and a strange-quark (H->bs). We found that, although (b->s gamma) restricts the decay rate (H->bs), the presence of different correlations in each observable still leaves room for a large decay rate (H->bs).

We have computed the associate production of a top-quark and a charm-quark at the LHC (pp->tc). The conclusion is that the prediction of NMFV SUSY models is several orders of magnitude larger that the SM one, and could be detected at the LHC. The detection of this process would be evidence of SUSY.

Distinguishing new physics at the LHC:
We addressed a theoretical and experimental study for the distinction of different kinds of new physics at the LHC, by studying the decay of charged Higgs bosons into tau-leptons and into heavy quarks. We found that the relation between these two decay channels is very sensitive to new physics. We have proven that the LHC will be able to measure this quantity, and tell whether this Higgs boson is supersymmetric or not.

Precision observables:
We studied two precision observables: the W-boson mass and the effective weak mixing angle, and their correlations in the split SUSY model. We compared this prediction with the SM, the SUSY prediction, and the present and future experimental precision. We found that present data does not allow to put restrictions on the split SUSY parameter space. However, the shifts induced by split SUSY are larger than the expected experimental accuracy of the ILC. The future ILC would be able to test the split SUSY model by measuring these precision observables.

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