MSSM Case Research Articles

Δr in the Two-Higgs-Doublet Model at full one loop level—and beyond

After the recent discovery of a Higgs-like boson particle at the CERN LHC-collider, it becomes more necessary than ever to prepare ourselves for identifying its standard or non-standard nature. The Electroweak parameter Delta r relating the values of the gauge boson masses [MW,MZ] and the Fermi constant [G_F] is the traditional observable encoding high precision information of the electroweak physics at the quantum level. In this work we present a complete quantitative study of Delta r in the framework of the general (unconstrained) Two-Higgs-Doublet Model (2HDM). First of all we report on a systematic analysis of Delta r at the full one loop level in the general 2HDM, which to our knowledge was missing in the literature. Thereby we extract a theoretical prediction for the mass of the W-boson in this model, taking MZ, \alpha_{em} and G_F as experimental inputs. We find typical corrections leading to mass shifts $\delta MW \sim 20-40 MeV$ which help to improve the agreement with the experimentally measured value, in a degree no less significant than in the MSSM case. In the second part of our study we extend our calculation beyond the mere one-loop order. We devise an effective Lagrangian approach that captures the dominant higher order quantum effects on delta rho (viz. that part of Delta r describing the breaking of the approximate SU(2) custodial symmetry) in the limit of large Higgs boson self-interactions. This limit constitutes a telltale property of the general 2HDM which is unmatched by e.g. the MSSM. Our conclusion is that the Electroweak precision program to be conducted at the LHC, and maybe at a future linear collider, can nicely complement the direct searches. Should these distinctive loop effects be eventually found they would signal a smoking gun hinting at non-standard Higgs physics.

Flavour physics constraints in the BMSSM

We study the implications of the presence of the two leading-order, non-renormalizable operators in the Higgs sector of the MSSM to flavour physics observables. We identify the constraints of flavour physics on the parameters of the BMSSM when we: a) focus on a region of parameters for which electroweak baryogenesis is feasible, b) use a CMSSM-like parametrization, and c) consider the case of a generic NUHM-type model. We find significant differences as compared to the standard MSSM case.

Light Higgs bosons in phenomenological NMSSM

We consider scenarios in the next-to-minimal supersymmetric model (NMSSM) where the CP-odd and charged Higgs bosons are very light. As we demonstrate, these can be obtained as simple deformations of existing phenomenological MSSM benchmarks scenarios with parameters defined at the weak scale. This offers a direct and meaningful comparison to the MSSM case. Applying a wide set of up-to-date constraints from both high-energy collider and flavour physics, the Higgs boson masses and couplings are studied in viable parts of parameter space. The LHC phenomenology of the light Higgs scenario for neutral and charged Higgs boson searches is discussed.

Phenomenology of SUSY SU(5) with type I+III seesaw

We consider a supersymmetric SU(5) model where two neutrino masses are obtained via a mixed type I+III seesaw mechanism induced by the component fields of a single SU(5) adjoint. We have analyzed the phenomenology of the model paying particular attention to flavour violating processes and dark matter relic density, assuming universal boundary conditions. We have found that, for a seesaw scale larger than $10^{12\div 13}$ GeV, BR$(\mu\to e \gamma)$ is in the reach of the MEG experiment in sizable regions of the parameter space. On the other side, current bounds on it force BR$(\tau \to \mu \gamma)$ to be well below the reach of forthcoming experiments, rendering thus the model disprovable if a positive signal is found. The same bounds still allow for a sizable positive contribution to $\epsilon_K$, while the CP violation in the $B_s$ mixing turns out to be too small to account for the di-muon anomaly reported by the D0 collaboration. Finally, the regions where the neutralino relic density is within the WMAP bounds can be strongly modified with respect to the constrained MSSM case. In particular, a peculiar coannihilation region, bounded from above, can be realized, which allows us to put an upper bound on the dark matter mass for certain set-ups of the parameters.

A PECULIAR PROPERTY OF SUSY AMPLITUDES AT HIGH ENERGY

We observe that the electroweak one-loop correction to the quark+gluon to quark+Higgs amplitude at high energy involves both single and quadratic logarithms of the energy in the SM case but only quadratic logarithms in the MSSM case. We explore the origin of this special SUSY cancellation, both in a diagrammatic way and through the splitting+parameter-renormalization procedure. We show that it is not an accident but a remarkable and general SUSY property of the renormalized Higgs–fermion–fermion and Higgsino–sfermion–fermion vertices which directly reflects in such processes, for example in bg→tH-, bg→bH0, bg→bh0, bg→bA0, and through equivalence in [Formula: see text], bg→bZ long , as well as in [Formula: see text]. This simplification of the high energy behavior (which only leaves quadratic logarithms involving pure gauge couplings without any free parameter) allows to write simple relations among these various processes which could constitute genuine tests of the assumed SUSY model.

Fine tuning as an indication of physics beyond the MSSM

We investigate the amount of fine tuning of the electroweak scale in the presence of new physics beyond the MSSM, parametrized by higher dimensional operators. We show that these significantly reduce the MSSM fine tuning to Δ < 10 for a Higgs mass between the LEPII bound and 130 GeV, and a corresponding scale M * of new physics as high as 30 to 65 times the higgsino mass. If the fine-tuning criterion is indeed of physical relevance, the findings indicate the presence of new physics in the form of new states of mass of O ( M * ) that generated the effective operators in the first instance. At small tan β these states can be a gauge singlet or a SU ( 2 ) triplet. We derive analytical results for the EW scale fine-tuning for the MSSM with higher dimensional operators, including the quantum corrections which are also applicable to the pure MSSM case in the limit the coefficients of the higher dimension operators vanish. A general expression for the fine-tuning is also obtained for an arbitrary two-Higgs doublet potential.