Marc Sher

Theory and phenomenology of two-Higgs-doublet models

Abstract We discuss theoretical and phenomenological aspects of two-Higgs-doublet extensions of the Standard Model. In general, these extensions have scalar mediated flavour changing neutral currents which are strongly constrained by experiment. Various strategies are discussed to control these flavour changing scalar currents and their phenomenological consequences are analysed. In particular, scenarios with natural flavour conservation are investigated, including the so-called type I and type II models as well as lepton-specific and inert models. Type III models are then discussed, where scalar flavour changing neutral currents are present at tree level, but are suppressed by either a specific ansatz for the Yukawa couplings or by the introduction of family symmetries leading to a natural suppression mechanism. We also consider the phenomenology of charged scalars in these models. Next we turn to the role of symmetries in the scalar sector. We discuss the six symmetry-constrained scalar potentials and their extension into the fermion sector. The vacuum structure of the scalar potential is analysed, including a study of the vacuum stability conditions on the potential and the renormalization-group improvement of these conditions is also presented. The stability of the tree level minimum of the scalar potential in connection with electric charge conservation and its behaviour under CP is analysed. The question of CP violation is addressed in detail, including the cases of explicit CP violation and spontaneous CP violation. We present a detailed study of weak basis invariants which are odd under CP. These invariants allow for the possibility of studying the CP properties of any two-Higgs-doublet model in an arbitrary Higgs basis. A careful study of spontaneous CP violation is presented, including an analysis of the conditions which have to be satisfied in order for a vacuum to violate CP. We present minimal models of CP violation where the vacuum phase is sufficient to generate a complex CKM matrix, which is at present a requirement for any realistic model of spontaneous CP violation.

Electroweak Higgs Potentials and Vacuum Stability

Abstract In electroweak models, radiative corrections to the scalar potential can have significant consequences. In the standard model, they can destabilize the standard model vacuum; the requirement of vacuum stability leads to severe bounds on Higgs and fermion masses. In supersymmetric models, they lead to the generation of the electroweak scale in terms of the unification scale. In this Report, the method of calculating radiative corrections to the scalar potentials is reviewed, with an emphasis on renormalization group improvement of the potential. Finite temperature corrections to the potential, calculation of tunneling rates and the nature of cosmological phase transitions are then discussed, and the results are then applied to the standard model to derive stringent bounds on Higgs and fermion passes. These results are then generalized to models with several Higgs fields. Finally, the scalar potential in supersymmetric models, including dimensional transmutation and no-scale model, is discussed.

Probing vacuum stability bounds at the fermilab collider

Abstract If the top quark mass is above 86 GeV, then a stringent lower bound on the mass of the Higgs boson arises from the requirement of vacuum stability. We calculate the bound as precisely as possible by numerically solving the renormalization group equations to two-loop order. If the lower bound on the top mass is 100 (110, 120) GeV, the lower bound to the Higgs mass is found to be 20 (34, 50) GeV. Thus, if the standard model is correct, then a nondiscovery of the top quark at the Fermilab Collider implies that the Higgs boson cannot be discovered at CUSB, SLC or LEP I.

Charge / Color Breaking Minima and a-Parameter Bounds in Supersymmetric Models

In supersymmetric models, the requirement that the global potential minimum not violate color and/or electric charge invariance implies restrictions on the parameters of the model. We reassess and improve upon constraints on the A parameter that have been obtained previously in the literature. In particular, we demonstrate that no universally applicable bound which is either necessary or sufficient can be given.

Precise vacuum stability bound in the standard model

This is an addendum to the paper of the above title published in Physics Letters B317, 159 (1993). In that paper, I found the lower bound to the Higgs mass as a function of the top quark mass one obtains by requiring that the standard model vacuum be stable. It included all higher order corrections to two-loops, precise definitions of the Higgs and top masses, etc. Unfortunately, the results were given for top quark masses between 120 and 160 GeV. In this addendum, I give the results for masses between 160 and 190 GeV. The main result, using a value of 0.117 for the strong coupling at the Z-mass, is M_Higgs > 132 + 2.2 (M_top-170) in GeV units. This increases (decreases) by 4.5 GeV if the strong coupling decreases (increases) by 0.007, and is accurate to 1 GeV in the top mass. This will be submitted to Phys. Lett. as an addendum.

Lifetime of the electroweak vacuum and sensitivity to Planck scale physics

If the Standard Model (SM) is valid up to extremely high energy scales, then the Higgs potential becomes unstable at approximately 1 0 11 GeV . However, calculations of the lifetime of the SM vacuum have shown that it vastly exceeds the age of the Universe. It was pointed out by two of us (V. B., E. M.) that these calculations are extremely sensitive to effects from Planck scale higher-dimensional operators and, without knowledge of these operators, firm conclusions about the lifetime of the SM vacuum cannot be drawn. The previous paper used analytical approximations to the potential and, except for Higgs contributions, ignored loop corrections to the bounce action. In this work, we do not rely on any analytical approximations and consider all contributions to the bounce action, confirming the earlier result. It is surprising that the Planck scale operators can have such a large effect when the instability is at 1 0 11 GeV . There are two reasons for the size of this effect. In typical tunneling calculations, the value of the field at the center of the critical bubble is much larger than the point of the instability; in the SM case, this turns out to be numerically within an order of magnitude of the Planck scale. In addition, tunneling is an inherently nonperturbative phenomenon and may not be as strongly suppressed by inverse powers of the Planck scale. We include effective Φ 6 and Φ 8 Planck-scale operators and show that they can have an enormous effect on the tunneling rate.

Bounds on {Delta}B = 1 couplings in the supersymmetric standard model

The most general supersymmetric model contains baryon number violating terms of the form {lambda}{sub ijk} {bar D}{sub i}{bar D}{sub j}{bar U}{sub k} in the superpotential. The authors reconsider the bounds on these couplings, assuming that lepton number conservation ensures proton stability. These operators can mediate n-{anti n} oscillations and double nucleon decay. They show that neutron oscillations do not, as previously claimed, constrain the {lambda}{sub dsu} coupling; they do provide a bound on the {lambda}{sub dbu} coupling, which they calculate. They find that the best bound on {lambda}{sub dsu} arises from double nucleon decay into two kaons; the calculation is discussed in detail. There are no published limits on this process; experimenters are urged to examine this nuclear decay mode. Finally, the other couplings can be bounded by the requirement of perturbative unification.

Vacuum stability bounds in the two Higgs doublet model

Abstract In the standard model, the requirements of vacuum stability and the validity of perturbation theory up to the unification scale force the mass of the Higgs boson to be approximately between 130 GeV and 180 GeV. We re-examine these requirements in the (non-supersymmetric) two-Higgs doublet model, in the light of the large top quark mass, and constrain the masses of the Higgs bosons in this model. It is found that the mass of the charged Higgs boson must be lighter than 150 GeV. This bound is below the lower bound in the popular model-II two-Higgs doublet model, and thus we conclude that this model cannot be valid up to the unification scale. The bounds on the neutral Higgs scalars are also discussed.

Implications of the LHC two-photon signal for two-Higgs-doublet models

We study the implications for two-Higgs-doublet models of the recent announcement at the LHC giving a tantalizing hint for a Higgs boson of mass 125 GeV decaying into two photons. We require that the experimental result be within a factor of 2 of the theoretical standard model prediction, and analyze the type I and type II models as well as the lepton-specific and flipped models, subject to this requirement. It is assumed that there is no new physics other than two Higgs doublets. In all of the models, we display the allowed region of parameter space taking the recent LHC announcement at face value, and we analyze the

Heavy Higgs Searches and Constraints on Two Higgs Doublet Models

{W}^{+}{W}^{\ensuremath{-}}