P. Q. Hung

A Model of electroweak-scale right-handed neutrino mass

Abstract If neutrino masses are realized through the see-saw mechanism, can the right-handed neutrinos be produced and detected at present and future colliders? The answer is negative in the most popular see-saw scenarios for the simple reason that they are too heavy in these models. However, a simple extension of the Standard Model (SM) particle content, including mirror fermions, two SU ( 2 ) L triplet and one singlet Higgs fields, leads to a scenario in which the see-saw mechanism is realized with the Majorana mass M R of the right-handed neutrino being of the order of the electroweak scale or smaller. A custodial SU ( 2 ) symmetry arising from the two triplet Higgs fields ensures that ρ = 1 at tree level even when their vacuum expectation values (VEV) which determine the value of M R , can be as large as the electroweak scale. M R is found to obey the bound M Z 2 ⩽ M R 246 GeV which makes it accessible experimentally (Tevatron, LHC or ILC) since, in our scenario, ν R s can couple directly to the Standard Model (SM) gauge bosons.

Renormalization group fixed point with a fourth generation: Higgs-induced bound states and condensates

Abstract In the Standard Model with four generations, the two-loop renormalization group equations for the Higgs quartic and Yukawa couplings show a quasi fixed point structure which does not appear at the one-loop level. This quasi fixed point behavior indicates a possible restoration of scale symmetry above some physical cut-off scale Λ FP . We conjecture that there exists a true fixed point which is reached at a similar energy scale. If the masses of the fourth family are sufficiently large, this cut-off scale, Λ FP , is situated in the range of a few TeV to the order of 10 2 TeV , above which the Higgs quartic and Yukawa couplings become practically constant. We found that around Λ FP the strong Yukawa couplings make it possible for the fourth generation to form bound states, including composite extra Higgs doublets. In this scenario the fourth generation condensates are obtained without introducing Technicolor or other unknown interactions.

Implication of a Quasi Fixed Point with a Heavy Fourth Generation: The emergence of a TeV-scale physical cutoff

Abstract It has been shown in a recent Letter that the Higgs quartic and Yukawa sectors of the Standard Model (SM) with a heavy fourth generation exhibit at a two-loop level a quasi fixed point structure instead of the one-loop Landau singularity and which could be located in the TeV region, a scale which is denoted by Λ FP in this Letter. This provides the possibility of the existence of a TeV-scale physical cutoff endowed with several implications. In the vicinity of this quasi fixed point bound states and Higgs-like condensates made up of the 4th generation quarks and leptons get formed. It implies the possibility of a dynamical electroweak symmetry breaking generated by 4th generation condensates. The quasi fixed points also hint at a possible restoration of scale symmetry at Λ FP and above and the emergence of a theory which could be deeper than the SM.

COSMO MSW EFFECT FOR MASS VARYING NEUTRINOS

We consider neutrinos with varying masses which arise in scenarios relating neutrino masses to the dark energy density in the universe. We point out that the neutrino mass variation can lead to level crossing and thus a cosmo MSW effect, having dramatic consequences for the flavor ratio of astrophysical neutrinos.

Dynamical Electroweak Symmetry Breaking with a Heavy Fourth Generation

Abstract A heavy fourth generation with a mass of the order of 400 GeV or more could trigger dynamical electroweak symmetry breaking by forming condensates through the exchange of a fundamental Higgs scalar doublet. The dynamics leading to these condensates is studied within the framework of the Schwinger–Dyson equation. This scenario leads to the presence of three (two composite and one fundamental) Higgs doublets, with interesting phenomenological implications. In addition, this dynamical phenomenon occurs in the vicinity of the energy scale where the restoration of scale symmetry might happen.

Almost pure phase mass matrices from six dimensions

Abstract A model of quark masses and mixing angles is constructed within the framework of two large extra compact dimensions. A “democratic” almost pure phase mass matrix arises in a rather interesting way. This type of mass matrix has often been used as a phenomenologically viable ansatz, albeit one which had very little dynamical justification. It turns out that the idea of large extra dimensions provides a fresh look at this interesting phenomenological ansatz as presented in this paper. Some possible interesting connections to the strong CP problem will also be presented.

Long-lived quarks?

Three lines of reasoning suggest that there might exist a nonsequential fourth generation of heavy quarks having very small mixing with light quarks and hence exceptionally long lifetimes. It is proposed to seek out quarks that travel between 100 {mu}m and 1thinspm in hadron colliders; they would have been overlooked in previous searches. {copyright} {ital 1998} {ital The American Physical Society}

Implications of a Higgs discovery at LEP

Abstract If the Higgs boson has a mass below 130 GeV, then the standard model vacuum is unstable; if it has a mass below 90 GeV (i.e. within reach of LEP within the next two years), then the instability will occur at a scale between 800 GeV and 10 TeV. We show that precise determinations of the Higgs and top quark masses as well as more detailed effective potential calculations will enable one to pin down the location of the instability to an accuracy of about 25 percent. It is often said that “the standard model must break down” or “new physics must enter” by that scale. However, by considering a toy model for the new physics, we argue that it is possible that the effects of such new physics would not be detectable until energies as much as an order of magnitude greater than the location of the instability are reached.

Anatomy of the Higgs mass spectrum

Abstract We analyze the implications of a Higgs discovery on possible “new-physics” scenarios, for mH up to ∼ 700 GeV. For this purpose we critically review lower and upper limits on the Higgs mass in the SM and in the MSSM, respectively. Furthermore, we discuss the general features of possible “heavy” (mH ≳ 2mz) Higgs scenarios by means of a simple heavy-fermion condensate model.

Kaluza-Klein structure associated with a fat brane

It is known that the imposition of orbifold boundary conditions on a background scalar field can give rise to a nontrivial vacuum expectation value along extra dimensions, which in turn generates fat branes and associated unconventional Kaluza-Klein (KK) towers of fermions. We study the structure of these KK towers in the limit of one large extra dimension and show that normalizable (bound) states of massless and massive fermions can exist at both orbifold fixed points. A closer look, however, indicates that orbifold boundary conditions act to suppress at least half of the bound KK modes, while periodic boundary conditions tend to drive high-lying modes to a conventional structure. By investigating the scattering of fermions on branes, we analytically compute the masses and wave functions of KK spectra in the presence of these boundary conditions up to the one-loop level. The implication of KK-number nonconservation couplings for the Coulomb potential is also examined.