Junhai Kang

Theory and phenomenology of exotic isosinglet quarks and squarks

Extensions of the minimal supersymmetric standard model often predict the existence of new fermions and their scalar superpartners which are vectorlike with respect to the standard model gauge group but may be chiral under additional gauge factors. In this paper we explore the production and decay of an important example, i.e., a heavy isosinglet charge -1/3 quark and its scalar partner, using the charge assignments of a 27-plet of E{sub 6} for illustration. We emphasize that, depending on the symmetries of the low-energy theory, such exotic particles may decay by the mixing of the fermion with the d, s, or b quarks; may decay by leptoquark or diquark couplings (which may nevertheless preserve a form of R parity); or may be stable with respect to renormalizable couplings but decay by higher-dimension operators on cosmological times scales. We discuss the latter two possibilities in detail for various assumptions concerning the relative masses of the exotic fermions, scalars, and the lightest neutralino, and emphasize the necessity of considering the collider signatures in conjunction with the normal minimal supersymmetric standard model processes. Existing and projected constraints from colliders, indirect experiments, proton decay, and big bang nucleosynthesis are considered.

Electroweak baryogenesis, CDM and anomaly-free supersymmetric U(1)′ models

We construct an anomaly-free supersymmetric U(1)′ model with a secluded U(1)′-breaking sector (sMSSM), with E6 embedding. Then we systematically study electroweak baryogenesis (EWBG) driven by spontaneous CP-violation (SCPV), using the sMSSM as an example. Unlike the Minimal Supersymmetric Standard Model (MSSM), CP violation in the Higgs sector can be both spontaneous and explicit in the sMSSM, even at tree level. These new CP sources do not introduce significant new contributions to electric dipole moments of the electron and neutron. Novel features are found in the EWBG driven by SCPV (significantly different from the ones in the MSSM). First, due to the space-dependence of the relevant CP phases, the CP-violating stop and chargino currents do not require a variation of tan β in the bubble wall to have a non-trivial structure at the lowest order of Higgs insertion. Second, there exists a new type of CP-violating current at the leading order. Because of this, in addition to leptons and charginos, top quarks and top squarks can also play a significant role in the EWBG. Numerical results show that the baryon asymmetry is large enough to explain the cosmological observation today. We illustrate that EWBG and neutralino cold dark matter can be accommodated in the same framework, i.e., there exists parameter space where a strong enough first order EWPT, large CP phase variations across the bubble wall, a reasonable baryon asymmetry as well as an acceptable neutralino dark matter relic density can be achieved simultaneously.

Neutrino Masses in Supersymmetric SU(3)_C x SU(2)_L x U(1)_Y x U(1)' Models

We consider various possibilities for generating neutrino masses in supersymmetric models with an additional U(1) ′ gauge symmetry. One class of models involves two extra U(1) ′ ×U(1) ′′ gauge symmetries, with U(1) ′′ breaking at an intermediate scale and yielding small Dirac masses through high-dimensional operators. The right-handed neutrinos N c i can naturally decouple from the low energy U(1) ′ , avoiding cosmological constraints. A variant version can generate large Majorana masses for N c i and an ordinary see-saw. We secondly consider models with a pair of heavy triplets which couple to left-handed neutrinos. After integrating out the heavy triplets, a small neutrino Majorana mass matrix can be generated by the induced non-renormalizable terms. We also study models involving the double-see-saw mechanism, in which heavy Majorana masses for N c i are associated with the TeV-scale of U(1) ′ breaking. We give the conditions to avoid runaway directions in such models and discuss simple patterns for neutrino masses.

Neutrino masses in supersymmetric SU(3)(C) x SU(2)(L) x U(1)(Y) x U(1)-prime models

We consider various possibilities for generating neutrino masses in supersymmetric models with an additional U(1) ′ gauge symmetry. One class of models involves two extra U(1) ′ ×U(1) ′′ gauge symmetries, with U(1) ′′ breaking at an intermediate scale and yielding small Dirac masses through high-dimensional operators. The right-handed neutrinos N c i can naturally decouple from the low energy U(1) ′ , avoiding cosmological constraints. A variant version can generate large Majorana masses for N c i and an ordinary see-saw. We secondly consider models with a pair of heavy triplets which couple to left-handed neutrinos. After integrating out the heavy triplets, a small neutrino Majorana mass matrix can be generated by the induced non-renormalizable terms. We also study models involving the double-see-saw mechanism, in which heavy Majorana masses for N c i are associated with the TeV-scale of U(1) ′ breaking. We give the conditions to avoid runaway directions in such models and discuss simple patterns for neutrino masses.

Neutrino masses in supersymmetricSU(3)C×SU(2)L×U(1)Y×U(1)′models

We consider various possibilities for generating neutrino masses in supersymmetric models with an additional U(1) ′ gauge symmetry. One class of models involves two extra U(1) ′ ×U(1) ′′ gauge symmetries, with U(1) ′′ breaking at an intermediate scale and yielding small Dirac masses through high-dimensional operators. The right-handed neutrinos N c i can naturally decouple from the low energy U(1) ′ , avoiding cosmological constraints. A variant version can generate large Majorana masses for N c i and an ordinary see-saw. We secondly consider models with a pair of heavy triplets which couple to left-handed neutrinos. After integrating out the heavy triplets, a small neutrino Majorana mass matrix can be generated by the induced non-renormalizable terms. We also study models involving the double-see-saw mechanism, in which heavy Majorana masses for N c i are associated with the TeV-scale of U(1) ′ breaking. We give the conditions to avoid runaway directions in such models and discuss simple patterns for neutrino masses.