H. Lew

Seesaw Neutrino Masses Induced by a Triplet of Leptons

Neutrinos can acquire mass through the see-saw mechanism by extending the lepton sector of the Standard Model. The most generalSU(2)L assignments allowed for the exotic leptons are the singlet and triplet representations. A model is constructed involving a triplet of exotic leptons. In this model lepton number is broken. Therefore lepton number violating processes, in particular those involving the lepton triplet, could provide distinctive experimental signatures.

A model with fundamental improper spacetime symmetries

Abstract It is shown how the standard model can be extended so that both the lagrangian and the vacuum are invariant under appropriately defined space- and time-inversion transformations. This means that parity and time-reversal may actually be unbroken symmetries of nature.

Unbroken versus broken mirror world: a tale of two vacua

If the lagrangian of nature respects parity invariance then there are two distinct possibilities: either parity is unbroken by the vacuum or it is spontaneously broken. We examine the two simplest phenomenologically consistent gauge models which have unbroken and spontaneously broken parity symmetries, respectively. These two models have a lagrangian of the same form, but a different parameter range is chosen in the Higgs potential. They both predict the existence of dark matter and can explain the MACHO events. However, the models predict quite different neutrino physics. Although both have light mirror (effectively sterile) neutrinos, the ordinary-mirror neutrino mixing angles are unobservably tiny in the broken parity case. The minimal broken parity model therefore cannot simultaneously explain the solar, atmospheric and LSND data. By contrast, the unbroken parity version can explain all of the neutrino anomalies. Furthermore, we argue that the unbroken case provides the most natural explanation of the neutrino physics anomalies (irrespective of whether evidence from the LSND experiment is included) because of its characteristic maximal mixing prediction.

CHARGE QUANTIZATION IN THE STANDARD MODEL AND SOME OF ITS EXTENSIONS

We review recent advances in the theoretical understanding of electric charge quantization in the Standard Model and some of its extensions. We discuss the roles played by classical constraints, gauge and mixed gauge-gravitational anomaly-cancellation and the demand of vector-like electromagnetic interactions. An attempt is made to clearly explain and contrast the points of view of various authors.

Model for a light Z-prime boson

A model of a light [ital Z][prime] boson is constructed and phenomenological bounds are derived. This [ital Z][prime] boson arises from a very simple extension to the standard model, and it is constrained to be light because the vacuum expectation values which generate its mass also break the electroweak gauge group. It is difficult to detect experimentally because it couples exclusively or primarily (depending on symmetry-breaking details) to second and third generation leptons. However, if the [ital Z][prime] boson is sufficiently light, then there exists the possibility of the two-body decay [tau][r arrow][mu][ital Z][prime] occurring. This will provide a striking signature to test the model.

Comments on grand unification and an alternative approach to unification of quarks and leptons

SummarySome of the original motivations behind grand unified theories are re-examined in the light of our present understanding of the standard model. We argue that there is no experimental evidence which uniquely implies grand unification. We then give an example of a non-GUT model which unifies quark and leptons.

Horizontal SU(2) symmetry revisited

Abstract One possible approach to understanding the pattern of masses and mixing angles in the quark sector is the concept of horizontal symmetry. We consider the SU(2) horizontal symmetry candidate, and examine the possibility of assigning the three generations of fermions to the doublet and singlet representations of SU(2)H. This assignment allows a natural explanation of the near masslessness of the first generation, and the smallness of the Kobayashi-Maskawa matrix elements connecting the third and first generation, and the second and third generation. We suggest that these masses and mixings may arise radiatively, and an example is given to illustrate how this may come about. In this example the inequality mu

Anomally cancellation in chiral colour models with a minimal Higgs sector

Abstract It has been proposed that the strong interactions arise from the chiral colour group SU(3)L⊗SU(3)R. In such models exotic fermions are required in order to cancel the anomalies. We propose a very simple extension of the fermion sector which involves no exotic colour quantum numbers, but instead involves an exotic SU(2)L⊗U(1)Y generation. This exotic generation preserves the lepton-quark symmetry and requires no additional exotic Higgs bosons to give mass to the exotic fermions.

ARE QUARKS AND LEPTONS RELATED BY C, P AND T SYMMETRIES?

It is possible that the similarities between the quarks and leptons arise from the spontaneous breaking of an exact quark-lepton Z2 discrete symmetry. We study the connection between this type of discrete symmetry and the discrete symmetries of charge conjugation, parity and time reversal: C, P and T.

A Theory of Quarks and Leptons

The gauge theory with gauge group [SU(3)]2 ⊗ [SU(2)]2 ⊗ [U(1)Y′]3 supplemented by quark-lepton, left-right, and generation discrete symmetries represents a new approach to the understanding of the particle content of the standard model. All of the fermion fields of the model are interconnected by the discrete symmetries. The classical symmetries and the symmetry breaking content of the model explain the electric charges of all of the observed particles. The theory also predicts the existence of new heavy mesons. Some implications of this theory are discussed.