Unification mechanism for gauge and spacetime symmetries

A group theoretical mechanism for unification of local gauge and spacetime symmetries is introduced. No-go theorems prohibiting such unification are circumvented by slightly relaxing the usual requirement on the gauge group: only the so called Levi factor of the gauge group needs to be compact semisimple, not the entire gauge group. This allows a non-conventional supersymmetry-like extension of the gauge group, glueing together the gauge and spacetime symmetries, but not needing any new exotic gauge particles. It is shown that this new relaxed requirement on the gauge group is nothing but the minimal condition for energy positivity. The mechanism is demonstrated to be mathematically possible and physically plausible on a U(1) based gauge theory setting. The unified group, being an extension of the group of spacetime symmetries, is shown to be different than that of the conventional supersymmetry group, thus overcoming the McGlinn and Coleman-Mandula no-go theorems in a non-supersymmetric way.Comment: 20 pages, 4 figure

Stable Superstring Relics and Ultrahigh Energy Cosmic Rays

One of the most intriguing experimental results of recent years is the observation of Ultrahigh Energy Cosmic Rays (UHECRs) above the GZK cutoff. Plausible candidates for the UHECR primaries are the decay products of a meta--stable matter state with mass of order O(10^{12-15 GeV}), which simultaneously is a good cold dark matter candidate. We study possible meta-stable matter states that arise from Wilson line breaking of GUT symmetries in semi-realistic heterotic string models. In the models that we study the exotic matter states can be classified according to patterns of SO(10) symmetry breaking. We show that cryptons, which are states that carry fractional electric charge $\pm1/2$, and are confined by a hidden gauge group cannot produce viable dark matter. This is due to the fact that, in addition to the lightest neutral bound state, cryptons give rise to meta-stable charged bound states. However, these states may still account for the UHECR events. We argue that the uniton, which is an exotic Standard Model quark but carries ``fractional'' U(1)_{Z'} charge, as well as the singleton, which is a Standard Model singlet with ``fractional'' U(1)_{Z'} charge do provide viable dark matter candidates and can at the same time explain the observed UHECR events.Comment: 24 pages. 5 figure

Broken symmetry and Yang-Mills theory

From its inception in statistical physics to its role in the construction and in the development of the asymmetric Yang-Mills phase in quantum field theory, the notion of spontaneous broken symmetry permeates contemporary physics. This is reviewed with particular emphasis on the conceptual issues.Comment: Latex 30 pages, 9 figures. Typo corrected. Contribution to "Fifty years of Yang Mills theory", editor G. 't Hooft, to be published by World Scientifi

E6 Models from F-theory

F-theory is a non-perturbative formulation of type IIB superstring theory which allows for the decoupling of gravity and for the formulation of GUT theories based on the gauge group E6. In this paper we explore F-theory models in which the low energy supersymmetric theory contains the particle content of three 27 dimensional representations of the underlying E6 gauge group, plus two extra right-handed neutrinos predicted from F and D flatness. The resulting TeV scale effective theory resembles either the E6SSM or the NMSSM+, depending on whether an additional Abelian gauge group does or does not survive. However there are novel features compared to both these models as follows: (i) If the additional Abelian gauge group is unbroken then it can have a weaker gauge coupling than in the E6SSM; (ii) If the additional Abelian gauge group is broken then non-perturbative effects can violate the scale invariance of the NMSSM+ leading to a generalised model; (iii) Unification is achieved not at the field theory level but at the F-theory level since the gauge couplings are split by flux effects, negating the need for any additional doublet states which are usually required; (iv) Proton decay is suppressed by the geometric coupling suppression of a singlet state, a mechanism peculiar to F-theory, which effectively suppresses the coupling of the exotic charge -1/3 colour triplet state D to quarks and leptons; (v) The D bar couples to left-handed leptoquarks, providing characteristic and striking signatures at the LHC.Comment: 31 pages, 7 figure

Advances in Old-Fashioned Heterotic String Model Building

I review findings of various research groups regarding perturbative heterotic string model building in the last 12 months. Attention is given to recent studies of extra U(1)'s and local discrete symmetries (LDS's) in generic string models. Issues covered include the role of U(1)'s and LDS's in limiting proton decay, developments in classification of models containing anomalous U(1), and possible complications resulting from kinetic mixing between observable and hidden sector U(1)'s. Additionally, recent string-derived and string-inspired models are briefly reviewed. Talk Presented at SUSY '97.Comment: Talk presented at SUSY'97. Latex w/ espcrc2.sty, 10 pages, reference corrections and update

Self-Interacting Dark Matter from the Hidden Heterotic-String Sector

It has been suggested recently that self-interacting dark matter fits better the observational characteristics of galaxy dynamics. We propose that the self-interacting dark matter is composed from the glueballs of the hidden sector non-Abelian gauge group, while the hidden matter states exist in vector-like representation and decouple from the light spectrum. It is shown that these glueballs are semi-stable with the life-time larger than the present age of the Universe, if their mass is 1 GeV or less. The constraint on their abundance today suggests that the energy was stored in the hidden sector soon after inflation. This imposes an upper limit on the reheating temperature. We further study the naturalness of this scenario in the context of the free-fermionic string models and point out a class of such models where the self-interacting dark matter from the hidden sector is indeed plausible.Comment: 16 pages. standard LaTe

A Brief Course in Spontaneous Symmetry Breaking II. Modern Times: The BEH Mechanism

The theory of symmetry breaking in presence of gauge fields is presented, following the historical track. Particular emphasis is placed upon the underlying concepts.Comment: LaTeX file, 25 pages 9 figures. Presented at the 2001 Corfu Summer Institute on Elementary Particle Physic