We identify a class of chiral models where the one-loop effective potential for Higgs scalar fields is finite without any requirement of supersymmetry. It corresponds to the case where the Higgs fields are identified with the components of a gauge field along compactified extra dimensions. We present a six dimensional model with gauge group U (3) × U (3) and quarks and leptons accomodated in fundamental and bi-fundamental representations. The model can be embedded in a D-brane configuration of type I string theory and, upon compactification on a T 2 /Z 2 orbifold, it gives rise to the standard model with two Higgs doublets. * On leave of absence from CPHT, Ecole Polytechnique, UMR du CNRS 7644.

We construct the minimal supersymmetric left-right theory and show that at the renormalizable level it requires the existence of an intermediate B −L breaking scale. The subsequent symmetry breaking down to MSSM automatically preserves R-symmetry. Furthermore, unlike in the nonsupersymmetric version of the theory, the see-saw mechanism takes its canonical form. The theory predicts the existence of a triplet of Higgs scalars much lighter than the B − L breaking scale. A. Introduction.There is no doubt that the Minimal Supersymmetric Standard Model (MSSM) has become the most popular extension of the Standard Model (SM). However, one of the most appealing features of the Standard Model is lost in its supersymmetric counterpart: automatic conservation of baryon and lepton numbers. In SUSY, unless some mechanism of suppression is found, baryon number violation, as is well known, is catastrophically fast.It turns out that another popular extension of the Standard Model, the Left-Right (L-R) symmetric theory [1] offers a natural solution to this MSSM problem. The B-L symmetry, which is a part of L-R models, automatically forbids all the baryon and lepton number violating operators [2]. L-R theories are interesting in their own right, for among other appealing features, they offer a simple and natural explanation of the smallness of neutrino mass through the so-called see-saw mechanism [3,4].In view of this, it becomes important to systematically study L-R supersymmetric theories, in order to arrive at a realistic minimal supersymmetric left-right model (MSLRM). Up to now, the only serious attempt in this direction is the work of Kuchimanchi and Mohapatra [5] which showed that in the minimal version of the theory no spontaneous symmetry breaking takes place [6]. Furthermore, when this is cured through the introduction of a parity-odd singlet, the soft SUSY breaking terms inevitably lead to the breaking of electromagnetic charge invariance. This is true at least for a scale of L-R symmetry breaking M R above 10T eV . In this letter we stick to the physically motivated assumption of M R being much larger than the scale of supersymmetry breaking M S taken to be not far from the electroweak scale: M S ≃ M W . We show that this problem disappears if one allows for an intermediate B − L breaking scale. Furthermore, the physically unappealing singlet becomes redundant.The most important result of our study is that at low energies the model reduces to the MSSM with an exact R-parity: its breaking is simply incompatible with phenomenology. A phenomenologically interesting feature of the theory is the possibility of a low-lying B − L scale, M BL > ∼ 1T eV . Furthermore, the see-saw mechanism in this theory takes its canonical form m ν ≃ m 2 D /M BL (where m D is the neutrino Dirac mass term), as opposed to the nonsupersymmetric version of L-R models or SO(10) GUTs. Namely, despite its generic see-saw form, the neutrino mass in ordinary L-R theories depends unfortunately on the unknown parameters of the Higgs potential.Another imp...

We reconsider superheavy dark matter candidates in string and M theory, in view of the possibility that inflation might generate superheavy particles with an abundance close to that required for a near-critical Universe. We argue that cryptons-stable or metastable bound states of matter in the hidden sector-are favored over other possible candidates in string or M theory, such as the Kaluza-Klein states associated with extra dimensions. We exhibit a specific string model that predicts cryptons as hidden-sector bound states weighing ϳ1012 GeV, and discuss their astrophysical observability. ͓S0556-2821͑98͒04824-3͔PACS number͑s͒: 95.35.ϩd, 11.25.Mj, 12.60.Jv There is striking evidence from astronomical observations for the existence of dark matter. The rotational velocities of galaxies, the dynamics of galaxy clusters, and theories of structure formation suggest that most of the matter in the Universe is invisible and largely composed of nonbaryonic particles. Many candidates have been proposed as constituents of this particle dark matter. An upper bound on the self-annihilation cross section, based on unitarity, suggests that particles much heavier than 1 TeV ϳͱM Pl ϫT CMBR that have been in thermal equilibrium would be left with such a large relic abundance that they would overclose the Universe ͓1͔. This rule could be evaded if there was significant entropy generation after they went out of thermal equilibrium. This possibility was raised in connection with one class of superheavy dark matter candidates: cryptons ͓2͔, which are stable or metastable bound states of matter in a hidden sector of string theory. However, there was until recently little reason to expect that the abundance of cryptons or other superheavy relics would be such as to constitute most of the mass density of dark matter.The question of the abundance of superheavy relics has recently been reexamined ͓3-5͔. In particular, a gravitational mechanism was suggested ͓4,5͔ whereby cosmological inflation may generate a desirable abundance of such massive and weakly interacting massive relic particles. Numerical analysis indicates that the process may be largely independent of the details of the models considered for most properties of the dark matter constituent, as well as of the details of the transition between the inflationary phase and the subsequent thermal radiation-dominated phase. In the light of this new proposal, it is interesting to reexamine the possibility that cryptons or other superheavy string relics may constitute an important part of the astrophysical dark matter.We argue below that the combination of metastability and a desirable relic density are more likely for cryptons than for superheavy Kaluza-Klein states associated with the extra dimensions compactified at short distance scales in string or M theory. We also discuss the possible observability of cryptons via their high-energy decay products.We first review some basic facts about superheavy dark matter and the new proposals ͓4,5͔ for relic production during inflation. To ...

Perturbative breaking of supersymmetry in four-dimensional string theories predict in general the existence of new large dimensions at the TeV scale. Such dimensions can be consistent with perturbative unification up to the Planck scale in a class of string models and open the exciting possibility of lowering a part of the massive string spectrum at energies accessible to future accelerators. The main signature is the production of Kaluza-Klein excitations which have a very particular structure, strongly correlated with the supersymmetry breaking mechanism. We present a model independent analysis of the physics of these states in the context of orbifold compactifications of the heterotic superstring. In particular, we compute the limits on the size of large dimensions used to break supersymmetry.Comment: 16 pages, CPTH-A257.079