Steven Abel

EDM constraints in supersymmetric theories

We systematically analyze constraints on supersymmetric theories imposed by the experimental bounds on the electron, neutron, and mercury electric dipole moments. We critically reappraise the known mechanisms to suppress the EDMs and conclude that only the scenarios with approximate CP-symmetry or flavour-off-diagonal CP violation remain attractive after the addition of the mercury EDM constraint.

Kinetic Mixing of the Photon with Hidden U(1)s in String Phenomenology

Embeddings of the standard model in type II string theory typically contain a variety of U(1) gauge factors arising from D-branes in the bulk. In general, there is no reason why only one of these - the one corresponding to weak hypercharge - should be massless. Observations require that standard model particles must be neutral (or have an extremely small charge) under additional massless U(1)s, i.e. the latter have to belong to a so called hidden sector. The exchange of heavy messengers, however, can lead to a kinetic mixing between the hypercharge and the hidden-sector U(1)s, that is testable with near future experiments. This provides a powerful probe of the hidden sectors and, as a consequence, of the string theory realisation itself. In the present paper, we show, using a variety of methods, how the kinetic mixing can be derived from the underlying type II string compactification, involving supersymmetric and nonsupersymmetric configurations of D-branes, both in large volumes and in warped backgrounds with fluxes. We first demonstrate by explicit example that kinetic mixing occurs in a completely supersymmetric set-up where we can use conformal field theory techniques. We then develop a supergravity approach which allows us to examine the phenomenon in more general backgrounds, where we find that kinetic mixing is natural in the context of flux compactifications. We discuss the phenomenological consequences for experiments at the low-energy frontier, searching for signatures of light, sub-electronvolt or even massless hidden-sector U(1) gauge bosons and minicharged particles.

Interactions in intersecting brane models

Abstract We discuss tree level three and four point scattering amplitudes in type II string models with matter fields localized at the intersections of D-brane wrapping cycles. Using conformal field theory techniques we calculate the four fermion amplitudes. These give “contact” interactions that can lead to flavour changing effects. We show how in the field theory limit the amplitudes can be interpreted as the exchange of Kaluza–Klein excitations, string oscillator states and stretched heavy string modes.

Dynamical breaking of U(1)(R) and supersymmetry in a metastable vacuum

Abstract We consider the metastable N = 1 QCD model of Intriligator, Seiberg and Shih (ISS), deformed by adding a baryon term to the superpotential. This simple deformation causes the spontaneous breaking of the approximate R -symmetry of the metastable vacuum. We then gauge the flavour SU ( 5 ) f and identify it with the parent gauge symmetry of the Standard Model (SM). This implements direct mediation of supersymmetry breaking without the need for an additional messenger sector. A reasonable choice of parameters leads to gaugino masses of the right order. Finally, we speculate that the entire “ISS × SM” model should be interpreted as a magnetic dual of an (unknown) asymptotically free theory.

N point amplitudes in intersecting brane models

We derive general and complete expressions for N-point tree-level amplitudes in Type II string models with matter fields localised at D-brane intersections.

SUSY breaking by a metastable ground state: why the early Universe preferred the non-supersymmetric vacuum

Supersymmetry breaking in a metastable vacuum is re-examined in a cosmological context. It is shown that thermal effects generically drive the Universe to the metastable minimum even if it begins in the supersymmetry-preserving one. This is a generic feature of the ISS models of metastable supersymmetry breaking due to the fact that SUSY preserving vacua contain fewer light degrees of freedom than the metastable ground state at the origin. These models of metastable SUSY breaking are thus placed on an equal footing with the more usual dynamical SUSY breaking scenarios.

Brane–antibrane kinetic mixing, millicharged particles and SUSY breaking

It is known that hidden U(1) gauge factors can couple to visible U(1)s through kinetic mixing. This phenomenon is shown to occur in non-supersymmetric string set-ups, between D-branes and D-branes. Kinetic mixing, if present, acts either to give millicharges (of, e.g., hypercharge) to would-be hidden sector fermions, or to generate an enhanced communication of supersymmetry breaking that dominates over the usual gravitational suppression. In either case, the conclusion is that the string scale in non-supersymmetric brane configurations has a generic upper bound of Ms≲108 GeV.

Illuminating the hidden sector of string theory by shining light through a magnetic field

Many models of physics beyond the Standard Model predict minicharged particles to which current and near future low-energy experiments are highly sensitive. Such minicharges arise generically from kinetic mixing in theories containing at least two U(1) gauge factors. Here, we point out that the required multiple U(1) factors, the size of kinetic mixing, and suitable matter representations to allow for a detection in the near future occur naturally in the context of string theory embeddings of the Standard Model. A detection of minicharged particles in a low energy experiment would likely be a signal of an underlying string theory and may provide a means of testing it.

Soft susy breaking, dilaton domination and intermediate scale string models.

We present an analysis of the low-energy implications of an intermediate scale ( ~ 1011 GeV) string theory. We mainly focus on the evolution of the physical parameters under the renormalisation group equations (RGEs) and find several interesting new features that differ from the standard GUT scale or Planck scale scenarios. We give a general discussion of soft supersymmetry breaking terms in type-I theories and then investigate the renormalization group running. In the dilaton domination scenario, we present the sparticle spectra, analysing constraints from charge and colour breaking, fine tuning and radiative electroweak symmetry breaking. We compare with the allowed regions of parameter space when the RGEs start running at the standard GUT or the intermediate scales, and find quite remarkably that the dilaton dominated supersymmetry breaking scenario, which is essentially ruled out from constraints on charge and colour breaking if the fundamental scale is close to the Planck mass, is allowed in a large region of parameter space if the fundamental scale is intermediate.

Phenomenology of Pure General Gauge Mediation

We investigate the phenomenology of general gauge mediation in the MSSM. We apply the strict definition of gauge mediated SUSY-breaking where B? is generated only through gauge interactions, and as a result is very close to zero at the messenger scale. In this setup tan ? is a prediction rather than an input. The input parameters are independent scales for the gaugino masses, the scalar masses and the messenger mass in accord with general gauge mediation. We investigate the spectra, the constraints on the parameter space from direct searches and indirect observables, as well as fine-tuning. The favoured region of parameter space includes and interpolates between non-split and mildly split SUSY, characteristic of ordinary gauge mediation and direct gauge mediation models, respectively.