Emilian Dudas

R -PARITY-VIOLATING SUPERSYMMETRY

Theoretical and phenomenological implications of R-parity violation in supersymmetric theories are discussed in the context of particle physics and cosmology. Fundamental aspects include the relation with continuous and discrete symmetries and the various allowed patterns of R-parity breaking. Recent developments on the generation of neutrino masses and mixings within different scenarios of R-parity violation are discussed. The possible contribution of Rparity-violating Yukawa couplings in processes involving virtual supersymmetric particles and the resulting constraints are reviewed. Finally, direct production of supersymmetric particles and their decays in the presence of R-parity-violating couplings is discussed together with a survey of existing constraints from collider experiments. To be submitted to Physics Reports

Grand unification at intermediate mass scales through extra dimensions

Abstract One of the drawbacks of conventional grand unification scenarios has been that the unification scale is too high to permit direct exploration. In this paper, we show that the unification scale can be significantly lowered (perhaps even to the TeV scale) through the appearance of extra space-time dimensions. Such extra dimensions are a natural consequence of string theories with large-radius compactifications. We show that extra space-time dimensions naturally lead to gauge coupling unification at intermediate mass scales, and moreover may provide a natural mechanism for explaining the fermion mass hierarchy by permitting the fermion masses to evolve with a power-law dependence on the mass scale. We also show that proton-decay constraints may be satisfied in our scenario due to the higher-dimensional cancellation of proton-decay amplitudes to all orders in perturbation theory. Finally, we extend these results by considering theories without supersymmetry; experimental collider signatures; and embeddings into string theory. The latter also enables us to develop several novel methods of explaining the fermion mass hierarchy via D-branes. Our results therefore suggest a new approach towards understanding the physics of grand unification as well as the phenomenology of large-radius string compactifications.

Extra Spacetime Dimensions and Unification

We study the effects of extra spacetime dimensions at intermediate mass scales, as expected in string theories with large-radius compactifications, and focus on the gauge and Yukawa couplings within the Minimal Supersymmetric Standard Model. We find that extra spacetime dimensions naturally lead to the appearance of grand unified theories at scales substantially below the usual GUT scale. Furthermore, we show that extra spacetime dimensions provide a natural mechanism for explaining the fermion mass hierarchy by permitting the Yukawa couplings to receive power-law corrections. We also discuss how proton-decay constraints may be addressed in this scenario, and suggest that proton-decay amplitudes may be exactly cancelled to all orders in perturbation theory as a result of new Kaluza-Klein selection rules corresponding to the extra spacetime dimensions.

On tadpoles and vacuum redefinitions in String Theory

“Issues on tadpoles and vacuum redefinitions in String Theory” M. Nicolosi This Thesis is devoted to the problem of NS-NS tadpoles, bosonic one-point functions going into the vacuum that typically emerge in String Theory after supersymmetry breaking. These theories contain bosonic fields in two sectors, commonly denoted with NS-NS and R-R. While R-R tadpoles typically signal an inconsistency, like the presence of quantum anomalies in the case of a compact internal space, and thus in general must be cancelled, NS-NS tadpoles are associated to redefinitions of the background, as first stressed by Fischler and Susskind in the eighties. In particular, in Type I String Theory NS-NS tadpoles emerge already at the disk level and, from a space-time viewpoint, correspond to configurations of D-branes and orientifold planes with a non-vanishing tension giving rise to a net gravitational attraction that curves the background space-time. Up to now one is able to perform efficient string computations only in a flat Minkowski background, a case that is allowed and protected by supersymmetry. Hence, the (closed) infrared divergences emerging after supersymmetry breaking in string amplitudes, due to the propagation of NS-NS massless states that are absorbed by tadpoles at vanishing momentum, are just the signal that the flat Minkowski background is no more a vacuum of the theory. In this context our proposal is to keep quantizing the string around the Minkowski background, recovering the proper results after suitable tadpole resummations that cancel the infrared divergences. This procedure is still very difficult to carry out in String Theory, because the higher-order tadpole corrections correspond to Riemann surfaces of increasing genus, and efficient calculations can be only carried out up to genus one (one-loop amplitudes). Moreover, in most models that realize supersymmetry breaking, tadpoles arise already at the disk level, and thus, even in a perturbative region of small string coupling, the first tadpole corrections can be large. Hence, it is interesting to search for models with “small” tadpoles. Examples of this kind seem are provided by models with suitable internal fluxes, for which reliable perturbative results can be recovered just considering the first tadpole corrections. Another line that one can pursue is to search for quantities that are protected against the infrared divergences. An example of this kind is provided by the one-loop string corrections to gauge couplings, commonly known as threshold corrections, for supersymmetry breaking models with parallel branes, a case that we have widely discussed in this Thesis. The Thesis is organized in the following way. There is a general Introduction to String Theory, where we summarize the main ideas of the Theory, trying to underline its successes and its open problems. Then in the first Chapter we recall the basic properties of string spectra and discuss some simple examples of toroidal and orbifold compactifications. The second Chapter is devoted to reviewing a number of different mechanisms to break supersymmetry. In the third Chapter we begin to analyze our resummation program in a number of field theory toy models, trying to recover the right results, at least at the classical level, starting from a “wrong vacuum”. The cases of cubic and quartic potentials are simple and interesting, and display some general features concerning tadpole resummations and convergence domains around inflection points of the potential, where the tadpole expansion breaks down. Our analysis shows that, starting from an arbitrary initial value of the field, classical tadpole resummations typically drive the quantities we are computing towards an extremum of the potential, not necessary a minimum. In addition, for the case of a quartic potential we find some very special “non-renormalization” points for which all higher order tadpole corrections cancel. We then analyze our procedure for a sting-inspired toy model with tadpoles localized on lower dimensional D-branes, performing explicitly the resummations. We also consider the introduction of gravity, that should give further complications related to the graviton mass terms, but seems to not affect substantially our program, and indeed tadpole resummations prove still to work in this case. Finally, in Chapter four we begin to face the tadpole problem in String Theory itself. In the first Section, we describe an example where the vacuum redefinition can be understood not only at the level of the low energy effective field theory, but also at the full string theory level. In particular, we show that the vacuum of a Type II orientifold with a compact dimension and local tadpoles is a Type 0 orientifold without compact dimensions. These results are contained in a paper to appear in Nuclear Physics B. Finally, in the last Section we begin the analysis of one-loop threshold corrections in a number of models with supersymmetry breaking with parallel branes and no closed tachyons propagating in the bulk. The result is that the one-loop threshold corrections in all these cases are always (closed) infrared finite, in spite of the presence of NS-NS tadpoles. These computations will be included in a paper that is currently in preparation.

Light Neutrinos without Heavy Mass Scales: A Higher-Dimensional Seesaw Mechanism

Recent theoretical developments have shown that extra spacetime dimensions can lower the fundamental GUT, Planck, and string scales. However, recent evidence for neutrino oscillations suggests the existence of light nonzero neutrino masses, which in turn suggests the need for a heavy mass scale via the seesaw mechanism. In this short note, we make two observations. First, we show that a higher-dimensional analogue of the seesaw mechanism may be capable of generating naturally light neutrino masses without the introduction of a heavy mass scale. In particular, we show that mixing between the left-handed neutrino and an entire Kaluza-Klein tower of right-handed neutrinos produces a naturally suppressed neutrino mass regardless of the intrinsic energy scale of the right-handed neutrinos. Second, we consider whether neutrino masses are actually required in order to explain neutrino oscillations, and show that our higher-dimensional seesaw mechanism may be able to permit neutrino oscillations even if the neutrino masses vanish. Essentially, neutrino oscillations can be induced indirectly via the masses of the Kaluza-Klein states. Thus, within the context of a higher-dimensional seesaw mechanism, we propose that neutrino oscillations can be explained without neutrino masses and without heavy mass scales. ∗ E-mail addresses: keith.dienes, emilian.dudas, tony.gherghetta@cern.ch † Laboratoire associé au CNRS-URA-D0063.Recent theoretical developments have shown that extra spacetime dimensions can lower the fundamental GUT, Planck, and string scales. However, recent evidence for neutrino oscillations suggests the existence of light non-zero neutrino masses, which in turn suggests the need for a heavy mass scale via the seesaw mechanism. In this paper, we make several observations in this regard. First, we point out that allowing the right-handed neutrino to experience extra spacetime dimensions naturally permits the left-handed neutrino mass to be power-law suppressed relative to the masses of the other fermions. This occurs due to the power-law running of the neutrino Yukawa couplings, and therefore does not require a heavy scale for the right-handed neutrino. Second, we show that a higher-dimensional analogue of the seesaw mechanism may also be capable of generating naturally light neutrino masses without the introduction of a heavy mass scale. Third, we show that such a higher-dimensional seesaw mechanism may even be able to explain neutrino oscillations without neutrino masses, with oscillations induced indirectly via the masses of the Kaluza-Klein states. Fourth, we point out that even when higher-dimensional right-handed neutrinos are given a bare Majorana mass, the higher-dimensional seesaw mechanism surprisingly replaces this mass scale with the radius scale of the extra dimensions. Finally, we also discuss a possible new mechanism for inducing lepton-number violation by shifting the positions of D-branes in Type I string theory.

The Volkov–Akulov–Starobinsky supergravity

Abstract We construct a supergravity model whose scalar degrees of freedom arise from a chiral superfield and are solely a scalaron and an axion that is very heavy during the inflationary phase. The model includes a second chiral superfield X , which is subject however to the constraint X 2 = 0 so that it describes only a Volkov–Akulov goldstino and an auxiliary field. We also construct the dual higher-derivative model, which rests on a chiral scalar curvature superfield R subject to the constraint R 2 = 0 , where the goldstino dual arises from the gauge-invariant gravitino field strength as γ m n D m ψ n . The final bosonic action is an R + R 2 theory involving an axial vector A m that only propagates a physical pseudoscalar mode.

Moduli stabilization and uplifting with dynamically generated F-terms

We use the F-term dynamical supersymmetry breaking models with metastable vacua in order to uplift the vacuum energy in the KKLT moduli stabilization scenario. The main advantage compared to earlier proposals is the manifest supersymmetric treatment and the natural coexistence of a TeV gravitino mass with a zero cosmological constant. We argue that it is generically difficult to avoid anti de-Sitter supersymmetric minima, however the tunneling rate from the metastable vacuum with zero vacuum energy towards them can be very suppressed. We briefly comment on the properties of the induced soft terms in the observable sector.

SUPERSYMMETRY BREAKING, OPEN STRINGS AND M-THEORY

Abstract We study supersymmetry breaking by Scherk-Schwarz compactifications in type I string theory. While in the gravitational sector all mass splittings are proportional to a (large) compactification radius, supersymmetry remains unbroken for the massless excitations of D-branes orthogonal to the large dimension. In this sector, supersymmetry breaking can then be mediated by gravitational interactions alone, that are expected to be suppressed by powers of the Planck mass. The mechanism is non-perturbative from the heterotic viewpoint and requires a compactification radius at intermediate energies of order 10 12 −10 14 GeV This can also explain the value of Newtons constant if the string scale is close to the unification scale, of order 10 16 GeV.

Anomalies, anomalous U(1)'s and generalized Chern-Simons terms

A detailed analysis of anomalous U(1)s and their effective couplings is performed both in field theory and string theory. It is motivated by the possible relevance of such couplings in particle physics, as well as a potential signal distinguishing string theory from other UV options. The most general anomaly related effective action is analyzed and parameterized. It contains Stuckelberg, axionic and Chern-Simons-like couplings. It is shown that such couplings are generically non-trivial in orientifold string vacua and are not in general fixed by anomalies. A similar analysis in quantum field theories provides similar couplings. The trilinear gauge boson couplings are also calculated and their phenomenological relevance is advocated. We do not find qualitative differences between string and field theory in this sector.

On hypercharge flux and exotics in F-theory GUTs

We study SU(5) Grand Unified Theories within a local framework in F-theory with multiple extra U(1) symmetries arising from a small monodromy group. The use of hypercharge flux for doublet-triplet splitting implies massless exotics in the spectrum that are protected from obtaining a mass by the U(1) symmetries. We find that lifting the exotics by giving vacuum expectation values to some GUT singlets spontaneously breaks all the U(1) symmetries which implies that proton decay operators are induced. If we impose an additional R-parity symmetry by hand we find all the exotics can be lifted while proton decay operators are still forbidden. These models can retain the gauge coupling unification accuracy of the MSSM at 1-loop. For models where the generations are distributed across multiple curves we also present a motivation for the quark-lepton mass splittings at the GUT scale based on a Froggatt-Nielsen approach to flavour.