Pascal Anastasopoulos

Orientifolds, hypercharge embeddings and the Standard Model

Abstract The embedding of the SM hypercharge into an orientifold gauge group is studied. Possible embeddings are classified, and a systematic construction of bottom–up configurations and top–down orientifold vacua is achieved, solving the tadpole conditions in the context of Gepner orientifolds. Some hypercharge embeddings are strongly preferred compared to others. Configurations with chiral anti-symmetric tensors are suppressed. We find among others, genuine examples of supersymmetric SU ( 5 ) , flipped SU ( 5 ) , Pati–Salam and trinification vacua with no chiral exotics.

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.

String constraints on discrete symmetries in MSSM type II quivers

A bstractWe study the presence of discrete gauge symmetries in D-brane semi-realistic compactifications. After establishing the constraints on the transformation behaviour of the chiral matter for the presence of a discrete gauge symmetry we perform a systematic search for discrete gauge symmetries within local semi-realistic D-brane realizations, based on four D-brane stacks, of the MSSM and the MSSM with three right-handed neutrinos. The systematic search reveals that Proton hexality, a discrete symmetry which ensures the absence of R-parity violating terms as well as the absence of dangerous dimension 5 proton decay operators, is only rarely realized. Moreover, none of the semi-realistic local D-brane configurations exhibit any family dependent discrete gauge symmetry.

On mass hierarchies in orientifold vacua

We analyze the problem of the hierarchy of masses and mixings in orientifold realizations of the Standard Model. We find bottom-up brane configurations that can generate such hierarchies.

Phenomenological analysis of D-brane Pati-Salam vacua

In the present work, we perform a phenomenological analysis of the effective low energy models with Pati-Salam (PS) gauge symmetry derived in the context of D-branes. The main issue in these models arises from the fact that the right-handed fermions and the PS-symmetry breaking Higgs field transform identically under the symmetry, causing unnatural matter-Higgs mixing effects. We argue that this problem can be solved in particular D-brane setups where these fields arise in different intersections. We further observe that whenever a large Higgs mass term is being generated in a particular class of mass spectra, a splitting mechanism –reminiscent of the doublet triplet splitting– may protect the neutral Higgs components from becoming heavy. We analyze the implications of each individual representation available in these models in order to specify the minimal spectrum required to build up a consistent model that reconciles the low energy data. A short discussion is devoted to the effects of stringy instanton corrections, particularly those generating missing Yukawa couplings and contributing to the fermion mass textures. We discuss the correlations of the intersecting D-brane spectra with those obtained from Gepner constructions and analyze the superpotential, the resulting mass textures and the low energy implications of some examples of the latter along the lines proposed above.

Light stringy states

A bstractWe carefully study the spectrum of open strings localized at the intersections of D6-branes and identify the lowest massive ‘twisted’ states and their vertex operators, paying particular attention to the signs of the intersection angles. We argue that the masses of the lightest states scale as

SU(5) D-brane realizations, Yukawa couplings and proton stability

M_{\theta }^2 \approx \theta M_s^2

Anomalous U(1)'s, Chern-Simons couplings and the Standard Model

and can thus be parametrically smaller than the string scale. Relying on previous analyses, we compute scattering amplitudes of massless ‘twisted’ open strings and study their factorization, confirming the presence of the light massive states as sub-dominant poles in one of the channels.

Anomalies, Chern-Simons terms and the Standard Model

We discuss SU(5) Grand Unified Theories in the context of orientifold compactifications. Specifically, we investigate two and three D-brane stack realizations of the Georgi-Glashow and the flipped SU(5) model and analyze them with respect to their Yukawa couplings. As pointed out in [1] the most economical Georgi-Glashow realization based on two stacks generically suffers from a disastrous large proton decay rate. We show that allowing for an additional U(1) D-brane stack this as well as other phenomenological problems can be resolved. We exemplify with globally consistent Georgi-Glashow models based on RCFT that these D-brane quivers can be indeed embedded in a global setting. These globally consistent realizations admit rigid O(1) instantons inducing the perturbatively missing coupling 10105H. Finally we show that flipped SU(5) D-brane realizations even with multiple U(1) D-brane stacks are plagued by severe phenomenological drawbacks which generically cannot be overcome.

Production of light stringy states

This proceeding is based on hep-th/0605225 and it shows that the most general anomaly related effective action contains Stuckelberg, axionic and Chern-Simons-like couplings. Such couplings are generically non-trivial in orientifold string vacua. A similar analysis in quantum field theories provides similar couplings. These Chern-Simons couplings generate new signals which might be visible at LHC.