No abstract

The orbifold GUT doublet-triplet splitting mechanism was discussed in 1994 in the framework of the NAHE-based free fermionic models in which the SO(10) GUT symmetry is broken to SO(6) × SO(4), SU (3) × SU (2) × U (1) 2 , or SU (3) × U (1) × SU (2) 2 . In this paper we study NAHE-based free fermionic models in which the SO(10) symmetry is broken at the string level to SU (4) × SU (2) × U (1). In addition to the doublet-triplet splitting this case also has the advantage of inducing the doublet-doublet splitting already at the string level. We demonstrate, however, that NAHE-based models with SU (4)×SU (2)×U (1) SO(10) subgroup are not viable. We show that, similarly to the LRS models, and in contrast to the FSU5, PS and SLM models, the SU421 case gives rise to models without an anomalous U (1) symmetry, and discuss the different cases in terms of their N = 4 origins.

The first particle physics observable whose origin may be sought in string theory is the triple replication of the matter generations. The class of Z 2 × Z 2 orbifolds of six dimensional compactified tori, that have been most widely studied in the free fermionic formulation, correlate the family triplication with the existence of three twisted sectors in this class. In this work we seek an improved understanding of the geometrical origin of the three generation free fermionic models. Using fermionic and orbifold techniques we classify the Z 2 × Z 2 orbifold with symmetric shifts on six dimensional compactified internal manifolds. We show that perturbative three generation models are not obtained in the case of Z 2 × Z 2 orbifolds with symmetric shifts on complex tori, and that the perturbative three generation models in this class necessarily employ an asymmetric shift. We present a class of three generation models in which the SO(10) gauge symmetry cannot be broken perturbatively, while preserving the Standard Model matter content. We discuss the potential implications of the asymmetric shift for strong-weak coupling duality and moduli stabilization. We show that the freedom in the modular invariant phases in the N = 1 vacua that control the chiral content, can be interpreted as vacuum expectation values of background fields of the underlying N = 4 theory, whose dynamical components are projected out by the Z 2 -fermionic projections. In this class of vacua the chiral content of the models is determined by the underlying N = 4 mother theory.

The observation potential of the decay B + → K + K + π − with the ATLAS detector at LHC is described in this paper. In the Standard Model this decay mode is highly suppressed, while in models beyond the Standard Model it could be significantly enhanced. To improve the selection of the K + K + π − final state, a charged hadron identification using Time-over-Threshold measurements in the ATLAS Transition Radiation Tracker was developed and used.