D-term spectroscopy in realistic heterotic-string models

Abstract

The recent emergence of free fermionic heterotic string models with solely the MSSM charged spectrum below the string scale reinforces the motivation to investigate the pheneomelogical characteristics of this class of string models, which possess an underlying Z 2 ϫZ 2 orbifold structure. An important property of the Z 2 ϫZ 2 orbifold is the cyclic permutation symmetry between the three twisted sectors. If preserved in the three-generation models the cyclic permutation symmetry results in a family universal anomalous U(1) A , which is instrumental in explaining squark degeneracy, provided that the dominant component of supersymmetry breaking arises from the U(1) A D term. Interestingly, the contribution of the family-universal D A term to the squark masses may be intrafamily nonuniversal, and may differ from the usual ͑universal͒ boundary conditions assumed in the MSSM. We contemplate how D A -term spectroscopy may be instrumental in studying superstring models irrespective of our ignorance of the details of supersymmetry breaking. We examine the possible effect of the intrafamily nonuniversality on the resulting SUSY spectrum and the values of the strong coupling, effective weak mixing angle, and W-gauge boson mass, up to a two-loop accuracy, in the two models ͑universal and nonuniversal͒. We find that nonuniversality relaxes the constraint of color and charge breaking minima which appears in the universal case. In addition, it predicts a 3% smaller value of ␣ s due to different threshold masses obtained in the latter scenario. Finally, we present the experimentally allowed predictions of the two models in an M 0 and M 1/2 parameter space.

String Phenomenology: Past, Present and Future Perspectives

“…Low scale supersymmetry is therefore not a necessary outcome of string theory, but certainly its observation will provide further evidence that the different structures of string constructions are realised in nature. Specific SUSY breaking scenarios in string models give rise to distinct supersymmetric spectra and that in turn will be used to constrain further the phenomenological string vacua [95]. It is further noted that R-parity is generically broken in string vacua [96] and that the LSP is not expected therefore to provide a viable dark matter candidate.…”

Self-Duality and Vacuum Selection

“…These studies are reviewed in [25], where further details can be found. These include among others: top quark mass prediction [31], several years prior to the actual observation by the CDF/D0 collaborations [37]; generations mass hierarchy [38]; CKM mixing [39]; superstring see-saw mechanism [40]; Gauge coupling unification [41]; Proton stability [42]; supersymmetry breaking and squark degeneracy [43,44]. Additionally, it was demonstrated in ref.…”

Quasi-fixed points and charge and colour breaking in low scale models

“…The second issue we consider is the possibility of minima that break charge and colour lying along F and D flat directions [10,13,14,15,16,17,18]. The constraints found by requiring that there be no such (CCB) minima are dependent on the distance from the QFP.…”

“…In particular, we consider the empirically derived lower bound upon the lightest CP-even MSSM Higgs mass, which in the canonical GUT scenarios has been shown to be a strong restriction upon the QFP scenario [12]. The second issue we consider is the possibility of minima that break charge and colour lying along F and D flat directions [10,13,14,15,16,17,18]. The constraints found by requiring that there be no such (CCB) minima are dependent on the distance from the QFP.…”