Universal Soft Terms in the MSSM on D-branes

In Type II string vacua constructed from intersecting/magnetized D-branes, the supersymmetry-breaking soft terms are genericaly non-universal. It is shown that universal supersymmetry-breaking soft terms may arise in a realistic MSSM constructed from intersecting/magnetized D-branes in Type II string theory. For the case of dilaton-dominated supersymmetry-breaking, it is shown that the universal scalar mass and trilinear coupling are fixed such that $m_0=(1/2)m_{3/2}$ and $A_0 = - m_{1/2}$. In addition, soft terms where the universal scalar mass $m_0$ is much larger than the universal gaugino mass $m_{1/2}$ may be easily obtained within the model, corresponding to the Focus Point (FP)/Hyperbolic Branch (HB) regions of the mSUGRA/CMSSM parameter space. Finally, it is shown that the special dilaton and no-scale strict moduli boundary conditions, which are well-known in heterotic string constructions, may also be obtained.Comment: Version published in Nuclear Physics

Flipped Cryptons and the UHECRs

Cryptons are metastable bound states of fractionally-charged particles that arise generically in the hidden sectors of models derived from heterotic string. We study their properties and decay modes in a specific flipped SU(5) model with long-lived four-particle spin-zero bound states called {\it tetrons}. We show that the neutral tetrons are metastable, and exhibit the tenth-order non-renormalizable superpotential operators responsible for their dominant decays. By analogy with QCD, we expect charged tetrons to be somewhat heavier, and to decay relatively rapidly via lower-order interactions that we also exhibit. The expected masses and lifetimes of the neutral tetrons make them good candidates for cold dark matter (CDM), and a potential source of the ultra-high energy cosmic rays (UHECRs) which have been observed, whereas the charged tetrons would have decayed in the early Universe.Comment: 8 Pages RevTex. New version with expanded introduction to flipped SU(5). Accepted for publication in PR

The Search for a Realistic String Model at LHC

We survey the low-energy supersymmetry phenomenology of a three-family Pati-Salam model constructed from intersecting D6-branes in Type IIA string theory on the T^6/(Z_2 x Z_2) orientifold which possesses many of the phenomenological properties desired in string model-building. In the model, there is no exotic matter in the low-energy spectrum, the correct mass hierarchies for quarks and leptons may be obtained, and the gauge couplings are automatically unified at the string scale. We calculate the supersymmetry breaking soft terms and the corresponding low-energy supersymmetry particle spectra for the model. We find the WMAP constrained dark matter density can be generated in this model in the stau-neutralino and chargino-neutralino coannihilation regions, with expected final states at LHC consisting of low energy leptons and O(GeV) neutrinos. Moreover, we expect final states in the supercritical string cosmology (SSC) scenario to comprise high energy leptons and O(GeV) neutrinos.Comment: 35 pages, 4 figures, accepted for publication in PR

Proton Stability and Dark Matter in a Realistic String MSSM

We demonstrate the existence of an extra nonanomalous U(1) gauge symmetry in a three-generation Pati-Salam model constructed with intersecting D6-branes in Type IIA string theory on a T^6/(Z_2 \times Z_2) orientifold. This extra U(1) forbids all dimension-4, 5, and 6 operators which mediate proton decay in the MSSM. Moreover, this results in the effective promotion of baryon and lepton number to local gauge symmetries, which can potentially result in leptophobic and leptophilic $Z'$ bosons observable at the LHC. Furthermore, it is not necessary to invoke R-parity to forbid the dimension-4 operators which allow rapid proton decay. However, R-parity may arise naturally from a spontaneously broken U(1)_{B-L}. Assuming the presence of R-parity, we then study the direct detection cross-sections for neutralino dark matter, including the latest constraints from the XENON100 experiment. We find that these limits are now within required range necessary to begin testing the model.Comment: Expanded discussion of Z' boson phenomenology. Accepted for publication to Physical Review D. arXiv admin note: text overlap with arXiv:1103.603

Natural Four-Generation Mass Textures in MSSM Brane Worlds

A fourth generation of Standard Model (SM) fermions is usually considered unlikely due to constraints from direct searches, electroweak precision measurements, and perturbative unitarity. We show that fermion mass textures consistent with all constraints may be obtained naturally in a model with four generations constructed from intersecting D6 branes on a T^6/(Z_2 x Z_2) orientifold. The Yukawa matrices of the model are rank 2, so that only the third- and fourth-generation fermions obtain masses at the trilinear level. The first two generations obtain masses via higher-order couplings and are therefore naturally lighter. In addition, we find that the third and fourth generation automatically split in mass, but do not mix at leading order. Furthermore, the SM gauge couplings automatically unify at the string scale, and all the hidden-sector gauge groups become confining in the range 10^{13}--10^{16} GeV, so that the model becomes effectively a four-generation MSSM at low energies.Comment: Accepted for publication in Physical Review