arXiv subjects

Dimitri V. Nanopoulos

Publications and source records attributed to Dimitri V. Nanopoulos.

At least 145 records · Page 8Linked to original sources

arXiv2009

D-Foam Phenomenology: Dark Energy, the Velocity of Light and a Possible D-Void

In a D-brane model of space-time foam, there are contributions to the dark energy that depend on the D-brane velocities and on the density of D-particle defects. The latter may also reduce the speeds of photons linearly with their energies, establishing a phenomenological connection with astrophysical probes of the universality of the velocity of light. Specifically, the cosmological dark energy density measured at the present epoch may be linked to the apparent retardation of energetic photons propagating from nearby AGNs. However, this nascent field of `D-foam phenomenology' may be complicated by a dependence of the D-particle density on the cosmological epoch. A reduced density of D-particles at redshifts z ~ 1 - a `D-void' - would increase the dark energy while suppressing the vacuum refractive index, and thereby might reconcile the AGN measurements with the relatively small retardation seen for the energetic photons propagating from GRB 090510, as measured by the Fermi satellite.

astro-ph.CO↗

arXiv2009

Quark Masses from Gaugino Condensation in String Theories

We present a mechanism able to generate the perturbatively absent up/down $<{\bf 10} \cdot {\bf 10} \cdot {\bf 5}^H>$ quark Yukawa couplings of SU(5)/flipped SU(5) GUTS in Type II orientifold compactifications with D-branes. The mechanism works when there are Sp(N) gauge groups involved. The ${\bf {\bar 5}}$'s get charged under the Sp(N) gauge groups and the generation of quark masses proceeds via the generation of the fermionic Sp(N) singlet condensate $<{\bf {\bar 5} \cdot {\bar 5} \cdot {\bar 5} \cdot {\bar 5}}>$ in the term $(1/{M_s^5}) {\bf 10} \cdot {\bf 10} \cdot < {\bf {\bar 5} \cdot {\bar 5} \cdot {\bar 5} \cdot {\bar 5}}>$. Also non-chiral states charged under Sp gauge groups may become constrained by the requirement of Sp's becoming strongly coupled.

hep-th↗

arXiv2009

Time Delays of Strings in D-particle Backgrounds and Vacuum Refractive Indices

Using standard techniques in string/D-brane scattering amplitude computations, we evaluate the scattering of open strings off D-particles in brane world scenarios. The D-particles are viewed as D3 branes wrapped up around three cycles, and their embedding in brane worlds constitutes a case of intersecting branes, among which strings are stretched, representing various types of excitations of the Standard Model (SM) particles in the low-energy limit. Our analysis, reveals interesting and novel selection rules for the resulting causal time delays, proportional to the energy of the incident matter state, from the processes of splitting, capture and re-emission of the latter by the D-particles. In particular, we show that there are relatively large time delays only for excitations that belong to the Cartan subalgebra of the SM gauge group, which notably includes photons. We discuss the possible relevance of these results to the models of space time foam predicting a non-trivial vacuum refractive index for photons and the associated cosmic γ-ray phenomenology. In particular, we demonstrate how low-string-scale models can be falsified already in this context, by current astrophysical observations of cosmic photons, and how such observations serve as a way to discriminate low- from high-string scale models.

hep-th↗

arXiv2009

Stringy WIMP Detection and Annihilation

We calculate the direct dark matter detection spin-independent and proton spin-dependent cross-sections for a semi-realistic intersecting D6-brane model. The cross-sections are compared to the latest constraints of the current dark matter direct detection experiments, as well as the projected results of future dark matter experiments. The allowed parameter space of the intersecting D6-brane model is shown with all current experimental constraints, including those regions satisfying the WMAP and Supercritical String Cosmology (SSC) limits on the dark matter density in the universe. Additionally, we compute the indirect detection gamma-ray flux resulting from neutralino annihilation for the D6-brane model and compare the flux to the projected sensitivity of the Fermi Gamma-ray Space Telescope. Finally, we compute the direct and indirect detection cross-sections as well as the gamma-ray flux resulting from WIMP annihilations for the one-parameter model for comparison, where the one-parameter model is a highly constrained subset of the mSUGRA parameter space such that the soft supersymmetry breaking terms are functions of the common gaugino mass, which is common to many string compactifications.

hep-ph↗

arXiv2009

F-SU(5)

We construct three flipped SU(5) X U(1)_X models from F-theory, and consider two such models from free fermionic string model building. To achieve the decoupling scenario in F-theory models and the string-scale gauge coupling unification in free fermionic models, we introduce vector-like particles at the TeV scale that can be observed at the Large Hadron Collider. We study gauge coupling unification, and find that proton decay is within the reach of the future Hyper-Kamiokande experiment. In these models, the doublet-triplet splitting problem and monopole problem can be solved, the neutrino masses and mixings can be explained via the double seesaw or seesaw mechanism, the observed baryon asymmetry can be obtained through leptogenesis, the hybrid inflation can be realized, and the correct cosmic primodial density fluctuations can be generated.

hep-th↗

arXiv2009

Supergravity and Superstring Signatures of the One-Parameter Model at LHC

Many string constructions have a classical no-scale structure, resulting in a one-parameter model (OPM) for the supersymmetry breaking soft terms. As a highly constrained subset of mSUGRA, the OPM has the potential to be predictive. Conversely, if the observed superpartner spectrum at LHC is a subset of the OPM parameter space, then this may provide a clue to the underlying theory at high energies. We investigate the allowed supersymmetry parameter space for a generic one-parameter model taking into account the most recent experimental constraints. We find that in the strict moduli scenario, there are no regions of the parameter space which may satisfy all constraints. However, for the dilaton scenario, there are small regions of the parameter space where all constraints may be satisfied and for which the observed dark matter density may be generated. We also survey the possible signatures which may be observable at the Large Hadron Collider (LHC). Finally, we compare collider signatures of OPM to those from a model with non-universal soft terms, in particular those of an intersecting D6-brane model. We find that it may be possible to distinguish between these diverse scenarios at LHC.

hep-ph↗

arXiv2008

Realistic Type IIB Supersymmetric Minkowski Flux Models without the Freed-Witten Anomaly

We show that there exist supersymmetric Minkowski vacua on Type IIB toroidal orientifold with general flux compactifications where the RR tadpole cancellation conditions can be relaxed and the Freed-Witten anomaly can be cancelled elegantly. We present a realistic Pati-Salam like flux model without the Freed-Witten anomaly. At the string scale, we can break the gauge symmetry down to the Standard Model (SM) gauge symmetry, achieve the gauge coupling unification naturally, and decouple all the extra chiral exotic particles. Thus, we have the supersymmetric SMs with/without SM singlet(s) below the string scale. Also, we can explain the SM fermion masses and mixings. In addition, the unified gauge coupling and the real parts of the dilaton and Kähler moduli are functions of the four-dimensional dilaton. The complex structure moduli and one linear combination of the imaginary parts of the Kähler moduli can be determined as functions of the fluxes and the dilaton.

hep-th↗

arXiv2008

Supercriticality of a Class of Critical String Cosmological Solutions

For a class of Friedmann-Robertson-Walker type string solutions with compact hyperbolic spatial slices formulated in critical dimension, we find the world sheet conformal field theory which involves the linear dilaton and Wess-Zumino-Witten type model with the compact hyperbolic target space. By analyzing the infrared spectrum, we conclude that the theory is actually supercritical due to the modular invariance of string theory. Thus, taking into account previous results, we conclude that all the simple nontrivial string cosmological solutions are supercritical. A possible explanation of why we are living in D=4 is provided. The interesting relation of this background with the Supercritical String Cosmology (SSC) is pointed out.

hep-th↗

arXiv2008

Flux Parameter Spaces in Type II Vacua

We study the flux parameter spaces for semi-realistic supersymmetric Pati-Salam models in the AdS vacua on Type IIA orientifold and realistic supersymmetric Pati-Salam models in the Minkowski vacua on Type IIB orientifold. Because the fluxes can be very large, we show explicitly that there indeed exists a huge number of semi-realistic Type IIA and realistic Type IIB flux models. In the Type IIA flux models, in the very large flux limit, the theory can become weakly coupled and the AdS vacua can approach to the Minkowski vacua. In a series of realistic Type IIB flux models, at the string scale, the gauge symmetry can be broken down to the Standard Model (SM) gauge symmetry, the gauge coupling unification can be achieved naturally, all the extra chiral exotic particles can be decoupled, and the observed SM fermion masses and mixings can be obtained as well. In particular, the real parts of the dilaton, Kähler moduli, and the unified gauge coupling are independent of the very large fluxes. The very large fluxes only affect the real and/or imaginary parts of the complex structure moduli, and/or the imaginary parts of the dilaton and Kähler moduli. However, these semi-realistic Type IIA and realistic Type IIB flux models can not be populated in the string landscape.

hep-th↗

arXiv2007

Realistic Type IIB Supersymmetric Minkowski Flux Vacua

We show that there exist supersymmetric Minkowski vacua on Type IIB toroidal orientifold with general flux compactifications where the RR tadpole cancellation conditions can be relaxed elegantly. Then we present a realistic Pati-Salam like model. At the string scale, the gauge symmetry can be broken down to the Standard Model (SM) gauge symmetry, the gauge coupling unification can be achieved naturally, and all the extra chiral exotic particles can be decoupled so that we have the supersymmetric SMs with/without SM singlet(s) below the string scale. The observed SM fermion masses and mixings can also be obtained. In addition, the unified gauge coupling, the dilaton, the complex structure moduli, the real parts of the Kähler moduli and the sum of the imaginary parts of the Kähler moduli can be determined as functions of the four-dimensional dilaton and fluxes, and can be estimated as well.

hep-th↗

arXiv2007

No-Scale Solution to Little Hierarchy

We show that the little hierarchy problem can be solved in the no-scale supergravity framework. In this model the supersymmetry breaking scale is generated when the electroweak symmetry breaking condition is satisfied and therefore, unlike usual supersymmetric models, the correlation between the electroweak symmetry breaking scale and the average stop mass scale can be justified. This correlation solves the little hierarchy puzzle. Using minimal supergravity boundary conditions, we find that the parameter space predicted by no-scale supergravity is allowed by all possible experimental constraints. The predicted values of supersymmetric particle masses are low enough to be very easily accessible at the LHC. This parameter space will also be probed in the upcoming results from the dark matter direct detection experiments.

hep-ph↗

arXiv2007

Variations of the Hidden Sector in a Realistic Intersecting Brane Model

Recently, we discussed the first example of a phenomenologically realistic intersecting D6-brane model. In this model, the gauge symmetry in the hidden sector is USp(2)_1 x USp(2)_2 x USp(2)_3 x USp(2)_4. However, we find that the USp(2)_1 x USp(2)_2 gauge symmetry can be replaced by an U(2)_{12} gauge symmetry, and/or the USp(2)_3 x USp(2)_4 gauge symmetry can be replaced by an U(2)_{34} gauge symmetry since the USp(2)^2 stacks of D6-branes contribute to the same Ramond-Ramond tadpoles as those of the U(2) stacks. Thus, there are three non-equivalent variations of the hidden sector, and the corresponding gauge symmetries are U(2)_{12} x USp(2)_3 x USp(2)_4, U(2)_{34} x USp(2)_1 x USp(2)_2, and U(2)_{12} x U(2)_{34}, respectively. Moreover, we study the hidden sector gauge symmetry breaking, discuss how to decouple the additional exotic particles, and briefly comment on the phenomenological consequences.

hep-th↗

arXiv2007

A Realistic World from Intersecting D6-Branes

We briefly describe a three-family intersecting D6-brane model in Type IIA theory on the T^6/(Z_2 x Z_2) orientifold with a realistic phenomenology. In this model, the gauge symmetry can be broken down to the Standard Model (SM) gauge symmetry close to the string scale, and the gauge coupling unification can be achieved. We calculate the supersymmetry breaking soft terms, and the corresponding low energy supersymmetric particle spectrum, which may be tested at the Large Hadron Collider (LHC). The observed dark matter density may also be generated. Finally, we can explain the SM quark masses and CKM mixings, and the tau lepton mass. The neutrino masses and mixings may be generated via the seesaw mechanism as well.

hep-th↗

arXiv2007

Testable Flipped SU(5) x U(1)_X Models

The little hierarchy between the GUT scale and the string scale may give us some hints that can be tested at the LHC. To achieve string-scale gauge coupling unification, we introduce additional vector-like particles. We require that these vector-like particles be standard, form complete GUT multiplets, and have masses around the TeV scale or close to the string scale. Interestingly, only the flipped SU(5) x U(1)_X models can work elegantly. We consider all possible sets of vector-like particles with masses around the TeV scale. And we introduce vector-like particles with masses close to the string scale which can mimic the string-scale threshold corrections. We emphasize that all of these vector-like particles can be obtained in the interesting flipped SU(5) x U(1)_X string models from the four-dimensional free fermionic string construction. Assuming the low-energy supersymmetry, high-scale supersymmetry, and split supersymmetry, we show that the string-scale gauge coupling unification can indeed be achieved in the flipped SU(5) x U(1)_X models. These models can be tested at the LHC by observing simple sets of vector-like particles at the TeV scale. Moreover, we discuss a simple flipped SU(5) x U(1)_X model with string-scale gauge coupling unification and high-scale supersymmetry by introducing only one pair of the vector-like particles at the TeV scale, and we predict the corresponding Higgs boson masses. Also, we briefly comment on the string-scale gauge coupling unification in the model with low-energy supersymmetry by introducing only one pair of the vector-like particles at the intermediate scale. And we briefly comment on the mixings among the SM fermions and the corresponding extra vector-like particles.

hep-ph↗

arXiv2006

CPT and Quantum Mechanics Tests with Kaons

In this review we first discuss the theoretical motivations for possible CPT violation and deviations from ordinary quantum-mechanical behavior of field-theoretic systems in the context of an extended class of quantum-gravity models. Then we proceed to a description of precision tests of CPT symmetry using mainly neutral kaons. We emphasize the possibly unique role of neutral meson factories in providing specific tests of models where the quantum-mechanical CPT operator is not well-defined, leading to modifications of Einstein-Podolsky-Rosen particle correlators. Finally, we present tests of CPT, T, and CP using charged kaons, and in particular K_l4 decays, which are interesting due to the high statistics attainable in experiments.

hep-ph↗

arXiv2006

Flipped and Unflipped SU(5) as Type IIA Flux Vacua

On Type IIA orientifolds with flux compactifications in supersymmetric AdS vacua, we for the first time construct SU(5) models with three anti-symmetric {\bf 10} representations and without symmetric {\bf 15} representations. We show that all the pairs of the anti-fundamental {\bf \bar 5} and fundamental {\bf 5} representations can obtain GUT/string-scale vector-like masses after the additional gauge symmetry breaking via supersymmetry preserving Higgs mechanism. Then we have exact three {\bf \bar 5}, and no other chiral exotic particles that are charged under SU(5) due to the non-abelian anomaly free condition. Moreover, we can break the SU(5) gauge symmetry down to the SM gauge symmetry via D6-brane splitting, and solve the doublet-triplet splitting problem. Assuming that the extra one (or several) pair(s) of Higgs doublets and adjoint particles obtain GUT/string-scale masses via high-dimensional operators, we only have the MSSM in the observable sector below the GUT scale. Then the observed low energy gauge couplings can be generated via RGE running if we choose the suitable grand unified gauge coupling by adjusting the string scale. Furthermore, we construct the first flipped SU(5) model with exact three {\bf 10}, and the first flipped SU(5) model in which all the Yukawa couplings are allowed by the global U(1) symmetries.

hep-th↗

arXiv2006

Type IIA Pati-Salam Flux Vacua

We show that for supersymmetric AdS vacua on Type IIA orientifolds with flux compactifications, the RR tadpole cancellation conditions can be completely relaxed, and then the four-dimensional N=1 supersymmetry conditions are the main constraints on consistent intersecting D6-brane model building. We construct two kinds of three-family Pati-Salam models. In the first kind of models, the suitable three-family SM fermion masses and mixings can be generated at the stringy tree level, and then the rank one problem for the SM fermion Yukawa matrices can be solved. In the second kind of models, only the third family of the SM fermions can obtain masses at tree level. In these models, the complex structure parameters can be determined by supersymmetric D6-brane configurations, and all the moduli may be stabilized. The initial gauge symmetries U(4)_C \times U(2)_L \times U(2)_R and U(4)_C \times USp(2)_L \times U(2)_R can be broken down to the SU(3)_C \times SU(2)_L \times U(1)_{B-L} \times U(1)_{I_{3R}} due to the Green-Schwarz mechanism and D6-brane splittings, and further down to the SM gauge symmetry around the string scale via the supersymmetry preserving Higgs mechanism. Comparing to the previous model building, we have less bidoublet Higgs fields. However, there generically exist some exotic particles.

hep-th↗

arXiv2005

Standard-Like Model Building on Type II Orientifolds

We construct new Standard-like models on Type II orientifolds. In Type IIA theory on $\mathbf{T^6/(\Z_2\times \Z_2)}$ orientifold with intersecting D6-branes, we first construct a three-family trinification model where the $U(3)_C\times U(3)_L\times U(3)_R$ gauge symmetry can be broken down to the $SU(3)_C\times SU(2)_L\times U(1)_{Y_L}\times U(1)_{I_{3R}}\times U(1)_{Y_R}$ gauge symmetry by the Green-Schwarz mechanism and D6-brane splittings, and further down to the SM gauge symmetry at the TeV scale by Higgs mechanism. We also construct a Pati-Salam model where we may explain three-family SM fermion masses and mixings. Furthermore, we construct for the first time a Pati-Salam like model with $U(4)_C \times U(2)_L \times U(1)' \times U(1)''$ gauge symmetry where the $U(1)_{I_{3R}}$ comes from a linear combination of U(1) gauge symmetries. In Type IIB theory on $\mathbf{T^6/(\Z_2\times \Z_2)}$ orientifold with flux compactifications, we construct a new flux model with $U(4)_C \times U(2)_L \times U(2)_R$ gauge symmetry where the magnetized D9-branes with large negative D3-brane charges are introduced in the hidden sector. However, we can not construct the trinification model with supergravity fluxes because the three SU(3) groups already contribute very large RR charges. The phenomenological consequences of these models are briefly discussed as well.

hep-th↗