1,601 research outputs found
Neutrino Masses, Baryon Asymmetry, Dark Matter and the Moduli Problem : A Complete Framework
Recent developments in string theory have led to "realistic" string
compactifications which lead to moduli stabilization while generating a
hierarchy between the Electroweak and Planck scales at the same time. However,
this seems to suggest a rethink of our standard notions of cosmological
evolution after the end of inflation and before the beginning of BBN. We argue
that within classes of realistic string compactifications, there generically
exists a light modulus with a mass comparable to that of the gravitino which
generates a large late-time entropy when it decays. Therefore, all known
mechanisms of generating the baryon asymmetry of the Universe in the literature
have to take this fact into account. In this work, we find that it is still
possible to naturally generate the observed baryon asymmetry of the Universe as
well as light left-handed neutrino masses from a period of Affleck-Dine(AD)
leptogenesis shortly after the end of inflation, in classes of realistic string
constructions with a minimal extension of the MSSM below the unification scale
(consisting only of right-handed neutrinos) and satisfying certain microscopic
criteria described in the text. The consequences are as follows. The lightest
left-handed neutrino is required to be virtually massless. The moduli
(gravitino) problem can be naturally solved in this framework both within
gravity and gauge mediation. The observed upper bound on the relic abundance
constrains the moduli-matter and moduli-gravitino couplings since the DM is
produced non-thermally within this framework. Finally, although not a definite
prediction, the framework naturally allows a light right-handed neutrino and
sneutrinos around the electroweak scale which could have important implications
for DM as well as the LHC.Comment: 41 pages, no figures, journal version adde
Sequestered Dark Matter
We show that hidden-sector dark matter is a generic feature of the type IIB
string theory landscape and that its lifetime may allow for a discovery through
the observation of very energetic gamma-rays produced in the decay. Throats or,
equivalently, conformally sequestered hidden sectors are common in flux
compactifications and the energy deposited in these sectors can be calculated
if the reheating temperature of the standard model sector is known. Assuming
that throats with various warp factors are available in the compact manifold,
we determine which throats maximize the late-time abundance of sequestered dark
matter. For such throats, this abundance agrees with cosmological data if the
standard model reheating temperature was 10^10 - 10^11 GeV. In two distinct
scenarios, the mass of dark matter particles, i.e. the IR scale of the throat,
is either around 10^5 GeV or around 10^10 GeV. The lifetime and the decay
channels of our dark matter candidates depend crucially on the fact that the
Klebanov-Strassler throat is supersymmetric. Furthermore, the details of
supersymmetry breaking both in the throat and in the visible sector play an
essential role. We identify a number of scenarios where this type of dark
matter can be discovered via gamma-ray observations.Comment: 36 pages, 3 figures; v2: references added, v3: introduction extended
and typos correcte
Astrophysical and Cosmological Implications of Large Volume String Compactifications
We study the spectrum, couplings and cosmological and astrophysical
implications of the moduli fields for the class of Calabi-Yau IIB string
compactifications for which moduli stabilisation leads to an exponentially
large volume V ~ 10^{15} l_s^6 and an intermediate string scale m_s ~
10^{11}GeV, with TeV-scale observable supersymmetry breaking. All K\"ahler
moduli except for the overall volume are heavier than the susy breaking scale,
with m ~ ln(M_P/m_{3/2}) m_{3/2} ~ (\ln(M_P/m_{3/2}))^2 m_{susy} ~ 500 TeV and,
contrary to standard expectations, have matter couplings suppressed only by the
string scale rather than the Planck scale. These decay to matter early in the
history of the universe, with a reheat temperature T ~ 10^7 GeV, and are free
from the cosmological moduli problem (CMP). The heavy moduli have a branching
ratio to gravitino pairs of 10^{-30} and do not suffer from the gravitino
overproduction problem. The overall volume modulus is a distinctive feature of
these models and is an M_{planck}-coupled scalar of mass m ~ 1 MeV and subject
to the CMP. A period of thermal inflation can help relax this problem. This
field has a lifetime ~ 10^{24}s and can contribute to dark matter. It may be
detected through its decays to 2\gamma or e^+e^-. If accessible the e^+e^-
decay mode dominates, with Br(\chi \to 2 \gamma) suppressed by a factor
(ln(M_P/m_{3/2}))^2. We consider the potential for detection of this field
through different astrophysical sources and find that the observed gamma-ray
background constrains \Omega_{\chi} <~ 10^{-4}. The decays of this field may
generate the 511 keV emission line from the galactic centre observed by
INTEGRAL/SPI.Comment: 31 pages, 2 figures; v2. refs adde
F-theory, GUTs, and the Weak Scale
In this paper we study a deformation of gauge mediated supersymmetry breaking
in a class of local F-theory GUT models where the scale of supersymmetry
breaking determines the value of the mu term. Geometrically correlating these
two scales constrains the soft SUSY breaking parameters of the MSSM. In this
scenario, the hidden SUSY breaking sector involves an anomalous U(1)
Peccei-Quinn symmetry which forbids bare mu and B mu terms. This sector
typically breaks supersymmetry at the desired range of energy scales through a
simple stringy hybrid of a Fayet and Polonyi model. A variant of the
Giudice-Masiero mechanism generates the value mu ~ 10^2 - 10^3 GeV when the
hidden sector scale of supersymmetry breaking is F^(1/2) ~ 10^(8.5) GeV.
Further, the B mu problem is solved due to the mild hierarchy between the GUT
scale and Planck scale. These models relate SUSY breaking with the QCD axion,
and solve the strong CP problem through an axion with decay constant f_a ~
M_(GUT) * mu / L, where L ~ 10^5 GeV is the characteristic scale of gaugino
mass unification in gauge mediated models, and the ratio \mu / L ~
M_(GUT)/M_(pl) ~ 10^(-3). We find f_a ~ 10^12 GeV, which is near the high end
of the phenomenologically viable window. Here, the axino is the goldstino mode
which is eaten by the gravitino. The gravitino is the LSP with a mass of about
10^1 - 10^2 MeV, and a bino-like neutralino is (typically) the NLSP with mass
of about 10^2 - 10^3 GeV. Compatibility with electroweak symmetry breaking also
determines the value of tan(beta) ~ 30 +/- 7.Comment: v3: 94 pages, 9 figures, clarification of Fayet-Polonyi model and
instanton corrections to axion potentia
FCNC Processes from D-brane Instantons
Low string scale models might be tested at the LHC directly by their Regge
resonances. For such models it is important to investigate the constraints of
Standard Model precision measurements on the string scale. It is shown that
highly suppressed FCNC processes like K0- bar K^0 oscillations or leptonic
decays of the D0-meson provide non-negligible lower bounds on both the
perturbatively and surprisingly also non-perturbatively induced string theory
couplings. We present both the D-brane instanton formalism to compute such
amplitudes and discuss various possible scenarios and their constraints on the
string scale for (softly broken) supersymmetric intersecting D-brane models.Comment: 28 pages, 13 figures, reference added, 1 typo corrected, style file
adde
On hypercharge flux and exotics in F-theory GUTs
We study SU(5) Grand Unified Theories within a local framework in F-theory
with multiple extra U(1) symmetries arising from a small monodromy group. The
use of hypercharge flux for doublet-triplet splitting implies massless exotics
in the spectrum that are protected from obtaining a mass by the U(1)
symmetries. We find that lifting the exotics by giving vacuum expectation
values to some GUT singlets spontaneously breaks all the U(1) symmetries which
implies that proton decay operators are induced. If we impose an additional
R-parity symmetry by hand we find all the exotics can be lifted while proton
decay operators are still forbidden. These models can retain the gauge coupling
unification accuracy of the MSSM at 1-loop. For models where the generations
are distributed across multiple curves we also present a motivation for the
quark-lepton mass splittings at the GUT scale based on a Froggatt-Nielsen
approach to flavour.Comment: 38 pages; v2: emphasised possibility of avoiding exotics in models
without a global E8 structure, added ref, journal versio
D-branes at Toric Singularities: Model Building, Yukawa Couplings and Flavour Physics
We discuss general properties of D-brane model building at toric
singularities. Using dimer techniques to obtain the gauge theory from the
structure of the singularity, we extract results on the matter sector and
superpotential of the corresponding gauge theory. We show that the number of
families in toric phases is always less than or equal to three, with a unique
exception being the zeroth Hirzebruch surface. With the physical input of three
generations we find that the lightest family of quarks is massless and the
masses of the other two can be hierarchically separated. We compute the CKM
matrix for explicit models in this setting and find the singularities possess
sufficient structure to allow for realistic mixing between generations and CP
violation.Comment: 55 pages, v2: typos corrected, minor comments adde
On two problems in graph Ramsey theory
We study two classical problems in graph Ramsey theory, that of determining
the Ramsey number of bounded-degree graphs and that of estimating the induced
Ramsey number for a graph with a given number of vertices.
The Ramsey number r(H) of a graph H is the least positive integer N such that
every two-coloring of the edges of the complete graph contains a
monochromatic copy of H. A famous result of Chv\'atal, R\"{o}dl, Szemer\'edi
and Trotter states that there exists a constant c(\Delta) such that r(H) \leq
c(\Delta) n for every graph H with n vertices and maximum degree \Delta. The
important open question is to determine the constant c(\Delta). The best
results, both due to Graham, R\"{o}dl and Ruci\'nski, state that there are
constants c and c' such that 2^{c' \Delta} \leq c(\Delta) \leq 2^{c \Delta
\log^2 \Delta}. We improve this upper bound, showing that there is a constant c
for which c(\Delta) \leq 2^{c \Delta \log \Delta}.
The induced Ramsey number r_{ind}(H) of a graph H is the least positive
integer N for which there exists a graph G on N vertices such that every
two-coloring of the edges of G contains an induced monochromatic copy of H.
Erd\H{o}s conjectured the existence of a constant c such that, for any graph H
on n vertices, r_{ind}(H) \leq 2^{c n}. We move a step closer to proving this
conjecture, showing that r_{ind} (H) \leq 2^{c n \log n}. This improves upon an
earlier result of Kohayakawa, Pr\"{o}mel and R\"{o}dl by a factor of \log n in
the exponent.Comment: 18 page
Dark Radiation and Dark Matter in Large Volume Compactifications
We argue that dark radiation is naturally generated from the decay of the
overall volume modulus in the LARGE volume scenario. We consider both
sequestered and non-sequestered cases, and find that the axionic superpartner
of the modulus is produced by the modulus decay and it can account for the dark
radiation suggested by observations, while the modulus decay through the
Giudice-Masiero term gives the dominant contribution to the total decay rate.
In the sequestered case, the lightest supersymmetric particles produced by the
modulus decay can naturally account for the observed dark matter density. In
the non-sequestered case, on the other hand, the supersymmetric particles are
not produced by the modulus decay, since the soft masses are of order the heavy
gravitino mass. The QCD axion will then be a plausible dark matter candidate.Comment: 27 pages, 4 figures; version 3: version published in JHE
Towards a Realistic Type IIA T^6/Z_4 Orientifold Model with Background Fluxes, Part 1: Moduli Stabilization
We apply the methods of DeWolfe et al. [hep-th/0505160] to a T^6/Z_4
orientifold model. This is the first step in an attempt to build a
phenomenologically interesting meta-stable de Sitter model with small
cosmological constant and standard model gauge groups.Comment: 1+30 pages, 2 figures, LaTeX, v2: minor corrections, stability
analysis of b_a moduli added, refs added, version accepted for publication in
JHE
- …