2,187 research outputs found
Moduli and multi-field inflation
Moduli with flat or run-away classical potentials are generic in theories
based on supersymmetry and extra dimensions. They mix between themselves and
with matter fields in kinetic terms and in the nonperturbative superpotentials.
As the result, interesting structure appears in the scalar potential which
helps to stabilise and trap moduli and leads to multi-field inflation. The new
and attractive feature of multi-inflationary setup are isocurvature
perturbations which can modify in an interesting way the final spectrum of
primordial fluctuations resulting from inflation.Comment: 8 pages, 5 figures, based on talks given at CTP Symposium on
Supersymmetry at LHC (Cairo, March 11-14 2007) and String Phenomenology 2007
(Frascati, June 4-8 2007
"Big" Divisor D3/D7 Swiss Cheese Phenomenology
We review progress made over the past couple of years in the field of Swiss
Cheese Phenomenology involving a mobile space-time filling D3-brane and
stack(s) of fluxed D7-branes wrapping the "big" (as opposed to the "small")
divisor in (the orientifold of a) Swiss-Cheese Calabi-Yau. The topics reviewed
include reconciliation of large volume cosmology and phenomenology, evaluation
of soft supersymmetry breaking parameters, one-loop RG-flow equations'
solutions for scalar masses, obtaining fermionic (possibly first two
generations' quarks/leptons) mass scales in the O(MeV-GeV)-regime as well as
(first two generations') neutrino masses (and their one-loop RG flow) of around
an eV. The heavy sparticles and the light fermions indicate the possibility of
"split SUSY" large volume scenario.Comment: Invited review for MPLA, 14 pages, LaTe
Seeing the Invisible Axion in the Sparticle Spectrum
I describe how under favourable circumstances the invisible axion may
manifest its existence at the LHC through the sparticle spectrum; in particular
through a gluino \sim \ln (M_P/m_{3/2}) times heavier than other gauginos.Comment: 4 pages, REVTe
The moduli problem at the perturbative level
Moduli fields generically produce strong dark matter -- radiation and baryon
-- radiation isocurvature perturbations through their decay if they remain
light during inflation. We show that existing upper bounds on the magnitude of
such fluctuations can thus be translated into stringent constraints on the
moduli parameter space m_\sigma (modulus mass) -- \sigma_{inf} (modulus vacuum
expectation value at the end of inflation). These constraints are complementary
to previously existing bounds so that the moduli problem becomes worse at the
perturbative level. In particular, if the inflationary scale H_{inf}~10^{13}
GeV, particle physics scenarios which predict high moduli masses m_\sigma >
10-100 TeV are plagued by the perturbative moduli problem, even though they
evade big-bang nucleosynthesis constraints.Comment: 4 pages, 3 figures (revtex) -- v2: an important correction on the
amplitude/transfer of isocurvature modes at the end of inflation, typos
corrected, references added, basic result unchange
Spin Dynamics in Pyrochlore Heisenberg Antiferromagnets
We study the low temperature dynamics of the classical Heisenberg
antiferromagnet with nearest neighbour interactions on the pyrochlore lattice.
We present extensive results for the wavevector and frequency dependence of the
dynamical structure factor, obtained from simulations of the precessional
dynamics. We also construct a solvable stochastic model for dynamics with
conserved magnetisation, which accurately reproduces most features of the
precessional results. Spin correlations relax at a rate independent of
wavevector and proportional to temperature.Comment: 4 pages, 4 figures, submitted to PR
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
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
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
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