31 research outputs found

### Back-door fine-tuning in supersymmetric low scale inflation

Low scale inflation has many virtues and it has been claimed that its natural
realisation in supersymmetric standard model can be achieved rather easily. In
this letter we have demonstrated that also in this case the dynamics of the
hidden sector responsible for supersymmetry breakdown and the structure of the
soft terms affects significantly, and in fact often spoils, the would-be
inflationary dynamics. Also, we point out that the issue if the cosmological
constant cancellation in the post-inflationary vacuum strongly affects
supersymmetric inflation. It is important to note the crucial difference
between freezing of the modulus and actually stabilising it - the first
approach misses parts of the scalar potential which turn out to be relevant for
inflation. We argue, that it is more likely that the low scale supersymmetric
inflation occurs at a critical point at the origin in the field space than at
an inflection point away from the origin, as the necessary fine-tuning in the
second case is typically larger.Comment: 10

### Radiatively induced leptogenesis in a minimal seesaw model

We study the possibility that the baryon asymmetry of the universe is
generated in a minimal seesaw scenario where two right-handed Majorana
neutrinos with degenerate masses are added to the standard model particle
content. In the usual framework of thermal leptogenesis, a nonzero $CP$
asymmetry can be obtained through the mass splitting induced by the running of
the heavy Majorana neutrino masses from their degeneracy scale down to the
seesaw scale. Although, in the light of the present neutrino oscillation data,
the produced baryon asymmetry turns out to be smaller than the experimental
value, the present mechanism could be viable in simple extensions of the
standard model.Comment: 6 pages, 2 figures, uses RevTeX4, calculations improved, comments
adde

### Suppressing Proton Decay in the Minimal SO(10) Model

We show that in a class of minimal supersymmetric SO(10) models which have
been found to be quite successful in predicting neutrino mixings, all proton
decay modes can be suppressed by a particular choice of Yukawa textures. This
suppression works for contributions from both left and right operators for
nucleon decay and for arbitrary \tan\beta. The required texture not only fits
all lepton and quark masses as well as CKM parameters but it also predicts
neutrino mixing parameter U_e3 and Dirac CP phase \sin|\delta_MNS| to be
0.07-0.09 and 0.3-0.7 respectively. We also discuss the relation between the
GUT symmetry breaking parameters for the origin of these textures.Comment: 7 pages, 2 figure

### Collective treatment of High Energy Thresholds in SUSY - GUTs

Supersymmetric GUTs are the most natural extension of the Standard model
unifying electroweak and strong forces. Despite their indubitable virtues,
among these the gauge coupling unification and the quantization of the electric
charge, one of their shortcomings is the large number of parameters used to
describe the high energy thresholds (HET), which are hard to handle. We present
a new method according to which the effects of the HET, in any GUT model, can
be described by fewer parameters that are randomly produced from the original
set of the parameters of the model. In this way, regions favoured by the
experimental data are easier to locate, avoiding a detailed and time consuming
exploration of the parameter space, which is multidimensional even in the most
economic unifying schemes. To check the efficiency of this method, we directly
apply it to a SUSY SO(10) GUT model in which the doublet-triplet splitting is
realized through the Dimopoulos-Wilczek mechanism. We show that the demand of
gauge coupling unification, in conjunction with precision data, locates regions
of the parameter space in which values of the strong coupling \astrong are
within the experimental limits, along with a suppressed nucleon decay, mediated
by a higgsino driven dimension five operators, yielding lifetimes that are
comfortably above the current experimental bounds. These regions open up for
values of the SUSY breaking parameters m_0, M_1/2 < 1 TeV being therefore
accessible to LHC.Comment: 21 pages, 8 figures, UA-NPPS/BSM-10/02 (added

### Kahler potentials for the MSSM inflation and the spectral index

Recently it has been argued that some of the fine-tuning problems of the MSSM
inflation associated with the existence of a saddle point along a flat
direction may be solved naturally in a class of supergravity models. Here we
extend the analysis and show that the constraints on the Kahler potentials in
these models are considerably relaxed when the location of the saddle point is
treated as a free variable. We also examine the effect of supergravity
corrections on inflationary predictions and find that they can slightly alter
the value of the spectral index. As an example, for flat direction field values
$|\bar{\phi}_0|=1\times10^{-4}M_P$ we find $n\sim0.92 ... 0.94$ while the
prediction of the MSSM inflation without any corrections is $n\sim0.92$.Comment: 13 pages, one figure. Typos corrected and a reference adde

### Quintessential Kination and Leptogenesis

Thermal leptogenesis induced by the CP-violating decay of a right-handed
neutrino (RHN) is discussed in the background of quintessential kination, i.e.,
in a cosmological model where the energy density of the early Universe is
assumed to be dominated by the kinetic term of a quintessence field during some
epoch of its evolution. This assumption may lead to very different
observational consequences compared to the case of a standard cosmology where
the energy density of the Universe is dominated by radiation. We show that,
depending on the choice of the temperature T_r above which kination dominates
over radiation, any situation between the strong and the super--weak wash--out
regime are equally viable for leptogenesis, even with the RHN Yukawa coupling
fixed to provide the observed atmospheric neutrino mass scale ~ 0.05 eV. For M<
T_r < M/100, i.e., when kination stops to dominate at a time which is not much
later than when leptogenesis takes place, the efficiency of the process,
defined as the ratio between the produced lepton asymmetry and the amount of CP
violation in the RHN decay, can be larger than in the standard scenario of
radiation domination. This possibility is limited to the case when the neutrino
mass scale is larger than about 0.01 eV. The super--weak wash--out regime is
obtained for T_r << M/100, and includes the case when T_r is close to the
nucleosynthesis temperature ~ 1 MeV. Irrespective of T_r, we always find a
sufficient window above the electroweak temperature T ~ 100 GeV for the
sphaleron transition to thermalize, so that the lepton asymmetry can always be
converted to the observed baryon asymmetry.Comment: 13 pages, 8 figure

### Detailed Analysis of Proton Decay Rate in the Minimal Supersymmetric SO(10) Model

We consider the minimal supersymmetric SO(10) model, where only one {\bf 10}
and one $\bar{\bf 126}$ Higgs multiplets have Yukawa couplings with matter
multiplets. This model has the high predictive power for the Yukawa coupling
matrices consistent with the experimental data of the charged fermion mass
matrices, and all the Yukawa coupling matrices are completely determined once a
few parameters in the model are fixed. This feature is essential for definite
predictions to the proton decay rate through the dimension five operators. We
analyze the proton decay rate for the dominant decay modes $p \to K^{+}
\bar{\nu}$ by including as many free parameters as possible and varying them.
There are two free parameters in the Yukawa sector, while five in the Higgsino
sector. It is found that an allowed region exists when the free parameters in
the Higgs sector are tuned so as to cancel the proton decay amplitude. The
resultant proton lifetime is proportional to $1/\tan^2 \beta$ and the allowed
region eventually disappears as $\tan \beta$ becomes large.Comment: 15 pages, 3 figures; the version to appear in JHE

### Charged Lepton Electric Dipole Moments from TeV Scale Right-handed Neutrinos

We study the connection between charged lepton electric dipole moments, $d_l$
$(l=e,\mu,\tau)$, and seesaw neutrino mass generation in a simple two Higgs
doublet extension of the Standard Model plus three right-handed neutrinos (RHN)
$N_a$, $a=1,2,3$. For RHN with hierarchical masses and at least one with mass
in the 10 TeV range we obtain the upper bounds of $|d_e|< 9\times 10^{-30}$
e-cm and $|d_{\mu}|<2 \times 10^{-26}$ e-cm. Our scenario favors the normal
mass hierarchy for the light neutrinos. We also calculated the cross section
for e^-e^- \ra W^- W^- in a high luminosity collider with constraints from
neutrinoless double beta decay of nuclei included. Among the rare muon decay
experiments we find that \mu\ra e\gamma is most sensitive and the upper limit
is $<8\times 10^{-13}$.Comment: references added, 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