133 research outputs found
Superheavy Dark Matter with Discrete Gauge Symmetries
We show that there are discrete gauge symmetries protect naturally heavy X
particles from decaying into the ordinary light particles in the supersymmetric
standard model. This makes the proposal very attractive that the superheavy X
particles constitute a part of the dark matter in the present universe. It is
more interesting that there are a class of discrete gauge symmetries which
naturally accommodate a long-lived unstable X particle. We find that in some
discrete Z_{10} models, for example, a superheavy X particle has lifetime
\tau_X \simeq 10^{11}-10^{26} years for its mass M_X \simeq 10^{13}-10^{14}
GeV. This long lifetime is guaranteed by the absence of lower dimensional
operators (of light particles) couple to the X. We briefly discuss a possible
explanation for the recently observed ultra-high-energy cosmic ray events by
the decay of this unstable X particle.Comment: 9 pages, Late
Gauging Away the Strong CP Problem
We propose a new solution to the strong-CP problem. It involves the existence
of an unbroken gauged symmetry whose gauge boson gets a Stuckelberg
mass term by combining with a pseudoscalar field . The latter has
axion-like couplings to so that the theta parameter may
be gauged away by a gauge transformation. This system leads to mixed
gauge anomalies and we argue that they are cancelled by the addition of an
appropriate Wess-Zumino term, so that no SM fermions need to be charged under
. We discuss scenarios in which the above set of fields and couplings
appear. The mechanism is quite generic, but a natural possibility is that the
the symmetry arises from bulk gauge bosons in theories with extra
dimensions or string models. We show that in certain D-brane Type-II string
models (with antisymmetric tensor field strength fluxes) higher dimensional
Chern-Simons couplings give rise to the required D=4 Wess-Zumino terms upon
compactification. In one of the possible string realizations of the mechanism
the gauge boson comes from the Kaluza-Klein reduction of the
eleven-dimensional metric in M-theory.Comment: 21 pages, latex, one eps figure; v2 improved discussio
Strongly Coupled Grand Unification in Higher Dimensions
We consider the scenario where all the couplings in the theory are strong at
the cut-off scale, in the context of higher dimensional grand unified field
theories where the unified gauge symmetry is broken by an orbifold
compactification. In this scenario, the non-calculable correction to gauge
unification from unknown ultraviolet physics is naturally suppressed by the
large volume of the extra dimension, and the threshold correction is dominated
by a calculable contribution from Kaluza-Klein towers that gives the values for
\sin^2\theta_w and \alpha_s in good agreement with low-energy data. The
threshold correction is reliably estimated despite the fact that the theory is
strongly coupled at the cut-off scale. A realistic 5d supersymmetric SU(5)
model is presented as an example, where rapid d=6 proton decay is avoided by
putting the first generation matter in the 5d bulk.Comment: 17 pages, latex, to appear in Phys. Rev.
Affleck-Dine baryogenesis in the local domain
For Affleck-Dine baryogenesis to proceed, there must have been two types of
phase transitions. One is the destabilized-stabilized phase transition of the
flat direction, which is in general induced by the Hubble parameter. The other
is the phase transition related to the A-term, which induces the misalignment
of the relative phase of the flat direction. In the conventional Affleck-Dine
baryogenesis they are supposed to start almost simultaneously. Of course these
phase transitions can take place separately, but the latter must not be later
than the former because the phase transition of the A-term can not produce any
baryon number when there is no condensate of the relative charge.
In this paper we try to construct models where the original idea of
Affleck-Dine baryogenesis is realized in a different way. We show examples in
which the local domain of the false vacuum with the required condensate is
formed after inflation and collapses in a safe way so that the domain wall
problem is avoided. We also show examples where the phase transition of the
A-term starts before the decay of the condensate. As in the conventional
Affleck-Dine mechanism, the phase transition of the A-term produces baryon
number in the local domain of the condensate. We construct scenarios where our
mechanism produces sufficient baryon asymmetry of the Universe.Comment: 18pages, latex2e, to appear in PR
Family Unification in Five and Six Dimensions
In family unification models, all three families of quarks and leptons are
grouped together into an irreducible representation of a simple gauge group,
thus unifying the Standard Model gauge symmetries and a gauged family symmetry.
Large orthogonal groups, and the exceptional groups and have been
much studied for family unification. The main theoretical difficulty of family
unification is the existence of mirror families at the weak scale. It is shown
here that family unification without mirror families can be realized in simple
five-dimensional and six-dimensional orbifold models similar to those recently
proposed for SU(5) and SO(10) grand unification. It is noted that a family
unification group that survived to near the weak scale and whose coupling
extrapolated to high scales unified with those of the Standard model would be
evidence accessible in principle at low energy of the existence of small
(Planckian or GUT-scale) extra dimensions.Comment: 13 pages, 2 figures, minor corrections, references adde
Gauge Coupling Unification from Unified Theories in Higher Dimensions
Higher dimensional grand unified theories, with gauge symmetry breaking by
orbifold compactification, possess SU(5) breaking at fixed points, and do not
automatically lead to tree-level gauge coupling unification. A new framework is
introduced that guarantees precise unification -- even the leading loop
threshold corrections are predicted, although they are model dependent. Precise
agreement with the experimental result, \alpha_s^{exp} = 0.117 \pm 0.002,
occurs only for a unique theory, and gives \alpha_s^{KK} = 0.118 \pm 0.004 \pm
0.003. Remarkably, this unique theory is also the simplest, with SU(5) gauge
interactions and two Higgs hypermultiplets propagating in a single extra
dimension. This result is more successful and precise than that obtained from
conventional supersymmetric grand unification, \alpha_s^{SGUT} = 0.130 \pm
0.004 \pm \Delta_{SGUT}. There is a simultaneous solution to the three
outstanding problems of 4D supersymmetric grand unified theories: a large mass
splitting between Higgs doublets and their color triplet partners is forced,
proton decay via dimension five operators is automatically forbidden, and the
absence of fermion mass relations amongst light quarks and leptons is
guaranteed, while preserving the successful m_b/m_\tau relation. The theory
necessarily has a strongly coupled top quark located on a fixed point and part
of the lightest generation propagating in the bulk. The string and
compactification scales are determined to be around 10^{17} GeV and 10^{15}
GeV, respectively.Comment: 29 pages, LaTe
The Supersymmetric Standard Models with Decay and Stable Dark Matters
We propose two supersymmetric Standard Models (SMs) with decaying and stable
dark matter (DM) particles. To explain the SM fermion masses and mixings and
have a heavy decay DM particle S, we consider the Froggatt-Nielsen mechanism by
introducing an anomalous U(1)_X gauge symmetry. Around the string scale, the
U(1)_X gauge symmetry is broken down to a Z_2 symmetry under which S is odd
while all the SM particles are even. S obtains a vacuum expectation value
around the TeV scale, and then it can three-body decay dominantly to the
second/third family of the SM leptons in Model I and to the first family of the
SM leptons in Model II. Choosing a benchmark point in the constrained minimal
supersymmetric SM with exact R parity, we show that the lightest neutralino DM
is consistent with the CDMS II experiment. Considering S three-body decay and
choosing suitable parameters, we show that the PAMELA and Fermi-LAT experiments
and the PAMELA and ATIC experiments can be explained in Model I and Model II,
respectively.Comment: RevTex4, 26 pages, 6 figures, references added, version to appear in
EPJ
Stringy Instantons and Cascading Quivers
D-brane instantons can perturb the quantum field theories on space-time
filling D-branes by interesting operators. In some cases, these D-brane
instantons are novel "stringy" effects (not interpretable directly as instanton
effects in the low-energy quantum field theory), while in others the D-brane
instantons can be directly interpreted as field theory effects. In this note,
we describe a situation where both perspectives are available, by studying
stringy instantons in quivers which arise at simple Calabi-Yau singularities.
We show that a stringy instanton which wraps an unoccupied node of the quiver,
and gives rise to a non-perturbative mass in the space-time field theory, can
be reinterpreted as a conventional gauge theory effect by going up in an
appropriate renormalization group cascade. Interestingly, in the cascade, the
contribution of the stringy instanton does not come from gauge theory
instantons but from strong coupling dynamics.Comment: 17 pages, 6 figures, harvma
Bounds on Four-Fermion Contact Interactions Induced by String Resonances
Based on tree-level open-string scattering amplitudes in the low string-scale
scenario, we derive the massless fermion scattering amplitudes. The amplitudes
are required to reproduce those of the Standard Model at tree level in the low
energy limit. We then obtain four-fermion contact interactions by expanding in
inverse powers of the string scale and explore the constraints on the string
scale from low energy data. The Chan-Paton factors and the string scale are
treated as free parameters. We find that data from the neutral and charged
current processes at HERA, Drell-Yan process at the Tevatron, and from LEP-II
put lower bounds on the string scale M_S >= 0.9-1.3 TeV for typical values of
the Chan-Paton factors, which are comparable to Tevatron bounds on Z' and W'
masses.Comment: 13 pages, 1 ps figure, version to appear in PR
Long Lived Superheavy Dark Matter with Discrete Gauge Symmetries
The recently observed ultra-high energy (UHE) cosmic rays beyond the
Greisen-Zatsepin-Kuzmin bound can be explained by the decays of some superheavy
particles forming a part of dark matter in our universe. We consider
various discrete gauge symmetries to ensure the required long
lifetime () of the particle to explain
the UHE cosmic rays in the minimal supersymmetric standard model (MSSM) with
massive Majorana neutrinos. We show that there is no anomaly-free discrete
gauge symmetry to make the lifetime of the particle sufficiently long in
the MSSM with the particle. We find, however, possible solutions to this
problem especially by enlarging the particle contents in the MSSM. We show a
number of solutions introducing an extra pair of singlets and
which have fractional (N=2,3) charges. The present experimental
constraints on the particle are briefly discussed.Comment: 27 pages, Late
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