1,620 research outputs found
Electronic circuitry for a high intensity flashlamp
Flash lamp with ballistic piston compressor for absorption spectroscopy measurements in ultraviolet regio
NAHE-based string models with SU(4) X SU(2) X U(1) SO(10) Subgroup
The orbifold GUT doublet-triplet splitting mechanism was discussed in 1994 in
the framework of the NAHE-based free fermionic models in which the SO(10) GUT
symmetry is broken to SO(6) X SO(4), SU(3) X SU(2) X U(1)^2, or SU(3) X U(1) X
SU(2)^2. In this paper we study NAHE-based free fermionic models in which the
SO(10) symmetry is broken at the string level to SU(4) X SU(2) X U(1). In
addition to the doublet-triplet splitting this case also has the advantage of
inducing the doublet-doublet splitting already at the string level. We
demonstrate, however, that NAHE-based models with SU(4) X SU(2) X U(1) SO(10)
subgroup are not viable. We show that, similarly to the LRS models, and in
contrast to the FSU5, PS and SLM models, the SU421 case gives rise to models
without an anomalous U(1) symmetry, and discuss the different cases in terms of
their N=4 origins.Comment: 25 pages. Standard Latex. Revised version to appear in NP
Stable Superstring Relics and Ultrahigh Energy Cosmic Rays
One of the most intriguing experimental results of recent years is the
observation of Ultrahigh Energy Cosmic Rays (UHECRs) above the GZK cutoff.
Plausible candidates for the UHECR primaries are the decay products of a
meta--stable matter state with mass of order O(10^{12-15 GeV}), which
simultaneously is a good cold dark matter candidate. We study possible
meta-stable matter states that arise from Wilson line breaking of GUT
symmetries in semi-realistic heterotic string models. In the models that we
study the exotic matter states can be classified according to patterns of
SO(10) symmetry breaking. We show that cryptons, which are states that carry
fractional electric charge , and are confined by a hidden gauge group
cannot produce viable dark matter. This is due to the fact that, in addition to
the lightest neutral bound state, cryptons give rise to meta-stable charged
bound states. However, these states may still account for the UHECR events. We
argue that the uniton, which is an exotic Standard Model quark but carries
``fractional'' U(1)_{Z'} charge, as well as the singleton, which is a Standard
Model singlet with ``fractional'' U(1)_{Z'} charge do provide viable dark
matter candidates and can at the same time explain the observed UHECR events.Comment: 24 pages. 5 figure
Towards String Predictions
The aim of superstring phenomenology is to develop the tools and methodology
needed to confront string theory with experimental data. The first mandatory
task is to find string solutions which reproduce the observable data. The
subsequent goal is to extract potential signatures beyond the observable data.
Recently, by studying exact flat directions of non-Abelian singlet fields, we
demonstrated the existence of free fermionic heterotic-string models in which
the -charged matter spectrum, just below the
string scale, consists solely of the MSSM spectrum. In this paper we study the
possibility that the exact flat directions leave a symmetry
unbroken at the Planck scale. We demonstrate in a specific example that such
unbroken is in general expected to be not of the GUT type but
of intrinsic stringy origin. We study its phenomenological characteristics and
the consequences in the case that remains unbroken down to
low energies. We suggest that observation in forthcoming colliders of a
, with universal couplings for the two light generations but
different couplings for the heavy generation may provide evidence for the
orbifold which underlies the free fermionic models.Comment: 18 pages. Standard Latex. References adde
String Inspired Z' Model With Stable Proton and Light Neutrino Masses
Grand unification, and its incarnation in the form of heterotic-string
unification, are the only extensions of the Standard Model that are rooted in
the structure of the Standard Model itself. In this context, it was proposed
that the exclusiveness of proton stability and suppression of neutrino masses
necessitates the existence of an additional U(1)_Z' symmetry, which is of
non-GUT origin and remains unbroken down to intermediate, or low, energies.
Realistic string models frequently give rise to non-GUT U(1) symmetries, which
arise from the flavor symmetries in the models. In this paper we demonstrate in
a string-inspired toy model that such a stringy Z' can indeed guarantee proton
longevity and viable phenomenology in the neutrino sector, as well as in the
quark and charged lepton sectors.Comment: 20 pages. Standard LaTex. Version to appear in NP
String Inspired Neutrino Mass Textures in Light of KamLAND and WMAP
Recent data from astrophysical and terrestrial experiments indicates large
mixing angles in the neutral lepton sector and restricts the allowed regions of
neutrino masses. In particular, the large mixing angles in the lepton sector
are disparate from the small mixing in the quark sector. This disparity is
unnatural from the point of view of grand unified theories, that are well
motivated by the Standard Model multiplet structure and logarithmic running of
its parameters. we examine the issue of this disparity from the perspective of
string derived SO(10) GUT models, in which the SO(10) symmetry is broken
directly at the string theory level. A characteristic feature of such models is
the appearance of numerous SO(10) singlet fields. We propose that the mismatch
between the quark and lepton mixing parameters arises due to this extended
singlet spectrum and its mixing with the right-handed neutrinos. We discuss a
string inspired effective parameterization of the extended neutrino mass matrix
and demonstrates that the coupling with the SO(10) singlet spectrum can readily
account for the neutrino flavor parameters. The mechanism implies that some
SO(10) singlet fields should exist at intermediate mass scales. We study the
possibilty of deriving the neutrino mass spectrum from string theory in a
specific string derived vacuum solution, and comment on the properties that
such a solution should possess.Comment: 16 pages. Standard Latex. Minor corrections. Version to appear in
Physics Letters
Self-Interacting Dark Matter from the Hidden Heterotic-String Sector
It has been suggested recently that self-interacting dark matter fits better
the observational characteristics of galaxy dynamics. We propose that the
self-interacting dark matter is composed from the glueballs of the hidden
sector non-Abelian gauge group, while the hidden matter states exist in
vector-like representation and decouple from the light spectrum. It is shown
that these glueballs are semi-stable with the life-time larger than the present
age of the Universe, if their mass is 1 GeV or less. The constraint on their
abundance today suggests that the energy was stored in the hidden sector soon
after inflation. This imposes an upper limit on the reheating temperature. We
further study the naturalness of this scenario in the context of the
free-fermionic string models and point out a class of such models where the
self-interacting dark matter from the hidden sector is indeed plausible.Comment: 16 pages. standard LaTe
Doublet-Triplet Splitting in Realistic Heterotic String Derived models
There has been recently a surge of interest in Grand Unified Theories on
orbifolds of higher dimensional spaces. In particular, the higher dimensional
doublet-triplet splitting mechanism has been of much interest. I revisit the
superstring doublet-triplet splitting mechanism in which the color triplets are
projected out by the GSO projections, while leaving the electroweak doublets in
the physical spectrum. The connection with the higher dimensional theories is
elucidated. It is shown that the doublet-triplet splitting depends crucially on
the assignment of boundary conditions in the compactified directions.Comment: 12 pages. Standard Latex. References adde
Yukawa couplings in SO(10) heterotic M-theory vacua
We demonstrate the existence of a class of N=1 supersymmetric nonperturbative
vacua of Horava-Witten M-theory compactified on a torus fibered Calabi-Yau
3-fold Z with first homotopy group \pi_{1}(Z)= Z2, having the following
properties: 1) SO(10) grand unification group, 2) net number of three
generations of chiral fermions in the observable sector, and 3) potentially
viable matter Yukawa couplings. These vacua correspond to semistable
holomorphic vector bundles V_{Z} over Z having structure group SU(4)_C, and
generically contain M5-branes in the bulk space. The nontrivial first homotopy
group allows Wilson line breaking of the SO(10) symmetry. Additionally, we
propose how the 11-dimensional Horava-Witten M-theory framework may be used to
extend the perturbative calculation of the top quark Yukawa coupling in the
realistic free-fermionic models to the nonperturbative regime. The basic
argument being that the relevant coupling couples twisted-twisted-untwisted
states and can be calculated at the level of the Z2 X Z2 orbifold without
resorting to the full three generation models.Comment: 24 pages. Standard LaTe
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