1,574 research outputs found
Physical States of the Quantum Conformal Factor
The conformal factor of the spacetime metric becomes dynamical due to the
trace anomaly of matter fields. Its dynamics is described by an effective
action which we quantize by canonical methods on the Einstein universe . We find an infinite tower of discrete states which satisfy the
constraints of quantum diffeomorphism invariance. These physical states are in
one-to-one correspondence with operators constructed by integrating integer
powers of the Ricci scalar.Comment: PlainTeX File, 34 page
Gauge Unification and Quark Masses in a Pati-Salam Model from Branes
We investigate the phase space of parameters in the Pati-Salam model derived
in the context of D-branes scenarios, requiring low energy string scale. We
find that a non-supersymmetric version complies with a string scale as low as
10 TeV, while in the supersymmetric version the string scale raises up to ~2 x
10^7 TeV. The limited energy region for RGE running demands a large tan(beta)
in order to have experimentally acceptable masses for the top and bottom
quarks.Comment: 11 pages, LaTeX, 7 figures include
Making Ends Meet: String Unification and Low-Energy Data
A long-standing problem in string phenomenology has been the fact that the
string unification scale disagrees with the GUT scale obtained by extrapolating
low-energy data within the framework of the minimal supersymmetric standard
model (MSSM). In this paper we examine several effects that may modify the
minimal string predictions and thereby bring string-scale unification into
agreement with low-energy data. These include heavy string threshold
corrections, non-standard hypercharge normalizations, light SUSY thresholds,
intermediate gauge structure, and thresholds arising from extra matter beyond
the MSSM. We explicitly evaluate these contributions within a variety of
realistic free-fermionic string models, including the flipped SU(5), SO(6) x
SO(4), and various SU(3) x SU(2) x U(1) models, and find that most of these
sources do not substantially alter the minimal string predictions. Indeed, we
find that the only way to reconcile string unification with low-energy data is
through certain types of extra matter. Remarkably, however, many of the
realistic string models contain precisely this required matter in their
low-energy spectra.Comment: 10 pages, standard LaTeX, 1 figure (Encapsulated PostScript), version
published in Phys. Rev. Lett. 75 (1995) 264
Bhabha Scattering with Radiated Gravitons at Linear Colliders
We study the process e+- e- -> e+- e- +- missing energy at a high-energy e+-
e- collider, where the missing energy arises from the radiation of Kaluza-Klein
gravitons in a model with large extra dimensions. It is shown that at a
high-energy linear collider, this process can not only confirm the signature of
such theories but can also sometimes be comparable in effectiveness to the
commonly discussed channel e+- e- -> gamma +- missing energy, especially for a
large number of extra dimensions and with polarized beams. We also suggest some
ways of distinguishing the signals of a graviton tower from other types of new
physics signals by combining data on our suggested channel with those on the
photon-graviton channel.Comment: 16 pages, LaTex, 8 figures embedded, typos, report no and references
correcte
Testable Flipped SU(5) x U(1)_X Models
The little hierarchy between the GUT scale and the string scale may give us
some hints that can be tested at the LHC. To achieve string-scale gauge
coupling unification, we introduce additional vector-like particles. We require
that these vector-like particles be standard, form complete GUT multiplets, and
have masses around the TeV scale or close to the string scale. Interestingly,
only the flipped SU(5) x U(1)_X models can work elegantly. We consider all
possible sets of vector-like particles with masses around the TeV scale. And we
introduce vector-like particles with masses close to the string scale which can
mimic the string-scale threshold corrections. We emphasize that all of these
vector-like particles can be obtained in the interesting flipped SU(5) x U(1)_X
string models from the four-dimensional free fermionic string construction.
Assuming the low-energy supersymmetry, high-scale supersymmetry, and split
supersymmetry, we show that the string-scale gauge coupling unification can
indeed be achieved in the flipped SU(5) x U(1)_X models. These models can be
tested at the LHC by observing simple sets of vector-like particles at the TeV
scale. Moreover, we discuss a simple flipped SU(5) x U(1)_X model with
string-scale gauge coupling unification and high-scale supersymmetry by
introducing only one pair of the vector-like particles at the TeV scale, and we
predict the corresponding Higgs boson masses. Also, we briefly comment on the
string-scale gauge coupling unification in the model with low-energy
supersymmetry by introducing only one pair of the vector-like particles at the
intermediate scale. And we briefly comment on the mixings among the SM fermions
and the corresponding extra vector-like particles.Comment: RevTex4, 25 pages, 4 figures, 4 tables, comments and references
added, version to appear in NP
Flipped and Unflipped SU(5) as Type IIA Flux Vacua
On Type IIA orientifolds with flux compactifications in supersymmetric AdS
vacua, we for the first time construct SU(5) models with three anti-symmetric
{\bf 10} representations and without symmetric {\bf 15} representations. We
show that all the pairs of the anti-fundamental {\bf \bar 5} and fundamental
{\bf 5} representations can obtain GUT/string-scale vector-like masses after
the additional gauge symmetry breaking via supersymmetry preserving Higgs
mechanism. Then we have exact three {\bf \bar 5}, and no other chiral exotic
particles that are charged under SU(5) due to the non-abelian anomaly free
condition. Moreover, we can break the SU(5) gauge symmetry down to the SM gauge
symmetry via D6-brane splitting, and solve the doublet-triplet splitting
problem. Assuming that the extra one (or several) pair(s) of Higgs doublets and
adjoint particles obtain GUT/string-scale masses via high-dimensional
operators, we only have the MSSM in the observable sector below the GUT scale.
Then the observed low energy gauge couplings can be generated via RGE running
if we choose the suitable grand unified gauge coupling by adjusting the string
scale. Furthermore, we construct the first flipped SU(5) model with exact three
{\bf 10}, and the first flipped SU(5) model in which all the Yukawa couplings
are allowed by the global U(1) symmetries.Comment: RevTex4, 34 pages, 17 table
Localized anomalies in orbifold gauge theories
We apply the path-integral formalism to compute the anomalies in general
orbifold gauge theories (including possible non-trivial Scherk-Schwarz boundary
conditions) where a gauge group G is broken down to subgroups H_f at the fixed
points y=y_f. Bulk and localized anomalies, proportional to \delta(y-y_f), do
generically appear from matter propagating in the bulk. The anomaly zero-mode
that survives in the four-dimensional effective theory should be canceled by
localized fermions (except possibly for mixed U(1) anomalies). We examine in
detail the possibility of canceling localized anomalies by the Green-Schwarz
mechanism involving two- and four-forms in the bulk. The four-form can only
cancel anomalies which do not survive in the 4D effective theory: they are
called globally vanishing anomalies. The two-form may cancel a specific class
of mixed U(1) anomalies. Only if these anomalies are present in the 4D theory
this mechanism spontaneously breaks the U(1) symmetry. The examples of five and
six-dimensional Z_N orbifolds are considered in great detail. In five
dimensions the Green-Schwarz four-form has no physical degrees of freedom and
is equivalent to canceling anomalies by a Chern-Simons term. In all other
cases, the Green-Schwarz forms have some physical degrees of freedom and leave
some non-renormalizable interactions in the low energy effective theory. In
general, localized anomaly cancellation imposes strong constraints on model
building.Comment: 30 pages, 4 figures. v2: reference adde
Effects of Extra Space-time Dimensions on the Fermi Constant
Effects of Kaluza-Klein excitations associated with extra dimensions with
large radius compactifications on the Fermi constant are explored. It is shown
that the current precision determinations of the Fermi constant, of the fine
structure constant, and of the W and Z mass put stringent constraints on the
compactification radius. The analysis excludes one extra space time dimension
below TeV, and excludes 2, 3 and 4 extra space dimensions opening
simultaneously below 3.5 TeV, 5.7 TeV and 7.8 TeV at the .
Implications of these results for future collider experiments are discussed.Comment: 12 pages including one figur
Conformal Invariance and Cosmic Background Radiation
The spectrum and statistics of the cosmic microwave background radiation
(CMBR) are investigated under the hypothesis that scale invariance of the
primordial density fluctuations should be promoted to full conformal
invariance. As in the theory of critical phenomena, this hypothesis leads in
general to deviations from naive scaling. The spectral index of the two-point
function of density fluctuations is given in terms of the quantum trace anomaly
and is greater than one, leading to less power at large distance scales than a
strict Harrison-Zel'dovich spectrum. Conformal invariance also implies
non-gaussian statistics for the higher point correlations and in particular, it
completely determines the large angular dependence of the three-point
correlations of the CMBR.Comment: 4 pages, Revtex file, uuencoded with one figur
Supersymmetry breaking on orbifolds from Wilson lines
We consider five dimensional theories compactified on the orbifold S^1/Z_2
and prove that spontaneous local supersymmetry breaking by Wilson lines and by
the Scherk-Schwarz mechanism are equivalent. Wilson breaking is triggered by
the SU(2)_R symmetry which is gauged in off-shell N=2 supergravity by auxiliary
fields. The super-Higgs mechanism disposes of the would-be Goldstinos which are
absorbed by the gravitinos to become massive. The breaking survives in the flat
limit, where we decouple all gravitational interactions, and the theory becomes
softly broken global supersymmetry.Comment: 9 pages, some comments in the discussion of the super-Higgs effect
and some references adde
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