47 research outputs found
Constraints on the Bulk Standard Model in the Randall-Sundrum Scenario
We derive constraints on the Randall-Sundrum scenario with the standard model
fields in the bulk. These result from tree level effects associated with the
deformation of the zero mode wave-functions of the W and the Z once electroweak
symmetry is broken. Recently Cs\'{a}ki, Erlich and Terning pointed out that
this implies large contributions to electroweak oblique parameters. Here we
find that when fermions are allowed in the bulk the couplings of the and
the to zero-mode fermions are also affected. We perform a fit to
electroweak observables assuming universal bulk fermion masses and including
all effects and find constraints that are considerably stronger than for the
case with fermions localized in the low energy boundary. These put the lowest
Kaluza-Klein excitation out of reach of the Large Hadron Collider. We then
relax the universality assumption and study the effects of flavor violation in
the bulk and its possible signatures.Comment: 18 pages, 2 ps figure
Search for TeV Strings and New Phenomena in Bhabha Scattering at LEP2
A combined analysis of the data on Bhabha scattering at centre-of-mass
energies 183 and 189 GeV from the LEP experiments ALEPH, L3 and OPAL is
performed to search for effects of TeV strings in quantum gravity models with
large extra dimensions. No statistically significant deviations from the
Standard Model expectations are observed and lower limit on the string scale
M_S = 0.631 TeV at 95 % confidence level is derived. The data are used to set
lower limits on the scale of contact interactions ranging from 4.2 to 16.2 TeV
depending on the model. In a complementary analysis we derive an upper limit on
the electron size of 2.8 x 10^{-19} m at 95 % confidence level.Comment: 10 pages, 1 table, 3 figure
Singularities In Scalar-Tensor Cosmologies
In this article, we examine the possibility that there exist special
scalar-tensor theories of gravity with completely nonsingular FRW solutions.
Our investigation in fact shows that while most probes living in such a
Universe never see the singularity, gravity waves always do. This is because
they couple to both the metric and the scalar field, in a way which effectively
forces them to move along null geodesics of the Einstein conformal frame. Since
the metric of the Einstein conformal frame is always singular for
configurations where matter satisfies the energy conditions, the gravity wave
world lines are past inextendable beyond the Einstein frame singularity, and
hence the geometry is still incomplete, and thus singular. We conclude that the
singularity cannot be entirely removed, but only be made invisible to most, but
not all, probes in the theory.Comment: 23 pages, latex, no figure
Opaque Branes in Warped Backgrounds
We examine localized kinetic terms for gauge fields which can propagate into
compact, warped extra dimensions. We show that these terms can have a relevant
impact on the values of the Kaluza-Klein (KK) gauge field masses, wave
functions, and couplings to brane and bulk matter. The resulting
phenomenological implications are discussed. In particular, we show that the
presence of opaque branes, with non-vanishing brane-localized gauge kinetic
terms, allow much lower values of the lightest KK mode than in the case of
transparent branes. Moreover, we show that if the large discrepancies among the
different determinations of the weak mixing angle would be solved in favor of
the value obtained from the lepton asymmetries, bulk electroweak gauge fields
in warped-extra dimensions may lead to an improvement of the agreement of the
fit to the electroweak precision data for a Higgs mass of the order of the weak
scale and a mass of the first gauge boson KK excitation most likely within
reach of the LHC.Comment: 37 pages, 12 figures, improved analysis of the precision electroweak
constraint
New extended superconformal sigma models and Quaternion Kahler manifolds
Quaternion Kahler manifolds are known to be the target spaces for matter
hypermultiplets coupled to N=2 supergravity. It is also known that there is a
one-to-one correspondence between 4n-dimensional quaternion Kahler manifolds
and those 4(n+1)-dimensional hyperkahler spaces which are the target spaces for
rigid superconformal hypermultiplets (such spaces are called hyperkahler
cones). In this paper we present a projective-superspace construction to
generate a hyperkahler cone M^{4(n+1)}_H of dimension 4(n+1) from a
2n-dimensional real analytic Kahler-Hodge manifold M^{2n}_K. The latter emerges
as a maximal Kahler submanifold of the 4n-dimensional quaternion Kahler space
M^{4n}_Q such that its Swann bundle coincides with M^{4(n+1)}_H. Our approach
should be useful for the explicit construction of new quaternion Kahler
metrics. The results obtained are also of interest, e.g., in the context of
supergravity reduction N=2 --> N=1, or alternatively from the point of view of
embedding N=1 matter-coupled supergravity into an N=2 theory.Comment: 30 page
D terms from D-branes, gauge invariance and moduli stabilization in flux compactifications
We elucidate the structure of D terms in N=1 orientifold compactifications
with fluxes. As a case study, we consider a simple orbifold of the type-IIA
theory with D6-branes at angles, O6-planes and general NSNS, RR and
Scherk-Schwarz geometrical fluxes. We examine in detail the emergence of D
terms, in their standard supergravity form, from an appropriate limit of the
D-brane action. We derive the consistency conditions on gauged symmetries and
general fluxes coming from brane-localized Bianchi identities, and their
relation with the Freed-Witten anomaly. We extend our results to other N=1
compactifications and to non-geometrical fluxes. Finally, we discuss the
possible role of U(1) D terms in the stabilization of the untwisted moduli from
the closed string sector.Comment: 1+31 pages, 1 figur
Quantized bulk fermions in the Randall-Sundrum brane model
The lowest order quantum corrections to the effective action arising from
quantized massive fermion fields in the Randall-Sundrum background spacetime
are computed. The boundary conditions and their relation with gauge invariance
are examined in detail. The possibility of Wilson loop symmetry breaking in
brane models is also analysed. The self-consistency requirements, previously
considered in the case of a quantized bulk scalar field, are extended to
include the contribution from massive fermions. It is shown that in this case
it is possible to stabilize the radius of the extra dimensions but it is not
possible to simultaneously solve the hierarchy problem, unless the brane
tensions are dramatically fine tuned, supporting previous claims.Comment: 25 pages, 1 figure, RevTe
Neutrinoless Double Beta Decay from Singlet Neutrinos in Extra Dimensions
We study the model-building conditions under which a sizeable
-decay signal to the recently reported level of~0.4 eV is due
to Kaluza--Klein singlet neutrinos in theories with large extra dimensions. Our
analysis is based on 5-dimensional singlet-neutrino models compactified on an
orbifold, where the Standard--Model fields are localized on a
3-brane. We show that a successful interpretation of a positive signal within
the above minimal 5-dimensional framework would require a non-vanishing shift
of the 3-brane from the orbifold fixed points by an amount smaller than the
typical scale (100 MeV) characterizing the Fermi nuclear momentum. The
resulting 5-dimensional models predict a sizeable effective Majorana-neutrino
mass that could be several orders of magnitude larger than the light neutrino
masses. Most interestingly, the brane-shifted models with only one bulk sterile
neutrino also predict novel trigonometric textures leading to mass scenarios
with hierarchical active neutrinos and large - and
- mixings that can fully explain the current atmospheric and
solar neutrino data.Comment: 33 pages, LaTeX, minor rewordings, references adde
Dark Matter with Dirac and Majorana Gaugino Masses
We consider the minimal supersymmetric extension of the Standard Model
allowing both Dirac and Majorana gauginos. The Dirac masses are obtained by
pairing up extra chiral multiplets: a singlet S for U(1)_Y, a triplet T for
SU(2) and an octet O for SU(3) with the respective gauginos. The electroweak
symmetry breaking sector is modified by the couplings of the new fields S and T
to the Higgs doublets. We discuss two limits: i) both the adjoint scalars are
decoupled with the main effect being the modification of the Higgs quartic
coupling; ii) the singlet remaining light, and due to its direct coupling to
sfermions, providing a new contribution to the soft masses and inducing new
decay/production channels. We discuss the LSP in this scenario; after
mentioning the possibility that it may be a Dirac gravitino, we focus on the
case where it is identified with the lightest neutralino, and exhibit
particular values of the parameter space where the relic density is in
agreement with WMAP data. This is illustrated for different scenarios where the
LSP is either a bino (in which case it can be a Dirac fermion) or
bino-higgsino/wino mixtures. We also point out in each case the peculiarity of
the model with respect to dark matter detection experiments.Comment: 43 pages, 5 figures; one reference added. Corresponds to published
version in JCA
Supersymmetric Regularization, Two-Loop QCD Amplitudes and Coupling Shifts
We present a definition of the four-dimensional helicity (FDH) regularization
scheme valid for two or more loops. This scheme was previously defined and
utilized at one loop. It amounts to a variation on the standard 't
Hooft-Veltman scheme and is designed to be compatible with the use of helicity
states for "observed" particles. It is similar to dimensional reduction in that
it maintains an equal number of bosonic and fermionic states, as required for
preserving supersymmetry. Supersymmetry Ward identities relate different
helicity amplitudes in supersymmetric theories. As a check that the FDH scheme
preserves supersymmetry, at least through two loops, we explicitly verify a
number of these identities for gluon-gluon scattering (gg to gg) in
supersymmetric QCD. These results also cross-check recent non-trivial two-loop
calculations in ordinary QCD. Finally, we compute the two-loop shift between
the FDH coupling and the standard MS-bar coupling, alpha_s. The FDH shift is
identical to the one for dimensional reduction. The two-loop coupling shifts
are then used to obtain the three-loop QCD beta function in the FDH and
dimensional reduction schemes.Comment: 44 pages, minor corrections and clarifications include