722 research outputs found
Matter-gravity interaction in a multiply warped braneworld,
The role of a bulk graviton in predicting the signature of extra dimensions
through collider-based experiments is explored in the context of a multiply
warped spacetime. In particular it is shown that in a doubly warped braneworld
model, the presence of the sixth dimension, results in enhanced concentration
of graviton Kaluza Klein (KK) modes compared to that obtained in the usual
5-dimensional Randall-Sundrum model. Also, the couplings of these massive
graviton KK modes with the matter fields on the visible brane turn out to be
appreciably larger than that in the corresponding 5- dimensional model. The
significance of these results are discussed in the context of KK graviton
search at the Large Hadron Collider (LHC).Comment: 13 pages, 2 table
Probing Large Extra Dimensions Using Top Production in Photon-Photon Collisions
Theories with large extra dimensions predict an infinite tower of
Kaluza-Klein states in the 1 TeV range, which can consequently have significant
implications for experimental observables. One such observable, which gets
affected by the exchange of spin-2 Kaluza-Klein particles, is the t t(bar)
production cross-section in photon-photon collisions at NLC energies. We study
this process and obtain bounds on the effective quantum gravity scale M_S
between 700 and 2400 GeV (depending on the centre-of-mass energy). We show that
the use of polarisation will further strengthen these bounds.Comment: 9 pages, latex, 2 ps figures, numerical errors corrected which has
improved the final result
Gap Equations and Electroweak Symmetry Breaking
Recently a new dynamical symmetry breaking model of electroweak interactions
was proposed based on interacting fermions. Two fermions of different SU(2)
representations form a symmetry breaking condensate and generate the lepton and
quark masses. The weak gauge bosons get their usual standard model masses from
a gauge invariant Lagrangian of a composite doublet scalar field. The new
fermion fields become massive by condensation. In this note the gap equations
are given in the linearized (mean field) approximation and the conditions for
symmetry breaking and mass generation are presented. Perturbative unitarity
constrains the self-couplings and the masses of the new fermions, a raw
spectrum is given.Comment: 10 pages, 4 figure
An A4 flavor model for quarks and leptons in warped geometry
We propose a spontaneous A4 flavor symmetry breaking scheme implemented in a
warped extra dimensional setup to explain the observed pattern of quark and
lepton masses and mixings. The main advantages of this choice are the
explanation of fermion mass hierarchies by wave function overlaps, the
emergence of tribimaximal neutrino mixing and zero quark mixing at the leading
order and the absence of tree-level gauge mediated flavor violations. Quark
mixing is induced by the presence of bulk flavons, which allow for cross-brane
interactions and a cross-talk between the quark and neutrino sectors, realizing
the spontaneous symmetry breaking pattern A4 --> nothing first proposed in
[X.G.\,He, Y.Y.\,Keum, R.R.\,Volkas, JHEP{0604}, 039 (2006)]. We show that the
observed quark mixing pattern can be explained in a rather economical way,
including the CP violating phase, with leading order cross-interactions, while
the observed difference between the smallest CKM entries V_{ub} and V_{td} must
arise from higher order corrections. We briefly discuss bounds on the
Kaluza-Klein scale implied by flavor changing neutral current processes in our
model and show that the residual little CP problem is milder than in flavor
anarchic models.Comment: 34 pages, 2 figures; version published in JHE
Flavour physics of the RS model with KK masses reachable at LHC
The version of the higher-dimensional Randall-Sundrum (RS) model with matter
in the bulk, which addresses the gauge hierarchy problem, has additional
attractive features. In particular, it provides an intrinsic geometrical
mechanism that can explain the origin of the large mass hierarchies among the
Standard Model fermions. Within this context, a good solution for the gauge
hierarchy problem corresponds to low masses for the Kaluza-Klein (KK)
excitations of the gauge bosons. Some scenarios have been proposed in order to
render these low masses (down to a few TeV) consistent with precision
electroweak measurements. Here, we give specific and complete realizations of
this RS version with small KK masses, down to 1 TeV, which are consistent with
the entire structure of the fermions in flavour space: (1) all the last
experimental data on quark/lepton masses and mixing angles (including massive
neutrinos of Dirac type) are reproduced, (2) flavour changing neutral current
constraints are satisfied and (3) the effective suppression scales of
non-renormalizable interactions (in the physical basis) are within the bounds
set by low energy flavour phenomenology. Our result, on the possibility of
having KK gauge boson modes as light as a few TeV, constitutes one of the first
theoretical motivations for experimental searches of direct signatures at the
LHC collider, of this interesting version of the RS model which accommodates
fermion masses.Comment: 27 pages, Latex file. References and comments adde
Leptons in Holographic Composite Higgs Models with Non-Abelian Discrete Symmetries
We study leptons in holographic composite Higgs models, namely in models
possibly admitting a weakly coupled description in terms of five-dimensional
(5D) theories. We introduce two scenarios leading to Majorana or Dirac
neutrinos, based on the non-abelian discrete group which is
responsible for nearly tri-bimaximal lepton mixing. The smallness of neutrino
masses is naturally explained and normal/inverted mass ordering can be
accommodated. We analyze two specific 5D gauge-Higgs unification models in
warped space as concrete examples of our framework. Both models pass the
current bounds on Lepton Flavour Violation (LFV) processes. We pay special
attention to the effect of so called boundary kinetic terms that are the
dominant source of LFV. The model with Majorana neutrinos is compatible with a
Kaluza-Klein vector mass scale TeV, which is roughly the
lowest scale allowed by electroweak considerations. The model with Dirac
neutrinos, although not considerably constrained by LFV processes and data on
lepton mixing, suffers from a too large deviation of the neutrino coupling to
the boson from its Standard Model value, pushing TeV.Comment: 37 pages, 4 figures; v2: Note added in light of recent T2K and MINOS
results, figures updated with new limit from MEG, references added, various
minor improvements, matches JHEP published versio
Composite Leptoquarks at the LHC
If electroweak symmetry breaking arises via strongly-coupled physics, the
observed suppression of flavour-changing processes suggests that fermion masses
should arise via mixing of elementary fermions with composite fermions of the
strong sector. The strong sector then carries colour charge, and may contain
composite leptoquark states, arising either as TeV scale resonances, or even as
light, pseudo-Nambu-Goldstone bosons. The latter, since they are coupled to
colour, get a mass of the order of several hundred GeV, beyond the reach of
current searches at the Tevatron. The same generic mechanism that suppresses
flavour-changing processes suppresses leptoquark-mediated rare processes,
making it conceivable that the many stringent constraints may be evaded. The
leptoquarks couple predominantly to third-generation quarks and leptons, and
the prospects for discovery at LHC appear to be good. As an illustration, a
model based on the Pati-Salam symmetry is described, and its embedding in
models with a larger symmetry incorporating unification of gauge couplings,
which provide additional motivation for leptoquark states at or below the TeV
scale, is discussed.Comment: 10 pp, version to appear in JHE
Effects of TeV Scale Gravity on e^+e^- --> W^+ W^-
We study the process e^+e^- --> W^+W^- to probe low-scale gravity at high
energy linear colliders. A characteristic signature of extra-dimensions models
is the forward-backward asymmetry due to the interference of spin-2 graviton
and spin-1 SM gauge boson exchange terms, even when right-polarised electron
beam is used. Our analysis shows that larger than 5% asymmetry is possible at a
linear collider with \sqrt{s}=500 (800) GeV if the mass scale, M_S is smaller
than 2.7 (4.5) TeV. W^- polarisation factors measured with a few percent
accuracy will also be able to put similar limits on M_S.Comment: 8 pages including figure
Baryon Number in Warped GUTs : Model Building and (Dark Matter Related) Phenomenology
In the past year, a new non-supersymmetric framework for electroweak symmetry
breaking (with or without Higgs) involving SU(2)_L * SU(2)_R * U(1)_{B-L} in
higher dimensional warped geometry has been suggested. In this work, we embed
this gauge structure into a GUT such as SO(10) or Pati-Salam. We showed
recently (in hep-ph/0403143) that in a warped GUT, a stable Kaluza-Klein
fermion can arise as a consequence of imposing proton stability. Here, we
specify a complete realistic model where this particle is a weakly interacting
right-handed neutrino, and present a detailed study of this new dark matter
candidate, providing relic density and detection predictions. We discuss
phenomenological aspects associated with the existence of other light (<~ TeV)
KK fermions (related to the neutrino), whose lightness is a direct consequence
of the top quark's heaviness. The AdS/CFT interpretation of this construction
is also presented. Most of our qualitative results do not depend on the nature
of the breaking of the electroweak symmetry provided that it happens near the
TeV brane.Comment: 61 pages, 12 figures; v2: minor changes; v3: Two additional diagrams
in Fig. 10; a numerical factor corrected in section 16.1 (baryogenesis
section), corresponding discussion slightly modified but qualitative results
unchange
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