22 research outputs found
Approximate gauge symmetry of composite vector bosons
It can be shown in a solvable field theory model that the couplings of the
composite vector bosons made of a fermion pair approach the gauge couplings in
the limit of strong binding. Although this phenomenon may appear accidental and
special to the vector boson made of a fermion pair, we extend it to the case of
bosons being constituents and find that the same phenomenon occurs in more an
intriguing way. The functional formalism not only facilitates computation but
also provides us with a better insight into the generating mechanism of
approximate gauge symmetry, in particular, how the strong binding and global
current conservation conspire to generate such an approximate symmetry. Remarks
are made on its possible relevance or irrelevance to electroweak and higher
symmetries.Comment: Correction of typos. The published versio
It is a Graviton! or maybe not
The discovery of Kaluza-Klein (KK) gravitons is a smoking gun of extra
dimensions. Other scenarios, however, could give rise to spin-two resonances of
a new strongly-coupled sector and act as impostors. In this paper we prove that
a spin-two resonance does not couple to the Standard Model through
dimension-four operators. We then show that the massive graviton and its
impostor both couple to the Standard Model through the same dimension-five
operators. Therefore the spin determination is identical. Nevertheless, we also
show that one can use the ratio of branching ratios to photons and to jets for
distinguishing between KK gravitons and their impostors. The capacity to
distinguish between KK gravitons and impostors is a manifestation of the
breakdown of the duality between AdS and strongly-coupled theories.Comment: 14 pages, 3 figures, 1 table. References added, typos correcte
Fermion Masses in Emergent Electroweak Symmetry Breaking
We consider the generation of fermion masses in an emergent model of
electroweak symmetry breaking with composite gauge bosons. A universal
bulk fermion profile in a warped extra dimension is used for all fermion
flavors. Electroweak symmetry is broken at the UV (or Planck) scale where
boundary mass terms are added to generate the fermion flavor structure. This
leads to flavor-dependent nonuniversality in the gauge couplings. The effects
are suppressed for the light fermion generations but are enhanced for the top
quark where the and couplings can deviate at the
level in the minimal setup. By the AdS/CFT correspondence our model
implies that electroweak symmetry is not a fundamental gauge symmetry. Instead
the Standard Model with massive fermions and gauge bosons is an effective
chiral Lagrangian for some underlying confining strong dynamics at the TeV
scale, where mass is generated without a Higgs mechanism.Comment: modified discussion in Sec 3.1, version published in JHE
Gravity-mediated (or composite) Dark Matter confronts astrophysical data
We consider the astrophysical bounds on a new form of dark matter, the so called Gravity-mediated Dark Matter. In this scenario, dark matter communicates with us through a mediator sector composed of gravitational resonances, namely a new scalar (radion) and a massive spin-two resonance (massive graviton). We consider specific models motivated by natural electroweak symmetry breaking or weak-scale dark matter in the context of models in warped extra-dimensions and their composite duals. The main Dark Matter annihilation mechanism is due to the interactions of KK gravitons to gauge bosons that propagate in bulk. We impose the bounds on monochromatic or continuum photons from Fermi-LAT and HESS. We also explore scenarios in which the Fermi gamma-ray line could be a manifestation of Gravity-mediated Dark Matter
Low-Energy Probes of a Warped Extra Dimension
We investigate a natural realization of a light Abelian hidden sector in an
extended Randall-Sundrum (RS) model. In addition to the usual RS bulk we
consider a second warped space containing a bulk U(1)_x gauge theory with a
characteristic IR scale of order a GeV. This Abelian hidden sector can couple
to the standard model via gauge kinetic mixing on a common UV brane. We show
that if such a coupling induces significant mixing between the lightest U(1)_x
gauge mode and the standard model photon and Z, it can also induce significant
mixing with the heavier U(1)_x Kaluza-Klein (KK) modes. As a result it might be
possible to probe several KK modes in upcoming fixed-target experiments and
meson factories, thereby offering a new way to investigate the structure of an
extra spacetime dimension.Comment: 26 pages, 1 figure, added references, corrected minor typos, same as
journal versio
Warped dipole completed, with a tower of Higgs bosons
In the context of warped extra-dimensional models which address both the Planck-weak- and flavor-hierarchies of the Standard Model (SM), it has been argued that certain observables can be calculated within the 5D effective field theory only with the Higgs field propagating in the bulk of the extra dimension, just like other SM fields. The related studies also suggested an interesting form of decoupling of the heavy Kaluza-Klein (KK) fermion states in the warped 5D SM in the limit where the profile of the SM Higgs approaches the IR brane. We demonstrate that a similar phenomenon occurs when we include the mandatory KK excitations of the SM Higgs in loop diagrams giving dipole operators for SM fermions, where the earlier work only considered the SM Higgs (zero mode). In particular, in the limit of a quasi IR-localized SM Higgs, the effect from summing over KK Higgs modes is unsuppressed (yet finite), in contrast to the naive expectation that KK Higgs modes decouple as their masses become large. In this case, a wide range of KK Higgs modes have quasi-degenerate masses and enhanced couplings to fermions relative to those of the SM Higgs, which contribute to the above remarkable result. In addition, we find that the total contribution from KK Higgs modes in general can be comparable to that from the SM Higgs alone. It is also interesting that KK Higgs couplings to KK fermions of the same chirality as the corresponding SM modes have an unsuppressed overall contribution, in contrast to the result from the earlier studies involving the SM Higgs. Our studies suggest that KK Higgs bosons are generally an indispensable part of the warped 5D SM, and their phenomenology such as signals at the LHC are worth further investigation
Anomaly Mediation and Cosmology
We consider an extension of the MSSM wherein anomaly mediation is the source
of supersymmetry breaking, and the tachyonic slepton problem is solved by a
gauged U(1) symmetry, which is broken at high energies in a manner preserving
supersymmetry, thereby also facilitating the see-saw mechanism for neutrino
masses and a natural source for the Higgs mu-term. We show that these
favourable outcomes can occur both in the presence and the absence of a large
Fayet-Iliopoulos (FI) D-term associated with the new U(1). We explore the
cosmological consequences of the model, showing that it naturally produces a
period of hybrid inflation, terminating in the production of cosmic strings. In
spite of the presence of a U(1) (even with an FI term), inflation is effected
by the F-term, with a D-flat tree potential (the FI term, if present, being
cancelled by non-zero squark and slepton fields). Calculating the 1-loop
corrections to the inflaton potential, we estimate the constraints on the
parameters of the model from Cosmic Microwave Background data. We will see that
a consequence of these constraints is that the Higgs mu-term necessarily small.
We briefly discuss the mechanisms for baryogenesis via conventional
leptogenesis, the out-of-equilibrium production of neutrinos from the cosmic
strings, or the Affleck-Dine mechanism. Cosmic string decays also boost the
relic density of dark matter above the low value normally obtained in AMSB
scenarios.Comment: 34 pages. Revised to incorporate discussion of the case when the
Fayet-Ilipoulos term is absen