22 research outputs found
On the spectra of scalar mesons from HQCD models
We determine the holographic spectra of scalar mesons from the fluctuations
of the embedding of flavor D-brane probes in HQCD models. The models we
consider include a generalization of the Sakai Sugimoto model at zero
temperature and at the "high-temperature intermediate phase", where the system
is in a deconfining phase while admitting chiral symmetry breaking and a
non-critical 6d model at zero temperature. All these models are based on
backgrounds associated with near extremal N_c D4 branes and a set of N_f<<N_c
flavor probe branes that admit geometrical chiral symmetry breaking. We point
out that the spectra of these models include a 0^{--} branch which does not
show up in nature.
At zero temperature we found that the masses of the mesons M_n depend on the
"constituent quark mass" parameter m^c_q and on the excitation number n as
M_n^2 m^c_q and M^2_n n^{1.7} for the ten dimensional case and as M_n m^c_q and
M_n n^{0.75} in the non-critical case. At the high temperature intermediate
phase we detect a decrease of the masses of low spin mesons as a function of
the temperature similar to holographic vector mesons and to lattice
calculations.Comment: 22 pages, 12 figure
Comments on Mesonic Correlators in the Worldline Formalism
We elaborate on how to incorporate mesonic correlators into the worldline
formalism. We consider possible applications to QCD-like theories in various
dimensions. We focus on large-N_c two dimensional QCD (the 't Hooft model) and
relate it to a single harmonic oscillator. We also discuss the dependence of
the Peskin S-parameter on the number of massless flavors and their
representation and compare our expression to the corresponding expression
obtained at weak coupling. Finally, we use the worldline formalism to discuss
how the Veneziano limit of QCD is realized in holography in the limit of small
N_f/N_c.Comment: 14 pages, LaTex. 6 eps figures. v2: minor changes, typos fixed. To
appear in Nuclear Physics
Holographic technicolor models and their S-parameter
We study the Peskin-Takeuchi S-parameter of holographic technicolor models.
We present the recipe for computing the parameter in a generalized holographic
setup. We then apply it to several holographic models that include: (a) the
Sakai-Sugimoto model and (b) its non-compactified cousin, (c) a non-critical
analog of (a) based on near extremal AdS_6 background, (d) the KMMW model which
is similar to model (a) but with D6 and anti-D6 flavor branes replacing the D8
and anti-D8 branes, (e) a model based on D5 branes compactified on two S^1s
with D7 and anti-7 probe branes and (f) the conifold model with the same probe
branes as in (e). The models are gravity duals of gauge theories with U(N_{TC})
gauge theory and with a breakdown of a flavor symmetry U(N_{TF})xU(N_{TF}) to
U_V(N_{TF}). The models (a), (c),(d) and (e) are duals of a confining gauge
theories whereas (b) and (f) associate with non confining models.
The S-parameter was found to be S=sN_{TC} where s is given by
0.017\lambda_{TC}, 0.016\lambda_{TC}, 0.095, 0.50 and 0.043 for the
(a),(b),(c),(d), (f) models respectively and for model (e) s is divergent.
These results are valid in the large N_{TC} and large \lambda_{TC} limit. We
further derive the dependence of the S-parameter on the "string endpoint" mass
of the techniquarks for the various models. We compute the masses of the low
lying vector technimesons.Comment: 37 pages, 2 figures V2: 2 coerrections in sectionss 4 and 5,
reference adde
Electroweak Symmetry Breaking from Gauge/Gravity Duality
We use the gauge/gravity duality to study a model of walking technicolor. The
latter is a phenomenologically promising framework for dynamical electroweak
symmetry breaking. A traditional problem for technicolor models has been the
need to address gauge theories at strong coupling. Recent developments in
gauge/gravity duality provide a powerful tool for handling this problem. First,
we revisit previously considered holographic models of QCD-like technicolor
from D-branes. In particular, we develop analytical understanding of earlier
numerical computations of the Peskin-Takeuchi S-parameter. Then we apply this
method to the investigation of a model of walking technicolor, obtained by
embedding D7 - anti-D7 probe branes in a recently discovered type IIB
background dual to walking behaviour. As a necessary step, we also show that
there is an embedding of the techniflavor branes, that realizes chiral symmetry
breaking. Finally, we show that the divergences that appear in the S-parameter
can be removed by using holographic renormalization.Comment: 33 pages; minor clarification, references added, journal versio
Constraining holographic technicolor
We obtain a new bound on the value of Peskin-Takeuchi S parameter in a wide
class of bottom-up holographic models for technicolor. Namely, we show that
weakly coupled holographic description in these models implies S>>0.2. Our
bound is in conflict with the results of electroweak precision measurements, so
it strongly disfavors the models we consider.Comment: 8 pages; journal versio
Scalar Mesons in Holographic Walking Technicolor
We study the spectrum of scalar mesons in the holographic dual of walking
technicolor, obtained by embedding D7 - anti-D7 probe branes in a certain type
IIB background. The scalar mesons arise from fluctuations of the probe
techniflavour branes and complement the (axial-)vector meson spectra that we
investigated in earlier work. By explicitly finding the spectrum of scalar
masses, we show that the nonsupersymmetric D7 - anti-D7 embedding is stable
with respect to such fluctuations. Interestingly, it turns out that the mass
splitting between the scalar and vector meson spectra is of subleading order in
a small parameter expansion. It is noteworthy that this near-degeneracy may not
be entirely due to a small amount of supersymmetry breaking and thus could
indicate the presence of some other (approximate) symmetry in the problem.Comment: 25 pages; minor improvements, references added, journal versio
The decay constant of the holographic techni-dilaton and the 125 GeV boson
We critically discuss the possibility that the 125 GeV boson recently
discovered at the LHC is the holographic techni-dilaton, a composite state
emerging from a strongly-coupled model of electroweak symmetry breaking. This
composite state differs from the SM for three main reasons. Its decay constant
is in general larger than the electroweak scale, hence suppressing all the
couplings to standard model particles with respect to an elementary Higgs
boson, with the exception of the coupling to photons and gluons, which is
expected to be larger than the standard-model equivalent.
We discuss three classes of questions. Is it possible to lower the decay
constant, by changing the geometry of the holographic model? Is it possible to
lower the overall scale of the strong dynamics, by modifying the way in which
electroweak symmetry breaking is implemented in the holographic model? Is there
a clear indication in the data that production mechanisms other than
gluon-gluon fusion have been observed, disfavoring models in which the
holographic techni-dilaton has a large decay constant?
We show that all of these questions are still open, given the present status
of theoretical as well as phenomenological studies, and that at present the
techni-dilaton hypothesis yields a fit to the data which is either as good as
the elementary Higgs hypothesis, or marginally better, depending on what sets
of data are used in the fit. We identify clear strategies for future work aimed
at addressing these three classes of open questions.
In the process, we also compute the complete scalar spectrum of the
two-scalar truncation describing the GPPZ model, as well as the decay constant
of the holographic techni-dilaton in this model.Comment: 23 pages, 7 figures. Two paragraphs of general comments added.
Several references added. Version accepted for publicatio
Holographic Conformal Window - A Bottom Up Approach
We propose a five-dimensional framework for modeling the background geometry
associated to ordinary Yang-Mills (YM) as well as to nonsupersymmetric gauge
theories possessing an infrared fixed point with fermions in various
representations of the underlying gauge group. The model is based on the
improved holographic approach, on the string theory side, and on the
conjectured all-orders beta function for the gauge theory one. We first analyze
the YM gauge theory. We then investigate the effects of adding flavors and show
that, in the holographic description of the conformal window, the geometry
becomes AdS when approaching the ultraviolet and the infrared regimes. As the
number of flavors increases within the conformal window we observe that the
geometry becomes more and more of AdS type over the entire energy range.Comment: 20 Pages, 3 Figures. v2: references adde
Attractive Holographic Baryons
We propose a holographic model of baryon interactions based on
non-supersymmetric D7-anti-D7 flavor branes embedded in the Klebanov-Strassler
background. The baryons are D3-branes wrapping the 3-sphere of the conifold
with M strings connecting the D3 and the flavor branes. Depending on the
location of the latter there are two possibilities: the D3 either remains
separate from the flavor branes or dissolves in them and becomes a flavor
instanton. The leading order interaction between the baryons is a competition
between the attraction and the repulsion due to the sigma and omega mesons. The
lightest 0++ particle sigma is a pseudo-Goldstone boson associated with the
spontaneous breaking of scale invariance. In a certain range of parameters it
is parametrically lighter than any other massive state. As a result at large
distances baryons attract each other. At short distances the potential admits a
repulsive core due to an exchange of the omega vector meson. We discuss baryon
coupling to glueballs, massive mesons and pions and point out the condition for
the model to have a small binding energy.Comment: 36 pages, 4 figure
Calculable mass hierarchies and a light dilaton from gravity duals
In the context of gauge/gravity dualities, we calculate the scalar and tensor mass spectrum of the boundary theory defined by a special 8-scalar sigma-model in five dimensions, the background solutions of which include the 1-parameter family dual to the baryonic branch of the Klebanov-Strassler field theory. This provides an example of a strongly-coupled, multi-scale system that yields a parametrically light mass for one of the composite scalar particles: the dilaton. We briefly discuss the implications of these findings towards identifying a satisfactory solution to both the big and little hierarchy problems of the electro-weak theory