57 research outputs found
Tantalizing dilaton tests from a near-conformal EFT
The dilaton low-energy effective field theory (EFT) of an emergent light
scalar is probed in the paradigm of strongly coupled near-conformal gauge
theories. These studies are motivated by models which exhibit small
-functions near the conformal window (CW), perhaps with slow
scale-dependent walking and a light scalar with quantum numbers.
We report our results from the hypothesis of a dilaton inspired EFT analysis
with two massless fermions in the two-index symmetric (sextet) representation
of the SU(3) color gauge group. With important caveats in our conclusions,
conformal symmetry breaking entangled with chiral symmetry breaking would drive
the near-conformal infrared behavior of the theory predicting characteristic
dilaton signatures of the light scalar from broken scale invariance when probed
on relevant scales of fermion mass deformations. From a recently reasoned
choice of the dilaton potential in the EFT description~\cite{Golterman:2016lsd}
we find an unexpectedly light dilaton mass in the chiral limit at , set in units of the pion decay constant . Subject to further
statistical and systematic tests of continued post-conference analysis, this
result is significantly lower than our earlier estimates from less controlled
extrapolations of the light scalar (the -particle) to the massless
fermion limit of chiral perturbation theory. We also discuss important
distinctions between the dilaton EFT analysis and the linear -model
without dilaton signatures. For comparative reasons, we comment on dilaton
tests from recent work with fermions in the fundamental representation with
flavors.Comment: 14 pages, 34 figures, Proceedings of the 36th International Symposium
on Lattice Field Theory (Lattice 2018), July 22-28, 2018, East Lancing, USA;
elimination of some fit redundancies with minor changes in related figure
Can a light Higgs impostor hide in composite gauge models?
The frequently discussed strongly interacting gauge theory with a fermion
flavor doublet in the two-index symmetric (sextet) representation of the SU(3)
color gauge group is investigated. In previous studies the chiral condensate
and the mass spectrum were shown to be consistent with chiral symmetry breaking
(SB) at vanishing fermion mass. The recently reported -function is
not inconsistent with this observation, suggesting that the model is very close
to the conformal window and a light "Higgs impostor" could emerge as a
composite state. In this work we describe the methodology and preliminary
results of studying the emergence of the light composite scalar with
quantum numbers.Comment: 7 pages, Proceedings of the 31st International Symposium on Lattice
Field Theory - LATTICE 2013 06
A new method for the beta function in the chiral symmetry broken phase
We describe a new method to determine non-perturbatively the beta function of
a gauge theory using lattice simulations in the p-regime of the theory. This
complements alternative measurements of the beta function working directly at
zero fermion mass and bridges the gap between the weak coupling perturbative
regime and the strong coupling regime relevant to the mass spectrum of the
theory. We apply this method to gauge theory with two
fermion flavors in the 2-index symmetric (sextet) representation. We find that
the beta function is small but non-zero at the renormalized coupling value , consistent with our previous independent investigation using
simulations directly at zero fermion mass. The model continues to be a very
interesting explicit realization of the near-conformal composite Higgs paradigm
which could be relevant for Beyond Standard Model phenomenology.Comment: 8 pages, 7 figures; Proceedings of the 35th International Symposium
on Lattice Field Theory, 18-24 June 2017, Granada, Spai
The chiral condensate from the Dirac spectrum in BSM gauge theories
The eigenvalues of the Dirac operator at finite volume encode whether or not
chiral symmetry is spontaneously broken in a massless theory. We apply this
framework in a particular BSM context, namely SU(3) gauge theory with N_f=2
massless flavors in the 2-index symmetric (sextet) representation. Our first
results are at a single lattice spacing. We find that both the density of
near-zero eigenvalues and the renormalization group invariant mode number
indicate spontaneous symmetry breaking. Quantitatively, there is a discrepancy
between the determination of the fermion condensate in the chiral limit via the
eigenvalue spectrum and the determinations from direct measurements of the
chiral condensate and the GMOR relation. We comment on possible explanations of
this discrepancy and further refinements of this study.Comment: 7 pages, Proceedings of the 31st International Symposium on Lattice
Field Theory - LATTICE 201
Extended investigation of the twelve-flavor -function
We report new results from high precision analysis of an important BSM gauge
theory with twelve massless fermion flavors in the fundamental representation
of the SU(3) color gauge group. The range of the renormalized gauge coupling is
extended from our earlier work {Fodor:2016zil} to probe the existence of an
infrared fixed point (IRFP) in the -function reported at two different
locations, originally in {Cheng:2014jba} and at a new location in
{Hasenfratz:2016dou}. We find no evidence for the IRFP of the -function
in the extended range of the renormalized gauge coupling, in disagreement with
{Cheng:2014jba,Hasenfratz:2016dou}. New arguments to guard the existence of the
IRFP remain unconvincing {Hasenfratz:2017mdh}, including recent claims of an
IRFP with ten massless fermion flavors {Chiu:2016uui,Chiu:2017kza} which we
also rule out. Predictions of the recently completed 5-loop QCD
-function for general flavor number are discussed in this context.Comment: 7 pages, 9 figure
Spectroscopy of the BSM sextet model
As part of our ongoing lattice study of SU(3) gauge theory with two flavors
of fermions in the two-index symmetric representation (the sextet model), we
present the current status of the pseudoscalar particle spectrum. We use a
mixed action approach based on the gradient flow to control lattice artifacts,
allowing a simultaneous extrapolation to the chiral and continuum limits. We
find strong evidence that the pseudoscalar is a Goldstone boson state, with
spontaneously broken chiral symmetry and a non-zero Goldstone decay constant in
the chiral limit. In agreement with our study of the gauge coupling
function, we find the sextet model appears to be a near-conformal gauge theory
and serves as a prototype of the composite Higgs BSM template.Comment: 8 pages, 6 figures, Proceeding of the 35th International Symposium on
Lattice Field Theory, 18-24 June 2017, Granada, Spai
Weakly coupled conformal gauge theories on the lattice
Results are reported for the beta-function of weakly coupled conformal gauge
theories on the lattice, SU(3) with Nf=14 fundamental and Nf=3 sextet fermions.
The models are chosen to be close to the upper end of the conformal window
where perturbation theory is reliable hence a fixed point is expected. The
study serves as a test of how well lattice methods perform in the weakly
coupled conformal cases. We also comment on the 5-loop beta-function of two
models close to the lower end of the conformal window, SU(3) with Nf=12
fundamental and Nf=2 sextet fermions.Comment: 8 pages, 20 figures, Proceedings of the 35th International Symposium
on Lattice Field Theory, 18-24 June 2017, Granada, Spai
Conformal finite size scaling of twelve fermion flavors
Extended simulation results and their analysis are reported in a strongly
coupled gauge theory with twelve fermion flavors in the fundamental SU(3) color
representation. The conformality of the model is probed using mass deformed
conformal finite size scaling (FSS) theory driven by the fermion mass anomalous
dimension. Two independent conformal FSS fitting procedures are used in the
analysis. The first one deploys physics motivated scaling functions,
complemented by a second fitting procedure with spline based general B-forms
for the scaling functions. The results at fixed gauge coupling show unresolved
problems with the conformal hypothesis.Comment: 7 pages, Proceedings of the 30th International Symposium on Lattice
Field Theory, June 24 - 29, 2012, Cairns, Australi
Confining force and running coupling with twelve fundamental and two sextet fermions
We investigate two models of much recent interest in lattice Beyond Standard
Model studies: N_f=2 fermions in the 2-index symmetric (sextet) representation,
and N_f=12 fermions in the fundamental representation, both with SU(3) gauge
symmetry. We present results at fixed lattice spacing for the static fermion
potential and force as measured via lattice simulations. We show indications
that both models are confining in the chiral limit and that neither theory is
conformal. This is consistent with our findings for the mass spectrum, which
indicate that chiral symmetry is spontaneously broken in both theories.Comment: 7 pages, Proceedings of the 30th International Symposium on Lattice
Field Theory, June 24 - 29, 2012, Cairns, Australi
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