130 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
New Higgs physics from the lattice
We report the first results from our comprehensive lattice tool set to
explore non-perturbative aspects of Higgs physics in the Standard Model. We
demonstrate in Higgs-Yukawa models that Higgs mass lower bounds and upper
bounds can be determined in lattice simulations when triviality requires the
necessity of a finite cutoff to maintain non-zero interactions. The vacuum
instability problem is investigated and the lattice approach is compared with
the traditional renormalization group procedure which sets similar goals to
correlate lower and upper Higgs mass bounds with the scale of new physics. A
novel feature of our lattice simulations is the use of Ginsparg-Wilson fermions
to represent the effects of Top quark loops in Higgs dynamics. The need for
chiral lattice fermions is discussed and the approach is extended to full
Top-Higgs-QCD dynamics. We also report results from our large analysis of
Top-Higgs Yukawa models to gain analytic insight and to verify our new lattice
tool set which is deployed in the simulations. The role of non-perturbative
lattice studies to investigate heavy Higgs particle scenarios is illustrated in
extensions of the Standard Model.Comment: 28 pages, based on contributions from K. Holland, J. Kuti, D.
Nogradi, and C. Schroeder at The XXV International Symposium on Lattice Field
Theory, July 30 - August 4 2007, Regensburg, German
Probing technicolor theories with staggered fermions
One exciting possibility of new physics beyond the Standard Model is that the
fundamental Higgs sector is replaced by a strongly-interacting gauge theory,
known as technicolor. A viable theory must break chiral symmetry dynamically,
like in QCD, to generate Goldstone bosons which become the longitudinal
components of the W and Z. By measuring the eigenvalues of the Dirac operator,
one can determine if chiral symmetry is in fact spontaneously broken. We
simulate SU(3) gauge theory with n_s=2 and 3 staggered flavors in the
fundamental representation, corresponding to N_f=8 and 12 flavors in the
continuum limit. Although our first findings show that both theories are
consistent with dynamically broken chiral symmetry and QCD-like behavior,
flavor breaking effects in the spectrum may require further clarifications
before final conclusions can be drawn. We also compare various improved
staggered actions, to suppress this potentially large flavor breaking.Comment: 7 pages, 5 figures, talk presented at The XXVI International
Symposium on Lattice Field Theory, July 14 - 19, 2008, Williamsburg,
Virginia, US
Chiral symmetry breaking in fundamental and sextet fermion representations of SU(3) color
We report new results for lattice gauge theories with twelve fermion flavors
in the fundamental representation and two fermion flavors in the two-index
symmetric (sextet) representation of the SU(3) color gauge group. Both models
are important in searching for a viable composite Higgs mechanism in the Beyond
the Standard Model (BSM) paradigm. We subject both models to opposite
hypotheses inside and outside of the conformal window. In the first hypothesis
we test chiral symmetry breaking () with its Goldstone spectrum,
, the condensate, and several composite hadron states as
the fermion mass is varied in a limited range with our best effort to control
finite volume effects. Supporting results for from the running
coupling based on the force between static sources is also presented. In the
second test for the alternate hypothesis we probe conformal behavior driven by
a single anomalous mass dimension under the assumption of unbroken chiral
symmetry. Our results show very low level of confidence in the conformal
scenario.Comment: 14 pages, 12 figures. Based on talks presented by J.Kuti and
K.Holland at the XXVIII International Symposium on Lattice Field Theory,
Lattice2010, June 14-19, 2010, Villasimius, Ital
Chiral symmetry breaking in nearly conformal gauge theories
We present new results on chiral symmetry breaking in nearly conformal gauge
theories with fermions in the fundamental representation of the SU(3) color
gauge group. The number of fermion flavors is varied in an extended range below
the conformal window with chiral symmetry breaking () for all
flavors between and . To identify we apply
several methods which include, within the framework of chiral perturbation
theory, the analysis of the Goldstone spectrum in the p-regime and the spectrum
of the fermion Dirac operator with eigenvalue distributions of random matrix
theory in the \eps-regime. Chiral condensate enhancement is observed with
increasing when the electroweak symmetry breaking scale is held fixed
in technicolor language. Important finite-volume consistency checks from the
theoretical understanding of the rotator spectrum of the
-regime are discussed. We also consider these gauge theories at
inside the conformal window. The importance of understanding finite
volume, zero momentum gauge field dynamics inside the conformal window is
pointed out. Staggered lattice fermions with supressed taste breaking are used
throughout the simulations.Comment: 21 pages, 9 figures. Presented at the XXVII International Symposium
on Lattice Field Theory, July 26-31, 2009, Peking University, Beijin
Calculating the running coupling in strong electroweak models
One possibility for Beyond Standard Model physics is a new
strongly-interacting gauge theory. One way to determine if a non-abelian gauge
theory is QCD-like or conformal is to measure the running of the renormalized
gauge coupling. We define the renormalized coupling from Wilson loop ratios,
and measure these ratios via lattice simulations. We test this method in SU(3)
pure gauge theory and show some first results for simulations with dynamical
fermions in the fundamental representation.Comment: 7 pages, 5 figures. Presented at the XXVII International Symposium on
Lattice Field Theory, July 26-31, 2009, Peking University, Beijin
Nearly conformal electroweak sector with chiral fermions
SU(3) gauge theory with dynamical overlap fermions in the 2-index symmetric
(sextet) representation is considered. This model may be a viable model of the
electroweak symmetry breaking sector along the lines of the walking technicolor
paradigm. The number of fermion species is chosen such that the theory is
expected to be below the conformal window. We will discuss how the
epsilon-regime and random matrix theory can be used to test whether at any
given set of parameters (N_c, N_f, representation) the theory is in the
conformal phase or indeed just below it. Quenched Monte Carlo results are
included in the fundamental representation and also preliminary dynamical ones
in the 2-index symmetric representation.Comment: 7 pages, 2 figures, talk presented at The XXVI International
Symposium on Lattice Field Theory, July 14 - 19, 2008, Williamsburg,
Virginia, US
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
- …