68 research outputs found

### Strong dynamics, composite Higgs and the conformal window

We review recent progress in the lattice investigations of near-conformal
non-abelian gauge theories relevant for dynamical symmetry breaking and model
building of composite Higgs models. The emphasis is placed on the mass spectrum
and the running renormalized coupling. The role of a light composite scalar
isosinglet particle as a composite Higgs particle is highlighted.Comment: 28 pages, 1 figure. Invited review for IJMPA special issue "Lattice
gauge theories beyond QCD.

### Multi-calorons and their moduli

Pure Yang-Mills instantons are considered on S^1 x R^3 -- so-called calorons.
The holonomy -- or Polyakov loop around the thermal S^1 at spatial infinity --
is assumed to be a non-centre element of the gauge group SU(n) as most
appropriate for QCD applications in the confined phase. It is shown that a
charge k caloron can be seen as a collection of nk massive magnetic monopoles
each carrying fractional topological charge. This interpretation offers a
physically appealing way of introducing monopole degrees of freedom into pure
gluodynamics: as constituents of finite temperature instantons. New and exact
solutions are found along with the fermionic zero-modes of the Dirac operator.
The properties of the zero-modes are analysed as well as the hyperkahler and
twistor geometry of the caloron moduli space. Lattice gauge theoretic
applications are also mentioned.Comment: PhD thesis, 109 pages, 24 figure

### SU(3) gauge theory with sextet fermions

SU(3) gauge theory coupled to N_f = 2 fermions in the sextet representation
is a promising candidate for a technicolor inspired Standard Model extension.
In this note the progress in the past few years aimed at understanding the
non-perturbative properties of the model is reviewed. The main difficulties
lying ahead in order to make robust conclusions from lattice simulations are
outlined.Comment: Contribution to the XXIX International Symposium on Lattice Field
Theory - Lattice 2011 (references added

### Computational efficiency of staggered Wilson fermions: A first look

Results on the computational efficiency of 2-flavor staggered Wilson fermions
compared to usual Wilson fermions in a quenched lattice QCD simulation on
$16^3\times32$ lattice at $\beta=6$ are reported. We compare the cost of
inverting the Dirac matrix on a source by the conjugate gradient (CG) method
for both of these fermion formulations, at the same pion masses, and without
preconditioning. We find that the number of CG iterations required for
convergence, averaged over the ensemble, is less by a factor of almost 2 for
staggered Wilson fermions, with only a mild dependence on the pion mass. We
also compute the condition number of the fermion matrix and find that it is
less by a factor of 4 for staggered Wilson fermions. The cost per CG iteration,
dominated by the cost of matrix-vector multiplication for the Dirac matrix, is
known from previous work to be less by a factor 2-3 for staggered Wilson
compared to usual Wilson fermions. Thus we conclude that staggered Wilson
fermions are 4-6 times cheaper for inverting the Dirac matrix on a source in
the quenched backgrounds of our study.Comment: v2: Major correction and revisions: we had overlooked a factor 1/4 in
the cost estimate for matrix-vector multiplication with the staggered Wilson
Dirac matrix. This gives an increased speed-up by a factor 4 for the overall
computation cost. 7 pages, 3 figures, presented at the 31st International
Symposium on Lattice Field Theory (Lattice 2013), 29 July - 3 August 2013,
Mainz, German

### 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 ($\chi{\rm SB}$) with its Goldstone spectrum,
$F_\pi$, the $\chi{\rm SB}$ 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 $\chi{\rm SB}$ 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

### 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

### 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

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