218 research outputs found
Numerical results for gauge theories near the conformal window
A novel strong interaction beyond the standard model could provide a
dynamical explanation of electroweak symmetry breaking. Experimental results
strongly constrain properties of models that realise this mechanism. Whether
these constraints are obeyed by any strongly interacting quantum field theory
is a non-perturbative problem that needs to be addressed by first-principle
calculations. Monte Carlo simulations of lattice regularised gauge theories is
a powerful tool that enables us to address this question. Recently various
lattice investigations have appeared that have studied candidate models of
strongly interacting dynamics beyond the standard model. After a brief review
of the main methods and of some recent results, we focus on the analysis of
SU(2) gauge theory with one adjoint Dirac fermion flavour, which is shown to
have a near-conformal behaviour with an anomalous dimension of order one. The
implications of our findings are also discussed
Universality of k-string Tensions from Holography and the Lattice
We consider large Wilson loops with quarks in higher representations in SU(N)
Yang-Mills theories. We consider representations with common N-ality and check
whether the expectation value of the Wilson loop depends on the specific
representation or only on the N-ality. In the framework of AdS/CFT we show that
= dim R exp -sigma_k A, namely that the string tension depends only on
the N-ality k but the pre-exponent factor is representation dependent. The
lattice strong coupling expansion yields an identical result at infinite N, but
shows a representation dependence of the string tension at finite N, a result
which we interpret as an artifact. In order to confirm the representation
independence of the string tension we re-analyse results of lattice simulations
involving operators with common N-ality in pure SU(N) Yang-Mills theory. We
find that the picture of the representation-independence of the string tension
is confirmed by the spectrum of excited states in the stringy sector, while the
lowest-lying states seem to depend on the representation. We argue that this
unexpected result is due to the insufficient distance of the static sources for
the asymptotic behaviour to be visible and give an estimate of the distance
above which a truly representation-independent spectrum should be observed.Comment: Refs. added, discussion in sect. 3 improved, other minor changes; to
appear in JHE
Topology of Minimal Walking Technicolor
We perform a lattice study of the topological susceptibility and instanton
size distribution of the \su{2} gauge theory with two adjoint Dirac fermions
(also known as Minimal Walking Technicolor), which is known to be in the
conformal window. In the theory deformed with a small mass term, by drawing a
comparison with the pure gauge theory, we find that topological observables are
decoupled from the fermion dynamics. This provides further evidence for the
infrared conformality of the theory. A study of the instanton size distribution
shows that this quantity can be used to detect the onset of finite size
effects.Comment: An error in the analysis has been corrected that does not affect the
result. Discussions have been expanded, comments and references added,
conclusions unchanged. Version to appear on EPJ
The Phase Diagram of the Three Dimensional Thirring Model
We present Monte Carlo simulation results for the three dimensional Thirring
model on moderate sized lattices using a hybrid molecular dynamics algorithm
which permits an odd or non-integer number Nf of fermion flavors. We find a
continuous chiral symmetry breaking transition for Nf approximately equal to 3
with critical exponents consistent with expectations from previous studies. For
Nf=5 the order of the transition is difficult to determine on the lattice sizes
explored. We present a phase diagram for the model in the (1/g^2,Nf) plane and
contrast our findings with expectations based on approximate solutions of the
continuum Schwinger-Dyson equations.Comment: 13 pages, 7 figure
Form the density-of-states method to finite density quantum field theory
During the last 40 years, Monte Carlo calculations based upon Importance
Sampling have matured into the most widely employed method for determinig first
principle results in QCD. Nevertheless, Importance Sampling leads to
spectacular failures in situations in which certain rare configurations play a
non-secondary role as it is the case for Yang-Mills theories near a first order
phase transition or quantum field theories at finite matter density when
studied with the re-weighting method. The density-of-states method in its LLR
formulation has the potential to solve such overlap or sign problems by means
of an exponential error suppression. We here introduce the LLR approach and its
generalisation to complex action systems. Applications include U(1), SU(2) and
SU(3) gauge theories as well as the Z3 spin model at finite densities and
heavy-dense QCD.Comment: 12 pages, 14 figures, based upon talks presented at Excited QCD 2016,
6-12 March 2016, Costa da Caparica, Portugal; typos corrected in this final
versio
The density of states in gauge theories
The density of states is calculated for a SU(2) and a compact U(1) lattice
gauge theory using a modified version of the Wang-Landau algorithm. We find
that the density of states of the SU(2) gauge theory can be reliably calculated
over a range of 120,000 orders of magnitude for lattice sizes as big as 20^4.
We demonstrate the potential of the algorithm by reproducing the SU(2) average
action, its specific heat and the critical couplings of the weak first order
transition in U(1).Comment: 4 pages, 6 figure
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