62 research outputs found
Infrared Evolution and Phase Structure of a Gauge Theory Containing Different Fermion Representations
We study the evolution of an asymptotically free vectorial SU() gauge
theory from the ultraviolet to the infrared and the resultant phase structure
in the general case in which the theory contains fermions transforming
according to several different representations of the gauge group. We discuss
the sequential fermion condensation and dynamical mass generation that occur,
and comment on the effect of bare fermion mass terms.Comment: 13 pages, late
Patterns of Dynamical Gauge Symmetry Breaking
We construct and analyze theories with a gauge symmetry in the ultraviolet of
the form , in which the vectorial, asymptotically free
gauge interaction becomes strongly coupled at a scale where the interaction
is weakly coupled and produces bilinear fermion condensates that dynamically
break the symmetry. Comparisons are given between Higgs and dynamical
symmetry breaking mechanisms for various models.Comment: 14 pages, late
Higher-Loop Corrections to the Infrared Evolution of a Gauge Theory with Fermions
We consider a vectorial, asymptotically free gauge theory and analyze the
effect of higher-loop corrections to the beta function on the evolution of the
theory from the ultraviolet to the infrared. We study the case in which the
theory contains copies of a fermion transforming according to the
fundamental representation and several higher-dimensional representations of
the gauge group. We also calculate higher-loop values of the anomalous
dimension of the mass, of at the infrared zero of the
beta function. We find that for a given theory, the values of
calculated to three- and four-loop order, and evaluated at the infrared zero
computed to the same order, tend to be somewhat smaller than the value
calculated to two-loop order. The results are compared with recent lattice
simulations.Comment: 22 pages, latex, matches Phys. Rev. D publicatio
Ion distribution and ablation depth measurements of a fs-ps laser-irradiated solid tin target
The ablation of solid tin surfaces by an 800-nanometer-wavelength laser is
studied for a pulse length range from 500 fs to 4.5 ps and a fluence range
spanning 0.9 to 22 J/cm^2. The ablation depth and volume are obtained employing
a high-numerical-aperture optical microscope, while the ion yield and energy
distributions are obtained from a set of Faraday cups set up under various
angles. We found a slight increase of the ion yield for an increasing pulse
length, while the ablation depth is slightly decreasing. The ablation volume
remained constant as a function of pulse length. The ablation depth follows a
two-region logarithmic dependence on the fluence, in agreement with the
available literature and theory. In the examined fluence range, the ion yield
angular distribution is sharply peaked along the target normal at low fluences
but rapidly broadens with increasing fluence. The total ionization fraction
increases monotonically with fluence to a 5-6% maximum, which is substantially
lower than the typical ionization fractions obtained with nanosecond-pulse
ablation. The angular distribution of the ions does not depend on the laser
pulse length within the measurement uncertainty. These results are of
particular interest for the possible utilization of fs-ps laser systems in
plasma sources of extreme ultraviolet light for nanolithography.Comment: 8 pages, 7 figure
Flavor in Minimal Conformal Technicolor
We construct a complete, realistic, and natural UV completion of minimal
conformal technicolor that explains the origin of quark and lepton masses and
mixing angles. As in "bosonic technicolor", we embed conformal technicolor in a
supersymmetric theory, with supersymmetry broken at a high scale. The exchange
of heavy scalar doublets generates higher-dimension interactions between
technifermions and quarks and leptons that give rise to quark and lepton masses
at the TeV scale. Obtaining a sufficiently large top quark mass requires strong
dynamics at the supersymmetry breaking scale in both the top and technicolor
sectors. This is natural if the theory above the supersymmetry breaking also
has strong conformal dynamics. We present two models in which the strong top
dynamics is realized in different ways. In both models, constraints from
flavor-changing effects can be easily satisfied. The effective theory below the
supersymmetry breaking scale is minimal conformal technicolor with an
additional light technicolor gaugino. We argue that this light gaugino is a
general consequence of conformal technicolor embedded into a supersymmetric
theory. If the gaugino has mass below the TeV scale it will give rise to an
additional pseudo Nambu-Goldstone boson that is observable at the LHC.Comment: 37 pages; references adde
Thermodynamics of lattice QCD with 2 sextet quarks on N_t=8 lattices
We continue our lattice simulations of QCD with 2 flavours of colour-sextet
quarks as a model for conformal or walking technicolor. A 2-loop perturbative
calculation of the -function which describes the evolution of this
theory's running coupling constant predicts that it has a second zero at a
finite coupling. This non-trivial zero would be an infrared stable fixed point,
in which case the theory with massless quarks would be a conformal field
theory. However, if the interaction between quarks and antiquarks becomes
strong enough that a chiral condensate forms before this IR fixed point is
reached, the theory is QCD-like with spontaneously broken chiral symmetry and
confinement. However, the presence of the nearby IR fixed point means that
there is a range of couplings for which the running coupling evolves very
slowly, i.e. it 'walks'. We are simulating the lattice version of this theory
with staggered quarks at finite temperature studying the changes in couplings
at the deconfinement and chiral-symmetry restoring transitions as the temporal
extent () of the lattice, measured in lattice units, is increased. Our
earlier results on lattices with show both transitions move to weaker
couplings as increases consistent with walking behaviour. In this paper
we extend these calculations to . Although both transition again move to
weaker couplings the change in the coupling at the chiral transition from
to is appreciably smaller than that from to .
This indicates that at we are seeing strong coupling effects and that
we will need results from to determine if the chiral-transition
coupling approaches zero as , as needed for the theory
to walk.Comment: 21 pages Latex(Revtex4) source with 4 postscript figures. v2: added 1
reference. V3: version accepted for publication, section 3 restructured and
interpretation clarified. Section 4 future plans for zero temperature
simulations clarifie
Chiral properties of SU(3) sextet fermions
SU(3) gauge theory with overlap fermions in the 2-index symmetric (sextet)
and fundamental representations is considered. A priori it is not known what
the pattern of chiral symmetry breaking is in a higher dimensional
representation although the general expectation is that if two representations
are both complex, the breaking pattern will be the same. This expectation is
verified for the sextet at N_f = 0 in several exact zero mode sectors. It is
shown that if the volume is large enough the same random matrix ensemble
describes both the sextet and fundamental Dirac eigenvalues. The number of zero
modes for the sextet increases approximately 5-fold relative to the fundamental
in accordance with the index theorem for small lattice spacing but zero modes
which do not correspond to integer topological charge do exist at larger
lattice spacings. The zero mode number dependence of the random matrix model
predictions correctly match the simulations in each sector and each
representation.Comment: 38 pages (12 pages text and gazillion tables/figures), minor
modification, references adde
Determining the conformal window: SU(2) gauge theory with N_f = 4, 6 and 10 fermion flavours
We study the evolution of the coupling in SU(2) gauge field theory with
, 6 and 10 fundamental fermion flavours on the lattice. These values are
chosen close to the expected edges of the conformal window, where the theory
possesses an infrared fixed point. We use improved Wilson-clover action, and
measure the coupling in the Schr\"odinger functional scheme. At four flavours
we observe that the couping grows towards the infrared, implying QCD-like
behaviour, whereas at ten flavours the results are compatible with a Banks-Zaks
type infrared fixed point. The six flavour case remains inconclusive: the
evolution of the coupling is seen to become slower at the infrared, but the
accuracy of the results falls short from fully resolving the fate of the
coupling. We also measure the mass anomalous dimension for the case.Comment: 22 pages, 12 figures. Proof readin
Sn ion energy distributions of ns- and ps-laser produced plasmas
Ion energy distributions arising from laser-produced plasmas of Sn are measured over a wide laser parameter space. Planar-solid and liquid-droplet targets are exposed to infrared laser pulses with energy densities between 1 J cm(-2) and 4 kJ cm(-2) and durations spanning 0.5 ps to 6 ns. The measured ion energy distributions are compared to two self-similar solutions of a hydrodynamic approach assuming isothermal expansion of the plasma plume into vacuum. For planar and droplet targets exposed to ps-long pulses, we find good agreement between the experimental results and the self-similar solution of a semi-infinite simple planar plasma configuration with an exponential density profile. The ion energy distributions resulting from solid Sn exposed to ns-pulses agrees with solutions of a limited-mass model that assumes a Gaussian-shaped initial density profile.</p
Conformality or confinement: (IR)relevance of topological excitations
We study aspects of the conformality to confinement transition for
non-supersymmetric Yang-Mills theories with fermions in arbitrary chiral or
vectorlike representations. We use the presence or absence of mass gap for
gauge fluctuations as an identifier of the infrared behavior. Present-day
understanding does not allow the mass gap for gauge fluctuations to be computed
on R*4. However, recent progress allows its non-perturbative computation on
R*3xS*1 by using either the twisted partition function or deformation theory,
for a range of S*1 sizes depending on the theory. For small number of fermions,
Nf, we show that the mass gap increases with increasing radius, due to the
non-dilution of monopoles and bions, the topological excitations relevant for
confinement on R*3xS*1. For sufficiently large Nf, we show that the mass gap
decreases with increasing radius. In a class of theories, we claim that the
decompactification limit can be taken while remaining within the region of
validity of semi-classical techniques, giving the first examples of
semiclassically solvable Yang-Mills theories at any size S*1. For general
non-supersymmetric vectorlike or chiral theories, we conjecture that the change
in the behavior of the mass gap on R*3xS*1 as a function of the radius occurs
near the lower boundary of the conformal window and give non-perturbative
estimates of its value. For vectorlike theories, we compare our estimates of
the conformal window with existing lattice results, truncations of the
Schwinger-Dyson equations, NSVZ beta function-inspired estimates, and degree of
freedom counting criteria. For multi-generation chiral gauge theories, to the
best of our knowledge, our estimates of the conformal window are the only known
ones.Comment: 40 pages, 3 figures; modified various comments, reference adde
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