621 research outputs found
Large-N reduction in QCD-like theories with massive adjoint fermions
Large-N QCD with heavy adjoint fermions emulates pure Yang-Mills theory at
long distances. We study this theory on a four- and three-torus, and
analytically argue the existence of a large-small volume equivalence. For any
finite mass, center symmetry unbroken phase exists at sufficiently small volume
and this phase can be used to study the large-volume limit through the
Eguchi-Kawai equivalence. A finite temperature version of volume independence
implies that thermodynamics on R^3 x S^1 can be studied via a unitary matrix
quantum mechanics on S^1, by varying the temperature. To confirm this
non-perturbatively, we numerically study both zero- and one-dimensional
theories by using Monte-Carlo simulation. Order of finite-N corrections turns
out to be 1/N. We introduce various twisted versions of the reduced QCD which
systematically suppress finite-N corrections. Using a twisted model, we observe
the confinement/deconfinement transition on a 1^3 x 2-lattice. The result
agrees with large volume simulations of Yang-Mills theory. We also comment that
the twisted model can serve as a non-perturbative formulation of the
non-commutative Yang-Mills theory.Comment: 34 pages, 12 figures, version accepted for publication in PR
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Measurements of the top quark mass at the Tevatron
The mass of the top quark (m{sub top}) is a fundamental parameter of the standard model (SM). Currently, its most precise measurements are performed by the CDF and D0 collaborations at the Fermilab Tevatron p{bar p} collider at a centre-of-mass energy of {radical}s = 1.96 TeV. We review the most recent of those measurements, performed on data samples of up to 8.7 fb{sup -1} of integrated luminosity. The Tevatron combination using up to 5.8 fb{sup -1} of data results in a preliminary world average top quark mass of m{sub top} = 173.2 {+-} 0.9 GeV. This corresponds to a relative precision of about 0.54%. We conclude with an outlook of anticipated precision the final measurement of m{sub top} at the Tevatron
Primordial non-Gaussianity and Dark Energy constraints from Cluster Surveys
Galaxy cluster surveys will be a powerful probe of dark energy. At the same
time, cluster abundance is sensitive to any non-Gaussianity of the primordial
density field. It is therefore possible that non-Gaussian initial conditions
might be misinterpreted as a sign of dark energy or at least degrade the
expected constraints on dark energy parameters. To address this issue, we
perform a likelihood analysis of an ideal cluster survey similar in size and
depth to the upcoming South Pole Telescope/Dark Energy Survey (SPT-DES). We
analyze a model in which the strength of the non-Gaussianity is parameterized
by the constant fNL; this model has been used extensively to derive Cosmic
Microwave Background (CMB) anisotropy constraints on non-Gaussianity, allowing
us to make contact with those works. We find that the constraining power of the
cluster survey on dark energy observables is not significantly diminished by
non-Gaussianity provided that cluster redshift information is included in the
analysis. We also find that even an ideal cluster survey is unlikely to improve
significantly current and future CMB constraints on non-Gaussianity. However,
when all systematics are under control, it could constitute a valuable cross
check to CMB observations.Comment: 10 pages, 4 figures. Corrected a minor discrepancy between our
earlier definition of fNL and CMB constraints. References adde
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Multiflavor QCD* on R_3 * S_1: Studying Transition From Abelian to Non-Abelian Confinement
The center-stabilized multiflavor QCD* theories formulated on R{sub 3} x S{sub 1} exhibit both Abelian and non-Abelian confinement as a function of the S{sub 1} radius, similar to the Seiberg-Witten theory as a function of the mass deformation parameter. For sufficiently small number of flavors and small r(S{sub 1}), we show occurrence of a mass gap in gauge fluctuations, and linear confinement. This is a regime of confinement without continuous chiral symmetry breaking ({chi}SB). Unlike one-flavor theories where there is no phase transition in r(S{sub 1}), the multiflavor theories possess a single phase transition associated with breaking of the continuous {chi}S. We conjecture that the scale of the {chi}SB is parametrically tied up with the scale of Abelian to non-Abelian confinement transition
Near-field coupling and SERS effects of palladium nanoparticle dimers
The linear optical properties and the surface-enhanced Raman scattering (SERS) effect of spherical palladium nanoparticle dimers are analyzed theoretically using generalized Mie theory. The calculation results demonstrate that the near-field coupling effect greatly influences the absorption, scattering and extinction spectra of nanoparticle dimers. The surface plasmon resonance wavelength red-shifts dramatically as the separation between nanoparticles decreases. Because of the near-field coupling between nanoparticles and the size effect, the maximum SERS enhancement factor at the' hot spot' between palladium nanoparticle dimers is as high as 10(7)-10(8), while the averaged SERS enhancement factor over the entire nanoparticle surface is in the range of 10(5)-10(6). The deviation between the position of the peak in the extinction spectrum and the wavelength for maximum surface-averaged enhancement for the Pd nanoparticle dimers indicates that localized surface plasmon resonance has different influences on the far and near fields. These theoretical results may help to reveal the relationship between the far and near fields, as well as understand the mechanism of electromagnetic enhancement in the surface-enhanced scattering of transition metals.National Natural Science Foundation of China [20703032]; National Basic Research Program of China [2009CB930703]; Natural Science Foundation of Fujian Province of China [E0710028
Spin Hall effect in the kagome lattice with Rashba spin-orbit interaction
We study the spin Hall effect in the kagom\'{e} lattice with Rashba
spin-orbit coupling. The conserved spin Hall conductance (see
text) and its two components, i.e., the conventional term
and the spin-torque-dipole term , are numerically
calculated, which show a series of plateaus as a function of the electron Fermi
energy . A consistent two-band analysis, as well as a Berry-phase
interpretation, is also given. We show that these plateaus are a consequence of
the various Fermi-surface topologies when tuning . In particular,
we predict that compared to the case with the Fermi surface encircling the
point in the Brillouin zone, the amplitude of the spin Hall
conductance with the Fermi surface encircling the points is twice
enhanced, which makes it highly meaningful in the future to systematically
carry out studies of the -valley spintronics.Comment: 7 pages, 3 figures. Phys. Rev. B (in press
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On Yang--Mills Theories with Chiral Matter at Strong Coupling
Strong coupling dynamics of Yang-Mills theories with chiral fermion content remained largely elusive despite much effort over the years. In this work, we propose a dynamical framework in which we can address non-perturbative properties of chiral, non-supersymmetric gauge theories, in particular, chiral quiver theories on S{sub 1} x R{sub 3}. Double-trace deformations are used to stabilize the center-symmetric vacuum. This allows one to smoothly connect smaller(S{sub 1}) to larger(S{sub 1}) physics (R{sub 4} is the limiting case) where the double-trace deformations are switched off. In particular, occurrence of the mass gap in the gauge sector and linear confinement due to bions are analytically demonstrated. We find the pattern of the chiral symmetry realization which depends on the structure of the ring operators, a novel class of topological excitations. The deformed chiral theory, unlike the undeformed one, satisfies volume independence down to arbitrarily small volumes (a working Eguchi-Kawai reduction) in the large N limit. This equivalence, may open new perspectives on strong coupling chiral gauge theories on R{sub 4}
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