884 research outputs found
How light can the Higgs be?
It is widely believed that, for a given Top mass, the Higgs mass has a lower
bound: if m_Higgs is too small, the Higgs vacuum is unstable due to Top
dynamics. From vacuum instability, the state-of-the-art calculation of the
lower bound is close to the current experimental limit. Using non-perturbative
simulations and large N calculations, we show that the vacuum is in fact never
unstable. Instead, we investigate the existence of a new lower bound, based on
the intrinsic cut-off of this trivial theory.Comment: 3 pages, 4 figures, uses espcrc2.sty, Lattice2003(higgs
The QCD String Spectrum and Conformal Field Theory
The low energy excitation spectrum of the critical Wilson surface is
discussed between the roughening transition and the continuum limit of lattice
QCD. The fine structure of the spectrum is interpreted within the framework of
two-dimensional conformal field theory.Comment: Lattice2001 (confinement),3 pages,1 figure,uses espcrc2.st
Zero Point Energy of Renormalized Wilson Loops
The quark antiquark potential, and its associated zero point energy, can be
extracted from lattice measurements of the Wilson loop. We discuss a unique
prescription to renormalize the Wilson loop, for which the perturbative
contribution to the zero point energy vanishes identically. A zero point energy
can arise nonperturbatively, which we illustrate by considering effective
string models. The nonperturbative contribution to the zero point energy
vanishes in the Nambu model, but is nonzero when terms for extrinsic curvature
are included. At one loop order, the nonperturbative contribution to the zero
point energy is negative, regardless of the sign of the extrinsic curvature
term.Comment: 14 pages, ReVTeX. Paper shortened, results unchange
The static hybrid potential in D dimensions at short distances
We compute the energy of a static hybrid, i.e. of a hybrid quarkonium with
static quark and antiquark, at short distances in D=4,3 dimensions. The soft
contribution to this energy is the static potential of a color octet
quark-antiquark pair at short distances, which is known at two loops for
arbitrary D. We have checked this expression employing thermal field theory
methods. Using the effective field theory pNRQCD we calculate the ultrasoft
contributions to the hybrid (and singlet) static energy at the two-loop level.
We then present new results for the static hybrid energy/potential and the
hybrid decay width in three and four dimensions. Finally we comment on the
meaning of the perturbative results in two space-time dimensions, where the
hybrid does not exist.Comment: 30 pages, 2 figure
Spin-String Interaction in QCD Strings
I consider the question of the interaction between a QCD string and the spin
of a quark or an antiquark on whose worldline the string terminates. The
problem is analysed from the point of view of a string representation for the
expectation value of a Wilson loop for a spin-half particle. A string
representation of the super Wilson loop is obtained starting from an effective
string representation of a Wilson Loop. The action obtained in this manner is
invariant under a worldline supersymmetry and has a boundary term which
contains the spin-string interaction. For rectangular loops the spin-string
interaction vanishes and there is no spin-spin term in the resulting heavy
quark potential. On the other hand if an allowance is made for the finite
intrinsic thickness of the flux-tube, by assuming that the spin-string
interaction takes place not just at the boundary of the string world-sheet but
extends to a distance of the order of the intrinsic thickness of the flux tube,
then we do obtain a spin-spin interaction which falls as the fifth power of the
distance. Such a term was previously suggested by Kogut and Parisi in the
context of a flux-tube model of confinement.Comment: 19 pages, 1 figure; Published version with added discussion and
references in section
The Heavy Hybrid Spectrum from NRQCD and the Born-Oppenheimer Approximation
The spectrum of heavy-quark hybrids is studied in the leading
Born-Oppenheimer (LBO) approximation and using leading-order NRQCD simulations
with an improved gluon action on anisotropic lattices. The masses of four
hybrid states are obtained from our simulations for lattice spacings 0.1 fm and
0.2 fm and are compared to the LBO predictions obtained using
previously-determined glue-excited static potentials. The consistency of
results from the two approaches reveals a compelling physical picture for
heavy-quark hybrid states.Comment: LATTICE99(Heavy Quarks),3 pages,2 figures,uses espcrc2.st
High-Precision Thermodynamics and Hagedorn Density of States
We compute the entropy density of the confined phase of QCD without quarks on
the lattice to very high accuracy. The results are compared to the entropy
density of free glueballs, where we include all the known glueball states below
the two-particle threshold. We find that an excellent, parameter-free
description of the entropy density between 0.7Tc and Tc is obtained by
extending the spectrum with the exponential spectrum of the closed bosonic
string.Comment: 4 pages, 3 figure
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