2,603 research outputs found
Gluon Chain Model of the Confining Force
We develop a picture of the QCD string as a chain of constituent gluons,
bound by attractive nearest-neighbor forces which may be treated
perturbatively. This picture accounts for both Casimir scaling at large N, and
the asymptotic center dependence of the static quark potential. We discuss the
relevance, to the gluon-chain picture, of recent three-loop results for the
static quark potential. A variational framework is presented for computing the
minimal energy and wavefunction of a long gluon chain, which enables us to
derive both the logarithmic broadening of the QCD flux tube (``roughening''),
and the existence of a Luscher -c/R term in the potential.Comment: 25 pages, 5 figures, latex2
Electron Beam Ion Sources
Electron beam ion sources (EBISs) are ion sources that work based on the
principle of electron impact ionization, allowing the production of very highly
charged ions. The ions produced can be extracted as a DC ion beam as well as
ion pulses of different time structures. In comparison to most of the other
known ion sources, EBISs feature ion beams with very good beam emittances and a
low energy spread. Furthermore, EBISs are excellent sources of photons (X-rays,
ultraviolet, extreme ultraviolet, visible light) from highly charged ions. This
chapter gives an overview of EBIS physics, the principle of operation, and the
known technical solutions. Using examples, the performance of EBISs as well as
their applications in various fields of basic research, technology and medicine
are discussed.Comment: 37 pages, contribution to the CAS-CERN Accelerator School: Ion
Sources, Senec, Slovakia, 29 May - 8 June 2012, edited by R. Baile
Defining the Force between Separated Sources on a Light Front
The Newtonian character of gauge theories on a light front requires that the
longitudinal momentum P^+, which plays the role of Newtonian mass, be
conserved. This requirement conflicts with the standard definition of the force
between two sources in terms of the minimal energy of quantum gauge fields in
the presence of a quark and anti-quark pinned to points separated by a distance
R. We propose that, on a light front, the force be defined by minimizing the
energy of gauge fields in the presence of a quark and an anti-quark pinned to
lines (1-branes) oriented in the longitudinal direction singled out by the
light front and separated by a transverse distance R. Such sources will have a
limited 1+1 dimensional dynamics. We study this proposal for weak coupling
gauge theories by showing how it leads to the Coulomb force law. For QCD we
also show how asymptotic freedom emerges by evaluating the S-matrix through one
loop for the scattering of a particle in the N_c representation of color
SU(N_c) on a 1-brane by a particle in the \bar N_c representation of color on a
parallel 1-brane separated from the first by a distance R<<1/Lambda_{QCD}.
Potential applications to the problem of confinement on a light front are
discussed.Comment: LaTeX, 15 pages, 12 figures; minor typos corrected; numerical
correction in equation 3.
The Outburst of the Blazar AO 0235+164 in 2006 December: Shock-in-Jet Interpretation
We present the results of polarimetric ( band) and multicolor photometric
() observations of the blazar AO 0235+16 during an outburst in 2006
December. The data reveal a short timescale of variability (several hours),
which increases from optical to near-IR wavelengths; even shorter variations
are detected in polarization. The flux density correlates with the degree of
polarization, and at maximum degree of polarization the electric vector tends
to align with the parsec-scale jet direction. We find that a variable component
with a steady power-law spectral energy distribution and very high optical
polarization (30-50%) is responsible for the variability. We interpret these
properties of the blazar withina model of a transverse shock propagating down
the jet. In this case a small change in the viewing angle of the jet, by
, and a decrease in the shocked plasma compression by a factor of
1.5 are sufficient to account for the variability.Comment: 22 pages, 8 figures, accepted for Ap
Scattering of Glue by Glue on the Light-cone Worldsheet II: Helicity Conserving Amplitudes
This is the second of a pair of articles on scattering of glue by glue, in
which we give the light-cone gauge calculation of the one-loop on-shell
helicity conserving scattering amplitudes for gluon-gluon scattering
(neglecting quark loops). The 1/p^+ factors in the gluon propagator are
regulated by replacing p^+ integrals with discretized sums omitting the p^+=0
terms in each sum. We also employ a novel ultraviolet regulator that is
convenient for the light-cone worldsheet description of planar Feynman
diagrams. The helicity conserving scattering amplitudes are divergent in the
infra-red. The infrared divergences in the elastic one-loop amplitude are shown
to cancel, in their contribution to cross sections, against ones in the cross
section for unseen bremsstrahlung gluons. We include here the explicit
calculation of the latter, because it assumes an unfamiliar form due to the
peculiar way discretization of p^+ regulates infrared divergences. In resolving
the infrared divergences we employ a covariant definition of jets, which allows
a transparent demonstration of the Lorentz invariance of our final results.
Because we use an explicit cutoff of the ultraviolet divergences in exactly 4
space-time dimensions, we must introduce explicit counterterms to achieve this
final covariant result. These counter-terms are polynomials in the external
momenta of the precise order dictated by power-counting. We discuss the
modifications they entail for the light-cone worldsheet action that reproduces
the ``bare'' planar diagrams of the gluonic sector of QCD. The simplest way to
do this is to interpret the QCD string as moving in six space-time dimensions.Comment: 56 pages, 21 figures, references added, minor typos correcte
Spontaneous Symmetry Breaking at Infinite Momentum without P+ Zero-Modes
The nonrelativistic interpretation of quantum field theory achieved by
quantization in an infinite momentum frame is spoiled by the inclusion of a
mode of the field carrying p+=0. We therefore explore the viability of doing
without such a mode in the context of spontaneous symmetry breaking (SSB),
where its presence would seem to be most needed. We show that the physics of
SSB in scalar quantum field theory in 1+1 space-time dimensions is accurately
described without a zero-mode.Comment: LaTeX, 8 pages, 3 eps figure
String Bit Models for Superstring
We extend the model of string as a polymer of string bits to the case of
superstring. We mainly concentrate on type II-B superstring, with some
discussion of the obstacles presented by not II-B superstring, together with
possible strategies for surmounting them. As with previous work on bosonic
string we work within the light-cone gauge. The bit model possesses a good deal
less symmetry than the continuous string theory. For one thing, the bit model
is formulated as a Galilei invariant theory in dimensional
space-time. This means that Poincar\'e invariance is reduced to the Galilei
subgroup in space dimensions. Naturally the supersymmetry present in the
bit model is likewise dramatically reduced. Continuous string can arise in the
bit models with the formation of infinitely long polymers of string bits. Under
the right circumstances (at the critical dimension) these polymers can behave
as string moving in dimensional space-time enjoying the full
Poincar\'e supersymmetric dynamics of type II-B superstring.Comment: 43 pages, phyzzx require
Quantum Newtonian Dynamics on a Light Front
We recall the special features of quantum dynamics on a light-front (in an
infinite momentum frame) in string and field theory. The reason this approach
is more effective for string than for fields is stressed: the light-front
dynamics for string is that of a true Newtonian many particle system, since a
string bit has a fixed Newtonian mass. In contrast, each particle of a field
theory has a variable Newtonian mass P^+, so the Newtonian analogy actually
requires an infinite number of species of elementary Newtonian particles. This
complication substantially weakens the value of the Newtonian analogy in
applying light-front dynamics to nonperturbative problems. Motivated by the
fact that conventional field theories can be obtained as infinite tension
limits of string theories, we propose a way to recast field theory as a
standard Newtonian system. We devise and analyze some simple quantum mechanical
systems that display the essence of the proposal, and we discuss prospects for
applying these ideas to large N_c QCD.Comment: 13 pages, 3 figures, LaTex, psfig, references added, APS copyrigh
-Dimensional Large QCD coupled to Adjoint Fermions
We consider 1+1-dimensional QCD coupled to Majorana fermions in the adjoint
representation of the gauge group . Pair creation of partons (fermion
quanta) is not suppressed in the large- limit, where the glueball-like bound
states become free. In this limit the spectrum is given by a linear \lc\ Schr\"
odinger equation, which we study numerically using the discretized \lcq. We
find a discrete spectrum of bound states, with the logarithm of the level
density growing approximately linearly with the mass. The wave function of a
typical excited state is a complicated mixture of components with different
parton numbers. A few low-lying states, however, are surprisingly close to
being eigenstates of the parton number, and their masses can be accurately
calculated by truncated diagonalizations.Comment: 22 pages + 9 figures (available by request from
[email protected]), uses phyzzx.tex + tables.tex PUPT-1413,
IASSNS-HEP-93/4
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