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 ($R$ band) and multicolor photometric ($BVRIJHK$) 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 $\lesssim 1^o$, and a decrease in the shocked plasma compression by a factor of $\sim$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 $(D-2)+1$ dimensional space-time. This means that Poincar\'e invariance is reduced to the Galilei subgroup in $D-2$ 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 $D$ dimensional space-time enjoying the full $N=2$ 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

1+11+1-Dimensional Large NN QCD coupled to Adjoint Fermions

We consider 1+1-dimensional QCD coupled to Majorana fermions in the adjoint representation of the gauge group $SU(N)$. Pair creation of partons (fermion quanta) is not suppressed in the large-$N$ 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