D-Brane Boundary State Dynamics

We construct the open string boundary states corresponding to various time-dependent deformations of the D-brane and explore several ways in which they may be used to study stringy soliton collective coordinate quantum dynamics. Among other things, we find that D-strings have exact moduli corresponding to arbitrary chiral excitations of the basic soliton. These are presumably the duals of the BPS-saturated excitations of the fundamental Type IIB string. These first steps in a systematic study of the dynamics and interactions of Dirichlet-brane solitons give further evidence of the consistency of Polchinski's new approach to string soliton physics.Comment: 14 pages, harvmac; reference added, end of section 3 modifie

Critical Theories of the Dissipative Hofstadter Model

It has recently been shown that the dissipative Hofstadter model (dissipative quantum mechanics of an electron subject to uniform magnetic field and periodic potential in two dimensions) exhibits critical behavior on a network of lines in the dissipation/magnetic field plane. Apart from their obvious condensed matter interest, the corresponding critical theories represent non-trivial solutions of open string field theory, and a detailed account of their properties would be interesting from several points of view. A subject of particular interest is the dependence of physical quantities on the magnetic field since it, much like $\theta_{\rm QCD}$, serves only to give relative phases to different sectors of the partition sum. In this paper we report the results of an initial investigation of the free energy, $N$-point functions and boundary state of this type of critical theory. Although our primary goal is the study of the magnetic field dependence of these quantities, we will present some new results which bear on the zero magnetic field case as well.Comment: 42 pages (25 reduced

Exact Solution of a Boundary Conformal Field Theory

We study the conformal field theory of a free massless scalar field living on the half line with interactions introduced via a periodic potential at the boundary. An SU(2) current algebra underlies this system and the interacting boundary state is given by a global SU(2) rotation of the left-moving fields in the zero-potential (Neumann) boundary state. As the potential strength varies from zero to infinity, the boundary state interpolates between the Neumann and the Dirichlet values. The full S-matrix for scattering from the boundary, with arbitrary particle production, is explicitly computed. To maintain unitarity, it is necessary to attribute a hidden discrete ``soliton'' degree of freedom to the boundary. The same unitarity puzzle occurs in the Kondo problem, and we anticipate a similar solution.Comment: harvmac and epsf, 36 pages with 5 figures; v2: the version which appeared in NPB including a Note Added on the band structure of open string

Brane Dynamics From the Born-Infeld Action

We use the abelian Born-Infeld action for the worldvolume gauge field and transverse displacement scalars to explore new aspects of D-brane structure and dynamics. We study several classic gauge field configurations, including point charges in any worldvolume dimension and vortices in two worldvolume dimensions, and show that, with an appropriate excitation of the transverse coordinate field, they are BPS-saturated solutions. The Coulomb point charge solutions turn out to represent, with considerable fidelity, fundamental strings attached to the brane (their magnetic counterparts describe D1-branes attached to D3-branes). We also show that S-matrix for small excitations propagating on the point charge solution is consistent with (and gives further illuminating information about) Polchinski's effective open string boundary condition.Comment: 16 pages, 3 figures. Minor typos fixe

Violations of Bjorken scaling in inclusive e+e- annihilation

We discuss the application of renormalization-group techniques to inclusive e+e- annihilation. It is shown by a modest extension of Mueller's techniques that annihilation structure functions have a behavior completely analogous to electroproduction structure functions: Their moments scale for large virtual photon mass, and this scaling is described by "anomalous dimensions" which have a singularity structure and general form very similar to the usual anomalous dimension, though there is no simple relation between the two. We show how information about the structure functions can be deduced from the moments and how, in appropriate limits, deviations from Bjorken scaling can be interpreted in terms of an underlying field theory

Baryons and String Creation from the Fivebrane Worldvolume Action

We construct BPS-exact solutions of the worldvolume Born-Infeld plus WZW action of a D5-brane in the background of N D3-branes. The non-trivial background metric and RR five-form field strength play a crucial role in the solution. When a D5-brane is dragged across a stack of N D3-branes a bundle of N fundamental strings joining the two types of branes is created, as in the Hanany-Witten effect. Our solutions give a detailed description of this bundle in terms of a D5-brane wrapped on a sphere. We discuss extensions of these solutions which have an interpretation in terms of gauge theory multi-quark states via the AdS/CFT correspondence.Comment: LaTeX, 18 pages, 5 eps figures; v2: added reference which had been inadvertently omitte

Quantifying selection in immune receptor repertoires

The efficient recognition of pathogens by the adaptive immune system relies on the diversity of receptors displayed at the surface of immune cells. T-cell receptor diversity results from an initial random DNA editing process, called VDJ recombination, followed by functional selection of cells according to the interaction of their surface receptors with self and foreign antigenic peptides. To quantify the effect of selection on the highly variable elements of the receptor, we apply a probabilistic maximum likelihood approach to the analysis of high-throughput sequence data from the $\beta$-chain of human T-cell receptors. We quantify selection factors for V and J gene choice, and for the length and amino-acid composition of the variable region. Our approach is necessary to disentangle the effects of selection from biases inherent in the recombination process. Inferred selection factors differ little between donors, or between naive and memory repertoires. The number of sequences shared between donors is well-predicted by the model, indicating a purely stochastic origin of such "public" sequences. We find a significant correlation between biases induced by VDJ recombination and our inferred selection factors, together with a reduction of diversity during selection. Both effects suggest that natural selection acting on the recombination process has anticipated the selection pressures experienced during somatic evolution