High Energy Physics

Recent PAMELA and ATIC results may represent a breakthrough in dark matter searches beyond its gravitational imprint. After briefly reviewing the possible (classes of) explanations for the observed excesses in positron and electron cosmic ray fluxes I will focus on a two component dark matter model that may provide an explanation for large boost factors needed in the dark matter annihilation interpretation of the signals.

We discuss various properties of holographic mesons in a deconfined strongly coupled plasma. We show that such mesons obtain a width from a non-perturbative effect. On the string theory side this is due to open string modes on a D-brane tunneling into a black hole through worldsheet instantons. On the field theory side these instantons have the simple interpretation as heavy thermal quarks. We also comment on how this non-perturbative effect has important consequences for the phase structure of the Yang-Mills theory obtained in the classical gravity limit.

We discuss holography for geometries that are asymptotic to non-conformal brane backgrounds. The near-horizon limit of all non-conformal branes, including D-branes and the fundamental string but excluding five-branes, is conformal to $AdS_{p+2} imes S^{8-p}$ with a linear dilaton. They exhibit a generalized conformal structure, both on the QFT and on the gravitational side. We develop holographic renormalization for all these cases and discuss a number of applications. We compute the holographic 2-point functions of the stress energy tensor and gluon operator and show they satisfy the expected Ward identities and the constraints of generalized conformal structure. The holographic results are also manifestly compatible with the M-theory uplift, with the asymptotic solutions, counterterms, one and two point functions etc. of the IIA F1 and D4 appropriately descending from those of M2 and M5 branes, respectively.

The interpretation of virtual gluons as ghosts in the non-linear gluonic structure of QCD permits the formulation and realization of a manifestly gauge-invariant and Lorentz covariant theory of interacting quarks/antiquarks, for all values of coupling. The simplest example of quark/antiquark scattering in a high-energy, quenched, eikonal model at large coupling is shown to be expressable as a set of finite, local integrals which may be evaluated numerically; and before evaluation, it is clear that the result will be dependent only on, and damped by increasing momentum transfer, while displaying physically-reasonable color dependence in a manner underlying the MIT Bag Model and an effective, asymptotic freedom. These results are compatible with an earlier, instanton, field-strength analysis of Reinhardt, et.al.

The discovery of integrability in the large N limit of the prototypical realization of the AdS/CFT correspondence has raised the hope that the spectrum of scale dimensions in N=4 SYM (and strings in AdS_5 x S^5) might be known exactly, i.e. to all orders in the coupling constant. So far, most of the efforts focused on closed strings and periodic boundary conditions. In this talk I will discuss how these ideas are extended to open string and open boundary conditions. In particular how to obtain exact expressions for the boundary scattering matrices, discuss the integrability of the boundary conditions and how finite size effects may be taken into account.

We review basic properties of effective worldvolume theory describing the dynamics of Dirichlet branes in type II supergravity backgrounds and then show that the SL(2,R) symmetry of IIB supergravity allows for the existence of new supersymmetric 7-brane configurations called Q7-branes. The Q7-branes differ from the D7-branes, in particular, by their coupling to the dilaton, axion and to `magnetic\' gauge field duals thereof. The appearance of Q7-branes and their instanton duals may indicate the existence of a new, yet unexplored, perturbative vacuum of type IIB string theory.

In frustrated systems, competing interactions lead to complex phase diagrams and sometimes entirely new states of matter. Frustration often arises from the lattice geometry, leaving the system delicately balanced between a variety of possible orders. A number of normally weak effects can lead to a lifting of this degeneracy. For example, I will discuss how quantum fluctuations can stabilize a supersolid phase, where the system is at once both a crystal and a superfluid. Frustrated magnets are promising candidates for realizing spin liquid phases with exotic \'topological order\', and new kinds of quantum phase transitions that have no classical analog. Bio: Ashvin Vishwanath received his MSc from IIT Kanpur in 1996, and PhD from Princeton in 2001. After holding the Papparlardo fellowship at MIT, he joined UC Berkeley in 2004, where he is currently associate professor of physics. He specializes in theoretical condensed matter physics, especially magnetism, superconductivity and other correlated quantum phenomena in solids and cold atomic gases. Ashvin is the recipient of several awards, most recently a Sloan fellowship (2004), a Hellman foundation fellowship (2006), and an NSF CAREER Award (2007).