High resolution beam monitoring with optical transition radiation at 3 MeV electron energy
In the framework of the plasma beat wave accelerator experiment at Ecole Polytechnique, high precision position and focussing monitoring of a 3 MeV electron beam is needed. A device is proposed that uses backward optical transition radiation (OTR) from a tilted metal foil held into the beam. For an electron energy of 3 MeV, OTR is emitted within a large solid angle (typical apex angle about 400) around the direction of specular reflection. The design requirements are a high resolution of the imaging optics (/spl  /spl mu/m), a high sensitivity (/spl ap/ 10 /spl mu/A beam current, not focussed), robustness, and low cost. A prototype has been constructed and successfully tested. A similar device will be used for adjusting a laser focal spot on an electron focal spot, and for monitoring the beam on entry and exit of a gas vessel.<<ETX>>

title: Branching fraction for $B^{+} \to \pi^0 \ell^{+} \nu$, measured in $\upsilon_{4S} \to B \bar{B}$ events tagged by $B^{-} \to D^0 \ell^{-}$ anti-v ($X$) decays

title: Improved Measurement of CP Violation in Neutral B Decays to c cs

title: Measurement of the CKM matrix element $|V_{ub}|$ with $B \to \rho e \nu$ decays

title: Measurements of branching fractions and CP-violating asymmetries in B-meson decays to the charmless two-body states K0 pi+, anti-K0 K+, and K0 anti-K0

title: Operation of the Cherenkov detector DIRC of BABAR at high luminosity
abstract: The DIRC (acronym for Detection of Internally Reflected Cherenkov (light)) is the ring imaging Cherenkov detector of the BABAR detector at the PEP-II ring of SLAC. It provides the identification of pions, kaons and protons for momenta up to 4 GeV/c with high efficiency. This is needed to reconstruct CP-violating B-decay final states and to provide B-meson flavour tagging for time dependent asymmetry measurements. The DIRC radiators consists of long rectangular bars made of synthetic fused silica and the photon detector is a water tank equipped with an array of 10.752 conventional photomultipliers. At the end of the year 2000 BABAR has recorded about 22 million B~B pairs reaching the design luminosity of L=3/spl middot/10/sup 33//cm/sup 2/s. The ability to keep the beam background level low at highest collision rates and the long term reliability of the DIRC components during continuous data taking are requirements of BABAR to accomplish its physics program.

title: Some Simple (Integrable) Models of Fractional Statistics
abstract: In the first part, we introduce the notion of fractional statistics in the sense of Haldane. We illustrate it on simple models related to anyon physics and to integrable models solvable by the Bethe ansatz. In the second part, we describe the properties of the long-range interacting spin chains. We describe its infinite dimensional symmetry, and we explain how the fractional statistics of its elementary excitations is an echo of this symmetry. In the third part, we review recent results on the Yangian representation theory which emerged from the study of the integrable long-range interacting models.

title: Renormalization group for network models of quantum Hall transitions
abstract: We analyze in detail the renormalization group flows which follow from the recently proposed all orders β functions for the Chalker–Coddington network model. The flows in the physical regime reach a true singularity after a finite scale transformation. Other flows are regular and we identify the asymptotic directions. One direction is in the same universality class as the disordered XY model. The all orders β function is computed for the network model of the spin quantum Hall transition and the flows are shown to have similar properties. It is argued that fixed points of general current–current interactions in 2d should correspond to solutions of the Virasoro master equation. Based on this we identify two coset conformal field theories osp (2 N |2 N ) 1 / u (1) 0 and osp (4 N |4 N ) 1 / su (2) 0 as possible fixed points and study the resulting multifractal properties. We also obtain a scaling relation between the typical amplitude exponent α 0 and the typical point contact conductance exponent X t which is expected to hold when the density of states is constant.

title: A quasi-Hopf algebra interpretation of quantum 3-j and 6-j symbols and difference equations
abstract: We consider the universal solution of the Gervais-Neveu-Felder equation in the U q ( sl 2 ) case. We show that it has a quasi-Hopf algebra interpretation. We also recall its relation to quantum 3- j and 6- j symbols. Finally, we use this solution to build a q -deformation of the trigonometric Lame equation.

title: Scrutinizing the Cosmological Constant Problem and a possible resolution
abstract: We suggest a new perspective on the Cosmological Constant Problem by scrutinizing its standard formulation. In classical and quantum mechanics without gravity, there is no definition of the zero point of energy. Furthermore, the Casimir effect only measures how the vacuum energy {\it changes} as one varies a geometric modulus. This leads us to propose that the physical vacuum energy in a Friedman-Lema\^itre-Robertson-Walker expanding universe only depends on the time variation of the scale factor $a(t)$. Equivalently, requiring that empty Minkowski space is gravitationally stable is a principle that fixes the ambiguity in the zero point energy. On the other hand, if there is a meaningful bare cosmological constant, this prescription should be viewed as a fine-tuning. 
We describe two different choices of vacuum, one of which is consistent with the current universe consisting only of matter and vacuum energy. The resulting vacuum energy density $\rhovac$ is constant in time and approximately $k_c^2 H_0^2$, where $k_c$ is a momentum cut-off and $H_0$ is the current Hubble constant; for a cut-off close to the Planck scale, values of $\rhovac$ in agreement with astrophysical measurements are obtained. Another choice of vacuum is more relevant to the early universe consisting of only radiation and vacuum energy, and we suggest it as a possible model of inflation.

title: Twisted self-duality of dimensionally reduced gravity and vertex operators
abstract: The Geroch group, isomorphic to the SL(2, R ) affine Kac-Moody group, is an infinite-dimensional solution generating group of Einstein's equations with two surface orthogonal commuting Killing vectors. We introduce another solution generating group for these equations, the dressing group, and discuss its connection with the Geroch group. We show that it acts transitively on a dense subset of moduli space. We use a new Lax pair expressing a twisted self-duality of this system and we study the dressing problem associated to it. We also describe how to use vertex operators to solve the reduced Einstein's equations.