TPC in gamma-ray astronomy above pair-creation threshold
We examine the performance of a TPC as a -ray telescope above the pair-creation threshold. The contributions to the photon angular resolution are studied and their dependence on energy is obtained. The e ective area per detector unit mass for such a thin detector is the conversion mass attenuation coe cient. The di erential sensitivity for the detection of a point-like source is then derived. Finally, the measurement of track momentum from deflections due to multiple scattering is optimized. These analytical results are exemplified numerically for a few sets of detector parameters. TPCs show an impressive improvement in sensitivity with respect to existing pair-creation-based telescopes in the (MeV - GeV) energy range, even with the modest detector parameters of this study. In addition, gas TPCs allow an improvement in angular resolution of about one order of magnitude. High-energy astrophysics focuses on the study of com- plex objects such as active galactic nuclei (AGN), pulsars and gamma-ray bursts (GRB), the spectral energy distributions (SEDs) of which comprise contributions from a number of pro- cesses (1), including synchrotron emission, synchrotron self- Compton scattering (SSC), the inverse scattering of external thermal photons from the accretion disk (ESC), and 0 decays. These studies su er from the existence of gaps in the sensitivity of the available instruments, such as that between the Compton- scattering-based telescopes, that are mostly sensitive below 1 MeV, and the pair-based telescopes, that are mostly sensitive above 100 MeV. Pair-based telescopes also su er from a poor angular resolution at low energy, making the analysis of dense regions of the sky di cult. In this paper we present a characterization of the use of a thin detector as an active target for cosmic -ray detection in the pair-conversion regime, with the aim of improving the angular resolution and the sensitivity at low energy. The past missions and the presently operated Fermi/LAT and AGILE use thick de- tectors for which the photon conversion probability is close to unity and the e ective area A e is the product of the geomet- rical area of the detector A and of the reconstruction e ciency r: A e = A r. In a thin detector for which the conversion probability is small, the e ective area becomes proportional to the pair conversion mass attenuation coe cient H and to the detector mass M, A e = H M r. In a thick detector, the optimization of the aspect ratio height/surface is therefore criti- cal, while for a thin detector it does not a ect the e ective area to first order. Note that in a thick detector, the various pos- sible conversion processes are in competition with each other, and Compton scattering prior to- or in place of- pair conversion

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

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

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: 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: Energy flow in non-equilibrium conformal field theory
abstract: We study the energy current and its fluctuations in quantum gapless 1d systems far from equilibrium modeled by conformal field theory, where two separated halves are prepared at distinct temperatures and glued together at a point contact. We prove that these systems converge towards steady states, and give a general description of such non-equilibrium steady states in terms of quantum field theory data. We compute the large deviation function, also called the full counting statistics, of energy transfer through the contact. These are universal and satisfy fluctuation relations. We provide a simple representation of these quantum fluctuations in terms of classical Poisson processes whose intensities are proportional to Boltzmann weights.

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: Poisson algebra of 2d dimensionally reduced gravity
abstract: Using a Lax pair based on twisted affine sl(2,) Kac-Moody and Virasoro algebras, we deduce an r-matrix formulation of two-dimensional reduced vacuum Einstein's equations. Whereas the fundamental Poisson brackets are non ultralocal, they lead to pure c-number modified Yang-Baxter equations. We also describe how to obtain classical observables by imposing reasonable boundary conditions.

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.

title: Vertex operator representations of the quantum affine algebra Uq(Br(1))
abstract: We construct the level one vertex operator representations of the q-deformation Uq(Br(1)) of the affine Kac-Moody algebra Br(1). Beside the q-deformed vertex operators introduced by Frenkel and Jing, this construction involves a q-deformation of free fermionic fields.