120714913
Low-pressure drift chamber with a built-in scintillator
Abstract A low-pressure drift chamber with a built-in scintillator has been developed. Pulse height characteristics and the velocity of electrons, drifting in various electric fields in isobutane, have been measured. A standard deviation of the coordinate determined by measuring the drift time of the electrons has been found to be around 100 μm for a 20 mm drift length at 50 Torr of isobutane. A standard deviation of the second coordinate measured by a delay line is better than 100 μm. The work was performed at the High Energy Laboratory, JINR.
122883967
Structure of neutron-rich oxygen isotopes
One-nucleon removal reactions at relativistic energies have been used as a spectroscopic tool for obtaining information about the ground state of neutron-rich oxygen isotopes. Using the FRS at GSI, the longitudinal momentum distributions of the emerging fragments after breakup were measured along with the one-nucleon removal cross-sections. The relative contributions of the remaining fragments in their ground and excited states have been determined for a few cases from measurements of γ rays in coincidence with the longitudinal momentum distribution. The controversial case of 23O has been directly addressed. Comparison of our exclusive momentum distributions for the one-neutron removal channel to theoretical momentum distributions calculated with an Eikonal model for the knockout process rendered a spin and parity of Iπ = 1/2+ for the 23O ground state. This result resolves the existent experimental discrepancy and supports sub-shell closure at N = 14 with a fairly pure 2s1/2 neutron in 23O.
15755675
First measurement of beta decay half-lives in neutron-rich Tl and Bi isotopes
Neutron-rich isotopes around lead, beyond N=126, have been studied exploiting the fragmentation of an uranium primary beam at the FRS-RISING setup at GSI. For the first time β-decay half-lives of 219 Bi and 211,212,213 Tl isotopes have been derived. The half-lives have been extracted using a numerical simulation developed for experiments in high-background conditions. Comparison with state of the art models used in r-process calculations is given, showing a systematic underestimation of the experimental values, at variance from close-lying nuclei.
28960903
One-neutron removal measurement reveals 24O as a new doubly magic nucleus.
The first measurement of the momentum distribution for one-neutron removal from (24)O at 920A MeV performed at GSI, Darmstadt is reported. The observed distribution has a width (FWHM) of 99 +/- 4 MeV/c in the projectile rest frame and a one-neutron removal cross section of 63 +/- 7 mb. The results are well explained with a nearly pure 2s_{1/2} neutron spectroscopic factor of 1.74 +/- 0.19 within the eikonal model. This large s-wave probability shows a spherical shell closure thereby confirming earlier suggestions that (24)O is a new doubly magic nucleus.
122123923
Time projection chambers for tracking and identification of radioactive beams
Time projection chambers (TPC) with delay-line readout have been built as position sensitive detectors for high-resolution experiments at the magnetic spectrometer FRS at GSI. The main qualities of TPCs are a small amount of material in the active area, a high tracking efficiency (≈98%), and a spatial resolution of σ∼115μm in x-direction, measured by a delay line, and σ∼50μm in y-direction, measured by drift time. The TPC works as a true two-dimensional detector. A multiple energy-loss measurement allows to identify light fragments (Z=4–6) with a resolution better then σZ<0.14. With the TPCs, several parameters of the spectrometer could be improved considerably and a higher momentum resolution of the FRS was obtained. The TPC construction, test results, and physical experiments are presented.
121806959
Measurements of interaction cross sections for light neutron-rich nuclei at relativistic energies and determination of effective matter radii
Abstract We measured the interaction cross sections ( σ I ) of 10,11B, 12–20C, 14–23N, 16–24O and 18–26F on carbon targets at energies of around 950 A MeV. We then deduced the effective matter radii of the nuclei by a Glauber-model analysis. Based on the assumption of a core plus a valence neutron structure, we applied a Glauber-model analysis for a few-body system adapted for nuclei with an odd neutron number. We also deduced the effective nucleus-matter densities as well as some spectroscopic information for selected nuclei. Evidence for a one-neutron halo structure was found for 22N, 23O and 24F, as well as 19C.
16946306
New Developments in Non-Hermitian Random Matrix Models
In this talk we go over several new developments regarding the techniques for a large class of non-hermitian matrix models with unitary randomness (complex random numbers). In particular, we discuss: 
(a) - A diagrammatic approach based on a $1/N$ expansion 
(b) - A generalization of the addition theorem (R-transformation) 
(c) - A conformal transformation on the position of pertinent singularities 
(d) - A `phase' analysis using appropriate partition functions 
(e) - A number of two-point functions and the issue of universality.
119096516
Chiral Disorder in QCD
Using the Gell{endash}Mann{endash}Oakes{endash}Renner relation and semiclassical arguments, we show that the bulk quark spectrum in QCD exhibits a variety of regimes including the ergodic one described by random matrix theory. We analyze the quark spectral form factor in the diffusive and ballistic regime. We suggest that a class of chiral transitions in QCD is possibly of the metal-insulator type, with a universal spectral statistics at the mobility edge. {copyright} {ital 1998} {ital The American Physical Society}
13930159
Infinite products of large random matrices and matrix-valued diffusion
Abstract We use an extension of the diagrammatic rules in random matrix theory to evaluate spectral properties of finite and infinite products of large complex matrices and large Hermitian matrices. The infinite product case allows us to define a natural matrix-valued multiplicative diffusion process. In both cases of Hermitian and complex matrices, we observe the emergence of a “topological phase transition”, when a hole develops in the eigenvalue spectrum, after some critical diffusion time τ crit is reached. In the case of a particular product of two Hermitian ensembles, we observe also an unusual localization–delocalization phase transition in the spectrum of the considered ensemble. We verify the analytical formulas obtained in this work by numerical simulation.
16527950
Backreaction and the rolling tachyon: an effective action point of view
Abstract We compute the decay of an unstable D9 brane in type IIA string theory including backreaction effects using an effective field theory approach. The open string tachyon on the brane is coupled consistently to the spacetime metric, the dilaton and the RR 9-form. The purpose of this note is to address the fate of the open string energy density, which remains constant if no interaction with the closed string modes is included. Our computations show that taking only into account the coupling to the massless closed strings the total energy stored in the open string sector vanishes asymptotically, independently how small one chooses g s . We find also the large time behaviour of the fields in the Einstein and string frames.
11697968
Minimal surfaces and Reggeization in the AdS / CFT correspondence
Abstract We address the problem of computing scattering amplitudes related to the correlation function of two Wilson lines and/or loops elongated along light-cone directions in strongly coupled gauge theories. Using the AdS/CFT correspondence in the classical approximation, the amplitudes are shown to be related to minimal surfaces generalizing the helicoid in various AdS 5 backgrounds. Infra-red divergences appearing for Wilson lines can be factorized out or can be cured by considering the IR finite case of correlation functions of two Wilson loops. In non-conformal cases related to confining theories, reggeized amplitudes with linear trajectories and unit intercept are obtained and shown to come from the approximately flat metrics near the horizon, which sets the scale for the Regge slope. In the conformal case the absence of confinement leads to a different solution. A transition between both regimes appears, in a confining theory, when varying impact parameter.