Tracking in Antiproton Annihilation Experiments

A major ingredient of the planned new accelerator complex FAIR, to be constructed at the GSI, Darmstadt, Germany, is the availability of antiproton beams with high quality and intensity. Among the experiments which will make use of this opportunity is PANDA, a dedicated experiment to study antiproton annihilations on nucleons and nuclei. This article gives an overview on the foreseen techniques to perform charged particle tracking in the high rate environment of this experiment.Comment: 1 tar.gz file containing 5 pages paper, 3 figures in 5 files; proceedings of the TIME05 worksho

Fractal dimension of domain walls in two-dimensional Ising spin glasses

We study domain walls in 2d Ising spin glasses in terms of a minimum-weight path problem. Using this approach, large systems can be treated exactly. Our focus is on the fractal dimension $d_f$ of domain walls, which describes via \simL^{d_f} the growth of the average domain-wall length with %% systems size $L\times L$. %% 20.07.07 OM %% Exploring systems up to L=320 we yield $d_f=1.274(2)$ for the case of Gaussian disorder, i.e. a much higher accuracy compared to previous studies. For the case of bimodal disorder, where many equivalent domain walls exist due to the degeneracy of this model, we obtain a true lower bound $d_f=1.095(2)$ and a (lower) estimate $d_f=1.395(3)$ as upper bound. Furthermore, we study the distributions of the domain-wall lengths. Their scaling with system size can be described also only by the exponent $d_f$, i.e. the distributions are monofractal. Finally, we investigate the growth of the domain-wall width with system size (``roughness'') and find a linear behavior.Comment: 8 pages, 8 figures, submitted to Phys. Rev. B; v2: shortened versio

On the merit of a Central Limit Theorem-based approximation in statistical physics

The applicability conditions of a recently reported Central Limit Theorem-based approximation method in statistical physics are investigated and rigorously determined. The failure of this method at low and intermediate temperature is proved as well as its inadequacy to disclose quantum criticalities at fixed temperatures. Its high temperature predictions are in addition shown to coincide with those stemming from straightforward appropriate expansions up to (k_B T)^(-2). Our results are clearly illustrated by comparing the exact and approximate temperature dependence of the free energy of some exemplary physical systems.Comment: 12 pages, 1 figur

A Spitzer view of protoplanetary disks in the gamma Velorum cluster

We present new Spitzer Space Telescope observations of stars in the young ~5 Myr gamma Velorum stellar cluster. Combining optical and 2MASS photometry, we have selected 579 stars as candidate members of the cluster. With the addition of the Spitzer mid-infrared data, we have identified 5 debris disks around A-type stars, and 5-6 debris disks around solar-type stars, indicating that the strong radiation field in the cluster does not completely suppress the production of planetesimals in the disks of cluster members. However, we find some evidence that the frequency of circumstellar primordial disks is lower, and the IR flux excesses are smaller than for disks around stellar populations with similar ages. This could be evidence for a relatively fast dissipation of circumstellar dust by the strong radiation field from the highest mass star(s) in the cluster. Another possibility is that gamma Velorum stellar cluster is slightly older than reported ages and the the low frequency of primordial disks reflects the fast disk dissipation observed at ~5 Myr.Comment: Accepted for publication in the Astrophysical Journal: 32 pages; 11 Figure

Quasar Jets and their Fields

Observations of jets from quasars and other types of accreting black hole are briefly summarized. The importance of beaming and $\gamma$-ray observations for understanding the origin of these jets is emphasised. It is argued that both the power source and the collimation are likely to be magnetic in origin, although the details remain controversial. Ultrarelativistic jets may be formed by the spinning hole and collimated by a hydromagnetic disc wind. Progress in understanding jets has been handicapped by our inadequate knowledge of how magnetic field really behaves under cosmic conditions. Fortunately, significant insights are coming from solar observations, numerical simulation and laboratory plasma experiments. Some possible, evolutionary ramifications are briefly discussed and it is suggested that it is the mass of the black hole relative to that of the galaxy which determines the eventual galaxy morphology.Comment: Latex. 17pages Proc Discusison Meeting on Magnetic Activity in Stars, Discs and Quasars. Ed. D. Lynden-Bell, E. R. Priest and N. O. Weiss. To appear in Phil. Trans. Roy. Soc.

Giant mass and anomalous mobility of particles in fermionic systems

We calculate the mobility of a heavy particle coupled to a Fermi sea within a non-perturbative approach valid at all temperatures. The interplay of particle recoil and of strong coupling effects, leading to the orthogonality catastrophe for an infinitely heavy particle, is carefully taken into account. We find two novel types of strong coupling effects: a new low energy scale $T^{\star}$ and a giant mass renormalization in the case of either near-resonant scattering or a large transport cross section $\sigma$. The mobility is shown to obey two different power laws below and above $T^{\star}$. For $\sigma\gg\lambda_f^2$, where $\lambda_f$ is the Fermi wave length, an exponentially large effective mass suppresses the mobility.Comment: 4 pages, 4 figure

Covalency effects on the magnetism of EuRh2P2

In experiments, the ternary Eu pnictide EuRh2P2 shows an unusual coexistence of a non-integral Eu valence of about 2.2 and a rather high Neel temperature of 50 K. In this paper, we present a model which explains the non-integral Eu valence via covalent bonding of the Eu 4f-orbitals to P2 molecular orbitals. In contrast to intermediate valence models where the hybridization with delocalized conduction band electrons is known to suppress magnetic ordering temperatures to at most a few Kelvin, covalent hybridization to the localized P2 orbitals avoids this suppression. Using perturbation theory we calculate the valence, the high temperature susceptibility, the Eu single-ion anisotropy and the superexchange couplings of nearest and next-nearest neighbouring Eu ions. The model predicts a tetragonal anisotropy of the Curie constants. We suggest an experimental investigation of this anisotropy using single crystals. From experimental values of the valence and the two Curie constants, the three free parameters of our model can be determined.Comment: 9 pages, 5 figures, submitted to J. Phys.: Condens. Matte

Toward accurate CO_2 and CH_4 observations from GOSAT

The column-average dry air mole fractions of atmospheric carbon dioxide and methane (X_(CO_2) and X_(CH_4)) are inferred from observations of backscattered sunlight conducted by the Greenhouse gases Observing SATellite (GOSAT). Comparing the first year of GOSAT retrievals over land with colocated ground-based observations of the Total Carbon Column Observing Network (TCCON), we find an average difference (bias) of −0.05% and −0.30% for X_(CO_2) and X_(CH_4) with a station-to-station variability (standard deviation of the bias) of 0.37% and 0.26% among the 6 considered TCCON sites. The root-mean square deviation of the bias-corrected satellite retrievals from colocated TCCON observations amounts to 2.8 ppm for X_(CO_2) and 0.015 ppm for X_(CH_4). Without any data averaging, the GOSAT records reproduce general source/sink patterns such as the seasonal cycle of X_(CO_2) suggesting the use of the satellite retrievals for constraining surface fluxes

Unified description of Fermi and non-Fermi liquid behavior in a conserving slave boson approximation for strongly correlated impurity models

We show that the presence of Fermi or non-Fermi liquid behavior in the SU(N) x SU(M) Anderson impurity models may be read off the infrared threshold exponents governing the spinon and holon dynamics in a slave boson representation of these models. We construct a conserving T-matrix approximation which recovers the exact exponents with good numerical accuracy. Our approximation includes both coherent spin flip scattering and charge fluctuation processes. For the single-channel case the tendency to form bound states drastically modifies the low energy behavior. For the multi-channel case in the Kondo limit the bound state contributions are unimportant.Comment: 4 pages, Latex, 3 postscript figures included Final version with minor changes in wording, to appear in Phys.Rev.Let