3,320 research outputs found
A prototype for the AMS-RICH experiment
The AMS spectrometer will be installed on the International Space Station in
2005. Among other improvements over the first version of the instrument, a ring
imaging Cherenkov detector (RICH) will be added and should open a new window
for cosmic-ray physics, allowing isotope separation up to A = 25 between 1 and
10 GeV/c and element identification up to Z = 25 between threshold and 1
TeV/c/nucleon. It should also contribute to the high level of redundancy
required for AMS and reject efficiency albedo particles. A second generation
prototype has been operated for a few months : the architecture and the first
results are presented.Comment: Proceedings of the 3rd International Conference on "New developments
in photodetection" (Beaune - France
Spectral Orbits and Peak-to-Average Power Ratio of Boolean Functions with respect to the {I,H,N}^n Transform
We enumerate the inequivalent self-dual additive codes over GF(4) of
blocklength n, thereby extending the sequence A090899 in The On-Line
Encyclopedia of Integer Sequences from n = 9 to n = 12. These codes have a
well-known interpretation as quantum codes. They can also be represented by
graphs, where a simple graph operation generates the orbits of equivalent
codes. We highlight the regularity and structure of some graphs that correspond
to codes with high distance. The codes can also be interpreted as quadratic
Boolean functions, where inequivalence takes on a spectral meaning. In this
context we define PAR_IHN, peak-to-average power ratio with respect to the
{I,H,N}^n transform set. We prove that PAR_IHN of a Boolean function is
equivalent to the the size of the maximum independent set over the associated
orbit of graphs. Finally we propose a construction technique to generate
Boolean functions with low PAR_IHN and algebraic degree higher than 2.Comment: Presented at Sequences and Their Applications, SETA'04, Seoul, South
Korea, October 2004. 17 pages, 10 figure
Dessins, their delta-matroids and partial duals
Given a map on a connected and closed orientable surface, the
delta-matroid of is a combinatorial object associated to which captures some topological information of the embedding. We explore how
delta-matroids associated to dessins d'enfants behave under the action of the
absolute Galois group. Twists of delta-matroids are considered as well; they
correspond to the recently introduced operation of partial duality of maps.
Furthermore, we prove that every map has a partial dual defined over its field
of moduli. A relationship between dessins, partial duals and tropical curves
arising from the cartography groups of dessins is observed as well.Comment: 34 pages, 20 figures. Accepted for publication in the SIGMAP14
Conference Proceeding
On the Classification of All Self-Dual Additive Codes over GF(4) of Length up to 12
We consider additive codes over GF(4) that are self-dual with respect to the
Hermitian trace inner product. Such codes have a well-known interpretation as
quantum codes and correspond to isotropic systems. It has also been shown that
these codes can be represented as graphs, and that two codes are equivalent if
and only if the corresponding graphs are equivalent with respect to local
complementation and graph isomorphism. We use these facts to classify all codes
of length up to 12, where previously only all codes of length up to 9 were
known. We also classify all extremal Type II codes of length 14. Finally, we
find that the smallest Type I and Type II codes with trivial automorphism group
have length 9 and 12, respectively.Comment: 18 pages, 4 figure
Algebraic Correlation Function and Anomalous Diffusion in the HMF model
In the quasi-stationary states of the Hamiltonian Mean-Field model, we
numerically compute correlation functions of momenta and diffusion of angles
with homogeneous initial conditions. This is an example, in a N-body
Hamiltonian system, of anomalous transport properties characterized by non
exponential relaxations and long-range temporal correlations. Kinetic theory
predicts a striking transition between weak anomalous diffusion and strong
anomalous diffusion. The numerical results are in excellent agreement with the
quantitative predictions of the anomalous transport exponents. Noteworthy, also
at statistical equilibrium, the system exhibits long-range temporal
correlations: the correlation function is inversely proportional to time with a
logarithmic correction instead of the usually expected exponential decay,
leading to weak anomalous transport properties
Quasi-local evolution of cosmic gravitational clustering in the weakly non-linear regime
We investigate the weakly non-linear evolution of cosmic gravitational
clustering in phase space by looking at the Zel'dovich solution in the discrete
wavelet transform (DWT) representation. We show that if the initial
perturbations are Gaussian, the relation between the evolved DWT mode and the
initial perturbations in the weakly non-linear regime is quasi-local. That is,
the evolved density perturbations are mainly determined by the initial
perturbations localized in the same spatial range. Furthermore, we show that
the evolved mode is monotonically related to the initial perturbed mode. Thus
large (small) perturbed modes statistically correspond to the large (small)
initial perturbed modes. We test this prediction by using QSO Ly
absorption samples. The results show that the weakly non-linear features for
both the transmitted flux and identified forest lines are quasi-localized. The
locality and monotonic properties provide a solid basis for a DWT
scale-by-scale Gaussianization reconstruction algorithm proposed by Feng & Fang
(Feng & Fang, 2000) for data in the weakly non-linear regime. With the
Zel'dovich solution, we find also that the major non-Gaussianity caused by the
weakly non-linear evolution is local scale-scale correlations. Therefore, to
have a precise recovery of the initial Gaussian mass field, it is essential to
remove the scale-scale correlations.Comment: 22 pages, 13 figures. Accepted for publication in the Astrophysical
Journa
Dynamics of pairwise motions
We derive a simple closed-form expression, relating \vs(r) -- the mean
relative velocity of pairs of galaxies at fixed separation -- to the
two-point correlation function of mass density fluctuations, . We
compare our analytic model for \vs(r) with N-body simulations, and find
excellent agreement in the entire dynamical range probed by the simulations
(0.1 \lsim \xi \lsim 1000). Our results can be used to estimate the
cosmological density parameter, \Om, directly from redshift-distance surveys,
like Mark III.Comment: 10 pages 2 Figs., submitted to ApJ Let
The Bispectrum of IRAS Galaxies
We compute the bispectrum for the galaxy distribution in the IRAS QDOT, 2Jy,
and 1.2Jy redshift catalogs for wavenumbers 0.05<k<0.2 h/Mpc and compare the
results with predictions from gravitational instability in perturbation theory.
Taking into account redshift space distortions, nonlinear evolution, the survey
selection function, and discreteness and finite volume effects, all three
catalogs show evidence for the dependence of the bispectrum on configuration
shape predicted by gravitational instability. Assuming Gaussian initial
conditions and local biasing parametrized by linear and non-linear bias
parameters b_1 and b_2, a likelihood analysis yields 1/b_1 =
1.32^{+0.36}_{-0.58}, 1.15^{+0.39}_{-0.39} and b_2/b_1^2=-0.57^{+0.45}_{-0.30},
-0.50^{+0.31}_{-0.51}, for the for the 2Jy and 1.2Jy samples, respectively.
This implies that IRAS galaxies trace dark matter increasingly weakly as the
density contrast increases, consistent with their being under-represented in
clusters. In a model with chi^2 non-Gaussian initial conditions, the bispectrum
displays an amplitude and scale dependence different than that found in the
Gaussian case; if IRAS galaxies do not have bias b_1> 1 at large scales, \chi^2
non-Gaussian initial conditions are ruled out at the 95% confidence level. The
IRAS data do not distinguish between Lagrangian or Eulerian local bias.Comment: 30 pages, 11 figure
Probability distribution of density fluctuations in the non-linear regime
We present a general procedure for obtaining the present density fluctuation
probability distribution given the statistics of the initial conditions. The
main difficulties faced with regard to this problem are those related to the
non-linear evolution of the density fluctuations and those posed by the fact
that the fields we are interested in are the result of filtering an underlying
field with structure down to scales much smaller than that of filtering. The
solution to the latter problem is discussed here in detail and the solution to
the former is taken from a previous work.
We have checked the procedure for values of the rms density fluctuation as
large as 3/2 and several power spectra and found that it leads to results in
excellent agreement with those obtained in numerical simulations. We also
recover all available exact results from perturbation theory.Comment: Accepted to be published in Ap
- âŠ