38,426 research outputs found
Estimate of crossed-boson-exchange contributions to the binding energy of two-body systems
Binding energies calculated from using the Bethe-Salpeter equation in the
simplest ladder approximation significantly differ from those obtained in the
non-relativistic standard instantaneous approximation. While they should a
priori be better, they turn out to be further away from an exact calculation in
the case of scalar neutral particles or from experiment in the case of the
Coulomb interaction. Part of the discrepancy is due to the omission in the
interaction kernel of contributions corresponding to crossed-boson-exchange
diagrams. The role of these contributions is examined numerically, using a
simple approximation. The sensitivity to both the coupling constant and the
mass of the exchanged boson is considered.Comment: 11 pages, 7 figures, file.ta
Cryptanalysis of an MPEG-Video Encryption Scheme Based on Secret Huffman Tables
This paper studies the security of a recently-proposed MPEG-video encryption
scheme based on secret Huffman tables. Our cryptanalysis shows that: 1) the key
space of the encryption scheme is not sufficiently large against
divide-and-conquer (DAC) attack and known-plaintext attack; 2) it is possible
to decrypt a cipher-video with a partially-known key, thus dramatically
reducing the complexity of the DAC brute-force attack in some cases; 3) its
security against the chosen-plaintext attack is very weak. Some experimental
results are included to support the cryptanalytic results with a brief discuss
on how to improve this MPEG-video encryption scheme.Comment: 8 pages, 4 figure
Data Exchange Problem with Helpers
In this paper we construct a deterministic polynomial time algorithm for the
problem where a set of users is interested in gaining access to a common file,
but where each has only partial knowledge of the file. We further assume the
existence of another set of terminals in the system, called helpers, who are
not interested in the common file, but who are willing to help the users. Given
that the collective information of all the terminals is sufficient to allow
recovery of the entire file, the goal is to minimize the (weighted) sum of bits
that these terminals need to exchange over a noiseless public channel in order
achieve this goal. Based on established connections to the multi-terminal
secrecy problem, our algorithm also implies a polynomial-time method for
constructing the largest shared secret key in the presence of an eavesdropper.
We consider the following side-information settings: (i) side-information in
the form of uncoded packets of the file, where the terminals' side-information
consists of subsets of the file; (ii) side-information in the form of linearly
correlated packets, where the terminals have access to linear combinations of
the file packets; and (iii) the general setting where the the terminals'
side-information has an arbitrary (i.i.d.) correlation structure. We provide a
polynomial-time algorithm (in the number of terminals) that finds the optimal
rate allocations for these terminals, and then determines an explicit optimal
transmission scheme for cases (i) and (ii)
The covarion model of molecular evolution : a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Biology at Massey University
Current methods for constructing evolutionary trees generally do not work well for sequences in which multiple substitutions have occurred. The covarion hypothesis may provide a solution to this problem. This hypothesis states that only a limited number of the codons in a given sequence are free to vary, but that the set of variable codons may change as mutations are fixed in the population. Although this is reasonable from a biological point of view, it is a difficult hypothesis to test scientifically because the apparent large number of parameters involved makes it very hard to analyse statistically. In this study, computer simulations were carried out on up to 51 machines running in parallel, using a simple covarion model based on a hidden Markov model (HMM) approach. This model required two new parameters—the proportion of sites that are variable at any given time, and the rate of exchange between fixed and variable states. These two parameters were both varied in the simulations. Sequence and distance data were simulated on a given tree under this covarion model, and these data were used to test the performance of standard tree-building methods at recovering the original tree The neighbour joining and maximum likelihood methods tested were found to perform better with data generated under the covarion model than with data generated under a simpler model in which all sites vary at the same rate. This suggests that current tree-building methods may perform better with biological data than computer simulation studies suggest
Higher-Spin Interactions: four-point functions and beyond
In this work we construct an infinite class of four-point functions for
massless higher-spin fields in flat space that are consistent with the gauge
symmetry. In the Lagrangian picture, these reflect themselves in a peculiar
non-local nature of the corresponding non-abelian higher-spin couplings implied
by the Noether procedure that starts from the fourth order. We also comment on
the nature of the colored spin-2 excitation present both in the open string
spectrum and in the Vasiliev system, highlighting how some aspects of String
Theory appear to reflect key properties of Field Theory that go beyond its low
energy limit. A generalization of these results to n-point functions, fermions
and mixed-symmetry fields is also addressed.Comment: 66 pages, 10 figures, 1 table, LaTex. Several statements clarified.
Final version to appear in JHE
Recovering Lost Local History: The Daily Record Project
This practitioner perspective describes a collaboration between students and teachers at three middle schools, along with community partners, to recover and digitize news stories from The Daily Record, an African American owned newspaper that was attacked and burned in the 1898 Wilmington coup d’état
Cronin effect and energy conservation constraints in pA collisions at LHC and RHIC
We evaluate the Cronin effect in pA collisions at the CERN LHC and at RHIC in
the framework of Glauber-Eikonal model of initial state multiparton
interactions. Taking carefully into account all kinematical constraints of each
multi-parton interaction process we obtain a softening of the spectrum of
produced partons, improving in this way the agreement of the model with the
recent measurements of pi0 production in d+Au collisions at sqrt{s}=200 AGeV.Comment: Proceeding of 34th International Symposium on Multiparticle dynamics
(ISMD), Sonoma California, 26th-1st Aug 2004, to be published in Acta Phisica
Polonica
- …