4,868 research outputs found
Laser-dressed vacuum polarization in a Coulomb field
We investigate quantum electrodynamic effects under the influence of an
external, time-dependent electromagnetic field, which mediates dynamic
modifications of the radiative corrections. Specifically, we consider the
quantum electrodynamic vacuum-polarization tensor under the influence of two
external background fields: a strong laser field and a nuclear Coulomb field.
We calculate the charge and current densities induced by a nuclear Coulomb
field in the presence of a laser field. We find the corresponding induced
scalar and vector potentials. The induced potential, in first-order
perturbation theory, leads to a correction to atomic energy levels. The
external laser field breaks the rotational symmetry of the system.
Consequently, the induced charge density is not spherically symmetric, and the
energy correction therefore leads to a "polarized Lamb shift." In particular,
the laser generates an additional potential with a quadrupole moment. The
corresponding laser-dressed vacuum-polarization potential behaves like 1/r**3
at large distances, unlike the Uehling potential that vanishes exponentially
for large r. Our investigation might be useful for other situations where
quantum field theoretic phenomena are subjected to external fields of a rather
involved structure.Comment: 13 pages, RevTe
A new method for calculating jet-like QED processes
We consider inelastic QED processes, the cross sections of which do not drop
with increasing energy. Such reactions have the form of two-jet processes with
the exchange of a virtual photon in the t-channel. We consider them in the
region of small scattering angles m/E <= theta << 1, which yield the dominant
contribution to their cross sections. A new effective method is presented to
calculate the corresponding helicity amplitudes. Its basic idea consists in
replacing spinor structures for real and weakly virtual intermediate leptons by
simple transition vertices for real leptons. The obtained compact amplitudes
are particularly suitable for numerical calculations in jet-like kinematics.Comment: 6 pages, 2 figures, Contribution presented by V.G. Serbo at PHOTON
2003, Frascati, Ital
Photon splitting in a laser field
Photon splitting due to vacuum polarization in a laser field is considered.
Using an operator technique, we derive the amplitudes for arbitrary strength,
spectral content and polarization of the laser field. The case of a
monochromatic circularly polarized laser field is studied in detail and the
amplitudes are obtained as three-fold integrals. The asymptotic behavior of the
amplitudes for various limits of interest are investigated also in the case of
a linearly polarized laser field. Using the obtained results, the possibility
of experimental observation of the process is discussed.Comment: 31 pages, 4 figure
O fim do subsĂdio do trigo e a utilização de farinhas mistas.
bitstream/item/119471/1/FOL-04309.pdfTrabalho apresentado na I Jornada Estadual de Tecnologia de Alimentos e Nutrição Humana, Passo Fundo, 1988
Distinguishing Hidden Markov Chains
Hidden Markov Chains (HMCs) are commonly used mathematical models of
probabilistic systems. They are employed in various fields such as speech
recognition, signal processing, and biological sequence analysis. We consider
the problem of distinguishing two given HMCs based on an observation sequence
that one of the HMCs generates. More precisely, given two HMCs and an
observation sequence, a distinguishing algorithm is expected to identify the
HMC that generates the observation sequence. Two HMCs are called
distinguishable if for every there is a distinguishing
algorithm whose error probability is less than . We show that one
can decide in polynomial time whether two HMCs are distinguishable. Further, we
present and analyze two distinguishing algorithms for distinguishable HMCs. The
first algorithm makes a decision after processing a fixed number of
observations, and it exhibits two-sided error. The second algorithm processes
an unbounded number of observations, but the algorithm has only one-sided
error. The error probability, for both algorithms, decays exponentially with
the number of processed observations. We also provide an algorithm for
distinguishing multiple HMCs. Finally, we discuss an application in stochastic
runtime verification.Comment: This is the full version of a LICS'16 pape
Partonic Energy Loss and the Drell-Yan Process
We examine the current status of the extraction of the rate of partonic
energy loss in nuclei from A dependent data. The advantages and difficulties of
using the Drell-Yan process to measure the energy loss of a parton traversing a
cold nuclear medium are discussed. The prospects of using relatively low energy
proton beams for a definitive measurement of partonic energy loss are
presented.Comment: 12 pages, 2 figure
Fractional Energy Loss and Centrality Scaling
The phenomenon of centrality scaling in the high-\pt spectra of
produced in Au-Au collisions at GeV is examined in the framework
of relating fractional energy loss to fractional centrality increase. A new
scaling behavior is found where the scaling variable is given a power-law
dependence on . The exponent specifies the fractional
proportionality relationship between energy loss and centrality, and is a
phenomenologically determined number that characterizes the nuclear suppression
effect. The implication on the parton energy loss in the context of
recombination is discussed.Comment: 4 pages in RevTe
Maximizing the Conditional Expected Reward for Reaching the Goal
The paper addresses the problem of computing maximal conditional expected
accumulated rewards until reaching a target state (briefly called maximal
conditional expectations) in finite-state Markov decision processes where the
condition is given as a reachability constraint. Conditional expectations of
this type can, e.g., stand for the maximal expected termination time of
probabilistic programs with non-determinism, under the condition that the
program eventually terminates, or for the worst-case expected penalty to be
paid, assuming that at least three deadlines are missed. The main results of
the paper are (i) a polynomial-time algorithm to check the finiteness of
maximal conditional expectations, (ii) PSPACE-completeness for the threshold
problem in acyclic Markov decision processes where the task is to check whether
the maximal conditional expectation exceeds a given threshold, (iii) a
pseudo-polynomial-time algorithm for the threshold problem in the general
(cyclic) case, and (iv) an exponential-time algorithm for computing the maximal
conditional expectation and an optimal scheduler.Comment: 103 pages, extended version with appendices of a paper accepted at
TACAS 201
Limit Synchronization in Markov Decision Processes
Markov decision processes (MDP) are finite-state systems with both strategic
and probabilistic choices. After fixing a strategy, an MDP produces a sequence
of probability distributions over states. The sequence is eventually
synchronizing if the probability mass accumulates in a single state, possibly
in the limit. Precisely, for 0 <= p <= 1 the sequence is p-synchronizing if a
probability distribution in the sequence assigns probability at least p to some
state, and we distinguish three synchronization modes: (i) sure winning if
there exists a strategy that produces a 1-synchronizing sequence; (ii)
almost-sure winning if there exists a strategy that produces a sequence that
is, for all epsilon > 0, a (1-epsilon)-synchronizing sequence; (iii) limit-sure
winning if for all epsilon > 0, there exists a strategy that produces a
(1-epsilon)-synchronizing sequence.
We consider the problem of deciding whether an MDP is sure, almost-sure,
limit-sure winning, and we establish the decidability and optimal complexity
for all modes, as well as the memory requirements for winning strategies. Our
main contributions are as follows: (a) for each winning modes we present
characterizations that give a PSPACE complexity for the decision problems, and
we establish matching PSPACE lower bounds; (b) we show that for sure winning
strategies, exponential memory is sufficient and may be necessary, and that in
general infinite memory is necessary for almost-sure winning, and unbounded
memory is necessary for limit-sure winning; (c) along with our results, we
establish new complexity results for alternating finite automata over a
one-letter alphabet
Quantitative multi-objective verification for probabilistic systems
We present a verification framework for analysing multiple quantitative objectives of systems that exhibit both nondeterministic and stochastic behaviour. These systems are modelled as probabilistic automata, enriched with cost or reward structures that capture, for example, energy usage or performance metrics. Quantitative properties of these models are expressed in a specification language that incorporates probabilistic safety and liveness properties, expected total cost or reward, and supports multiple objectives of these types. We propose and implement an efficient verification framework for such properties and then present two distinct applications of it: firstly, controller synthesis subject to multiple quantitative objectives; and, secondly, quantitative compositional verification. The practical applicability of both approaches is illustrated with experimental results from several large case studies
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