175 research outputs found
The structure of Green functions in quantum field theory with a general state
In quantum field theory, the Green function is usually calculated as the
expectation value of the time-ordered product of fields over the vacuum. In
some cases, especially in degenerate systems, expectation values over general
states are required. The corresponding Green functions are essentially more
complex than in the vacuum, because they cannot be written in terms of standard
Feynman diagrams. Here, a method is proposed to determine the structure of
these Green functions and to derive nonperturbative equations for them. The
main idea is to transform the cumulants describing correlations into
interaction terms.Comment: 13 pages, 6 figure
Auditory distraction during reading: A Bayesian meta-analysis of a continuing controversy
Everyday reading occurs in different settings, such as on the train to work, in a busy cafeteria, or at home, while listening to music. In these situations, readers are exposed to external auditory stimulation from nearby noise, speech, or music that may distract them from their task and reduce their comprehension. Although many studies have investigated auditory distraction effects during reading, the results have proved to be inconsistent and sometimes even contradictory. Additionally, the broader theoretical implications of the findings have not always been explicitly considered. In the present study, we report a Bayesian meta-analysis of 65 studies on auditory distraction effects during reading and use meta-regression models to test predictions derived from existing theories. The results showed that background noise, speech, and music all have a small, but reliably detrimental effect on reading performance. The degree of disruption in reading comprehension did not generally differ between adults and children. Intelligible speech and lyrical music resulted in the biggest distraction. While this last result is consistent with theories of semantic distraction, there was also reliable distraction by noise. It is argued that new theoretical models are needed that can account for distraction by both background speech and noise
(Borel) convergence of the variationally improved mass expansion and the O(N) Gross-Neveu model mass gap
We reconsider in some detail a construction allowing (Borel) convergence of
an alternative perturbative expansion, for specific physical quantities of
asymptotically free models. The usual perturbative expansions (with an explicit
mass dependence) are transmuted into expansions in 1/F, where
for while for m \lsim \Lambda,
being the basic scale and given by renormalization group
coefficients. (Borel) convergence holds in a range of which corresponds to
reach unambiguously the strong coupling infrared regime near , which
can define certain "non-perturbative" quantities, such as the mass gap, from a
resummation of this alternative expansion. Convergence properties can be
further improved, when combined with expansion (variationally improved
perturbation) methods. We illustrate these results by re-evaluating, from
purely perturbative informations, the O(N) Gross-Neveu model mass gap, known
for arbitrary from exact S matrix results. Comparing different levels of
approximations that can be defined within our framework, we find reasonable
agreement with the exact result.Comment: 33 pp., RevTeX4, 6 eps figures. Minor typos, notation and wording
corrections, 2 references added. To appear in Phys. Rev.
Open and Hidden Charm Production in 920 GeV Proton-Nucleus Collisions
The HERA-B collaboration has studied the production of charmonium and open
charm states in collisions of 920 GeV protons with wire targets of different
materials. The acceptance of the HERA-B spectrometer covers negative values of
xF up to xF=-0.3 and a broad range in transverse momentum from 0.0 to 4.8
GeV/c. The studies presented in this paper include J/psi differential
distributions and the suppression of J/psi production in nuclear media.
Furthermore, production cross sections and cross section ratios for open charm
mesons are discussed.Comment: 5 pages, 9 figures, to be published in the proceedings of the 6th
International Conference on Hyperons, Charm & Beauty Hadrons (BEACH04),
Chicago, IL, June 27 - July 3, 200
Spintronics: Fundamentals and applications
Spintronics, or spin electronics, involves the study of active control and
manipulation of spin degrees of freedom in solid-state systems. This article
reviews the current status of this subject, including both recent advances and
well-established results. The primary focus is on the basic physical principles
underlying the generation of carrier spin polarization, spin dynamics, and
spin-polarized transport in semiconductors and metals. Spin transport differs
from charge transport in that spin is a nonconserved quantity in solids due to
spin-orbit and hyperfine coupling. The authors discuss in detail spin
decoherence mechanisms in metals and semiconductors. Various theories of spin
injection and spin-polarized transport are applied to hybrid structures
relevant to spin-based devices and fundamental studies of materials properties.
Experimental work is reviewed with the emphasis on projected applications, in
which external electric and magnetic fields and illumination by light will be
used to control spin and charge dynamics to create new functionalities not
feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes
from the published versio
Charged pions from Ni on Ni collisions between 1 and 2 AGeV
Charged pions from Ni + Ni reactions at 1.05, 1.45 and 1.93 AGeV are measured with the FOPI detector. The mean multiplicities per mean number of participants increase with beam energy, in accordance with earlier studies of the Ar + KCl and La + La systems. The pion kinetic energy spectra have concave shape and are fitted by the superposition of two Boltzmann distributions with different temperatures. These apparent temperatures depend only weakly on bombarding energy. The pion angular distributions show a forward/backward enhancement at all energies, but not the enhancement which was observed in case of the Au + Au system. These features also determine the rapidity distributions which are therefore in disagreement with the hypothesis of one thermal source. The importance of the Coulomb interaction and of the pion rescattering by spectator matter in producing these phenomena is discussed
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