228 research outputs found
Scale-Invariant Curvature Fluctuations from an Extended Semiclassical Gravity
We present an extension of the semiclassical Einstein equations which couples
n-point correlation functions of a stochastic Einstein tensor to the n-point
functions of the quantum stress-energy tensor. We apply this extension to
calculate the quantum fluctuations during an inflationary period, where we take
as a model a massive conformally coupled scalar field on a perturbed de Sitter
space and describe how a renormalization independent, almost-scale-invariant
power spectrum of the scalar metric perturbation is produced. Furthermore, we
discuss how this model yields a natural basis for the calculation of
non-Gaussianities of the considered metric fluctuations.Comment: 16 pages, 2 figures; final versio
Thermal bremsstrahlung probing the thermodynamical state of multifragmenting systems
Inclusive and exclusive hard-photon (E 30 MeV) production in five
different heavy-ion reactions (Ar+Au, Ag, Ni,
C at 60{\it A} MeV and Xe+Sn at 50{\it A} MeV) has been
studied coupling the TAPS photon spectrometer with several charged-particle
multidetectors covering more than 80% of 4. The measured spectra, slope
parameters and source velocities as well as their target-dependence, confirm
the existence of thermal bremsstrahlung emission from secondary nucleon-nucleon
collisions that accounts for roughly 20% of the total hard-photon yield. The
thermal slopes are a direct measure of the temperature of the excited nuclear
systems produced during the reaction.Comment: 4 pages, 3 figures, Proceedings CRIS 2000, 3rd Catania Relativistic
Ion Studies, "Phase Transitions in Strong Interactions: Status and
Perspectives", Acicastello, Italy, May 22-26, 2000 (to be published in Nuc.
Phys. A
Quantum Energy Inequalities in Pre-Metric Electrodynamics
Pre-metric electrodynamics is a covariant framework for electromagnetism with a general constitutive law. Its lightcone structure can be more complicated than that of Maxwell theory as is shown by the phenomenon of birefringence. We study the energy density of quantized pre-metric electrodynamics theories with linear constitutive laws admitting a single hyperbolicity double-cone and show that averages of the energy density along the worldlines of suitable observers obey a Quantum Energy Inequality (QEI) in states that satisfy a microlocal spectrum condition. The worldlines must meet two conditions: (a) the classical weak energy condition must hold along them, and (b) their velocity vectors have positive contractions with all positive frequency null covectors (we call such trajectories `subluminal'). After stating our general results, we explicitly quantize the electromagnetic potential in a translationally invariant uniaxial birefringent crystal. Since the propagation of light in such a crystal is governed by two nested lightcones, the theory shows features absent in ordinary (quantized) Maxwell electrodynamics. We then compute a QEI bound for worldlines of inertial `subluminal' observers, which generalizes known results from the Maxwell theory. Finally, it is shown that the QEIs fail along trajectories that have velocity vectors which are timelike with respect to only one of the lightcones
Evidence for Thermal Equilibration in Multifragmentation Reactions probed with Bremsstrahlung Photons
The production of nuclear bremsstrahlung photons (E 30 MeV) has
been studied in inclusive and exclusive measurements in four heavy-ion
reactions at 60{\it A} MeV. The measured photon spectra, angular distributions
and multiplicities indicate that a significant part of the hard-photons are
emitted in secondary nucleon-nucleon collisions from a thermally equilibrated
system. The observation of the thermal component in multi-fragment
Ar+Au reactions suggests that the breakup of the thermalized
source produced in this system occurs on a rather long time-scale.Comment: Revised version, accepted for publication in Physical Review Letters.
4 pages, 4 fig
Alpha-multiplicity in 12C induced reactions
Average a-multiplicities have been determined from a-a coincidence data in the 12C + 16°Gd reaction at bombarding energies of 120 MeV and 200 MeV. The results show that the incomplete fusion reactions a(12C, a) and a(12C, 2a), and the 3a-particle breakup a(12C, 3a) can account for the large inclusive c~-production cross sectio
Alpha-decay branching ratios of near-threshold states in <sup>19</sup>Ne and the astrophysical rate of <sup>15</sup> O(α, γ )<sup>19</sup>Ne
The 15O(α,γ)19Ne reaction is one of two routes for breakout from the hot CNO cycles into the rp process in accreting neutron stars. Its astrophysical rate depends critically on the decay properties of excited states in 19Ne lying just above the 15O + α threshold. We have measured the α-decay branching ratios for these states using the p(21lNe,t)19Ne reaction at 43 MeV/u.</p
Alpha-decay branching ratios of near-threshold states in 19Ne and the astrophysical rate of 15O(alpha,gamma)19Ne
The 15O(alpha,gamma)19Ne reaction is one of two routes for breakout from the
hot CNO cycles into the rp process in accreting neutron stars. Its
astrophysical rate depends critically on the decay properties of excited states
in 19Ne lying just above the 15O + alpha threshold. We have measured the
alpha-decay branching ratios for these states using the p(21Ne,t)19Ne reaction
at 43 MeV/u. Combining our measurements with previous determinations of the
radiative widths of these states, we conclude that no significant breakout from
the hot CNO cycle into the rp process in novae is possible via
15O(alpha,gamma)19Ne, assuming current models accurately represent their
temperature and density conditions
Suppression of soft nuclear bremsstrahlung in proton-nucleus collisions
Photon energy spectra up to the kinematic limit have been measured in 190 MeV
proton reactions with light and heavy nuclei to investigate the influence of
the multiple-scattering process on the photon production. Relative to the
predictions of models based on a quasi-free production mechanism a strong
suppression of bremsstrahlung is observed in the low-energy region of the
photon spectrum. We attribute this effect to the interference of photon
amplitudes due to multiple scattering of nucleons in the nuclear medium.Comment: 12 pages, 3 figures, submitted to Phys. Rev. Let
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