364 research outputs found
Perturbative Quantum Field Theory at Positive Temperatures: An Axiomatic Approach
It is shown that the perturbative expansions of the correlation functions of
a relativistic quantum field theory at finite temperature are uniquely
determined by the equations of motion and standard axiomatic requirements,
including the KMS condition. An explicit expression as a sum over generalized
Feynman graphs is derived. The canonical formalism is not used, and the
derivation proceeds from the beginning in the thermodynamic limit. No doubling
of fields is invoked. An unsolved problem concerning existence of these
perturbative expressions is pointed out.Comment: 17pages Late
Emission patterns of neutral pions in 40 A MeV Ta+Au reactions
Differential cross sections of neutral pions emitted in 181Ta + 197Au
collisions at a beam energy of 39.5A MeV have been measured with the photon
spectrometer TAPS. The kinetic energy and transverse momentum spectra of
neutral pions cannot be properly described in the framework of the thermal
model, nor when the reabsorption of pions is accounted for in a
phenomenological model. However, high energy and high momentum tails of the
pion spectra can be well fitted through thermal distributions with unexpectedly
soft temperature parameters below 10 MeV.Comment: 16 pages (double-spaced), 5 figures; corrections after referee's
comments and suggestion
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
Dynamic IMF production in at intermediate energies
The azimuthal correlations and polar-angle distributions of intermediate-mass fragments (IMFs) produced in Mg+Al at 45 an 95 AMeV were studied. Measurements of -particles and IMFs with emmitted in the mid-rapidity region for mid-central events were compared to IQMD results and results from a static-source model. A maximum in the azimuthal-correlation function at 180\degree\/ can not be described by independently emmitted particles. Momentum conservation of a small source as well as target-projectile correlations from IQMD show the same azimuthal correlations as the experimental data. The polar-angle distributions in the experimental data show a target-projectile seperation, thus giving evidence of dynamic IMF production.\\ {\it Keywords:} dynamic multifragmentation, IMF, IQMD, azimuthal correlations
Deterministic delivery of externally cold and precisely positioned single molecular ions
We present the preparation and deterministic delivery of a selectable number
of externally cold molecular ions. A laser cooled ensemble of Mg^+ ions
subsequently confined in several linear Paul traps inter-connected via a
quadrupole guide serves as a cold bath for a single or up to a few hundred
molecular ions. Sympathetic cooling embeds the molecular ions in the
crystalline structure. MgH^+ ions, that serve as a model system for a large
variety of other possible molecular ions, are cooled down close to the Doppler
limit and are positioned with an accuracy of one micrometer. After the
production process, severely compromising the vacuum conditions, the molecular
ion is efficiently transfered into nearly background-free environment. The
transfer of a molecular ion between different traps as well as the control of
the molecular ions in the traps is demonstrated. Schemes, optimized for the
transfer of a specific number of ions, are realized and their efficiencies are
evaluated. This versatile source applicable for broad charge-to-mass ratios of
externally cold and precisely positioned molecular ions can serve as a
container-free target preparation device well suited for diffraction or
spectroscopic measurements on individual molecular ions at high repetition
rates (kHz).Comment: 11 pages, 8 figure
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
Photon Production in Heavy-ion Collisions Close to the Pion Threshold
We report on a measurement of hard photons (Eg>30 MeV) in the reaction Ar+Ca
at 180A MeV at an energy in which photons from the decay of pi0 mesons are
dominating. Simultaneous measurement with the TAPS spectrometer of the photon
spectrum and photon-photon coincidences used for the identification of pi0
enabled the subtraction of pi0 contribution. The resulting photon spectrum
exhibits an exponential shape with an inverse slope of
E0=(53+-0.03(stat)-5+8(syst)) MeV. The photon multiplicity, equal to
(1.21+-0.03(stat)+0.3-0.2(syst))10E0-2, is roughly one order of magnitude
larger than the value extrapolated from existing systematics. This enhancement
of the hard photon production is attributed to a strong increase in the
contribution of secondary np collisions to the total photon yield. We conclude
that, on average, the number of np collisions which contribute to the hard
photon production is 7 times larger than the number of first chance np
collisions in the reaction Ar+Ca at 180A MeV.Comment: 15 pages, 4 figures, references adde
IMF-IMF azimuthal correlations as a tool to probe reaction dynamics in Ar-36+Ti-48 at 45 A MeV
IMF-IMF azimuthal correlation functions have been measured in 36Ar + 48Ti collisions at 45 A MeV. The experimental data as well as the events generated with IQMD were sorted with respect to impact parameter and the analysis was restricted to the mid-rapidity region. We find strong anisotropies and asymmetries in the measured correlation functions, which cannot be described by independently emitted particles. The measured correlation functions are, however, well reproduced by IQMD model calculations, although decay of a small mid-rapidity source cannot be excluded experimentally
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