1,108 research outputs found
Effect of the source charge on charged-beam interferometry
We investigate quantal perturbations of the interferometric correlations of
charged bosons by the Coulomb field of an instantaneous, charged source. The
source charge increases the apparent source size by weakening the correlation
at non-zero relative momenta. The effect is strongest for pairs with a small
total momentum and is stronger for kaons than for pions of the same momenta.
The experimental data currently available are well described by this effect
without invoking Pratt's exploding source model. A simple expression is
proposed to account for the effect.Comment: 9 pages TEX, 3 Postscript figures available at
http://www.krl.caltech.edu/preprints/MAP.htm
Neutrons from multiplicity-selected Au-Au collisions at 150, 250, 400, and 650 AMeV
We measured neutron triple-differential cross sections from
multiplicity-selected Au-Au collisions at 150, 250, 400, and 650 \AMeV. The
reaction plane for each collision was estimated from the summed transverse
velocity vector of the charged fragments emitted in the collision. We examined
the azimuthal distribution of the triple-differential cross sections as a
function of the polar angle and the neutron rapidity. We extracted the average
in--plane transverse momentum and the normalized
observable , where is the neutron
transverse momentum, as a function of the neutron center-of-mass rapidity, and
we examined the dependence of these observables on beam energy. These
collective flow observables for neutrons, which are consistent with those of
protons plus bound nucleons from the Plastic Ball Group, agree with the
Boltzmann--Uehling--Uhlenbeck (BUU) calculations with a momentum--dependent
interaction. Also, we calculated the polar-angle-integrated maximum azimuthal
anisotropy ratio R from the value of .Comment: 20 LaTeX pages. 11 figures to be faxed on request, send email to
sender's addres
Causality in relativistic many body theory
The stability of the nuclear matter system with respect to density
fluctuations is examined exploring in detail the pole structure of the
electro-nuclear response functions. Making extensive use of the method of
dispersion integrals we calculate the full polarization propagator not only for
real energies in the spacelike and timelike regime but also in the whole
complex energy plane. The latter proved to be necessary in order to identify
unphysical causality violating poles which are the consequence of a neglection
of vacuum polarization. On the contrary it is shown that Dirac sea effects
stabilize the nuclear matter system shifting the unphysical pole from the upper
energy plane back to the real axis. The exchange of strength between these real
timelike collective excitations and the spacelike energy regime is shown to
lead to a reduction of the quasielastic peak as it is seen in electron
scattering experiments. Neglecting vacuum polarization one also obtains a
reduction of the quasielastic peak but in this case the strength is partly
shifted to the causality violating pole mentioned above which consequently
cannot be considered as a physical reliable result. Our investigation of the
response function in the energy region above the threshold of nucleon
anti-nucleon production leads to another remarkable result. Treating the
nucleons as point-like Dirac particles we show that for any isospin independent
NN-interaction RPA-correlations provide a reduction of the production amplitude
for -pairs by a factor 2.Comment: 19 pages Latex including 12 postscript figure
Neutrons from multiplicity-selected La-La and Nb-Nb collisions at 400A MeV and La-La collisions at 250A MeV
Triple-differential cross sections for neutrons from high-multiplicity La-La
collisions at 250 and 400 MeV per nucleon and Nb-Nb collisions at 400 MeV per
nucleon were measured at several polar angles as a function of the azimuthal
angle with respect to the reaction plane of the collision. The reaction plane
was determined by a transverse-velocity method with the capability of
identifying charged-particles with Z=1, Z=2, and Z > 2. The flow of neutrons
was extracted from the slope at mid-rapidity of the curve of the average
in-plane momentum vs the center-of-mass rapidity. The squeeze-out of the
participant neutrons was observed in a direction normal to the reaction plane
in the normalized momentum coordinates in the center-of-mass system.
Experimental results of the neutron squeeze-out were compared with BUU
calculations. The polar-angle dependence of the maximum azimuthal anisotropy
ratio was found to be insensitive to the mass of the colliding
nuclei and the beam energy. Comparison of the observed polar-angle dependence
of the maximum azimuthal anisotropy ratio with BUU calculations for
free neutrons revealed that is insensitive also to the
incompressibility modulus in the nuclear equation of state.Comment: ReVTeX, 16 pages, 17 figures. To be published in Physical Review
Gene Discovery and Functional Genomics in the Pig
Advances in gene mapping and genomics in farm animals have been considerable over the past decade. Medium resolution linkage and physical maps have been reported, and specific chromosomal regions and genes associated with traits of biological and economic interest have been identified. We have reached an exciting stage in gene identification, mapping and quantitative trait locus discovery in pigs, as new molecular information is accumulating rapidly. Significant progress has been made by identifying candidate gene associations and low-resolution regions containing quantitative trail loci (QTL). However, we are still disadvantaged by the lack of tools available to efficiently use much of this new information. For example, current pig maps are neither of high enough resolution nor sufficiently informative at the comparative level for positional candidate gene cloning within QTL regions. As well, studying biological mechanisms underlying economically important traits such as reproduction is limited by the lack of molecular resources. This is especially important, as reproduction is very difficult to genetically improve by classical breeding methods due to the relatively low heritability and high expense in data collection. Thus, an improved understanding of porcine reproductive biology is of crucial economic importance, yet reproductive processes are poorly characterized at the molecular level. Recently, new methodologies have been brought to bear on a better understanding of pig molecular biology for accelerating genetic improvement in pigs. Several groups are developing molecular information in the pig, and the total Genbank sequence entries for porcine expressed genes have recently topped 100,000. Our Midwest EST Consortium has produced cDNA libraries containing the majority of genes expressed in major female reproductive tissues, and we have deposited nearly 15,000 gene sequences into public databases. These sequences represent over 8,900 different genes, based on sequence comparison among these data. Furthermore, we have developed computer software to automatically extract sequence similarity of these pig genes with their human counterparts, as well as the mapping information of these human homologues. Within our data set, we have identified nearly 1,500 pig genes with strong similarity to mapped human genes, and we are in the process of mapping 700 of these genes to improve the human-pig comparative map. This work and the complementary work of others can now be used to more rapidly understand and identify the genes controlling reproduction, so that genetic improvement of reproduction phenotypes can accelerate
Maximum Azimuthal Anisotropy of Neutrons from Nb-Nb Collisions at 400 AMeV and the Nuclear Equation of State
We measured the first azimuthal distributions of triple--differential cross
sections of neutrons emitted in heavy-ion collisions, and compared their
maximum azimuthal anisotropy ratios with Boltzmann--Uehling--Uhlenbeck (BUU)
calculations with a momentum-dependent interaction. The BUU calculations agree
with the triple- and double-differential cross sections for positive rapidity
neutrons emitted at polar angles from 7 to 27 degrees; however, the maximum
azimuthal anisotropy ratio for these free neutrons is insensitive to the size
of the nuclear incompressibility modulus K characterizing the nuclear matter
equation of state.Comment: Typeset using ReVTeX, with 3 ps figs., uuencoded and appende
Collective Flow from the Intranuclear Cascade Model
The phenomenon of collective flow in relativistic heavy ion collisions is
studied using the hadronic cascade model ARC. Direct comparison is made to data
gathered at the Bevalac, for Au+Au at GeV/c. In contrast to the
standard lore about the cascade model, collective flow is well described
quantitatively without the need for explicit mean field terms to simulate the
nuclear equation of state. Pion collective flow is in the opposite direction to
nucleon flow as is that of anti-nucleons and other produced particles. Pion and
nucleon flow are predicted at AGS energies also, where, in light of the higher
baryon densities achieved, we speculate that equation of state effects may be
observable.Comment: 9 pages, 2 figures include
Detectability of Strange Matter in Heavy Ion Experiments
We discuss the properties of two distinct forms of hypothetical strange
matter, small lumps of strange quark matter (strangelets) and of hyperon matter
(metastable exotic multihypernuclear objects: MEMOs), with special emphasis on
their relevance for present and future heavy ion experiments. The masses of
small strangelets up to A = 40 are calculated using the MIT bag model with
shell mode filling for various bag parameters. The strangelets are checked for
possible strong and weak hadronic decays, also taking into account multiple
hadron decays. It is found that strangelets which are stable against strong
decay are most likely highly negative charged, contrary to previous findings.
Strangelets can be stable against weak hadronic decay but their masses and
charges are still rather high. This has serious impact on the present high
sensitivity searches in heavy ion experiments at the AGS and CERN facilities.
On the other hand, highly charged MEMOs are predicted on the basis of an
extended relativistic mean-field model. Those objects could be detected in
future experiments searching for short-lived, rare composites. It is
demonstrated that future experiments can be sensitive to a much wider variety
of strangelets.Comment: 26 pages, 5 figures, uses RevTeX and epsf.st
Rapidity and centrality dependence of particle production for identified hadrons in Cu+Cu collisions at GeV
The BRAHMS collaboration has measured transverse momentum spectra of pions,
kaons, protons and antiprotons at rapidities 0 and 3 for Cu+Cu collisions at
GeV. As the collisions become more central the collective
radial flow increases while the temperature of kinetic freeze-out decreases.
The temperature is lower and the radial flow weaker at forward rapidity. Pion
and kaon yields with transverse momenta between 1.5 and 2.5 GeV/c are
suppressed for central collisions relative to scaled collisions. This
suppression, which increases as the collisions become more central is
consistent with jet quenching models and is also present with comparable
magnitude at forward rapidity. At such rapidities initial state effects may
also be present and persistence of the meson suppression to high rapidity may
reflect a combination of jet quenching and nuclear shadowing. The ratio of
protons to mesons increases as the collisions become more central and is
largest at forward rapidities.Comment: 19 pages, 11 figures and 6 table
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