394 research outputs found
Heavy Flavor Enhancement as a Signal of Color Deconfinement
We argue that the color deconfinement in heavy ion collisions may lead to
enhanced production of hadrons with open heavy flavor (charm or bottom). We
estimate the upper bound of this enhancement.Comment: 7 pages, LaTeX, 3 PS-figure
Inherited multifocal RPE-diseases: mechanisms for local dysfunction in global retinoid cycle gene defects
AbstractAlterations of retinoid cycle genes are known to cause retinal diseases characterized by focal white dot fundus lesions. Fundus appearances reveal circumscribed RPE-changes, although generalized metabolic defects and global functional abnormalities are present. As a possible explanation, topographic inhomogeneities of the human photoreceptor mosaic and the role of a cone specific visual cycle will be discussed. Due to particular characteristics of photoreceptor subtypes as well as different pathways for photopigment regeneration the metabolic demand of individual RPE cells might differ. In “flecked retina diseases” heterogeneity of metabolic demand in individual RPE cells could therefore be responsible for their multifocal appearance
On Justification, Idealization, and Discursive Purchase
Conceptions of acceptability-based moral or political justification take it that authoritative acceptability, widely conceived, constitutes, or contributes to, validity, or justification. There is no agreement as to what bar for authoritativeness such justification may employ. The paper engages the issue in relation to (i) the level of idealization that a bar for authoritativeness, ψ, imparts to a standard of acceptability-based justification, S, and (ii) the degree of discursive purchase of the discursive standing that S accords to people when it builds ψ. I argue that (i) and (ii) are interdependent: high idealization values entail low discursive purchase, while high degrees of purchase require low idealization values. I then distinguish between alethic conceptions of justification that prioritize ends that commit to high idealization values, and recognitive conceptions that favor high discursive purchase. On this basis, I argue for a moderately recognitivist constraint on idealization. To render the recognitive discursive minimum available to relevant people at the site of justification, S should set ψ low enough so that it is a genuine option for actual people to reject relevant views in ways that S recognizes as authoritative. (The Appendix applies this to a Forst-type view of reciprocity of reasons to draw out some limitations of this view.) [Draft available from author on request.
High Precision Ionization Chamber for Relative Intensity Monitoring of Synchrotron Radiation
A single channel, high precision ionization chamber has been built for
monitoring the relative intensity of X-rays in the energy range above 5 keV. It
can be used in experiments, such as EXAFS, where simultaneous high precision
monitoring of the relative intensity during the actual experiment is required.
In this paper the construction of the chamber and its performance during test
measurements with an X-ray tube are presented.Comment: 10 pages, 8 figure
Open charm enhancement by secondary interactions in relativistic nucleus-nucleus collisions?
We calculate open charm production in reactions at SPS energies
within the HSD transport approach - which is based on string, quark, diquark
() and hadronic degrees of freedom - including
the production of open charm pairs from secondary 'meson'-'baryon' (or
quark-diquark and antiquark-diquark) collisions. It is argued that at collision
energies close to the pair threshold the dominant production
mechanism is related to the two body (or quasi two body) reactions . Estimates within the framework of
the Quark-Gluon String model suggest cross sections of a few for in the region of 1 GeV above threshold. The dynamical
transport calculations for at 160 AGeV indicate that the open
charm enhancement reported by the NA50 Collaboration might be due to such
secondary reaction mechanisms.Comment: 14 pages, LaTeX, including 5 postscript figures, submitted to Phys.
Lett.
Parton model versus color dipole formulation of the Drell-Yan process
In the kinematical region where the center of mass energy is much larger than
all other scales, the Drell-Yan process can be formulated in the target rest
frame in terms of the same color dipole cross section as low Bjorken-x deep
inelastic scattering. Since the mechanisms for heavy dilepton production appear
very different in the dipole approach and in the conventional parton model, one
may wonder whether these two formulations really represent the same physics. We
perform a comparison of numerical calculations in the color dipole approach
with calculations in the next-to-leading order parton model. For proton-proton
scattering, the results are very similar at low x_2 from fixed target to RHIC
energies, confirming the close connection between these two very different
approaches. We also compare the transverse momentum distributions of Drell-Yan
dileptons predicted in both formulations. The range of applicability of the
dipole formulation and the impact of future Drell-Yan data from RHIC for
determining the color dipole cross section are discussed. A detailed derivation
of the dipole formulation of the Drell-Yan process is also included.Comment: 20 pages, 5 figure
Hard diffraction in hadron--hadron interactions and in photoproduction
Hard single diffractive processes are studied within the framework of the
triple--Pomeron approximation. Using a Pomeron structure function motivated by
Regge--theory we obtain parton distribution functions which do not obey
momentum sum rule. Based on Regge-- factorization cross sections for hard
diffraction are calculated. Furthermore, the model is applied to hard
diffractive particle production in photoproduction and in
interactions.Comment: 13 pages, Latex, 13 uuencoded figure
Open charm production in relativistic nucleus-nucleus collisions
We calculate excitation functions for open charm mesons in reactions
from AGS to RHIC energies within the HSD transport approach which is based on
string, quark, diquark () and hadronic degrees
of freedom. The open charm cross sections from and reactions are
fitted to results from PYTHIA and scaled in magnitude to the available
experimental data. From our dynamical calculations we find an approximate
-scaling for pions, kaons, -mesons and -- when discarding
final state elastic scattering of kaons and -mesons with pions -- in
central collisions of at 160 AGeV (with an apparent slope of
176 MeV) without employing the assumption of a Quark-Gluon Plasma (QGP). We
demonstrate that this result is essentially due to a relative -scaling in
collisions at 17.3 GeV. At lower bombarding energies of
25 AGeV a suppression of -mesons by a factor of 10 relative to
a global -scaling with slope 143 MeV is expected. However, when
incorporating attractive -meson self energies as suggested by QCD sum rules,
an approximate -scaling is regained even at 25 AGeV. The effects of
-meson rescattering and charmonium absorption are discussed, furthermore,
with respect to rapidity and transverse mass distributions in central
collisions of at 25, 160 AGeV and 21.3 ATeV.Comment: 46 pages, LaTeX, including 19 postscript figures, to be published in
Nucl. Phys.
Vector Meson Photoproduction with an Effective Lagrangian in the Quark Model
A quark model approach to the photoproduction of vector mesons off nucleons
is proposed. Its starting point is an effective Lagrangian of the interaction
between the vector meson and the quarks inside the baryon, which generates the
non-diffractive s- and u- channel resonance contributions. Additional t-channel
and exchanges are included for the and
production respectively to account for the large diffractive behavior in the
small region as suggested by Friman and Soyeur. The numerical results are
presented for the and productions in four isospin channels with
the same set of parameters, and they are in good agreement with the available
data not only in and productions but also in the charged
productions where the additional t-channel exchange does not
contribute so that it provides an important test to this approach. The
investigation is also extended to the photoproduction, and the initial
results show that the non-diffractive behavior of the productions in the
large region can be described by the s- and u- channel contributions with
significantly smaller coupling constants, which is consistent with the findings
in the similar studies in the QHD framework. The numerical investigation has
also shown that polarization observables are essential for identifying
so-called "missing resonances".Comment: 36 pages, 10 PS figures, extended version of nucl-th/9711061 and
nucl-th/9803021, submitted to PR
Cross Section Measurement of Charged Pion Photoproduction from Hydrogen and Deuterium
We have measured the differential cross section for the gamma n --> pi- p and
gamma p --> pi+ n reactions at center of mass angle of 90 degree in the photon
energy range from 1.1 to 5.5 GeV at Jefferson Lab (JLab). The data at photon
energies greater than 3.3 GeV exhibit a global scaling behavior for both pi-
and pi+ photoproduction, consistent with the constituent counting rule and the
existing pi+ photoproduction data. Possible oscillations around the scaling
value are suggested by these new data The data show enhancement in the scaled
cross section at a center-of-mass energy near 2.2 GeV. The cross section ratio
of exclusive pi- to pi+ photoproduction at high energy is consistent with the
prediction based on one-hard-gluon-exchange diagrams
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