4,288 research outputs found
Formation of color-singlet gluon-clusters and inelastic diffractive scattering
This is the extensive follow-up report of a recent Letter in which the
existence of self-organized criticality (SOC) in systems of interacting soft
gluons is proposed, and its consequences for inelastic diffractive scattering
processes are discussed. It is pointed out, that color-singlet gluon-clusters
can be formed in hadrons as a consequence of SOC in systems of interacting soft
gluons, and that the properties of such spatiotemporal complexities can be
probed experimentally by examing inelastic diffractive scattering. Theoretical
arguments and experimental evidences supporting the proposed picture are
presented --- together with the result of a systematic analysis of the existing
data for inelastic diffractive scattering processes performed at different
incident energies, and/or by using different beam-particles. It is shown in
particular that the size- and the lifetime-distributions of such gluon-clusters
can be directly extracted from the data, and the obtained results exhibit
universal power-law behaviors --- in accordance with the expected
SOC-fingerprints. As further consequences of SOC in systems of interacting soft
gluons, the -dependence and the -dependence of the double
differential cross-sections for inelastic diffractive scattering off
proton-target are discussed. Here stands for the four-momentum-transfer
squared, for the missing mass, and for the total c.m.s.
energy. It is shown, that the space-time properties of the color-singlet
gluon-clusters due to SOC, discussed above, lead to simple analytical formulae
for and for , and that the obtained
results are in good agreement with the existing data. Further experiments are
suggested.Comment: 67 pages, including 11 figure
Gravitational Collapse in Turbulent Molecular Clouds. I. Gasdynamical Turbulence
Observed molecular clouds often appear to have very low star formation
efficiencies and lifetimes an order of magnitude longer than their free-fall
times. Their support is attributed to the random supersonic motions observed in
them. We study the support of molecular clouds against gravitational collapse
by supersonic, gas dynamical turbulence using direct numerical simulation.
Computations with two different algorithms are compared: a particle-based,
Lagrangian method (SPH), and a grid-based, Eulerian, second-order method
(ZEUS). The effects of both algorithm and resolution can be studied with this
method. We find that, under typical molecular cloud conditions, global collapse
can indeed be prevented, but density enhancements caused by strong shocks
nevertheless become gravitationally unstable and collapse into dense cores and,
presumably, stars. The occurance and efficiency of local collapse decreases as
the driving wave length decreases and the driving strength increases. It
appears that local collapse can only be prevented entirely with unrealistically
short wave length driving, but observed core formation rates can be reproduced
with more realistic driving. At high collapse rates, cores are formed on short
time scales in coherent structures with high efficiency, while at low collapse
rates they are scattered randomly throughout the region and exhibit
considerable age spread. We suggest that this naturally explains the observed
distinction between isolated and clustered star formation.Comment: Minor revisions in response to referee, thirteen figures, accepted to
Astrophys.
Charm, Beauty and Top at HERA
Results on open charm and beauty production and on the search for top
production in high-energy electron-proton collisions at HERA are reviewed. This
includes a discussion of relevant theoretical aspects, a summary of the
available measurements and measurement techniques, and their impact on improved
understanding of QCD and its parameters, such as parton density functions and
charm- and beauty-quark masses. The impact of these results on measurements at
the LHC and elsewhere is also addressed.Comment: 103 pages, 60 figures, to be published in Prog. Part. Nucl. Phy
Nonlinear Criterion for the Stability of Molecular Clouds
Dynamically significant magnetic fields are routinely observed in molecular
clouds, with mass-to-flux ratio lambda = (2 pi sqrt{G}) (Sigma/B) ~ 1 (here
Sigma is the total column density and B is the field strength). It is widely
believed that ``subcritical'' clouds with lambda < 1 cannot collapse, based on
virial arguments by Mestel and Spitzer and a linear stability analysis by
Nakano and Nakamura. Here we confirm, using high resolution numerical models
that begin with a strongly supersonic velocity dispersion, that this criterion
is a fully nonlinear stability condition. All the high-resolution models with
lambda <= 0.95 form ``Spitzer sheets'' but collapse no further. All models with
lambda >= 1.02 collapse to the maximum numerically resolvable density. We also
investigate other factors determining the collapse time for supercritical
models. We show that there is a strong stochastic element in the collapse time:
models that differ only in details of their initial conditions can have
collapse times that vary by as much as a factor of 3. The collapse time cannot
be determined from just the velocity dispersion; it depends also on its
distribution. Finally, we discuss the astrophysical implications of our
results.Comment: 11 pages, 5 figure
The Color Dipole Picture of low-x DIS: Model-Independent and Model-Dependent Results
We present a detailed examination of the color-dipole picture (CDP) of
low- deep inelastic scattering. We discriminate model-independent results,
not depending on a specific parameterization of the dipole cross section, from
model-dependent ones. The model-independent results include the ratio of the
longitudinal to the transverse photoabsorption cross section at large , or
equivalently the ratio of the longitudinal to the unpolarized proton structure
function, , as well as the low- scaling
behavior of the total photoabsorption cross section as for
, and as for .
Here, denotes the low- scaling variable, with being
the saturation scale. The model-independent analysis also implies
at any for asymptotically
large energy, . Consistency with pQCD evolution determines the underlying
gluon distribution and the numerical value of in the expression
for the saturation scale, . In the
model-dependent analysis, by restricting the mass of the actively contributing
fluctuations by an energy-dependent upper bound, we extend the
validity of the color-dipole picture to . The
theoretical results agree with the world data on DIS for .Comment: 77 pages, 30 figure
- …