6,991 research outputs found

### Vortex breakdown incipience: Theoretical considerations

The sensitivity of the onset and the location of vortex breakdowns in concentrated vortex cores, and the pronounced tendency of the breakdowns to migrate upstream have been characteristic observations of experimental investigations; they have also been features of numerical simulations and led to questions about the validity of these simulations. This behavior seems to be inconsistent with the strong time-like axial evolution of the flow, as expressed explicitly, for example, by the quasi-cylindrical approximate equations for this flow. An order-of-magnitude analysis of the equations of motion near breakdown leads to a modified set of governing equations, analysis of which demonstrates that the interplay between radial inertial, pressure, and viscous forces gives an elliptic character to these concentrated swirling flows. Analytical, asymptotic, and numerical solutions of a simplified non-linear equation are presented; these qualitatively exhibit the features of vortex onset and location noted above

### Conatraints on $\Delta G$ from Prompt Photon plus Jet Production at HERA-$\vec{N}$

The utility of prompt photon plus associated jet production for constraining
the size and shape of the polarized gluon density of the proton $\Delta G$ is
examined at $\sqrt{S}=40$ GeV, appropriate for the proposed HERA-$\vec{N}$
polarized $\vec{p}\vec{p}$ collider experiment. The calculation is performed at
next-to-leading order (O($\alpha\alpha_s^2$)) in QCD. The reliability of the
predictions are examined in some detail.Comment: 16 pages, 3 figure

### Prompt Photon plus Charm Production at Next-to-Leading Order in QCD

The two particle inclusive cross section for the reaction $p
+\bar{p}\rightarrow \gamma + c + X$ is studied in perturbative quantum
chromodynamics at order $O(\alpha ^2_s)$. Differential distributions are
provided for various observables, and a comparison is made with data from the
CDF collaboration.Comment: 4 pages, LaTeX, uses stwol.sty, 6 postscript figures. Invited paper
presented by E. L. Berger at the 28th International Conference on High Energy
Physics, Warsaw, Poland, July 25 - 31, 199

### Associated Production of Charm and a Hard Photon

The two particle inclusive cross section for the reaction $p
+\bar{p}\rightarrow \gamma + c + X$ is studied in perturbative quantum
chromodynamics at order $O(\alpha ^2_s)$, for large values of the transverse
momentum of the prompt photon and charm quark. Two different techniques are
used in performing the phase-space integrals; the first is purely analytical,
and the second is a combination of analytic and Monte Carlo integration
methods. The second, more versatile technique facilitates imposition of photon
isolation restrictions and other selections of relevance in experiments.
Differential distributions are provided for various observables, and a
comparison is made with preliminary data from the CDF collaboration.Comment: 6 pages. Latex file plus 4 ps files of figures submitted togther in
uuencoded form; invited talk presented by L. E. Gordon at the XXXI Rencontres
de Moriond on QCD and High Energy Hadronic Interactions, Les Arcs, France,
March, 199

### Production of a Prompt Photon in Association with Charm at Next-to-Leading Order in QCD

A second order, $O(\alpha ^2_s)$, calculation in perturbative quantum
chromodynamics of the two particle inclusive cross section is presented for the
reaction $p +\bar{p}\rightarrow \gamma + c + X$ for large values of the
transverse momentum of the prompt photon and charm quark. The combination of
analytic and Monte Carlo integration methods used here to perform phase-space
integrations facilitates imposition of photon isolation restrictions and other
selections of relevance in experiments. Differential distributions are provided
for various observables. Positive correlations in rapidity are predicted.Comment: 27 pages in RevTex plus 14 figures in one compressed PS fil

### A note on the production of photons at RHIC

I study the production of prompt photons in polarized hadronic collisions,
considering different isolation prescriptions. In particular, I focus on the
problem of the measurability of the polarized gluon density in the proton
through isolated-photon data.Comment: 4 pages Latex; espcrc2.sty and one ps figure included. Talk given at
QCD99, 7 - 13 July 1999, Montpellier, Franc

### Bayesian Calibrated Significance Levels Applied to the Spectral Tilt and Hemispherical Asymmetry

Bayesian model selection provides a formal method of determining the level of
support for new parameters in a model. However, if there is not a specific
enough underlying physical motivation for the new parameters it can be hard to
assign them meaningful priors, an essential ingredient of Bayesian model
selection. Here we look at methods maximizing the prior so as to work out what
is the maximum support the data could give for the new parameters. If the
maximum support is not high enough then one can confidently conclude that the
new parameters are unnecessary without needing to worry that some other prior
may make them significant. We discuss a computationally efficient means of
doing this which involves mapping p-values onto upper bounds of the Bayes
factor (or odds) for the new parameters. A p-value of 0.05 ($1.96\sigma$)
corresponds to odds less than or equal to 5:2 which is below the `weak' support
at best threshold. A p-value of 0.0003 ($3.6\sigma$) corresponds to odds of
less than or equal to 150:1 which is the `strong' support at best threshold.
Applying this method we find that the odds on the scalar spectral index being
different from one are 49:1 at best. We also find that the odds that there is
primordial hemispherical asymmetry in the cosmic microwave background are 9:1
at best.Comment: 5 pages. V2: clarifying comments added in response to referee report.
Matches version to appear in MNRA

### Spin Dependence of Massive Lepton Pair Production in Proton-Proton Collisions

We calculate the transverse momentum distribution for the production of
massive lepton-pairs in longitudinally polarized proton-proton reactions at
collider energies within the context of perturbative quantum chromodynamics.
For values of the transverse momentum Q_T greater than roughly half the pair
mass Q, Q_T > Q/2, we show that the differential cross section is dominated by
subprocesses initiated by incident gluons, provided that the polarized gluon
density is not too small. Massive lepton-pair differential cross sections
should be a good source of independent constraints on the polarized gluon
density, free from the experimental and theoretical complications of photon
isolation that beset studies of prompt photon production. We provide
predictions for the spin-averaged and spin-dependent differential cross
sections as a function of Q_T at energies relevant for the Relativistic Heavy
Ion Collider (RHIC) at Brookhaven, and we compare these with predictions for
real prompt photon production.Comment: 34 pages, RevTeX including 17 figures in .ps file

### A Quillen model structure for Gray-categories

A Quillen model structure on the category Gray-Cat of Gray-categories is
described, for which the weak equivalences are the triequivalences. It is shown
to restrict to the full subcategory Gray-Gpd of Gray-groupoids. This is used to
provide a functorial and model-theoretic proof of the unpublished theorem of
Joyal and Tierney that Gray-groupoids model homotopy 3-types. The model
structure on Gray-Cat is conjectured to be Quillen equivalent to a model
structure on the category Tricat of tricategories and strict homomorphisms of
tricategories.Comment: v2: fuller discussion of relationship with work of Berger;
localizations are done directly with simplicial set

### On the exploitability of thermo-charged capacitors

Recently [Physics Letters A, 374, (2010) 1801] the concept of vacuum
capacitor spontaneously charged harnessing the heat from a single thermal
reservoir at room temperature has been introduced, along with a mathematical
description of its functioning and a discussion on the main paradoxical feature
that seems to violate the Second Law of Thermodynamics. In the present paper we
investigate the theoretical and practical possibility of exploiting a such
thermo-charged capacitor as voltage/current generator: we show that if very
weak provisos on the physical characteristics of the capacitor are fulfilled,
then a non-zero current should flow across the device, allowing the generation
of potentially usable voltage, current and electric power out of a single
thermal source at room temperature. Preliminary results show that the power
output is tiny but non-zero.Comment: 23 Pages, 10 Figures. Physica A 390/6 (2011) 482 + Addendum (2013

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