45,593 research outputs found
Soft High Energy Scattering in Nonperturbative QCD
In this report diffractive high energy reactions are discussed in a
functional integral approach where hadronic amplitudes are calculated from
vacuum expectation values of lightlike Wegner-Wilson loops. In the first part
we calculate elastic differential cross sections for high energy and small
momentum transfer elastic proton-proton (pp) scattering which are in reasonable
agreement with the experimental data. In the second part we consider exclusive
production in ep-scattering. At high energies photon and odderon
exchange contribute to this reaction. We show that odderon exchange leads to a
much larger inelastic than elastic production cross section, dominating
the contribution by orders of magnitude. Observing our process at HERA
would establish the soft odderon.Comment: Talk given at the International Conference on Elastic and Diffractive
Scattering (EDS 99), Protvino, Russia, 28 June-2 July 199
Differential cross sections for high energy elastic hadron-hadron scattering in nonperturbative QCD
Total and differential cross sections for high energy and small momentum
transfer elastic hadron-hadron scattering are studied in QCD using a functional
integral approach. The hadronic amplitudes are governed by vacuum expectation
values of lightlike Wegner-Wilson loops, for which a matrix cumulant expansion
is derived. The cumulants are evaluated within the framework of the Minkowskian
version of the model of the stochastic vacuum. Using the second cumulant, we
calculate elastic differential cross sections for hadron-hadron scattering. The
agreement with experimental data is good.Comment: 30 pages, 14 figure
Timelike Compton scattering: exclusive photoproduction of lepton pairs
We investigate the exclusive photoproduction of a heavy timelike photon which
decays into a lepton pair, gamma p -> l+ l- p. This can be seen as the analog
of deeply virtual Compton scattering, and we argue that the two processes are
complementary for studying generalized parton distributions in the nucleon. In
an unpolarized experiment the angular distribution of the leptons readily
provides access to the real part of the Compton amplitude. We estimate the
possible size of this effect in kinematics where the Compton process should be
dominated by quark exchange.Comment: 31 pages, 17 figure
Broad-band Modeling of GRB Afterglows
Observations of GRB afterglows ranging from radio to X-ray frequencies
generate large data sets. Careful analysis of these broad-band data can give us
insight into the nature of the GRB progenitor population by yielding such
information like the total energy of the burst, the geometry of the fireball
and the type of environment into which the GRB explodes. We illustrate, by
example, how global, self-consistent fits are a robust approach for
characterizing the afterglow emission. This approach allows a relatively simple
comparison of different models and a way to determine the strengths and
weaknesses of these models, since all are treated self-consistently. Here we
quantify the main differences between the broad-band, self-consistent approach
and the traditional approach, using GRB000301C and GRB970508 as test cases.Comment: Appears in "Gamma-Ray Bursts in the Afterglow Era" proceedings of the
Roma 2000 GRB Workshop; 3 pages; 2 figure
Spitzer Observations of Gamma-Ray Burst Host Galaxies: A Unique Window into High Redshift Chemical Evolution and Star-formation
We present deep Spitzer 3.6 micron observations of three z~5 GRB host
galaxies. Our observations reveal that z~5 GRB hosts are a factor of 3 less
luminous than the median rest-frame V-band luminosity of spectroscopically
confirmed z~5 galaxies in the GOODS fields and the UDF. The strong connection
between GRBs and massive star formation implies that not all star-forming
galaxies at these redshifts are currently being accounted for in deep surveys
and GRBs provide a unique way to measure the contribution to the star-formation
rate density from galaxies at the faint end of the galaxy luminosity function.
By correlating the co-moving star-formation rate density with co-moving GRB
rates at lower redshifts, we estimate a lower limit to the star-formation rate
density of 0.12+/-0.09 and 0.09+/-0.05 M_sun/yr/Mpc^3 at z~4.5 and z~6,
respectively. Finally, we provide evidence that the average metallicity of
star-forming galaxies evolves as (stellar mass density)^(0.69+/-0.17) between
and , probably indicative of the loss of a significant
fraction of metals to the intergalactic medium, particularly in low-mass
galaxies.Comment: ApJ, in pres
Production of a Prompt Photon in Association with Charm at Next-to-Leading Order in QCD
A second order, , calculation in perturbative quantum
chromodynamics of the two particle inclusive cross section is presented for the
reaction 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
Representation theory of super Yang-Mills algebras
We study in this article the representation theory of a family of super
algebras, called the \emph{super Yang-Mills algebras}, by exploiting the
Kirillov orbit method \textit{\`a la Dixmier} for nilpotent super Lie algebras.
These super algebras are a generalization of the so-called \emph{Yang-Mills
algebras}, introduced by A. Connes and M. Dubois-Violette in \cite{CD02}, but
in fact they appear as a "background independent" formulation of supersymmetric
gauge theory considered in physics, in a similar way as Yang-Mills algebras do
the same for the usual gauge theory. Our main result states that, under certain
hypotheses, all Clifford-Weyl super algebras \Cliff_{q}(k) \otimes A_{p}(k),
for , or and , appear as a quotient of all super
Yang-Mills algebras, for and . This provides thus a family
of representations of the super Yang-Mills algebras
Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. III. X-ray, Radio, and H-alpha Activity Trends in M and L Dwarfs
[Abridged] As part of our on-going investigation into the magnetic field
properties of ultracool dwarfs, we present simultaneous radio, X-ray, and
H-alpha observations of three M9.5-L2.5 dwarfs (BRI0021-0214,
LSR060230.4+391059, and 2MASSJ052338.2-140302). We do not detect X-ray or radio
emission from any of the three sources, despite previous detections of radio
emission from BRI0021 and 2M0523-14. Steady and variable H-alpha emission are
detected from 2M0523-14 and BRI0021, respectively, while no H-alpha emission is
detected from LSR0602+39. Overall, our survey of nine M8-L5 dwarfs doubles the
number of ultracool dwarfs observed in X-rays, and triples the number of L
dwarfs, providing in addition the deepest limits to date, log(L_X/L_bol)<-5.
With this larger sample we find the first clear evidence for a substantial
reduction in X-ray activity, by about two orders of magnitude, from mid-M to
mid-L dwarfs. We find that the decline in both X-rays and H-alpha roughly
follows L_{X,Halpha}/L_bol ~ 10^[-0.4x(SP-M6)] for SP>M6. In the radio band,
however, the luminosity remains relatively unchanged from M0 to L4, leading to
a substantial increase in L_rad/L_bol. Our survey also provides the first
comprehensive set of simultaneous radio/X-ray/H-alpha observations of ultracool
dwarfs, and reveals a clear breakdown of the radio/X-ray correlation beyond
spectral type M7, evolving smoothly from L_{\nu,rad}/L_X ~ 10^-15.5 to
~10^-11.5 Hz^-1 over the narrow spectral type range M7-M9. This breakdown
reflects the substantial reduction in X-ray activity beyond M7, but its
physical origin remains unclear since, as evidenced by the uniform radio
emission, there is no drop in the field dissipation and particle acceleration
efficiency.Comment: Submitted to ApJ; 19 pages, 10 figures, 5 table
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