57,693 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
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
Constraints on the Gluon Density from Lepton Pair Production
The hadroproduction of lepton pairs with mass Q and finite transverse
momentum Q_T is described in perturbative QCD by the same partonic subprocesses
as prompt photon production. We demonstrate that, like prompt photon
production, lepton pair production is dominated by quark-gluon scattering in
the region Q_T>Q/2. This feature leads to sensitivity to the gluon density in
kinematical regimes accessible in collider and fixed target experiments, and it
provides a new independent method for constraining the gluon density.Comment: 5 pages, 7 figures, to appear in the Proceedings of the Fermilab
Physics at Run II Workshop on ``QCD and Weak Boson Physics
QCD calculation of J/psi+gamma mass distributions
We compute the J/psi+gamma invariant-mass distributions from the QCD
subprocess g + g --> J/psi+gamma. At large masses, this subprocess is the
dominant mechanism for J/psi+gamma production, and data could provide a good
test of QCD. The mass distribution peaks at relatively small masses (3.4 - 4.0
GeV) and the subprocess could, therefore, represent a significant QCD
background to J/psi+gamma decay of heavier charmonia. We also analyze the J/psi
angular distribution in the J/psi+gamma rest frame.Comment: 7 pages LaTex, 4 figures available on request. CERN-TH.6974/93,
ANL-HEP-PR-93.6
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
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
- …