49,176 research outputs found
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
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
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
Next-to-leading Order SUSY-QCD Calculation of Associated Production of Gauginos and Gluinos
Results are presented of a next-to-leading order calculation in perturbative
QCD of the production of charginos and neutralinos in association with gluinos
at hadron colliders. Predictions for cross sections are shown at the energies
of the Fermilab Tevatron and CERN Large Hadron Collider for a typical
supergravity (SUGRA) model of the sparticle mass spectrum and for a light
gluino model.Comment: 3 pages, latex, 4 figures, paper presented by E. L. Berger at ICHEP
2000, the XXXth International Conference on High Energy Physics July 27 -
August 2, 2000, Osaka, Japa
Spatio-Temporal Scaling of Solar Surface Flows
The Sun provides an excellent natural laboratory for nonlinear phenomena. We
use motions of magnetic bright points on the solar surface, at the smallest
scales yet observed, to study the small scale dynamics of the photospheric
plasma. The paths of the bright points are analyzed within a continuous time
random walk framework. Their spatial and temporal scaling suggest that the
observed motions are the walks of imperfectly correlated tracers on a turbulent
fluid flow in the lanes between granular convection cells.Comment: Now Accepted by Physical Review Letter
Voyager Mars planetary quarantine Basic math model report
Basic math model study of planetary quarantine effects on Voyager Mars missio
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
Periodic Radio and H-alpha Emission from the L Dwarf Binary 2MASSW J0746425+200032: Exploring the Magnetic Field Topology and Radius of an L Dwarf
[Abridged] We present an 8.5-hour simultaneous radio, X-ray, UV, and optical
observation of the L dwarf binary 2MASSW J0746+20. We detect strong radio
emission, dominated by short-duration periodic pulses at 4.86 GHz with
P=124.32+/-0.11 min. The stability of the pulse profiles and arrival times
demonstrates that they are due to the rotational modulation of a B~1.7 kG
magnetic field. A quiescent non-variable component is also detected, likely due
to emission from a uniform large-scale field. The H-alpha emission exhibits
identical periodicity, but unlike the radio pulses it varies sinusoidally and
is offset by exactly 1/4 of a phase. The sinusoidal variations require
chromospheric emission from a large-scale field structure, with the radio
pulses likely emanating from the magnetic poles. While both light curves can be
explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out
a symmetric dipole topology since it would result in a phase lag of 1/2
(poloidal field) or zero (toroidal field). We therefore conclude that either
(i) the field is dominated by a quadrupole configuration, which can naturally
explain the 1/4 phase lag; or (ii) the H-alpha and/or radio emission regions
are not trivially aligned with the field. Regardless of the field topology, we
use the measured period along with the known rotation velocity (vsini=27 km/s),
and the binary orbital inclination (i=142 deg), to derive a radius for the
primary star of 0.078+/-0.010 R_sun. This is the first measurement of the
radius of an L dwarf, and along with a mass of 0.085+/-0.010 M_sun it provides
a constraint on the mass-radius relation below 0.1 M_sun. We find that the
radius is about 30% smaller than expected from theoretical models, even for an
age of a few Gyr.Comment: Submitted to Ap
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