2,653 research outputs found
Emittance growth in linear induction accelerators
The Dual-Axis Radiographic Hydrotest (DARHT) facility uses bremsstrahlung
radiation source spots produced by the focused electron beams from two linear
induction accelerators (LIAs) to radiograph large hydrodynamic experiments
driven by high explosives. Radiographic resolution is determined by the size of
the source spot, and beam emittance is the ultimate limitation to spot size. On
the DARHT Axis-II LIA we measure an emittance higher than predicted by
theoretical simulations, and even though this axis produces sub-millimeter
source spots, we are exploring ways to improve the emittance. Some of the
possible causes for the discrepancy have been investigated using
particle-in-cell (PIC) codes, although most of these are discounted based on
beam measurements. The most likely source of emittance growth is a mismatch of
the beam to the magnetic transport, which can cause beam halo.Comment: 20th Int. Conf. on High-Power Particle Beams, Washington, DC, May,
201
Decay of a Yukawa fermion at finite temperature and applications to leptogenesis
We calculate the decay rate of a Yukawa fermion in a thermal bath using
finite temperature cutting rules and effective Green's functions according to
the hard thermal loop resummation technique. We apply this result to the decay
of a heavy Majorana neutrino in leptogenesis. Compared to the usual approach
where thermal masses are inserted into the kinematics of final states, we find
that deviations arise through two different leptonic dispersion relations. The
decay rate differs from the usual approach by more than one order of magnitude
in the temperature range which is interesting for the weak washout regime. We
discuss how to arrive at consistent finite temperature treatments of
leptogenesis.Comment: 16 pages, 5 figure
Flavor ordering of elliptic flows at high transverse momentum
Based on the quark coalescence model for the parton-to-hadron phase
transition in ultra-relativistic heavy ion collisions, we relate the elliptic
flow () of high \pt hadrons to that of high \pt quarks. For high \pt
hadrons produced from an isospin symmetric and quark-antiquark symmetric
partonic matter, magnitudes of their elliptic flows follow a flavor ordering as
if strange quarks have a
smaller elliptic flow than light quarks. The elliptic flows of high \pt
hadrons further follow a simple quark counting rule if strange quarks and light
quarks have same high \pt spectrum and coalescence probability.Comment: 4 pages, 1 figure, revte
Non-Perturbative Dilepton Production from a Quark-Gluon Plasma
The dilepton production rate from the quark-gluon plasma is calculated from
the imaginary part of the photon self energy using a quark propagator that
contains the gluon condensate. The low mass dilepton rate obtained in this way
exhibits interesting structures (peaks and gaps), which might be observable at
RHIC and LHC.Comment: 16 pages, REVTEX, 8 PostScript figure
Vacuum Polarization and the Electric Charge of the Positron
We show that higher-order vacuum polarization would contribute a measureable
net charge to atoms, if the charges of electrons and positrons do not balance
precisely. We obtain the limit for the sum of
the charges of electron and positron. This also constitutes a new bound on
certain violations of PCT invariance.Comment: 9 pages, 1 figure attached as PostScript file, DUKE-TH-92-38. Revised
versio
Wakes in the quark-gluon plasma
Using the high temperature approximation we study, within the linear response
theory, the wake in the quark-gluon plasma by a fast parton owing to dynamical
screening in the space like region. When the parton moves with a speed less
than the average speed of the plasmon, we find that the wake structure
corresponds to a screening charge cloud traveling with the parton with one sign
flip in the induced charge density resulting in a Lennard-Jones type potential
in the outward flow with a short range repulsive and a long range attractive
part. On the other hand if the parton moves with a speed higher than that of
plasmon, the wake structure in the induced charge density is found to have
alternate sign flips and the wake potential in the outward flow oscillates
analogous to Cerenkov like wave generation with a Mach cone structure trailing
the moving parton. The potential normal to the motion of the parton indicates a
transverse flow in the system. We also calculate the potential due to a color
dipole and discuss consequences of possible new bound states and
suppression in the quark-gluon plasma.Comment: 20 pages, 14 figures (high resolution figures available with
authors); version accepted for publication in Phys. Rev.
- …