34 research outputs found
Prompt Quark Production by exploding Sphalerons
Following recent works on production and subsequent explosive decay of QCD
sphaleron-like clusters, we discuss the mechanism of quark pair production in
this process. We first show how the gauge field explosive solution of Luscher
and Schechter can be achieved by non-central conformal mapping from the
O(4)-symmetric solution. Our main result is a new solution to the Dirac
equation in real time in this configuration, obtained by the same inversion of
the fermion O(4) zero mode. It explicitly shows how the quark acceleration
occurs, starting from the spherically O(3) symmetric zero energy chiral quark
state to the final spectrum of non-zero energies.
The sphaleron-like clusters with any Chern-Simons number always produce quarks, and the antisphaleron-like clusters the
chirality opposite.
The result are relevant for hadron-hadron and nucleus-nucleus collisions at
large , wherein such clusters can be produced
Prompt Multi-Gluon Production in High Energy Collisions from Singular Yang-Mills Solutions
We study non-perturbative parton-parton scattering in the Landau method using
singular O(3) symmetric solutions to the Euclidean Yang-Mills equations. These
solutions combine instanton dynamics (tunneling) and overlap (transition)
between incoming and vacuum fields. We derive a high-energy solution at small
Euclidean times, and assess its susequent escape and decay into gluons in
Minkowski space-time. We describe the spectrum of the {\it outgoing} gluons and
show that it is related through a particular rescaling to the Yang-Mills
sphaleron explosion studied earlier. We assess the number of {\it incoming}
gluons in the same configuration, and argue that the observed scaling is in
fact more general and describes the energy dependence of the spectra and
multiplicities at {\it all} energies. Applications to hadron-hadron and
nucleus-nucleus collisions are discussed elsewhere
Spontaneous annihilation of high-density matter in the electroweak theory
In the presence of fermionic matter the topologically distinct vacua of the
standard model are metastable and can decay by tunneling through the sphaleron
barrier. This process annihilates one fermion per doublet due to the anomalous
non-conservation of baryon and lepton currents and is accompanied by a
production of gauge and Higgs bosons. We present a numerical method to obtain
local bounce solutions which minimize the Euclidean action in the space of all
configurations connecting two adjacent topological sectors. These solutions
determine the decay rate and the configuration of the fields after the
tunneling. We also follow the real time evolution of this configuration and
analyze the spectrum of the created bosons. If the matter density exceeds some
critical value, the exponentially suppressed tunneling triggers off an
avalanche producing an enormous amount of bosons.Comment: 38 pages, 6 Postscript figure
Relaxational dynamics in 3D randomly diluted Ising models
We study the purely relaxational dynamics (model A) at criticality in
three-dimensional disordered Ising systems whose static critical behaviour
belongs to the randomly diluted Ising universality class. We consider the
site-diluted and bond-diluted Ising models, and the +- J Ising model along the
paramagnetic-ferromagnetic transition line. We perform Monte Carlo simulations
at the critical point using the Metropolis algorithm and study the dynamic
behaviour in equilibrium at various values of the disorder parameter. The
results provide a robust evidence of the existence of a unique model-A dynamic
universality class which describes the relaxational critical dynamics in all
considered models. In particular, the analysis of the size-dependence of
suitably defined autocorrelation times at the critical point provides the
estimate z=2.35(2) for the universal dynamic critical exponent. We also study
the off-equilibrium relaxational dynamics following a quench from T=\infty to
T=T_c. In agreement with the field-theory scenario, the analysis of the
off-equilibrium dynamic critical behavior gives an estimate of z that is
perfectly consistent with the equilibrium estimate z=2.35(2).Comment: 38 page
Sphaleron Effects Near the Critical Temperature
We discuss one-loop radiative corrections to the sphaleron-induced baryon
number-violating transition rate near the electroweak phase transition in the
standard model. We emphasize that in the case of a first-order transition a
rearrangement of the loop expansion is required close to the transition
temperature. The corresponding expansion parameter, the effective 3-dimensional
gauge coupling approaches a finite dependent value at the critical
temperature.
The
(Higgs mass) dependence of the 1-loop radiative corrections is discussed in
the framework of the heat kernel method. Radiative corrections are small
compared to the leading sphaleron contribution as long as the Higgs mass is
small compared to the W mass. To 1-loop accuracy, there is no Higgs mass range
compatible with experimental limits where washing-out of a B+L asymmetry could
be avoided for the minimal standard model with one Higgs doublet.Comment: 17 pages, RevTeX, (4 figures in a separate uuencoded file),
HD-THEP-93-23re
Ising model on 3D random lattices: A Monte Carlo study
We report single-cluster Monte Carlo simulations of the Ising model on
three-dimensional Poissonian random lattices with up to 128,000 approx. 503
sites which are linked together according to the Voronoi/Delaunay prescription.
For each lattice size quenched averages are performed over 96 realizations. By
using reweighting techniques and finite-size scaling analyses we investigate
the critical properties of the model in the close vicinity of the phase
transition point. Our random lattice data provide strong evidence that, for the
available system sizes, the resulting effective critical exponents are
indistinguishable from recent high-precision estimates obtained in Monte Carlo
studies of the Ising model and \phi^4 field theory on three-dimensional regular
cubic lattices.Comment: 35 pages, LaTex, 8 tables, 8 postscript figure
Computational Study of Baryon Number Violation in High Energy Electroweak Collisions
We use semiclassical methods to study processes which give rise to change of
topology and therefore to baryon number violation in the standard model. We
consider classically allowed processes, i.e.~energies above the sphaleron
barrier. We develop a computational procedure that allows us to solve the Yang
Mills equations of motion for spherically symmetric configurations and to
identify the particle numbers of the in- and out-states. A stochastic sampling
technique is then used to map the region spanned by the topology changing
solutions in the energy versus incoming particle number plane and, in
particular, to determine its lower boundary. A lower boundary which approaches
small particle number would be a strong indication that baryon number violation
would occur in high energy collisions, whereas a lower asymptote at large
particle number would be evidence of the contrary. With our method and the
computational resources we have had at our disposal, we have been able to
determine the lower boundary up to energies approximately equal to one and a
half time times the sphaleron energy and observed a 40\% decrease in particle
number with no sign of the particle number leveling off. However encouraging
this may be, the decrease in incoming particle number is only from particles 50
down to approximately 30. Nevertheless, the formalism we have established will
make it possible to extend the scope of this investigation and also to study
processes in the classically forbidden region, which we plan to do in the
future.Comment: 57 pages, LaTeX, 14 figues. Four of these figures are gray scale 3D
surface plots too large to distribute in color. The full color figures can be
viewed or downloaded from http://cthulu.bu.edu/~bobs/bviolate.htm
New AMS 14C dates track the arrival and spread of broomcorn millet cultivation and agricultural change in prehistoric Europe
Broomcorn millet (Panicum miliaceum L.) is not one of the founder crops domesticated in Southwest Asia in the early Holocene, but was domesticated in northeast China by 6000 bc. In Europe, millet was reported in Early Neolithic contexts formed by 6000 bc, but recent radiocarbon dating of a dozen 'early' grains cast doubt on these claims. Archaeobotanical evidence reveals that millet was common in Europe from the 2nd millennium bc, when major societal and economic transformations took place in the Bronze Age. We conducted an extensive programme of AMS-dating of charred broomcorn millet grains from 75 prehistoric sites in Europe. Our Bayesian model reveals that millet cultivation began in Europe at the earliest during the sixteenth century bc, and spread rapidly during the fifteenth/fourteenth centuries bc. Broomcorn millet succeeds in exceptionally wide range of growing conditions and completes its lifecycle in less than three summer months. Offering an additional harvest and thus surplus food/fodder, it likely was a transformative innovation in European prehistoric agriculture previously based mainly on (winter) cropping of wheat and barley. We provide a new, high-resolution chronological framework for this key agricultural development that likely contributed to far-reaching changes in lifestyle in late 2nd millennium bc Europe