501 research outputs found
Structure Functions are not Parton Probabilities
The common view that structure functions measured in deep inelastic lepton
scattering are determined by the probability of finding quarks and gluons in
the target is not correct in gauge theory. We show that gluon exchange between
the fast, outgoing partons and target spectators, which is usually assumed to
be an irrelevant gauge artifact, affects the leading twist structure functions
in a profound way. This observation removes the apparent contradiction between
the projectile (eikonal) and target (parton model) views of diffractive and
small x_{Bjorken} phenomena. The diffractive scattering of the fast outgoing
quarks on spectators in the target causes shadowing in the DIS cross section.
Thus the depletion of the nuclear structure functions is not intrinsic to the
wave function of the nucleus, but is a coherent effect arising from the
destructive interference of diffractive channels induced by final state
interactions. This is consistent with the Glauber-Gribov interpretation of
shadowing as a rescattering effect.Comment: 35 pages, 8 figures. Discussion of physical consequences of final
state interactions amplified. Material on light-cone gauge choices adde
Level Eulerian Posets
The notion of level posets is introduced. This class of infinite posets has
the property that between every two adjacent ranks the same bipartite graph
occurs. When the adjacency matrix is indecomposable, we determine the length of
the longest interval one needs to check to verify Eulerianness. Furthermore, we
show that every level Eulerian poset associated to an indecomposable matrix has
even order. A condition for verifying shellability is introduced and is
automated using the algebra of walks. Applying the Skolem--Mahler--Lech
theorem, the -series of a level poset is shown to be a rational
generating function in the non-commutative variables and .
In the case the poset is also Eulerian, the analogous result holds for the
-series. Using coalgebraic techniques a method is developed to
recognize the -series matrix of a level Eulerian poset
Exploratory Behavior, Trap Models and Glass Transitions
A random walk is performed on a disordered landscape composed of sites
randomly and uniformly distributed inside a -dimensional hypercube. The
walker hops from one site to another with probability proportional to , where is the inverse of a formal temperature and
is an arbitrary cost function which depends on the hop distance .
Analytic results indicate that, if and , there
exists a glass transition at . Below
, the average trapping time diverges and the system falls into an
out-of-equilibrium regime with aging phenomena. A L\'evy flight scenario and
applications to exploratory behavior are considered.Comment: 4 pages, 1 figure, new versio
Effect on costs of ACC/AHA guidelines for preoperative cardiac risk assessment before aortic surgery
Initial-State Interactions in the Unpolarized Drell-Yan Process
We show that initial-state interactions contribute to the
distribution in unpolarized Drell-Yan lepton pair production and , without suppression. The asymmetry is expressed as a
product of chiral-odd distributions , where the quark-transversity function
is the transverse momentum dependent, light-cone
momentum distribution of transversely polarized quarks in an {\it unpolarized}
proton. We compute this (naive) -odd and chiral-odd distribution function
and the resulting asymmetry explicitly in a quark-scalar diquark
model for the proton with initial-state gluon interaction. In this model the
function equals the -odd (chiral-even) Sivers
effect function . This suggests that the
single-spin asymmetries in the SIDIS and the Drell-Yan process are closely
related to the asymmetry of the unpolarized Drell-Yan process,
since all can arise from the same underlying mechanism. This provides new
insight regarding the role of quark and gluon orbital angular momentum as well
as that of initial- and final-state gluon exchange interactions in hard QCD
processes.Comment: 22 pages, 6 figure
Continuum theory of vacancy-mediated diffusion
We present and solve a continuum theory of vacancy-mediated diffusion (as
evidenced, for example, in the vacancy driven motion of tracers in crystals).
Results are obtained for all spatial dimensions, and reveal the strongly
non-gaussian nature of the tracer fluctuations. In integer dimensions, our
results are in complete agreement with those from previous exact lattice
calculations. We also extend our model to describe the vacancy-driven
fluctuations of a slaved flux line.Comment: 25 Latex pages, subm. to Physical Review
Social Experimentation as Reflection-in-A ction
We present the results of our review of some forty community-level interventions undertaken in the developing world over the past twenty years m order to reduce malnourishment in children. We argue that such interventions, if they are considered as social experiments, cannot be assimilated to models of quasi-experimental method. We propose an alternative model of experimentation, which we call "reflection-in-action", which seems to us better suited to account for the kinds ofvahdity and rigor attainable in situations such as these.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68568/2/10.1177_107554708400600101.pd
The U(1)-Higgs Model: Critical Behaviour in the Confinig-Higgs region
We study numerically the critical properties of the U(1)-Higgs lattice model,
with fixed Higgs modulus, in the region of small gauge coupling where the Higgs
and Confining phases merge. We find evidence of a first order transition line
that ends in a second order point. By means of a rotation in parameter space we
introduce thermodynamic magnitudes and critical exponents in close resemblance
with simple models that show analogous critical behaviour. The measured data
allow us to fit the critical exponents finding values in agreement with the
mean field prediction. The location of the critical point and the slope of the
first order line are accurately given.Comment: 21 text pages. 12 postscript figures available on reques
Transitions between Inherent Structures in Water
The energy landscape approach has been useful to help understand the dynamic
properties of supercooled liquids and the connection between these properties
and thermodynamics. The analysis in numerical models of the inherent structure
(IS) trajectories -- the set of local minima visited by the liquid -- offers
the possibility of filtering out the vibrational component of the motion of the
system on the potential energy surface and thereby resolving the slow
structural component more efficiently. Here we report an analysis of an IS
trajectory for a widely-studied water model, focusing on the changes in
hydrogen bond connectivity that give rise to many IS separated by relatively
small energy barriers. We find that while the system \emph{travels} through
these IS, the structure of the bond network continuously modifies, exchanging
linear bonds for bifurcated bonds and usually reversing the exchange to return
to nearly the same initial configuration. For the 216 molecule system we
investigate, the time scale of these transitions is as small as the simulation
time scale ( fs). Hence for water, the transitions between each of
these IS is relatively small and eventual relaxation of the system occurs only
by many of these transitions. We find that during IS changes, the molecules
with the greatest displacements move in small ``clusters'' of 1-10 molecules
with displacements of nm, not unlike simpler liquids.
However, for water these clusters appear to be somewhat more branched than the
linear ``string-like'' clusters formed in a supercooled Lennar d-Jones system
found by Glotzer and her collaborators.Comment: accepted in PR
Escaping from cycles through a glass transition
A random walk is performed over a disordered media composed of sites
random and uniformly distributed inside a -dimensional hypercube. The walker
cannot remain in the same site and hops to one of its neighboring sites
with a transition probability that depends on the distance between sites
according to a cost function . The stochasticity level is parametrized by
a formal temperature . In the case , the walk is deterministic and
ergodicity is broken: the phase space is divided in a number of
attractor basins of two-cycles that trap the walker. For , analytic
results indicate the existence of a glass transition at as . Below , the average trapping time in two-cycles diverges and
out-of-equilibrium behavior appears. Similar glass transitions occur in higher
dimensions choosing a proper cost function. We also present some results for
the statistics of distances for Poisson spatial point processes.Comment: 11 pages, 4 figure
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