195 research outputs found
Thermodynamics of nano-cluster phases: a unifying theory
We propose a unifying, analytical theory accounting for the self-organization
of colloidal systems in nano- or micro-cluster phases. We predict the
distribution of cluter sizes with respect to interaction parameters and colloid
concentration. In particular, we anticipate a proportionality regime where the
mean cluster size grows proportionally to the concentration, as observed in
several experiments. We emphasize the interest of a predictive theory in soft
matter, nano-technologies and biophysics.Comment: 4 pages, 1 figur
Statistical ensemble of scale-free random graphs
A thorough discussion of the statistical ensemble of scale-free connected
random tree graphs is presented. Methods borrowed from field theory are used to
define the ensemble and to study analytically its properties. The ensemble is
characterized by two global parameters, the fractal and the spectral
dimensions, which are explicitly calculated. It is discussed in detail how the
geometry of the graphs varies when the weights of the nodes are modified. The
stability of the scale-free regime is also considered: when it breaks down,
either a scale is spontaneously generated or else, a "singular" node appears
and the graphs become crumpled. A new computer algorithm to generate these
random graphs is proposed. Possible generalizations are also discussed. In
particular, more general ensembles are defined along the same lines and the
computer algorithm is extended to arbitrary (degenerate) scale-free random
graphs.Comment: 10 pages, 6 eps figures, 2-column revtex format, minor correction
A Remark on the Renormalization Group Equation for the Penner Model
It is possible to extract values for critical couplings and gamma_string in
matrix models by deriving a renormalization group equation for the variation of
the of the free energy as the size N of the matrices in the theory is varied.
In this paper we derive a ``renormalization group equation'' for the Penner
model by direct differentiation of the partition function and show that it
reproduces the correct values of the critical coupling and gamma_string and is
consistent with the logarithmic corrections present for g=0,1.Comment: LaTeX, 5 pages, LPTHE-Orsay-94-5
Local charge compensation from colour preconfinement as a key to the dynamics of hadronization
If, as is commonly accepted, the colour-singlet, `preconfined', perturbative
clusters are the primary units of hadronization, then the electric charge is
necessarily compensated locally at the scale of the typical cluster mass. As a
result, the minijet electric charge is suppressed at scales that are greater
than the cluster mass. We hence argue, and demonstrate by means of Monte Carlo
simulations using HERWIG, that the scale at which charge compensation is
violated is close to the mass of the clusters involved in hadronization, and
its measurement would provide a clue to resolving the nature of the dynamics.
We repeat the calculation using PYTHIA and find that the numbers produced by
the two generators are similar. The cluster mass distribution is sensitive to
soft emission that is considered unresolved in the parton shower phase. We
discuss how the description of the splitting of large clusters in terms of
unresolved emission modifies the algorithm of HERWIG, and relate the findings
to the yet unknown underlying nonperturbative mechanism. In particular, we
propose a form of that follows from a power-enhanced beta function,
and discuss how this that governs unresolved emission may be related
to power corrections. Our findings are in agreement with experimental data.Comment: 37 pages, 20 figure
Tree Networks with Causal Structure
Geometry of networks endowed with a causal structure is discussed using the
conventional framework of equilibrium statistical mechanics. The popular
growing network models appear as particular causal models. We focus on a class
of tree graphs, an analytically solvable case. General formulae are derived,
describing the degree distribution, the ancestor-descendant correlation and the
probability a randomly chosen node lives at a given geodesic distance from the
root. It is shown that the Hausdorff dimension of the causal networks is
generically infinite, in contrast to the maximally random trees, where it is
generically finite.Comment: 9 pages, 2-column revtex format, 1 eps figure, misprints correcte
Can pions created in high-energy heavy-ion collisions produce a Centauro-type effect?
We study a Centauro-type phenomenon in high-energy heavy-ion collisions by
assuming that pions are produced semiclassically both directly and in pairs
through the isovector channel. The leading-particle effect and the
factorization property of the scattering amplitude in the impact-parameter
space are used to define the classical pion field. By analyzing the joint
probability function for producing neutral
and negative pions from a definite isospin state of the
incoming leading-particle system we show that only direct production of pions
without isovector pairs favors Centauro-type behavior. The presence of
isovector pairs seems to destroy the effect. Our conclusion is supported
through the calculation of two pion correlation parameters, and
, and the average number of neutral pions as
a function of negative pions produced.Comment: 12 pages, 3 pictures, late
Nuclear Dependence in Direct Photon Production
We calculate the nuclear dependence of direct photon production in
hadron-nucleus collisions. In terms of a multiple scattering picture, we
factorize the cross section for direct photon production into calculable
short-distance partonic parts times multiparton correlation functions in
nuclei. We present the hadron-nucleus cross section as times the
hadron-nucleon cross section. Using information on the multiparton correlation
functions extracted from photon-nucleus experiments, we compute the value of
as a function of transverse momentum of the direct photon. We also
compare our results with recent data from Fermilab experiment E706.Comment: 24 pages text in RevTex, 9 Postscript figure
Decoherence, einselection, and the quantum origins of the classical
Decoherence is caused by the interaction with the environment. Environment
monitors certain observables of the system, destroying interference between the
pointer states corresponding to their eigenvalues. This leads to
environment-induced superselection or einselection, a quantum process
associated with selective loss of information. Einselected pointer states are
stable. They can retain correlations with the rest of the Universe in spite of
the environment. Einselection enforces classicality by imposing an effective
ban on the vast majority of the Hilbert space, eliminating especially the
flagrantly non-local "Schr\"odinger cat" states. Classical structure of phase
space emerges from the quantum Hilbert space in the appropriate macroscopic
limit: Combination of einselection with dynamics leads to the idealizations of
a point and of a classical trajectory. In measurements, einselection replaces
quantum entanglement between the apparatus and the measured system with the
classical correlation.Comment: Final version of the review, with brutally compressed figures. Apart
from the changes introduced in the editorial process the text is identical
with that in the Rev. Mod. Phys. July issue. Also available from
http://www.vjquantuminfo.or
Centauro- and anti-Centauro-type events
Assuming that leading particles in high-energy hadronic and nuclear
collisions become sources of a classical pion field, we show that the direct
production of pions favors Centauro (mainly charged) events and that the
production of pions through the -type channel favors anti-Centauro
(mainly neutral) events. We also observe a strong negative neutral-charged
correlation in both cases.Comment: 14 pages, 2 pictures, late
Discrete approaches to quantum gravity in four dimensions
The construction of a consistent theory of quantum gravity is a problem in
theoretical physics that has so far defied all attempts at resolution. One
ansatz to try to obtain a non-trivial quantum theory proceeds via a
discretization of space-time and the Einstein action. I review here three major
areas of research: gauge-theoretic approaches, both in a path-integral and a
Hamiltonian formulation, quantum Regge calculus, and the method of dynamical
triangulations, confining attention to work that is strictly four-dimensional,
strictly discrete, and strictly quantum in nature.Comment: 33 pages, invited contribution to Living Reviews in Relativity; the
author welcomes any comments and suggestion
- …