2,396 research outputs found
Random walk, cluster growth, and the morphology of urban conglomerations
We propose a new model of cluster growth according to which the probability
that a new unit is placed in a point at a distance from the city center is
a Gaussian with mean equal to the cluster radius and variance proportional to
the mean, modulated by the local density . The model is analytically
solvable in dimensions, where the density profile varies as a
complementary error function. The model reproduces experimental observations
relative to the morphology of cities, determined via an original analysis of
digital maps with a very high spatial resolution, and helps understanding the
emergence of vehicular traffic.Comment: Physica A. To appea
Order, Disorder and Confinement
Studying the order of the chiral transition for is of fundamental
importance to understand the mechanism of color confinement. We present results
of a numerical investigation on the order of the transition by use of a novel
strategy in finite size scaling analysis. The specific heat and a number of
susceptibilities are compared with the possible critical behaviours. A second
order transition in the O(4) and O(2) universality classes are excluded.
Substantial evidence emerges for a first order transition. Results are in
agreement with those found by studying the scaling properties of a disorder
parameter related to the dual superconductivity mechanism of color confinement.Comment: 7 pages, 11 figures. Talk given at the International Workshop on
Quantum Chromodynamics: QCD@Work 2005, Conversano, Italy, 16-20 June 200
The first jamming crossover: geometric and mechanical features
The jamming transition characterizes athermal systems of particles
interacting via finite range repulsive potentials, and occurs on increasing the
density when particles cannot avoid making contacts with those of their first
coordination shell. We have recently shown [M. Pica Ciamarra and P. Sollich,
arXiv:1209.3334] that the same systems are also characterized by a series of
jamming crossovers. These occur at higher volume fractions as particles are
forced to make contact with those of subsequent coordination shells. At finite
temperature, the crossovers give rise to dynamic and thermodynamic density
anomalies, including a diffusivity anomaly and a negative thermal expansion
coefficient. Density anomalies may therefore be related to structural changes
occurring at the jamming crossovers. Here we elucidate these structural
changes, investigating the evolution of the structure and of the mechanical
properties of a jammed system as its volume fraction varies from the jamming
transition to and beyond the first jamming crossover. We show that the first
jamming crossover occurs at a well defined volume fraction, and that it induces
a rearrangement of the force network causing a softening of the system. It also
causes qualitative changes in the normal mode density of states and the spatial
properties of the normal mode vectors.Comment: submitted to J. Chem. Phys. for "Special Topic Issue on the Glass
Transition
The chiral transition in two-flavor QCD
QCD with N_f=2 is a specially interesting system to investigate the chiral
transition. The order of the transition has still not been established. We
report the results of an in-depth numerical investigation performed with
staggered fermions on lattices with L_t=4 and L_s=12,16,20,24,32 and quark
masses am_q ranging from 0.01335 to 0.307036. Using finite-size techniques we
compare the scaling behavior of a number of thermodynamical susceptibilities
with the expectations of O(4) and O(2) universality classes. Clear disagreement
is observed. Indications of a first order transition are found.Comment: 6 pages, 4 figures, PoS style. Talk presented at Lattice 2005
(Nonzero temperature and density), Dublin, Ireland, July 25-3
Exceptional and Spinorial Conformal Windows
We study the conformal window of gauge theories containing fermionic matter
fields, where the gauge group is any of the exceptional groups with the
fermions transforming according to the fundamental and adjoint representations
and the orthogonal groups where the fermions transform according to a spinorial
representation. We investigate the phase diagram using a purely perturbative
four loop analysis, the all-orders beta function and the ladder approximation.Comment: 23 pages, 8 figures, 8 table
A test of first order scaling in Nf=2 QCD
We complete our analysis of Nf=2 QCD based on the lattice staggered fermion
formulation. Using a series of Monte Carlo simulations at fixed (amq*Ls^yh) one
is able to test the universality class with given critical exponent yh. This
strategy has been used to test the O(4) universality class and it has been
presented at the previous Lattice conferences. No agreement was found with
simulations in the mass range amq=[0.01335,0.15] using lattices with Ls=16 up
to 32 and Lt=4. With the same strategy, we now investigate the possibility of a
first order transition using a new set of Monte Carlo data corresponding to
yh=3 in the same mass and volume range as the one used for O(4). A substantial
agreement is observed both in the specific heat scaling and in the scaling of
the chiral condensate, while the chiral susceptibilities still presents visible
deviation from scaling in the mass range explored.Comment: 5 pages, 6 figures, Presented at the XXV International Symposium on
Lattice Field Theory, July 30 - August 4 2007, Regensburg, German
Two flavor QCD and confinement - II
This paper is part of a program of investigation of the chiral transition in
Nf=2 QCD, started in Phys.Rev.D72:114510,2005. Progress is reported on the
understanding of some possible systematic errors. A direct test of first order
scaling is presented.Comment: 7 pages, 6 figure
Conformal vs confining scenario in SU(2) with adjoint fermions
The masses of the lowest-lying states in the meson and in the gluonic sector
of an SU(2) gauge theory with two Dirac flavors in the adjoint representation
are measured on the lattice at a fixed value of the lattice coupling for values of the bare fermion mass that span a range
between the quenched regime and the massless limit, and for various lattice
volumes. Even for light constituent fermions the lightest glueballs are found
to be lighter than the lightest mesons. Moreover, the string tension between
two static fundamental sources strongly depends on the mass of the dynamical
fermions and becomes of the order of the inverse squared lattice linear size
before the chiral limit is reached. The implications of these findings for the
phase of the theory in the massless limit are discussed and a strategy for
discriminating between the (near--)conformal and the confining scenario is
outlined.Comment: 5 pages, 4 figures using RevTeX4, Typos corrected, references added.
Versions to appear on PR
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