8,427 research outputs found
Instanton-Induced Correlations in Hadrons
QCD instantons generate non-perturbative spin- and flavor- dependent forces
between quarks. We review the results of a series of studies on
instanton-induced correlations in hadrons. We first present some evidence for
instanton-mediated interactions in QCD, based on lattice simulations. Then we
show that the Instanton Liquid Model can reproduce the available data on proton
and pion form factors at large momentum transfer and explain the delay of the
onset of the perturbative regime in several hard reactions. We also show that
instantons generate a deeply bound scalar color anti-triplet diquark, with a
mass of about 450 MeV and size comparable with that of the proton. The strong
attraction in the anti-triplet scalar diquark channel leads to a quantitative
description of non-leptonic weak decays of hyperons and provides a microscopic
dynamical explanation of the Delta I=1/2 rule.Comment: Summary of the results presented at the "8th Workshop on
Non-perturbative Quantum Chromodynamics", Paris 7-11 June 2003, the "26th
International School on Nuclear Physics", Erice 16-24 September 2004, and the
"X Convegno sui Problemi della Fisica Nucleare Teorica", Cortona, 6-9 October
200
Molecular Dynamics at Low Time Resolution
The internal dynamics of macro-molecular systems is characterized by widely
separated time scales, ranging from fraction of ps to ns. In ordinary molecular
dynamics simulations, the elementary time step dt used to integrate the
equation of motion needs to be chosen much smaller of the shortest time scale,
in order not to cut-off important physical effects. We show that, in systems
obeying the over-damped Langevin Eq., the fast molecular dynamics which occurs
at time scales smaller than dt can be analytically integrated out and gives
raise to a time-dependent correction to the diffusion coefficient, which we
rigorously compute. The resulting effective Langevin equation describes by
construction the same long-time dynamics, but has a lower time resolution
power, hence it can be integrated using larger time steps dt. We illustrate and
validate this method by studying the diffusion of a point-particle in a
one-dimensional toy-model and the denaturation of a protein.Comment: 12 pages, 5 figure
Instanton Contribution to the Proton and Neutron Electric Form Factors
We study the instanton contribution to the proton and neutron electric form
factors. Using the single instanton approximation, we perform the calculations
in a mixed time-momentum representation in order to obtain the form factors
directly in momentum space. We find good agreement with the experimentally
measured electric form factor of the proton. For the neutron, our result falls
short of the experimental data. We argue that this discrepancy is due to the
fact that we neglect the contribution of the sea quarks. We compare to lattice
calculations and a relativistic version of the quark-diquark model.Comment: 8 pages, 5 figures, updated references, to appear in Phys. Lett.
Gaussian quantum fluctuations in the superfluid-Mott phase transition
Recent advances in cooling techniques make now possible the experimental
study of quantum phase transitions, which are transitions near absolute zero
temperature accessed by varying a control parameter. A paradigmatic example is
the superfluid-Mott transition of interacting bosons on a periodic lattice.
From the relativistic Ginzburg-Landau action of this superfluid-Mott transition
we derive the elementary excitations of the bosonic system, which contain in
the superfluid phase a gapped Higgs mode and a gappless Goldstone mode. We show
that this energy spectrum is in good agreement with the available experimental
data and we use it to extract, with the help of dimensional regularization,
meaningful analytical formulas for the beyond-mean-field equation of state in
two and three spatial dimensions. We find that, while the mean-field equation
of state always gives a second-order quantum phase transition, the inclusion of
Gaussian quantum fluctuations can induce a first-order quantum phase
transition. This prediction is a strong benchmark for next future experiments
on quantum phase transitions.Comment: 7 pages, 4 figures, to be published in Physical Review
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