237 research outputs found
Constraints on the Universe as a Numerical Simulation
Observable consequences of the hypothesis that the observed universe is a numerical simulation performed on a cubic space-time lattice or grid are explored. The simulation scenario is first motivated by extrapolating current trends in computational resource requirements for lattice QCD into the future. Using the historical development of lattice gauge theory technology as a guide, we assume that our universe is an early numerical simulation with unimproved Wilson fermion discretization and investigate potentially-observable consequences. Among the observables that are considered are the muon g-2 and the current differences between determinations of alpha, but the most stringent bound on the inverse lattice spacing of the universe, b−1 \u3e ~ 10^11 GeV, is derived from the high-energy cut off of the cosmic ray spectrum. The numerical simulation scenario could reveal itself in the distributions of the highest energy cosmic rays exhibiting a degree of rotational symmetry breaking that reflects the structure of the underlying lattice
Meson Masses in High Density QCD
The low-energy effective theories for the two- and three-flavor
color-superconductors arising in the high density limit of QCD are discussed.
Using an effective field theory to describe quarks near the fermi surface, we
compute the masses of the pseudo-Goldstone bosons that dominate the
low-momentum dynamics of these systems.Comment: 13 pages, 3 figures, late
From Hadrons to Nuclei: Crossing the Border
The study of nuclei predates by many years the theory of quantum
chromodynamics. More recently, effective field theories have been used in
nuclear physics to ``cross the border'' from QCD to a nuclear theory. We are
now entering the second decade of efforts to develop a perturbative theory of
nuclear interactions using effective field theory. This work describes the
current status of these efforts.Comment: 141 pages, 58 figs, latex. To appear in the Boris Ioffe Festschrift,
ed. by M. Shifman, World Scientifi
I=2 pi-pi Scattering from Fully-Dynamical Mixed-Action Lattice QCD
We compute the I=2 pi-pi scattering length at pion masses of m_pi ~ 294, 348
and 484 MeV in fully-dynamical lattice QCD using Luscher's finite-volume
method. The calculation is performed with domain-wall valence-quark propagators
on asqtad-improved MILC configurations with staggered sea quarks at a single
lattice spacing, b ~ 0.125 fm. Chiral perturbation theory is used to perform
the extrapolation of the scattering length from lattice quark masses down to
the physical value, and we find m_pi a_2 = -0.0426 +- 0.0006 +- 0.0003 +-
0.0018, in good agreement with experiment. The I=2 pi-pi scattering phase shift
is calculated to be delta = -43 +- 10 +- 5 degrees at |p| ~ 544 MeV for m_pi ~
484 MeV.Comment: 16 pages, 4 figure
Multi-Pion Systems in Lattice QCD and the Three-Pion Interaction
The ground-state energies of 2, 3, 4 and 5 \pi^+'s in a spatial volume V (2.5
fm)^3 are computed with lattice QCD. By eliminating the leading contribution
from three-\pi^+ interactions, particular combinations of these n-\pi^+
ground-state energies provide precise extractions of the \pi^+\pi^+ scattering
length in agreement with that obtained from calculations involving only two
\pi^+'s. The three-\pi^+ interaction can be isolated by forming other
combinations of the n-\pi^+ ground-state energies. We find a result that is
consistent with a repulsive three-\pi^+ interaction for m_\pi < 352 MeV.Comment: 4 pages, 5 figure
Unitary Limit of Two-Nucleon Interactions in Strong Magnetic Fields
Two-nucleon systems are shown to exhibit large scattering lengths in strong
magnetic fields at unphysical quark masses, and the trends toward the physical
values indicate that such features may exist in nature. Lattice QCD
calculations of the energies of one and two nucleons systems are performed at
pion masses of and 806 MeV in uniform, time-independent
magnetic fields of strength {\bf B}| \sim 10^{19}10^{20}$ Gauss to determine
the response of these hadronic systems to large magnetic fields. Fields of this
strength may exist inside magnetars and in peripheral relativistic heavy ion
collisions, and the unitary behavior at large scattering lengths may have
important consequences for these systems.Comment: Accepted journal versio
Ab initio calculation of the radiative capture process
Lattice QCD calculations of two-nucleon systems are used to isolate the
short-distance two-body electromagnetic contributions to the radiative capture
process , and the photo-disintegration processes
. In nuclear potential models, such contributions are
described by phenomenological meson-exchange currents, while in the present
work, they are determined directly from the quark and gluon interactions of
QCD. Calculations of neutron-proton energy levels in multiple background
magnetic fields are performed at two values of the quark masses, corresponding
to pion masses of and 806 MeV, and are combined with pionless
nuclear effective field theory to determine these low-energy inelastic
processes. Extrapolating to the physical pion mass, a cross section of
is obtained at an incident neutron speed of $v=2,200\
m/s\sigma^{expt}(np \to d\gamma)
= 334.2(0.5)\ mb$
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