1,438 research outputs found
The Lambda_Q-Lambda_Q Potential
Lattice QCD simulations of the potential between two baryons, each containing
a heavy quark and two light quarks, such as the Lambda_Q-Lambda_Q potential,
will provide insight into the nucleon-nucleon interaction. As one-pion exchange
does not contribute to the Lambda_Q-Lambda_Q potential, the long-distance
behavior is dominated by physics that contributes to the intermediate-range
attraction between two nucleons. We compute the leading long-distance
contributions to the Lambda_Q-Lambda_Q potential in QCD and in
partially-quenched QCD in the low-energy effective field theory.Comment: 10 pages LaTeX, 3 eps figs, 3 ps fig
To bind or not to bind: The H-dibaryon in light of chiral effective field theory
We analyse the quark mass dependence of the binding energy of the H-dibaryon
in the framework of chiral effective field theory. We show that the SU(3)
breaking effects induced by the differences of the pertinent two-baryon
thresholds (Lambda-Lambda, Xi-N, Sigma-Sigma) have a very pronounced impact
that need to be incorporated properly in future lattice QCD simulations. We
also point out that if the H-dibaryon is a two-baryon bound state, its dominant
component is Xi-N rather than Lambda-Lambda, which is a consequence of the
approximate SU(3) flavor symmetry of the two-baryon interactions.Comment: 8 pages, 2 figures; Results updated to the new H binding energy
reported by NPLQCD, conclusions remain unchanged, several references adde
Pions in the Pionless Effective Field Theory
We show that processes involving pions that remain very near their mass-shell
can be reliably computed in the pionless effective field theory, with the pion
integrated in as a heavy field. As an application, we compute the pion-deuteron
scattering amplitude near threshold to next-to-leading order in the momentum
expansion. This amplitude is formally dominated by an infrared logarithm of the
form log(gamma/mpi), where gamma is the deuteron binding momentum, and mpi is
the mass of the pion. The coefficient of this logarithm is determined by the
S-wave pion-nucleon scattering lengths.Comment: 10 pages LaTeX, 4 eps fig
A Conjecture about Hadrons
We conjecture that in the chiral limit of QCD the spectrum of hadrons is
comprised of decoupled, reducible chiral multiplets. A simple rule is developed
which identifies the chiral representations filled out by the ground-state
hadrons. Our arguments are based on the algebraic structure of superconvergence
relations derived by Weinberg from the high-energy behavior of pion-hadron
scattering amplitudes.Comment: 15 pages LaTe
Bridging over p-wave pi-production and weak processes in few-nucleon systems with chiral perturbation theory
I study an aspect of chiral perturbation theory (\chi PT) which enables one
to ``bridge'' different reactions. That is, an operator fixed in one of the
reactions can then be used to predict the other. For this purpose, I calculate
the partial wave amplitude for the p-wave pion production (pp\to pn\pi^+) using
the pion production operator from the lowest and the next nonvanishing orders.
The operator includes a contact operator whose coupling has been fixed using a
matrix element of a low-energy weak process (pp\to de^+\nu_e). I find that this
operator does not reproduce the partial wave amplitude extracted from
experimental data, showing that the bridging over the reactions with
significantly different kinematics is not necessarily successful. I study the
dependence of the amplitude on the various inputs such as the NN potential, the
\pi N\Delta coupling, and the cutoff. I argue the importance of a higher order
calculation. In order to gain an insight into a higher order calculation, I add
a higher order counter term to the operator used above, and fit the couplings
to both the low-energy weak process and the pion production. The energy
dependence of the partial wave amplitude for the pion production is described
by the operator consistently with the data. However, I find a result which
tells us to be careful about the convergence of the chiral expansion for the
pp\to pn\pi^+ reaction.Comment: 30 pages, 13 figures, figures changed, compacted tex
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
Electroweak Matrix Elements in the Two-Nucleon Sector from Lattice QCD
We demonstrate how to make rigorous predictions for electroweak matrix
elements in nuclear systems directly from QCD. More precisely, we show how to
determine the short-distance contributions to low-momentum transfer electroweak
matrix elements in the two-nucleon sector from lattice QCD. In potential model
descriptions of multi-nucleon systems, this is equivalent to uniquely
determining the meson-exchange currents, while in the context of nuclear
effective field theory, this translates into determining the coefficients of
local, gauge-invariant, multi-nucleon-electroweak current operators. The
energies of the lowest-lying states of two nucleons on a finite volume lattice
with periodic boundary conditions in the presence of a background magnetic
field are sufficient to determine the local four-nucleon operators that
contribute to the deuteron magnetic moment and to the threshold cross-section
of n + p -> d + gamma. Similarly, the energy-levels of two nucleons immersed in
a background isovector axial weak field can be used to determine the
coefficient of the leading local four-nucleon operator contributing to the
neutral- and charged-current break-up of the deuteron. This is required for the
extraction of solar neutrino fluxes at SNO and future neutrino experiments.Comment: 22 pages, 4 figure
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