158 research outputs found

### Thermalization and pinch singularities in non-equilibrium quantum field theory

I argue that, within the Closed-Time-Path formalism, pinch singularities do
not appear in truly out of equilibrium situations.Comment: 9 pages, plain TEX, no figures (TEX mistakes corrected

### Nucleon-Deuteron Scattering from an Effective Field Theory

We use an effective field theory to compute low-energy nucleon-deuteron
scattering. We obtain the quartet scattering length using low energy constants
entirely determined from low-energy nucleon-nucleon scattering. We find
$a_{th}=6.33$ fm, to be compared to $a_{exp}=6.35\pm 0.02$ fm.Comment: 8 pages, Latex, epsfig, figures include

### Narrow Resonances in Effective Field Theory

We discuss the power counting for effective field theories with narrow
resonances near a two-body threshold. Close to threshold, the effective field
theory is perturbative and only one combination of coupling constants is
fine-tuned. In the vicinity of the resonance, a second, ``kinematic''
fine-tuning requires a nonperturbative resummation. We illustrate our results
in the case of nucleon-alpha scattering.Comment: 11 pages, revtex4, 3 ps figure

### More on the infrared renormalization group limit cycle in QCD

We present a detailed study of the recently conjectured infrared
renormalization group limit cycle in QCD using chiral effective field theory.
It was conjectured that small increases in the up and down quark masses can
move QCD to the critical trajectory for an infrared limit cycle in the
three-nucleon system. At the critical quark masses, the binding energies of the
deuteron and its spin-singlet partner are tuned to zero and the triton has
infinitely many excited states with an accumulation point at the three-nucleon
threshold. We exemplify three parameter sets where this effect occurs at
next-to-leading order in the chiral counting. For one of them, we study the
structure of the three-nucleon system in detail using both chiral and contact
effective field theories. Furthermore, we investigate the matching of the
chiral and contact theories in the critical region and calculate the influence
of the limit cycle on three-nucleon scattering observables.Comment: 17 pages, 7 figures, discussion improved, results unchanged, version
to appear in EPJ

### Low lying modes of triplet-condensed neutron matter and their effective theory

The condensation of neutrons into a 3P2 superfluid phase occurs at densities relevant for the interior of neutron stars. The triplet pairing breaks rotational symmetry spontaneously and leads to the existence of gapless modes (angulons) that are relevant for many transport coefficients and to the star's cooling properties. We derive the leading terms of the low-energy effective field theory, including the leading coupling to electroweak currents, valid for a variety of possible 3P2 phases

### On Parity-Violating Three-Nucleon Interactions and the Predictive Power of Few-Nucleon EFT at Very Low Energies

We address the typical strengths of hadronic parity-violating three-nucleon
interactions in "pion-less" Effective Field Theory in the nucleon-deuteron
(iso-doublet) system. By analysing the superficial degree of divergence of loop
diagrams, we conclude that no such interactions are needed at leading order.
The only two linearly independent parity-violating three-nucleon structures
with one derivative mix two-S and two-P-half waves with iso-spin transitions
Delta I = 0 or 1. Due to their structure, they cannot absorb any divergence
ostensibly appearing at next-to-leading order. This observation is based on the
approximate realisation of Wigner's combined SU(4) spin-isospin symmetry in the
two-nucleon system, even when effective-range corrections are included.
Parity-violating three-nucleon interactions thus only appear beyond
next-to-leading order. This guarantees renormalisability of the theory to that
order without introducing new, unknown coupling constants and allows the direct
extraction of parity-violating two-nucleon interactions from three-nucleon
experiments.Comment: 20 pages LaTeX2e, including 9 figures as .eps file embedded with
includegraphicx. Minor modifications and stylistic corrections. Version
accepted for publication in Eur. Phys. J.

### Causality bounds for neutron-proton scattering

We consider the constraints of causality and unitarity for the low-energy
interactions of protons and neutrons. We derive a general theorem that
non-vanishing partial-wave mixing cannot be reproduced with zero-range
interactions without violating causality or unitarity. We define and calculate
interaction length scales which we call the causal range and the Cauchy-Schwarz
range for all spin channels up to J = 3. For some channels we find that these
length scales are as large as 5 fm. We investigate the origin of these large
lengths and discuss their significance for the choice of momentum cutoff scales
in effective field theory and universality in many-body Fermi systems.Comment: 36 pages, 10 figures, 7 tables, version to appear in Eur. Phys. J.

### Singular Potentials and Limit Cycles

We show that a central $1/r^n$ singular potential (with $n\geq 2$) is
renormalized by a one-parameter square-well counterterm; low-energy observables
are made independent of the square-well width by adjusting the square-well
strength. We find a closed form expression for the renormalization-group
evolution of the square-well counterterm.Comment: 15 pages LaTex, 5 eps figures, error in figures and text correcte

### A renormalisation group approach to two-body scattering in the presence of long-range forces

We apply renormalisation-group methods to two-body scattering by a
combination of known long-range and unknown short-range potentials. We impose a
cut-off in the basis of distorted waves of the long-range potential and
identify possible fixed points of the short-range potential as this cut-off is
lowered to zero. The expansions around these fixed points define the power
countings for the corresponding effective field theories. Expansions around
nontrivial fixed points are shown to correspond to distorted-wave versions of
the effective-range expansion. These methods are applied to scattering in the
presence of Coulomb, Yukawa and repulsive inverse-square potentials.Comment: 22 pages (RevTeX), 4 figure

### Massive and massless modes of the triplet phase of neutron matter

Neutron matter at densities on the order of the nuclear saturation density is believed to have neutrons paired in the P23 channel. We study the low-lying modes of this phase and find two massless modes (angulons), resulting from the spontaneous breaking of rotational symmetry as well as three other gapped modes. We compute their masses at arbitrary temperatures. © 2015 American Physical Society

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