2,547 research outputs found
Impact of the Delta (1232) resonance on neutral pion photoproduction in chiral perturbation theory
We present an ongoing project to assess the importance of D-waves and the
resonance for descriptions of neutral pion photoproduction in
Heavy Baryon Chiral Perturbation Theory. This research has been motivated by
data published by the A2 and CB-TAPS collaborations at MAMI [1]. This data has
reached unprecedented levels of accuracy from threshold through to the
resonance. Accompanying the experimental work, there has also been a series of
publications studying the theory that show that, to go beyond an energy of
MeV, it is necessary to include other aspects, in particular the
as a degree of freedom [2] and possibly higher partial waves
[3].Comment: Proceedings to the 8th International Workshop on Chiral Dynamics 201
Compton scattering from the proton: An analysis using the delta expansion up to N3LO
We report on a chiral effective field theory calculation of Compton
scattering from the proton. Our calculation includes pions, nucleons, and the
Delta(1232) as explicit degrees of freedom. It uses the "delta expansion", and
so implements the hierarchy of scales m_pi < M_Delta-M_N < Lambda_chi. In this
expansion the power counting in the vicinity of the Delta peak changes, and
resummation of the loop graphs associated with the Delta width is indicated.
We have computed the nucleon Compton amplitude in the delta expansion up to
N3LO for photon energies of the order of m_pi. This is the first order at which
the proton Compton scattering amplitudes receive contributions from contact
operators which encode contributions to the spin-independent polarisabilities
from states with energies of the order of Lambda_chi. We fit the coefficients
of these two operators to the experimental proton Compton data that has been
taken in the relevant photon-energy domain, and are in a position to extract
new results for the proton polarisabilities alpha and beta.Comment: 6 pages. Proceeding of Sixth International Workshop on Chiral
Dynamics, Bern (Switzerland), 6th -- 10th July 2009. To be published in Po
Using EFT to analyze low-energy Compton scattering from protons and light nuclei
We discuss the application of an effective field theory (EFT) which
incorporates the chiral symmetry of QCD to Compton scattering from the proton
and deuteron. We describe the chiral EFT analysis of the proton Compton
scattering database presented in our recent review (arXiv:1203.6834), which
gives: alpha^{(p)}=10.5 +/- 0.5(stat) +/- 0.8(theory); beta^{(p)}= 2.7 +/-
0.5(stat) +/- 0.8(theory), for the electric and magnetic dipole polarizability
of the proton. We also summarize the chiral EFT analysis of the world data on
coherent Compton scattering from deuterium presented in arXiv:1203.6834. That
yields: alpha^{(s)}=10.5 +/- 2.0(stat) +/- 0.8(theory); beta^{(s)}=3.6 +/-
1.0(stat) +/- 0.8(theory).Comment: 5 pages. Invited talk, presented by Phillips at the 11th Conference
on the Intersections of Nuclear and Particle Physics (CIPANP 2012), St.
Petersburg, FL, May 201
Comprehensive Study of Observables in Compton Scattering on the Nucleon
We present an analysis of observables in Compton scattering on the
proton. Cross sections, asymmetries with polarised beam and/or targets, and
polarisation-transfer observables are investigated for energies up to the
resonance to determine their sensitivity to the proton's dipole
scalar and spin polarisabilities. The Chiral Effective Field Theory Compton
amplitude we use is complete at NLO, , for photon
energies , and so has an accuracy of a few per cent there. At
photon energies in the resonance region it is complete at NLO,
, and so its accuracy there is about \%. We find
that for energies from pion-production threshold to about ,
multiple asymmetries have significant sensitivity to presently ill-determined
combinations of proton spin polarisabilities. We also argue that the broad
outcomes of this analysis will be replicated in complementary theoretical
approaches, e.g., dispersion relations. Finally, we show that below the
pion-production threshold, observables suffice to reconstruct the Compton
amplitude, and above it are required. Although not necessary for
polarisability extractions, this opens the possibility to perform "complete"
Compton-scattering experiments. An interactive Mathematica notebook, including
results for the neutron, is available from [email protected] .Comment: 75 pages LaTeX2e (pdflatex) including 37 figures as .pdf files using
includegraphics; minor corrections. Text-identical to published version but
including the Online Supplement. Higher-resolution figures are available at
http://home.gwu.edu/~hgrie/Compton/one-N-comprehensive-observables-delta4.v2.0.high-resolution-figures.tg
Nucleon Polarisabilities at and Beyond Physical Pion Masses
We examine the results of Chiral Effective Field Theory (EFT) for the
scalar- and spin-dipole polarisabilities of the proton and neutron, both for
the physical pion mass and as a function of . This provides chiral
extrapolations for lattice-QCD polarisability computations. We include both the
leading and sub-leading effects of the nucleon's pion cloud, as well as the
leading ones of the resonance and its pion cloud. The analytic
results are complete at NLO in the -counting for pion masses close
to the physical value, and at leading order for pion masses similar to the
Delta-nucleon mass splitting. In order to quantify the truncation error of our
predictions and fits as \% degree-of-belief intervals, we use a Bayesian
procedure recently adapted to EFT expansions. At the physical point, our
predictions for the spin polarisabilities are, within respective errors, in
good agreement with alternative extractions using experiments and
dispersion-relation theory. At larger pion masses we find that the chiral
expansion of all polarisabilities becomes intrinsically unreliable as
approaches about MeV---as has already been seen in other observables.
EFT also predicts a substantial isospin splitting above the physical
point for both the electric and magnetic scalar polarisabilities; and we
speculate on the impact this has on the stability of nucleons. Our results
agree very well with emerging lattice computations in the realm where EFT
converges. Curiously, for the central values of some of our predictions, this
agreement persists to much higher pion masses. We speculate on whether this
might be more than a fortuitous coincidence.Comment: 39 pages LaTeX2e (pdflatex) including 12 figures as 16 .pdf files
using includegraphics. Version approved for publication in EPJA includes
modifications, clarifications and removal of typographical errors in
refereeing and publication proces
Compton scattering from the proton in an effective field theory with explicit Delta degrees of freedom
We analyse the proton Compton-scattering differential cross section for
photon energies up to 325 MeV using Chiral Effective Field Theory and extract
new values for the electric and magnetic polarisabilities of the proton. Our
EFT treatment builds in the key physics in two different regimes: photon
energies around the pion mass ("low energy") and the higher energies where the
Delta(1232) resonance plays a key role. The Compton amplitude is complete at
N4L0, O(e^2 delta^4), in the low-energy region, and at NLO, O(e^2 delta^0), in
the resonance region. Throughout, the Delta-pole graphs are dressed with pi-N
loops and gamma-N-Delta vertex corrections. A statistically consistent database
of proton Compton experiments is used to constrain the free parameters in our
amplitude: the M1 gamma-N-Delta transition strength b_1 (which is fixed in the
resonance region) and the polarisabilities alpha and beta (which are fixed from
data below 170 MeV). In order to obtain a reasonable fit we find it necessary
to add the spin polarisability gammaM1 as a free parameter, even though it is,
strictly speaking, predicted in chiral EFT at the order to which we work. We
show that the fit is consistent with the Baldin sum rule, and then use that sum
rule to constrain alpha+beta. In this way we obtain
alpha=[10.65+/-0.35(stat})+/-0.2(Baldin)+/-0.3(theory)]10^{-4} fm^3, and beta
=[3.15-/+0.35(stat)-/+0.2(Baldin)-/+0.3(theory)]10^{-4} fm^3, with chi^2 =
113.2 for 135 degrees of freedom. A detailed rationale for the theoretical
uncertainties assigned to this result is provided.Comment: 36 pages, 15 figures Version 2 is shortened for publication; version
1 is more self-contained. Results section unchange
Compton Scattering and Nucleon Polarisabilities in Chiral EFT: Update and Future
We review theoretical progress and prospects for determining the nucleon's
static dipole polarisabilities from Compton scattering on few-nucleon targets,
including new values; see Refs. [1-5] for details and a more thorough
bibliography.Comment: 6 pages LaTeX2e (pdflatex) including 11 figures as .pdf files. First
presented by Griesshammer at the 12th Conference on the Intersections of
Nuclear and Particle Physics CIPANP2015, 19-24 May 2015, Vail (CO), USA;
updated for 22nd International Spin Symposium (SPIN 2016), University of
Illinois, Urbana (USA), 26-30 September 2016. Corrected 2 figures, added
clarifying tex
Study to design and develop remote manipulator system
Modeling of human performance in remote manipulation tasks is reported by automated procedures using computers to analyze and count motions during a manipulation task. Performance is monitored by an on-line computer capable of measuring the joint angles of both master and slave and in some cases the trajectory and velocity of the hand itself. In this way the operator's strategies with different transmission delays, displays, tasks, and manipulators can be analyzed in detail for comparison. Some progress is described in obtaining a set of standard tasks and difficulty measures for evaluating manipulator performance
Using effective field theory to analyse low-energy Compton scattering data from protons and light nuclei
Compton scattering provides important insight into the structure of the
nucleon. For photons up to about 300 MeV, it is parameterised by six dynamical
dipole polarisabilities which characterise the response of the nucleon to a
monochromatic photon of fixed frequency and multipolarity. Their zero-energy
limit yields the well-known static electric and magnetic dipole
polarisabilities \alpha and \beta, and the four dipole spin polarisabilities.
Chiral Effective Field Theory (ChiEFT) describes nucleon, deuteron and 3-He
Compton scattering, using consistent nuclear currents, rescattering and wave
functions. It can thus also be used to extract useful information on the
neutron amplitude from Compton scattering on light nuclei. We summarise past
work in ChiEFT on all of these reactions and compare with other theoretical
approaches. We also discuss all proton experiments up to about 400 MeV, as well
as the three modern elastic deuteron data sets, paying particular attention to
precision and accuracy of each set. Constraining the Delta(1232) parameters
from the resonance region, we then perform new fits to the proton data up to
omega(lab)=170 MeV, and a new fit to the deuteron data. After checking in each
case that a two-parameter fit is compatible with the respective Baldin sum
rules, we obtain, using the sum-rule constraints in a one-parameter fit,
\alpha=10.7\pm0.3(stat)\pm0.2(Baldin)\pm0.8(theory),
\beta=3.1\mp0.3(stat)\pm0.2(Baldin)\pm0.8(theory), for the proton
polarisabilities, and \alpha =10.9\pm 0.9(stat)\pm0.2(Baldin)\pm0.8(theory),
\beta =3.6\mp 0.9(stat)\pm0.2(Baldin)\pm0.8(theory), for the isoscalar
polarisabilities, each in units of 10^(-4) fm^3. We discuss plans for polarised
Compton scattering, their promise as tools to access spin polarisabilities, and
other future avenues for theoretical and experimental investigation.Comment: 82 pages LaTeX2e including 24 figures as .eps file embedded with
includegraphicx; review for Prog. Part Nucl Phys. Final version identical to
published areticle; spelling and grammar correcte
Elastic Compton Scattering from 3He and the Role of the Delta
We report observables for elastic Compton scattering from He in Chiral
Effective Field Theory with an explicit degree of freedom
(EFT) for energies between 50 and 120 MeV. The He amplitude
is complete at N3LO, , and in general converges well
order by order. It includes the dominant pion-loop and two-body currents, as
well as the Delta excitation in the single-nucleon amplitude. Since the cross
section is two to three times that for deuterium and the spin of polarised
He is predominantly carried by its constituent neutron, elastic Compton
scattering promises information on both the scalar and spin polarisabilities of
the neutron. We study in detail the sensitivities of 4 observables to the
neutron polarisabilities: the cross section, the beam asymmetry and two double
asymmetries resulting from circularly polarised photons and a longitudinally or
transversely polarised target. Including the Delta enhances those asymmetries
from which neutron spin polarisabilities could be extracted. We also correct
previous, erroneous results at N2LO, i.e.~without an explicit Delta, and
compare to the same observables on proton, neutron and deuterium targets. An
interactive Mathematica notebook of our results is available from
[email protected]: 37 pages LaTeX2e (pdflatex) including 16 figures as .pdf files using
includegraphics; minor corrections, triggered by referee comments, plus
typographical errors; text-identical to version published as EPJA 54 (2018)
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