180 research outputs found
Status of Nucleon Resonances with Masses
We discuss different interpretations of peaks observed a few years ago by
Tatischeff et al. in missing mass spectra of the reaction , which
were declared as new exited nucleon states with small masses. A study of the
possible production of such states in the process by analyzing the invariant mass spectrum of
is proposed. It is shown that the data, obtained recently at MAMI-B, can allow
to analyze this process and to get information about an existence of exited
nucleon states with small masses.Comment: 4 pages, 2 figures, LaTeX with ws-p8-50x6-00.cls. Talk presented at
the NSTAR2001 Workshop, Mainz, Germany, March 7-10, 200
A method to polarise antiprotons in storage rings and create polarised antineutrons
An intense circularely polarised photon beam interacts with a cooled
antiproton beam in a storage ring. Due to spin dependent absorption cross
sections for the reaction gamma+antiproton > pi- + antineutron a built-up of
polarisation of the stored antiprotons takes place. Figures-of-merit around 0.1
can be reached in principle over a wide range of antiproton energies. In this
process antineutrons with Polarisation > 70% emerge. The method is presented
for the case of 300 MeV/c cooled antiproton beam
A surprising method for polarising antiprotons
We propose a method for polarising antiprotons in a storage ring by means of
a polarised positron beam moving parallel to the antiprotons. If the relative
velocity is adjusted to the cross section for spin-flip is
as large as about barn as shown by new QED-calculations of
the triple spin-cross sections. Two possibilities for providing a positron
source with sufficient flux density are presented. A polarised positron beam
with a polarisation of 0.70 and a flux density of approximately /(mm s) appears to be feasible by means of a radioactive C
dc-source. A more involved proposal is the production of polarised positrons by
pair production with circularly polarised photons. It yields a polarisation of
0.76 and requires the injection into a small storage ring. Such polariser
sources can be used at low (100 MeV) as well as at high (1 GeV) energy storage
rings providing a time of about one hour for polarisation build-up of about
antiprotons to a polarisation of about 0.18. A comparison with other
proposals show a gain in the figure-of-merit by a factor of about ten.Comment: 13 pages, 8 figures; v2: minor language and signification corrections
v3: (14 pages, 12 figures) major error, nonapplicable polarisation transfer
cross sections replaced by the mandatory spin-flip cross section
Separated cross sections in \pi^0 electroproduction at threshold at Q^2 = 0.05 GeV^2/c^2
The differential cross sections \sigma_0=\sigma_T+\epsilon \sigma_L,
\sigma_{LT}, and \sigma_{TT} of \pi^0 electroproduction from the proton were
measured from threshold up to an additional center of mass energy of 40 MeV, at
a value of the photon four-momentum transfer of Q^2= 0.05 GeV^2/c^2 and a
center of mass angle of \theta=90^\circ. By an additional out-of-plane
measurement with polarized electrons \sigma_{LT'} was determined. This showed
for the first time the cusp effect above the \pi^+ threshold in the imaginary
part of the s-wave. The predictions of Heavy Baryon Chiral Perturbation Theory
are in disagreement with these data. On the other hand, the data are somewhat
better predicted by the MAID phenomenological model and are in good agreement
with the dynamical model DMT.Comment: 6 pages, 4 figure
A Coherent Interpretation of the Form Factors of the Nucleon in Terms of a Pion Cloud and Constituent Quarks
The recent unbiased measurements of the electric form factor of the neutron
suggest that its shape may be interpreted as a smooth broad distribution with a
bump at Q^2 \approx 0.3(GeV/c)^2 superimposed. As a consequence the
corresponding charge distribution in the Breit frame shows a negative charge
extending as far out as 2fm. It is natural to identify this charge with the
pion cloud. This realisation is then used to reanalyse all old and new data of
the electric and magnetic from factors of the proton and the neutron by a
phenomenological fit and by a fit based on the constituent quark model. It is
shown that it is possible to fit all form factors coherently with both
ansaetzen and that they all show the signal of the pion cloud.Comment: 17 pages, 17 figure
Recoil polarization and beam-recoil double polarization measurement of \eta electroproduction on the proton in the region of the S_{11}(1535) resonance
The beam-recoil double polarization P_{x'}^h and P_{z'}^h and the recoil
polarization P_{y'} were measured for the first time for the
p(\vec{e},e'\vec{p})\eta reaction at a four-momentum transfer of Q^2=0.1
GeV^2/c^2 and a center of mass production angle of \theta = 120^\circ at MAMI
C. With a center of mass energy range of 1500 MeV < W < 1550 MeV the region of
the S_{11}(1535) and D_{13}(1520) resonance was covered. The results are
discussed in the framework of a phenomenological isobar model (Eta-MAID). While
P_{x'}^h and P_{z'}^h are in good agreement with the model, P_{y'} shows a
significant deviation, consistent with existing photoproduction data on the
polarized-target asymmetry.Comment: 4 pages, 1 figur
The f_LT Response Function of D(e,e'p)n at Q^2=0.33(GeV/c)^2
The interference response function f_LT (R_LT) of the D(e,e'p)n reaction has
been determined at squared four-momentum transfer Q^2 = 0.33 (GeV/c)^2 and for
missing momenta up to p_miss= 0.29 (GeV/c). The results have been compared to
calculations that reproduce f_LT quite well but overestimate the cross sections
by 10 - 20% for missing momenta between 0.1 (GeV/c) and 0.2 (GeV/c) .Comment: 12 Pages, 10 figure
Precise Neutron Magnetic Form Factors
Precise data on the neutron magnetic form factor G_{mn} have been obtained
with measurements of the ratio of cross sections of D(e,e'n) and D(e,e'p) up to
momentum transfers of Q^2 = 0.9 (GeV/c)^2. Data with typical uncertainties of
1.5% are presented. These data allow for the first time to extract a precise
value of the magnetic radius of the neutron.Comment: 10 pages, 2 figures, submitted to Physics Letters
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