65 research outputs found
The size of the proton - closing in on the radius puzzle
We analyze the recent electron-proton scattering data from Mainz using a
dispersive framework that respects the constraints from analyticity and
unitarity on the nucleon structure. We also perform a continued fraction
analysis of these data. We find a small electric proton charge radius, r_E^p =
0.84_{-0.01}^{+0.01} fm, consistent with the recent determination from muonic
hydrogen measurements and earlier dispersive analyses. We also extract the
proton magnetic radius, r_M^p = 0.86_{-0.03}^{+0.02} fm, consistent with
earlier determinations based on dispersion relations.Comment: 4 pages, 2 figures, fit improved, small modifications, section on
continued fractions modified, conclusions on the proton charge radius
unchanged, version accepted for publication in European Physical Journal
Particle tracking in kaon electroproduction with cathode-charge sampling in multi-wire proportional chambers
Wire chambers are routinely operated as tracking detectors in magnetic
spectrometers at high-intensity continuous electron beams. Especially in
experiments studying reactions with small cross-sections the reaction yield is
limited by the background rate in the chambers. One way to determine the track
of a charged particle through a multi-wire proportional chamber (MWPC) is the
measurement of the charge distribution induced on its cathodes. In practical
applications of this read-out method, the algorithm to relate the measured
charge distribution to the avalanche position is an important factor for the
achievable position resolution and for the track reconstruction efficiency. An
algorithm was developed for operating two large-sized MWPCs in a strong
background environment with multiple-particle tracks. Resulting efficiencies
were determined as a function of the electron beam current and on the signal
amplitudes. Because of the different energy-losses of pions, kaons, and protons
in the momentum range of the spectrometer the efficiencies depend also on the
particle species
Real and Virtual Compton Scattering: the nucleon polarisabilities
We give an overview of low-energy Compton scattering (gamma^(*) p --> gamma
p) with a real or virtual incoming photon. These processes allow the
investigation of one of the fundamental properties of the nucleon, i.e. how its
internal structure deforms under an applied static electromagnetic field. Our
knowledge of nucleon polarisabilities and their generalization to non-zero
four-momentum transfer will be reviewed, including the presently ongoing
experiments and future perspectives.Comment: 20 pages, 12 figures. Minireview/Proceedings of "Many-Body Structure
of Strongly Interacting Systems", Mainz, Germany, Feb. 23-25 2011 . V2: typos
corrected. version to appear in EPJ Special Topic
Exclusive electroproduction of K+ Lambda and K+ Sigma^0 final states at Q^2 = 0.030-0.055 (GeV/c)^2
Cross section measurements of the exclusive p(e,e'K+)Lambda,Sigma^0
electroproduction reactions have been performed at the Mainz Microtron MAMI in
the A1 spectrometer facility using for the first time the Kaos spectrometer for
kaon detection. These processes were studied in a kinematical region not
covered by any previous experiment. The nucleon was probed in its third
resonance region with virtual photons of low four-momenta, Q^2= 0.030-0.055
(GeV/c)^2. The MAMI data indicate a smooth transition in Q^2 from
photoproduction to electroproduction cross sections. Comparison with
predictions of effective Lagrangian models based on the isobar approach reveal
that strong longitudinal couplings of the virtual photon to the N* resonances
can be excluded from these models.Comment: 16 pages, 7 figure
A novel technique for determining luminosity in electron-scattering/positron-scattering experiments from multi-interaction events
Differential cross section measurement of the 12C(e,e’pp)10Beg.s. reaction
The differential cross section was measured for the 12C(e, e
pp)10Beg.s. reaction at energy and
momentum transfers of 163MeV and 198MeV/c, respectively. The measurement was performed at the
Mainz Microtron by using two high-resolution magnetic spectrometers of the A1 Collaboration and a
newly developed silicon detector telescope. The overall resolution of the detector system was sufficient to
distinguish the ground state from the first excited state in 10Be. We chose a super-parallel geometry that
minimizes the effect of two-body currents and emphasizes the effect of nucleon-nucleon correlations. The
obtained differential cross section is compared to the theoretical results of the Pavia reaction code in which
different processes leading to two-nucleon knockout are accounted for microscopically. The comparison
shows a strong sensitivity to nuclear-structure input and the measured cross section is seen to be dominated
by the interplay between long- and short-range nucleon-nucleon correlations. Microscopic calculations based
on the ab initio self-consistent Green’s function method give a reasonable description of the experimental
cross section.
Subtracted dispersion relation formalism for the two-photon exchange correction to elastic electron-proton scattering: Comparison with data
Measurement and tricubic interpolation of the magnetic field for the OLYMPUS experiment
The OLYMPUS experiment used a 0.3 T toroidal magnetic spectrometer to measure
the momenta of outgoing charged particles. In order to accurately determine
particle trajectories, knowledge of the magnetic field was needed throughout
the spectrometer volume. For that purpose, the magnetic field was measured at
over 36,000 positions using a three-dimensional Hall probe actuated by a system
of translation tables. We used these field data to fit a numerical magnetic
field model, which could be employed to calculate the magnetic field at any
point in the spectrometer volume. Calculations with this model were
computationally intensive; for analysis applications where speed was crucial,
we pre-computed the magnetic field and its derivatives on an evenly spaced grid
so that the field could be interpolated between grid points. We developed a
spline-based interpolation scheme suitable for SIMD implementations, with a
memory layout chosen to minimize space and optimize the cache behavior to
quickly calculate field values. This scheme requires only one-eighth of the
memory needed to store necessary coefficients compared with a previous scheme
[1]. This method was accurate for the vast majority of the spectrometer volume,
though special fits and representations were needed to improve the accuracy
close to the magnet coils and along the toroid axis.Comment: 7 pages, 8 figure
Strange hadrons – strangeness in strongly interacting particles
In 2007 the Mainz Microtron MAMI has been upgraded to 1.5 GeV electron beam energy, crossing the energy threshold for open strangeness production. The strangeness quantum number, as carried by the strange quark, provides valuable information on the contribution of individual quark flavours to hadronic processes. Theoretically, the strange quark with its rest energy of order 150 MeV is particularly interesting because it can neither be treated as a massless nor as a heavy quark. Experimentally, an instrument of central importance for the charged kaon electro-production off the proton or light nuclei at MAMI is the magnetic spectrometer Kaos that was installed recently and is now routinely operated by the A1 collaboration
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