550 research outputs found
Combined Description of Scattering and Annihilation With A Hadronic Model
A model for the nucleon-antinucleon interaction is presented which is based
on meson-baryon dynamics. The elastic part is the -parity transform of the
Bonn potential. Annihilation into two mesons is described in terms of
microscopic baryon-exchange processes including all possible combinations of
. The remaining
annihilation part is taken into account by a phenomenological energy- and state
independent optical potential of Gaussian form. The model enables a
simultaneous description of nucleon-antinucleon scattering and annihilation
phenomena with fair quality.Comment: revised version, REVTEX, 9 pages, 10 figures available from this URL
ftp://ikp113.ikp.kfa-juelich.de/pub/kph140/nucl-th.9411014.u
New pixelized Micromegas detector for the COMPASS experiment
New Micromegas (Micro-mesh gaseous detectors) are being developed in view of
the future physics projects planned by the COMPASS collaboration at CERN.
Several major upgrades compared to present detectors are being studied:
detectors standing five times higher luminosity with hadron beams, detection of
beam particles (flux up to a few hundred of kHz/mm^2, 10 times larger than for
the present detectors) with pixelized read-out in the central part, light and
integrated electronics, and improved robustness. Studies were done with the
present detectors moved in the beam, and two first pixelized prototypes are
being tested with muon and hadron beams in real conditions at COMPASS. We
present here this new project and report on two series of tests, with old
detectors moved into the beam and with pixelized prototypes operated in real
data taking condition with both muon and hadron beams.Comment: 11 pages, 5 figures, proceedings to the Micro-Pattern Gaseous
Detectors conference (MPGD2009), 12-15 June 2009, Kolympari, Crete, Greece
Minor details added and language corrections don
Realtime calibration of the A4 electromagnetic lead fluoride calorimeter
Sufficient energy resolution is the key issue for the calorimetry in particle
and nuclear physics. The calorimeter of the A4 parity violation experiment at
MAMI is a segmented calorimeter where the energy of an event is determined by
summing the signals of neighbouring channels. In this case the precise matching
of the individual modules is crucial to obtain a good energy resolution. We
have developped a calibration procedure for our total absorbing electromagnetic
calorimeter which consists of 1022 lead fluoride (PbF_2) crystals. This
procedure reconstructs the the single-module contributions to the events by
solving a linear system of equations, involving the inversion of a 1022 x
1022-matrix. The system has shown its functionality at beam energies between
300 and 1500 MeV and represents a new and fast method to keep the calorimeter
permanently in a well-calibrated state
Eta photoproduction off the neutron at GRAAL: Evidence for a resonant structure at W=1.67 GeV
New (preliminary) data on eta photoproduction off the neutron are presented.
These data reveal a resonant structure at W=1.67 GeV.Comment: 8 pages, 4 figures. Published in Proceedings of Workshop on the
Physics of Excited Nucleons NSTAR2004, Grenoble, France, March 24 - 27,
pg.19
Eta photoproduction off the neutron at GRAAL
The gamma n -> eta n quasi-free cross section reveals a resonant structure at
W ~ 1.675 GeV. This structure may be a manifestation of a baryon resonance. A
priori its properties, the possibly narrow width and the strong photocoupling
to the neutron, look surprising. This structure may also signal the existence
of a narrow state.Comment: To appear in Proceedings of Workshop on the Physics of Excited
Nucleons NSTAR2005, 12 - 15 October 2005, Tallahassee, Florida, US
New pixelized Micromegas detector with low discharge rate for the COMPASS experiment
New Micromegas (Micro-mesh gaseous detectors) are being developed in view of
the future physics projects planned by the COMPASS collaboration at CERN.
Several major upgrades compared to present detectors are being studied:
detectors standing five times higher luminosity with hadron beams, detection of
beam particles (flux up to a few hundred of kHz/mm^{2}, 10 times larger than
for the present Micromegas detectors) with pixelized read-out in the central
part, light and integrated electronics, and improved robustness. Two solutions
of reduction of discharge impact have been studied, with Micromegas detectors
using resistive layers and using an additional GEM foil. Performance of such
detectors has also been measured. A large size prototypes with nominal active
area and pixelized read-out has been produced and installed at COMPASS in 2010.
In 2011 prototypes featuring an additional GEM foil, as well as an resistive
prototype, are installed at COMPASS and preliminary results from those
detectors presented very good performance. We present here the project and
report on its status, in particular the performance of large size prototypes
with an additional GEM foil.Comment: 11 pages, 5 figures, proceedings to the Micro-Pattern Gaseous
Detectors conference (MPGD2011), 29-31 August 2011, Kobe, Japa
Lowering the Light Speed Isotropy Limit: European Synchrotron Radiation Facility Measurements
The measurement of the Compton edge of the scattered electrons in GRAAL
facility in European Synchrotron Radiation Facility (ESRF) in Grenoble with
respect to the Cosmic Microwave Background dipole reveals up to 10 sigma
variations larger than the statistical errors. We now show that the variations
are not due to the frequency variations of the accelerator. The nature of
Compton edge variations remains unclear, thus outlining the imperative of
dedicated studies of light speed anisotropy
Bacteriophage DNA glucosylation impairs target DNA binding by type I and II but not by type V CRISPR-Cas effector complexes
Prokaryotes encode various host defense systems that provide protection against mobile genetic elements. Restriction-modification (R-M) and CRISPR-Cas systems mediate host defense by sequence specific targeting of invasive DNA. T-even bacteriophages employ covalent modifications of nucleobases to avoid binding and therefore cleavage of their DNA by restriction endonucleases. Here, we describe that DNA glucosylation of bacteriophage genomes affects interference of some but not all CRISPR-Cas systems. We show that glucosyl modification of 5-hydroxymethylated cytosines in the DNA of bacteriophage T4 interferes with type I-E and type II-A CRISPR-Cas systems by lowering the affinity of the Cascade and Cas9-crRNA complexes for their target DNA. On the contrary, the type V-A nuclease Cas12a (also known as Cpf1) is not impaired in binding and cleavage of glucosylated target DNA, likely due to a more open structural architecture of the protein. Our results suggest that CRISPR-Cas systems have contributed to the selective pressure on phages to develop more generic solutions to escape sequence specific host defense systems
Eta photoproduction on the neutron at GRAAL: Measurement of the differential cross section
In this contribution, we will present our first preliminary measurement of
the differential cross section for the reaction gamma+n->eta+n. Comparison of
the reactions gamma+p->eta+p for free and bound proton (D2 target) will also be
discussed.Comment: 6 pages, 4 figures, Proceedings of the 10th International Symposium
on Meson-Nucleon Physics and the Structure of the Nucleon, August
29-September 4 2004, Beijing, Chin
Measurement of the Transverse Beam Spin Asymmetry in Elastic Electron Proton Scattering and the Inelastic Contribution to the Imaginary Part of the Two-Photon Exchange Amplitude
We report on a measurement of the asymmetry in the scattering of transversely
polarized electrons off unpolarized protons, A, at two Q values of
\qsquaredaveragedlow (GeV/c) and \qsquaredaveragedhighII (GeV/c) and a
scattering angle of . The measured transverse
asymmetries are A(Q = \qsquaredaveragedlow (GeV/c)) =
(\experimentalasymmetry alulowcorr \statisticalerrorlow
\combinedsyspolerrorlowalucor) 10 and
A(Q = \qsquaredaveragedhighII (GeV/c)) = (\experimentalasymme
tryaluhighcorr \statisticalerrorhigh
\combinedsyspolerrorhighalucor) 10. The first
errors denotes the statistical error and the second the systematic
uncertainties. A arises from the imaginary part of the two-photon
exchange amplitude and is zero in the one-photon exchange approximation. From
comparison with theoretical estimates of A we conclude that
N-intermediate states give a substantial contribution to the imaginary
part of the two-photon amplitude. The contribution from the ground state proton
to the imaginary part of the two-photon exchange can be neglected. There is no
obvious reason why this should be different for the real part of the two-photon
amplitude, which enters into the radiative corrections for the Rosenbluth
separation measurements of the electric form factor of the proton.Comment: 4 figures, submitted to PRL on Oct.
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