The PANDA spectrometer will be a state of the art universal detector for strong interaction studies at the High-Energy Storage Ring (HESR) in the future international Facility for Anti-proton and Ion ...Research (FAIR) at GSI, Darmstadt. The detector is designed to take advantage of the extraordinary physics potential which will be available utilizing high intensity, phase space cooled anti-proton beams. This facility will provide a cooled anti-proton beam−2with momenta of 1.5-15 GeV/c, a maximal luminosity of 2×1032cms−1 that translates into 2×107 pp annihilations per second. A GEM based Time Projection Chamber (TPC) was one of the central tracker candidates for the PANDA experiment. To check the feasibility of such a detector system, a large prototype GEM-TPC was built and tested inside the FOPI Spectrometer at GSI. A set of three Gas Electron Multiplier (GEM) foils were used as amplification stage instead of Multi Wire Proportional Chambers. In this report, the design, construction, characterization of the prototype-detector system will be discussed in detail along with the results of recent beam tests.
The reaction $\gamma p \rightarrow p \pi^0 \eta$ has been studied with the
CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for
incident photon energies from threshold up to ...3.1 GeV. This paper has been
motivated by the recently claimed observation of a narrow structure in the
M$_{N\eta}$ invariant mass distribution at a mass of 1678 MeV/$c^2$. The
existence of this structure cannot be confirmed in the present work. Instead,
for E$_{\gamma}$ = 1400 - 1500 MeV and the cut M$_{p\pi^0} \le 1190 $ MeV/$c^2$
a statistically significant structure in the M$_{p\eta}$ invariant mass
distribution near 1700 MeV/$c^2$ is observed with a width of $\Gamma\approx 35$
MeV/$c^2$ while the mass resolution is $\sigma_{res}$ = 5 MeV/$c^2$. Increasing
the incident photon energy from 1420 to 1540 MeV this structure shifts in mass
from $\approx$ 1700MeV/c$^2$ to $\approx$ 1725 MeV/$c^2$; the width increases
to about 50 MeV/$c^2$ and decreases thereafter. The cross section associated
with this structure reaches a maximum of $\approx$ 100 nb around E$_{\gamma}
\approx$ 1490 MeV (W $\approx $ 1920 MeV), which coincides with the $p a_0$
threshold. Three scenarios are discussed which might be the origin of this
structure in the M$_{p\eta}$ invariant mass distribution. The most likely
interpretation is that it is due to a triangular singularity in the $\gamma p
\rightarrow p a_0 \rightarrow p \pi^0 \eta$ reaction
Phys. Rev. Lett. 126, 162501 (2021) We report the first measurement of the average of the electron-proton and
positron-proton elastic scattering cross sections. This lepton charge-averaged
cross ...section is insensitive to the leading effects of hard two-photon
exchange, giving more robust access to the proton's electromagnetic form
factors. The cross section was extracted from data taken by the OLYMPUS
experiment at DESY, in which alternating stored electron and positron beams
were scattered from a windowless gaseous hydrogen target. Elastic scattering
events were identified from the coincident detection of the scattered lepton
and recoil proton in a large-acceptance toroidal spectrometer. The luminosity
was determined from the rates of M{\o}ller, Bhabha and elastic scattering in
forward electromagnetic calorimeters. The data provide some selectivity between
existing form factor global fits and will provide valuable constraints to
future fits.
A measurement of the double-polarization observable $E$ for the reaction
$\gamma p\to \pi^0 p$ is reported. The data were taken with the CBELSA/TAPS
experiment at the ELSA facility in Bonn using the ...Bonn frozen-spin butanol
(C$_4$H$_9$OH) target, which provided longitudinally-polarized protons.
Circularly-polarized photons were produced via bremsstrahlung of
longitudinally-polarized electrons. The data cover the photon energy range from
$E_\gamma =600$~MeV to $E_\gamma =2310$~MeV and nearly the complete angular
range. The results are compared to and have been included in recent partial
wave analyses.
The polarization observables $T, E, P, H$, and $G$ in photoproduction of
$\eta$ mesons off protons are measured for photon energies from threshold to
$W=2400\,$MeV ($T$), 2280 MeV ($E$), 1620 MeV ...($P, H$), or 1820 MeV ($G$),
covering nearly the full solid angle. The data are compared to predictions from
the SAID, MAID, J\"uBo, and BnGa partial-wave analyses. A refit within the BnGa
approach including further data yields precise branching ratios for the $N\eta$
decay of nucleon resonances. A $N\eta$-branching ratio of $0.33\pm 0.04$ for
$N(1650)1/2^-$ is found, which reduces the large and controversially discussed
$N\eta$-branching ratio difference of the two lowest mass
$J^P=1/2^-$-resonances significantly.
The reaction \(\gamma p \rightarrow p \pi^0 \eta\) has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to ...3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the M\(_{N\eta}\) invariant mass distribution at a mass of 1678 MeV/\(c^2\). The existence of this structure cannot be confirmed in the present work. Instead, for E\(_{\gamma}\) = 1400 - 1500 MeV and the cut M\(_{p\pi^0} \le 1190 \) MeV/\(c^2\) a statistically significant structure in the M\(_{p\eta}\) invariant mass distribution near 1700 MeV/\(c^2\) is observed with a width of \(\Gamma\approx 35\) MeV/\(c^2\) while the mass resolution is \(\sigma_{res}\) = 5 MeV/\(c^2\). Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from \(\approx\) 1700MeV/c\(^2\) to \(\approx\) 1725 MeV/\(c^2\); the width increases to about 50 MeV/\(c^2\) and decreases thereafter. The cross section associated with this structure reaches a maximum of \(\approx\) 100 nb around E\(_{\gamma} \approx\) 1490 MeV (W \(\approx \) 1920 MeV), which coincides with the \(p a_0\) threshold. Three scenarios are discussed which might be the origin of this structure in the M\(_{p\eta}\) invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the \(\gamma p \rightarrow p a_0 \rightarrow p \pi^0 \eta\) reaction
We report the first measurement of the average of the electron-proton and positron-proton elastic scattering cross sections. This lepton charge-averaged cross section is insensitive to the leading ...effects of hard two-photon exchange, giving more robust access to the proton's electromagnetic form factors. The cross section was extracted from data taken by the OLYMPUS experiment at DESY, in which alternating stored electron and positron beams were scattered from a windowless gaseous hydrogen target. Elastic scattering events were identified from the coincident detection of the scattered lepton and recoil proton in a large-acceptance toroidal spectrometer. The luminosity was determined from the rates of Møller, Bhabha and elastic scattering in forward electromagnetic calorimeters. The data provide some selectivity between existing form factor global fits and will provide valuable constraints to future fits.
Photoproduction of \(\eta\) mesons from neutrons} \abstract{Results from measurements of the photoproduction of \(\eta\) mesons from quasifree protons and neutrons are summarized. The experiments ...were performed with the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn using the \(\eta\to3\pi^{0}\to6\gamma\) decay. A liquid deuterium target was used for the measurement of total cross sections and angular distributions. The results confirm earlier measurements from Bonn and the MAMI facility in Mainz about the existence of a narrow structure in the excitation function of \(\gamma n\rightarrow n\eta\). The current angular distributions show a forward-backward asymmetry, which was previously not seen, but was predicted by model calculations including an additional narrow \(P_{11}\) state. Furthermore, data obtained with a longitudinally polarized, deuterated butanol target and a circularly polarized photon beam were analyzed to determine the double polarization observable \(E\). Both data sets together were also used to extract the helicity dependent cross sections \(\sigma_{1/2}\) and \(\sigma_{3/2}\). The narrow structure in the excitation function of \(\gamma n\rightarrow n\eta\) appears associated with the helicity-1/2 component of the reaction.
The OLYMPUS collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, \(R_{2\gamma}\), a direct measure of the contribution of hard ...two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01~GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight scintillators detected elastically scattered leptons in coincidence with recoiling protons over a scattering angle range of \(\approx 20\degree\) to \(80\degree\). The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved GEM and MWPC detectors at \(12\degree\), as well as symmetric Møller/Bhabha calorimeters at \(1.29\degree\). A total integrated luminosity of 4.5~fb\(^{-1}\) was collected. In the extraction of \(R_{2\gamma}\), radiative effects were taken into account using a Monte Carlo generator to simulate the convolutions of internal bremsstrahlung with experiment-specific conditions such as detector acceptance and reconstruction efficiency. The resulting values of \(R_{2\gamma}\), presented here for a wide range of virtual photon polarization \(0.456<\epsilon<0.978\), are smaller than some hadronic two-photon exchange calculations predict, but are in reasonable agreement with a subtracted dispersion model and a phenomenological fit to the form factor data.
This paper reports on a measurement of the double-polarization observable G in \(\pi^0\) photoproduction off the proton using the CBELSA/TAPS experiment at the ELSA accelerator in Bonn. The ...observable G is determined from reactions of linearly-polarized photons with longitudinally-polarized protons. The polarized photons are produced by bremsstrahlung off a properly oriented diamond radiator. A frozen spin butanol target provides the polarized protons. The data cover the photon energy range from 617 to 1325 MeV and a wide angular range. The experimental results for G are compared to predictions by the Bonn-Gatchina (BnGa), J\"ulich-Bonn (J\"uBo), MAID and SAID partial wave analyses. Implications of the new data for the pion photoproduction multipoles are discussed.
PDF