arXiv subjects

A. Winnebeck

Publications and source records attributed to A. Winnebeck.

18 recordsLinked to original sources

arXiv2023

Measurement of the Charge-Averaged Elastic Lepton-Proton Scattering Cross Section by the OLYMPUS Experiment

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.

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arXiv2021

Observation of a structure in the M$_{pη}$ invariant mass distribution near 1700 MeV/$c^2$ in the $\mathbf{γp \rightarrow p π^0 η} $ reaction

The reaction $γp \rightarrow p π^0 η$ 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$_{Nη}$ 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$_γ$ = 1400 - 1500 MeV and the cut M$_{pπ^0} \le 1190 $ MeV/$c^2$ a statistically significant structure in the M$_{pη}$ invariant mass distribution near 1700 MeV/$c^2$ is observed with a width of $Γ\approx 35$ MeV/$c^2$ while the mass resolution is $σ_{res}$ = 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$_γ \approx$ 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$_{pη}$ invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the $γp \rightarrow p a_0 \rightarrow p π^0 η$ reaction

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arXiv2019

New data on $\vecγ \vec{p}\rightarrow ηp$ with polarized photons and protons and their implications for $N^* \to Nη$ decays

The polarization observables $T, E, P, H$, and $G$ in photoproduction of $η$ 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, JüBo, and BnGa partial-wave analyses. A refit within the BnGa approach including further data yields precise branching ratios for the $Nη$ decay of nucleon resonances. A $Nη$-branching ratio of $0.33\pm 0.04$ for $N(1650)1/2^-$ is found, which reduces the large and controversially discussed $Nη$-branching ratio difference of the two lowest mass $J^P=1/2^-$-resonances significantly.

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arXiv2019

Measurement of the Helicity Asymmetry $E$ for the reaction $ γp\to π^0 p$

A measurement of the double-polarization observable $E$ for the reaction $γp\to π^0 p$ 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 $E_γ=600$~MeV to $E_γ=2310$~MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.

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arXiv2017

Photoproduction of eta mesons from the neutron: cross sections and double polarization observable E

Photoproduction of $η$ mesons from neutrons} \abstract{Results from measurements of the photoproduction of $η$ 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 $η\to3π^{0}\to6γ$ 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 $γn\rightarrow nη$. 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 $σ_{1/2}$ and $σ_{3/2}$. The narrow structure in the excitation function of $γn\rightarrow nη$ appears associated with the helicity-1/2 component of the reaction.

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arXiv2016

Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering: Determined by the OLYMPUS Experiment

The OLYMPUS collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, $R_{2γ}$, 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 $R_{2γ}$, 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 $R_{2γ}$, presented here for a wide range of virtual photon polarization $0.456<ε<0.978$, 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.

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arXiv2016

Double-polarization observable G in neutral-pion photoproduction off the proton

This paper reports on a measurement of the double-polarization observable G in $π^0$ 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), Jülich-Bonn (JüBo), MAID and SAID partial wave analyses. Implications of the new data for the pion photoproduction multipoles are discussed.

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arXiv2015

Measurement of double polarisation asymmetries in $ω$-photoproduction

The first measurements of the beam-target-helicity-asymmetries $E$ and $G$ in the photoproduction of $ω$-mesons off protons at the CBELSA/TAPS experiment are reported. $E$ ($G$) was measured using circularly (linearly) polarised photons and a longitudinally polarised target. $E$ was measured over the photon energy range from close to threshold ($E_γ= 1108$~MeV) to $E_γ= 2300$~MeV and $G$ at a single energy interval of $1108 < E_γ<1300$~MeV. Both measurements cover the full solid angle. The observables $E$ and $G$ are highly sensitive to the contribution of baryon resonances, with $E$ acting as a helicity filter in the $s$-channel. The new results indicate significant $s$-channel resonance contributions together with contributions from $t$-channel exchange processes. A partial wave analysis reveals strong contributions from the partial waves with spin-parity $J^P=3/2^+, 5/2^+$, and $3/2^-$.

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arXiv2015

The polarization observables T, P, and H and their impact on $γp \to pπ^0$ multipoles

Data on the polarization observables T, P, and H for the reaction $γp\to pπ^0$ are reported. Compared to earlier data from other experiments, our data are more precise and extend the covered range in energy and angle substantially. The results were extracted from azimuthal asymmetries measured using a transversely polarized target and linearly polarized photons. The data were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS detector. Within the Bonn-Gatchina partial wave analysis, the new polarization data lead to a significant narrowing of the error band for the multipoles for neutral-pion photoproduction.

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arXiv2014

The N(1520) 3/2- helicity amplitudes from an energy-independent multipole analysis based on new polarization data on photoproduction of neutral pions

New data on the polarization observables T, P, and H for the reaction $γp \to pπ^0$ are reported. The results are extracted from azimuthal asymmetries when a transversely polarized butanol target and a linearly polarized photon beam are used. The data were taken at the Bonn electron stretcher accelerator ELSA using the CBELSA/TAPS detector. These and earlier data are used to perform a truncated energy-independent partial wave analysis in sliced-energy bins. This energy-independent analysis is compared to the results from energy-dependent partial wave analyses.

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arXiv2014

Experimental constraints on the $ω$-nucleus real potential

In a search for $ω$ mesic states, the production of $ω$-mesons in coincidence with forward going protons has been studied in photon induced reactions on $^{12}$C for incident photon energies of 1250 - 3100 MeV. The $π^0 γ$ pairs from decays of bound or quasi-free $ω$-mesons have been measured with the CBELSA/TAPS detector system in coincidence with protons registered in the MiniTAPS forward array. Structures in the total energy distribution of the $π^0 γ$ pairs, which would indicate the population and decay of bound $ω~^{11}$B states, are not observed. The $π^0 γ$ cross section of 0.3 nb/MeV/sr observed in the bound state energy regime between -100 and 0 MeV may be accounted for by yield leaking into the bound state regime because of the large in-medium width of the $ω$-meson. A comparison of the measured total energy distribution with calculations suggests the real part $V_0$ of the $ω~^{11}$B potential to be small and only weakly attractive with $V_0(ρ=ρ_0) = -15\pm$ 35(stat) $\pm$20(syst) MeV in contrast to some theoretical predictions of attractive potentials with a depth of 100 - 150 MeV.

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arXiv2014

First measurement of the helicity asymmetry for $γp\rightarrow pπ^0$ in the resonance region

The first measurement of the helicity dependence of the photoproduction cross section of single neutral pions off protons is reported for photon energies from 600 to 2300\,MeV, covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, and BnGa partial wave analyses. Strikingly large differences between data and predictions are observed which are traced to differences in the helicity amplitudes of well known and established resonances. Precise values for the helicity amplitudes of several resonances are reported.

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arXiv2013

The OLYMPUS Experiment

The OLYMPUS experiment was designed to measure the ratio between the positron-proton and electron-proton elastic scattering cross sections, with the goal of determining the contribution of two-photon exchange to the elastic cross section. Two-photon exchange might resolve the discrepancy between measurements of the proton form factor ratio, $μ_p G^p_E/G^p_M$, made using polarization techniques and those made in unpolarized experiments. OLYMPUS operated on the DORIS storage ring at DESY, alternating between 2.01~GeV electron and positron beams incident on an internal hydrogen gas target. The experiment used a toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight detectors to measure rates for elastic scattering over the polar angular range of approximately $25^\circ$--$75^\circ$. Symmetric Møller/Bhabha calorimeters at $1.29^\circ$ and telescopes of GEM and MWPC detectors at $12^\circ$ served as luminosity monitors. A total luminosity of approximately 4.5~fb$^{-1}$ was collected over two running periods in 2012. This paper provides details on the accelerator, target, detectors, and operation of the experiment.

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arXiv2013

Determination of the η'-nucleus optical potential

The excitation function and momentum distribution of $η^\prime$ mesons have been measured in photon induced reactions on $^{12}{}$C in the energy range of 1250-2600 MeV. The experiment was performed with tagged photon beams from the ELSA electron accelerator using the Crystal Barrel and TAPS detectors. The data are compared to model calculations to extract information on the sign and magnitude of the real part of the $η^\prime$-nucleus potential. Within the model, the comparison indicates an attractive potential of -($37 \pm 10(stat)\pm10(syst)$) MeV depth at normal nuclear matter density. Since the modulus of this depth is larger than the modulus of the imaginary part of the $η^\prime$-nucleus potential of -($10\pm2.5$) MeV, determined by transparency ratio measurements, a search for resolved $η^\prime$-bound states appears promising.

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arXiv2013

DarkLight: A Search for Dark Forces at the Jefferson Laboratory Free-Electron Laser Facility

We give a short overview of the DarkLight detector concept which is designed to search for a heavy photon A' with a mass in the range 10 MeV/c^2 < m(A') < 90 MeV/c^2 and which decays to lepton pairs. We describe the intended operating environment, the Jefferson Laboratory free electon laser, and a way to extend DarkLight's reach using A' --> invisible decays.

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arXiv2012

Technical Design Report for the: PANDA Micro Vertex Detector

This document illustrates the technical layout and the expected performance of the Micro Vertex Detector (MVD) of the PANDA experiment. The MVD will detect charged particles as close as possible to the interaction zone. Design criteria and the optimisation process as well as the technical solutions chosen are discussed and the results of this process are subjected to extensive Monte Carlo physics studies. The route towards realisation of the detector is outlined.

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arXiv2012

Well-established nucleon resonances revisited by double-polarization measurements

The first measurement is reported of the double-polarization observable G in photoproduction of neutral pions off protons, covering the photon energy range from 620 to 1120 MeV and the full solid angle. G describes the correlation between the photon polarization plane and the scattering plane for protons polarized along the direction of the incoming photon. The observable is highly sensitive to contributions from baryon resonances. The new results are compared to the predictions from SAID, MAID, and BnGa partial wave analyses. In spite of the long-lasting efforts to understand γp -> pπ 0 as the simplest photoproduction reaction, surprisingly large differences between the new data and the latest predictions are observed which are traced to different contributions of the N (1535) with spin-parity J^P = 1/2^- and N (1520) with J^P = 3/2^- . In the third resonance region, where N (1680) with J^P = 5/2^+ production dominates, the new data are reasonably close to the predictions.

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arXiv2010

The PANDA GEM-based TPC Prototype

We report on the development of a GEM-based TPC prototype for the PANDA experiment. The design and requirements of this device will be illustrated, with particular emphasis on the properties of the recently tested GEM-detector, the characterization of the read-out electronics and the development of the tracking software that allows to evaluate the GEM-TPC data.

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