arXiv subjects

W. Meyer

Publications and source records attributed to W. Meyer.

At least 19 recordsLinked to original sources

arXiv2023

The new APD-Based Readout of the Crystal Barrel Calorimeter -- An Overview

The Crystal Barrel is an electromagnetic calorimeter consisting of 1380 CsI(Tl) scintillators, and is currently installed at the CBELSA/TAPS experiment where it is used to detect decay products from photoproduction of mesons. The readout of the Crystal Barrel has been upgraded in order to integrate the detector into the first level of the trigger and to increase its sensitivity for neutral final states. The new readout uses avalanche photodiodes in the front-end and a dual back-end with branches optimized for energy and time measurement, respectively. An FPGA-based cluster finder processes the whole hit pattern within less than 100 ns. The important downside of APDs -- the temperature dependence of their gain -- is handled with a temperature stabilization and a compensating bias voltage supply. Additionally, a light pulser system allows the APDs' gains to be measured during beamtimes.

physics.ins-det↗

arXiv2022

Polarization observables in double neutral pion photoproduction

Measurements of target asymmetries and double-polarization observables for the reaction $\gamma p\to p\pi^0\pi^0$ are reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility (Bonn University) using the Bonn frozen-spin butanol (C$_4$H$_9$OH) target, which provided transversely polarized protons. Linearly polarized photons were produced via bremsstrahlung off a diamond crystal. The data cover the photon energy range from $E_{\gamma}$=650 MeV to $E_{\gamma}$=2600 MeV and nearly the complete angular range. The results have been included in the BnGa partial wave analysis. Experimental results and the fit agree very well. Observed systematic differences in the branching ratios for decays of $N^*$ and $\Delta^*$ resonances are attributed to the internal structure of these excited nucleon states. Resonances which can be assigned to SU(6)$\times$O(3) two-oscillator configurations show larger branching ratios to intermediate states with non-zero intrinsic orbital angular momenta than resonances assigned to one-oscillator configurations.

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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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arXiv2020

Spin polarizabilities of the proton by measurement of Compton double-polarization observables

The Compton double-polarization observable $Σ_{2z}$ has been measured for the first time in the $Δ(1232)$ resonance region using a circularly polarized photon beam incident on a longitudinally polarized target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using dispersion relation and chiral perturbation theory calculations, with the former resulting in: $γ_{E1E1} = -3.18 \pm 0.52$, $γ_{M1M1} = 2.98 \pm 0.43$, $γ_{E1M2} = -0.44 \pm 0.67$ and $γ_{M1E2} = 1.58 \pm 0.43$, in units of $10^{-4}~\mathrm{fm}^{4}$.

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arXiv2020

Helicity-dependent cross sections for the photoproduction of $π^0$ pairs from nucleons

The double-polarization observable $E$ and helicity-dependent cross sections $σ_{1/2}$, $σ_{3/2}$ have been measured for the photoproduction of $π^0$ pairs off quasi-free protons and neutrons at the Mainz MAMI accelerator with the Crystal Ball/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polarized deuterated butanol target. The reaction products were detected with an almost $4π$ covering calorimeter. The results reveal for the first time the helicity- and isospin-dependent structure of the $γN\rightarrow Nπ^0π^0$ reaction. They are compared to predictions from reaction models in view of nucleon resonance contributions and also to a refit of one model that predicted results for the proton and for the neutron target. The comparison of the prediction and the refit demonstrate the large impact of the new data.

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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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arXiv2018

First measurement of helicity-dependent cross sections in pi0-eta photoproduction from quasi-free nucleons

The helicity-dependent cross sections for the photoproduction of $π^0η$ pairs have been measured for the first time. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS calorimeter. The experiment used a polarized deuterated butanol target and a circularly polarized photon beam. This arrangement allowed the $σ_{1/2}$ (photon and target spin antiparallel) and $σ_{3/2}$ (parallel spins) components to be measured for quasi-free production of $π^0η$ pairs off protons and neutrons. The main finding is that the two helicity components contribute identically, within uncertainties, for both participant protons and neutrons. The absolute couplings for protons and neutrons are also identical. This means that nucleon resonances contributing to this reaction in the investigated energy range have almost equal electromagnetic helicity couplings, $A_{1/2}^{n,p}$ and $A_{3/2}^{n,p}$. Identical couplings for protons and neutrons are typical for $Δ$ resonances and identical $A_{1/2}$ and $A_{3/2}$ components are only possible for $J\geq 3/2$ states, which constrains possible contributions of nucleon resonances.

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arXiv2017

Polarised target for Drell-Yan experiment in COMPASS at CERN, part I

In the polarised Drell-Yan experiment at the COMPASS facility in CERN pion beam with momentum of 190 GeV/c and intensity about $10^8$ pions/s interacted with transversely polarised NH$_3$ target. Muon pairs produced in Drel-Yan process were detected. The measurement was done in 2015 as the 1st ever polarised Drell-Yan fixed target experiment. The hydrogen nuclei in the solid-state NH$_3$ were polarised by dynamic nuclear polarisation in 2.5 T field of large-acceptance superconducting magnet. Large helium dilution cryostat was used to cool the target down below 100 mK. Polarisation of hydrogen nuclei reached during the data taking was about 80 %. Two oppositely polarised target cells, each 55 cm long and 4 cm in diameter were used. Overview of COMPASS facility and the polarised target with emphasis on the dilution cryostat and magnet is given. Results of the polarisation measurement in the Drell-Yan run and overviews of the target material, cell and dynamic nuclear polarisation system are given in the part II.

physics.ins-det↗

arXiv2017

First measurement of the polarization observable $E$ and helicity-dependent cross sections in single $π^{0}$ photoproduction from quasi-free nucleons

The double-polarization observable $E$ and the helicity-dependent cross sections $σ_{1/2}$ and $σ_{3/2}$ have been measured for the first time for single $π^{0}$ photoproduction from protons and neutrons bound in the deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the $π^0$ meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the $π^{0}N$ final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry $E$ since the $σ_{1/2}$ and $σ_{3/2}$ components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry $E$ measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons.

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arXiv2017

Helicity-dependent cross sections and double-polarization observable E in eta photoproduction from quasi-free protons and neutrons

Precise helicity-dependent cross sections and the double-polarization observable $E$ were measured for $η$ photoproduction from quasi-free protons and neutrons bound in the deuteron. The $η\rightarrow 2γ$ and $η\rightarrow 3π^0\rightarrow 6γ$ decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz. A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were detected with the electromagnetic calorimeters Crystal Ball and TAPS, which covered 98\% of the full solid angle. The results show that the narrow structure observed earlier in the unpolarized excitation function of $η$ photoproduction off the neutron appears only in reactions with antiparallel photon and nucleon spin ($σ_{1/2}$). It is absent for reactions with parallel spin orientation ($σ_{3/2}$) and thus very probably related to partial waves with total spin 1/2. The behavior of the angular distributions of the helicity-dependent cross sections was analyzed by fitting them with Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which uses an interference of $P_{11}$ and $S_{11}$ partial waves to explain the narrow structure.

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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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arXiv2017

Insight into the narrow structure in {\boldmath{$η$}}-photoproduction on the neutron from helicity dependent cross sections

The double polarization observable $E$ and the helicity dependent cross sections $σ_{1/2}$ and $σ_{3/2}$ were measured for $η$ photoproduction from quasi-free protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost $4π$ detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from $η\rightarrow 2γ$ and $η\rightarrow 3π^0$. The results show that the narrow structure previously observed in $η$ photoproduction from the neutron is only apparent in $σ_{1/2}$ and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that contribute to this partial wave in $η$ production are only $N1/2^-$ ($S_{11}$) and $N1/2^+$ ($P_{11}$). Furthermore, the extracted Legendre coefficients of the angular distributions for $σ_{1/2}$ are in good agreement with recent reaction model predictions assuming a narrow resonance in the $P_{11}$ wave as the origin of this structure.

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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

Measurement of pi^0 photoproduction on the proton at MAMI C

Differential cross sections for the gamma p -> pi^0 p reaction have been measured with the A2 tagged-photon facilities at the Mainz Microtron, MAMI C, up to the center-of-mass energy W=1.9 GeV. The new results, obtained with a fine energy and angular binning, increase the existing quantity of pi^0 photoproduction data by ~47%. Owing to the unprecedented statistical accuracy and the full angular coverage, the results are sensitive to high partial-wave amplitudes. This is demonstrated by the decomposition of the differential cross sections in terms of Legendre polynomials and by further comparison to model predictions. A new solution of the SAID partial-wave analysis obtained after adding the new data into the fit is presented.

hep-ex↗

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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arXiv2015

First measurement of target and beam-target asymmetries in the $γp\to π^0ηp$ reaction

The first data on target and beam-target asymmetries for the $γp\toπ^0ηp$ reaction at photon energies from 1050 up to 1450 MeV are presented. The measurements were performed using the Crystal Ball and TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. The general assumption that the reaction is dominated by the $Δ3/2^-$ amplitude is confirmed. The data are in particular sensitive to small contributions from other partial waves.

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