0000000001299331
AUTHOR
F. Zehr
showing 36 related works from this author
New determination of theηtransition form factor in the Dalitz decayη→e+e−γwith the Crystal Ball/TAPS detectors at the Mainz Microtron
2014
The Dalitz decay eta -> e(+) e(-) gamma has been measured in the gamma p -> eta p reaction with the Crystal Ball and TAPS multiphoton spectrometers, together with the photon-tagging facility at the Mainz Microtron MAMI. The experimental statistic used in this work is one order of magnitude greater than in any previous measurement of eta -> e(+) e(-) gamma. The value obtained for the slope parameter Lambda(-2) of the eta transition form factor, Lambda(-2) = (1.95 +/- 0.15(stat) +/- 0.10(syst)) GeV-2, is in good agreement with recent measurements conducted in eta -> e(+) e(-) gamma and eta -> mu(+) mu(-) gamma decays, as well as with recent form-factor calculations. The uncertainty obtained i…
Double pion photoproduction off nuclei - Are there effects beyond final-state interaction?
2013
Photoproduction of $\pi^{0}\pi^{0}$ and $\pi^{0}\pi^{\pm}$ pairs from nuclei has been measured over a wide mass range ($^2$H, $^{7}$Li, $^{12}$C, $^{40}$Ca, and $^{\rm nat}$Pb) for photon energies from threshold to 600 MeV. The experiments were performed at the MAMI accelerator in Mainz, using the Glasgow photon tagging spectrometer and a 4$\pi$ electromagnetic calorimeter consisting of the Crystal Ball and TAPS detectors. A shift of the pion-pion invariant mass spectra for heavy nuclei to small invariant masses has been observed for $\pi^0$ pairs but also for the mixed-charge pairs. The precise results allow for the first time a model-independent analysis of the influence of pion final-sta…
Coherent photoproduction of η-mesons off 3He – search for η-mesic nuclei
2012
Coherent photoproduction of $\eta$-mesons off $^3$He, i.e. the reaction $\gamma ^3{He}\rightarrow \eta ^3{He}$, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS detector. Angular distributions and the total cross section were measured using the $\eta\rightarrow \gamma\gamma$ and $\eta\rightarrow 3\pi^0\rightarrow 6\gamma$ decay channels. The observed extremely sharp rise of the cross section at threshold and the behavior of the angular distributions are evidence for a strong $\eta {^3{He}}$ final state interaction, pointing to the existence of a resonant …
Measurement of theγp→K0Σ+reaction with the Crystal Ball/TAPS detectors at the Mainz Microtron
2013
The gamma p -> K-0 Sigma(+) reaction has been measured from threshold to E-gamma = 1.45 GeV (W-CM = 1.9 GeV) using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. In the present experiment, this reaction was searched for in the 3 pi(0)p final state, by assuming K-S(0) -> pi(0)pi(0) and Sigma(+) -> pi(0)p. The experimental results include total and differential cross sections as well as the polarization of the recoil hyperon. The new data significantly improve empirical knowledge about the gamma p -> K-0 Sigma(+) reaction in the measured energy range. The results are compared to previous measurements and model predict…
Neutron Skin ofPb208from Coherent Pion Photoproduction
2014
Information on the size and shape of the neutron skin on (208)Pb is extracted from coherent pion photoproduction cross sections measured using the Crystal Ball detector together with the Glasgow tagger at the MAMI electron beam facility. On exploitation of an interpolated fit of a theoretical model to the measured cross sections, the half-height radius and diffuseness of the neutron distribution are found to be c(n)=6.70±0.03(stat.) fm and a(n)=0.55±0.01(stat.)(-0.03)(+0.02)(sys.) fm, respectively, corresponding to a neutron skin thickness Δr(np)=0.15±0.03(stat.)(-0.03)(+0.01)(sys.) fm. The results give the first successful extraction of a neutron skin thickness with an electromagnetic p…
Measurement of the $^{1}H$($\vec{\gamma}$, $\vec{p}$)$\pi^{0}$ reaction using a novel nucleon spin polarimeter
2013
We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon, pioneering a novel polarimetry technique with wide application to future nuclear and hadronic physics experiments. The commissioning measurement of polarization transfer in the $^{1}H$($\vec{\gamma}$,$\vec{p}$)$\pi^{0}$ reaction in the range $0.4<E_{\gamma}<1.4$ GeV is highly selective regarding the basic parameterizations used in partial wave analyses to extract the nucleon excitation spectrum. The new data strongly favor the recently proposed Chew-Mandelstam formalism.
Determination of the η-transition form factor in the γp→pη→pγe+e− reaction
2011
The Dalitz decay η→γe+e− has been measured using the combined Crystal Ball and TAPS photon detector setup at the electron accelerator MAMI-C. Compared to the most recent transition form-factor measurement in the e+e− channel, statistics have been improved by one order of magnitude. The e+e− invariant mass distribution shows a deviation from the QED prediction for a point-like particle, which can be described by a form-factor. Using the usual monopole transition form-factor parameterization, F(m2)=(1−m2/Λ2)−1, a value of Λ−2=(1.92±0.35(stat)±0.13(syst)) GeV−2 has been determined. This value is in good agreement with a recent measurement of the η Dalitz decay in the μ+μ− channel and with rece…
Beam-Helicity Asymmetries in Double-Pion Photoproduction off the Proton
2009
Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz in the three isospin channels $\vec{\gamma}p\to \pi^{+}\pi^0n$, $\vec{\gamma}p\to \pi^{0}\pi^0p$ and $\vec{\gamma}p\to \pi^{+}\pi^{-}p$ . The circularly polarized photons, produced from bremsstrahlung of longitudinally polarized electrons, were tagged with the Glasgow magnetic spectrometer. Charged pions and the decay photons of $\pi^0$ mesons were detected in a $~4\pi$ electromagnetic calorimeter which combined the Crystal Ball detector with the TAPS detector. The precisely measured asymmetries are very sensitive to details of the production processes and are thus key observables in the modeling of the reaction d…
Measurement of the beam-helicity asymmetry I⊙ in the photoproduction of π0-pairs off the proton and off the neutron
2013
Abstract Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of neutral pion pairs in the reactions γ → p → p π 0 π 0 and γ → d → ( n ) p π 0 π 0 , γ → d → ( p ) n π 0 π 0 off free protons and off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. Circularly polarized photons were produced from bremsstrahlung of longitudinally polarized electrons and tagged with the Glasgow magnetic spectrometer. Decay photons from the π 0 mesons, recoil protons, and recoil neutrons were detected in the 4π covering electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. After kinematic reconstruction of…
K+Λ and K+Σ0 photoproduction with fine center-of-mass energy resolution
2014
Measurements of γp→K+Λγp→K+Λ and γp→K+Σ0γp→K+Σ0 cross-sections have been obtained with the photon tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement uses a novel K+K+ meson identification technique in which the weak decay products are characterized using the energy and timing characteristics of the energy deposit in the calorimeter, a method that has the potential to be applied at many other facilities. The fine center-of-mass energy (W) resolution and statistical accuracy of the new data results in a significant impact on partial wave analyses aiming to better establish the excitation spectrum of the nucleon. The new analyses disfavor a strong role for quark–diqua…
Incoherent neutral pion photoproduction on 12C.
2007
We present the first detailed measurement of incoherent photoproduction of neutral pions to a discrete state of a residual nucleus. The C-12(gamma,pi(0))C-12(4.4 MeV)* reaction has been studied with the Glasgow photon tagger at MAMI employing a new technique which uses the large solid angle Crystal Ball detector both as a pi(0) spectrometer and to detect decay photons from the excited residual nucleus. The technique has potential applications to a broad range of future nuclear measurements with the Crystal Ball and similar detector systems elsewhere. Such data are sensitive to the propagation of the Delta in the nuclear medium and will give the first information on matter transition form fa…
Experimental study of theγp→ηpreaction with the Crystal Ball detector at the Mainz Microtron (MAMI-C)
2010
The gamma p - = W >= 1.87 GeV). Bremsstrahlung photons produced by the 1.5-GeV electron beam of the Mainz Microtron MAMI-C and momentum analyzed by the Glasgow tagging spectrometer were used for the eta-meson production. Our accumulation of 3.8 x 10(6) gamma p -< eta p -< 3 pi(0) p -< 6 gamma p events allows a detailed study of the reaction dynamics. The gamma p -< eta p differential cross sections were determined for 120 energy bins and the full range of the production angles. Our data show a dip near W = 1680 MeV in the total cross section caused by a substantial dip in eta production at forward angles. The data are compared to predictions of previous SAID and MAID partial-wave analyses a…
Photoproduction of theωmeson on the proton near threshold
2015
An experimental study of omega photoproduction on the proton was conducted by using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. The gamma p -> omega p differential cross sections are measured from threshold to the incident-photon energy E-gamma = 1.40 GeV (W = 1.87 GeV for the center-of-mass energy) with 15-MeV binning in E-gamma and full production-angle coverage. The quality of the present data near threshold gives access to a variety of interesting physics aspects. As an example, an estimation of the omega N scattering length alpha(omega p) is provided.
Measurement of theH1(γ⃗, p⃗)π0Reaction Using a Novel Nucleon Spin Polarimeter
2014
We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon. The measurement pioneers a novel polarimetry technique, which can be applied to many other nuclear and hadron physics experiments. The commissioning reaction of $^{1}H(\mathrm{\ensuremath{\gamma}P\vec},\text{ }\mathrm{pP\vec}){\ensuremath{\pi}}^{0}$ in the range $0.4l{E}_{\ensuremath{\gamma}}l1.4\text{ }\text{ }\mathrm{GeV}$ validates the technique and provides essential new data to constrain the excitation spectrum of the nucleon.
First measurement of the circular beam asymmetry in the γ→p→π0ηp reaction
2010
Abstract The circular photon asymmetry for π 0 η photoproduction on the proton was measured for the first time at the tagged photon facility of the MAMI C accelerator using the Crystal Ball/TAPS photon spectrometer. The experimental results are interpreted within a phenomenological isobar model that confirms the dominant role of the Δ ( 1700 ) D 33 resonance. The measured asymmetry allows us to identify small contributions from positive-parity resonances via interference terms with the dominant D 33 amplitude.
"dcs09" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.7891 GeV
"dcs02" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.7319 GeV
"dcs13" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.8192 GeV
"dcs04" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.7490 GeV
"dcs05" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.7568 GeV
"dcs19" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.8657 GeV
"dcs06" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.7645 GeV
"dcs11" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.8045 GeV
"dcs12" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.8119 GeV
"tcs" of "Photoproduction of the omega meson off the proton near threshold"
2022
Total cross section as a function of c.m. energy W.
"dcs17" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.8504 GeV
"dcs01" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.7245 GeV
"dcs20" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.8720 GeV
"dcs18" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.8581 GeV
"dcs16" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.8424 GeV
"dcs07" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.7729 GeV
"dcs03" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.7401 GeV
"dcs14" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.8269 GeV
"dcs10" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.7968 GeV
"dcs08" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.7811 GeV
"dcs15" of "Photoproduction of the omega meson off the proton near threshold"
2022
Differential cross section at W= 1.8348 GeV