arXiv subjects

J. R. M. Annand

Publications and source records attributed to J. R. M. Annand.

At least 109 records · Page 6Linked to original sources

arXiv2013

Measurement of the $^{1}H$($\vecγ$, $\vec{p}$)$π^{0}$ reaction using a novel nucleon spin polarimeter

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 $^{1}H$($\vecγ$,$\vec{p}$)$π^{0}$ reaction in the range $0.4<E_γ<1.4$ 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.

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arXiv2013

Accurate Test of Chiral Dynamics in the \boldmath$\vecγ p \rightarrow π^0p$ Reaction

A precision measurement of the differential cross sections $dσ/dΩ$ and the linearly polarized photon asymmetry $Σ\equiv (dσ_\perp - dσ_\parallel) \slash (dσ_\perp + dσ_\parallel)$ for the $\vecγ p \rightarrow π^0p$ reaction in the near-threshold region has been performed with a tagged photon beam and almost $4π$ detector at the Mainz Microtron. The Glasgow-Mainz photon tagging facility along with the Crystal Ball/TAPS multi-photon detector system and a cryogenic liquid hydrogen target were used. These data allowed for a precise determination of the energy dependence of the real parts of the $S$- and all three $P$-wave amplitudes for the first time and provide the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of agreement with experiment.

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arXiv2013

Measurement of the beam-helicity asymmetry $I^{\odot}$ in the photoproduction of $π^0$-pairs off the proton and off the neutron

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of neutral pion pairs in the reactions $\vecγp\rightarrow pπ^0π^0$ and $\vecγd\rightarrow (n)pπ^0π^0$, $\vecγd\rightarrow (p)nπ^0π^0$ 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 4$π$ covering electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. After kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons. This demonstrates that the free-nucleon behavior of such observables can be extracted from measurements with quasi-free nucleons, which is the only possibility for the neutron. Contrary to expectations, the measured asymmetries are very similar for reactions off protons and neutrons. The results are compared to the predictions from the Two-Pion-MAID reaction model and (for the proton) also to the Bonn-Gatchina coupled channel analysis.

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arXiv2013

Double pion photoproduction off nuclei - are there effects beyond final-state interaction?

Photoproduction of $π^{0}π^{0}$ and $π^{0}π^{\pm}$ 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$π$ 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 $π^0$ 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-state interactions. The corresponding effects are found to be large and must be carefully considered in the search for possible in-medium modifications of the $σ$-meson. Results from a transport model calculation reproduce the shape of the invariant-mass distributions for the mixed-charge pairs better than for the neutral pairs, but also for the latter differences between model results and experiment are not large, leaving not much room for $σ$-in-medium modification.

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arXiv2013

Coherent photoproduction of pi^0- and eta-mesons off 7Li

Coherent photoproduction of $π^0$-mesons from threshold ($E_{th} \approx$ 136 MeV) throughout the $Δ$-resonance region and of $η$-mesons close to the production threshold ($E_{th} \approx$ 570 MeV for $η$) has been measured for $^7$Li nuclei. The experiment was performed using the tagged-photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4$π$ solid-angle electromagnetic calorimeter. The reactions were identified by a combined invariant-mass and missing-energy analysis. A comparison of the pion data to plane-wave impulse modelling tests the nuclear mass form factor. So far coherent $η$-production had been only identified for the lightest nuclear systems ($^2$H and $^3$He). For $^3$He a large enhancement of the cross section above plane-wave approximations had been reported, indicating the formation of a quasi-bound state. The present Li-data for $η$-production agree with a plane-wave approximation. Contrary to $^3$He, neither a threshold enhancement of the total cross section nor a deviation of the angular distributions from the expected form-factor dependence were observed.

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arXiv2012

Determination of the η mass with the Crystal Ball at MAMI-B

A new precise determination of the η meson mass is presented. It is based on a measurement of the threshold for the γp -> pη reaction using the tagger focal-plane microscope detector at the MAMI-B facility in Mainz. The tagger microscope has a higher energy resolution than the standard tagging spectrometer and, hence, allowed an improvement in the accuracy compared to the previous η mass measurement at MAMI-B. The result mη = (547.851 +- 0.031 stat. +- 0.062 syst.) MeV agrees very well with the precise values of the NA48, KLOE and CLEO collaborations and deviates by about 5σ from the smaller, but also very precise value obtained by the GEM collaboration at COSY.

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arXiv2012

Photoproduction of pi0pi0 and pi0pi+-pairs off the proton from threshold to the second resonance region

Precise total cross-sections and invariant-mass distributions have been measured for photoproduction of pion pairs off the proton producing $pπ^0π^0$ and $nπ^+π^0$ final states from the threshold region up to 800 MeV incident photon energy. Additionally, beam helicity asymmetries have been measured in the second resonance region (550 MeV - 820 MeV). The experiment was performed at the tagged photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4$π$ solid-angle electromagnetic calorimeter. The results are much more precise than any previous measurements and confirm the chiral perturbation theory predictions for the threshold behavior of these reactions. In the second resonance region, the invariant-mass distributions of meson-meson and meson-nucleon pairs are in reasonable agreement with model predictions, but none of the models reproduce the asymmetries for the mixed-charge channel.

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arXiv2012

Study of the gamma p --> pi0 pi0 p reaction with the Crystal Ball/TAPS at the Mainz

The gamma p --> pi0 pi0 p reaction has been measured from threshold to 1.4 GeV using the Crystal Ball and TAPS photon spectrometers together with the photon tagging facility at the Mainz Microtron. The experimental results include total and differential cross sections as well as specific angular distributions, which were used to extract partial-wave amplitudes. In particular, the energy region below the D13(1520) resonance was studied.

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arXiv2012

Methods for Optical Calibration of the BigBite Hadron Spectrometer

The techniques for optical calibration of Jefferson Lab's large-acceptance magnetic hadron spectrometer, BigBite, have been examined. The most consistent and stable results were obtained by using a method based on singular value decomposition. In spite of the complexity of the optics, the particles' positions and momenta at the target have been precisely reconstructed from the coordinates measured in the detectors by means of a single back-tracing matrix. The technique is applicable to any similar magnetic spectrometer and any particle type. For 0.55 GeV/c protons, we have established the vertex resolution of 1.2 cm, angular resolutions of 7 mrad and 16 mrad (in-plane and out-of-plane, respectively), and a relative momentum resolution of 1.6%.

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arXiv2012

Beam-Target Double Spin Asymmetry A_LT in Charged Pion Production from Deep Inelastic Scattering on a Transversely Polarized He-3 Target at 1.4<Q^2<2.7 GeV^2

We report the first measurement of the double-spin asymmetry $A_{LT}$ for charged pion electroproduction in semi\nobreakdash-inclusive deep\nobreakdash-inelastic electron scattering on a transversely polarized $^{3}$He target. The kinematics focused on the valence quark region, $0.16<x<0.35$ with $1.4<Q^{2}<2.7\,\textrm{GeV}^{2}$. The corresponding neutron $A_{LT}$ asymmetries were extracted from the measured $^{3}$He asymmetries and proton over $^{3}$He cross section ratios using the effective polarization approximation. These new data probe the transverse momentum dependent parton distribution function $g_{1T}^{q}$ and therefore provide access to quark spin-orbit correlations. Our results indicate a positive azimuthal asymmetry for $π^{-}$ production on $^{3}$He and the neutron, while our $π^{+}$ asymmetries are consistent with zero.

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arXiv2012

Coherent photoproduction of eta-mesons off 3He - search for eta-mesic nuclei

Coherent photoproduction of $η$-mesons off $^3$He, i.e. the reaction $γ^3{He}\rightarrow η^3{He}$, 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 $η\rightarrow γγ$ and $η\rightarrow 3π^0\rightarrow 6γ$ 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 $η{^3{He}}$ final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of $π^0$-proton back-to-back emission in the $γ^3{He}\rightarrow π^0 pX$ reaction revealed a very complicated structure of the background and could not support previous conclusions.

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arXiv2011

Single Spin Asymmetries in Charged Pion Production from Semi-Inclusive Deep Inelastic Scattering on a Transversely Polarized $^3$He Target

We report the first measurement of target single spin asymmetries in the semi-inclusive $^3{He}(e,e'π^\pm)X$ reaction on a transversely polarized target. The experiment, conducted at Jefferson Lab using a 5.9 GeV electron beam, covers a range of 0.14 $< x <$ 0.34 with 1.3 $<Q^2<$ 2.7 GeV$^2$. The Collins and Sivers moments were extracted from the azimuthal angular dependence of the measured asymmetries. The extracted $π^\pm$ Collins moments for $^3$He are consistent with zero, except for the $π^+$ moment at $x=0.34$, which deviates from zero by 2.3$σ$. While the $π^-$ Sivers moments are consistent with zero, the $π^+$ Sivers moments favor negative values. The neutron results were extracted using the nucleon effective polarization and the measured cross section ratio of proton to $^3$He, and are largely consistent with the predictions of phenomenological fits and quark model calculations.

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arXiv2011

Measurement of 3pi^0 photoproduction on the proton from threshold to 1.4 GeV

The total cross section for gamma p -> 3pi0 p has been measured for the first time from threshold to 1.4 GeV using the tagged photon beam of the Mainz Microtron. The equipment utilized the Crystal Ball multiphoton spectrometer, the TAPS forward detector and a particle identification detector. The gamma p -> 3pi0 p total cross section has two broad enhancements at sqrt{s}~1.5 GeV and 1.7 GeV. We obtained the ratio of the total cross sections gamma p -> 3pi0 p to gamma p -> eta p equal to 0.014 \pm 0.001 at sqrt{s}~1.5 GeV.

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arXiv2010

Measurements of the Electric Form Factor of the Neutron up to Q2=3.4 GeV2 using the Reaction He3(e,e'n)pp

The electric form factor of the neutron was determined from studies of the reaction He3(e,e'n)pp in quasi-elastic kinematics in Hall A at Jefferson Lab. Longitudinally polarized electrons were scattered off a polarized target in which the nuclear polarization was oriented perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons that were registered in a large-solid-angle detector. More than doubling the Q2-range over which it is known, we find GEn = 0.0225 +/- 0.0017 (stat) +/- 0.0024 (syst), 0.0200 +/- 0.0023 +/- 0.0018, and 0.0142 +/- 0.0019 +/- 0.0013 for Q2 = 1.72, 2.48, and 3.41 GeV2, respectively.

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arXiv2010

Study of the gp-->etap reaction with the Crystal Ball detector at the Mainz Microtron(MAMI-C)

The gp-->etap reaction has been measured with the Crystal Ball and TAPS multiphoton spectrometers in the energy range from the production threshold of 707 MeV to 1.4 GeV (1.49 =< 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 gp-->etap-->3pi0p-->6gp events allows a detailed study of the reaction dynamics. The gp-->etap 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 and to thelatest SAID and MAID fits that have included our data.

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arXiv2010

First measurement of the circular beam asymmetry in the gamma p --> pi0 eta p reaction

The circular photon asymmetry for pi0 eta 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 Delta(1700)D33 resonance. The measured asymmetry allows us to identify small contributions from positive-parity resonances via interference terms with the dominant D33 amplitude.

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arXiv2010

Radiative pi0 photoproduction on protons in the Delta+(1232) region

The reaction gamma p -> p pi0 gamma' has been measured with the Crystal Ball / TAPS detectors using the energy-tagged photon beam at the electron accelerator facility MAMI-B. Energy and angular differential cross sections for the emitted photon gamma' and angular differential cross sections for the pi0 have been determined with high statistics in the energy range of the Delta+(1232) resonance. Cross sections and the ratio of the cross section to the non-radiative process gamma p -> p pi0 are compared to theoretical reaction models, having the anomalous magnetic moment kappa_Delta+ as free parameter. As the shape of the experimental distributions is not reproduced in detail by the model calculations, currently no extraction of kappa_Delta+ is feasible.

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arXiv2009

Photoproduction of pi^0 eta on protons and the Delta(1700)D_{33} resonance

Total and differential cross sections for the reaction gamma p -> pi^o eta p have been measured with the Crystal Ball/TAPS detector using the tagged photon facility at the MAMI C accelerator in Mainz. In the energy range E_gamma=0.95-1.4 GeV the reaction is dominated by the excitation and sequential decay of the Delta(1700)D33 resonance. Angular distributions measured with high statistics allow us to determine the ratio of hadronic decay widths Γ_{ηΔ}/Γ_{πS11} and the ratio of the helicity amplitudes A_{3/2}/A_{1/2} for this resonance.

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