K+ΛK^+\Lambda and K+Σ0K^+\Sigma^0 photoproduction with fine center-of-mass energy resolution

Measurements of $\gamma p \rightarrow K^{+} \Lambda$ and $\gamma p \rightarrow K^{+} \Sigma^0$ cross-sections have been obtained with the photon tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement uses a novel $K^+$ 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-diquark dynamics in the nucleon.Comment: 7 pages, 8 figure

Measurements of 12C(→γ,pp) photon asymmetries for Eγ= 200–450 MeV

The 12C (→γ ,pp) reaction has been studied in the photon energy range 200-450 MeV at the Mainz microtron MAMI-C, where linearly polarised photons were energy-tagged using the Glasgow-Mainz Tagged Photon Spectrometer and protons were detected in the Crystal Ball detector. The photon asymmetry Σ has been measured over a wider Eγ range than previous measurements. The strongest asymmetries were found at low missing energies where direct emission of nucleon pairs is expected. Cuts on the difference in azimuthal angles of the two ejected protons increased the magnitude of the observed asymmetries. At low missing energies the Σ data exhibit a strong angular dependence, similar to deuteron photodisintegration

Determination of the eta-transition form factor in the gamma p -< p eta -< p gamma e(+)e(-) reaction

The Dalitz decay eta -&gt; gamma e(+)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(m(2)) = (1 - m(2)/Lambda(2))(-1), a value of Lambda(-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 eta Dalitz decay in the mu(+)mu(-) channel and with recent form-factor calculations. An improved value of the branching ratio BR(eta -&gt; gamma e(+)e(-)) = (6.6 +/- 0.4(stat) +/- 0.4(syst)) . 10(-3) has been determined. (C) 2011 Elsevier B.V. All rights reserved

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

The Compton double-polarization observable $\Sigma_{2z}$ has been measured for the first time in the $\Delta(1232)$ 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: $\gamma_{E1E1} = -3.18 \pm 0.52$, $\gamma_{M1M1} = 2.98 \pm 0.43$, $\gamma_{E1M2} = -0.44 \pm 0.67$ and $\gamma_{M1E2} = 1.58 \pm 0.43$, in units of $10^{-4}~\mathrm{fm}^{4}$.Comment: 7 pages, 5 figures, 2 tables, accepted for publication in PR

Measurement of the decay η′ →π0π0η at MAMI

An experimental study of the η′→π0π0η→6γ decay has been conducted with the best up-to-date statistical accuracy, by measuring η′ mesons produced in the γp→η′p reaction with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The results obtained for the standard parametrization of the η′→π0π0η matrix element are consistent with the most recent results for η′→ππη decays, but have smaller uncertainties. The available statistics and experimental resolution allowed, for the first time, an observation of a structure below the π+π- mass threshold, the magnitude and sign of which, checked within the framework of the nonrelativistic effective-field theory, demonstrated good agreement with the cusp that was predicted based on the ππ scattering length combination, a0-a2, extracted from K→3π decays

First Measurement of Near-Threshold J /ψ Exclusive Photoproduction off the Proton

We report on the measurement of the γp→J/ψp cross section from Eγ=11.8 GeV down to the threshold at 8.2 GeV using a tagged photon beam with the GlueX experiment. We find that the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section dσ/dt has an exponential slope of 1.67±0.39 GeV-2 at 10.7 GeV average energy. The LHCb pentaquark candidates Pc+ can be produced in the s channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions B(Pc+→J/ψp) and cross sections σ(γp→Pc+)×B(Pc+→J/ψp)

Machine morality An enquiry concerning the ethical status of expert systems

SIGLEAvailable from British Library Document Supply Centre-DSC:D196761 / BLDSC - British Library Document Supply CentreGBUnited Kingdo