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Register a new userThreshold $pi^-$ Photoproduction on the Neutron
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W.J. Briscoe
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Recent data from the PIONS$@$MAX-lab Collaboration, measuring the total cross section of the pion incoherent photoproduction $gamma dtopi^-pp$ near threshold, have been used to extract the E$_{0+}$ multipole and total cross section of the reaction $gamma n!to!pi^-p$, also near threshold. These are the first measurements of the reaction $gamma dtopi^-pp$ in the threshold region. The value of E$_{0+}$ is extracted through a fit to the deuteron data in a photoproduction model accounting for final-state interactions. The model takes an $S$-wave approximation for the elementary reaction $gamma n!to!pi^-p$ with E$_{0+}! = $ const in the threshold region. The obtained value E$_{0+} = -31.86pm 0.8$ (in $10^{-3}/m_{pi^+}$ units) is in agreement with other existing results. Model predictions for the total cross section $sigma(gamma n!to!pi^-p)$ are also given.
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We discuss the possibility of extracting the neutron-neutron scattering length $a_{nn}$ and effective range $r_{nn}$ from cross section data ($d^2sigma/dM_{nn}/dOmega_pi$), as a function of the $nn$ invariant mass $M_{nn}$, for $pi^+$ photoproduction on the deuteron ($gamma dto pi^+nn$). The analysis is based on a $gamma dto pi^+nn$ reaction model in which realistic elementary amplitudes for $gamma pto pi^+n$, $NNto NN$, and $pi Nto pi N$ are built in. We show that $M_{nn}$ dependence (lineshape) of a ratio $R_{rm th}$, $d^2sigma/dM_{nn}/dOmega_pi$ normalized by $dsigma/dOmega_pi$ for $gamma ptopi^+ n$ and the nucleon momentum distribution inside the deuteron, at the kinematics with $theta_pi=0^circ$ and $E_gammasim 250$ MeV is particularly useful for extracting $a_{nn}$ and $r_{nn}$ from the corresponding data $R_{rm exp}$. It is found that $R_{rm exp}$ with 2% error, resolved into the $M_{nn}$ bin width of 0.04 MeV (corresponding to the $p_pi$ bin width of 0.05 MeV$/c$), can determine $a_{nn}$ and $r_{nn}$ with uncertainties of $pm 0.21$ fm and $pm 0.06$ fm, respectively, for the case of $a_{nn}=-18.9$ fm and $r_{nn}=2.75$ fm. The requirement of such narrow bin widths indicates that the momenta of the incident photon and the emitted $pi^+$ have to be measured with high resolutions. This can be achieved by utilizing virtual photons of very small $Q^2$ from electron scattering at Mainz MAMI facility. The proposed method for determining $a_{nn}$ and $r_{nn}$ from $gamma dto pi^+ nn$ has a great experimental advantage over the previous one utilizing $pi^- dtogamma nn$ for being free from the formidable task of controlling the neutron detection efficiency and its uncertainty.
The first experimental investigation of the near-threshold cross section for incoherent $pi^-$ photoproduction on the deuteron $gamma$ d -> $pi^-$ pp is presented. The experimental technique involved detection of the ~131 MeV gamma ray resulting from the radiative capture of photoproduced $pi^-$ in the target. The total cross section has been measured using an unpolarized tagged-photon beam, a liquid-deuterium target, and three very large NaI(Tl) spectrometers. The data are compared to theoretical models that give insight into the elementary reaction $gamma$ n -> $pi^-$ p and pion-nucleon and nucleon-nucleon final-state interactions.
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 $gamma ptoomega p$ 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.
The reaction $gamma p to pi^0 pi^0 p$ has been measured using the TAPS BaF$_2$ calorimeter at the tagged photon facility of the Mainz Microtron accelerator. Chiral perturbation theory (ChPT) predicts that close to threshold this channel is significantly enhanced compared to double pion final states with charged pions. In contrast to other reaction channels, the lower order tree terms are strongly suppressed in 2$pi^0$ photoproduction. The consequence is the dominance of pion loops in the 2$pi^0$ channel close to threshold - a result that opens new prospects for the test of ChPT and in particular its inherent loop terms. The present measurement is the first which is sensitive enough for a conclusive comparison with the ChPT calculation and is in agreement with its prediction. The data also show good agreement with a calculation in the unitary chiral approach.
The double-polarization observable $E$ was studied for the reaction $gamma pto pomega$ using the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B at the Thomas Jefferson National Accelerator Facility and the longitudinally-polarized frozen-spin target (FROST). The observable was measured from the charged decay mode of the meson, $omegatopi^+pi^-pi^0$, using a circularly-polarized tagged-photon beam with energies ranging from the $omega$ threshold at 1.1 to 2.3 GeV. A partial-wave analysis within the Bonn-Gatchina framework found dominant contributions from the $3/2^+$ partial wave near threshold, which is identified with the sub-threshold $N(1720),3/2^+$ nucleon resonance. To describe the entire data set, which consisted of $omega$ differential cross sections and a large variety of polarization observables, further contributions from other nucleon resonances were found to be necessary. With respect to non-resonant mechanisms, $pi$ exchange in the $t$-channel was found to remain small across the analyzed energy range, while pomeron $t$-channel exchange gradually grew from the reaction threshold to dominate all other contributions above $W approx 2$ GeV.
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