arXiv 1507.018332015-07-07nucl-ex

阈区核子光生π-η对的同位旋结构

The isospin structure of photoproduction of pi-eta pairs from the nucleon in the threshold region

A. Käser, J. Ahrens, J. R. M. Annand, H. J. Arends, K. Bantawa, P. A. Bartolome, R. Beck, V. Bekrenev, H. Berghäuser, A. Braghieri, D. Branford, W. J. Briscoe, … 展开作者

A. Käser, J. Ahrens, J. R. M. Annand, H. J. Arends, K. Bantawa, P. A. Bartolome, R. Beck, V. Bekrenev, H. Berghäuser, A. Braghieri, D. Branford, W. J. Briscoe, J. Brudvik, S. Cherepnya, S. Costanza, B. Demissie, M. Dieterle, E. J. Downie, P. Drexler, L. V. Fil'kov, A. Fix, D. I. Glazier, D. Hamilton, E. Heid, D. Hornidge, D. Howdle, G. M. Huber, O. Jahn, I. Jaegle, T. C. Jude, V. L. Kashevarov, I. Keshelashvili, R. Kondratiev, M. Korolija, S. P. Kruglov, B. Krusche, V. Lisin, K. Livingston, I. J. D. MacGregor, Y. Maghrbi, J. Mancell, D. M. Manley, Z. Marinides, J. C. McGeorge, E. McNicoll, D. Mekterovic, V. Metag, S. Micanovic, D. G. Middleton, A. Mushkarenkov, A. Nikolaev, R. Novotny, M. Oberle, M. Ostrick, P. Otte, B. Oussena, P. Pedroni, F. Pheron, A. Polonski, S. N. Prakhov, J. Robinson, G. Rosner, T. Rostomyan, S. Schumann, M. H. Sikora, D. I. Sober, A. Starostin, I. Supek, M. Thiel, A. Thomas, M. Unverzagt, D. P. Watts, D. Werthmüller, L. Witthauer

备注：: accepted for publication in Phys. Lett. B
发表信息：: Phys. Lett. B 748 (2015) 244

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摘要：

从阈能到入射光子能量为1.4~GeV，研究了核子光生$πη$ ~-对.准自由反应$γp\rightarrow pπ^0η$、$γn\rightarrow nπ^0η$、$γp\rightarrow nπ^+η$和$γn\rightarrow pπ^-η$首次从结合在氘核中的核子中测量出来。还研究了自由质子靶的相应反应，以研究终态相互作用效应（对于中性π介子，自由质子的结果可以与以前的测量结果进行比较; $γp\rightarrow nπ^+η$反应是首次测量）。对于$π^0η$，还研究了通过$γd\rightarrow dπ^0η$反应的终态相干产生。实验在美因茨MAMI加速器的标记光子束上进行，使用由Crystal Ball和TAPS探测器组成的几乎$4π$覆盖的电磁量能器。四种不同终态的总截面服从关系式$σ(pπ^0η)$ $\approx$ $σ(nπ^0η)$ $\approx$ $2σ(pπ^-η)$ $\approx$ $2σ(nπ^+η)$，正如预期的那样，来自$Δ^{\star}\rightarrowηΔ(1232)\rightarrowπηN$反应链的主要贡献，这也得到了核子-介子和$π$ - $η$对的不变质量分布形状的支持。实验结果进行了比较，从同量异位素反应模型的预测。

英文摘要：

Photoproduction of $πη$-pairs from nucleons has been investigated from threshold up to incident photon energies of $\approx$~1.4~GeV. The quasi-free reactions $γp\rightarrow pπ^0η$, $γn\rightarrow nπ^0η$, $γp\rightarrow nπ^+η$, and $γn\rightarrow pπ^-η$ were for the first time measured from nucleons bound in the deuteron. The corresponding reactions from a free-proton target were also studied to investigate final-state interaction effects (for neutral pions the free-proton results could be compared to previous measurements; the $γp\rightarrow nπ^+η$ reaction was measured for the first time). For the $π^0η$ final state coherent production via the $γd\rightarrow dπ^0η$ reaction was also investigated. The experiments were performed at the tagged photon beam of the Mainz MAMI accelerator using an almost $4π$ coverage electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The total cross sections for the four different final states obey the relation $σ(pπ^0η)$ $\approx$ $σ(nπ^0η)$ $\approx$ $2σ(pπ^-η)$ $\approx$ $2σ(nπ^+η)$ as expected for a dominant contribution from a $Δ^{\star}\rightarrowηΔ(1232)\rightarrowπηN$ reaction chain, which is also supported by the shapes of the invariant-mass distributions of nucleon-meson and $π$-$η$ pairs. The experimental results are compared to the predictions from an isobar reaction model.

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