Study of deeply-bound pionic atoms

本研究的目的是从理论和实验上研究深束缚的π原子态，特别是1 s和2 p态，并获得核中有效π质量的信息。我们还发展了一种新的方法来研究介质中的η介子和Ω介子的质量。两年来取得的主要成果如下：1.为了寻找π原子的束缚态，我们利用^<206>Pb（d，^3He）反应进行了实验。束缚在重核上的π介子的波函数与原子核有很大的重叠，因此适合于研究π介子在原子核中的行为。然而，没有这样的状态已被实验观察到。理论研究表明，如果选择Pb 2 O 6作为目标，则可以将2 p以及is状态识别为激发能谱中的清晰峰。因此我们进行了实验，并成功地观察到了2 p和is峰。当详细的分析完成后，我们期望能高精度地确定π介子的有效质量。π介子原子实验的成功促使我们研究利用无反冲（d，^3He）反应研究其他介子（如η介子和ω介子）行为的可行性。我们首先估算了介子-核光学势，并通过求解Klein-Gordon方程确定了介子的结合能和宽度。在这两种情况下，潜在的强烈吸引力，并存在束缚态的预测。然后，我们使用绿色函数方法和Eikonal近似估计的反应截面。我们发现，可以观察到的束缚态峰，即使考虑到各种背景过程。基于本研究的实验方案已提交GSI，并被批准为S214。

The purpose of the research is to study theoretically as well as experimentally the deeply-bound pionic atom states, in particular is and 2p states, and to obtain information on the effective pion mass in nuclei. We also developed a new method to study the in-medium masses of eta and omega mesons. The major achievements during the two years are as follows :1. In order to search for the is bound state of pionic atom, we carried out an experiment using the ^<206>Pb(d, ^3He) reaction. The wavefunction of the pionic is state bound to a heavy nucleus has a large overlap with the nucleus, hence suitable to study the behavior of pions in nuclei. However, no such states have been experimentally observed. Theoretical studies have suggested that it is possible to identify 2p as well as is states as clear peaks in the excitation-energy spectrum, if Pb2O6 is chosen as the target. We therefore carried out the expriment, and successfully observed the 2p and is peaks. When the detailed analysis, being carried out, is completed, we expect to determine the pion effective mass with a high precision.2. The success of the pionic atom experiment motivated us to study the feasibility of studying the behavior of other mesons, such as eta and omega, by using the recoilless (d, ^3He) reaction. We first estimated the meson-nucleus optical potential, and determined the binding energies and widths of mesons by solving the Klein-Gordon equation. In both cases, the potentials were strongly attractive, and the existence of bound states are predicted. We then used the Green function method and the Eikonal approximation to estimate the reaction cross sections. We found that the bound-state peaks can be observed, even after taking account of various background processes. An experimental proposal based on this study was submitted to GSI, and was approved as S214.