Search for the σ meson by utilizing the Primakov effect

本研究的目的是利用普里马科夫效应来测量σ→2γ的逆过程。待测的主要过程为γA→σX→π^0π^0X。因此，为了寻找σ介子，我们一直致力于γ探测器的设计、建造和性能测试。这是研究σ→π^0π^0过程的主要准备部分。在2000财政年度，我们已经研究了200个PWO晶体用作正向γ探测器。在SPring-8上成功地进行了激光-电子光子束的测试实验。然而，事实证明，征集提案的工作将在2001年夏天进行。我们希望我们的研究项目的实验将在之后进行。因此，我们不得不改变日程安排，以免浪费时间。也就是说，我们开始准备测量后角区域的σ→π^0π^0过程。这个测量是对σ介子研究中Primakov产生的补充。我们构建了一个由252个Pb/SCIFI块组成的后向γ探测器，覆盖了30°至100°的极角。这是一种采样探测器的电磁量热计，覆盖了约2π sr的大立体角。利用π^0事件对探测器进行了能量校准。然后观察到η峰。目前，π^0和η的质量分辨率分别为~ 14%和~ 10%。

The aim of this research project was to measure the inverse process of σ→2γ by employing the Primakov effect. The main process to be measured is then γA→σX→π^0π^0X.Thus, in search for the σ meson, we have been working on design, construction and the performance test of a γ detector. That is the main part of the preparation for the study of the σ→π^0π^0 process. We have investigated 200 PWO crystals to be used as a forward γ detector in the fiscal year 2000.A test experiment with a laser-electron photon beam is now successfully conducted at SPring-8. It turned out, however, that the call for proposals would be made in summer, 2001. And we expect that the experiment for our research project with the beam will be performed afterward. Therefore we could not help changing the schedule so as not to waste time. Namely, we started preparation for a measurement of the σ→π^0π^0 process at a backward angular region also. This measurement is complementary to that of Primakov production in the study of the σ meson.We have constructed a backward γ detector consisting of 252 Pb/SCIFI blocks which covers the polar angle from 30° to 100°. This is an EM calorimeter of a kind of sampling detector covering a large solid angle of about 2π sr. Energy calibration for the detector has been made by utilizing π^0 events. Then the η peak is also observed. The mass resolution for π^0 and η are obtained to be 〜14% and 〜10%, respectively, for the moment.