Precise measurement of x-rays from kaonic helium atom

为了研究低能区K介子与核的强相互作用，我们对K介子~ 4 He原子3d→ 2 p跃迁发射的X射线能量进行了精确测量.该实验是KEK-PS（E570）的最后一次实验，我们在2005财政年度收集了数据。我们对2006财政年度的数据进行了深入的分析。在我们的实验之前，对3d→ 2 p跃迁的X射线能量进行了三次独立的测量，测量结果的平均值约为40± 8 eV的排斥位移，而大多数理论计算预测的能量位移要小得多。在我们的实验（E570）中，我们改进了以下几点：（1）采用了新研制的硅漂移探测器（SDDs）。SDD比传统的Si（Li）探测器具有更好的能量分辨率。它也有很好的能力，以减少软康普顿X射线背景。(2)我们可以通过参考放置在束上的Ti/Ni箔的特征X射线的能量来校准绝对能量标度。首次实现了现场能量标定。(3)利用漂移室可以同时跟踪入射的K介子和出射的带电粒子，这使得我们能够清晰地识别氦靶上K反应停止的事件，在X射线谱中，我们大大提高了统计、分辨率和信噪比，能量测量精度达到2 eV左右。我们发现K介子氦2 p态的能级位移几乎与零一致，其宽度也与0一致（小于16 eV（95%CL））。这些研究结果已发表在一份参考期刊上。

In order to study the strong interaction between K meson and nucleus in the low energy region, we carried out a precise measurement of the energy of x-rays emitted from the 3d→2p transition of kaonic 4He atom. The experiment was carried out as the last experiment of KEK-PS (E570) and we took data in fiscal year 2005. We intensively analyzed the data in fiscal year 2006.There were three independent measurements for this 3d→2p transition x-ray energy before our experiment and the average of those results was about 40±8eV repulsive shift, while most of the theoretical calculations predict much smaller energy shift. The disagreement of this 2p level shift between them has been known for a long term as "the puzzle of kaonic helium atom" and the new precision measurement has been awaited.In our experiment(E570), we improved following points :(1) We utilized newly developed Silicon Drift Detectors (SDDs). The SDD has better energy resolution than the conventional Si(Li) detector. It also has good capability to reduce soft-compton x-rays background.(2) We can calibrate absolute energy scale by referring the energy of characteristic x-ray from Ti/Ni foils placed on the beam. In-situ energy calibration has been achieved for the first time.(3) We can track both of incoming kaons and outgoing charged particles by drift chambers, which enabled us to cleanly identify the events from stopped K-reaction on helium target.In the x-ray spectrum, we drastically improved statistics, resolution and signal-to-noise ratio and we achieved about 2eV accuracy for the energy measurement. We found that the level shift of kaonic helium 2p state is almost consistent with zero and its width is also consistent with 0 (less than 16eV(95% CL). Those results has been published in a referred journal.