PWO晶体测量和PANDA的超子极化模拟研究
PWO Crystal Measurements and Simulation Studies of Lambdabar Hyperon Polarisation for PANDA
摘要：
在瑞典Lund的MAX实验室，使用能量在19和56 MeV之间的标记光子束，用3个$\times$ 3 PbWO 4晶体的阵列进行了两次能量分辨率的测量。将晶体冷却至-15和-25摄氏度。测得的相对能量分辨率，sigma/E，从20 MeV时的约12%下降到55 MeV时的7%。因此，α的值被固定为从光收集产额的泊松统计（50 phe/MeV）中预期的值，并且对于4月和9月的测量，来自拟合的结果分别为sigma/E= 0.45%/sqrt（E）+0.18%/E +8.63%和sigma/E= 0.45%/sqrt（E）+0.21%/E +6.12%。五个PbWO 4晶体，三个锥形和两个非锥形的光输出均匀性的研究，也已进行。锥形晶体提供的光输出随着与光电倍增管（PM管）的距离增加而增加。对于pbar p -> Lambdabar Lambda模拟，使用来自LEAR PS185实验的修改后的发生器生成事件。在100%偏振垂直于散射平面的情况下，重建（99 ± 1.8）%的偏振。沿着由出射超子确定的轴以及散射平面中的轴的轻微非零偏振也被重建。分别为（4.1 ± 2.1）%和（2.6 ± 2.0）%。从这个调查中，它表明，探测器的效率是不均匀的，慢π介子是难以重建。
英文摘要：
Two measurements of the energy resolution were performed at MAX-Lab in Lund, Sweden, with an array of 3$\times$3 PbWO4 crystals using a tagged photon beam with energies between 19 and 56 MeV. The crystals were cooled down to -15 and -25 degrees C. The measured relative energy resolution, sigma/E, is decreasing from approximately 12% at 20 MeV to 7% at 55 MeV. The value of a was therefore fixed to that one would expect from Poisson statistics of the light collection yield (50 phe/MeV) and the results from fits were sigma/E= 0.45%/sqrt(E) + 0.18%/E + 8.63% and sigma/E= 0.45%/sqrt(E) + 0.21%/E + 6.12% for the April and September measurements, respectively. Light yield uniformity studies of five PbWO4 crystals, three tapered and two non-tapered ones, have also been performed. The tapered crystals delivered a light output which increased with increasing distance from the Photo Multiplier Tube (PM tube). For the pbar p -> Lambdabar Lambda simulations, events were generated using a modified generator from the PS185 experiment at LEAR. With a 100% polarisation perpendicular to the scattering plane, a polarisation of (99+-1.8)% was reconstructed. Slight non-zero polarisations along the axis determined by the outgoing hyperon as well as the axis in the scattering plane, were also reconstructed. These were (4.1+-2.1)% and (2.6+-2.0)% respectively. From this investigation it was shown that the detector efficiency was not homogeneous and that slow pions are difficult to reconstruct.
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