Precision measurement of proton recoil spectrum and proton asymmetry in neutron decay using the spectrometer aSPECT (ILL contribution)

我们建议在格勒诺布尔ILL的冷中子束设施PF 1b使用光谱仪aSPECT对两个中子衰变可观测量进行精确测量。通过质子反冲谱测量反中微子电子角相关系数“a”，准确度为0.3%;通过质子反冲谱测量质子发射相对于中子自旋方向的不对称性“C”，准确度为0.1%。在粒子物理学的标准模型中，它们都是弱轴矢量与矢量耦合常数之比λ = gA/gV的函数。这个值是计算许多半轻子弱截面的重要输入，例如与大爆炸核合成、恒星聚变和中微子探测有关的那些。它也进入确定的CKM矩阵的参数化混合内的夸克的弱带电电流相互作用的最大元素。另一方面，两个系数对标准模型之外的物理学具有不同的敏感性，例如，标量或张量相互作用。我们的实验将提供有价值的输入，全球分析弱相互作用参数的应用程序和测试的物理超出标准模型。光谱仪aSPECT是美因茨和慕尼黑的大学与ILL合作开发和委托的。在上一个资助期间，aSPECT已经配备了一个大的表面硅漂移探测器，提供了一个很好的分离质子加速到只有几个千电子伏的信号从噪声。对光谱仪及其性能的理解得到了显着提高，因此aSPECT现在已经准备好开发。拟议的实验不仅将提高物理观测量的知识超过一个数量级，但也提供了重要的经验，在FRM II建设中的仪器PERC的操作，其二级光谱仪的设计。

We propose to perform accurate measurements of two neutron decay observables using the spectrometer aSPECT at the cold neutron beam facility PF1b at the ILL in Grenoble. The antineutrino-electron angular correlation coefficient “a” shall be measured via the proton recoil spectrum with an accuracy of 0.3%, and the asymmetry “C” of proton emission with respect to the neutron spin direction with accuracy 0.1%. Within the standard model of particle physics, they are both functions of the ratio of the weak axial-vector and vector coupling constants λ = gA/gV. This value is an important input to calculate many semi-leptonic weak cross sections such as those relevant for big-bang nucleosynthesis, stellar fusion and detection of neutrinos. It also enters the determination of the largest element of the CKM matrix which parameterizes mixing within the weak charged current interactions of quarks. On the other hand, both coefficients have different sensitivity to physics beyond the standard model as, e.g., scalar or tensor interactions. Our experiments will provide valuable input to global analyses of weak interaction parameters for applications and for tests of physics beyond the standard model. The spectrometer aSPECT has been developed and commissioned in collaboration of Universities from Mainz and Munich with the ILL. During the last funding period aSPECT has been equipped with a large surface silicon drift detector offering an excellent separation of the signal from noise for protons accelerated to only a few keV. The understanding of the spectrometer and its performance were significantly improved, so that aSPECT is now ready for exploitation. The proposed experiments will not only improve the knowledge of the physics observables by more than an order of magnitude but also deliver important experience for the operation of the instrument PERC under construction at the FRM II, and the design of its secondary spectrometers.