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PF1b上使用光谱仪aSPECT对两个中子衰变观测值进行精确测量。通过质子反冲谱测量反中微子-电子角相关系数“a”的精度为0.3%，质子发射相对于中子自旋方向的不对称性“C”的精度为0.1%。在粒子物理的标准模型中，它们都是弱轴矢量和矢量耦合常数λ = gA/gV之比的函数。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相关的截面。它还进入了CKM矩阵中最大元素的确定，该矩阵参数化了夸克弱带电电流相互作用中的混合。另一方面，这两个系数对标准模型之外的物理具有不同的敏感性，例如标量或张量相互作用。我们的实验将为应用和标准模型之外的物理测试的弱相互作用参数的全局分析提供有价值的输入。光谱仪aSPECT是美因茨和慕尼黑的大学与伊利诺伊大学合作开发和委托的。在上一个资助期间，aSPECT已经配备了一个大型表面硅漂移探测器，为加速到几keV的质子提供了极好的信号与噪声分离。对光谱仪及其性能的认识有了显著的提高，因此aSPECT现已准备好进行开发。提出的实验不仅将提高对物理观测的认识，而且还将为FRM II正在建设的PERC仪器的操作及其二次光谱仪的设计提供重要的经验。

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.