Transport of Polarized Ultra Cold Neutrons for UCN-A Fundamental Symmetry Study

核衰变揭示了自然界确实违反了被认为是非常基本的对称性：宇称（通过镜子看世界）。这种测量的精度大大提高，使我们能够再次探索“标准模型”之外的东西。通过测量极化中子与衰变为质子时发射的电子之间的方向相关性，再加上中子和介子的寿命，我们可以测试我们对夸克“混合”的理解是否完整。（一个中子有两个“下”夸克和一个“上”夸克，而质子有两个“上”夸克和一个“下”夸克——在中子的十日中，一个“下”夸克变成了一个“上”夸克。）如果当前的图像是正确的，那么夸克混合矩阵应该是酉的（就像矢量的长度在旋转其坐标系时不会改变一样）。任何对统一性的偏离都将表明新的物理学在起作用，并且具有根本的重要性。为了测量这种相关性，我们在洛斯阿拉莫斯国家实验室创造了“超冷中子”（UCN），它经历全反射，就像光纤中的光一样，这意味着这些中子可以在特殊的波导中长距离传输，也很容易极化。这大大减少了系统误差，否则存在由于光束诱导的背景和不完全极化。我们已经证明，最好的导板是金刚石涂层的，这是我们在弗吉尼亚理工大学使用“脉冲激光沉积”的“类金刚石碳”制造的。在中子极化的上游，我们也可以用镍-58涂层，我们也可以。该试验正处于最后的调试阶段，并且已经进行了几次试运行。我们期望在今年完成对中子β -不对称性的第一次1%的测量，并利用UCN实验的所有优势，在完成之前达到0.2%的目标。这笔拨款也支持我们为运行实验的数据采集系统所做的努力。

Nuclear beta decay revealed that Nature indeed violates what had been thought to be a very fundamental symmetry: parity (looking at the world through a mirror). Greatly improved precision of this measurement lets us again probe beyond the 'Standard Model'.By measuring the directional correlation between a polarized neutron and the electron emitted when it decays into a proton - coupled with the neutron and muon lifetimes - we can test whether our understanding of quark 'mixing' is complete. (A neutron has two 'down' and one 'up' quarks, while the proton has two 'up' and one 'down' quarks - in neutron betadecay a 'down' quark becomes an 'up' quark.)If the current picture is correct, the quark mixing matrix should be unitary (much like the length of a vector doesn't change when you rotate its coordinate system). Any deviationfrom unitarity would show new physics at work and be of fundamental importance.To measure this correlation, we create 'Ultra-Cold Neutrons' (UCN) at the Los Alamos National Laboratory which undergo total reflection - much like light in fiber optics - meaning these neutrons can be transported long distances in special guides and are also easily polarized. This dramatically reduces systematic errors otherwise present due to beam-induced backgrounds and incomplete polarization.We have shown that the best guides are diamond coated, which we fabricate at Virginia Tech (with this grant) using 'pulsed laser deposition' of 'diamond-like carbon'. Upstream of where the neutrons get polarized we can also use Nickel-58 coatings, which we are also able to to.The experiment in in its final commissioning stages, and several test runs have already been made. We anticipate getting our first one percent measurement of the neutron's beta asymmetry this year, and bringing to bear all the advantages of a UCN experiment to reachthe goal of 0.2% before we are finished.This grant also supports our efforts for the data acquisition system to run the experiment.