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

核β衰变揭示了自然界确实违反了一个被认为是非常基本的对称性：平等性(通过镜子看世界)。这种测量的精确度大大提高，让我们再次超越了标准模型。通过测量极化的中子和衰变成质子时发出的电子之间的方向关联--加上中子和缪子的寿命--我们可以测试我们是否对夸克“混合”的理解是完整的。(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--在中子中，一个‘下’夸克可以变成一个‘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么正的(就像当你旋转它的坐标系时，向量的长度不会改变一样)。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)，这种中子可以进行全反射--就像光纤中的光一样--这意味着这些中子可以在特殊的导轨中长距离传输，而且很容易被极化。这大大减少了由于光束诱导背景和不完全偏振而存在的系统误差。我们已经证明，最好的导向器是钻石涂层，我们在弗吉尼亚理工大学(用这笔赠款)使用‘脉冲激光沉积’的‘类钻石碳’。在中子极化的上游，我们也可以使用镍-58涂层，这也是我们能够做到的。实验已经进入最后的调试阶段，已经进行了几次测试。我们预计今年将首次对中子的β不对称性进行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。这笔赠款也支持我们为数据采集系统运行实验所做的努力。

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.