PREX II and CREX: Precision Parity Violating Measurement of the Neutron Skin of Heavy Nuclei at Jefferson Laboratory

核结构模型可以预测的最基本的性质之一是原子核应该有多大，也就是说，“原子核内质子和中子的分布是怎样的？”这个问题的完整答案确实具有深远的影响，不仅对物理学的一系列领域，而且对我们来说，通过告诉我们中子星，意义深远。原子核中中子和质子的半径之差对中子星的结构有直接影响，影响它们的大小、外壳的厚度以及冷却所需的时间。这些信息还将帮助我们预测一颗恒星是会坍缩成中子星还是黑洞。我们传统上所说的原子核的大小，在大多数情况下，实际上是电荷的分布。这已经用电子散射很好地测量了：电子-核散射的横截面是在不同的运动学下测量的，然后用来获得电荷半径。也就是说，通过测量原子核在不同的四动量传递下的形状因子，然后进行傅里叶变换，就可以得到原子核内的电荷密度作为半径的函数。它告诉你的是原子核内质子的分布，因为质子给原子核提供电荷。但问题仍然是中子的分布是否与质子的分布相同。北美首屈一指的电子散射设施，杰弗逊实验室，是找到这个问题答案的理想场所。Pb半径实验（PREX）和Ca半径实验（CREX）将利用弱相互作用中的宇称破坏特性测量铅（Pb）和钙（Ca）的特定同位素的中子半径，以测量原子核的弱电荷。宇称破坏类似于在镜子中观察到的散射不同。因为质子几乎没有弱电荷，而中子有最大的弱电荷，测量弱电荷的分布告诉你中子的分布。杰斐逊实验室的电子加速器使电子的运动速度超过光速的0.99999倍。探测器之所以能工作，是因为电子在构成活跃区域的石英中实际上比光传播得快，导致它们发出切伦科夫辐射。我们测量的不对称比一桶55加仑水中4滴墨水的浓度还小！PREX和CREX合作由全球20多个机构和50多个合作者组成。我们的加拿大团队将在PREX和CREX实验中发挥关键作用，通过设计、建造、测试和部署关键设备，包括模拟背景和设计用于测量电子束极化程度的探测器。

One of the most fundamental properties that a nuclear structure model could predict is how big a nucleus should be, that is, "What is the distribution of protons and neutrons within the nucleus?". The full answer to this question has truly far-reaching consequences, both for a range of fields in physics and also, literally far-reaching - by telling us about neutron stars. The difference between the radius of neutrons and protons in a nucleus has direct consequences for the structure of neutron stars, affecting how big they can be, how thick their crusts are and how long they will take to cool. This information will also help us predict whether a star will collapse into a neutron star or a black hole. What we traditionally call the size of a nucleus is really, in most cases, the electric charge distribution. This has been measured extremely well using electron scattering: the cross section in electron-nucleus scattering is measured at different kinematics and then used to obtain the charge radius. That is, by measuring the form factor of the nucleus at different four-momentum transfer, and then taking the Fourier transform, you can obtain the density of charge within the nucleus as a function of radius. What this tells you is the distribution of protons within the nucleus, because the proton is what gives the nucleus its electric charge. But the question remains whether the distribution of neutrons is the same as that of the protons. North America's premier electron scattering facility, Jefferson Laboratory, is the ideal place to find the answer to this question. The Pb Radius Experiment (PREX) and the Ca Radius Experiment (CREX) will measure the neutron radius of specific isotopes of both lead (Pb) and calcium (Ca) using the property of parity-violation in the weak interaction to measure the weak charge of the nucleus. Parity violation is analogous to the scattering being different when it is observed in a mirror. Because the proton has almost no weak charge, while the neutron has maximum weak charge, measuring the weak charge distribution tells you the neutron distribution. Jefferson Lab's electron accelerator makes the electrons travel greater than 0.99999 times the speed of light. The detectors work because the electrons are actually traveling faster than light in the quartz that makes up the active area, causing them to emit Cherenkov radiation. The asymmetry we measure is smaller than the concentration of 4 drops of ink in a 55 gallon barrel of water! The PREX and CREX collaborations consist of over 20 institutions world-wide with over 50 collaborators. Our Canadian group will play a key role in the PREX and CREX experiments through design, construction, testing and deployment of key pieces of equipment including simulation of backgrounds and design of detectors used to measure the degree of polarization of the electron beam.