PREX ll and CREX: Precision parity violating measurement of neutron skin of heavy nuclei at Jefferson Laboratory

PREX ll 和 CREX：杰斐逊实验室重核中子皮的精确宇称破坏测量

• 批准号: SAPPJ-2019-00050
• 负责人: Mammei, Juliette
• 金额: $ 10.93万
• 依托单位: University of Manitoba
• 依托单位国家: 加拿大
• 项目类别: Subatomic Physics Envelope - Project
• 财政年份: 2020
• 资助国家: 加拿大
• 起止时间: 2020-01-01 至 2021-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=718950
• 关键词: PREX; ll; CREX; Precision; parity

“How do the protons and neutrons fit together to make a nucleus?” is one of the most fundamental questions that a nuclear structure theory could answer. It has truly far-reaching consequences, both for a large range of fields in physics and also literally far-reaching - by telling us about the structure of neutron stars. LIGO has given us a new window on the universe by observing gravitational waves from neutron star mergers. In order to fully understand the results from these measurements, the Pb Radius Experiment (PREX II) and the Ca Radius Experiment (CREX) will provide information about the properties of neutron matter. 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. The "textbook" size of a nucleus is the electric charge distribution - measured extremely well in electron scattering at different kinematics and then used to obtain the charge radius. 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. PREX II and 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 II 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 II 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.

“质子和中子是如何结合在一起形成原子核的？”是核结构理论可以回答的最基本问题之一。 它具有真正深远的影响，不仅对物理学的大范围领域，而且通过告诉我们中子星的结构，也具有深远的意义。 通过观测中子星星合并产生的引力波，LIGO为我们提供了一个了解宇宙的新窗口。 为了充分理解这些测量的结果，铅半径实验（PREX II）和钙半径实验（CREX）将提供有关中子物质性质的信息。 原子核中中子和质子半径的差异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的时间。这些信息也将帮助我们预测一颗星星是否会坍缩成中子星星星或黑洞。原子核的“教科书”尺寸是电荷分布--在不同运动学的电子散射中测量得非常好，然后用来获得电荷半径。通过测量不同四动量转移下原子核的形状因子，然后进行傅里叶变换，可以得到原子核内电荷密度随半径的变化。这告诉你的是质子在原子核内的分布，因为质子是原子核的电荷来源。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斐逊实验室是找到这个问题答案的理想场所。PREX II和CREX将利用弱相互作用中宇称破坏的性质测量铅（Pb）和钙（Ca）的特定同位素的中子半径，以测量核的弱电荷。宇称破坏类似于在镜子中观察到的不同散射。因为质子几乎没有弱电荷，而中子有最大的弱电荷，测量弱电荷分布可以告诉你中子分布。 杰斐逊实验室的电子加速器使电子的速度超过光速的0.99999倍。 探测器之所以能工作，是因为电子在构成有源区的石英中的速度实际上比光快，从而使它们发出切伦科夫辐射。 我们测量的不对称性比一桶55加仑的水中4滴墨水的浓度还要小！ PREX II和CREX合作项目由全球20多个机构和50多个合作者组成。我们的加拿大小组将在PREX II和CREX实验中发挥关键作用，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偏振度的探测器。