Illuminating strongly interacting matter

照亮强相互作用物质

• 批准号: ST/G008582/1
• 负责人: Daniel Watts
• 金额: $ 61.96万
• 依托单位: University of Edinburgh
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=ST%2FG008582%2F1
• 关键词: Illuminating; strongly; interacting; matter

The work will give new information addressing fundamental gaps in our understanding of the structure of the nucleon (proton and neutron) and strongly interacting matter using the world's most intense energy tagged photon beams of appropriate wavelength (delivering ~1 billion polarised photons per second). The photon probe is an ideal way to investigate the structure of matter as its interaction is well understood and it probes the whole volume of the object under study. Nucleons are composite structures made of 3 much lighter particles called quarks, which exist in a sea of virtual gluons and quark-antiquark pairs. This system can be excited into a number of very short lived excited states which can be detected from their subsequent decay back to a nucleon via the emission of mesons. The spectrum of these excited states and their properties are fundamental observables reflecting the dynamics of the nucleon's internal constituents. Despite being studied for decades in various experiments information on the mass, lifetime and photon coupling of the known excited states have been obtained only with large uncertainties for many parts of the spectrum. For example the lifetime of the second excited state of the proton is only known to 50% accuracy. Some excited states only show up inconsistently in different theoretical analyses of the same experimental data and many 'missing' excited states predicted by quark models are not yet observed.This situation is particularly disappointing given the recent advances in theoretical descriptions of the spectra, such as first predictions from Lattice QCD. We will obtain unique new data to constrain the structure of the nucleon utilising polarised photon beams with (i) polarised neutron targets in which the spin of the target nucleons an be oriented in a chosen direction with respect to the beam (ii) measurement of the transfer of polarisation from the photon to the recoiling nucleon(s) in photoreactions. The first aspect utilises new HD target technology while the latter uses the Edinburgh nucleon polarimeter which employs a novel and highly cost-effective design to measure the polarisation of nucleons with a large angular acceptance. The polarimeter will enable the long sought 'complete measurement' of experimental observables in meson photoproduction from the nucleon. This will be a real milestone in the field, providing sufficient experimental constraints to allow a model independent analysis for the first time in the extraction of the fundamental excitation spectrum. We will also carry out world's first measurements of polarisation transfer in deuterium photodisintegration using the Edinburgh nucleon polarimeter. Polarisation transfer provides the most sensitive information on whether the appropriate degrees of freedom in the reaction process are the nucleon or the underlying quark structure of the deuteron. This will be the benchmark measurement to address intriguing indications of this transition at unexpectedly low photon energies. Finally, we will obtain important new information on how nucleons combine to make nuclei by accurately measuring the size of neutron skins. Neutron skins with thicknesses of up to ~1/3 of a neutron diameter are predicted to exist for heavy nuclei, and accurate measurement would give long sought constraints on the poorly established density dependence of the asymmetry parameter for neutron rich matter. This asymmetry parameter fixes the scale of the binding energy penalty for moving away from symmetric matter (equal numbers of protons and neutrons) and has fundamental consequences for neutron star physics including establishing the feasibility of cooling mechanisms and constraining mass-radius relationships. The size of the skin is also a crucial constraint on modern theoretical descriptions of the nucleus, which we will challenge by determining the change in skin thickness when the neutron number increases for a fixed proton number.

这项工作将提供新的信息，解决我们对核子（质子和中子）结构的理解中存在的根本差距，并使用世界上最强的能量标记的适当波长的光子束（每秒提供约10亿个极化光子）强烈相互作用的物质。光子探针是研究物质结构的理想方法，因为它的相互作用已经很好地理解，并且它可以探测研究对象的整个体积。核子是由3个轻得多的夸克粒子组成的复合结构，它们存在于虚胶子和夸克-反夸克对的海洋中。这个系统可以被激发成许多非常短暂的激发态，这些激发态可以通过介子的发射从它们随后的衰变回到核子中被检测到。这些激发态的光谱及其性质是反映核子内部成分动力学的基本观测量。尽管已经在各种实验中研究了几十年，但关于已知激发态的质量、寿命和光子耦合的信息仅在光谱的许多部分具有很大的不确定性。例如，质子第二激发态的寿命只有50%的准确度。有些激发态在相同实验数据的不同理论分析中只出现不一致的情况，而且夸克模型预测的许多“缺失”激发态尚未观察到。考虑到光谱理论描述的最新进展，这种情况尤其令人失望，例如晶格QCD的首次预测。我们将获得独特的新数据，以限制核子的结构，利用极化光子束与（i）极化中子靶，其中靶核子的自旋可以定向在一个选定的方向相对于光束（ii）测量的极化转移从光子反冲核子（S）在光反应。第一个方面利用新的高清目标技术，而后者使用爱丁堡核子偏振计，采用了一种新颖的和高成本效益的设计，以测量核子的偏振与大的角度接受。偏振计将使长期寻求的'完整的测量'的实验观测介子从核子的光生。这将是该领域的一个真实的里程碑，提供了足够的实验约束，允许在提取基本激发光谱中首次进行模型独立分析。我们还将进行世界上第一次测量的极化转移在氘光衰变使用爱丁堡核子偏振计。极化转移提供了反应过程中适当的自由度是核子还是氘核的夸克结构的最敏感的信息。这将是解决在出乎意料的低光子能量下这种转变的有趣迹象的基准测量。最后，通过精确测量中子皮的大小，我们将获得关于核子联合收割机如何组合成原子核的重要新信息。预测重核存在厚度高达中子直径的约1/3的中子皮，并且精确测量将对富中子物质的不对称性参数的差建立的密度依赖性给出长期寻求的约束。这个不对称参数确定了从对称物质（相等数量的质子和中子）移动的结合能惩罚的尺度，并对中子星星物理学产生了根本性的影响，包括建立冷却机制的可行性和约束质量-半径关系。皮肤的大小也是一个重要的约束现代理论描述的核，我们将通过确定皮肤厚度的变化时，中子数增加一个固定的质子数的挑战。

项目成果

Measurement of p 0 photoproduction on the proton at MAMI C

MAMI C 质子 p 0 光产生的测量

• DOI: 10.1103/physrevc.92.024617
• 发表时间: 2015
• 期刊: Physical Review C
• 影响因子: 3.1
• 作者: Adlarson P

Measurement of the p 0 ? e + e - ? Dalitz decay at the Mainz Microtron

p 0 的测量？

• DOI: 10.1103/physrevc.95.025202
• 发表时间: 2017
• 期刊: Physical Review C
• 影响因子: 3.1
• 作者: Adlarson P

Measurement of single- and double-spin asymmetries in deep inelastic pion electroproduction with a longitudinally polarized target.

使用纵向极化靶测量深部非弹性π介子电生产中的单自旋和双自旋不对称性。

• DOI: 10.1103/physrevlett.105.262002
• 发表时间: 2010
• 期刊: Physical review letters
• 影响因子: 8.6
• 作者: Avakian H

T and F asymmetries in p 0 photoproduction on the proton

质子 p 0 光生产中的 T 和 F 不对称性

• DOI: 10.1103/physrevc.93.055209
• 发表时间: 2016
• 期刊: Physical Review C
• 影响因子: 3.1
• 作者: Annand J

First measurement of target and beam-target asymmetries in the ? p ? p 0 ? p reaction

首次测量目标和光束目标不对称性？

• DOI: 10.1103/physrevc.91.055208
• 发表时间: 2015
• 期刊: Physical Review C
• 影响因子: 3.1
• 作者: Annand J