Studies of Nucleon Structure and Medium Modifications with Electroweak Probes

本研究的目的是提供关于自由质子和中子（核子）以及束缚在原子核内的质子和中子的基本结构的重要实验信息。从具有大动量转移的核子散射轻子的过程可以用微扰QCD来描述，其中散射发生在具有良好定义的纵向动量分布的渐近自由夸克上。另一方面，夸克被限制在介子和重子内部，在低动量转移时，这些复合粒子是有效自由度。这项研究的一个重点是更好地理解这些制度之间的过渡。特别是，这笔拨款将支持质子到大动量转移的磁弹性形状因子的精确测定，以研究向微扰尺度的转变，以及质子和氘核在大Bjorken-x下的F_2结构函数的测量，其中大部分核子动量由单个价夸克携带。前者实验将在Jefferson Lab实验大厅A进行，后者实验将在c大厅进行。此外，本研究将研究核子在核介质中的修饰，目前还没有普遍接受的物理模型。虽然在电子和介子散射中已经进行了许多关于核结构函数与氘核结构函数之比的研究，但实际上没有中微子散射的实验数据。这些数据将为介质修改模型提供重要的测试，并将成为即将进行的MINERvA中能束运行的重点，而对成功结束的低能量运行的持续分析主要集中在弹性散射和共振产生上。从更广泛的角度来看，本项目旨在通过新的实验数据进一步阐明普通物质核成分的结构。虽然量子色动力学（QCD）已被接受为强相互作用理论，它负责将组成夸克和胶子限制在组成普通物质的复合强子中，但所产生的结构仍然存在许多问题。这些研究将通过电磁相互作用（利用杰斐逊实验室的电子束）和弱相互作用（利用费米实验室MINERvA实验中的中微子束）的互补探针进行。杰斐逊实验室的两项实验预计将是加速器能量升级后的首批实验。这将利用该小组的重要专业知识来调试A厅和C厅，以支持这些实验，这也将广泛支持更大的杰斐逊实验室核物理项目。预计这一研究项目还将解决培养下一代物理学研究人员的关键需求，在这一领域，非裔美国人和女性的代表性都明显不足。

The objective of this research is to provide important experimental information on the fundamental structure of both free protons and neutrons (nucleons) and those bound inside the atomic nucleus. When scattering leptons from nucleons with large momentum transfer the process can be described within perturbative QCD in which the scattering takes place on an asymptotically free quark with a well defined longitudinal momentum distribution. On the other hand, the quarks are confined inside mesons and baryons, and at low momentum transfer it is these composite particles which are the effective degrees of freedom. One focus of this research is on better understanding the transition between these regimes. In particular, this grant will support work on a precision determination of the magnetic elastic form factor of the proton to large momentum transfer to study the transition to perturbative scaling, in addition to a measurement of the F_2 structure function of the proton and deuteron at large Bjorken-x, for which most of the nucleon momentum is carried by a single valence quark. The former experiment will be performed in Jefferson Lab experimental Hall A and the latter in Hall C. In addition, this research will study the modification of the nucleons in the nuclear medium, for which no generally accepted physics model currently exists. While many studies of the ratio of structure functions measured in nuclei to those on the deuteron have been performed in electron and muon scattering, practically no experimental data exists for neutrino scattering. Such data would provide important tests of models of medium modifications and will be the focus of the upcoming MINERvA medium energy beam run, while the continuing analysis of the successfully concluded low energy run is primarily focused on elastic scattering and resonance production. From a broader perspective, this project aims to further elucidate the structure of the nuclear constituents of ordinary matter via new experimental data. While, Quantum Chromodynamics (QCD) has been accepted as the theory of the strong interaction, which is responsible for confining the constituent quarks and gluons into the composite hadrons which make up ordinary matter, many questions of the resulting structure still remain.These studies will be performed with complementary probes via the electromagnetic interaction, utilizing the electron beam at Jefferson Lab, and the weak interaction, utilizing a neutrino beam in the MINERvA experiment at Fermilab.Both Jefferson Lab experiments are expected to be among the first performed after the energy upgrade of the accelerator. This will leverage the significant expertise of this group to the commissioning of both Hall A and Hall C in support of these experiments, which will also broadly support the larger Jefferson Lab nuclear physics program. It is expected that this research program will also address a critical need in the training of the next generation of researchers in physics, where African-Americans and women are both significantly under-represented.