The Transition to a Quark-Gluon Description of the Nucleon

在QCD作为夸克和胶子理论建立三十年后，理解QCD的工作原理仍然是核物理学中的一大难题。一个主要的挑战来自这样一个事实，即在自然界中观察到的自由度是由夸克(和胶子)组成的束缚物体：质子和中子等强子。QCD在大距离上的显著而独特的特征-夸克禁闭-防止组成非强子束缚态的单个夸克和胶子成分从系统中被移除并被孤立地检查。作为推论，夸克在强子内部以相对论速度运动，这种运动占人类质量的99.9%。在另一个极端，QCD的性质，即夸克在(非常)短距离内弱相互作用，允许人们很容易地用夸克和胶子来描述强子和原子核在极高能量下的某些性质。尽管强子和夸克胶子制度之间存在明显的差异，但在自然界中，在适当的能量区间内平均的低能强子截面的行为非常类似于以夸克胶子自由度计算的渐近高能强子截面的行为。这种现象被称为夸克-强子对偶，反映了QCD中禁闭和渐近自由之间的关系。令人惊讶的是，这种现象已经在相当低的能量下发生，并在相对较小的能量间隔(或几个强子状态)上平均出现。这个方案研究了质子、中子和原子核的电子散射反应，以努力理解这一现象的起源。该方案包括几个实验，将电子从质子目标散射，以推动和理解产生夸克-强二象性的极限。它还包括开发一种新的探测器，以尽可能接近地模拟电子与非中子靶的散射实验。根据许多理论，质子和中子的夸克-强子对偶的不同起始点将指向对偶现象的起源。最后，在原子核中也出现了有趣的问题：束缚在原子核中的质子会因为其潜在的夸克-胶子结构而改变结构吗？在QCD中，不可能得到像原子核那样的物质，其中的成分(在正常生命中，质子和中子)不会随着平均密度的增加而变化。同样，当强子穿过原子核时，人们能看到它们背后的夸克-胶子结构吗？提出建议的物理学家带头研究夸克-强子二元性，更广泛地说，在质子和原子核的强子和夸克-胶子描述之间的转换。批准的项目将使这项研究取得成果，同时将为代表不足的少数群体提供先进的、具有国际竞争力的科学培训。其中两名提名者和两名博士生是女性。此外，其中一名学生是非裔美国人，两名是非洲人。

Three decades after the establishment of QCD as the theory of quarks andgluons, understanding QCD works remains one of the great puzzles innuclear physics. A major challenge arises from the fact that the degreesof freedom observed in Nature are bound objects composed of quarks (andgluons): hadrons such as protons and neutrons. The remarkable andunique feature of QCD at large distances --- quark confinement ---prevents the individual quark and gluon constituents making up ahadronic bound state to be removed from the system and examined inisolation. As a corollary, quarks move with relativistic speeds insidethe hadrons and this motion makes up 99.9\% of the human mass.At the other extreme, the property of QCD known as asymptotic freedom,in which quarks interact weakly at (very) short distances, allows one toeasily describe certain properties of hadrons and atomic nuclei atextremely high energies in terms of quarks and gluons.Despite the apparent differences between the hadronic and quark-gluonregimes, in Nature there exist instances where the behavior oflow-energy hadronic cross sections, averaged over appropriate energyintervals, closely resembles that at asymptotically high energies,calculated in terms of quark-gluon degrees of freedom. This phenomenonis referred to as quark-hadron duality, and reflects the relationshipbetween confinement and asymptotic freedom in QCD. Surprisingly, thisphenomenon already takes place at rather low energies and averaged overrelatively small energy intervals (or a few hadronic states). Thisproposal investigates electron scattering reactions off protons,neutrons and nuclei in an effort to understand the origin of thisphenomenon.The proposal encompasses several experiments scattering electrons off aproton target to push and understand the limits where quark-hadronduality arises. It also includes the development of a novel detector toas closely as possible mimic an experiment scattering electrons off aneutron target. According to many theories the different onset ofquark-hadron duality for protons and neutrons will point to the originof the duality phenomenon. Lastly, interesting questions also arise inatomic nuclei: does a proton bound in a nucleus change structure due toits underlying quark-gluon structure? Within QCD, there is no way toderive anything like a nucleus in which the constituents (in normallife, the protons and neutrons) do not change as the mean densityincreases. Similarly, can one witness the underlying quark-gluonstructure of hadrons when they traverse a nucleus?The proposing physicists have spearheaded the research thrust ofquark-hadron duality and more generally the transition between thehadronic and quark-gluon descriptions of protons and nuclei. Theapproved project will allow this research to come to fruition and willsimultaneously provide advanced, internationally-competitive, scientifictraining for underrepresented minorities. Two of the proposers, and twoPh.D. students, are female. In addition, one of the students isAfro-american and two are African.