The Transition to a Quark-Gluon Description of the Nucleon

核子向夸克-胶子描述的转变

• 批准号: 0400332
• 负责人: Cynthia Keppel
• 金额: $ 58.42万
• 依托单位: Hampton University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-09-01 至 2007-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0400332&HistoricalAwards=false
• 关键词: Transition; Quark; Gluon; Description; Nucleon

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.

在QCD作为夸克和胶子的理论建立三十年后，理解QCD的工作仍然是核物理学中的一个重大难题。一个主要的挑战来自于这样一个事实，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。 大距离QCD的显著而独特的特性--夸克禁闭--阻止了构成强子束缚态的单个夸克和胶子组分从系统中被移除并被孤立地研究。作为一个推论，夸克在强子中以相对论速度运动，这种运动占人体质量的99.9%。在另一个极端，QCD的性质被称为渐近自由，其中夸克在（非常）短的距离上相互作用很弱，可以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。尽管强子和胶子之间存在明显的差异，和夸克胶子区，在自然界中存在这样的例子，即低能强子截面的行为，在适当的能量区间上平均，非常类似于在渐近高能量下的行为，用夸克胶子自由度计算。 这种现象被称为夸克-强子对偶性，反映了量子色动力学中禁闭与渐近自由的关系。令人惊讶的是，这种现象已经发生在相当低的能量和平均相对较小的能量间隔（或几个强子状态）。这个提议研究了电子从质子、中子和原子核上散射的反应，试图理解这一现象的起源。这个提议包括几个实验，将电子从质子靶上散射，以推动和理解夸克-强子二象性出现的极限。它还包括一种新型探测器的开发，以尽可能接近地模拟从非中子靶散射电子的实验。 根据许多理论，质子和中子的夸克强子二象性的不同起始点将指向二象性现象的起源。 最后，有趣的问题也出现在原子核中：束缚在原子核中的质子是否会因其潜在的夸克-胶子结构而改变结构？在量子色动力学中，不可能推导出像原子核那样的东西，其中的成分（在正常情况下，质子和中子）不会随着平均密度的增加而改变。同样地，当强子穿过原子核时，我们能看到它的夸克胶子结构吗？提出这一观点的物理学家们带头研究了夸克-强子对偶性，更广泛地说，研究了质子和原子核的强子描述和夸克-胶子描述之间的过渡。批准的项目将使这项研究取得成果，同时为代表性不足的少数民族提供先进的，具有国际竞争力的科学培训。两个提议者，两个博士。学生，都是女的。 此外，其中一名学生是非裔美国人，两名是非洲人。