RUI: Nucleon Structure and Fundamental Interactions

9901570Decowski从纵向极化核子散射的纵向极化电子（即自旋与其运动方向平行或反平行的电子）的数量取决于电子和核子自旋的相对取向。由于相互作用中交换的虚光子带走了自旋，相互作用电子的自旋和核子撞击夸克中的自旋反平行的构型更有利。因此，对纵向极化电子与纵向极化核子散射的不对称性的精确测量可以提供核子自旋组成的信息。基于对称性论证的一个非常简单的图景表明，当核子自旋的方向与电子自旋的方向成横向时，这种不对称性不应该在小角度下观察到。 然而，涉及细微干扰效应的高阶效应导致小的非零不对称性。不同的理论计算不收敛于这种效应的大小，这是非常敏感的假设，在各种理论方法。SLAC的E155x实验的目的是精确测量质子和中子的这种不对称性。这将是对基于标准模型的现有理论的一次重要检验。 同样，在SLAC计划的E158实验中，对纵向极化电子与非极化电子散射中宇称违反混合物引起的非常小的不对称性进行非常精确的测量，将通过精确追踪这种混合物对转移动量的依赖性来对标准模型进行另一次检验。 该奖项将资助史密斯学院的一些女学生参加这两项前沿实验。他们将参与E158实验的大部分方面，包括在夏季建造和测试探测系统的“动手”工作，以及E155x数据分析的一部分。该奖项为美国首屈一指的女子学院的年轻女性提供了参与尖端科学研究的机会。

9901570DecowskiNumber of longitudinally polarized electrons (i.e. electrons with spinsparallel or antiparallel to the direction of their motion) scattered from longitudinally polarized nucleons depends on relative orientation of electron and nucleon spins. Configuration in which the spin of the interacting electron and the spin of a residing in the nucleon struck quark are antiparallel is more favorable because of the spin carried away by virtual photon exchanged in the interaction. Hence, the precise measurement of asymmetry in scatttering of longitudinally polarized electrons from longitudinally polarized nucleons yields information on the composition of the nucleon spin. A very naive picture, based on symmetry arguments, suggests that such asymmetry should not be observed at small angles when the nucleon spin is oriented in the direction transverse to the direction of electron spin. However, higher order effects involving subtle interference effects lead to a small nonzero asymmetry. Different theoretical calculations do not converge on the magnitude of this effect which is extremely sensitive to assumptions in various theoretical approaches. The purpose of the E155x experiment at SLAC is the precise measurement of this asymmetry in protons and neutrons. It will be an important test of the existing theories based on the Standard Model. Similarly, the very precise measurement of very small asymmetry caused by parity violating admixture in scattering of longitudinally polarized electrons from unpolarized electrons, the goal of the planned E158 experiment at SLAC, will yield another test of the Standard Model by precise tracing the dependence of this admixture on the transfered momentum. The award will grant participation of a number of Smith College women students in both forefront experiments. They will be involved in most aspects of the E158 experiement, including "hands-on" work on building and testing the detection system during the summer months, and in part of the E155x data analysis. This awards affords an opportunity for young women at one of the premier womens colleges in the U.S. to participate in scientific research at the cutting edge.