RUI: Nucleon Structure and Fundamental Interactions

从纵向极化核子散射的纵向极化电子（即自旋平行或反平行于其运动方向的电子）的数目取决于电子和核子自旋的相对方向。由于相互作用中交换的虚光子带走了自旋，使相互作用电子的自旋与位于核子撞击夸克中的自旋反平行的构型更为有利。因此，从纵向极化的核子中精确测量纵向极化电子散射的不对称性，可以得到有关核子自旋组成的信息。一个基于对称性论证的非常朴素的图像表明，当核子自旋方向与电子自旋方向横向时，在小角度上不应该观察到这种不对称性。然而，涉及微妙干涉效应的高阶效应导致小的非零不对称性。不同的理论计算对这种效应的大小并不一致，这种效应对各种理论方法中的假设极为敏感。在SLAC进行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。同样地，对纵向极化电子从非极化电子散射中违反奇偶性的杂合引起的非常小的不对称性进行非常精确的测量，这是SLAC计划中的E158实验的目标，将通过精确追踪这种杂合对传递动量的依赖，产生对标准模型的另一个测试。该奖项将授予史密斯学院的一些女学生参与这两个前沿实验。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，以及部分E155x数据分析。该奖项为美国顶尖女子学院的年轻女性提供了参与前沿科学研究的机会。

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.