Large Rapidity Polarized Proton Measurements and Gluon Nuclear Saturation Effects at STAR

我们已经建造并将升级布鲁克海文国家实验室恒星探测器最前沿区域的电磁量热仪。众所周知，当高能粒子与入射光束轴成大角度(以高横向动量)产生时，下面的过程涉及到成对的单个夸克或胶子之间的散射。然而，产生相对于光束轴成前角的高能粒子的散射很可能涉及单个高能夸克与一个或多个低能胶子的相互作用。这些正向过程的测量对于理解单个快夸克如何与软多胶子场相互作用是很重要的。从一般的论证来看，当参与胶子的数量很大时，预计这些胶子场就会饱和。理论模型预测了这种效应。为了产生前向强子，我们将在pp散射和DAU散射之间进行比较。类似的pp测量，其中一个质子在横跨光束的方向(UP)极化，导致在前向截面上出现左右不对称。我们计划对这种不对称性进行新的测量，并详细描述相关粒子的产生，这将区分需要初态夸克轨道角动量的模型和涉及高极化夸克碎裂为终态喷注碎片的模型。这个问题的解决是理解质子组分的自旋和角动量的作用的重要一步。

We have built and we will upgrade electromagnetic calorimeters for thevery forward region of the STAR detector at Brookhaven National Lab. Itis well known that when energetic particles are produced at large anglesto the incident beam axis (at high transverse momentum), the underlyingprocess involves scattering between pairs of individual quarks or gluons.However, scattering that produces energetic particles at forward angles,relative to the beam axis, is likely to involve the interaction of asingle high energy quark with one or more low energy gluons. Measurementsof these forward processes are important to understand how a single fastquark interacts with a soft multi-gluon field. From general arguments,saturation of these gluon fields is expected to occur when the number ofparticipating gluons is large. Theoretical models predict such effects.Comparisons will be made between pp scattering and dAu scattering forproduction of forward hadrons.A similar pp measurement, with one proton polarized in a directiontransverse to the beam (up), leads to a left-right asymmetry in theforward cross section. We plan new measurements of this asymmetry, withdetailed characterization of the correlated particle production that willdistinguish between models that require initial state quark orbitalangular momentum from models that involve the fragmentation of highlypolarized quarks into final state jet fragments. The resolution of thisquestion is an important step in understanding the roles of spin andangular momentum for proton constituents.