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的星星探测器最前方区域的电磁量热计。 众所周知，当高能粒子与入射束轴成大角度（高横动量）时，其基本过程涉及到单个夸克或胶子对之间的散射。然而，产生高能粒子的散射与束轴成前向角时，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。这些前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。从一般的论点来看，当参与的胶子数量很大时，这些胶子场预计会发生饱和。理论模型预测了这种效应。我们将比较pp散射和dAu散射产生的前向强子。一个类似的pp测量，一个质子在横向于束的方向（向上）极化，导致前向截面的左右不对称。 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，以区分需要初始状态夸克轨道角动量的模型和涉及高度极化夸克碎裂成最终状态喷流碎片的模型。这个问题的解决是理解质子成分自旋和角动量作用的重要一步。

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.