arXiv 1111.07552011-11-03hep-ex

横向自旋不对称性的STAR结果和展望

STAR results and perspectives on transverse spin asymmetries

N. Poljak

备注：: 6 pages, 4 figures, proceedings of the Transversity 2011 workshop
DOI：: 10.1393/ncc/i2012-11199-3

摘要：

STAR合作报告了在$\sqrt{s}=$ 200 GeV的极化质子碰撞中产生正向$π^0$介子的横向单自旋不对称性的精确测量结果。为了解开对测量到的前向不对称性的贡献，我们必须超越包含性的$π^0$产生，看到前向喷流或直接光子的产生。在2006年，配备了正向π介子探测器++（FPD++）的STAR收集了6.8 pb $^{-1}$的正向数据，平均偏振度为60%。FPD++对“喷射状”物体有足够的接受度，这些物体是电磁量热计的聚集响应，主要对入射光子、电子和正电子敏感。对于这些对象，相对于碎裂部分子的传出的领先$π^0$的角度被重建，从而使我们能够解开的贡献向前$π^0$不对称性。模拟数据集显示，平均每个“喷流”物体大约有2.5个分裂介子，使它们合理地“码头”。初步结果没有提供任何证据表明，从射流破碎的不对称测量的贡献，这意味着西弗斯分布函数发挥了重要作用，在生产大包容性的前$π^0$不对称。在STAR探测器的中快度$(|η|<1)$区域也做了类似的工作，在那里收集了2.2 pb $^{-1}$的数据。我们目前取得的进展，通过横向极化质子碰撞产生的喷流的领先带电π介子的方位角不对称性的测量。

英文摘要：

The STAR collaboration reported precision measurements on the transverse single spin asymmetries for the production of forward $π^0$ mesons from polarized proton collisions at $\sqrt{s}=$ 200 GeV. To disentangle contributions to measured forward asymmetries one has to look beyond inclusive $π^0$ production to the production of forward jets or direct photons. In 2006, STAR with the Forward Pion Detector++ (FPD++) in place, collected 6.8 pb$^{-1}$ of forward data with an average polarization of 60%. FPD++ had sufficient acceptance for "jet-like" objects, which are clustered responses of an electromagnetic calorimeter primarily sensitive to incident photons, electrons and positrons. For these objects, the angle of the outgoing leading $π^0$ with respect to the fragmenting parton was reconstructed, thus enabling us to disentangle the contributions to the forward $π^0$ asymmetries. The simulated data set shows that on average there are approximately 2.5 fragmenting mesons per one "jet-like" object, making them reasonably "jetty". Preliminary results provide no evidence of measured contributions to the asymmetry from jet fragmentation, implying the Sivers distribution functions play a substantial role in producing the large inclusive forward $π^0$ asymmetries. A similar effort was made in the mid-rapidity $(|η|<1)$ region of the STAR detector, where 2.2 pb$^{-1}$ of data was collected. We present progress made by making measurements of the azimuthal asymmetry of leading charged pions in jets produced by transversely polarized proton collisions.

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