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arXiv 0912.4158nucl-th
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低能中性π介子光生中D波的意外作用

The unexpected role of D waves in low-energy neutral pion photoproduction

C. Fernandez-Ramirez

备注：
Talk presented in the 6th International Workshop on Chiral Dynamics, July 6-10 2009, Bern, Switzerland. To be published in Proceedings of Science. Uses class Pos.cls (included). 6 pages and 3 figures
发表信息：
PoS CD09:055,2009
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摘要：

一般认为质子的低能中性π介子光生只能用S波和P波来描述，而更高的分波则与此无关。我们已经发现，这种假设是不正确的，并列入D波是必要的，以获得一个可靠的提取的$E_{0+}$多极从实验数据。这在很大程度上是由于QCD中手征对称性的自发破缺导致了S波的贡献非常小。这使得通常的分波展开不太准确，尽管D波很小，但它们的贡献通过与P波的干涉而增强，如果不考虑D波，则会损害从数据中提取S波。在我们的工作中，我们使用重子手征微扰理论的一个循环，并高达${\cal O}(q^4)$，以考虑的S和P波，而D波添加在一个几乎独立的方式使用标准的玻恩项和矢量介子。我们还表明，较高的分波不起重要作用。

英文摘要：

It has been commonly assumed that low-energy neutral pion photoproduction from the proton can be described accounting only for S and P waves, and that higher partial waves are irrelevant. We have found that this assumption is not correct and that the inclusion of D waves is necessary to obtain a reliable extraction of the $E_{0+}$ multipole from experimental data. This is due in large measure to the spontaneous breaking of chiral symmetry in QCD which leads to very small S-wave contributions. This makes the usual partial wave expansion less accurate and although D waves are small, their contribution is enhanced through the interference with P waves, which compromises the S-wave extraction from data if D waves are not taken into account. In our work we have used Heavy Baryon Chiral Perturbation Theory to one loop, and up to ${\cal O}(q^4)$, to account for the S and P waves, while D waves are added in an almost model-independent way using standard Born terms and vector mesons. We also show that higher partial waves do not play an important role.

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