Meson-Baryon Scattering from Threshold to the Second Resonance Region

在非微扰区的强相互作用仍然是标准模型中最不被理解的部分之一。特别是，夸克是如何被限制在物理上可观察到的介子和重子状态中的，这一点还不清楚。目前，理论和实验的努力，以更好地了解激发重子（共振）和他们的衰变。在我们研究的最初几个月里，我们进一步发展了一个描述介子-重子散射的理论模型。尤利希模型是一种基于介子交换的解析么正模型。尤利希模型的振幅是从拉格朗日服从手征约束的场论方法中推导出来的。在各种分波中以高精度描述数据。我们发现，有了这些成分，再加上对分析性质和三体切割的彻底处理，就有可能可靠而精确地提取极点位置和残差。我们还表明，分解成极点和非极点的一部分，在文献中广泛使用的概念，是模型相关的，可靠的共振参数给出的极点位置和残留物，而不是打扮顶点或Breit-Wigner参数化。我们的研究结果已于最近发表。基于我们在调查过程中获得的见解和新开发的方法，我们计划扩展这项工作。我们的下一个目标是详细研究对πN <$ππN反应很重要的三体奇异性;现在有大量关于这些反应的最新数据。此外，我们计划在低能极限下将Jülich模型的振幅与手征微扰理论相匹配。额外的限制将来自匹配的Regge理论在高能量和包括额外的通道高达2 GeV的能量。

Strong interactions in the non-perturbative regime are still one of the least understood parts of the Standard Model. In particular it is not clear how quarks get confined into physically observable states called mesons and baryons. Currently, theoretical and experimental efforts are made to better understand excited baryons (resonances) and their decays. During the first months of our investigations, we have further developed a theoretical model of the description of meson-baryon scattering. The so-called Jülich model is an analytic unitary model based on meson exchange. The amplitudes of the Jülich model are derived within a field theoretical approach from Lagrangians obeying chiral constraints. Data are described to a high precision in the various partial waves. We found that with these ingredients, in combination with a thorough treatment of the analytic properties and three body cuts, a reliable and precise extraction of pole positions and residues becomes possible. We have also shown that the decomposition into pole and non-pole part, a widely used concept in the literature, is model dependent and that reliable resonance parameters are given by pole positions and residues rather than dressed vertices or Breit-Wigner parameterizations. Our results have been sent for publication recently. Based on the insights obtained during our investigation and the newly developed methods, we plan to extend the work. Our next target is the detailed study of three-body singularities important for the πN  ππN reaction; there is a large amount of recent data available now for these reactions. Moreover, we plan to match the amplitudes of the Jülich model to chiral perturbation theory in the low energy limit. Additional constraints will come from a matching to Regge theory at high energy and the inclusion of additional channels up to an energy of 2 GeV.