能量在4.2GeV附近的qqqcc-bar五夸克态的理论研究

With the development of the high energy experiments, especially after the new measurement about the contributions of strange quarks to the charge and magnetization distributions of the proton by JLab, the physical picture of multiquark components in the baryon is widely accepted. However, the structure and properties of the multiquark-picture are not very clear. In the light quark sector, qqqqq-bar picture for many resonance, such as N(1535) and Λ(1405), can not be clearly distinguished from the excited qqq picture. After the curious episode of Θ(1540), the structure and properties of pentaquark are more fascinating to researchers. We first study the mass spectrum of the heavy qqqcc-bar systems in the constituent quark model by using three popular interactions between quark and quark, respectively. Then we study their decay properties within the framework of the coupled channel unitary approach. The decay widths of qqqcc-bar pentaquark systems around 4.2 GeV are very small. Therefore, they definitely cannot be accommodated by the conventional 3q quark models. The study of the qqqcc-bar pentaquark systems around 4.2 GeV can help us to understand pentaquark more clearly. The existence of these new resonances may also have important implications to problems such as the strikingly large spin-spin correlation observed in pp elastic scattering near charm production threshold and difficulties in reproducing the cross sections and polarization observables of J/ψ production from high energy pp and γp reactions.

随着精确的高能物理实验的进行，特别是最近JLab测量了质子中奇异夸克的电磁分布，重子的多夸克图像已被广泛接受。目前，重子中多夸克图像中的夸克结合方式、特点等还有待研究。在轻夸克区域，很多共振态比如N(1535)和Λ(1405)，不能准确区分是三夸克qqq的激发态还是五夸克态qqqqq-bar。经历了Θ(1540)的插曲，人们对探索五夸克的性质、结构等更感兴趣。本项目将首先运用三种常用的组分夸克模型，分别研究含有重的隐粲cc-bar的能量在4.2GeV附近的qqqcc-bar五夸克系统的能量谱；然后通过耦合道效应研究它们较低能量态的衰变宽度。这类重的五夸克，衰变宽度很小，等能量的qqq激发态衰变宽度很大。qqqcc-bar五夸克系统的研究对于进一步澄清五夸克图像有重要意义，对于研究粲夸克产生阈值的pp散射中自旋-自旋相互关系以及高能pp和 γp反应中J/ψ的产生截面都有重要参考意义。

运用常用的三种组分夸克模型，分别研究了qqqcc-bar五夸克系统的能量谱；通过耦合道效应计算了这类五夸克中较低能量态的衰变宽度。计算结果表明这类重的五夸克的衰变宽度是100 MeV左右。2015年7月LHCb实验组报导他们发现了含有隐粲(cc-bar)的五夸克态Pc(4380)和Pc(4450)，这两个共振态的夸克组分都是uudcc-bar。这是实验上首次获得普遍认可的重味五夸克态，表明了我们选题的前瞻性；能谱范围和衰变宽度与我们的计算相符，对我们是极大的鼓励。我们将继续深入研究各个量子数的qqqcc-bar类型的五夸克态，并根据实验上测得共振态质量和衰变分支比研究新发现的五夸克候选态的结构、性质、自旋-宇称安排等。