Experimental research on penta-quark baryons with the STB ring

在本研究项目中，我们观测到了一个窄重子共振N^*（1670）。根据目前最新的分析结果，其质量和共振宽度分别为1665-1675 MeV/c^2和10-40 MeV/c^2。用固体氘靶进行的η介子光生实验中，在总截面数据中出现了共振峰。在相同的实验条件下，在固体氢靶的情况下没有出现凸起。因此，新的共振通过辐射跃迁耦合到中子，但不耦合到质子。我们检查了自旋1/2和3/2的状态与两个宇称，拟合这些状态的数据一个接一个。对总截面数据和角分布的cosθ系数进行了拟合。一个负的宇称状态，如S11或D13已被证明比正宇称状态更好地再现数据，虽然很难确定的自旋和宇称的共振与目前的数据。然而，有几个宽共振，S11（1650），D15（1675），F15（1680），D13（1700），P11（1700）和P13（1720），其宽度约为100-200 MeV/c^2。这些宽共振都位于1670 MeV/c^2附近的质量区域。与这些宽态之一具有相同量子数的窄共振不能被构造为三个夸克态的成员。考虑到共振与核子的耦合情况，我们自然地把窄共振看作是五夸克态重子反十重态的一员。在这里，U自旋守恒允许γn→ N^*（1670）的过程，但禁止γp→N^<*^+>（1670）的跃迁。因为在反十重态中，γ、n和p的U自旋分别是0、1和1/2，而N^*（1670）的U自旋是1和3/2，对于N^<*^+>（1670）。

We have observed a narrow baryon resonance N^*(1670) in this research project. The mass and the width of the resonance are 1665-1675 MeV/c^2 and 10-40 MeV/c^2,respectively, according to the latest analysis up to now. The resonance bump appeared in the total cross section data obtained in an η meson photoproduction experiment using a solid deuterium target. The bump did not show up in the case with a solid hydrogen target under the same experimental condition. Thus, the new resonance couples to the neutron, but not to the proton through radiative transitions. We examined the spin 1/2 and 3/2 states with both parities, fitting these states to the data one by one. The fitting was made for the data of the total cross section and the cosθ coefficients of the angular distributions. A negative parity state such as S11 or D13 has turned out to reproduce the data better than positive parity states, although it is difficult to determine the spin and parity of the resonance definitely with the present data. However, there are several broad resonances, S11(1650),D15(1675),F15(1680),D13(1700),P11(1700), and P13(1720), the width of which is roughly 100-200 MeV/c^2. And any of these broad resonances is placed over the mass region around 1670 MeV/c^2. The narrow resonance with the same quantum numbers as one of these broad states cannot be constructed as a member of three quark states. Taking the coupling situation of the resonance to the nucleons into account, it is natural, therefore, to regard the narrow resonance as a member of the antidecuplet of pentaquark baryons. Here U-spin conservation allows the process γn→ N^*(1670), but prohibits the transition γp→N^<*^+>(1670). Because the U-spin of γ,n, and p is 0,1,and 1/2,respectively, while that of N^*(1670) is 1 and 3/2 for N^<*^+>(1670) in the antidecuplet.