双介子Nuclear Fusion中的ABC效应及其可能的pn共振
The ABC Effect in Double-Pionic Nuclear Fusion and a pn Resonance as its Possible Origin
摘要：
ABC效应--双介子聚变中一个长期存在的难题--已经通过第一次使用WASA探测器对聚变反应$pn \to dπ^0π^0$、$pd \to ^3$ He $ππ$和$dd \to ^4$ He $ππ$的固体统计进行了独家和运动学上完整的测量而得到了重新检验，第一次是在CELSIUS，最近是在COSY --后者的统计又增加了两个数量级。在所有的情况下，我们观察到一个巨大的低质量增强的$ππ$不变的质量伴随着一个明显的$ΔΔ$激发。对于最基本的聚变反应，$pn \to dπ^0π^0$反应，我们观察到另外一个非常明显的共振类能量的依赖性，在总截面与最大90兆电子伏以下的$ΔΔ$质量和宽度只有50兆电子伏，这是五倍小于预期从传统的$t$通道$ΔΔ$激发。这揭示了ABC效应是s-通道共振的结果，该重子态的形状因子反映在$ππ$不变质量的低质量增强中。从$^3$ He和$^4$ He的融合反应中，我们了解到这种共振即使在原子核中也足够强大。
英文摘要：
The ABC effect -- a long-standing puzzle in double-pionic fusion -- has been reexamined by the first exclusive and kinematically complete measurements of solid statistics for the fusion reactions $pn \to dπ^0π^0$, $pd \to ^3$He$ππ$ and $dd \to ^4$He$ππ$ using the WASA detector, first at CELSIUS and recently at COSY -- the latter with a statistics increased by another two orders of magnitude. In all cases we observe a huge low-mass enhancement in the $ππ$-invariant mass accompanied by a pronounced $ΔΔ$ excitation. For the most basic fusion reaction, the $pn \to dπ^0π^0$ reaction, we observe in addition a very pronounced resonance-like energy dependence in the total cross section with a maximum 90 MeV below the $ΔΔ$ mass and a width of only 50 MeV, which is five times smaller than expected from a conventional $t$-channel $ΔΔ$ excitation. This reveals the ABC effect to be the consequence of a s-channel resonance with the formfactor of this dibaryonic state being reflected in the low-mass enhancement of the $ππ$-invariant mass. From the fusion reactions to $^3$He and $^4$He we learn that this resonance is robust enough to survive even in nuclei.
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