摘要
利用北京谱仪(BESⅡ)在北京正负电子对撞机(BEPC)上收集的58×10~6J/φ和14×10~6φ(2S)事例,对J/φ→Λ(?)π~0, J/φ→Λ(?)η,φ(2S)→Λ(?)π~0,φ(2S)→Λ(?)η, J/φ→∑~+(?) and J/φ→Ξ~0(?)等重子衰变道的分支比进行了精确测量。
在分析J/φ→Λ(?)π~0过程中,发现本底J/φ→∑~0π~0(?)+c.c.和J/φ→∑~+π~-(?) +c.c.对Λ(?)π~0信号构成严重污染。考虑这一本底贡献后,J/φ→Λ(?)π~0的衰变分支比比以前的测量结果明显减小。2006年粒子数据组(PDG)给出J/φ→Λ(?)π~0的分支比结果为(2.2±0.6)×10~(-4),而我们测得90%置信度下的分支比上限为
在寻找J/φ→Λ(?)η时,通过优化事例筛选条件,首次在两光子不变质量谱上观察到η信号,其统计显著性为4.80σ,并测得其分支比为其中第一项误差为统计误差,第二项为系统误差。对比J/φ→Λ(?)π~0和J/φ→Λ(?)η分支比,我们发现同位旋守恒过程(Λ(?)η)与同位旋破坏过程(Λ(?)π~0)分支比显著不同,与理论预期的强衰变分支比比电磁过程要大一致,表明J/φ→Λ(?)π~0和J/φ→Λ(?)η中不存在反常行为。
同样,我们还对φ(2S)衰变到Λ(?)π~0和Λ(?)η进行了分析和研究。在扣除已知本底后,没有发现明显的信号事例,测得90%置信度下的分支比上限为
本文还首次测量了重子对衰变过程J/φ→∑~+(?)和J/φ→Ξ~0(?)的分支比,测量结果如下:其中第一项误差为统计误差,第二项误差为系统误差。这些测量结果和它们的同位旋多重态J/φ→∑~0(?)和J/φ→Ξ~-(?)的分支比在误差范围内是一致的。利用CLEO合作组最新测得的φ(2S)→∑~+(?)和φ(2s)→Ξ~0(?)分支比结果,我们发现J/φ和φ(2S)衰变到∑~+(?)和Ξ~0(?)基本符合微扰QCD预言的“12%规则”。
With 58 million J/φand 14 millionφ(2S) events collected with the BESⅡdetector at the BEPC, branching fractions or upper limits on branching fractions for the baryonic decays J/φ→Λ(?)π~0, J/φ→Λ(?)η,φ(2S)→Λ(?)π~0,φ(2S)→Λ(?)η, J/φ→∑~+(?) and J/φ→Ξ~0(?) are measured.
In the analysis of J/φ→Λ(?)π~0, we found that the observedπ~0 was seriously contaminated by the backgrounds J/φ→∑~0π~0(?) + c.c. and J/φ→∑~+π~-(?) + c.c. Considering the contributions of these crucial backgrounds, we found that the branching fraction of J/φdecaying toΛ(?)π~0 is smaller than previous measurements. In contrast to the large value ((2.2±0.6)×10~(-4)) of branching fraction of J/φ→Λ(?)π~0 in PDG 2006, we determined, at the 90% confidence level,
In the measurement J/φ→Λ(?)η, we optimized the event selections, and observed the signal ofηin theγγinvariant mass for the first time. The statistical significance is 4.8σ, and the branching fraction is measured to be,where the first error is statistical and the second systematic. Comparing the distinct values of the branching fraction of J/φ→Λ(?)π~0 and J/φ→Λ(?)η, we found that our results are in accord with the theoretical predictions of the relative large branching fraction of strong interaction. Therefore, our results indicate that there is no abnormal physics in J/φ→Λ(?)π~0, and J/φ→Λ(?)ηdecays.
Similarly, we also analyzedφ(2S) decaying intoΛ(?)π~0, andΛ(?)η. No signals were found inφ(2S) decays, and the upper limits at the 90% confidence level are determined to be,
Furthermore, we observed the baryon pair processes J/φ→∑~+(?) and J/φ→Ξ~0(?) for the first time, and measured the branching fractions to be,where the first error is statistical and the second systematic. These results are consistent with the branching fractions of their isospin partners J/φ→∑~0(?) and J/φ→Ξ~-(?) with expectations of isospin symmetry. Using the recent branching fractions ofφ(2S)→∑~+(?) andφ(2S)→Ξ~0(?) measured by CLEO collaboration, we found that the ratio of the branching fraction ofφ(2S) and J/φdecaying to∑~+(?) andΞ~0(?) agree with the so called " 12% rule" predicted by perturbative QCD within1σ.
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