B→π跃迁形状因子的计算

重到轻跃迁形状因子是非常重要的参量，它在B介子衰变中起重要作用.在光锥QCD求和规则中，形状因子的计算结果的不确定性主要来自于光锥波函数，twist-2, twist-3的波函数是主要贡献的波函数.现在仅有twist-2的波函数被系统地分析并较精确地确定下来.然而，对twist-3, twist-4的波函数确定不够好.但twist-3波函数对形状因子的贡献与twist-2波函数贡献相当，Twist-4波函数贡献很小，这样twist-3波函数给计算结果带来较大的不确定性.为此，通过构造手征流关联函数，消除了twist-3波函数的不确定性所带来的影响，用改进的光锥QCD求和规则计算B→π跃迁形状因子，与标准的光锥QCD求和规则的结果基本吻合，从而更能精确地抽取CKM矩阵元｜Vub｜.各参数对计算结果f(q2)的不确定度的影响范围是：Borel质量参数M2的影响为±(3～5)%，光锥波函数±10%，高扭度波函数的影响不超过5%.

The Calculation of B→π Transitions Form Factor

The heavy-to-light transition form factors are very important parameters，which play an important role in B-meson decays. The uncertainties of calculation of form factors in light-cone QCD sum rules mainly come from light-cone wave function among which twist-2 and twist-3 wave function make significant contributions to form factors. Now only twist-2 wave function has been systematically analyzed and twist-3 and twist-4 wave function have not satisfactorily determined but twist-4 contribution is very small, therefore, it is twist-3 wave function that cause large uncertainty of form factors. So, through construction adequate chiral current correlation function and consideration of all twist 2 and 4 contribution, the heavy-to-light B→π transitions form factor was calculated in improved light-cone QCD sum rules, and was compared with the result in standard light-cone QCD sum rules. They are consistent with each other, but the uncertainties are eliminated about twist-3 wave function, so the CKM matrix element ｜Vub｜ precisely can be extracted.The contributions of every parameter to uncertainties of calculation result f(q2) are discussed and presented. For example, the contributions of Borel parameter is ±(3～5)%; light-cone wave function ±10%; The higher twist wave functions are less than 5%.