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arXiv 2103.00402hep-ph
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$B$介子在QCD和$B\to π$跃迁形状因子中的重整化群演化

Renormalization-Group Evolution of the None-local Matrix Element of $B$-meson In QCD and $B\to π$ transition form factor

Sheng Lü, Mao-Zhi Yang

摘要:

我们考虑了$\langle 0| \bar{q}(z)_β[z,0]b(0)_α| \bar{B}\rangle$的矩阵元的重整化群演化,它可以用来定义$B$介子的分布振幅,并广泛应用于$B$介子衰变的研究.非定域算符$\bar{q}(z)_β[z,0]b(0)_α$对重整化常数的贡献被考虑到QCD中的单圈阶。由于该算符中的夸克场不是直接耦合的场,动量不能通过该非定域算符自由流动。这个算符中涉及的动量可以在坐标空间中严格处理。我们发现,由尺寸参数$ε$调节的紫外发散相互抵消,演化效应消失。矩阵元$\langle 0| \bar{q}(z)_β[z,0]b(0)_α| \bar{B}\rangle$从重整化群演化中逃逸。然后将该矩阵元应用于计算$B\toπ$跃迁形状因子,其中矩阵元是由QCD激发势模型中计算的$B$介子波函数得到的。通过与$B\to π\ellν$半轻子衰变的实验数据和光锥求和规则计算的比较,在微扰QCD方法框架下分析了微扰和非微扰对$B\toπ$跃迁形状因子的贡献.我们发现Sudakov因子对软贡献的抑制效果取决于$B$介子波函数的端点行为,而采用$B$ -介子波函数时,软贡献不能得到很好的抑制.在朴素计算中,对$B\toπ$转变形状因子的硬贡献约为59%,并且软贡献可高达41%。为了使微扰计算可靠,必须在计算中引入软动量截断和软形状因子。

英文摘要:

We consider the renormalization-group evolution of the matrix element of $\langle 0| \bar{q}(z)_β[z,0]b(0)_α| \bar{B}\rangle$, which can be used to define the distribution amplitudes for $B$ meson and widely applied in studies of $B$ meson decays. The contribution to the renormalization constant of the non-local operator $\bar{q}(z)_β[z,0]b(0)_α$ is considered up to one-loop order in QCD. Since the quark fields in this operator are not directly coupled fields, momentum can not flow freely through this non-local operator. Momentum involved in this operator can be treated stringently in coordinate space. We find that the ultraviolet divergences regulated by dimensional parameter $ε$ cancel with each other, and the evolution effect vanishes. The matrix element $\langle 0| \bar{q}(z)_β[z,0]b(0)_α| \bar{B}\rangle$ escapes from the renormalization-group evolution. We then apply the matrix element in calculating $B\toπ$ transition form factor, where the matrix element is obtained by using the $B$ meson wave function calculated in QCD-inspired potential model. By comparing with experimental data for the semileptonic decay of $B\to π\ellν$ and light-cone sum rule calculation, we analyse the perturbative and non-perturbative contributions to $B\toπ$ transition form factor in the frame work of perturbative QCD approach. We find that the effectiveness of the suppression of Sudakov factor to soft contribution depends on the end-point behavior of $B$ meson wave function, and with the $B$-meson wave function used in this work, soft contribution can not be well suppressed. The hard contribution to the $B\toπ$ transition form factor is about 59\%, and soft contribution can be as large as 41\% in the naive calculation. To make the perturbative calculation reliable, a soft momentum cutoff in the calculation and soft form factor have to be introduced.

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