Bertone V, Britzger D, Camarda S, Cooper-Sarkar A, Geiser A, Giuli F, Glazov A, Godat E, Kusina A, Luszczak A, Lyonnet F, Olness F, Plačakytė R, Radescu V, Schienbein I, Zenaiev O
1Department of Physics and Astronomy, VU University, 1081 HV Amsterdam, The Netherlands.
2Nikhef Theory Group Science Park 105, 1098 XG Amsterdam, The Netherlands.
Eur Phys J C Part Fields. 2017;77(12):837. doi: 10.1140/epjc/s10052-017-5407-3. Epub 2017 Dec 7.
We investigate the impact of displaced heavy-quark matching scales in a global fit. The heavy-quark matching scale determines at which energy scale the QCD theory transitions from to in the variable flavor number scheme (VFNS) for the evolution of the parton distribution functions (PDFs) and strong coupling . We study the variation of the matching scales, and their impact on a global PDF fit of the combined HERA data. As the choice of the matching scale effectively is a choice of scheme, this represents a theoretical uncertainty; ideally, we would like to see minimal dependence on this parameter. For the transition across the charm quark (from to 4), we find a large dependence of the global fit at NLO, but this is significantly reduced at NNLO. For the transition across the bottom quark (from to 5), we have a reduced dependence of the at both NLO and NNLO as compared to the charm. This feature is now implemented in xFitter 2.0.0, an open source QCD fit framework.
我们在全局拟合中研究了位移重夸克匹配尺度的影响。重夸克匹配尺度决定了在可变味数方案（VFNS）中，对于部分子分布函数（PDFs）的演化和强耦合α_s，QCD理论从一种状态转变到另一种状态时的能量尺度。我们研究了匹配尺度的变化及其对HERA组合数据的全局PDF拟合的影响。由于匹配尺度μ的选择实际上是一种方案的选择，这代表了一种理论不确定性；理想情况下，我们希望看到对该参数的依赖性最小。对于跨越粲夸克的转变（从n_f = 3到4），我们发现在NLO时全局拟合χ²对μ有很大的依赖性，但在NNLO时这种依赖性显著降低。对于跨越底夸克的转变（从n_f = 4到5），与粲夸克相比，在NLO和NNLO时χ²对μ的依赖性都有所降低。这一特性现已在开源QCD拟合框架xFitter 2.0.0中实现。
