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由于风味方案的变化和更高的扭曲贡献对PDF和$α_S(M_Z^2)$的影响

The effect on PDFs and $α_S(M_Z^2)$ due to changes in flavour scheme and higher twist contributions

R. S. Thorne

备注：
23 pages, 19 figures. Final version. Sioke slightly extended discussion
发表信息：
Eur. Phys. J. C (2014) 74: 2958
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摘要：

我认为MSTW部分子分布函数（PDF）的影响，由于在拟合中使用的理论程序的选择的变化。我首先考虑使用3味固定味数计划，而不是标准的一般质量可变味数计划中使用的MSTW分析。这导致轻夸克在所有相对较小的$x$值处增加，胶子分布在$x$高值处变小，在$x$小值处变大，耦合常数$α_S(M_Z^2)$的优选值下降，特别是在NNLO处，并且拟合质量变差。我还考虑降低DIS数据的$W^2$的运动学切割，同时引入更高的扭曲项，这是适合数据。这导致对PDF和$α_S(M_Z^2)$的影响比方案改变小得多，除了在非常高的$x$下的夸克。我表明，从一个固定的输入集的PDF使用固定的风味计划和可变风味计划获得的结构函数不同显着$x \sim 0.01$在高$Q^2$，这是由于这样一个事实，即在固定的风味计划有一个缓慢的收敛的大对数项的形式$(α_S\ln(Q^2/m_c^2))^n$相关的这一制度。我的结论是，PDF集的一些最显着的差异在很大程度上是由于选择使用的风味方案。

英文摘要：

I consider the effect on MSTW partons distribution functions (PDFs) due to changes in the choices of theoretical procedure used in the fit. I first consider using the 3-flavour fixed flavour number scheme instead of the standard general mass variable flavour number scheme used in the MSTW analysis. This results in the light quarks increasing at all relatively small $x$ values, the gluon distribution becoming smaller at high values of $x$ and larger at small $x$, the preferred value of the coupling constant $α_S(M_Z^2)$ falling, particularly at NNLO, and the fit quality deteriorates. I also consider lowering the kinematic cut on $W^2$ for DIS data and simultaneously introducing higher twist terms which are fit to data. This results in much smaller effects on both PDFs and $α_S(M_Z^2)$ than the scheme change, except for quarks at very high $x$. I show that the structure function one obtains from a fixed input set of PDFs using the fixed flavour scheme and variable flavour scheme differ significantly for $x \sim 0.01$ at high $Q^2$, and that this is due to the fact that in the fixed flavour scheme there is a slow convergence of large logarithmic terms of the form $(α_S\ln(Q^2/m_c^2))^n$ relevant for this regime. I conclude that some of the most significant differences in PDF sets are largely due to the choice of flavour scheme used.

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