[论文解读] A Determination of the Charm Content of the Proton

Richard D. Ball, Valerio Bertone+7 more|arXiv (Cornell University)|May 20, 2016

Particle physics theoretical and experimental studies参考文献 98被引用 25

一句话总结

本文提出了一种质子部分子分布函数（PDFs）的全局拟合，将底夸克PDF与轻夸克和胶子在同等基础上处理，采用推广至重味底夸克的FONLL方案。结果发现，在Q ~ 1.5 GeV时，质子中存在约1%的非微扰底夸克成分，该结果对底夸克质量变化不敏感，且显著降低了其他PDF对m_c的依赖性。

ABSTRACT

We present an unbiased determination of the charm content of the proton, in which the charm parton distribution function (PDF) is parametrized on the same footing as the light quarks and the gluon in a global PDF analysis. This determination relies on the calculation of deep-inelastic structure functions in the FONLL scheme, generalized to account for massive charm-initiated contributions. In contrast to the usual situation in which the charm PDF is assumed to be generated perturbatively by pair radiation off gluons and light quarks, vanishing at a scale set by the value of the charm mass m_c, we find that the fitted charm PDF vanishes within uncertainties at a scale Q~1.5 GeV for all x ~0.1 and low scales does not vanish, but rather has an intrinsic component, very weakly scale dependent and almost independent of the value of m_c, carrying about 1% of the total momentum of the proton. The uncertainties in all other PDFs are only slightly increased by the inclusion of fitted charm, while the dependence of these PDFs on m_c is significantly reduced. When the EMC charm structure function dataset is included, it is well described by the fit, and PDF uncertainties in the fitted charm PDF are significantly reduced, though we verify that excluding the EMC data does not qualitatively modify any of our findings. The increased stability with respect to m_c persists at high scales and is the main implication of our results for LHC phenomenology. Fitting the charm PDF modifies the predictions for processes such as high p_T and large rapidity charm pair production and Z+c production, and thus we expect that future LHC data will further constrain the charm content of the proton.

研究动机与目标

• 在不假设底夸克PDF在低能标下为零的前提下，确定质子的底夸克含量。
• 减少PDF对任意底夸克质量标度m_c的依赖性。
• 通过引入非微扰底夸克成分，提高LHC物理拟合结果的稳定性和可靠性。
• 评估EMC底结构函数数据集对拟合底PDF及其不确定性的约束作用。
• 评估拟合底PDF对高pT和大快度底夸克对及Z+c产生过程预测的影响。

提出的方法

• 执行全局PDF分析，其中底PDF与轻夸克和胶子以相同方式参数化。
• 使用推广至包含重味底夸克引发的深度非弹性散射贡献的FONLL方案。
• 在低标度Q ~ 1.5 GeV处将底PDF作为自由参数拟合，允许存在内在的非微扰成分。
• 引入EMC底结构函数数据集以约束拟合的底PDF并减小不确定性。
• 通过对比包含与不包含EMC数据的拟合结果，以及在不同m_c值下的表现，评估结果的稳定性。
• 评估拟合底PDF对LHC物理中高能过程预测的影响。

实验结果

研究问题

• RQ1 当底PDF在低标度下不被假设为零时，质子的内在底夸克含量是多少？
• RQ2 包含非微扰底PDF成分如何影响其他PDF的不确定性及其对m_c的依赖性？
• RQ3 EMC底结构函数数据集在多大程度上约束了拟合的底PDF？
• RQ4 结果对底夸克质量m_c的变化有多大的稳定性？
• RQ5 当底PDF被拟合而非微扰生成时，高pT和大快度底夸克对及Z+c产生过程的预测如何变化？

主要发现

• 在Q ~ 1.5 GeV时，拟合的底PDF具有非零的内在成分，在x ~ 0.1处动量分数约为1%。
• 该内在底夸克成分的标度依赖性微弱，且几乎与底夸克质量m_c无关。
• 引入底PDF仅略微增加了其他PDF的不确定性，但显著降低了其对m_c的依赖性。
• EMC底结构函数数据集被拟合结果良好描述，其引入减小了底PDF的不确定性。
• 排除EMC数据不会定性改变结果，证实了内在底夸克成分的稳健性。
• 在高标度下，PDF对m_c的稳定性得以保持，这对可靠的LHC物理研究至关重要。