标题： 介子的强子断层扫描及其引力形式因子 显示英文标题

标题： Hadron tomography for pion and its gravitational form factors

摘要： 广义部分子分布（GPDs）是强子的三维结构函数，它们可以揭示轨道角动量对核子自旋的贡献。因此，GPDs对于解决质子自旋之谜非常重要。广义分布振幅（GDAs）是GPDs的$s$-$t$交叉量，GDAs可以在两光子过程（$\gamma^* \gamma \to h\bar h$）中进行研究，该过程可在KEKB中实现。通过分析Belle实验对$\pi^0 \pi^0$产生在$e^+ e^-$碰撞中的测量结果得到了π介子的GDAs。从获得的GDAs中，计算了π介子在类时区域的能量-动量张量形式因子。为了研究π介子的引力半径，利用色散关系在类空区域获得了能量-动量张量的形式因子。然后，通过类空形式因子计算出质量半径为0.32 $\sim$0.39 fm，π介子的力学半径也估计为0.82 $\sim$0.88 fm。这是首次通过实际实验测量得到强子引力形式因子和半径的研究结果。 在未来不久，我们可以期待对$\gamma^* \gamma \to h\bar h$的更精确测量，因为 Belle II 已经开始通过更高亮度的 Super KEKB 进行数据采集，因此也可以研究其他强子的 GDA。 显示英文摘要

摘要： Generalized parton distributions (GPDs) are three-dimensional structure functions of hadrons, and they can reveal the orbital-angular-momentum contributions to the nucleon spin. Therefore, GPDs are important for solving the proton spin puzzle. The generalized distribution amplitudes (GDAs) are the $s$-$t$ crossed quantities of the GPDs, and the GDAs can be investigated in two-photon process ($\gamma^* \gamma \to h\bar h$) which is accessible at KEKB. The pion GDAs were obtained by analyzing the Belle measurements for $\pi^0 \pi^0$ production in the $e^+ e^-$ collision. From the obtained GDAs, the form factors of energy-momentum tenor were calculated for pion in the timelike region. In order to study the gravitational radii for the pion, the form factors of energy-momentum tenor were obtained in the spacelike region by using the dispersion relation. Then, the mass radius was calculated as 0.32 $\sim$ 0.39 fm and the mechanical radius was also estimated for the pion as 0.82 $\sim$ 0.88 fm by using the spacelike form factors. This is the first finding on gravitational form factors and radii of hadrons from actual experimental measurements. In the near future we can expect more precise measurements of $\gamma^* \gamma \to h\bar h$ as the Belle II started data taking by the higher luminosity Super KEKB, so that the GDAs of other hadrons could be studied as well.