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arXiv 2209.02149hep-lathep-ph
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前向光-光散射与强子真空极化的电磁修正

Forward light-by-light scattering and electromagnetic correction to hadronic vacuum polarization

Volodymyr Biloshytskyi, En-Hung Chao, Antoine Gérardin, Jeremy R. Green, Franziska Hagelstein, Harvey B. Meyer, Julian Parrino, Vladimir Pascalutsa

AI总结:

针对格点QCD计算强子真空极化时电磁修正不可忽略的问题,提出引入分离标度拆分长短程部分的方案,结合色散求和规则关联前向强子光-光散射与γ*γ*融合截面,验证关系后预测了相关格点QCD被积函数。

摘要:

强子真空极化(HVP)的格点QCD计算已达到电磁(e.m.)修正不可忽视的精度。该修正因会引发紫外(UV)发散以及与无质量光子相关的显著红外(IR)效应,在计算上极具挑战且难以验证。我们利用算符乘积展开精确确定了紫外发散,同时提出在内部光子传播子中引入一个约400 MeV的分离标度Λ,由此计算被拆分为短程部分和紫外有限的长程部分:短程部分的紫外由格点规制、红外由标度Λ规制,长程部分则采用坐标空间方法处理,从而完全避免幂律有限体积效应。为预测长程部分,我们通过前向强子光-光(HLbL)散射振幅表示经紫外规制的HVP电磁修正,并通过色散求和规则将该振幅与γ*γ*融合截面关联起来。我们通过从树级γ*γ*→e+e-截面重现双圈QED真空极化(VP)验证了该关系,随后预测了由γ*γ*→π0过程产生的预期格点QCD被积函数。

英文摘要:

Lattice QCD calculations of the hadronic vacuum polarization (HVP) have reached a precision where the electromagnetic (e.m.) correction can no longer be neglected. This correction is both computationally challenging and hard to validate, as it leads to ultraviolet (UV) divergences and to sizeable infrared (IR) effects associated with the massless photon. While we precisely determine the UV divergence using the operator-product expansion, we propose to introduce a separation scale $Λ\sim400\;$MeV into the internal photon propagator, whereby the calculation splits into a short-distance part, regulated in the UV by the lattice and in the IR by the scale $Λ$, and a UV-finite long-distance part to be treated with coordinate-space methods, thereby avoiding power-law finite-size effects altogether. In order to predict the long-distance part, we express the UV-regulated e.m. correction to the HVP via the forward hadronic light-by-light (HLbL) scattering amplitude and relate the latter via a dispersive sum rule to $γ^*γ^*$ fusion cross-sections. Having tested the relation by reproducing the two-loop QED vacuum polarization (VP) from the tree-level $γ^*γ^*\to e^+e^-$ cross-section, we predict the expected lattice-QCD integrand resulting from the $γ^*γ^*\toπ^0$ process.

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