Lattice QCD calculations of nucleon form factors are restricted to discrete values of the Euclidean four-momentum transfer. Therefore, the extraction of radii typically relies on parametrizing and fitting the lattice QCD data to obtain its slope close to zero momentum transfer. We investigate a new method, which allows to compute the nucleon radius directly from existing lattice QCD data, without assuming a functional form for the momentum dependence of the underlying form factor. The method is illustrated for the case of the isovector mean square charge radius of the nucleon $\langle r^2_\mathrm{isov} \rangle$ and the quark-connected contributions to $\langle r^2_p\rangle$ and $\langle r^2_n \rangle$ for the proton and neutron, respectively. Computations are performed using a single gauge ensemble with $N_f=2+1+1$ maximally twisted mass clover-improved fermions at physical quark mass and a lattice spacing of $a=0.08\mathrm{fm}$.
The pion-nucleon coupling constants determine the strength of the long-range nuclear forces and play a fundamental part in our understanding of nuclear physics. While the charged- and neutral-pion couplings to protons and neutrons are expected to be very similar, owing to the approximate isospin symmetry of the strong interaction, the different masses of the up- and down-quarks and electromagnetic effects may result in their slightly different values. Despite previous attempts to extract these coupling constants from different systems, our knowledge of their values is still deficient. In this …
On the pi pi continuum in the nucleon form factors and the proton radius puzzle
本文提出了一种改进的确定$ππ$连续谱对核子电磁形状因子的等矢谱函数的贡献的方法。我们的分析包括最新的结果为$ππ\to\bar N N$分波从罗伊-施泰纳方程，一致的π矢量形状因子的输入，并彻底讨论违反同位旋效应和不确定性估计。作为一个应用程序，我们认为$ππ$贡献的等矢量电和磁半径的总和规则，其中，在准确已知的中子电半径相结合，被发现稍微喜欢一个小的质子电荷半径。
Calculations of neutrino-nucleus cross sections begin with the neutrino-nucleon interaction, making the latter critically important to flagship neutrino oscillation experiments, despite limited measurements with poor statistics. Alternatively, lattice QCD (LQCD) can be used to determine these interactions from the Standard Model with quantifiable theoretical uncertainties. Recent LQCD results of $g_{\mathrm{A}}$ are in excellent agreement with data, and results for the (quasi-)elastic nucleon form factors with full uncertainty budgets are expected within a few years. We review the status of th…
We present the first exploratory lattice quantum chromodynamics (QCD) calculation of the polarized gluon Ioffe-time pseudo-distribution in the nucleon. The Ioffe-time pseudo-distribution provides a frame-independent and gauge-invariant framework to determine the gluon helicity in the nucleon from first principles. We employ a high-statistics computation using a $32^3\times 64$ lattice ensemble characterized by a $358$ MeV pion mass and a $0.094$ fm lattice spacing. We establish the pseudo-distribution approach as a feasible method to address the proton spin puzzle with successive improvements …
Nucleon axial-vector form factor and radius from radiatively-corrected antineutrino scattering data
Institute of Theoretical Physics, Chinese Academy of Sciences（中国科学院理论物理研究所）；Theoretical Division, Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室理论部）；Nuclear & Chemical Sciences Division, Lawrence Livermore National Laboratory（劳伦斯利弗莫尔国家实验室核与化学科学部）；Department of Physics, Imperial College London（伦敦帝国理工学院物理系）；Department of Physics and Astronomy, University of Rochester（罗切斯特大学物理与天文学系）；Department of Physics and Astronomy, York University（约克大学物理与天文学系）；Department of Physics and Astronomy, University of Kentucky（肯塔基大学物理与天文学系）；Fermilab（费米实验室）
The nucleon axial coupling, $g_A$, is a fundamental property of protons and neutrons, dictating the strength with which the weak axial current of the Standard Model couples to nucleons, and hence, the lifetime of a free neutron. The prominence of $g_A$ in nuclear physics has made it a benchmark quantity with which to calibrate lattice QCD calculations of nucleon structure and more complex calculations of electroweak matrix elements in one and few nucleon systems. There were a number of significant challenges in determining $g_A$, notably the notorious exponentially-bad signal-to-noise problem …
Using dispersion theory with an improved description of the two-pion continuum based on the precise Roy-Steiner analysis of pion-nucleon scattering, we analyze recent data from electron-proton scattering. This allows for a high-precision determination of the electric and magnetic radius of the proton, $r_E = (0.838^{+0.005}_{-0.004}{}^{+0.004}_{-0.003})\,$fm and $r_M = (0.847\pm{0.004}\pm{0.004})\,$fm, where the first error refers to the fitting procedure using bootstrap and the data while the second one refers to the systematic uncertainty related to the underlying spectral functions.
We extract the proton magnetic radius from the high-precision electron-proton elastic scattering cross section data. Our theoretical framework combines dispersion analysis and chiral effective field theory and implements the dynamics governing the shape of the low-$Q^2$ form factors. It allows us to use data up to $Q^2\sim$ 0.5 GeV$^2$ for constraining the radii and overcomes the difficulties of empirical fits and $Q^2 \rightarrow 0$ extrapolation. We obtain a magnetic radius $r_M^p$ = 0.850 $\pm$0.001 (fit 68%) $\pm$0.010 (theory full range) fm, significantly different from earlier results ob…
We present a determination of the neutral current weak axial charge $G^Z_A(0)=-0.654(3)_{\rm stat}(5)_{\rm sys}$ using the strange quark axial charge $G^s_A(0)$ calculated with lattice QCD. We then perform a phenomenological analysis, where we combine the strange quark electromagnetic form factor from lattice QCD with (anti)neutrino-nucleon scattering differential cross section from MiniBooNE experiments in a momentum transfer region $0.24\lesssim Q^2 \lesssim 0.71$ GeV$^2$ to determine the neutral current weak axial form factor $G^Z_A(Q^2)$ in the range of $0\lesssim Q^2\leq 1$ GeV$^2$. This …
We present results for the isovector electromagnetic form factors of the nucleon computed on the CLS ensembles with $N_f=2+1$ flavors of $\mathcal{O}(a)$-improved Wilson fermions and an $\mathcal{O}(a)$-improved vector current. The analysis includes ensembles with four lattice spacings and pion masses ranging from 130 MeV up to 350 MeV and mainly targets the low-$Q^2$ region. In order to remove any bias from unsuppressed excited-state contributions, we investigate several source-sink separations between 1.0 fm and 1.5 fm and apply the summation method as well as explicit two-state fits. The ch…
A percent-level determination of the nucleon axial coupling from Quantum Chromodynamics
$\textit{axial coupling of the nucleon}$，$g_A$，是它与粒子物理学标准模型的$\textit{weak}$轴向电流的耦合强度，就像电荷是与电磁流的耦合强度一样。这种轴向耦合决定了中子衰变为质子的速率、核子之间长程引力的强度以及核物理的其他特征。在核环境中精确测试标准模型需要对扎根于量子色动力学的核物理学有定量的了解，量子色动力学是标准模型的支柱。$g_A$的重要性使它成为理论上确定的基准量-这是一项困难的任务，因为量子色动力学是非微扰的，排除了已知的分析方法。格点量子色动力学提供了一个严格的，非微扰的量子色动力学定义，可以数值实现。据估计，如果克服两个挑战，到2020年，2%的精度将是可能的：在计算中必须控制激发态对$g_A$的污染，并且必须显著提高统计精度。在这里，我们报告的计算$g_A^{QCD} = 1.271\pm0.013$，使用一种非传统的方法，灵感来自费曼-赫尔曼定理，克服了这些挑战。
In this proceedings, we lay the foundation for computing the contribution of quark chromo-electric dipole moment (qCEDM) operator to the nucleon electric dipole moment. By applying the gradient flow technique, we can parameterize the renormalization and operator mixing issues associated with the qCEDM operator on the lattice. As the nucleon mixing angle $α_N$ is a key component for determining the neutron and proton electric dipole moments induced by the qCEDM operator, we present the formalism and preliminary results for $α_N$ with respect to the gradient flow time $t_f$. The results are comp…
The form factors of the nucleon provide key information on nucleon properties. When confronted with precisely measured observables from experiments, they serve as benchmark quantities for lattice calculations. On the other hand lattice determinations may serve as vital theory input for the interpretation of experiments, e.g. in neutrino-nucleus scattering. I review recent progress in the calculation of nucleon form factors on the lattice and its relevance to future experiments.
In this study, we present a determination of the unpolarized gluon Ioffe-time distribution in the nucleon from a first principles lattice quantum chromodynamics calculation. We carry out the lattice calculation on a $32^3\times 64$ ensemble with a pion mass of $358$ MeV and lattice spacing of $0.094$ fm. We construct the nucleon interpolating fields using the distillation technique, flow the gauge fields using the gradient flow, and solve the summed generalized eigenvalue problem to determine the glounic matrix elements. Combining these techniques allows us to provide a statistically well-cont…
The transversity and the tensor charge of the nucleon, currently under active investigation experimentally and theoretically, are fundamental quantities in hadron physics as well as for our comprehension of the nucleon structure. Some tension between the values of the tensor charge, as computed on the basis of phenomenological extractions and lattice QCD calculations, has been observed. In this letter, by means of an explicit example, we study the role of assumptions, usually adopted in phenomenological parametrizations, and we show that, by relaxing some of them, such a tension could be eased…
The strange and flavor-singlet axial form factors of the nucleon from lattice QCD
Helmholtz-Institut Mainz, Johannes Gutenberg-Universität Mainz（美因茨亥姆霍兹研究所）；GSI Helmholtzzentrum für Schwerionenforschung（GSI重离子研究中心）；Theoretical Physics Department, CERN（欧洲核子研究中心理论物理部）；Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn（波恩大学辐射与核物理亥姆霍兹研究所）
Precision determination of nucleon iso-vector scalar and tensor charges at the physical point
Institute of Theoretical Physics, Chinese Academy of Sciences（中国科学院理论物理研究所）；University of Chinese Academy of Sciences, School of Physical Sciences（中国科学院大学物理科学学院）；Institute of Modern Physics, Chinese Academy of Sciences（中国科学院近代物理研究所）；Department of Physics, University of Maryland, College Park（马里兰大学帕克分校物理系）；CAS Key Laboratory of Theoretical Physics（中国科学院理论物理重点实验室）；School of Fundamental Physics and Mathematical Sciences, Hangzhou Institute for Advanced Study, UCAS（杭州高等研究院基础物理与数学学院，UCAS）；International Centre for Theoretical Physics Asia-Pacific（亚太理论物理中心）
