Precision measurement of the weak charge of the proton
Precision measurement of the weak charge of the proton
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DOI：
10.1038/s41586-018-0096-0
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发表时间：
2018-05
期刊：
影响因子：
64.8
通讯作者：
D. Androić;D. Armstrong;A. Asaturyan;T. Averett;J. Balewski;K. Bartlett;J. Beaufait;R. Beminiwattha-R.-Bemin
中科院分区：
文献类型：
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作者：
D. Androić;D. Armstrong;A. Asaturyan;T. Averett;J. Balewski;K. Bartlett;J. Beaufait;R. Beminiwattha-R.-Bemin
核物理和粒子物理领域的大型实验计划寻找现有理论无法解释的物理学证据。希格斯玻色子的观测完成了标准模型预测的粒子集，标准模型目前提供了基本粒子和力的最佳描述。然而，该理论的局限性包括未能预测基本参数，如希格斯玻色子的质量，以及无法解释暗物质和能量，引力和宇宙中的物质-反物质不对称性等现象。这些限制激发了人们在后希格斯时代通过在高能加速器上直接产生额外粒子来寻找超越标准模型的物理学，但迄今为止还没有成功。例子包括寻找超对称粒子，它将玻色子（整数自旋粒子）与费米子（半整数自旋粒子）联系起来，以及寻找轻夸克，它将基本夸克与轻子混合在一起。另外，使用精确测量预测的标准模型可观测量的间接搜索允许对标准模型之外的物理进行高度针对性的替代测试，因为它们可以达到超越当今高能加速器直接访问的质量和能量尺度。这种间接搜索的目的是确定质子的弱电荷，这定义了质子通过众所周知的中性电弱力与其他粒子相互作用的强度。由于宇称对称性（在空间反演（x，y，z）→（−x，−y，−z）下的不变性）只在弱相互作用中被破坏，它提供了一个工具来隔离弱相互作用，从而测量质子的弱电荷。在这里，我们报告的值为0.0719 ± 0.0045，其中不确定度是一个标准差，来自我们测量的极化电子在质子上散射的宇称违反不对称性，即-226.5 ± 9.3ppm（不确定度是一个标准差）。我们的质子的弱电荷值与标准模型非常吻合，并对标准模型中没有描述的任何半轻子宇称违反物理学设置了多太电子伏特尺度的约束。我们的研究结果表明，精确的宇称违反测量，使搜索物理超越标准模型，可以竞争的直接搜索在高能加速器和天文观测，可以提供肥沃的方法来探测更高的质量尺度。
Large experimental programmes in the fields of nuclear and particle physics search for evidence of physics beyond that explained by current theories. The observation of the Higgs boson completed the set of particles predicted by the standard model, which currently provides the best description of fundamental particles and forces. However, this theory’s limitations include a failure to predict fundamental parameters, such as the mass of the Higgs boson, and the inability to account for dark matter and energy, gravity, and the matter–antimatter asymmetry in the Universe, among other phenomena. These limitations have inspired searches for physics beyond the standard model in the post-Higgs era through the direct production of additional particles at high-energy accelerators, which have so far been unsuccessful. Examples include searches for supersymmetric particles, which connect bosons (integer-spin particles) with fermions (half-integer-spin particles), and for leptoquarks, which mix the fundamental quarks with leptons. Alternatively, indirect searches using precise measurements of well predicted standard-model observables allow highly targeted alternative tests for physics beyond the standard model because they can reach mass and energy scales beyond those directly accessible by today’s high-energy accelerators. Such an indirect search aims to determine the weak charge of the proton, which defines the strength of the proton’s interaction with other particles via the well known neutral electroweak force. Because parity symmetry (invariance under the spatial inversion (x,y,z) → (−x, −y, −z)) is violated only in the weak interaction, it provides a tool with which to isolate the weak interaction and thus to measure the proton’s weak charge. Here we report the value 0.0719 ± 0.0045, where the uncertainty is one standard deviation, derived from our measured parity-violating asymmetry in the scattering of polarized electrons on protons, which is −226.5 ± 9.3 parts per billion (the uncertainty is one standard deviation). Our value for the proton’s weak charge is in excellent agreement with the standard model and sets multi-teraelectronvolt-scale constraints on any semi-leptonic parity-violating physics not described within the standard model. Our results show that precision parity-violating measurements enable searches for physics beyond the standard model that can compete with direct searches at high-energy accelerators and, together with astronomical observations, can provide fertile approaches to probing higher mass scales.