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Improved measurement of the shape of the electron
《 Nature》 2011-05-25
Authors：J. J. Hudson; D. M. Kara; I. J. Smallman; B. E. Sauer; M. R. Tarbutt; E. A. Hinds
The electron is predicted to be slightly aspheric1, with a distortion characterized by the electric dipole moment (EDM), de. No experiment has ever detected this deviation. The standard model of particle physics predicts that de is far too small to detect2, being some eleven orders of magnitude smaller than the current experimental sensitivity. However, many extensions to the standard model naturally predict much larger values of de that should be detectable3. This makes the search for the electron EDM a powerful way to search for new physics and constrain the possible extensions. In particular, the popular idea that new supersymmetric particles may exist at masses of a few hundred GeV/c2 (where c is the speed of light) is difficult to reconcile with the absence of an electron EDM at the present limit of sensitivity2, 4. The size of the EDM is also intimately related to the question of why the Universe has so little antimatter. If the reason is that some undiscovered particle interaction5 breaks the symmetry between matter and antimatter, this should result in a measurable EDM in most models of particle physics2. Here we use cold polar molecules to measure the electron EDM at the highest level of precision reported so far, providing a constraint on any possible new interactions. We obtain de = (−2.4 ± 5.7stat ± 1.5syst) × 10−28e cm, where e is the charge on the electron, which sets a new upper limit of |de| < 10.5 × 10−28e cm with 90 per cent confidence. This result, consistent with zero, indicates that the electron is spherical at this improved level of precision. Our measurement of atto-electronvolt energy shifts in a molecule probes new physics at the tera-electronvolt energy scale2.
5月25日发表于《自然》杂志，伦敦帝国理工学院的科学家对电子的形状进行了迄今最为精确的测量，即便在10的负27次方厘米的精度上，电子仍然是完美的球形。如果将电子放大到太阳系的尺度，其圆度的偏差甚至小于人类发丝的宽度。
这与现有理论预测结果并不相同，对于探索宇宙奥秘的科学家来说有着深远影响。根据粒子物理标准模型的预测，电子是非常接近球形的椭圆形状，这种变形轻微到不可测量，因而可忽略不计；而在超对称性理论的框架内，每个粒子都有一个比自己重的“同胞兄弟”，这就要求电子的形状必须为椭圆形。
科学家一直试图对电子的圆度进行更加准确的测量，因为电子出现任何拉伸变形的迹象，都可能预示着重大发现，揭示出自然界中可能存在的未知粒子，甚至解释宇宙为什么由物质而非反物质构成。
伦敦帝国理工学院的物理学家乔尼·哈德森带领的研究小组通过测量电子在电场中的晃动情况来倒推其圆度，晃动越小，电子就越圆。实验中，他们将电子与氟化镱分子“绑定”，然后施加电场，并用激光束进行测量，每次测量耗时仅千分之一秒。
在整整3个月时间内，研究小组对电子共进行了2500万次测量，然后取其平均值加以分析，结果发现，电子在电场中没有任何晃动迹象，这证明电子比此前研究认为的要圆。
哈德森表示，这一结论对超对称性理论提出了挑战，欧洲大型强子对撞机实验中所“发现”的某些超对称粒子可能并不存在。他们将对设备加以改进，希望能在未来数年内对电子进行更精确的测量。“如果我们的精度能进一步提高，就可以很确凿地验证或淘汰超对称性理论了。”
美国密歇根大学的亚伦·莱昂纳特评价说，这项研究工作提供了一个了解“宇宙高能精髓”的窗口，并最终可能有助于解释宇宙的结构。 |
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