低维Lorentz流形的次极大共形对称超代数
Submaximal conformal symmetry superalgebras for Lorentzian manifolds of low dimension
摘要：
我们考虑一类三维和四维光滑定向洛伦兹流形，它有一个无处消失的共形Killing向量和一个在共形Killing向量的李代数下不变的闭二形式.不变二形式以特定的方式受到流形的共形几何的约束。在三维空间中，共形基灵向量必须处处是因果的（或者如果不变的二形式相同地消失，则为零）。在四维空间中，如果几何处处为彼得罗夫型N或O，则共形Killing向量处处为零，不变二形式同为零。对于任何这样的几何的共形类，可以指定一个特定的李超代数结构，称为共形对称超代数。这个超代数的偶数部分包含共形Killing向量和常数R-对称，而奇数部分包含（带电）扭量旋量。一个共形对称超代数的最大可能维数仅在局部共形平坦的几何中才能实现。我们精确地确定哪些非平凡的共形类的度量承认一个共形对称超代数与下一个最大的可能尺寸，并计算所有相关的次极大共形对称超代数。在四维空间中，我们还计算了一类不属于彼得罗夫型N或O的Ricci平坦洛伦兹几何的对称超代数。
英文摘要：
We consider a class of smooth oriented Lorentzian manifolds in dimensions three and four which admit a nowhere vanishing conformal Killing vector and a closed two-form that is invariant under the Lie algebra of conformal Killing vectors. The invariant two-form is constrained in a particular way by the conformal geometry of the manifold. In three dimensions, the conformal Killing vector must be everywhere causal (or null if the invariant two-form vanishes identically). In four dimensions, the conformal Killing vector must be everywhere null and the invariant two-form vanishes identically if the geometry is everywhere of Petrov type N or O. To the conformal class of any such geometry, it is possible to assign a particular Lie superalgebra structure, called a conformal symmetry superalgebra. The even part of this superalgebra contains conformal Killing vectors and constant R-symmetries while the odd part contains (charged) twistor spinors. The largest possible dimension of a conformal symmetry superalgebra is realised only for geometries that are locally conformally flat. We determine precisely which non-trivial conformal classes of metrics admit a conformal symmetry superalgebra with the next largest possible dimension, and compute all the associated submaximal conformal symmetry superalgebras. In four dimensions, we also compute symmetry superalgebras for a class of Ricci-flat Lorentzian geometries not of Petrov type N or O which admit a null Killing vector.
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