没有更多的墙！1/4 BPS双离子的模性、对称性和穿壁性
No More Walls! A Tale of Modularity, Symmetry, and Wall Crossing for 1/4 BPS Dyons
摘要：
我们确定了一类由$K3 \times T^2$的CHL轨道折叠产生的4d $\mathcal{N}=4$弦真空中的1/4 BPS双子的生成函数。我们表明，所有这样的生成函数服从一些简单的物理一致性条件，往往足以使他们唯一。我们施加的主要约束是没有非物理的边缘稳定性墙：1/4 BPS简并的不连续性只能发生在1/4 BPS双子衰变成1/2 BPS状态对时。在形式上，时空中的这些生成函数可以描述为K3上相应的非线性西格玛模型的世界表上某些超对称指数（缠绕生成）的乘法提升。因此，我们的程序也导致了几乎所有这些缠绕属的明确推导。正如最近的一个猜想所预测的那样，以这种方式挑选出来的世界表指数与一组对月光感兴趣的函数精确匹配。
英文摘要：
We determine the generating functions of 1/4 BPS dyons in a class of 4d $\mathcal{N}=4$ string vacua arising as CHL orbifolds of $K3 \times T^2$, a classification of which has been recently completed. We show that all such generating functions obey some simple physical consistency conditions that are very often sufficient to fix them uniquely. The main constraint we impose is the absence of unphysical walls of marginal stability: discontinuities of 1/4 BPS degeneracies can only occur when 1/4 BPS dyons decay into pairs of 1/2 BPS states. Formally, these generating functions in spacetime can be described as multiplicative lifts of certain supersymmetric indices (twining genera) on the worldsheet of the corresponding nonlinear sigma model on K3. As a consequence, our procedure also leads to an explicit derivation of almost all of these twining genera. The worldsheet indices singled out in this way match precisely a set of functions of interest in moonshine, as predicted by a recent conjecture.
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