用中文解释一下：The selection criteria that define the search class C3 reduce the background by introducing a constraint on the signal morphology. In order to illustrate the significance of GW150914 against a background of events with arbitrary shapes, we also show the results of a search that uses the same set of events as the one described above but without this constraint. Specifically, we use only two search classes: the C1 class and the union of C2 and C3 classes (C2 þ C3). In this two-class search the GW150914 event is found in the C2 þ C3 class. The left panel of Fig. 4 shows the C2 þ C3 class results and background. In the background of this class there are four events with ηc ≥ 32.1, yielding a false alarm rate for GW150914 of 1 in 8 400 years. This corresponds to a false alarm probability of 5 × 10−6 equivalent to 4.4σ.

定义搜索类别C3的选择标准通过引入信号形态的约束减少了背景噪声。为了说明GW150914在任意形状事件背景下的显著性，我们还展示了一种使用与上述相同的事件集进行搜索但没有这种约束的搜索结果。具体而言，我们只使用了两个搜索类别：C1类和C2和C3类别的并集（C2+C3）。在这种两类搜索中，GW150914事件被发现在C2+C3类别中。图4的左面板显示了C2+C3类别的结果和背景。在这个类别的背景中，有4个检测统计量ηc≥32.1的事件，导致GW150914的误报率为8,400年1次。这相当于误报概率为5×10^-6，相当于4.4σ。