Two-hop QA retrieval splits queries into two regimes determined by whether the hop-2 entity is explicitly named in the question (Q-dominant) or only in the bridge passage (B-dominant). We formalize this split with three theorems: (T1) per-query AUC is a monotone function of the cosine separation margin, with R^2 >= 0.90 for six of eight type-encoder pairs; (T2) regime is characterized by two surface-text predicates, with P1 decisive for routing and P2 qualifying the B-dominant case, holding across three encoders and three datasets; and (T3) bridge advantage requires the relation-bearing sentence, not entity name alone, with removal causing an 8.6-14.1 pp performance drop (p < 0.001). Building on this theory, we propose RegimeRouter, a lightweight binary router that selects between question-only and question-plus-relation-sentence retrieval using five text features derived directly from the predicate definitions. Trained on 2WikiMultiHopQA (n = 881, 5-fold cross-fitted) and applied zero-shot to MuSiQue and HotpotQA, RegimeRouter achieves +5.6 pp (p < 0.001), +5.3 pp (p = 0.002), and +1.1 pp (non-significant, no-regret) R@5 improvement, respectively, with artifact-driven.

翻译：双跳问答检索将查询划分为两类状态：取决于跳-2实体是在问题中显式命名（Q主导型），还是仅出现在桥接段落中（B主导型）。我们通过三个定理形式化这一划分：（T1）逐查询AUC是余弦分离间隔的单调函数，在八个类型-编码器对中的六个上R²≥0.90；（T2）状态由两个表层文本谓词刻画，其中P1决定路由方向，P2限定B主导型情况，该结论在三个编码器和三个数据集上均成立；（T3）桥接优势依赖于携带关系的句子而非仅实体名称，移除该句子会导致8.6-14.1个百分点的性能下降（p<0.001）。基于该理论，我们提出RegimeRouter——一种轻量级二元路由机制，利用直接从谓词定义导出的五个文本特征，在仅问题检索与问题加关系句子检索之间进行选择。该路由在2WikiMultiHopQA上训练（n=881，5折交叉拟合），零样本迁移至MuSiQue和HotpotQA后，R@5分别提升+5.6个百分点（p<0.001）、+5.3个百分点（p=0.002）和+1.1个百分点（无统计显著性，无遗憾），且具有可解释性。