用于四维 displaced-vertex 搜索的矢量玻色子融合定时参考
Vector-boson-fusion timing references for four-dimensional displaced-vertex searches
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中文总结 AI 辅助
本文提出利用 HL-LHC 的精密定时层，通过矢量玻色子融合产生的希格斯玻色子衰变事例，将 displaced vertex 转化为四维对象，使搜索灵敏度提升约 2.8 倍，拓展了长寿命粒子的搜索范围。
中文摘要 AI 辅助
在径迹探测器内部衰变的长寿命粒子可通过其衰变顶点的空间位移进行搜索，但这类衰变的信号粒子到达时间偏晚，这一特性可通过高亮度大型强子对撞机（HL-LHC）的精密定时层加以利用，而现有的 displaced-vertex 搜索并未用到该特性。本文研究了矢量玻色子融合（VBF）产生的希格斯玻色子的奇特衰变过程，即质子-质子碰撞产生希格斯玻色子伴随两个喷注（$pp\to hjj$），其中希格斯玻色子衰变至一对长寿命标量粒子（$h\to ss$），长寿命标量粒子再衰变至一对底夸克与反底夸克（$s\to b\bar b$）。我们利用 VBF 标记喷注识别硬散射顶点，其瞬时中心径迹由类似 CMS 径迹机定时探测器（MTD）的桶层层定时，该层同时也对 displaced vertex 的中心径迹定时，从而为每个事例定义一个起始时间，将每个 displaced vertex 转化为四维对象。由于快速模拟无法确定重味和仪器 displaced-vertex 本底的绝对归一化，我们采用无需归一化的品质因数作为主要结果：定时增强灵敏度与仅空间灵敏度的比值。在标称工作点，添加定时层可使该比值提升约 2.8 倍，若采用更严格的延迟顶点要求，提升幅度可达一个数量级，这是因为通过空间选择的重味本底在碰撞时间上是瞬时的。该提升效果随衰变长度增加而增大，将灵敏度延伸至长寿命区域，而纯空间搜索在该区域会失去径迹探测器的接受度。在衰变长度 $\tau\beta\bar{\tau}\tau\beta\bar{\tau}\tau\beta\bar{\tau}\tau\beta\bar{\tau}$ 约为 100 毫米时，提升效果基本与质量无关，此时标量质量的变化趋势发生反转：较轻标量在短寿命时因更大的 boost 受益更多，而较重标量在长寿命时受益更多，因为较轻状态会从径迹探测器中泄漏出去。
英文摘要
Long-lived particles that decay inside the tracker are searched for via the spatial displacement of their decay vertices. However, the same decays also arrive late, providing a handle made available by a precision timing layer at the HL-LHC that displaced-vertex searches do not exploit. We study the exotic decay of a Higgs boson produced through vector-boson fusion (VBF), $pp\to hjj$ with $h\to ss$, in which the long-lived scalar decays as $s\to b\bar b$, and use the VBF tag jets to identify the hard-scatter vertex. Its prompt central tracks, timed by a CMS-MTD-like barrel layer that also times the central tracks of the displaced vertex, define a per-event start time, turning each displaced vertex into a four-dimensional object. Because a fast simulation cannot determine the absolute normalization of the heavy-flavour and instrumental displaced-vertex backgrounds, we take as the primary result a normalization-free figure of merit: the ratio of the timing-enhanced sensitivity to the spatial-only sensitivity. Adding the timing layer improves this ratio by a factor of $\simeq2.8$ at a nominal working point and by up to an order of magnitude for a tighter delayed-vertex requirement, because the heavy-flavour background that survives the spatial selection is prompt in collision time. The improvement increases with decay length, extending sensitivity into the long-lifetime regime in which purely spatial searches lose tracker acceptance. The gain is essentially mass-independent at a decay length of $\ctau\simeq100\mm$, where the trend with scalar mass reverses: lighter scalars benefit more at short lifetimes because of their larger boost, whereas heavier scalars benefit more at long lifetimes as the lighter states leak out of the tracker.
