用$llqq$末态在Pb-Pb超周边碰撞中寻找低质量轻夸克
Search for Low-Mass Leptoquarks using the $llqq$ final states in Pb-Pb Ultra-Peripheral Collisions
摘要：
在回顾了过去的实验和低能味物理的理论要求后，我们认为，由于测量中所做的假设，不能完全排除低质量轻夸克（LQ）的可能性。因此，我们提出在最小模型依赖下（假设耦合常数$\lesssim0.1$较小）寻找Pb-Pb超周边碰撞中低质量细夸克对的产生.存在几个优点：1）高光子通量为低质量LQ提供高的产生速率; 2）背景污染比$p$ - $p$碰撞中的背景污染低得多。分析策略允许轻子夸克衰变到所有可能的轻子加夸克模式。以电荷为$|q|=\frac{4}{3}e$的标量LQ（$S_3$）为例，利用$\sqrt{s}=5.02$ TeV的4pb $^{-1}$ Pb-Pb超周边碰撞数据集和ATLAS探测器在Run 2的性能，在95%置信水平下可以排除100 GeV的质点.所提出的方法也适用于搜索高质量的LQ在$p$ - $p$碰撞，只要LQ对生产机制占主导地位。
英文摘要：
After a review over past experiments and theoretical requirements from low-energy flavour physics, we argue that the possibility of low-mass leptoquarks (LQ) cannot be fully excluded due to the assumptions made in the measurements. Therefore we propose to search for the pair production of low-mass leptoquarks in Pb-Pb ultra-peripheral collisions with the least model dependence (assuming a small coupling constant $\lesssim0.1$). There are a couple of advantages: 1) high photon flux provides high production rate for low mass LQs; 2) the background contamination is much lower than that in $p$-$p$ collisions. The analysis strategy permits a leptoquark to decay to all possible lepton-plus-quark modes. Taking the scalar LQ, $S_3$, with an electric charge $|q|=\frac{4}{3}e$, as example, the mass point of 100 GeV can be excluded at the 95~\% confidence level using a dataset of 4~pb$^{-1}$ Pb-Pb ultra-peripheral collisions at $\sqrt{s}=5.02$ TeV and the performance of the ATLAS detector in Run~2. The proposed method also applies to searching for high-mass LQs in $p$-$p$ collisions as long as the LQ pair production mechanism dominates.
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