[论文解读] Implication of the ALEPH 30 GeV dimuon resonance at the LHC

Pyungwon Ko, Jinmian Li+1 more|arXiv (Cornell University)|Oct 24, 2016

Particle physics theoretical and experimental studies被引用 3

一句话总结

本文研究了ALEPH数据中报告的30 GeV二mu子共振的含义，提出了三种简化模型来解释该过量：一种与新矢量律夸克$B$耦合的矢量共振$Z'$，一种具有希格斯门户的$U(1)'$规范模型，以及一种直接与$b\bar{b}$和$\mu^+\mu^-$耦合的共振。尽管这些模型能够拟合分支比，但均无法重现观测到的运动学分布，表明该过量可能并非源自共振衰变。

ABSTRACT

Recent reanalysis of ALEPH data on Z -> b bar{b} + X seems to indicate an existence of the dimuon excess around 30 GeV with a branching fraction for Z -> b bar{b} mu^+ mu^- around 1.1 X 10^{-5}. In this letter, we discuss three different types of simplified models for this possible excess. In the first class of models, we assume a new resonance couples to both b bar{b} and mu^+ mu^-. Within the allowed parameter space for the ALEPH data, this type of models is excluded because of too large Drell-Yan production of dimuon from the b bar{b} collision at the LHC. In the second model, we assume that the 30 GeV excess is a new gauge boson Z' that couples to the SM b and a new vectorlike singlet B quark heavier than Z and not to b bar{b}. Then one can account for the ALEPH data without conflict with the DY constraint. The new vectorlike quark B can be pair produced at the LHC 8/13 TeV by QCD with sigma(BB) ~ O(100-1000) pb, and Bq production rate is sigma(Bq) ~ a few pb which is larger than sigma(Bb) roughly by an order of magnitude. Their signatures at the LHC would be 2b + 4mu, bj + 2mu and 2b + 2mu, respectively, which however might have been excluded already by LHC run I and II data since the multi-muon events have low SM background and are rare at the LHC. In the third model, we consider Z -> Z' phi followed by Z' -> mu^+ mu^- and phi -> b bar{b} assuming that the Higgs field for Z' mass is also charged under the SM U(1)_Y gauge symmetry. In this class of model, we could accommodate the Br(Z -> b bar{b} mu^+ mu^-) ~ 1.1 X 10^{-5} if we assume very large U(1)' charge for the U(1)' breaking Higgs field. Finally, we study various kinematic distributions of muons and b jets in all the three models, and find that none of the models we consider in this paper are not compatible with the kinematic distributions extracted from the ALEPH data.

研究动机与目标

• 通过分支比为$1.1 \times 10^{-5}$的ALEPH $Z \to b\bar{b}\mu^+\mu^-$数据，解释报告的30 GeV二mu子共振。
• 探索可在不违反LHC约束（尤其是Drell-Yan产生）的前提下解释该共振的可行简化模型。
• 测试所提模型与ALEPH数据中观测到的运动学分布（如muon-jet夹角相关性和$p_T$分布）的兼容性。
• 评估新粒子在LHC中的现象学特性，包括产生截面和最终态信号（如$2b+4\mu$和$bj+2\mu$）。

提出的方法

• 构建三种简化模型：(1) 自旋0或自旋1共振耦合至$b\bar{b}$和$\mu^+\mu^-$，(2) 耦合至$b$和新矢量律$B$夸克的$Z'$玻色子，(3) 具有希格斯门户的$U(1)'$规范模型，实现$Z\to Z'\phi\to b\bar{b}\mu^+\mu^-$衰变。
• 对$X \to b\bar{b}\mu^+\mu^-$、Drell-Yan $b\bar{b} \to X \to \mu^+\mu^-$以及$t \to bWX$过程进行现象学分析，以约束参数空间。
• 计算LHC中通过$\sigma(BB) \sim O(100-1000)$ pb和$\sigma(Bq) \sim$ 几pb产生$B$夸克的截面，其衰变链可产生多muon末态。
• 利用自发$U(1)'$对称性自发破缺产生的$Z$-$Z'$-$\phi$顶点，生成$Z \to Z'\phi \to b\bar{b}\mu^+\mu^-$衰变，要求希格斯场具有较大的$U(1)'$电荷。
• 将所有模型的运动学分布（如$\cos\theta_{\mu}^*$、muon-jet夹角和相对$p_T$）与从ALEPH数据中提取的结果进行比较。

实验结果

研究问题

• RQ1 能否通过一种同时耦合至$b\bar{b}$和$\mu^+\mu^-$的新共振来解释ALEPH中30 GeV二mu子过量，且不违反LHC约束？
• RQ2 耦合至新矢量律$B$夸克的$Z'$玻色子能否在避开Drell-Yan约束的前提下重现ALEPH信号？
• RQ3 具有希格斯门户的$U(1)'$规范模型是否可行，以实现$Z \to Z'\phi \to b\bar{b}\mu^+\mu^-$衰变并达到观测到的速率？
• RQ4 三种模型预测的运动学分布是否与ALEPH数据中观测到的角相关性和动量关联一致？

主要发现

• 第一种模型（新共振直接耦合至$b\bar{b}$和$\mu^+\mu^-$）因LHC中来自$b\bar{b}$初态的过量二mu子产生而被排除。
• 第二种模型（涉及$Z'$玻色子和新矢量律$B$夸克）成功重现了ALEPH的分支比，并预测在LHC中$\sigma(BB) \sim O(100-1000)$ pb和$\sigma(Bq) \sim$ 几pb。
• 第三种模型（基于具有希格斯门户的$U(1)'$规范对称性）仅当希格斯场的$U(1)'$电荷极大（$\sim O(10^4)$）时，才能容纳分支比$\sim 1.1 \times 10^{-5}$。
• 三种模型均无法重现ALEPH数据中观测到的运动学分布，如$\cos\theta_{\mu}^*$在$\pm1$附近的峰以及muons与$b$喷流之间的小夹角。
• 模型预测的多muon末态（$2b+4\mu$、$bj+2\mu$）很可能已被LHC运行I和II的数据排除，因为标准模型背景较低且此类事件极为稀有。
• 运动学不兼容性表明，30 GeV二mu子过量可能并非源自共振衰变，而更可能源于非共振贡献，如半轻子$b$衰变。