标准模型之外新规范玻色子的对撞机信号研究

Extra gauge bosons naturally arise in many new physics model beyond the Standard Model (SM). For instance, extra charged and neutral gauge bosons appear in the new physics with extended gauge structure. Searching for those new gauge bosons is one of the major goals of Large Hadron Collider (LHC). The null results of searching for those heavy gauge bosons at the 8 TeV LHC indicate that new gauge bosons are much heavier than one expect. With the upgrade to the operation energy of 13 TeV the LHC would probe heavy resonances around 3-5 TeV. Once produced those new gauge bosons will soon decay into the SM particles. As the production cross section decreases rapidly with new gauge bosons, it is desirable to keep the signal event as much as possible. Therefore, the hadronical modes are preferred. Unfortunately, the SM particles from heavy gauge boson decays are highly boosted such that all the hadrons in the final state are accumulated in a small angle and form a "fat" jet in the detector. The so-called jet substructure method is used to identify the fat jet. However the method fails again in the case of extremely boosted fat jet. Improving the current jet substructure method is a must for discovering a heavy gauge boson around 5 TeV. In this proposal the principle investigator is going to systematically examine the discovery potential of new gauge bosons in three kinds of new physics models at the LHC and future hadron colliders. The first kind is the so-called U(1)_X model which is a Abelian extension the SM gauge group. The second kind of NP models is the so-called G(221) model with a gauge symmetry of SU(2)_L x SU_R x U(1)_X. The third type of NP models exhibits a gauge symmetry of SU(3)_L x U(1)_x which is often named as G(331) model. The topics are summarized as follows: (1) generate the complete model files of the three kinds of NP models using FeynRule package; (2) perform a comprehensive study of the collider phenomenology of heavy gauge bosons in NP models; (3) explore the potential of the LHC and future hadron collider on discovering those new gauge bosons; (3) improve the jet substructure technique to enhance the tagging efficiency of the W-jet, Z-jet, Top-jet and Higgs-jet; (4) utilize the jet substructure method to measure the spin correlation effect in heavy gauge boson productions and decays; (5) study the possibility of observing heavy gauge boson in the LHT and UED models in the collider signature of two fat-jets plus a large missing transverse momentum. The above study would shed lights on new physics beyond the SM.

新规范玻色子常见于超出标准模型之外的新物理模型。寻找新规范玻色子是当前和未来对撞机实验的核心任务之一。本课题将结合当前国内外新规范玻色子的研究现状，以及申请人的科研基础，围绕大型强子对撞机上新规范玻色子的产生过程来探索新物理存在的证据。此项目的主要研究内容包含：系统研究G(331)模型和G(221)模型中新规范玻色子的对撞机唯象学；精确计算新物理信号产生过程的微扰量子色动力学修正；完善测量顶夸克极化的方法，将之推广到多个不可测量末态粒子（如暗物质候选者）的情况；通过标准模型中W、Z、顶夸克和希格斯玻色子等粒子的强子衰变产物形成的“胖喷注”信号寻找新物理粒子。通过本项目的研究，力求完善改进探测胖喷注的喷注内部子结构的算法，提高胖喷注的探测效率，优化大型强子对撞机和未来强子对撞机寻找新规范玻色子的实验方案，深入了解新物理模型的对称性结构，拓宽对新物理的理解。

众多的超出标准模型之外新物理模型都含有新规范玻色子，寻找新规范玻色子是大型强子对撞机的核心任务。2017年LHC给出新规范玻色子质量的严格限制，使得在LHC上直接探测新规范玻色子信号变得非常困难。因此，本研究项目聚焦于精确检验标准模型，探讨间接探测新规范玻色子的潜力。本研究项目总计发表论文14篇，研究了新规范玻色子可诱导的各种低能精确实验观测量，探讨了欧洲大型强子对撞机上的新物理唯象学。1）通过单顶夸克过程精确检验Wtb耦合顶点，给出新规范玻色子的质量、耦合顶点限制，区分不同新物理模型。2）指出ZZ对的产生过程中Z玻色子的纵向极化分量可以探测Ztt耦合顶点中的轴矢流分量。3）通过3个顶夸克产生过程探测味改变的中性规范玻色子。4）建议通过四顶夸克的产生过程来探测希格斯粒子和顶夸克的耦合顶点，理论计算结果被CMS实验采用并用于实际测量中，首次获得模型无关的实验限制。5）研究了新规范玻色子诱导的暗物质和核子的有效耦合算符，比对暗物质直接探测实验和对撞机探测实验的潜力。工作多次被实验组正面引用。主办/协办国际会议7次，主办/协办国内会议11次，在国际会议上应邀做2次大会报告，多次在国内研究生暑期学校授课。