Research Interests

My research interests:

The standard model of particle physics provides a well-established theory to describe the most basic building blocks of the universe called fundamental particles, governed by four fundamental forces. Over time and through many experiments, it has successfully explained almost all experimental results and precisely predicted a wide variety of phenomena. Despite its success, there are important questions that it does not answer:

  1. Origin of masses: why the Higgs boson mass is stabilized at the electroweak scale, and why the neutrino masses are so tiny? The Higgs self-interaction coupling (the last-missed parameter), is still not determined yet.

  2. Origin of matter: the standard model cannot explain the observed 5% ordinary matter (the matter antimatter asymmetry). It also fails to explain the dark matter and dark energy, around 95% of the matter density in the standard model of cosmology.

  3. Scale of new physics: although many null experimental results, we are always curious to know the scale of new physics. Effective field theory provides a framework to study new physics without new particles even new physics scale is unknown.

To understand our universe and investigate these questions, we rely on two solid theoretical frameworks: quantum field theory, and general relativity. On the other hand, we know physics is essentially an experimental science: a beautify theory, with predictions, needs to be supported by various experimental evidences. My current research interests focus on the effective field theories framework and application at the particle, hadronic, and nuclear levels, to answer questions of the Higgs and neutrino masses and potential, the matter-antimatter asymmetry, and dark matter nature.

1. The energy frontier: Higgs Boson to explain origin of mass and matter

The discovery of the Higgs boson at the large hadron collider (LHC) is a milestone in the history of particle physics. The properties of the Higgs boson, such as the mass, width, and couplings, have been measured very precisely and agree with the predictions from the standard model within experimental error-bar. However, the Higgs self-interaction coupling is still not observed within 5 sigma confidence level at the LHC run II, and thus the shape of the Higgs potential still cannot be determined. The Higgs boson physics still has the following puzzles: (1) why the Higgs boson vacuum expectation value is stabilized at the electroweak scale, instead of the Planck scale? (2) Can the shape of the Higgs potential be different from the prediction from the standard model? (3) Whether a microscopic mechanism similar to superconductor exists in the Higgs mechanism? (4) Is the electroweak phase transition in the early universe the second order (cross-over) or the first order? (5) Can the Higgs boson provide successful baryogenesis mechanism, electroweak baryogenesis and how to test this mechanism? To answer these questions, we rely on the experimental data from colliders, such as large hadron collider, and future lepton colliders. The shape of the Higgs potential, and the phase transition pattern can be verified at the future Higgs factory.

2. The intensity frontier: neutrino to explain origin of mass and matter

The neutrino oscillation, observed by the SuperK and SNO, shows that neutrinos are massive particle, which provides the first firm evidence of beyond the standard model. Currently neutrino physics has vast experimental progress because of many on-going/planned neutrino experiments: NoVA/Dune, T2K/T2HK, etc. However, these experimental results can still not address theoretical puzzles from the tiny neutrino masses: (1) why neutrino masses are far smaller than the proton mass and the electron mass? (2) Are neutrinos Dirac or Majorana-type fermion? (3) Whether the lepton number is violated? (4) Whether there are large CP violation in the neutrino sector? (5) Can the neutrino provide successful baryogenesis mechanism, the leptogenesis, and how to test this mechanism? These question can be answered by the low-energy but high-luminosity experiments, such as neutrino-less double beta decay, neutrino long and short baseline, nuclear electric dipole moment, neutron-antineutron oscillation, etc. These experiments have relatively lower cost and would be one of the important directions in future.

3. The cosmic frontier: dark matter to explain the early universe

The existence of a vast amount of dark matter in the Universe is supported by many astrophysical and cosmological observations. The latest CMB measurements indicate that it constitutes approximately a 27% of the Universe energy density. Given that the standard model of particle physics does not contain any viable candidate to account for it, dark matter, if it is a fundamental particle, can be regarded as one of the clearest hints of new physics. Currently all the evidences of dark matter comes from its gravitational interaction, and we do not know any info on its particle nature. Thus there are lots of questions related to its particle nature: (1) Is the dark matter fundamental particle? (2) If so, what is its properties, such as spin, mass and couplings? (3) Whether dark matter ever has thermal equilibrium with the standard model particles? (4) Whether the dark matter is cold or warm after it freeze-out or freeze-in? (5) How dark matter affects the growth of the cosmological perturbation and the structure formation? To address dark matter’s particle nature, the underground direct detection experiments are running to exclude more and more parameter regions for the wimp particle. On the other hand, the light dark matter, which is easy to escape from the direct detection, provide richer cosmological signatures in the cosmic microwave background and Lyman alpha forests, and so on.

4. Effective Field Theories: interplay among particle, hadronic and nuclear physics

Problems with separated scales often appear in nature, and we intuitively know that it is most convenient to only work with degrees of freedom that are relevant for a particular scale. You never worry about physics of the atoms when designing bridges, nor try to track each and every molecule of a gas through phase space. As modern view of quantum field theory, effective field theory (EFT) is a successful paradigm to understand particle physics at different scales: most of the details of small distance physics are irrelevant for the description of longer-distance phenomena. EFT is a tool which has been applied to, but not limited to, new physics beyond the standard model (SMEFT), chiral symmetry breaking (chiral EFT) and heavy particle EFT at the hadronic and nuclear scale, and effective description of quantum gravity (gravity EFT), etc. The null experimental data at the LHC motivate us adapt the SMEFT-LEFT-ChiralPT-nuclearEFT pipeline to describe new physics effects at various scales: the Higgs - top quark - W/Z boson physics at the electroweak scale, the bottom - charm - kaon physics at the hadronic scale, and the nucleon - pion - nuclear physics at the nuclear scale. The low energy probe of high energy physics gives us an interplay among several different scales and is thus quite suitable to be implemented in the EFT framework.


我的主要研究兴趣如下:

目前已知的物质世界的基础是粒子物理的“标准模型”,包括17种基本粒子,描述自然界中存在的强、弱和电磁相互作用。尽管粒子物理标准模型已经取得了辉煌成功,标准模型仍然存在理论和实验观测上的挑战:

  1. 质量起源问题:标准模型还存在最后一个未测量的参数,就是希格斯自相互作用;希格斯粒子和中微子的质量起源仍然存在疑难;

  2. 物质起源问题:粒子物理标准模型不能解释物质反物质不对称性,也不能解释暗物质的来源。

为了研究这些问题,需要理论的描述,理论基础是量子场论和群论。同时物理是一门实验的科学,优美的理论需要实验的支持,理论的实验验证十分关键。我的研究兴趣如下:

1. 希格斯和中微子的质量起源:希格斯本质和势能,电弱相变和引力波,超对称/复合/额外维

大型强子对撞机(LHC)上希格斯粒子的发现是粒子物理的一个里程碑式的进展。目前,希格斯粒子的质量和性质,已经被测量到很高的精度且和标准模型的预言基本一致。但是,LHC还没有确定希格斯粒子的自相互作用,因此也就不能确定希格斯的势能形式。希格斯物理仍然面临如下疑难:为什么势能最小值稳定在电弱能标? 希格斯的势能形式是否可以和标准模型预言的形状不同? 希格斯势能是否存在类似于超导理论的微观机制? 早期宇宙中的电弱破缺引起的电弱相变是一阶还是二阶相变?

与此同时,近些年来中微子的实验进展十分迅速,中微子振荡的发现确定了中微子的非零质量,这是目前唯一可以确定的对标准模型的扩展。这一进展加深了理论上的疑难,为什么中微子的质量远远小于标准模型的强和电弱能标,中微子是狄拉克或马约拉纳型费米子,是否存在较大的CP和轻子数破坏等。

2. 有效场论理论框架:标准模型有效场论,手征有效场论,非平衡量子场论及宇宙学应用

我们对物质世界的认识本质上是不断对不同能标的有效场论的理解,我们也许永远无法达到一个终极理论。不同能标之间相互退耦合,例如研究宏观力学并不需要量子力学的知识,原子分子并不需要核物理。目前的高能物理实验已经推进到TeV能标,但是并没有观测到新物理的迹象。标准模型有效场论以洛伦兹对称性和规范对称性为基础,系统地参数化了高能标新物理的贡献。在目前无新物理信号的时期,这会是未来高能物理的重要研究方向。

对称性虽然在高能新物理中起主导作用,可是现实世界是不断发生对称性破缺的结果,任何对称性破缺的理论都可以用手征有效场论来描述,例如复合希格斯、电弱手征理论、QCD低能手征微扰论,这些描述加深我们对希格斯粒子的本质、低能介子核子动力学等的理解。同样地,在早期宇宙,各种不同的粒子不断从热平衡中脱离出来,因此非平衡态场论描述和闭时路径积分形式,在早期宇宙演化包括重子轻子生成,非热暗物质等过程中十分重要。

3. 宇宙正物质和暗物质的起源:重子轻子生成,非热暗物质/惰性中微子/轴子,及其宇宙学

通过对星系旋转曲线、子弹星系团、大尺度结构、以及宇宙微波背景辐射的研究,人们已经确证了暗物质的存在(占宇宙能量密度的25%),但是也基本确定粒子物理标准模型并不能解释暗物质。我们并不清楚,暗物质是否是基本粒子,暗物质是否和标准模型热浴有过热平衡,暗物质怎样影响宇宙微扰的增长和大尺度结构的形成?

另一方面,我们的可观测宇宙是正物质主导的,那么反物质到哪里去了? 萨克哈罗夫提出了重子生成机制,为了实现正反物质不对称性需要三个条件:重子数破坏,C和CP破缺,和偏离热平衡。粒子物理标准模型无法提供较大的CP破缺强度,且并不存在明显的偏离热平衡过程,因此为了解释正反物质不对称性,需要对标准模型进行扩展,希格斯粒子和中微子是解决重子生成问题的主流机制。

4. 能量和亮度前沿:对撞机信号,暗物质直接探测/中微子相干散射,无中微子双贝塔衰变

物理学本质上是一门的实验科学,理论物理脱离实验会变成无水之源,因此理论的实验检验至关重要。希格斯的本质和势能形状需要未来对撞机实验的直接验证,电弱相变可以由希格斯工厂的实验结果给出佐证,实验的不同信号可以用来区分不同的新物理模型。这些都是能量前沿实验关心的重点。

在高能对撞机进展平缓的同时,低能高亮度实验由于其低成本开始显现出其探测新物理的优势来。各种暗物质直接探测试验不断刷新其探测截面的下限,中微子相干散射作为背景的重要性凸显出来。新一轮吨级无中微子双贝塔衰变正在开展,这会直接验证中微子的马约拉纳本质以及轻子生成机制,电偶极矩实验可以观测电弱重子生成相关的CP破缺效应。