# Research

My research focuses on the deepest open questions in elementary particle physics and Nature: what is the nature of dark matter, which constitutes 85% of all the matter in our Universe? Does dark matter interact with ordinary matter beyond gravity? What is the origin of electroweak symmetry breaking that is key to understand the microscopic origin of masses? Why is the Higgs boson, the ground-breaking discovery at the Large Hadron Collider (LHC), light?

At the very moment, there is no clear confirmed evidence of the right answers to these tough questions. Instead we have a whole zoo of theories and experimental probes to address them. While it is unlikely that a narrow single route could be the right direction to pursue, it is, however, much more promising to explore a wide range of theoretical and experimental possibilities to leave no stone unturned and dig out a potential clue. More importantly, these questions may be intrinsically intertwined with each other though they are usually studied as separate subjects by different groups of researchers.

I have been actively pursuing a variety of directions to tackle these problems. My research is at the interface between elementary particle physics, astrophysics, and cosmology. In particular, I have always been developing novel ideas and pushing myself to work on new topics beyond my comfort zone, which may not belong to the most popular trends in the community at the moment but are intellectually intriguing and meritorious. My research will also continue to incorporate a model-building approach and a phenomenologically oriented/numerically driven approach, which complement each other.

You could find my publications/preprints here.

**Some Research highlights**

**1. Dark matter dynamics**

*Disspative dark matter and dark disk:*I have proposed novel dark matter paradigms, such as “Partially Interacting Dark Matter” and “Double Disk Dark Matter” models, investigated thoroughly energy dissipation rates relevant for the structure formation in the scenario, as well as observational and experimental prospects, including solar capture and astrometric probe such as Gaia measurements.*QCD axion and axion-like particles:*I have

– proposed a whole new suite of early universe observables for axion dark matter isocurvature, including correlated clock signals in the curvature and isocurvature spectra, and mixed cosmological-collider non-Gaussianities involving both curvature and isocurvature fluctuations;

– worked out a novel astrophysical probe of axion dark matter, searching for axion echos from supernovea graveyard.

– pointed out a new source of axion potential for axion coupling to Abelian gauge field through loops of magnetic monopoles.

– shown that the conventional boundary of inflationary/postinflationary QCD axion could be modified dramatically, taking into account of interactions between PQ field and inflaton.

– constructed several new types of models for axion-like particles and the QCD axion.

– examined implications of new axion models for terrestrial direct detection experiments, muon g-2 measurement, cosmological inflation, and gravitational wave observations.

– studied constraints on axion couplings from cosmic distance measurements.*Observational dark matter probes:*– I have proposed to use a novel dataset from planetary science, collected by Jupiter missions, to probe dark sectors with long-lived dark mediators.

– I have been among the first set of researchers to use the gamma ray data, to set the strongest constraint on a classic supersymmetric dark matter benchmark model, wino dark matter, and ruled out the simplest non-thermal supersymmetric dark matter scenario, the Moroi-Randall model. I have also worked out implications of a general supersymmetric non-thermal history for structure formation in the Universe.

– I have developed for the first time a simple model-independent framework, the non-relativistic effective theory for dark matter direct detection, which has been followed in ~ 300 papers and widely used in recent experimental analyses.

**2. Cosmological probes of fundamental physics**

*Cosmic Higgs dynamics:*I have been developing new classes of cosmological models with novel signatures and potentially interesting connections to particle physics such as Higgs physics and modulating fields. These include

– “Higgscitement” with exponential Higgs particle production and generation of gravitational waves in the early Universe;

– “cosmic Higgs switching” in which an oscillating electroweak phase leads to characteristic oscillations on the primordial spectrum and CMB;

– “cosmic microscope” in which spatial variations of a light field, which is generated during inflation, imprint nonlinear dynamics at tiny scales on large scale fluctuations and provide us a unique probe through non-Gaussianities into the preheating era.*New preheating model:*I have proposed a novel particle production meachanism, “spillway preheating”, that could improve the depletion of the inflaton energy density by up to four orders of magnitude, compared to canonical mechanisms in the literature.

**3. Electroweak and collider physics**

*Stealth Supersymmetry:*I have proposed a new class of supersymmetric model with highly unconventional signals for the LHC, “stealth supersymmetry”, which is now a benchmark model for the LHC searches.*Higgs physics:*I have made important contributions to understand the implications of the Higgs data from LHC for new physics beyond the Standard Model such as natural supersymmetry (1, 2) and scenarios containing vector-like fermions.*Electroweak precision tests and future colliders:*I have carried out the first electroweak precision analyses (1 , 2) for the future Circular Electron-Positron Collider (CEPC) and contributed to the studies of the physics potentials for several other possible future colliders including the muon collider (1, 2), the Compact Linear Collider (CLIC) and the Future Circular Collider (FCC) (1, 2) based in Europe.