Research Interests

Chen-Yu Liu head shot

My research focus is on symmetry tests and fundamental neutron physics; in particular, I specialize in techniques with ultra-cold neutrons (UCN). These neutrons, due to their very low kinetic energies, can be stored in material bottles or magnetic traps for durations of up to hundreds of seconds. The long storage time allows for measurements of many fundamental properties of this charge-neutral hadronic system, with improved precision. Using UCN as the principle tool (but not limited to it), my research group has been developing experimental techniques to measure the neutron β-decay lifetime, the neutron decay asymmetries, and the electric dipole moment of the neutron---with the aim to reduce experimental uncertainties to a level sufficient to challenge the predictions of the electroweak theory and, by scrutinizing the low-energy neutron observables, to search for long-awaited evidence of physics beyond the Standard Model of particle physics.

The techniques with UCN allow experimenters to carry out precision measurements on many fundamental properties of this simple baryonic system. A neutron, comprised of a sea of quarks and gluons, has valence quarks of one "up" and two "down" flavors. So far, physicists have discovered six kinds of quarks: up, down, charm, strange, top, and bottom.  Among them, the up and down quarks are the lightest and thus the most stable and abundant in ordinary matter. All evidence suggests that quarks, together with electrons, are indivisible particles; they are the fundamental building blocks of matter. Quarks bind together to form neutrons and protons; neutrons and protons bind to form various nuclei; nuclei and electrons attract to form atoms; atoms attract to form molecules; molecules interact to create all forms of substances, making the material world around us.  While the basic forces at the molecular and atomic scales are well understood, figuring out how the quarks bind together to form neutrons at the subatomic scale (via Quantum Chromodynamics, which describes how quarks of different types interact to form bound states) has been an important task in nuclear physics.

This and many other interesting questions in modern physics can be addressed by studying neutrons. One question asks why our observed universe is made of matter; in other words, where is all the anti-matter which, according to the best "Big-Bang" models of the universe, should have been created in equal abundance with ordinary matter? In physics, we call this observation the Baryon Asymmetry of the Universe (BAU); the degree of BAU as observed contradicts many symmetry principles of the physical laws that describe how particles interact at the individual level.

If matter and anti-matter were created in equal amounts, then they would have subsequently annihilated to energy (in the form of photons). 

After 13.8 billion years since the Big Bang, enough time for the primordial particles and anti-particles to scatter many times and annihilate, the Universe would have been left with pure light and---nothing else. The fact that our telescopes reveal hundreds of billions of galaxies, each filled with hundreds of billions of burning stars, each surrounded by a handful of planets, of which one sustains life---all of them made of matter---suggests that some fundamental physical process must have violated the symmetry between matter and anti-matter. 

This symmetry violation resulted in a slight imbalance of matter and anti-matter. 

Calculations suggest that this imbalance was only several parts in ten billion, a number minuscule and yet not at all inconsequential. This matter excess seeded our Universe, set forth its evolution, and made the Universe as we observe today.

My research program uses neutrons to test fundamental symmetries and set limits on the possible sizes of symmetry violations. In particular, the search for the neutron electric dipole moment (EDM) is a powerful means to test symmetry violations that are needed to explain the BAU. So far, the elusive electric dipole moments of the neutron and the electron have either ruled out or constrained many theoretical models which suggest possible extensions to the Standard Model. Measurements of the neutron lifetime help to refine our understanding of the nuclear processes that generate the primordial elements in Big-Bang nucleosynthesis; the precise knowledge of the neutron lifetime is needed to make consistency checks between the predictions and the direct astronomical observations. Both the EDM search and the neutron lifetime measurement were discussed in the 2015 Long Range Plan of the US Nuclear Sciences Advisory Committee (NSAC -- the principal board of nuclear scientists which advise national funding agencies); they are a part of the fundamental symmetry program which was ranked high priority and was recommended for continuing investment. 

The goal of physics research, since Galileo's time, has been set to discover the laws governing how fundamental particles in the material world interact and react. With the knowledge of these laws, we hope to understand our past by studying the present in order to predict the future. To this tradition I adhere my work and hope to make some contributions to the collective effort in advancing our knowledge of the physical world.