Symmetry Tests

Studies of various discrete symmetries -- such as parity (P), time reversal (T), CP, and lepton number (L) - have been an important area of my research for some time. In particular, I have focused on the use of atomic nuclei as "laboratories" in which tests of these symmetries can be performed. Considerable experimental effort is being devoted to measurements of parity-violating (PV) interactions of electrons with nucleons and nuclei and various accelerators; studies PV interactions between nucleons in many-body nuclei and few-body systems that probe the underlying weak interaction between quarks; searches for permanent electric dipole moments (EDMs) of the neutron, electron and neutral atoms that would result from T, and P violating interactions involving quarks and leptons; and the searches for neutrinoless double b-decay (0nbb) that would signal violation of lepton number conservation. The theoretical challenge is to develop the appropriate effective theory that characterizes the manifestation of symmetry-violating interactions involving quarks in strongly interacting systems.

Most recently, my collaborators and I have developed effective field theory (EFT) frameworks for the analysis of PV in hadronic systems and 0nbb (see the discussion in the Neutrinos page). In the case of hadronic PV, the EFT framework allows one to relate PV interactions between nucleons and pions to the underlying PV weak interactions of quarks in a systematic and model-independent manner. The development of this framework represents a major advance in the field, which has used a meson-exchange model for theoretical interpretation of hadronic PV experiments for more than two decades. A new program of few-body measurements to be carried out at Los Alamos, NIST, and the Spallation Neutron Source will be guided by this formulation. The theoretical task is now to apply it to calculations of few-body PV observables - a task for which considerable theoretical work remains.

In the case of EDMs, a major task is to properly analyze the way T and P violating interactions between quarks in nuclei give rise to atomic EDMs, since experiments are typically carried out with neutral atoms. In these experiments, one does not directly probe the nuclear EDM, since its effect is partially screened out by the atomic electrons - an effect first noted by Schiff many years ago. Instead, one probes the so-called "Schiff moment" that arises from a subtle interplay between T,P-violating forces inside the nucleus and electrons that penetrate it. My collaborators and I are reformulating Schiff's original treatment of this effect to make for more systematic computations of atomic EDMs and thereby sharpen the theoretical interpretation of the experiments. As discussed in the page on Baryogenesis, these measurements - and their proper theoretical interpretation - will have important implications for our understanding of the baryon asymmetry of the universe. A remaining open problem is to formulate an EFT for T,P-violating nuclear forces in the spirit of our recent work on hadronic PV.