Tests of fundamental symmetries in atomic nuclei play an important role in uncovering nature's basic forces. The observation of parity-violation in the beta-decay of the ⁶⁰Co nucleus by C.S. Wu and collaborators was a turning point, ultimately leading to the standard model picture of charge changing weak interactions involving only left-handed leptons and quarks. More recently, the observation of a shortage of solar neutrinos in Ray Davis' chlorine detector in the Homestake Mine established the solar neutrino problem whose resolution ultimately lay in the observation of neutrino oscillations. Today, tests of time reversal (T) symmetry and lepton number (LN) in nuclei are addressing key challenges for the new standard model: Where is the CP-violation that is needed to explain the excess of matter over antimatter in the universe? And Are neutrinos their own antiparticles? (see Baryogenesis and Neutrino Properties).

Among the most powerful tools for uncovering new CP-violation is through the search for permanent electric dipole moments (EDMs) of the neutron, electron, and neutral atoms. If an atomic EDM were observed, it would signal the presence of both parity (P) and T-violation. According to the CPT theorem, the observation of this effect would then imply the existence of CP-violation. Standard model predictions for the magnitude of atomic EDMs - based on the CP-violation seen in kaon and B-meson systems -- are well below the sensitivities of present and future experimental EDM searches. So, if an atomic EDM is observed, it would imply either a CP-violating effect due to the so-called "theta term" in the QCD Lagrangian, or new electroweak CP-violation that could be the ingredient needed for successful baryogenesis. A variety of experiments are searching for atomic EDMs in mercury, radium, xenon, and radon at facilities around the world, and these experiments are hoping to increase the sensitivity to yet unseen violations of time-reversal by several orders of magnitude.

NPAC theorists are developing the tools to compute atomic EDMs. Doing so is a challenging but exciting theoretical problem due to the interplay of a variety of physical scales (atomic, nuclear, hadronic, elementary particle), the many-body complexity of atoms and nuclei, and the variety of possible sources of CP-violating effects in the atom. A particularly interesting possibility is that CP-violation arises from interactions involving quarks and gluons in the nucleus. If so, then an observable atomic EDM would arise only due to the finite size of the nucleus through a quantity called the nuclear Schiff moment. We have recently reformulated the appropriate nuclear operator for the Schiff moment and are now pursuing new nuclear computations of the operator matrix element. Along with our collaborators, we are also planning to develop an effective field theory (EFT) framework for characterizing the effects of T- and P-violating interactions within the nucleus that could generate a nuclear Schiff moment.

In addition to analyzing the possible T- and P-violating effects in nuclei, we are also carrying out a program to calculate T-conserving but P-violating effects in few-nucleon systems. Historically, hadronic and nuclear parity-violation (PV) provided our only window on the strangeness-conserving weak interactions of quarks. Today, we believe the standard model description of this interaction, so we can use it to study the way the strong interaction modifies the weak interaction at low-energies. Together with our collaborators, we have developed an EFT framework for characterizing these low-energy hadronic PV effects (see the review article by Ramsey-Musolf and Page and the Effective Field Theory page). During the coming decade, a variety of exquisitely precise experiments aimed at studying these effects will be carried out at the Fundamental Neutron Physics Beamline (FNPB) at the Spallation Neutron Source, National Institutes of Standards and Technology (NIST), and elsewhere. Our work on this problem is aimed at providing theoretical guidance to this experimental program and helping to interpret the results at the most fundamental level.