Physics Division

# Fundamental Symmetries

Argonne’s Physics Division conducts precision experiments aimed at testing the fundamental symmetries inherent in the basic laws of physics. In their core, these experiments search for signatures of phenomena that lie beyond the Standard Model of physics.

While describing many aspects of the universe from its smallest components to the largest structures with amazing precision, the Standard Model still fails to account for some major phenomena, such as the dominance of matter over antimatter in the universe, the apparent observation of effects caused by so called dark matter and dark energy, and the structure of gravity.

Our experiments exploit many different techniques in atomic and nuclear physics. These include the manipulation of neutral atoms and ions to make precision measurements of nuclear decays and to measure their masses and moments. Others relate to the search for exotic decay modes such as neutrinoless double beta decay.

Specific projects include:

#### Electric dipole moments of Radium-225

We are investigating the electric dipole moment of Radium-225, a short-lived isotope of the element radium. According to the Standard Model, this property is expected to be unmeasurably small. However, Beyond Standard Model theories predict this property to be much larger. Our search is based on laser manipulation of neutral Radium-225 atoms to cool them to near absolute zero, and trap them in a laser beam for a sensitive measurement of their electric dipole moment.

#### Beta-neutrino angular correlation

We search for signatures of physics beyond the Standard Model by measuring the relative angle between the electron and the neutrino that results from the beta decay of unstable nuclei. These experiments require precision control of the initial state and sensitive detection of all outgoing particles, since the direction of the unobserved neutrino can only be deduced from the direction and energy of all other particles. We employ ion and atom traps to cool and capture the nuclei and to precicely know where and when the decay happened.

Isotopes that are currently studied are Helium-6, Lithium-8, and Boron-8, all with lifetimes of less than one second. Hence, these isotopes need to be produced at accelerator facilities, captured, and detected within a fraction of a second.

#### Nuclear matrix elements for rare decays

Neutrinoless double beta decay is an hypothesized decay mode in which a nucleus undergoes two simultaneous beta decays, emitting two electrons, but no neutrinos. Such a decay would imply that the neutrinos annihilate, being their own antiparticle and violating lepton number conservation.

Current experimental searches place limits of around 1026 years in the half-life of this decay, which corresponds to a neutrino mass of around 100 meV. However, the nuclear matrix elements connecting the half-life and mass are uncertain by factors of 2-3. The calculations can be constrained by experimental data from various nuclear reactions based on single-nucleon transfer data.