Double Beta Decay and the Fundamental Symmetries of the Standard Model

The Standard Model of particle physics is one of the most successful theories ever developed. (see the wikipedia entry or for more technical detail, reviews from the Particle Data Group). It describes almost all of the known fundamental particles that make up the universe and governs the way they have interacted with each other since nearly the beginning of time. Tantalizing evidence, however, exists for physics beyond the Standard Model that the theory cannot account for. Dark matter, which seems to control the universe's large-scale structure, is an example of a form of matter whose nature is unknown and therefore not described by the Standard Model.

The known particles and interactions of the Standard Model are constrained by a set of symmetries that the universe has thus far been observed to obey. However, our current universe presents certain puzzles that seem to require that some of these symmetries be violated. One of the biggest unanswered questions in physics is, "Why does the universe contain matter at all?" One of the Fundamental Symmetries of the Standard Model, called charge-conjugation/parity symmetry (or CP for short) insists that matter and anti-matter should be present in nearly equal parts. If this were true, however, then early on in the history of the universe matter and antimatter would have collided and annihilated to produce a universe of pure radiation (photons). The presence of matter in our current universe seems to indicate that, at least at some point in our history, the symmetry between matter and anti-matter was broken. The excess of matter over antimatter early on is reflected in the "primordial baryon-to-photon ratio," measured to be $6.10\pm0.04\times10^{-10}$; that is, there is one excess proton or neuturon for roughly every one billion photons. While this excess is very small, it is still more than a thousand times larger than one would predict from the Standard Model alone.

Neutrinoless Double Beta-Decay ($0\nu\beta\beta$)

One way that CP symmetry may be violated is through one of nature's most elusive particles, the neutrino. It is still unknown whether the anti-neutrino --- the antimatter partner of the neutrino --- even exists. If it does not, then it is also-called Majorana particle, and may have helped produce an abundance of matter in the early universe. Majorana neutrinos would also leave an unmistakable signature in experiments involving huge amounts of nuclear material that search for neutrinoless double-beta decay, a special nuclear decay process. Ordinary beta decay, which is very common, produces both an electron and a neutrino as a nucleus changes its identity. A free neutron, for example, beta decays in about 10 minutes on average to a proton (a hydrogen nucleus), creating an electron and an anti-neutrino in the process. (If that anti-neutrino is the same as a neutrino, then it is a Majorana particle). Neutrinoless double-beta decay, which is much rarer, involves the occurrence of two simultaneous beta decays within a nucleus with no resulting anti-neutrinos, something that is possible only if the neutrino is its own anti-particle --- a Majorana particle ---and can annihilate with itself.

Experiments world-wide have so far been unable to detect any neutrinoless double-beta decay, indicating that larger experiments with tons of nuclear material may be necessary. While these next-generation experiments are being planned and built, theorists are working hard to better understand the complex nuclear processes behind the decay in order to help guide experimentalists and gain as much information as possible about what lies beyond the Standard Model if and when a decay is observed. The extremely difficult calculations needed to reach a better understanding require that physicists in different research areas collaborate in order to understand the physics of very short distances (fundamental particles) and very large distances (big experiments with tons of nuclear material). Our collaboration is working hard to reach this understanding.

Electric Dipole Moments

Another place to look for clues when trying to understand the excess of matter over antimatter is in electric dipole moments (EDMs) of atoms and smaller particles. An electric dipole moment is an asymmetry in the distribution of charge inside objects. Any theory that explains the matter excess (through the violation of CP symmetry) also produces EDMs in elementary particles and the objects they make up. We already know that the Standard Model violates CP symmetry, but at levels that are too low to explain the observed matter excess. There ought to be another source of CP violation that the Standard Model doesn't account for, and it ought to produce measurable EDMs. The experiments that look for EDMs are smaller than their double-beta-decay counterparts, but just as important.

Once again, however, understanding the results of these experiments requires theoretical progress. How does CP violation work its way from elementary particles, such as quarks, to create EDMs for protons, neutrons, nuclei, and atoms? If we are to understand the significance of a measured EDM for the matter-antimatter problem, we will need to understand the particle, nuclear, and atomic physics that plays a role in EDM experiments. Here too, the theorists that make up our collaboration are working together to achieve that understanding.

Other Symmetry Tests (hadronic parity violation, cold neutron decays, etc.)

Even within the Standard Model, some fundamental symmetries are violated. An example is "parity" symmetry, between left and right, or clockwise and counterclockwise. It turns out, for example, that neutrons, which can spin either clockwise or counterclockwise when viewed from head on, must be spinning clockwise in order to undergo beta decay. This violation of parity symmetry is one of the keystones of the Standard Model. The force that act breaks this symmetry is called "weak" because it has much less of an effect than the "strong force" between quarks and gluons, holding atomic nuclei together.

The problem with the weak force is that we don't fully understand how it behaves inside a nucleus, where its effects are small but important in some experiments that seek to reveal physics beyond the Standard Model. The problem doesn't arise in beta decay but it does in experiments that measure the interaction of nuclei with the electrons in atoms. The way parity violation is transmitted from an atom's nucleus to its electrons is similar to the way CP violation in the nucleus produces atomic EDMs. (The P in CP, by way is the same parity we're talking about here). Until we understand the simpler Standard-Model physics that violates parity, it will be hard to have confidence in our predictions of atomic EDMs coming from beyond the Standard Model. Parity symmetry is another area in which our collaboration of theorists is working hard.