Neutrinos have mass but are they their own antimatter partner?

The fortunate thing about international flights in and out of the US is that, likely, it is long enough for me to slip in a quick post. Today’s article is about the search for Majorana neutrinos.

Neutrinos are a class of elementary particles that do not carry a color charge or electric charge, meaning that they do not interact with the strong nuclear force or electromagnetism. Though they are known to possess mass, their masses are so small experimentalists have not yet measured them. We are certain that they have mass because of neutrino oscillation data.

This history of neutrinos is rich. They were first proposed as a solution to the mystery of nuclear beta (β)-decay, a type of radioactive decay. Radioactive decay is the spontaneous and random disintegration of an unstable nucleus in an atom into two or more longer-lived, or more stable, nuclei. A free neutron (which is made up of two down-type quarks, one up-type quark, and lots of gluons holding everything together) is unstable and will eventually undergo radioactive decay. Its half-life is about 15 minutes, meaning that given a pile of free neutrons, roughly half will decay by the end of those 15 minutes. A neutron in a bound system, for example in a nucleus, is much more stable. When a neutron decays, a down quark will become an up-type quark by radiating a (virtual) W- boson. Two up-type quarks and a down-type quark are what make a proton, so when a neutron decays, it turns into a proton and a (virtual) W- boson. Due to conservation of energy, the boson is very restricted into what it can decay; the only choice is an electron and an antineutrino (the antiparticle partner of a neutrino). The image below represents how neutrons decay.

Since neutrinos are so light, and interact very weakly with other matter, when neutron decay was first observed, only the outgoing electron and proton (trapped inside of a nucleus) were ever observed. As electrons were historically called β-rays (β as in the Greek letter beta), this type of process is known as nuclear beta-decay (or β-decay). Observing only the outgoing electron and transmuted atom but not the neutrino caused much confusion at first. The process

Nucleus A → Nucleus B + electron

predicts, by conservation of energy and linear momentum, that the electron carries the same fixed amount of energy in each and every decay. However, outgoing electrons in β-decay do not always have the same energy: very often they come out with little energy, but other times they come out with a lot of energy. The plot below is an example distribution of how often (vertical axis) an electron in β-decay will be emitted carrying away a particular amount of energy (horizontal axis).

Scientists at the time, including Wolfgang Pauli, noted that the distribution was similar to the decay process where a nucleus decays into three particles instead of two:

Nucleus A → Nucleus B + electron + a third particle.

Furthermore, if the third particle had no mass, or at least an immeasurably small mass, then the energy spectrum of nuclear β-decay could be explained. This mysterious third particle is what we now call the neutrino.

One reason for neutrinos being so interesting is that they are chargeless. This is partially why neutrinos interact very weakly with other matter. However, since they carry no charge, they are actually nearly indistinguishable from their antiparticle partners. Antiparticles carry equal but opposite charges of their partners. For example: Antielectrons (or positrons) carry a +1 electric charge whereas the electron carries a -1 electric charge. Antiprotons carry a -1 electric charge were as protons carry a +1 electric charge. Etc. Neutrinos carry zero charge, so the charges of antineutrinos are still zero. Neutrinos and antineutrinos may in fact differ thanks to some charge that they both possess, but this has not been verified experimentally. Hence, it is possible that neutrinos and antineutrinos are actually the same particle. Such particles are called Majorana particles, named after the physicist Ettore Majorana, who first studied the possibility of neutrinos being their own antiparticles.

The Majorana nature of neutrinos is an open question in particle physics. We do not yet know the answer, but this possibility is actively being studied. One consequence of light Majorana neutrinos is the phenomenon called neutrinoless double β-decay (or 0νββ-decay). In the same spirit as nuclear β-decay (discussed above), double β-decay is when two β-decays occur simultaneously, releasing two electrons and two antineutrinos. Double β-decay proceeds through the following diagram (left):

Neutrinoless double β-decay is a special process that can only occur if neutrinos are Majorana. In this case, neutrinos and antineutrinos are the same and we can connect the two outgoing neutrino lines in the double β-decay diagram, as shown above. In 0νββ-decay, a neutrino/antineutrino is exchanged between the two decaying neutrons instead of escaping like the electrons.

Having only four particles in the final state for 0νββ-decay (two protons and two electrons) instead of six in double β-decay (two protons, two electrons, and two neutrinos) has an important effect on the kinematics, or motion, of the electrons, i.e., the energy and momentum distributions. In double β-decay:

Nucleus A → Nucleus B + electron + electrons + neutrino + neutrino

the two protons are so heavy compared to the energy released by the decaying neutrons that there is hardly any energy to give them a kick. So for the most part, the protons remain at rest. The neutrinos and electrons then shoot off in various directions and various energies. In neutrinoless double β-decay:

Nucleus A → Nucleus B + electron + electrons

since the remnant nucleus are still roughly at rest, the electron pair take away all the remaining energy allowed by energy conservation. There are no neutrinos to take energy away from the electrons and broaden their distribution. This difference between ββ-decay and 0νββ-decay is stark, particularly in the likelihood of how often (vertical axis) the electrons in β-decay will be emitted carrying away a particular amount of energy (horizontal axis). As seen below, the electron energy distribution in double β-decay is very wide and is centered around smaller energies, whereas the 0νββ-decay is very narrow and is peaked at the maximum of the 2νββ-decay curve.

Unfortunately, searches for 0νββ-decay have not yielded any evidence for Majorana neutrinos. This could be because neutrinos are not their own antiparticle, in which case we will never observe the decay. Alternatively, it could be the case that current experiments are simply not yet sensitive to how rarely 0νββ-decay occurs. The rate at which the decay occurs is proportional to the mass of the intermediate neutrino: a zero neutrino mass implies a zero 0νββ-decay rate.

Experiments such as KATRIN hope to measure the mass of neutrinos in the next coming years. If a mass measurement is obtained, it would be a very impressive and impacting result. Furthermore, definitive predictions for 0νββ-decay can be made, at which point the current generation of experiments, such as MAJORANA, COURE, and EXO will be in a mad dash for testing whether or not neutrinos are indeed their own antiparticle.

Happy Hunting and Happy Colliding,

Richard Ruiz (@BraveLittleMuon)

PS Much gratitude to Yury Malyshkin, Susanne Mertens, Gastón Moreno, and Martti Nirkko for discussions and inspiration for this post. Cheers!

Update 2015 September 25: Photos of the Cryogenic Underground Observatory for Rare Events (CUORE) experiment have been added. Much appreciate to QD-er Laura Gladstone.

Tags: @bravelittlemuon, CUORE, EXO, KATRIN, majorana, MAJORANA Experiment, majorana fermions, majorana neutrinos, neutrinoless double beta decay, neutrinos