Professor Messier’s research focuses on the experimental exploration of the basic properties of the neutrino. Much of his research uses neutrino beams produced at the Fermi National Accelerator Laboratory (“Fermilab”) pictured above.

What is a neutrino?

Neutrinos are a group of fundamental particles. They are unique in that they do not feel either of the forces that hold matter together: the electric force which binds electrons to the nucleus inside atoms nor the strong force which binds protons and neutrons together inside the nucleus. Since neutrinos don’t feel either of these forces they are almost oblivious to the presence of matter and can travel enormous distances without bumping into anything. For example, a neutrino could travel through 10 light years of lead before it interacted once. Since they can apparently walk through walls, neutrinos are sometimes called “the ghosts of the universe”.

Of course if neutrinos never interacted, they would be of no consequence. But they do feel two very tiny forces: the “weak” force and gravity. You might not be used to thinking about gravity as a small force -
it certainly doesn’t seem small when you have to carry a bag of groceries up a flight of stairs - but it is. Consider the fact that when you are lifting that bag you are overcoming the gravitation force of the entire Earth and that the Earth is very big. The “weak” force is actually quite a bit stronger than gravity and is responsible for nuclear fusion (which powers the Sun, for example) and nuclear decay. The weak force is what we use to detect neutrinos. The pictures at the right show an example of a neutrino interaction recorded by a detector at Argonne National Lab in 1970. The neutrino comes
in unseen from the left and bumps into a nucleus. The nucleus recoils, ejecting a proton and a particle called a pion. The weak force (represented by the W+ in the schematic) transforms the neutrino into its charged partner in what is essentially a decay process run backward. Since this partner (call a muon) is electrically charged it leaves a visible track in the detector. The job of the detector and the experimenter is to collect as much information about the particles we can see  so that we can accurately infer the properties of the neutrino that we couldn’t see. Its something like watching a game of pool being played with an invisible cue ball.

Neutrino mass and oscillations

In the standard model of particle physics neutrinos are assumed to be exactly massless and attempts to directly measure their mass consistently turned up results that were consistent with zero. However, in 1998 the Super-Kamiokande (SK) experiment in Japan conclusively showed that neutrinos must have some small amount of mass. Rather than try to measure the neutrino mass directly, SK looked for a phenomenon called neutrino oscillation. Neutrino oscillation is a quantum mechanical process by which a neutrino changes its type. In the case of SK, muon-type neutrinos produced in the atmosphere were seen to change to tau-type neutrinos. To understand how this process is related to neutrino mass, we need to bring in an idea from Einstein. According to Einstein’s special relativity, the rate at which a clock ticks depends on how fast the clock is moving through space; the faster the clock moves, the slower the clock ticks. If a clock could get up to light speed, it would appear to stop altogether. The only particles which can get up to light speed are particles that, like light, have zero mass; if a particle has mass, its speed will always be less than light speed and its clock will always be ticking. Oscillations are a way to show that a neutrino’s clock ticks; it starts a muon-type, changes to a tau-type, then back to muon-type etc. etc. its identity swaying back and forth like a pendulum. Since the neutrino’s clock is ticking, it must be traveling at a speed less than light’s speed and therefore it must have a mass. I was lucky back in 1998: this result from SK was the topic of my doctoral thesis.

MINOS and NOvA Experiments

The discovery of neutrino oscillations was an important first step, but it left many questions to be answered. The neutrino masses implied by the oscillation data are tiny compared to the other particles in the Standard Model - the heaviest neutrino seems to be almost a million times lighter than the lightest particle in the Standard Model - and this suggests that neutrinos get their mass via a mechanism which is very different that the other particles in the Standard Model. Also unique to neutrinos, is the pattern of the masses have a very different form from the other particles in the Standard Model. Whereas one can speak of “the mass of an electron”, the neutrinos seem to be quite confused about their identity. For example, the muon neutrino is a combination of roughly equal parts of two neutrinos of definite mass and possibly a small amount of a third. Theorists suspect that there is a relatively simple structure underlying this seemingly complex set of relationships between the electron, muon, and tau neutrinos and their masses and there are several possibilities. The goal of the current generation of neutrino oscillation experiments is to make precise measurements of the mass-splittings and “mixings” (the combinations of masses that compose the electron, muon, and tau neutrinos and the mass states) with the hope that and underlying simpler structure can be found. I work on two of these experiments, MINOS and NOvA.

The MINOS experiment is currently running. It uses a neutrino beam produced at the Fermilab accelerator and two detectors separated by 734 km (456 miles). The large distance between the two detectors is a reflection of the smallness of the neutrino mass. MINOS is designed (primarily) to precisely measure the mass splitting responsible for the oscillations of muon neutrinos first seen by the Super-Kamiokande experiment. The detector, pictured at the left, is constructed from 5 kilo-tons of steel arranged in 8m x 8m (26 ft x 26 ft) octagons. MINOS has recorded a considerable amount of data, and currently has the best measurement in the world of this mass splitting. The experiment finishing running in 2012. A new collaboration, called MINOS+, will continue running the experiment to search for new physics in neutrino oscillations.

The NOvA experiment is a follow-on to the MINOS experiment and re-uses the neutrino beam built for the MINOS experiment. The picture at the right shows the relative locations of the MINOS and NOvA detectors relative to the neutrino beam produced at Fermilab. NOvA’s primary goal is to search for appearance of electron neutrinos in the muon neutrino beam produced at Fermilab. This process holds the key to the remaining questions about oscillations. First, while other oscillation processes seem large, this process appears to be small and if it turns out to be considerably smaller than the other processes it is likely a reflection of an underlying symmetry of the neutrino masses. Second, this process is key to measuring the so-called “neutrino mass hierarchy”. The neutrino mass models constructed by theorists put the neutrinos in either a “normal” ordering or an “inverted” ordering but current data provides no way to choose between the two. NOvA hopes to measure the hierarchy and help theorists home in on the correct model of neutrino mass. Finally, the process of muon neutrino to electron neutrino conversion is sensitive to matter/anti-matter differences in neutrinos (so call “CP” violation). The universe today is completely dominated by matter rather than anti-matter, but from what we know we think the Big Bang should have produced equal amounts of matter and anti-matter. Neutrinos may hold the key to explaining why more matter survived the Big Bang than anti-matter, leaving enough matter left over to make stars, planets, you, and me. NOvA is under construction now and will begin taking data this year in June 2013 while construction continues to completion in 2014.