The NOvA Project

The NOvA Project is an experiment designed by Fermilab, using the NuMI Off-Axis Electron Neutrino Appearance (NOvA) detector facility to examine neutrinos sent from Fermilab’s Neutrinos at the Main Injector (NuMI) neutrino beam. The detector, which is being constructed in Ash River, Minnesota, broke ground May 1st, 2009, and the approximate date for completion is January 2014. The Main Injector at Fermilab, located just outside Batavia, Illinois, creates the NuMI beam.

Purpose

With the NOvA Project research, Fermilab has three main goals it aims to accomplish: To observe the oscillation of muon neutrinos to electron neutrinos; to determine the ordering of the neutrino masses; and to determine the symmetry between matter and antimatter . Accomplishing one of these goals would provide evidence for some neutrino theories as opposed to others, and answer deeper questions about the nature of our universe. Neutrinos, meaning ”little neutral one” in Italian, are a charge-less fundamental particle that interacts minimally with matter. Neutrinos’ limited interaction with any other particles makes them incredibly hard to measure. There are three types of neutrinos, called flavors, which are each related to a different fundamental particle: the electron; the muon; and the tau.

Oscillation of Muon Neutrinos to Electron Neutrinos

There is a large amount of evidence that neutrinos oscillate from one type to another. However, while oscillations have been observed for muon neutrinos to tau neutrinos, the muon neutrino to electron neutrino has never been observed. The goal is to understand the unknown factors that govern neutrino oscillation. Understanding neutrino oscillation is important, because neutrinos were always considered the one fundamental particle that was massless. If neutrinos have mass, it would be a property of all matter.

Ordering of the Neutrino Masses

Originally, the Standard Model considered neutrinos to be massless. However, having observed neutrino oscillation, the explanation required neutrinos to have a mass. Making a neutrino of a specific mass takes a mixture of the three flavors of neutrino. Neutrinos have three masses: one is mainly electron flavor, one is approximately equal parts electron, muon, and tau, and the third is roughly half muon and half tau. However, the mass hierarchy of neutrinos, which is the ordering of the masses of the neutrino types, is currently unknown, as well as the values of the masses of the different neutrino types. Knowing the correct mass hierarchy allows scientists to toss out neutrino theories that require other mass hierarchies.

The closely related question of whether neutrinos are their own antiparticles, possible because the neutrino and antineutrino are neutral particles, could possibly be answered by knowing the mass hierarchy. If they are in fact the same particle, the neutrino-less double beta decay process is allowed. The double beta decay process is when two neutrons in a nucleus decay into two protons and emit two electrons and two anti-neutrinos. If this process occurred without neutrinos, it wouldn’t converse lepton number. However, if neutrinos are their own anti-particle (thus are Majorana particles), and have a non-zero mass, then the process is allowed. Observation of this process is very important to researchers, as it is the only known way to test if the neutrino is its own anti-particle along with its absolute mass scale.

Matter-Antimatter Symmetry

The NOvA Project will also attempt to measure the various oscillation rates for the neutrino and the antineutrino. If the oscillation rates are found to be different, this would suggest the symmetry between neutrinos and anti-neutrinos is broken. Evidence of this asymmetry was seen in the MiniBooNE experiment . Particle-antiparticle asymmetry will help to answer why almost all of the matter we observe in the universe is matter, as opposed to anti-matter. Breaking charge-parity (CP) symmetry is called CP violation. Charge conjugation (C) is the reversal of all internal quantum numbers of a particle, including electric charge. Parity (P) is the reversal of the spatial coordinates of a particle, excluding time. The operators C and P combined were thought to hold all electromagnetic, strong, and weak interactions invariant, though kaon decay has been proven to have a small degree of CP violation. If the oscillation rates for neutrinos and anti-neutrinos are different, this is another example of CP violation. Observing this CP violation could allow physicists to bolster the theory that neutrinos and antineutrinos were a reason that matter “won out” over anti-matter.

Importance

In our observations of the universe, there is vastly more matter than antimatter. However, in the Standard Model of physics, there is no discernable reason why this should be so. The neutrino analysis that Fermilab hopes to accomplish in the NOvA Project all adds together to try and decipher a reason – if neutrino oscillation is understood, it may help find the mass hierarchy. Knowing the mass hierarchy can help determine whether neutrinos are Majorana particles. Knowing that can help determine if there is CP violation, which can help answer the matter-antimatter asymmetry question.

Operation

Since neutrinos have no charge and seldom interact with other particles, to observe them requires an incredibly intense beam focused on a substantial detector for a prolonged period of time. This is why the NOvA experiment apparatus is so enormous. The beam is created at Fermilab, passes through a near-detector at Fermilab, then travels roughly 500 miles to the detector. The trip takes less than three milliseconds. The beam widens from roughly six feet at the beginning to a few miles at the detector. The amounts of each flavor of neutrino are recorded at each detector to look for signs of changing flavor. To create the neutrino beam, protons are fired from the Main Injector into a graphite rod, causing the emitting of all types of particles. Since neutrinos have no charge, they cannot be steered, but pions are charged, and decay into muon neutrinos. Thus magnets are used to steer pions into the direction of the detector. When the pions decay, the muon neutrinos continue in that direction.

The detector in Ash River is 15 metric-kilotons, while the near-detector at Fermilab is 222 metric-tons. The detectors consist of 385,000 cells of plastic PVC filled with liquid scintillator. By using photo-detectors attached to wavelength-shifting fibers, scientists can measure the energy of the particles released when a neutrino strikes an atom in the scintillator. From this they can determine the type of neutrino and its energy.