NOvA Physics Goals / The NuMI Neutrino Beam / Accelerator and NuMI Upgrades / The NOvA Detectors

The NOvA Experiment

NOνA is collaboration of 180 scientists and engineers from 28 institutions which plans to study neutrino oscillations using the existing NuMI neutrino beam at Fermilab. The NOνA experiment is designed to search for oscillations of muon neutrinos to electron neutrinos by comparing the electron neutrino event rate measured at the Fermilab site with the electron neutrino event rate measured at a location just south of International Falls, MN 810 kilometers distant from Fermilab. If oscillations occur, the far site will see the appearance of electrons in the muon neutrino beam produced at Fermilab.

The NuMI neutrino beam starts at Fermilab and points toward the MINOS experiment in northern Minnesota. The NOνA detector site is just off the axis of this beam and to the north of MINOS.

The NOνA project consists of three main elements:

An upgrade of the Fermilab accelerator complex Currently the Fermilab accelerators are capable of delivering 400 kW of beam power to the NuMI beam. As part of NOνA, this will be upgraded to 700 kW. Upgrades beyond 700 kW (to 1.2 MW or 2.3 MW) are being discussed.

A 222 metric-ton near detector will be placed in a new, small, underground cavern adjacent to the existing underground cavern that houses the MINOS experiment. The near detector will measure the electron-neutrino and non-electron neutrino backgrounds to the search for electron neutrino appearance.

A 15 metric-kiloton far detector will be located in a new facility located in Ash River, MN, just south of International Falls, MN and the U.S.-Canada border. The detector will be composed of 385,000 cells of extruded PVC plastic. Each cell is 3.9 cm wide by 6.0 cm deep and is 15.5 meters long. The cells are filled with 3.3 million gallons of liquid scintillator and scintillation light will be guided to APD photo-detectors using wavelength shifting fiber.

NOvA physics goals

Neutrinos are produced and detected via what is known as the weak interaction. Neutrinos have a charge with respect to the weak force, also known as a "flavor", which is labeled e, μ, and τ. Through a quantum mechanical process called superposition, the neutrino states of define flavor are made by combining the neutrinos states that have definite mass. Mathematically, this is expressed as:

The matrix is specified in terms of the sines (s) and cosines (c) of three angles (θ12, θ23, and θ13) and a single phase, δ. The terms containing the phase δ change sign under the exchange of matter and anti-matter and are said to violate CP symmetry.

The flavor content of the neutrino beam used by NOνA. At the source, the neutrino beam is composed almost entirely of muon neutrinos (green). As the beam propagates away from the source, oscillations increase the tau (blue) and electron (red) content of the beam. By roughly 1000 km the beam is entirely composed of electron and tau neutrinos. The relatively pure muon neutrino beam is recovered at 2000 km, and the process repeats. NOνA is located at L=810 km, near the peak in the electron neutrino oscillation.

The fact that the flavor states are composed from a superposition of mass eigenstates leads to a phenomenon called neutrino oscillations. As neutrinos propagate through space, their flavor content changes as a result of a change in the relative phases of the mass states from which they are composed. Thus, a muon neutrino beam produced at Fermilab, may contain electron and tau neutrinos after the beam has been allowed to propagate several kilometers. The effect is illustrated in the figure at the right.

Much is known already about the neutrino states and their mixing. Studies of neutrinos produced in the Sun and on the Earth at nuclear reactors, tell us that θ12 = 35o and m22-m12 = +80 μeV2. Studies of neutrinos produced in the Earth's atmosphere and at particle accelerators (like the MINOS experiment at Fermilab) tell us that θ23 = 45o and |m32-m22| = 2.4 meV2. The remaining unmeasured angle, θ13, is the main target of the NOνA experiment. A glance at the matrix above will reveal that if this remaining angle is not zero, then there is a roll for the CP-violating phase δ to play in the neutrino mixing matrix, and hence, there is a possibility that neutrino mixing violates matter/anti-matter symmetry. This is an exciting possibility as it may be connected to the overall matter/anti-matter symmetry which exists in the universe today. Another unknown in this picture is the ordering of the neutrino mass states. It is possible that m3 is the lightest of the heaviest neutrino state. This question, referred to as the mass hierarchy, can be addressed by NOνA and is key to understanding future neutrino oscillation results and an important feature that distinguishes theoretical models of neutrino mass and mixing.

The sensitivity of the NOνA experiment to numu-nue oscillations as a function of the amplitude of the oscillations (sin213) and the CP-violating phase δ. Blue curves assume the normal mass ordering (m3 heaviest), red curves assume the inverted mass ordering (m3 lightest). From right to left, the curves assume 700 kW, 1.2 MW, and 2.3 MW beam power delivered to the NuMI beam line.

The NuMI Neutrino Beam

Neutrino event rates as a function of energy and off-axis angle. The NOvA detector will be located at 14 mrad which is shown in red.

NOvA will use the existing "Neutrinos at the Main Injector" (NuMI) beam at Fermilab that is currently producing neutrinos for the MINOS experiment. Unlike MINOS, which is located on the centerline of the neutrino beam, NOvA will locate its detector slightly off the centerline. This off-axis location produces a large neutrino flux that peaks at 2 GeV, the energy where oscillations to electron neutrinos is expected to be a maximum. The relative narrowness of the off-axis beam aids in rejecting backgrounds to the electron neutrino appearance search. Currently NuMI is running in a configuration optimized for low energy neutrino production which is optimal for the MINOS experiment. The highest neutrino rates for NOvA are achieved with the distance between the two horns is increased to what is called the medium energy configuration. On-axis, this beam produces a neutrino flux peaked at 7 GeV. Off-axis, the peak location is 2 GeV.

Accelerator and NuMI Upgrades

As part of the NOvA project, the accelerator infrastructure and the NuMI beam line will be upgraded to provide higher neutrino intensities than are currently possible in the NuMI beam. With the conclusion of the collider run at Fermilab, the Recycler ring, which is currently used to store anti-protons, can be converted to pre-injector to the Main Injector. This allows the cycle time to be reduced from 2 seconds down to 1.33 seconds, yielding an 80% increase in beam power while only increasing the number of protons per bunch by 10%. This upgrade requires the construction of new transfer lines and additional RF stations in the Recycler and Main Injector. In addition, the NuMI line must be upgraded to handle the higher proton beam powers. This involves construction of a new target, upgrades to the cooling systems and improvements to the primary proton line.

These upgrades will occur during two shutdowns. During the first shutdown in summer/fall of 2010 the work to convert the Recycler ring to a proton storage ring will be completed and work will begin on the NuMI modifications. During a second shutdown in summer/fall of 2011 the NuMI horns will be reconfigured to achieve the optimal beam setting in preparation for the start of the NOvA physics run.

Future upgrades to the NuMI proton intensity are possible. One possibility, "SNuMI" is to use the Accumulator ring to momentum stack additional proton batches from the Booster ring. This would achieve a beam power of 1.2 MW. Another possibility is to replace the Booster ring with a new proton linac. This "Project X" would increase the beam intensity to 2.3 MW.

The NOvA Detectors

The NOvA detectors are constructed from planes of PVC modules alternating between vertical and horizontal orientations. The far detector is 15.6 x 15.6 meters in size and 78 meters long. In addition to the far detector, NOvA will construct an "Integration Prototype Near Detector" which will operate in the NuMI surface building in 2008-2009. This detector will see the NuMI beam at an angle of 110 milliradians. For the physics run, NOvA will construct a near detector to be placed underground at Fermilab at the same off-axis angle as the far detector. All detector are identical in their construction, but differ in size.

NOvA will use two detectors, one located 810 km from Fermilab on the US-Canada border in northern Minnesota and one located underground at Fermilab in the NuMI tunnels. The far detector will be 15.6 m wide 15.6 m tall and 78 m long, weighing in at 15 kilotons. Due to the much higher rates at the near site, the near detectors can be considerably smaller, 2.9 m x 4.2 m x 14.3 m, weighing in at a total of 222 tons.

The NOvA detectors are assembled from modules of extruded PVC which is loaded with titanium dioxide to enhance reflectivity. The technology is similar to that used commercially for garage doors and fencing. Modules of 32 cells are assembled by gluing two 16-cell extrusion together. Each cell has an interior size of 3.8 cm transverse to the beam direction and 5.9 cm along the direction. The length of the modules ranges from 15.6 m for the far detector to 4.2 m and 2.9 m for the NOvA near detectors.

Each cell is filled with liquid scintillator and scintillation light is collected on a 0.7 mm diameter wave-length shifting fiber. The fiber is looped inside the cell and both ends are routed to a single pixel of an avalanche photo-diode (APD) detector. At the far end of a module, NOvA will collect an average of 28 photo-electrons per muon crossing about the APD threshold of 15 photoelectrons.

The performance of the NOvA design is illustrated in the event displays. Muon-neutrino quasi-elastic events typically show two clear tracks. Electron neutrino events are typified by the presence of a "fuzzy" track with roughly 3 cells hit transversely per plane.

Sample νμ charged-current (left) and νe charged-current (right) events as simulated in the NOvA detectors. In each display, the Monte-Carlo information is shown in the top two panels and the simulated response of the detector is shown in the bottom two panels. The x-vs-z view of the event is shown in the left two panels and the y-vs-z view of the event is shown in the right two panels. In both cases the recoil proton track is seen as a short track and the lepton is seen as a long track. The electron shower is distinguished from the muon track by its "fuzzy" transverse profile.