 Photo of the NOvA Far Detector | |
| Organization | NOvA collaboration |
|---|---|
| Location | Ash River, Minnesota, United States |
| Coordinates | 48°22′45″N92°50′1″W / 48.37917°N 92.83361°W /48.37917; -92.83361 |
| Website | novaexperiment |
  Location of NOvA Neutrino Experiment | |
 Related media on Commons | |
TheNOνA (NuMI Off-Axis νe Appearance) experiment is aparticle physics experiment designed to detectneutrinos inFermilab'sNuMI (Neutrinos at the Main Injector)beam. Intended to be the successor toMINOS, NOνA consists of two detectors, one at Fermilab (thenear detector), and one in northern Minnesota (thefar detector). Neutrinos fromNuMI pass through 810 km of Earth to reach the far detector. NOνA's main goal is to observe theoscillation of muon neutrinos to electron neutrinos. The primary physics goals of NOvA are:[1]
Neutrino oscillation is parameterized by thePMNS matrix and the mass squared differences between the neutrino masseigenstates. Assuming that threeflavors of neutrinos participate in neutrino mixing, there are six variables that affect neutrino oscillation: the three anglesθ12,θ23, andθ13, a CP-violating phaseδ, and any two of the three mass squared differences. There is currently no compelling theoretical reason to expect any particular value of, or relationship between, these parameters.
θ23 andθ12 have been measured to be non-zero by several experiments but the most sensitive search for non-zeroθ13 by theChooz collaboration yielded only an upper limit. In 2012,θ13 was measured atDaya Bay to be non-zero to astatistical significance of5.2 σ.[2] The following year,T2K discovered the transition excluding the non-appearance hypothesis with a significance of 7.3 σ.[3] No measurement ofδ has been made. The absolute values of two mass squared differences are known, but because one is very small compared to the other, the ordering of the masses has not been determined.
NOνA is an order of magnitude more sensitive toθ13 than the previous generation of experiments, such asMINOS. It will measure it by searching for the transition in the FermilabNuMI beam. If a non-zero value ofθ13 is resolvable by NOνA, it will be possible to obtain measurements ofδ and the mass ordering by also observing The parameterδ can be measured because it modifies the probabilities of oscillation differently for neutrinos and anti-neutrinos. The mass ordering, similarly, can be determined because the neutrinos pass through the Earth, which, through theMSW effect, modifies the probabilities of oscillation differently for neutrinos and anti-neutrinos.[4]
The neutrino masses and mixing angles are, to the best of our knowledge, fundamental constants of the universe. Measuring them is a basic requirement for our understanding of physics. Knowing the value of theCP violating parameterδ will help us understand why the universe has amatter-antimatter asymmetry. Also, according to theSeesaw mechanism theory, the very small masses of neutrinos may be related to very large masses of particles that we do not yet have the technology to study directly. Neutrino measurements are then an indirect way of studying physics at extremely high energies.[4]
In our current theory of physics, there is no reason why the neutrino mixing angles should have any particular values. And yet, of the three neutrino mixing angles, onlyθ12 has been resolved as being neither maximal or minimal. If the measurements of NOνA and other future experiments continue to showθ23 as maximal andθ13 as minimal, it may suggest some as yet unknown symmetry of nature.[4]
NOνA can potentially resolve the mass hierarchy because it operates at a relatively high energy. Of the experiments currently running it has the broadest scope for making this measurement unambiguously with least dependence on the value ofδ. Many future experiments that seek to make precision measurements of neutrino properties will rely on NOνA's measurement to know how to configure their apparatus for greatest accuracy, and how to interpret their results.
An experiment similar to NOνA isT2K, a neutrino beam experiment in Japan similar to NOνA. Like NOνA, it is intended to measureθ13 andδ. It will have a 295 km baseline and will use lower energy neutrinos than NOνA, about 0.6 GeV. Sincematter effects are less pronounced both at lower energies and shorter baselines, it is unable to resolve the mass ordering for the majority of possible values ofδ.[5]
The interpretation ofNeutrinoless double beta decay experiments will also benefit from knowing the mass ordering, since the mass hierarchy affects the theoretical lifetimes of this process.[4]
Reactor experiments also have the ability to measureθ13. While they cannot measureδ or the mass ordering, their measurement of the mixing angle is not dependent on knowledge of these parameters. The three experiments that have measured a value forθ13, in deceasing order of sensitivity areDaya Bay in China,RENO in South Korea andDouble Chooz in France, which use 1-2 km baselines, optimized for observation of the firstθ13-controlled oscillation maximum.[6]
In addition to its primary physics goals, NOνA will be able to improve upon the measurements of the already measured oscillation parameters. NOνA, likeMINOS, is well suited to detecting muon neutrinos and so will be able to refine our knowledge ofθ23.
The NOνA near detector will be used to conduct measurements of neutrino interactioncross sections which are currently not known to a high degree of precision. Its measurements in this area will complement other similar upcoming experiments, such asMINERνA, which also uses theNuMI beam.[7]
Since it is capable of detectingneutrinos fromgalacticsupernovas, NOνA will form part of theSupernova Early Warning System. Supernova data from NOνA can be correlated with that from Super-Kamiokande to study the matter effects on the oscillation of these neutrinos.[4]
To accomplish its physics goals, NOνA needs to be efficient at detecting electron neutrinos, which are expected to appear in theNuMIbeam (originally made only of muon neutrinos) as the result of neutrino oscillation.
Previous neutrino experiments, such asMINOS, have reduced backgrounds fromcosmic rays by being underground. However, NOνA is on the surface and relies on precise timing information and a well-defined beam energy to reduce spurious background counts. It is situated 810 km from the origin of theNuMI beam and 14 milliradians (12 km) west of the beam's central axis. In this position, it samples a beam that has a much narrower energy distribution than if it were centrally located, further reducing the effect of backgrounds.[4]
The detector is designed as a pair of finely grained liquid scintillator detectors. The near detector is at Fermilab and samples the unoscillated[check spelling] beam. The far detector is in northern Minnesota, and consists of about 500,000 cells, each 4 cm × 6 cm × 16 m, filled with liquidscintillator. Each cell contains a loop of barefiber optic cable to collect the scintillation light, both ends of which lead to anavalanche photodiode for readout.
The near detector has the same general design, but is only about1⁄200 as massive. This 222 ton detector is constructed of 186 planes of scintillator-filled cells (6 blocks of 31 planes) followed by amuon catcher. Although all the planes are identical, the first 6 are used as a veto region; particle showers which begin in them are assumed to not be neutrinos and ignored. The next 108 planes serve as the fiducial region; particle showers beginning in them are neutrino interactions of interest. The final 72 planes are a "shower containment region" which observe the trailing portion of particle showers which began in the fiducial region. Finally, a 1.7 meter long "muon catcher" region is constructed of steel plates interleaved with 10 active planes of liquid scintillator.
The NOνA experiment includes scientists from a large number of institutions. Different institutions take on different tasks. The collaboration, and subgroups thereof, meets regularly via phone for weekly meetings, and in person several times a year. Participating institutions as of May 2024 are:[9]
In late 2007, NOνA passed aDepartment of Energy "Critical Decision 2" review, meaning roughly that its design, cost, schedule, and scientific goals had been approved. This also allowed the project to be included in the Department of Energy congressional budget request. (NOνA still required a "Critical Decision 3" review to begin construction.)
On 21 December 2007, President Bush signed anomnibus spending bill, H.R. 2764, which cut the funding for high energy physics by 88 million dollars from the expected value of 782 million dollars.[10] The budget ofFermilab was cut by 52 million dollars.[11] This bill explicitly stated that "Within funding for Proton Accelerator-Based Physics, no funds are provided for the NOνA activity in Tevatron Complex Improvements."[12][13] So although the NOνA project retained its approval from both the Department of Energy and Fermilab,Congress left NOνA with no funds for the 2008 fiscal year to build its detector, pay its staff, or to continue in the pursuit of scientific results. However, in July 2008, Congress passed, and the President signed, a supplemental budget bill,[14] which included funding for NOνA, allowing the collaboration to resume its work.
The NOνA prototype near detector (Near Detector on Surface, or NDOS) began running at Fermilab in November and registered its first neutrinos from theNuMI beam on 15 December 2010.[15] As a prototype, NDOS served the collaboration well in establishing a use case and suggesting improvements in the design of detector components that were later installed as a near detector at Fermilab, and a far detector at Ash River, MN (48°22′45″N92°49′54″W / 48.37912°N 92.83164°W /48.37912; -92.83164 (NOνA far detector)).
Once construction of the NOvA building was complete, construction of the detector modules began. On 26 July 2012 the first module was laid in place. Placement and gluing of the modules continued over a year until the detector hall was filled.
The first detection occurred on 11 February 2014 and construction completed in September that year. Full operation began in October 2014.[16]