The NOvA Far Detector (red) and surface building 'placed' inside Soldier Field stadium in Chicago, for a sense of scale of the detector size. The Far Detector measures 51.2 feet wide by 51.2 feet high by 206.7 feet long, or 15.6 meters wide, 15.6 meters high and 63 meters long.
Let me set the scene for you. The NFL season has been cancelled so in an effort to raise money the Chicago Bears have rented out their Soldier Field stadium. The DOE obtained the lease and entrusted a host of physicists to build a particle detector inside the 61,500 seater.
None of this is true of course (well the coming NFL season may be in a lockout) but it gives you a sense of scale of the NOvA experiment if you compare the size of one of its detectors, the Far Detector, to the football stadium.
The NOvA (NuMI Off-axis electron-neutrino [νe]Appearance) experiment is a neutrino oscillation experiment designed to search for muon-neutrino to electron-neutrino oscillations and is the flagship project for the Fermi National Accelerator Laboratory (Fermilab) Intensity Frontier initiative. NOvA is a two-detector experiment with the smallest of the two a 200 ton Near Detector at Fermilab and the second a 15 kiloton Far Detector situated 503 miles, or 810 kilometers away in Ash River, Minnesota.
The experiment proceeds with an intense beam of muon neutrinos from the NuMI (Neutrinos at the Main Injector) beam at Fermilab. The neutrinos are then directed to travel along a trajectory such that they can be observed by the Near and Far Detectors. The neutrinos that reach Ash River, on the Canadian border, are compared to the neutrinos detected by the Near Detector. We know that neutrinos ”oscillate” or change type as they travel which is why NOvA is searching for the number of neutrinos that have oscillated from muon neutrinos to electron neutrinos, hence electron neutrino appearance: essentially measuring how many electron neutrinos have appeared compared to what is detected at the Near Detector.
So what is so great about knowing that, you may ask. Well, in neutrino physics our understanding of neutrino oscillations is governed by the PMNS matrix – a mathematical description of the probability of the different neutrinos changing from one type to another.
There are six different parameters that are derived from the PMNS matrix. Firstly, you have the three mixing angles theta-13, theta-23 and theta-12. These are essentially the proportions of each of the three known types of neutrinos that combine to form each type like Neapolitan ice cream. For example, electron neutrinos make up the largest share of the mixing angle for the electron neutrino. Second, you have a CP-violating phase which is the breaking of particle-antiparticle (charge conjugation – C) and mirror (parity – P) symmetries . Lastly, you have any two of three mass-squared differences which measure the difference between the masses of the neutrino types. The true nature of these parameters is beyond the scope of this introductory blog but, in short, NOvA aims to make the first measurement of the mixing angle theta-13 and push the search for electron neutrino appearance beyond the current scientific community’s limits by more than an order of magnitude. For a non-zero theta-13, it is possible for NOvA to observe CP violation in neutrinos, which will help us understand why the universe has a matter-antimatter asymmetry, and to establish the neutrino mass ordering or ”hierarchy” of neutrino types from lightest to heaviest.
Before NOvA can make any physics measurements it needs two fully assembled and calibrated detectors, which basically means that we understand what our detector is telling us!
The detectors are totally active, segmented and deploy the technology of liquid scintillator (mineral oil plus 5 percent pseudocumene) contained in highly reflective, rigid PVC extrusion cells to detect neutrino interactions.
The charged particles produced by a neutrino interaction inside the detector cause the liquid scintillator to produce light that is captured by optical fibers and carried to light-sensitive detectors at one end of each cell. The Far Detector will consist of about 400,000 1.6 inch by 2.4 inch by 52.5 feet, or 4 centimeter by 6 centimeter by 16 meter, cells that require approximately 3.2 gallons, or 12 million liters, of scintillator and 8,078 miles, or 13,000 kilometers, of .07-centimeter, or 0.7-millimeter, optical fiber. That is roughly equivalent to having enough fiber to feed through the Earth from Fermilab, near Chicago, to Sydney, Australia! The Near Detector will have the same design but will only be about 1/200th as massive.
The Far Detector is under construction and will begin taking data in early 2013. Due to the segmented nature of the detectors, data can be collected as soon as a section of readout has been installed.
Event display of the first NuMI neutrino event observed by NOvA's NDOS detector. The colored squares are a representation of time and location of the hits recorded by the detector cells. Click on image to see a larger version.
The Near Detector eventually will sit underground at Fermilab in the NuMI beamline but a portion of it has been built as a prototype on the surface. This prototype detector, named NDOS, began running at Fermilab in November and registered its first neutrinos from the NuMI beam in December 2010. The full installation of NDOS was completed in March 2011, at which point the detector entered an ongoing commissioning phase. NDOS is fundamental to understanding the fabrication and assembly procedures to be used in the construction of the Near and Far Detectors as well as inferring detector response and fine-tuning data acquisition systems and event reconstruction algorithms.
This is only the beginning for NOvA and future blog entries will aim to expand on some of the details brushed over here (in particular the underlying physics) as well as provide an insight into the daily activities of NOvA physicists. Who knows, maybe sometime soon NOvA will be putting neutrino physics firmly in the spotlight! For now I leave you with a picture of an event topology display showing the first NuMI beam neutrino event observed by NOvA’s NDOS.
NOvA really is a super experiment!!
— Gavin S. Davies
Tags: Chicago Bears, Intensity Frontier, neutrinos, NOvA, Solider Field
