About the Short-Baseline Neutrino Program

The SBN Program at Fermilab will measure properties of neutrinos, specifically the phenomena called neutrino oscillation, where the flavor of a neutrino changes as it moves through space and matter. The rate at which the flavor change happens is related to the masses of the neutrinos. So, studying neutrino oscillations is a way to measure their mass, which is so teeny it is very difficult to measure more directly. Measuring the rate of flavor change is more direct - you count how many neutrinos you have of a given flavor at point A, and do the same some distance away at point B. Thus neutrino oscillation experiments generally have more than one detector, and this is the case for SBN. There is SBND, the near detector, ICARUS, the far detector, and the MicroBooNE detector between. The "short" in Short-Baseline indicates the distance between the detectors - which is not very far, less than one kilometer. By comparison, the detectors in the Long-Baseline neutrino oscillation experiments are separated by hundreds of kilometers.

Neutrinos are all around us but are diffcult to study because they have a very small chance of interacting with ordinary matter - they can pass through the entire earth without disturbing any other particles. Neutrinos produced in the sun, or by cosmic rays hitting the atmosphere, arrive at a fixed rate - just so many per day (month, year). We cannot change the sun's nuclear reactions or how many cosmic rays there are in space! Experiments studying solar or atmospheric neutrinos operate for many years, waiting until a sufficient quantity of neutrino interactions have been gathered. So, another strategy to study properties of neutrinos is to create a LOT of them, in a relatively focused beam, and put your detector in their path. Neutrinos can be created by using intense and/or high energy particles from an accelerator - this method generally results in a beam of nearly 100% muon flavor neutrinos.

The SBN Program will use its detectors to measure the energy spectrum and flavor of the neutrinos produced by the Fermilab Booster Neutrino Beam at each detector location. As the neutrinos travel between detectors, some may morph from a muon neutrino to some other flavor, for example an electron neutrino. The SBN detectors need to be equally capable in detecting both muon and electron flavor neutrinos. When a neutrino interacts with (hits) an atom of matter, various elementary particles are produced by the reaction. These various elementary particles are what we see in our detectors - pions, protons, gammas, electrons and muons for the most part. An electron is the "signature" telling us that the neutrino was an electron-flavor type, and likewise a muon is the signature that the neutrino was a muon-flavor type. But sometimes both neutrino flavors interact in ways that do not produce their signature particle. In these cases the muon neutrino might produce a neutral pion which then decays to two gammas. Some types of detector have difficulty telling the difference between the gammas and the electrons, making it difficult to get an accurate count of how many muon versus electron flavor neutrinos are passing through. In addition, nature may have a trick up her sleeve. There could be a fourth type of neutrino out there, which never interacts with matter, but which does participate in flavor changing with the other three neutrinos. Because it never interacts with matter, it gets called "sterile". If a sterile neutrino exists, there might be an unexpected number of electron flavor neutrinos getting counted - some morphed from muon neutrinos as expected, but others came from the fourth sterile neutrino. Did an experiment count an excess of electron neutrinos because of detector issues, or because there is a fourth neutrino? This is one of the subjects which the SBN Program will explore.

There are many types of detectors used to study neutrinos - they have different strengths and weaknesses, and which type is best for a particular experiment can depend on the energy of the neutrinos being studied and the properties being measured. All three SBN Program detectors are Liquid Argon Time Projection Chambers, or LArTPC. These detectors provide a detailed "picture" of every charged particle exiting the nucleus where the interaction occurred, and are particularly good at distinguishing between gammas and electrons. The SBN Far Detector is the ICARUS T600, the largest LArTPC built to date; the detector operated for some years in Italy as a far detector in a long-baseline experiment. When that experiment completed, the ICARUS detector was taken to CERN and refurbished; in 2017 it was shipped to Fermilab in 2017 to become the SBN far detector. The SBN near detector, SBND, is brand new, and as of 2018 in fabrication; SBND incorporates some technical changes in LArTPC detector design which have developed since the ICARUS T600 was built. Both these detectors are planned to be fully installed and operating in 2020.

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