Neutrino Detectors - Theory

Theory

Neutrinos are omnipresent in nature such that in just one second, tens of billions of them "pass through every square centimetre of our bodies without us ever noticing." Despite this, they are extremely "difficult to detect" and may originate from events in the universe such as "colliding black holes, gamma ray bursts from exploding stars, and violent events at the cores of distant galaxies," according to some speculation by scientists. There are three types of neutrinos or what scientists term "flavors": electron, muon and tau neutrinos, which are named after the type of particle that arises after neutrino collisions; as neutrinos propagate through space, the neutrinos "oscillate between the three available flavours." Neutrinos only have a "smidgen of weight" according to the laws of physics, perhaps less than a "millionth as much as an electron." Neutrinos can interact via the neutral current (involving the exchange of a Z boson) or charged current (involving the exchange of a W boson) weak interactions.

• In a neutral current interaction, the neutrino leaves the detector after having transferred some of its energy and momentum to a target particle. If the target particle is charged and sufficiently light (e.g. an electron), it may be accelerated to a relativistic speed and consequently emit Cherenkov radiation, which can be observed directly. All three neutrino flavors can participate regardless of the neutrino energy. However, no neutrino flavor information is left behind.
• In a charged current interaction, the neutrino transforms into its partner lepton (electron, muon, or tau). However, if the neutrino does not have sufficient energy to create its heavier partner's mass, the charged current interaction is unavailable to it. Solar and reactor neutrinos have enough energy to create electrons. Most accelerator-based neutrino beams can also create muons, and a few can create taus. A detector which can distinguish among these leptons can reveal the flavor of the incident neutrino in a charged current interaction. Because the interaction involves the exchange of a charged boson, the target particle also changes character (e.g., neutron → proton).