How One Detects Neutrinos

Matt Strassler 9/25/11

This layperson’s explanation of to how to detect neutrinos is a companion post to the one that explains how to make a neutrino beam.

Neutrinos are passing through your body in vast numbers at all times. They are flooding out of the sun, from its central furnace, and even if it is nighttime where you are, those neutrinos are passing right through the earth and through your body as though the earth wasn’t even there. Cosmic rays (high energy particles flying in from deep space) often strike atoms in the high atmosphere and produce a number of neutrinos.  These too go right through you.

Almost always.  But a tiny, tiny fraction of these neutrinos do actually hit something.

If a neutrino enters the nucleus of an atom, passes into one of the protons or neutrons, and (roughly speaking) comes very close to a quark (or anti-quark) in the proton or neutron, then there is a moderate chance that the neutrino and quark (or anti-quark) will strike each other.  The same goes for a neutrino hitting an electron on the outskirts of an atom.  But this process doesn’t happen very often, because it  involves the weak nuclear force, and (especially for low-energy neutrinos) the weakness of that force assures such collisions are very rare.

Suppose a neutrino does hit a quark or anti-quark inside an atomic nucleus; what happens next?   If the neutrino has enough energy, it will blow the nucleus apart into its protons and neutrons, and often, if the neutrino’s energy is quite high, also make some pions (another types of hadron: a particle made out of quarks, antiquarks and gluons like the proton and neutron).  The neutrino will continue on undetected, but the recoiling protons, neutrons and pions can be observed as they too run into other atomic nuclei, smashing them apart in turn.  Precisely how observation this is done depends on the individual detector.  [You can read here a bit about detecting particles, including pions and protons, at the LHC.]

Actually there is one other possibility.  Sometimes, during the collision with a quark or anti-quark, the neutrino may change into a charged lepton, such as an electron, a muon or a tau.  [Which type of lepton depends on which type of neutrino was created in the first place, and may even depend on what experiences the neutrino has had before it arrives.   This is a long story which will have to be told elsewhere.] That this is possible is a special feature of the weak nuclear force, which implements this transformation [through the W field, whose ripples are W particles.]  In this case, not only can the spray of protons, neutrons and pions from the original and subsequent disrupted nuclei be detected, but one may also detect the electron, the muon, or the decay products of the tau from the converted neutrino.  [In the case of a tau, its decay products include either an electron, a muon, or a pion and some photons, all of which we can detect.]

Thus, although we cannot detect neutrinos easily and reliably the way we can detect electrons or muons (which hit lots of atoms on their way through matter) or protons and neutrons (which hit lots of atomic nuclei on their way through matter), we can still observe them now and then. If there are enough neutrinos around, such as after a nearby star goes supernova, or inside a neutrino beam,  or even just streaming in from the sun on a daily basis, we can detect those rare neutrinos that happen to hit one [and only one] atomic nucleus inside our detector.  That’s because even a collision with one measly nucleus can create a shower of protons and neutrons and pions (which we can easily detect) and perhaps an electron or a muon (which we can easily detect.)

One way to study neutrinos, then, is to create powerful neutrino beams (such as the one that CERN sent to Gran Sasso for the OPERA experiment to observe), build a detector that can observe protons, neutrons, pions, muons and/or electrons sailing out of a nucleus that has been smashed by a neutrino, and be patient (it took 3 years for the OPERA experiment to detect about 16000 neutrinos, which represents a handful per day.)  There are many other neutrino detectors around the world, using different materials and different strategies. A common technique is to build a huge detector full of water or some other clear liquid, stick it far underground away from ordinary cosmic ray debris, and wait patiently for the occasional neutrino from the sun, or one from a cosmic ray, or a few from a supernova to “make a splash”.  And make a splash they have; there have been several important discoveries involving neutrino physics recently. Perhaps (just perhaps) OPERA has made the greatest of all.