Physics Question #4773

Yevgeniy Perev, a 25 year old male from Staten Island asks on September 23, 2009,

How much energy did the highest energy neutrino detected so far have?

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The answer

Fraser Duncan, Research Scientist, SNOLAB answered on October 6, 2009

At SNOLAB we have been involved primarily in understanding the mass of the neutrino which is a fundamental property rather than it's energy which varies depending on how the neutrino is created. We know that there are 3 types of neutrino and that they have no electric charge. Each neutrino is partnered with a charged particle, the most familiar of which is the electron. The other charged particles are called the muon and the tau which are heavy versions of the electron. Each neutrino is named after its charged partner so we have the electron neutrino, muon neutrino and tau neutrino. Together these 6 particles (the 3 charged particles and 3 neutrinos) are called "leptons" (Greek or "light" as in not heavy). There are also antiparticles for each of these leptons (anti-electron, anti-muon, anti-tau and electron anti-neutrino, muon anti-neutrino and tau anti-neutrino).

We know from the Sudbury Neutrino Observatory (an experiment that ran at SNOLAB until 2006) and other experiments with names like Super Kamiokande and KamLAND that neutrinos have mass by observing what is called neutrino oscillations. Unfortunately neutrino oscillations can only tell you about the differences in masses between the types of neutrino and not the absolute mass of any one of them. You need to directly observe one of the neutrino masses to set the scale and know the masses of the other two. There are experiments that have directly attempted to measure the mass of electron neutrinos (well actually electron anti-neutrinos but the mass of the anti-neutrino is believed to be the same as the mass of the neutrino) but have only put an upper bound on the mass. The best measurements to date sets an upper limit of about 2eV (pronounced "two Eee Vee") or 2 electron volts.

Now coming around to energy. I think everybody hears about Einstein and that famous equation "E = m c2" or "energy equals mass times the speed of light squared". Einstein showed that mass and energy are equivalent.  So when we talk about the mass of the neutrino being less than 2 electron volts we can talk about the equivalent amount of energy. 2 eV is slightly more energy than you would get from a flashlight battery. This is very small. For comparison the mass of an electron (the partner to the electron neutrino) is 510,998.92 eV which is about 1,800 times lighter than the mass of a single hydrogen atom. That makes the electron neutrino *at least* 500,000,000 (five hundred million) times lighter than a hydrogen atom. When we talk about the total energy of a particle (or any object) we have to include both the mass and the energy of motion. Because a neutrino is so very light we can almost always completely ignore the mass and only consider the energy given to it by its motion. The energy of the neutrino will depend on how it was created. In processes such as radioactive decay the energy of the neutrino may be a few million electron volts (written as MeV for "mega electron volts" or in scientific notation as 106 eV). The neutrinos detected in the SNO experiment were created in the Sun and had energies of about 10 MeV. Particle accelerators at particle physics labs such as Fermi Lab in the United States create beams of neutrinos with energies of a few billion electron volts (written GeV or "giga electron volts"). To get to *really* high energies however, you have to look for neutrinos that come from outer space beyond our solar system. Out there in the cosmos are very violent processes such as matter falling into black holes, active galactic nuclei, hypernova and more. We know that some of these processes create very energetic particles - some of which are neutrinos. All together the particles produced by these comic exotica are called "cosmic rays". A simple form of cosmic ray from the Sun (called the solar wind) produce the Northern Lights.

There are dedicated facilities that are looking for these very high energy neutrinos and they require very big detectors. The detectors are big for two reasons.
1) The chance of detecting these neutrinos is very rare.
2) You need a big detector to contain all the energy that is deposited by these neutrinos.

It turns out that water is a good detecting medium which is convenient since the detectors are large. Large means of order 1 km by 1 km by 1 km. In particular there is a detector under construction at the South Pole called Ice Cube which is instrumenting a cubic kilometer of ice as a neutrino detector. Alas I don't know what the most energetic neutrino that has been detected by Ice Cube or similar experiments is but I do know that the goal of Ice Cube is to look for neutrinos with energies up to 1017 eV (that is 100 000 000 000 000 000 or 100 million billion electron volts).

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