Several months ago, I blogged about the experiment being done by a neutrino detector called IceCube at the South Pole to try to determine the nature of cosmic rays. A couple of weeks ago it was announced by the IceCube team that they had detected, for only the second time ever, neutrinos coming from beyond our Solar System.
What are neutrinos?
Neutrinos are amongst the most mysterious and elusive particles in nature. They were first proposed back in 1930 by Wolgang Pauli to solve a problem to do with radioactive beta decay. In radioactive beta decay, a neutron will turn into a proton, spitting out a high energy beta particle (which is actually an electron) from the nucleus. Experiments showed that the energy of these electrons varied, which seemed to violate the principle of the conservation of energy.
Pauli suggested that the energy was actually being shared between two particles, the electron and a new particle which he dubbed the neutrino, which means “little neutral one” in Italian. However, it was not until 1956 that they were first actually detected. The reason they took so long to detect is that they do not interact very much with matter. They have no electrical charge, so do not feel the electromagnetic force. They have next to no mass so do not feel the gravitational force, and they do not feel the strong nuclear force which keeps atomic nuclei together.
The only force they feel is the weak nuclear force. As a consequence of how little neutrinos interact with matter, they can pass through the Earth essentially unimpeded. Every seconds, billions pass through your body without interacting at all with any of the atoms in your body. However, very rarely, a neutrino will directly strike an atomic nucleus, and this collision enables us to detect them. IceCube uses huge columns of very pure water buried below the ice-sheet in Antarctica to shield the neutrino detectors from the background radiation and cosmic rays.
Neutrinos from the Sun
The Sun converts Hydrogen to Helium in its core, in a process known as the proton-proton chain. During this process, in addition to large amounts of energy being produced, neutrinos are generated.
The Sun is the strongest source of neutrinos beyond our terrestrial laboratories, but when physicists first started detecting neutrinos from the Sun in the 1960s they discovered a problem. It seemed that the Sun was only producing one third of the neutrinos that calculations predicted, or at least we were only detecting one third. This became known as the solar neutrino problem, and was not solved until the last 15 years. As this is quite a fascinating and involved story, I will talk about the solar neutrino problem and its resolution in more detail in a future blog.
In February 1987 a star was seen to explode in the nearby Large Magellanic Cloud, a satellite galaxy of the Milky Way. It was seen independently by Ian Shelton and Oscar Duhalde on the same evening whilst both were observing at the Las Campanas Observatory in Chile.
This was the first naked-eye supernova since the early 17th Century, and of course allowed astronomers to study supernovae in detail for the first time. But, 3 hours before anyone had seen the supernova, a burst of neutrinos was detected by 3 separate neutrino detectors, the Kamiokande II detector in Japan, the Irive-Michigan-Brooklyn detector in the USA and the Baksan detector in Russia. These neutrinos (strictly speaking, anti-neutrinos) were produced when the core of the dying star collapsed to form a neutron star. In this process, protons and electrons combine to produce neutrons and anti-neutrinos, in a process known as reverse beta decay. The detection of this burst of anti-neutrinos from supernova 1987A was the first time neutrinos were detected from beyond our Solar System.
The IceCube detections
In the announcement from the IceCube team, they have stated that IceCube has detected 28 cosmic neutrinos to date, but as of yet they do not know from which objects these neutrinos have come. This does, however, bring us a step closer to realising the promise of using neutrinos to better understand the nature of astrophysical objects. In particular, as I described in my previous blog, neutrinos hold the promise of enabling us to understand the origin of high energy cosmic rays. Because the rays themselves are bent by interstellar magnetic fields, tracing their origin is night-on impossible. But, neutrinos are not affected by the magnetic fields, and so should travel to us from their cosmic source in a straight line.
To date, IceCube is the only neutrino detector in the World which is capable of detecting cosmic neutrinos, but with other neutrino detectors being planned and built, we may indeed soon be entering a new era of astronomy.