In the sun, 4 hydrogens are being fused into Helium by means of the proton-proton chain. Neutrinos are important because they allow scientists to peek into the interior of the sun and learn about the processes there. All other information about the sun is from electromagnetic radiation that has to pass through the many layers of the sun interacting and changing along the way before traveling through space to us.
This whole process can take up 10 5 to 10 6 years. However, the neutrinos pass cleanly through the sun in a few seconds without interacting and take a mere 8 minutes to travel from the core where they are created to us. The greatest source of neutrinos happened some 15 billion years ago. The neutrino was first created 10 -4 seconds after the big bang.
Then at only 1 second after the big bang the universe became transparent to the neutrino allowing them to travel freely through space. Since the time of the big bang the universe expanded and cooled and continues to expand to this day. There are about million of these neutrinos per m 3 ; however, these neutrinos have very low energy.
They form a cosmic background radiation that is only 2. By studying these neutrinos scientists are able to learn about the universe when it was forming.
Chaisson, McMillan. Astronomy Today. Scientists are constantly coming up with new and ingenious ways to study neutrinos from space. Neutrino telescopes like Super-Kamiokande in Japan use huge vats of water to detect neutrinos see Figure 3.
The inside of the tank is lined with 11, photo-multiplier tubes that detect Cherenkov light. Cherenkov light is emitted by the neutrinos as they pass through the water at near the speed of light.
Therefore the detector detects the effect of the neutrinos interacting with the water and not the neutrinos themselves. Telescopes like the Super-Kamiokande are deep underground in order to avoid detecting other particles from cosmic rays. Currently, scientists are building a better neutrino telescope by using the clear polar ice as a medium by which to detect the neutrinos.
IceCube is a one-cubic-kilometer new neutrino telescope being built in the South Pole see Figure 4. It will be an array of 80 strings, each string having 60 optical moduals that are desigened to detect the cherenkov light from emitted from muons, which are a byproduct of the neutrinos interacting with the ice.
Scientists are building this new telescope under the South Pole because it allows them to make it incredibly large, to have a very stable place for the detectors, to keep a stable temperature, and to built it deep enough to avoid interference from cosmic rays. It featured a large chlorine-based detector located a mile underground in the Homestake Mine, which provided shielding from cosmic rays.
In , the Davis experiment detected solar neutrinos for the first time, but the results were puzzling. Astrophysicist John Bahcall had calculated the expected flux of neutrinos from the sun—that is, the number of neutrinos that should be detected over a certain area in a certain amount of time.
However, the experiment was only detecting about one-third the number of neutrinos predicted. Slowly, scientists began to suspect that it was actually an issue with the neutrinos. Scientists theorized that neutrinos might oscillate, or change from one type to another, as they travel through space. In , the Super-Kamiokande experiment in Japan first detected atmospheric neutrino oscillations.
Then, in , the Sudbury Neutrino Observatory in Canada announced the first evidence of solar neutrino oscillations, followed by conclusive evidence in After more than 30 years, scientists were able to confirm that neutrinos oscillate, thus solving the solar neutrino problem. The Standard Model—the theoretical model that describes elementary particles and their interactions—does not include a mechanism for neutrinos to have mass. The discovery of neutrino oscillation put a serious crack into an otherwise extremely accurate picture of the subatomic world.
After 60 years of studying neutrinos, several mysteries remain that could provide windows into physics beyond the Standard Model. The neutrino is unique in that it has the potential to be its own antiparticle. If the neutrino is not its own antiparticle, there must be something other than charge that makes antimatter different from matter.
Scientists are trying to determine if the neutrino is its own antiparticle by searching for neutrinoless double beta decay. These experiments look for events in which two neutrons decay into protons at the same time. The standard double beta decay would produce two electrons and two antineutrinos. However, if the neutrino is its own antiparticle, the two antineutrinos could annihilate, and only electrons would come out of the decay.
Several upcoming experiments will look for neutrinoless double beta decay. We know that neutrinos have mass and that the three neutrino mass states differ slightly, but we do not know which is the heaviest and which is the lightest. Scientists are aiming to answer this question through experiments that study neutrinos as they oscillate over long distances. For these experiments, a beam of neutrinos is created at an accelerator and sent through the Earth to far-away detectors.
To try to measure the absolute mass of neutrinos, scientists are returning to the reaction that first signaled the existence of the neutrino—beta decay. The KATRIN experiment in Germany aims to directly measure the mass of the neutrino by studying tritium an isotope of hydrogen that decays through beta decay. To look for evidence of sterile neutrinos, scientists are studying neutrinos as they travel over short distances.
Gran Sasso will also host an upcoming experiment called SOX that will look for sterile neutrinos. Scientists are also using long-baseline experiments to search for something called CP violation.
If equal amounts of matter and antimatter were created in the Big Bang, it all should have annihilated. Because the universe contains matter, something must have led to there being more matter than antimatter. If neutrinos violate CP symmetry, it could help explain why there is more matter. Their first idea for an experiment involved detonating a nuclear bomb. But they realized that a neutrino turning a proton into a neutron meant the neutrino was interacting with the world instead of just passing through.
That particular interaction resulted simultaneously in a neutron and a positron. Unlike a neutrino, a neutron and a positron were both nearly guaranteed to interact with the world sooner or later. The positron would interact sooner. It would hit an electron and annihilate, giving off a gamma ray. The neutron would interact later, when it hit the right nucleus at the right time. It would also give off a gamma ray. So if they built a detector, put it next to a nuclear reactor which would be giving off neutrons, and got flashes five microseconds apart, they were seeing the evidence of neutrinos.
Cowan and Reines built a tank, filled it full of liquid that would send off flashes of light when disturbed, and surrounded the tank with light detectors. The flashes came as predicted, five microseconds apart.
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