2015 Nobel Prize in Physics: Secret of Neutrinos Uncovered

     The Nobel Prize in physics this year was announced several days ago, awarded to two scientists who had solved the puzzles of neutrinos.

What we know before:

1. In 1930, Wolfgang Pauli first predicted a particle that is extremely light, uncharged, and inactive exist in beta decay due to the conservation of energy.

beta decay equation:  p → n + e+ + ν(neutrino)

2.In 1938, an Italian scientist proposed an elegant theory of neutrino.

3.In 1956, two Americans discovered neutrinos in the detector of nuclear power

plant. The breakthrough proved that neutrinos are real particles.

4.In standard model, it is said that "There are three kinds of neutrinos, and all of  them are massless."

  electron-neutrino (e)

  muon-neutrino (μ)

  tau-neutrino (τ)

Something unknown:

    Neutrinos remained plenty of secrets since 1930, among them, there was a bizarre phenomenon that confused scientists for decades. We can calculate the total neutrinos created from the Sun due to nuclear reactions. However, we only observe 1/3 of the numbers of neutrinos. Where is the missing 2/3 ?

Brief introduction of 2015 Nobel Prize in physics:

There are two major experiments to answer the question.

1.Super-Kamiokande (Japan)

    It is a 40m-in -depth water tank 1000m below earth surface. When neutrinos from cosmic radiation (宇宙射線) pass through the tank, some of them will collide with water, and particular blue light (Cherenkov light) will be emitted. Since electron neutrino and muon neutrino produce light with different intensity, we can differentiate the two particles. In the end, they discover that muon neutrinos from the top of the tank are more than the ones from the bottom of the tank( which means it comes from the other side of earth and travel through the core of earth).However, the number of electron neutrinos from both ways are equal. Therefore, scientists speculate that muon neutrinos transform into tau neutrinos when passing through the earth. Because the muon neutrinos from the bottom of the tank travel through earth in longer distance, more muon neutrinos change into tau neutrinos, and less muon neutrinos are left.

2.Sudburg Neutrino Observatory (SNO, Canada)

    The experimental device is similar to the one above, but the water inside is D2O instead of H2O for a higher possibility of collisions. We know that sun can only create electron neutrinos, and we can calculate the total theoretically. In the result, they detect only 1/3 of electron neutrinos comparing to the calculation. However, if they detect three kinds of neutrinos together, the number matches the calculation. So, the explanation to the bizarre phenomenon mentioned above is that neutrinos do not disappear, they change identities.

What's the influence:

    The two experiments lead to the same results-------three kinds of neutrinos can change identity with one another. If a particle can change identity, it can not be massless. Therefore, neutrinos have mass. This discovery conflict with standard model, so the model must be modified. Besides, neutrinos are the second numerous particle in the universe (photons are the most), so the fact that they have mass will change our understanding of universe significantly.

This is the announcement video:

If you want to read the whole article, please visit :

http://www.nobelprize.org/nobel_prizes/physics/laureates/2015/press.pdf