Since the theory of neutrino oscillation, many experiments have been conducted to measure the oscillation parameters (θij and ∆m2ij) in equation 1.11 over a wide range of energy. In this section, we shall review the methods and implications of the results from some of these experiments.
In equation 1.11, we can see that the oscillation probability depends on the factor L/E.
This factor is essential to the detection of neutrino oscillation because we can now place
detectors at different distances with respect to the sources of neutrinos and measure the oscillation probability P (να → νβ) as a function of L/E and the oscillation parameters. In general, the probability can be measured in two ways. First, the appearance method where we measure the number of neutrinos with unexpected flavours, for example the detection of νµ from a pure source of νe. On the other hand, one can also measure the probability by the disappearance method where one compare the expected number of neutrinos in a certain flavour to the actual number of neutrinos in the same flavour. In reality, the choice of method depends on the purity of the source and the capability of the detectors.
The sources of neutrinos can be divided into two categories, namely artificially generated sources and naturally generated sources. One of the major artificial sources of neutrinos is reactor neutrinos where electron antineutrinos are generated by neutron-rich nuclei from nuclear fissions undergoing beta decay. Another artificial source is accelerator neutrinos where one uses particle accelerators to collide nuclei to produce unstable particles that have neutrinos as their daughters. For example, in the MINOS experiment protons were collided with a graphite target to produce charged kaons and pions which have νµ or ¯νµ as their daughters[8]. As for the major natural sources, we have geoneutrinos from radioactive nuclei in the Earth, atmospheric neutrinos from cosmic ray’s interactions with air nuclei and the solar neutrinos from the Sun generated by nuclear fusions.
Below we shall review some of the most important experiments in the history of neutrino.
1.3.1 Solar neutrino experiments
The Homestake experiment, started around 1970, was the first experiment that aimed to measure the flux of solar neutrinos. Based on the inverse beta decay process (37Cl + ν→ e−+37Ar), the experiment was able to measure neutrinos with energy above 0.81 MeV[9].
However, the flux measured by the experiment was much smaller than the flux predicted by the standard solar model and this was known as the solar neutrino problem. The deficit was later also confirmed by two other experiements, namely the Super-Kamiokande (Super-K) and Sudbury Neutrino Observatory (SNO).
The Super-K experiment used a 50 kton water Cherenkov detector utilizing the
charged-current interactions of neutrinos on nuclei
ν + N → l + X (1.13)
Notice that the process is sensitive to all types of neutrinos, although the sensitivity for νµand ντ is much lower than νe. With this setup, they were able to measure electron-like events and muon-like events to determine the ratio of incoming νeand νµand determine if there was a deficit between the predictions and the observations. In 1998, with over 500 days of exposure, they reported a non zero mass different at 90% confidence level assuming a two-flavor (νµ↔ ντ) oscillation model[10].
On the other hand, the SNO experiment used a 1000 tons heavy water (D2O) detector to measure the solar neutrino flux. Unlike Super-K, the use of heavy water allowed the experiment to observe neutrinos through neutral current (NC) reactions
νx+ d→ p + n + νx, (x = e, µ, τ ) (1.14)
which is sensitive to all flavours of neutrinos and charged current (CC) reactions
νe+ d→ e−+ p + p (1.15)
which is sensitive only to electron-type neutrinos. This is crucial to the resolution of the solar neutrino problem for if the deficit was caused by flavour oscillations, then the fluxes between CC and NC should be different. With 306 days of live time, the team reported a significant excess of NC flux over CC and elastic scattering flux in favor of neutrino oscillations. Furthermore, the NC flux (sensitive to all neutrinos) is in agreement with the solar model predictions thus solving the solar neutrino problem.[11]
In summary, both experiments have shown strong evidences of neutrino oscillations.
But this is only the beginning of a new chapter in the history of neutrino oscillation. A natural question to ask is what are the values of the mixing angles and, most importantly, whether the CP violating term is non zero.
1.3.2 Reactor neutrino experiments
Since the discovery of neutrino oscillation from solar neutrino experiments, several reactor neutrino experiments were proposed to measure the mixing angles θij. By placing the neutrino detectors at different distances relative to the nuclear reactors, we can measure the survival probabilities of the electron anti-neutrinos originated from the reactor cores and hence the oscillation parameters (θij and ∆m2ij). Unlike solar neutrinos, reactor neutrinos are much more controllable and the reactions of the reactors are better understood.
In the Kamioka Liquid Scintillator Anti-Neutrino Detector (KamLAND) experiment, a 13-m-diameter spherical detector filled with liquid scintillator was used to detect the neutrinos from the reactors via inverse beta decay. By identifying coincidence of a prompt signal of e+ and a delayed signal of 2.2MeV γ-ray neutron capture on a hydrogen from IBD, they were able to greatly reduce the backgrounds. With nearly 150 days of running time, KamLAND showed disappearances of electron antineutrino with high significance in favor of neutrino oscillation. Furthermore, they successfully measured the mixing angle θ12assuming the three flavour oscillation.[12]
With θ12 measured by KamLand and θ23 measured by solar neutrino experiments, it remained to measure the last, and arguably the most important, mixing angle θ13. The mixing angle θ13 is of great physical interest because the CP violation term (eiδCP) is directly coupled with sin θ13 as shown in equation 1.5, so a nonzero sin θ13 is necessary for CP violation in the lepton sector.
In 1999 the Chooz experiment, a short baseline (L ∼ 1 km) reactor neutrino experiment, found strong evidences that there is no disappearances of electron antineutrino under certain assumptions. [13] In other words, the experiment set a limit on how small the oscillation parameter sin θ13could be. Therefore, to test whether sin θ13̸= 0 would be very challenging and require precise measurements.
Until recently, three other short baseline reactor neutrino experiments were done by three independent groups to measure the mixing angle θ13, namely the Double Chooz experiment (successor of the Chooz experiment), the Reactor Experiment for Neutrino Oscillation (RENO) and the Daya Bay Reactor Neutrino experiment. All of the above experiments have reported strong evidences of neutrino oscillation and a nonzero sin θ13[14, 15].