1.2.1 Postulation of the neutrino
Table 1.1 indicates that the neutrinos can only interact via the weak force, since they have zero electric and colour charge. Because they only interact weakly, the experimental detection of neutrinos can only happen indirectly, in processes where charged particles are created. By recording the directly detectable charged particles and applying conservation laws, the properties of the neutrino, which appear as ”missing” energy, momentum, spin, ... can be studied.
It was through this signature of a ”missing” particle, that the neutrino was first postulated in 1930, by the Austrian-born physicist Wolfgang Pauli.
He had been studying the process of beta decay, that describes the radioactive decay of an atomic nucleus (A,Z)to a lighter one(A,Z+1)by the emission of an electron. This essentially boils down to the conversion of a neutron to a proton inside the nucleus:
n→ p+e−. (1.1)
The laws of momentum and energy conservation state that for such a two-body decay, the outgoing particles should have a fixed energy and their sum should be equal to the Q-value of the reaction, which is related to the mass difference between the original nucleus and the reaction products:
Q= MN(A,Z)−MN(A,Z+1)−me−. (1.2) In radioactive decay, the proton will stay confined in the nucleus and the much lighter electron will escape, carrying nearly all available energy. Ex-periments, however, showed that the electron energy did not have a fixed
1.2. STANDARD MODEL NEUTRINOS 7 value Q, but instead was found to have a continuous energy spectrum, with electron energies ranging from zero to just under the Q-value (cf. figure 1.3).
Figure 1.3: The expected and observed energy spectra for the electron inβ-decay.
To explain these results, Pauli suggested that - besides the electron - a second light, but neutral particle was emitted in the reaction. The available energy would thus be distributed over these two particles, leading to a con-tinuous energy spectrum as seen in the experiments. The neutral particle had to be very light, since the maximal detected electron energy did not deviate much from the Q-value calculated from equation 1.2. At that time there were no known particles that could fit these requirements and Pauli stated that a new particle had to be involved. However, it was not Pauli, but the Italian physicist Enrico Fermi who created a first fully comprehensive theory of beta decay and it was Edoardo Amaldi who jokingly baptised the new particle neutrino.
1.2.2 Discovery of the neutrino
After Fermi had developed a mature theory of weak interactions in 1934, it took another two decades until neutrinos were experimentally observed.
The weak interaction theory predicted that antineutrinos would be able to interact with protons and undergo a reaction called inverse beta decay (IBD):
νe+p→e++n. (1.3)
8 CHAPTER 1. NEUTRINOS: FROM STANDARD TO STERILE However, soon after Fermi had published his theory, Bethe and Peierls made a first estimation of the cross section (σ) of this process and they found that σ<10−44cm2[9]. To be able to detect such a feeble reaction, the combination of a highly intense source of neutrinos and a very large amount of detector material would thus be needed.
It was only after the Second World War, when scientists had discovered nuclear fission, that neutrino detection came in reach. The physicists Clyde Cowan and Frederick Reines, who had been working at the famous Los Alamos site, believed that an atomic bomb explosion was the best antineu-trino source for a first attempt to detect these, up to then, elusive particles.
They would detect the IBD process by letting the antineutrinos from the ex-plosion interact with protons of a liquid organic scintillator volume. This scintillator would also serve as detection medium of the charged positron, emitting light when the positron would deposit its energy and annihilate. The plans for their experiment were becoming very concrete, see figure 1.4, when Cowan and Reines realized that they could improve the signal-to-background discrimination by also detecting the neutron from the IBD interaction. This significantly reduced the required flux of antineutrinos, such that a nuclear fission reactor would be a sufficiently intense source of antineutrinos for the experiment to succeed.
This new inspiration made the experiment much more feasible and the set-up was soon installed at the Savannah River reactor, where the first ex-perimental evidence of the neutrino was recorded in 1956 [11].
A few years later, in 1962, L.M. Lederman and his co-workers were able to discover the muon neutrino [12]. They conducted an experiment to see whether neutrinos produced in reactions that involve muons differ from the neutrinos created in association with electrons and positrons. They used the brand new Alternating Gradient Synchrotron (AGS) of the Brookhaven National Laboratory to create pions and study their decay:
π± →µ±+νµ. (1.4)
If the neutrinos produced in the decay were similar to the ones produced in β-decay, it should be possible to convert them into electrons. The exper-iment found, however, that only muons were produced by these neutrinos and therefore proved that the electron neutrino and muon neutrino are dis-tinct particles.
The discovery of the muon neutrino gave an indication that leptons come in flavour doublets, each of which seemed to couple a charged lepton with a certain neutrino. In the development of the Standard Model in the
sub-1.2. STANDARD MODEL NEUTRINOS 9
Figure 1.4: Physicists Cowan and Reines would detect antineutrinos from a nuclear bomb explosion with a liquid scintillator detector. [10]
sequent years, this doublet form was nicely incorporated and generally ac-cepted. This meant that, when the τ-lepton was discovered at the Stanford electron-positron collider by M. Perl and his collaborators in 1977 [13], the community strongly expected the existence of a third neutrino: the ντ. How-ever, it lasted another 23 years, before there was an experimental observation of this neutrino by the DONUT Collaboration at Fermilab [14]. For this ex-periment, the Tevatron accelerator was used as the source of tau neutrinos.
The DONUT detector then recorded the neutrino interactions using a large volume of nuclear emulsion sheets that acted as a tracker for charged par-ticles. The resulting particle tracks were analysed with image recognition techniques to select the tau neutrino candidates.
With the discovery of the third generation, all lepton flavours were now known, as it was determined by the Large Electron-Positron (LEP) collider experiments that there would be only three light neutrinos. This was done by measuring the decay of the Z-boson: the number of decay channels that is available for this boson, influences the lifetime and thus the width of the Z-resonance [15]. Every additional decay channel of the type
Z→ νl+νl (1.5)
10 CHAPTER 1. NEUTRINOS: FROM STANDARD TO STERILE makes the resonance wider by some hundreds of MeV. It can be seen from figure 1.5 that a valueNν=3 best matches the experimental results.
Figure 1.5: The results of the OPAL experiment, conducted at the LEP collider at CERN, show that the Z boson couples to 3 types of neutrino flavours. [16]