SNO is a Water Cerenkov Detector, but neutrinos interact in its heavy water in two additional ways not possible in ordinary H2O.

The first of these reactions is the absorption of a neutrino by the additional neutron in each deuteron. When the neutrino is absorbed by the neutron, the neutron emits an electron and becomes a proton. As in the electron scattering reaction, the electron moves through the heavy water fast enough that it creates detectable Cerenkov light. This neutrino absorption reaction can only happen if the neutrino is an electron neutrino---a muon neutrino or a tau neutrino interacting in this way must create either a muon or a tau rather than an electron, and solar neutrinos do not have nearly enough energy to do this. The neutrino absorbtion reaction  counts electron neutrinos, the neutrinos which the Sun produces in its central fusion reactions.

ne + d ----> p + p + e-

The second of SNO's neutrino-deuteron reactions is the deutron breakup reaction. Here, the neutrino splits the deuteron, liberating the neutron from the proton. In this reaction, no new charged particle is created and it occurs with equal probability for all neutrino flavors---a muon or tau neutrino will break the deuteron apart as easily as will a electron neutrino. The free neutron cannot by itself create Cerenkov light, but after scattering off of the nuclei in the heavy water it is eventually captured by another deuteron, creating a tritium nucleus and releasing a high energy gamma ray. This gamma ray then scatters an electron in the heavy water and it is this secondary electron which creates the Cerenkov light detected by the PMTs.

nx + d ----> p + n + nx

Of course, in addition to these two SNO-specific reactions, the electron scattering reaction seen in light water Cerenkov detectors (which only requires the presence of electrons) also occurs in SNO's heavy water. Although any neutrino flavor can scatter an electron (no new charged particles are created in the process) the probability for a electron neutrino to do so is much higher than either a muon or a tau neutrino (this is just a consequence of the fact that the electron neutrino is a `partner of the electron; a muon neutrino would scatter a muon more often than an electron neutrino would).

e- + nx ----> e- + nx

Demonstrating that neutrinos other than the electron type are arriving from the Sun is reduced to a simple comparison: if the number of neutrinos counted using either the deuteron breakup or electron scattering reactions is significantly higher than the number counted with the neutrino absorption reaction, then non-electron flavor neutrinos must be present in the solar flux.

The Sun can only produce electron neutrinos (its fusion processes lack the energy to produce any of the other flavors) and therefore detecting solar muon or tau neutrinos directly shows that electron neutrinos can transform into one or both of the other flavors, in violation of the basic assumptions of the Standard Model.

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Andrea T. Hughes - March 2004