358
21. CP Violation and Oscillation Phenomena
21.4.3 Neutrino oscillations: experimental results
Historically, the search for neutrino oscillations began when experiments by
Davis et al. (1968) detected solar neutrinos (from
8 B decays) at a rate approximately one third of that predicted by the solar model calculations of Bahcall
et al. (1968). Pontecorvo (1946) had proposed the experiment, in which the
−
neutrinos are detected by the inverse β-decay process ν e +
37 Cl → e +
37 Ar.
The Davis experiment used 520 metric tons of liquid tetrachloroethylene
(C 2 Cl 4 ), buried 4850 feet underground in the Homestake gold mine, in South
Dakota. Davis’ findings provided the impetus to study solar neutrinos using Kamiokande, a 3000 ton imaging water Cerenkov detector situated about
one kilometre underground in the Kamioka mine in Japan. Indeed,
8 B solar
neutrinos were observed, and at a rate consistent with that of the Davis experiment (Hirata et al. 1989). Later results from the Homestake mine (Cleveland
et al. 1998) reported a solar neutrino detection rate almost exactly one third
of the updated calculations of Bahcall et al. (2001).
In a separate development, Kamiokande also reported (Hirata et al. 1988)
an anomaly in the atmospheric neutrino flux. Atmospheric neutrinos are
produced as decay products in hadronic showers which result from collisions
of cosmic rays with nuclei in the upper atmosphere of the Earth. Production
+
of electron and muon neutrinos is dominated by the decay chain π
+
→ μ +
+
ν μ , μ → e
+ + ¯
ν μ + ν e (and its charge-conjugate), which gives an expected
value of about 2 for the ratio of (ν μ + ¯
ν μ ) flux to (ν e + ¯
ν e ) flux.
3 While the
number of electron-like events was in good agreement with the Monte Carlo
calculations based on atmospheric neutrino interactions in the detector, the
number of muon-like events was about one half of the expected number, at
the 4σ level.
This muon-like defect (and the lack of an electron-like defect) was later
confirmed at the 9σ level by Super-Kamiokande (Fukuda et al. 1998). In this
experiment, a marked dependence was observed on the zenith angle of the
muon neutrinos. This angle is simply related to the distance travelled by the
neutrinos from their point of production, which varies from about 20 km (from
above the detector) to over 10,000 km (from below the detector). The SuperKamiokande data was the first compelling evidence for neutrino oscillations.
Interpreting their data in terms of a simple 2-state ν μ ↔ ν τ model, as in
(21.129), Fukuda et al. (1998) reported the values sin
2 2θ μ3 > 0.82, and
5 × 10
−4 < Δm
2
6 × 10
−3 eV
2 at 90% CL.
31 <
We will postpone further discussion of the solar neutrino deficit for the
moment, since it is complicated by interactions of the neutrinos with the
Sun’s matter (see the following subsection). We proceed to describe some of
the main results which have come from the analysis of data from neutrinos
produced in terrestrial accelerators and reactors.
3 The detector could not measure the charge of the final state leptons, and therefore ν
and ¯
ν events could not be discriminated.
21. CP Violation and Oscillation Phenomena
21.4.3 Neutrino oscillations: experimental results
Historically, the search for neutrino oscillations began when experiments by
Davis et al. (1968) detected solar neutrinos (from
8 B decays) at a rate approximately one third of that predicted by the solar model calculations of Bahcall
et al. (1968). Pontecorvo (1946) had proposed the experiment, in which the
−
neutrinos are detected by the inverse β-decay process ν e +
37 Cl → e +
37 Ar.
The Davis experiment used 520 metric tons of liquid tetrachloroethylene
(C 2 Cl 4 ), buried 4850 feet underground in the Homestake gold mine, in South
Dakota. Davis’ findings provided the impetus to study solar neutrinos using Kamiokande, a 3000 ton imaging water Cerenkov detector situated about
one kilometre underground in the Kamioka mine in Japan. Indeed,
8 B solar
neutrinos were observed, and at a rate consistent with that of the Davis experiment (Hirata et al. 1989). Later results from the Homestake mine (Cleveland
et al. 1998) reported a solar neutrino detection rate almost exactly one third
of the updated calculations of Bahcall et al. (2001).
In a separate development, Kamiokande also reported (Hirata et al. 1988)
an anomaly in the atmospheric neutrino flux. Atmospheric neutrinos are
produced as decay products in hadronic showers which result from collisions
of cosmic rays with nuclei in the upper atmosphere of the Earth. Production
+
of electron and muon neutrinos is dominated by the decay chain π
+
→ μ +
+
ν μ , μ → e
+ + ¯
ν μ + ν e (and its charge-conjugate), which gives an expected
value of about 2 for the ratio of (ν μ + ¯
ν μ ) flux to (ν e + ¯
ν e ) flux.
3 While the
number of electron-like events was in good agreement with the Monte Carlo
calculations based on atmospheric neutrino interactions in the detector, the
number of muon-like events was about one half of the expected number, at
the 4σ level.
This muon-like defect (and the lack of an electron-like defect) was later
confirmed at the 9σ level by Super-Kamiokande (Fukuda et al. 1998). In this
experiment, a marked dependence was observed on the zenith angle of the
muon neutrinos. This angle is simply related to the distance travelled by the
neutrinos from their point of production, which varies from about 20 km (from
above the detector) to over 10,000 km (from below the detector). The SuperKamiokande data was the first compelling evidence for neutrino oscillations.
Interpreting their data in terms of a simple 2-state ν μ ↔ ν τ model, as in
(21.129), Fukuda et al. (1998) reported the values sin
2 2θ μ3 > 0.82, and
5 × 10
−4 < Δm
2
6 × 10
−3 eV
2 at 90% CL.
31 <
We will postpone further discussion of the solar neutrino deficit for the
moment, since it is complicated by interactions of the neutrinos with the
Sun’s matter (see the following subsection). We proceed to describe some of
the main results which have come from the analysis of data from neutrinos
produced in terrestrial accelerators and reactors.
3 The detector could not measure the charge of the final state leptons, and therefore ν
and ¯
ν events could not be discriminated.