Subatomic Physics

7.4. Lepton and Lepton Flavor Number 209
but not γ → e − p. (Remember that these processes can happen only in the field of
a nucleus that takes up momentum; see Problem 3.22 .)
Evidence for lepton conservation comes partially from neutrino reactions. Consider
first antineutrino capture,
νep −→ e + n. (7.34)
This process is allowed by lepton conservation because the lepton number on both
sides of the equation is −1. Antineutrino capture has been observed by Reines,
Cowan, and collaborators with antineutrinos from a nuclear reactor. (16) A reactor
produces predominantly antineutrinos because fission yields neutron-rich nuclides.
These decay through processes involving the mode
n −→ pe − νe. (7.35)
Since the neutron has L = 0, the right-hand side must also have L =0,andthe
particle emitted together with the negative electron must be an antineutrino. The
observation of the reaction Eq. (7.34) is in agreement with Eq. (7.35). However, reactions
of the type νen → e − p and νep → e + n are forbidden by lepton conservation.
Davis has searched for a reaction of this type,
νe 37 Cl −→ e −37 Ar, (7.36)
again using antineutrinos from reactors. Here, L = −1 on the left-hand side and
L = +1 on the right-hand side, and lepton conservation would be violated if the
reaction were observed. Davis did not see reaction (7.36) and thus was able to
set a limit (17) (2 × 10 −42 cm 2 /atom) on the cross section of the reaction caused by
antineutrinos. Note, however, that the reaction
νe 37 Cl −→ e −37 Ar (7.37)
should occur and was observed by Davis. This result shows that antineutrinos and
neutrinos have different characteristics.
16F.Reines,C.L.Cowan,F.B.Harrison,A.D.McGuire,andH.W.Kruse,Phys. Rev. 117,
159 (1960).
17R. Davis, Phys. Rev. 97, 766 (1955); J. K. Rowley et al., in Solar Neutrinos and Neutrino
Astronomy, (M. L. Cherry, K. Lande and W. A. Fowler, eds) American Institute of Physics, New
York, 1985) p. 1.