9 The Neutrino is Discovered
69
9.4 More Neutrino Types Found
Reines and Cowan discovered the electron type anti-neutrino ( ¯
ν e ) that is
emitted in nuclear beta decays. In 1962, a new, second kind of neutrino
with properties very similar to the previous neutrino ¯
ν e was discovered by
Lederman, Schwarz, Steinberger and collaborators, that involved the muon.
To accomplish this discovery, Lederman et al realized that the neutrino
interaction rate depended on how energetic the incoming neutrinos were. The
neutrinos emitted from the nuclei that Reines and Cowan used had much
lower energy (energies in the few MeV range). Lederman et al employed
a new method to create a beam of higher energy neutrinos. They started
with faster moving pions in the Alternating Gradient Synchrotron accelerator
at Brookhaven National Laboratory to create fast moving pions, which in
their decay produced fast moving neutrinos. In the first step, protons were
accelerated to high speeds and directed at a target of beryllium nucleus. This
produced high energy pi mesons, which in turn produced higher energy
neutrinos. These higher energy neutrinos helped to increase the rate at which
the inverse beta decay type reaction could take place in the detector.
The question then was: were the neutrinos impinging the detector in the
Brookhaven experiment of Lederman et al same as the ν e discovered by Reines
and Cowan? What Lederman et al found is that these neutrinos produced
muons and not electrons. This proved that they were a new kind of neutrino,
and were called ν μ or muon type neutrino.
Japanese physicists Ziro Maki, Masami Nakagawa, and Shoichi Sakata
proposed that the ν e and ν μ neutrinos could mix with each other and
transform from one to the other kind. This was a revolutionary suggestion,
similar to Pontecorvo’s suggestion of a neutrino–anti-neutrino transmutation.
The Maki et al suggestion was confirmed in 1998, by the observation of the
so-called neutrino oscillations, as we will see below. This discovery altered the
landscape of neutrino physics forever. The discovery involved the neutrinos
from the atmosphere of the Earth. Collisions of protons in the atmosphere
produce neutrinos, as already mentioned. The atmospheric neutrinos were
first observed by the joint collaboration of the Tata Institute group headed by
Indian physicist M. G. K. Menon, and the Durham group of A. Wolfendale,
in Kolar Gold Field in India, in 1965 [4]. The same year that the Kolar Gold
field experiment observed the atmospheric neutrinos, a group headed by Fred
Reines also discovered the atmospheric neutrinos in an experiment, carried out
in the East Rand Proprietary Mine in South Africa [87].
69
9.4 More Neutrino Types Found
Reines and Cowan discovered the electron type anti-neutrino ( ¯
ν e ) that is
emitted in nuclear beta decays. In 1962, a new, second kind of neutrino
with properties very similar to the previous neutrino ¯
ν e was discovered by
Lederman, Schwarz, Steinberger and collaborators, that involved the muon.
To accomplish this discovery, Lederman et al realized that the neutrino
interaction rate depended on how energetic the incoming neutrinos were. The
neutrinos emitted from the nuclei that Reines and Cowan used had much
lower energy (energies in the few MeV range). Lederman et al employed
a new method to create a beam of higher energy neutrinos. They started
with faster moving pions in the Alternating Gradient Synchrotron accelerator
at Brookhaven National Laboratory to create fast moving pions, which in
their decay produced fast moving neutrinos. In the first step, protons were
accelerated to high speeds and directed at a target of beryllium nucleus. This
produced high energy pi mesons, which in turn produced higher energy
neutrinos. These higher energy neutrinos helped to increase the rate at which
the inverse beta decay type reaction could take place in the detector.
The question then was: were the neutrinos impinging the detector in the
Brookhaven experiment of Lederman et al same as the ν e discovered by Reines
and Cowan? What Lederman et al found is that these neutrinos produced
muons and not electrons. This proved that they were a new kind of neutrino,
and were called ν μ or muon type neutrino.
Japanese physicists Ziro Maki, Masami Nakagawa, and Shoichi Sakata
proposed that the ν e and ν μ neutrinos could mix with each other and
transform from one to the other kind. This was a revolutionary suggestion,
similar to Pontecorvo’s suggestion of a neutrino–anti-neutrino transmutation.
The Maki et al suggestion was confirmed in 1998, by the observation of the
so-called neutrino oscillations, as we will see below. This discovery altered the
landscape of neutrino physics forever. The discovery involved the neutrinos
from the atmosphere of the Earth. Collisions of protons in the atmosphere
produce neutrinos, as already mentioned. The atmospheric neutrinos were
first observed by the joint collaboration of the Tata Institute group headed by
Indian physicist M. G. K. Menon, and the Durham group of A. Wolfendale,
in Kolar Gold Field in India, in 1965 [4]. The same year that the Kolar Gold
field experiment observed the atmospheric neutrinos, a group headed by Fred
Reines also discovered the atmospheric neutrinos in an experiment, carried out
in the East Rand Proprietary Mine in South Africa [87].