Author: Shao-Feng Ge
This achievement from 60 years ago is still guiding the direction of particle physics today.
Neutrinos are a very special and important fundamental particles in the Standard Model of particle physics. Being able to only participate weak interactions and hence penetrate several light years of lead plate, neutrinos are known as ghost particles for being very difficult to be detected. However, it is neutrino that gave the first new physics beyond the Standard Model, neutrino oscillation, which has been verified by various experiments and was awarded the Nobel Prize in Physics 2015. Neutrino oscillation has become an important guidance for the search of new physics beyond the Standard Model. The basic idea of high energy accelerator neutrino experiment that was proposed by Melvin Schwartz, Tsung-Dao Lee, Chen-Ning Yang during the golden era of particle physics in the 50-60s of last center is an evolutionary idea that has been continuously pushing forward the development of particle physics, including neutrino studies.
On November 24, 1950, Prof. Tsung-Dao Lee was leading the discussion of the possible experimental probe of weak interactions at high energy during the afternoon coffee hour of Physics Department of Columbia University. At that time, the study of weak interaction can already understand beta decay perfectly well with the four-fermion interaction established by Fermi in 1933 and the V-A theory by Gell-Mann, Feynman, Marshark, and Sudarshan in 1958. Unfortunately, the four-fermion interaction is not renormalizable and receives infinite divergence at high energy. This means that the four-fermion interaction is just effective operator at low energy as an approximation of the full theory at high energy. To obtain a complete picture of weak interaction, it is necessary to find a high energy experimental probe. Prof. Tsung-Dao Lee lead his fellow colleagues to explore all possible particles that high energy accelerator can produce, including proton, electron, pion, muon, and so on. Unfortunately, all these candidates are vetoed since they unavoidably involve the electromagentic and strong interactions that can disturb the detection of weak interaction.
Although the discussions that afternoon ended with nothing, Melvin Schwartz got strongly interested in this question. In the evening of that day, Schwartz suddenly realized that neutrinos only participate weak interaction and hence can naturally serve as an ideal probe of weak interaction without interference from electromagnetic or strong interaction. Despite the fact that neutrinos interact very weakly, probing neutrino is not absolutely impossible. Being overwhelmed, Schwartz called Tsung-Dao Lee during mid-night. Tsung-Dao Lee was every excited and quickly started calculation to confirm Schwartz’s idea. On March 15, 1960, M. Schwartz, Tsung-Dao Lee, and Chen-Ning Yang published two back-to-back papers on Physical Review Letters.
Phys.Rev.Lett. 4 (1960) 306-307
The experimental paper “Feasibility of using high-energy neutrinos to study the weak interactions" [1] by Schwartz proposed using high energy proton beam to hit target to produce pions which would decay to produce high energy neutrino beam. At the same time, Schwartz estimated that the neutrinos from pion decay can carry roughly half of the pion energy at most. By presenting the basic concept design of the experiment and estimating the signal event rate, Schwartz demonstrated the possibility and requirement of high energy neutrino experiment. At the end of his paper, Schwartz delivered acknowledgement to Tsung-Dao Lee and Chen-Ning Yang.
Phys.Rev.Lett. 4 (1960) 307-311
The paper “Theoretical discussions on possible high-energy neutrino experiments” [2] by Tsung-Dao Lee and Chen-Ning Yang discussed broadly the possible applications of high energy accelerator neutrino beam in 9 directions:
1) Identify the neutrino associated with electron and the one with muon are two different neutrinos;
2) Test lepton number conservation;
3) Test whether leptons participate neutral current interaction other than the electromagnetic one;
4) Test whether leptons are point particles;
5) Test lepton universality: whether electron and muon have exactly the same interactions;
6) Test whether there is S-symmetry between neutrino and anti-neutrino;
7) Test the conservation of vector current;
8) Probe the charged gauge boson W;
9) Probe the interaction with large momentum transfer.
The end of this paper also has an acknowledgement dedicated to M. Schwartz. This theory paper has been reprinted first in Chen-Ning Yang’s Selected Papers 1945-1980 with Commentary [3] and then in Tsung-Dao Lee’s Selected Papers [4].
These two papers established the revolutionary idea of high energy accelerator neutrino experiment and pointed out a clear direction for neutrino study. Following the suggestion in the theory paper by Lee and Yang, Leon Lederman, Melvin Schwartz, and Jack Steinberger verified that there are two different neutrinos, the electron neutrino and the muon neutrino, with high energy neutrino experiment in 1962 and won their Nobel Prize in 1988. In his Nobel Lecture [5], Schwartz recalled: “The Columbia University Physics Department had a tradition of a coffee hour …… At one of these Professor T. D. Lee was leading such a discussion of the possibilities for investigating weak interactions at high energies. …… That evening the key notion came to me …… I called T. D. Lee at home with the news and his enthusiasm was overwhelming. The next day planning for the experiment began in earnest”. Schwartz also gratefully mentioned in his bibliography [6] prepared for his Nobel Prize webpage: “T.D. Lee …... was the inspirer of this experiment”. On behalf of the three Nobel Laureates, Lederman specially thanked Tsung-Dao Lee in his Nobel Banquet Speech [7]: “We are also agreed that we owe much to many others, and I would like to mention Professor T. D. Lee, our Columbia colleague, for his guidance and inspiration.”
This achievement from 60 years ago is still guiding the direction of particle physics today. For example, the T2K/T2HK experiments in Japan, the NOvA and DUNE experiment in US, and the MOMENT experiment under design in China use high energy accelerator neutrino to measure the leptonic CP phase in the neutrino mixing matrix, which can help to under the baryon asymmetry in our Universe: why there are a lot of matter but almost no matter. Since the idea was proposed 60 years ago, high energy accelerator neutrino experiments have been playing an irreplaceable role at the frontier of particle physics.
References:
[1] M. Schwartz, “Feasibility of using high-energy neutrinos to study the weak interactions”, Phys.Rev.Lett. 4 (1960) 306-307
[2] T.D. Lee & C.N. Yang, “Theoretical Discussions on Possible High-Energy Neutrino Experiments”, Phys.Rev.Lett. 4 (1960) 307-311
[3] C.N. Yang, Selected Papers 1945-1980 with Commentary, San Francisco, 1983; Page 281-285 (commentary on Page 46), World Scientific Series In 20th Century Physics Vol 36, 2005
[4] T.D. Lee, Selected Papers, Vol. 1, Page 45-49. Contemporary Physicists Vol 1, 1986
[5] Melvin Schwartz , Nobel Lecture “The First High Energy Neutrino Experiment”: https://www.nobelprize.org/uploads/2018/06/schwartz-lecture.pdf
[6] Melvin Schwartz bibliography on Nobel Prize website: https://www.nobelprize.org/prizes/physics/1988/schwartz/biographical/
[7] Leon Lederman Nobel Prize Banquet speech https://www.nobelprize.org/prizes/physics/1988/lederman/speech/
Cover: http://particlecentral.com/neutrinos_page.html