1
September 12, 2006
Elaboration of an After-Dinner Talk at Stony Brook Workshop on Deep Underground Science and Engineering Laboratory
May 5, 2006
Chung Kee Jung has asked me to tell you how proton decay searches started. I begin with some prepared remarks and add some background information.
In the summer of 1954 my wife and I spent a month at Los Alamos. One day, whilst walking, I had the following thought: Might there also exist the reverse of the continuous creation of protons postulated by Bondi, Gold and Hoyle to preserve a steady state of the expanding Universe: some continuous disappearance of protons. On second thought, I preferred energy conservation to their disappearance, implying proton decay, as well as both baryon and lepton non-conservation, which play a role in understanding why our Universe is asymmetric.
I right away walked up to the laboratory, where Fred Reines and Clyde Cowan, Jr. had built a large scintillation counter to detect cosmic ray neutrinos, and convinced them that it was worthwhile also using their counter to search for proton decay.
For many years Chadwick was interested in the existence of a neutron, stimulated by Rutherford’s idea that a “neutral atom” might exist where the electron is bound more strongly to the proton than in the hydrogen atom. When Chadwick in 1932 did discover the neutron, his first estimate of its mass was lower than that of the hydrogen atom, apparently confirming Rutherford’s idea. However, in 1934 Chadwick and I found a more accurate way to measure the neutron’s mass by photo-disintegration of the deuteron. We proved that the neutron was not Rutherford’s “neutral atom,” but actually heavier than the hydrogen atom, a particle sui generis, as fundamental a particle as the proton!
To my surprise I was less shocked with the thought that the proton might decay than I was in 1934 when I realized one day with a shock that a free neutron might decay. The decay of particles had at that time not yet been observed, and I was not in the habit of publishing ideas. In 1940 Blackett still found it unusual that a particle produced by cosmic rays, now called the muon, was found to decay. After WWII, by which time many examples of the decay of particles produced by cosmic rays had been detected, the intense neutron beams from reactors made it possible to measure the lifetime of the neutron. The value found was compatible with an upper limit of 30 min I had deduced for its lifetime from the empirical Sargent relation for β-decay of complex nuclei, whose matrix elements could not be corrected for.
The simplest proton decay mode might be p → e+ π˚, though we looked for any energy release up to the possible limit of about 940 MeV. Some known nuclear phenomena allowed me to estimate a lower limit of 1020 years for the disappearance of bound nucleons. After subtracting the estimated cosmic ray background we arrived with the counter at a limit of 1022 years. [1] When Georgi and Glashow [2] predicted from the Grand Unified Theory (GUT) some partial proton decay rates, they found that
e+ + π˚ was the most prominent decay mode. To avoid confusion with the β-decay of nuclei, I began using the term proton decay as a code word, meant to also include neutrons and this has been generally adopted.
A group of researchers, whose members included David Cline, Claudio Rubbia and Larry Sulak planned a detector to test the GUT predictions, and invited me to join them. Sulak was a junior member of the Harvard faculty who knew that he would soon leave there and asked me whether he could work with me, and I agreed.
I had given a talk on the possibility of proton decay at Michigan, and Jack Van der Velde and his collaborators decided to look for it, as did several others. Michigan hired Sulak, and so I also decided to work with that group. Reines, who in the meantime had moved to Irvine, also decided to join our collaboration and proposed that it be called IMB for Irvine, Michigan and Brookhaven. Of the several detectors planned, the one we decided to build was the largest, a Water Cherenkov counter of 100 M tons, more than capable to test the SU5 GUT prediction.
To study proton decay it was necessary to study the neutrino background. Though in the first search at Los Alamos the search for proton decay was parasitic to a neutrino experiment the two experiments became symbiotic. Such experiments also proceeded at Kamiokande (Kamioka Nuclear Decay Experiment). Both Kamiokande and IMB discovered SN1986. In Kamioka a 5 times larger detector was built, dubbed Superkamiokande, which IMB members and Chung Kee also joined, and where neutrino oscillations were discovered, a good example of the saying “seek and you shall find – something!”
I did not succeed in finding collaborators at Brookhaven for the proton decay experiment. Those I asked preferred to continue working at the AGS, at that time the world’s foremost High Energy Accelerator.
There is no known interaction that would induce proton decay, but grand unification SU5 (GUT) promised to achieve this: Protons and leptons would become interchangeable at a high energy, where the extrapolated gravitational, electro-weak and strong interactions, when plotted on a logarithmic energy scale, converged at one point, from whose energy the proton lifetime was predicted by Georgi and Glashow. However, the results from IMB soon showed that the predicted lifetime was shorter than our experimental lower limit of >1030 years.
Some people were uneasy about a lifetime much longer than the age of the Universe, but clearly there is no contradiction there.
Assuming that supersymmetry plays a role in proton decay and choosing the energy from which to start extrapolations, a meeting at one point is achieved. Various higher supersymmetric formulations are being pursued by J.C. Pati, in case higher limits are found experimentally.
Somebody once asked me, who at Brookhaven works at IMB? Pointing at myself, I modestly answered: I’m B!