EXTRACT
In quest of cosmic particles
Biman Basu

Back at TIFR Yash Pal had very good graduate students and collaborators. Among them were Ramnath Cowsik, Shyam Tandon, R.P. Verma, T.N. Rengarajan, and many others. One of his first students was K. Babu who went to Caltech, did his PhD in theoretical physics and then shifted to molecular biology. Yash Pal hesitates to call them his students. He says, "I don’t know whether I should call them students because they were learning from me; but I was also learning from them. We had a continuous kind of a seminar going on all the time, day and night. It was such a productive period at TIFR, experimentally and theoretically."

And then it turned out that simultaneously at that time, Bernard Peters was on a visit to TIFR. Peters invited Yash Pal to the Niels Bohr Institute in Copenhagen, first for a few months and then, after a year or so, as a visiting professor. "It was primarily because who both started thinking in terms of the importance of excited states in high-energy interactions," he reminisces. They recognised that excited states of nucleons are very important in nuclear interactions, and they came to this conclusion partly from accelerator results, but largely looking at the cosmic ray phenomena; for example, the ratio of positive mu mesons (also known as ‘muons’) and negative muons in cosmic rays. It was clear that there was positive excess in interactions of cosmic rays, which could come only if they were preferentially produced by the forward moving nucleons. The positive excess in the incoming cosmic rays could come only if they preferentially transferred their energy to positive particles; otherwise positve excess would not be noticed.

They quantitatively calculated the positive excess and that really led them to the result that the energy in an interaction of high-energy cosmic rays is preferentially shared amongst a few particles — only 3-4 particles. Cosmic ray energy spectrum is very steep; that is, if one goes to higher energy then the number of particles is very small. The combination of the fact that the spectrum of cosmic rays is steep and the number of the particles that share the charge is few clearly showed that the domination was due to excited states. All the other large number of particles produced, they calculated, would get a small fraction of the energy and there would not be any positive excess. It was this kind of argument, which was carried through in detail, and then the spectra of all particles in the atmosphere were calculated, and neutrinos were one of them. It turned out to be a significant piece of work because at that time it was not known which excited states were involved.

The initial thinking was that only a couple of excited states were involved in general. Later it was found that this was essentially true; one could have very high excited states and the results would be similar. This was called the "isobar fireball model" of Pal and Peters. ‘Isobar’, because it essentially involved nucleons, and ‘fireball’, because there is also a cloud. For cosmic rays what they had done was to take essentially this model with a couple of hypothetical isobars, and so on, and then calculate the fluxes of muons, the spectra of muons, the spectra of neutrinos; that is, underground high-energy spectrum of everything that was observed in cosmic rays.

Yash Pal and Peters presented their work at the 1963 International Conference on Cosmic Rays in Jaipur, which was dominated by discussions of this paper. And later on, what emerged in particle physics was that actually the excited states go very high. And if they did, one should also get large transverse momenta of the particles. And values of a few transverse momenta were measured, and all that was consistent with the model proposed. As a result, the importance of excited states in cosmic rays became very important, and that really went into this model and which, in all probability, affected the thinking further in high-energy physics. However, the "Isobar-fireball model" did not receive universal acceptance immediately; some questioned its veracity. But Pal and Peters had enough reasons to be sure about their work, because "if everything fitted, roughly speaking, the model had to be correct."

Yash Pal’s next forays were into the study of gamma rays in the atmosphere, and neutrinous underground, because TIFR was beginning to do nautrino experiments.

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Yash Pal went abroad for a year to work with Gaurang Yodh at the University of Maryland in USA. They worked together using a method that had been earlier suggested by the Russians, of trying to determine how the cross-section of nucleon interactions would go at very high energies.

They used available experimental data and a cosmic ray propagation module to drive the behaviour of cross-section as function of energy; the manner of increase was obtained unambiguously. It goes to their credit that a few years later the same results came from accelerator experiments.

— Excerpted from Yashpal: A Life in Science by Biman Basu. Vigyan Prasar, Department of Science and Technology. Pages 109. Rs 225 hardbound, Rs 150 paperback

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