Four decades ago an important milestone in international physics was achieved

Experiment 36
July 1972. Experiment 36 at the C0 location in the Fermilab Main Ring. L to R: Nikitin, Malamud, Strauss.From the Fermilab history archives.

The context: in 1972 a fruitful international collaboration began. Scientists from the US and the Soviet Union joined forces to carry out a series of internal target experiments on pp and pHe elastic and inelastic scattering at the new accelerator at the National Accelerator Laboratory (later named Fermilab). The remarkable thing is that Experiment 36 described below was the first experiment carried out on a brand new machine,  A significant quantity of equipment was shipped to the US and connected to the US supplied components. All of this was carried out during the cold war. These initial experiments began a long and successful collaboration that continues to this day.

On the Romantic Tevatron Era
Memoire by Vladimir Nikitin

It was a romantic time and there were exciting adventures in high-energy physics at the beginning of the Tevatron era. One of the earliest studies at this new accelerator was carried out just as the proton beam of the world’s highest energy accelerator was making its first revolutions.

This was in March of 1972.

A joint team of scientists and engineers from the National Accelerator Laboratory (Batavia) and several US Universities, and the Joint Institute for Nuclear Research (Dubna) started experiment E-36 on proton proton elastic and inelastic scattering. The goal was to learn the properties of the strong interaction at the highest possible energy and study the so called asymptotic behavior. Such a study would make it possible to check the validity of the fundamental axioms of quantum field theory: micro causality, unitarity of the scattering matrix, analyticity of amplitudes and postulates of special relativity.

These experiments, carried out during the first three years of operation of the new machine greatly extended our knowledge of particle properties. We obtained the following results in the laboratory energy range 8 to 400 GeV.
  1. The real part of the scattering amplitude rises and crosses zero and becomes positive at an energy ∼ 280 GeV.  This result was unexpected and required a correction in the asymptotic behavior of the total proton proton cross section.
  2. The slope of the diffraction cone rises logarithmically, thus implying that the proton radius increases with energy.  From this it was possible to determine the slope of the Pomeron trajectory, an important parameter in the model of the strong interaction.
  3. Diffraction dissociation of the proton was measured. This inelastic process is sensitive to another important theoretical parameter, the constant of the triple Pomeron interaction. This was the first time such a measurement was done.

An important and novel element of the experimental technique was the use of a supersonic hydrogen jet target. This device creates localized gas flow in the accelerator vacuum chamber. It was created in JINR and was previously used at the Serpukhov 70 GeV accelerator. Using a pure hydrogen target was crucially important for detecting the inelastic diffraction reaction, where kinematic criteria are not sufficient to reject background.

It is interesting to recollect that the joint proposal for the E-36 experiment was submitted to the NAL Program Advisory Committee in 1971. The committee recommended postponing the project because the novel and not yet thoroughly tested gas jet target risked degrading accelerator performance. The NAL founding director, Robert R. Wilson met with the project initiators Vladimir Nikitin and Ernest Malamud, looked at photographs of the apparatus and made a personal decision to approve the experiment. He took a risk approving this contrary to advice from his Advisory Committee, on a brand new accelerator that at the time was having many difficulties in the commissioning process, and during the height of the cold war. Wilson was influenced by the elegant simplicity and originality of the technique and by his evaluation that the E-36 team would do a thorough professional job. Possibly Wilson was guided by his art intuition. Besides being one of the giants of accelerators, Wilson was also an accomplished professional artist.  There is a philosophy in art that says a beautiful idea should be true.

Vladimir Nikitin is Professor of Physics at Moscow University and is a senior research scientist at the Joint Institute for Nuclear Research (J.I.N.R.) in Dubna, Russia.

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