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A confluence of anniversaries

A retired physicist's first encounter with the structure of the proton took place in a university library in India 56 years ago.

The year 2014 seems to be a propitious one. CERN is celebrating its 60th anniversary. It is also the 50th anniversary of Murray Gell-Mann's and George Zweig's invention of the quark model and the 50th anniversary of James Cronin and Val Fitch's discovery of CP violation.

An anniversary is a useful thing. The memories it can stir are particularly helpful in retirement, when rumination is one's main occupation.

I will begin by recalling a incident that happened long before I had heard of CERN or quarks or CP violation. The event may have had a silent influence on the direction of my later career.

The incident took place on 8 May 1958. In a pause between lectures, I was standing in the verandah outside the lecture room of the physics department. I was 18 and completing the first year of study in physics. The place was Hoshiarpur, a small town in the Indian state of Punjab, which was the temporary location of the University at the time. Three months later, the university would move to a brand-new campus in the brand-new city of Chandigarh.

I was looking out languidly in the direction of the physics library, a one-story building about 50 meters away. Two Indian Air Force fighter-bombers—Dassault Ouragans—were carrying out aerial exercises. One of the planes went into a steep dive in the direction of the campus. I waited to see it complete the maneuver and begin its ascent, but it kept going lower and lower. The plane barely grazed the roof of the library building. Moments later, I heard a thud. Fragments of metal flew into the air. An explosion followed.

The Indian Air Force took delivery of its first four Dassault Ouragans in 1954. The IAF renamed the plane Toofani, whose meaning in Hindi is the same as

The Indian Air Force took delivery of its first four Dassault Ouragans in 1954. The IAF renamed the plane Toofani, whose meaning in Hindi is the same as "ouragan" in French: hurricane. CREDIT: Indian Air Force

Students ran toward the crash site. The plane had hit a wall on the perimeter of the campus. It had then veered towards a grove of fruit trees. Two small children playing in the orchard were killed. Beyond the grove, there was an open field where we found the body of the pilot. The smoking wreckage of the plane lay 100 meters away. Overhead, the companion plane was flying in a slow circle, as if in mourning.

I returned to the campus, shaken. The frailty of life, the rude finality of death, the iniquities of existence—such were the thoughts in my head. Looking to recover my equanimity, I turned to the physics library that had just had such a close brush with disaster.

The shelves were full of books on heat, light, sound, mechanics, electricity, and other classical topics. I went to a corner where new arrivals were kept. I picked up one that had a title like “new developments in modern physics.”

One chapter caught my attention. It concerned an experiment by a physicist named Robert Hofstadter. He had studied the scattering of a beam of electrons off a hydrogen target. From the distribution of the scattered electrons, he had concluded that the proton was not a point object. It had a size of about 0.8 femtometers (0.8 × 10−15 m).

What, I wondered, is inside the proton? That innocent question, unspoken and unanswered, lodged itself in the recesses of my memory.

What neutrinos can tell us about partons

Twenty-one years later, I was asked to give a rapporteur talk at the 1979 High-Energy Physics Conference, which was held that year in Geneva, Switzerland. The venue and date were chosen so that the conference would coincide with CERN's 25th anniversary.

My lecture was titled “Neutrinos and Nucleon Structure.” I had organized it in four parts: the nucleon at rest; the excited nucleon; the nucleon at infinite momentum; and the hadronic vacuum. The arrangement enabled me to discuss the diverse aspects of nucleon structure, such as weak form factors, resonance excitation, quark-parton distributions, and the evolution of parton distributions predicted by QCD. ("Parton" is generic name for nearly free quarks, antiquarks, and gluons; Richard Feynman had introduced it in 1969 to describe high-energy hadron collisions.)

The part I enjoyed talking about the most was Feynman's idea of partons, which he described at the Neutrino '72 Conference in Balatonfüred, a resort on the shore of Hungary's Lake Balaton. In the famous talk, which I heard in person, Feynman discussed the scaling behavior observed in inclusive electron scattering from a proton, e + p → e + X or in its neutrino analog ν + p → μ + X. Such an interaction, he suggested, could be understood if one assumed that in a frame in which the proton moves with very high momentum, the proton can be viewed as a system of essentially massless collinearly moving partons that interact independently with the incoming lepton.

The structure functions measured at high energies are then interpretable as linear combinations of q(x) and q(x), number densities of quarks and antiquarks present in the proton as function of parton momentum in units of the proton momentum, x. In a memorable analogy, Feynman remarked:

The spectrum of scattered leptons determines the distribution in x of the parts inside, in a manner analogous to the way the frequency distribution of radar scattered from a swarm of bees determines the velocity distribution of the bees in the swarm.

By combining information from electron and neutrino scattering, it was possible to determine the functions that describe the momentum spectrum of quarks and antiquarks in the proton.

The lecture in Geneva was in the city's International Conference Centre. A thousand people were in the audience. I had used up the time allotted to me, but there was a clamor for more discussion. Don Perkins, who chaired the session, allowed the proceedings to go on for another 10 minutes. From the ardor of the questions and the smiles and thumbs-up signs I got later, I sensed that the lecture had made an impact.

The following evening, the city of Geneva held a special symphony concert in honor of CERN's silver jubilee. It was a rousing concert, and I felt elated that a crowd of particle physicists was being honored in this manner. After the concert, I was walking down the steps from the concert hall to the street, when I saw two people just ahead of me—one a head taller than the other. They were the theorists Lev Okun and James Bjorken.

They complimented me on my talk. The books they had written (Relativistic Quantum Mechanics by Bjorken and Sidney Drell and Weak Interactions of Elementary Particles by Okun) were the ones from which I had learned particle physics as a graduate student.

Bjorken and Drell's book was published in 1964 and continues to be used today. Its 50th anniversary would be a worthy addition to the parade of anniversaries that we celebrate this year.

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