Old soldiers by Michael Riordan

Twenty years ago this month, an this deep inelastic region in excru­ experiment began at the Stanford ciating detail, the new quark-parton Linear Accelerator Center in Cali­ picture of a nucleon's innards fornia that would eventually redraw gradually took a firmer and firmer the map of high energy physics. hold upon the particle physics com­ In October 1967, MIT and SLAC munity. These two massive spec­ physicists started shaking down trometers were our principal 'eyes' their new 20 GeV spectrometer; into the new realm, by far the best by mid-December they were log­ ones we had until more powerful ging electron-proton scattering in muon and neutrino beams became the so-called deep inelastic region available at Fermilab and CERN. where the electrons probed deep They were our Geiger and Marsd- inside the protons. The huge ex­ en, reporting back to Rutherford cess of scattered electrons they the detailed patterns of ricocheting encountered there-about ten times projectiles. Through their magnetic the expected rate-was later inter­ lenses we 'observed' quarks for preted as evidence for pointlike, the very first time, hard 'pits' inside fractionally charged objects inside hadrons. the proton. These two goliaths stood reso­ Michael Riordan (above) did The quarks we take for granted lutely at the front as a scientific research using the 8 GeV today were at best 'mathematical' revolution erupted all about them spectrometer at SLAC as an entities in 1967 - if one allowed during the late 1960s and early MIT graduate student during them any true existence at all. The 1970s. The harbingers of a new the early 1970s. Now a re­ majority of physicists did not. Their age in particle physics, they helped search physicist with the Uni­ failure to turn up in a large number pioneer the previously radical idea versity of Rochester, he was of intentional searches had con­ that leptons, weakly interacting spokesman of the recent vinced most of us that Murray Gell- particles, of all things, could be axion search done with this Mann's whimsical entities could used to plumb the mysteries of device. He is also author of not possibly be 'real' particles in the strong force. Who would have The Hunting of the Quark', the usual sense, just as he had guessed, in 1967, that such spind­ an illustrated history of parti­ insisted from the very first. ly particles would eventually ferret cle physics being published Jerome Friedman, Henry Kendall, out their more robust cousins, the this month in the US by Si­ mon and Schuster. Richard Taylor and the other MIT- quarks? Nobody, except perhaps SLAC physicists were not looking Bjorken-and he wasn't too sure for quarks that year. SLAC Experi­ himself. ment 4B had originally been de­ In the late 1970s, following the role in particle physics that they signed to study the electroproduc- momentous discoveries at the were allowed to remain largely tion of resonances. But the prod- SPEAR electron-positron ring at intact, silent relics of the glory dings of a young SLAC theorist, SLAC, these venerable old spec­ years to be shown to the busloads James Bjorken, who had been trometers fell into disuse and disre­ of tourists who visit SLAC every working in current algebra (then pair. Were it not for the efforts of month. an esoteric field none of the exper­ a dedicated group of nuclear phy­ Whatever the case, it is indeed imenters really understood), helped sicists from American University gratifying to see one of these convince them to make additional (Washington DC), who doggedly vingtagenarians sharing the spot­ measurements in the deep inelastic kept the study of nucléon structure light again, in the autumn of its region, too. alive at SLAC, they would most years. When the 'EMC effect' sur­ Over the next six years, as first probably have followed their con­ faced in 1983, suggesting that the 20 GeV spectrometer and then temporary detectors into oblivion, inside nuclei quarks might come its 8 GeV counterpart swung out scavenged for parts to be used bagged in sixes as well as threes, to larger angles and cycled up and elsewhere. Or perhaps it was out it was the 8 GeV spectrometer down in momentum, mapping out of respect for their truly pivotal that supplied the quickest and best

CERN Courier, October 1987 9 The big spectrometers at End Station A of the Stanford Linear Accelerator Centre (SLAC) in 1967: right of the beam line, the 8 GeV spectrometer, and on the other side its 20 GeV and 1.6 GeV (extreme left) counterparts. confirmation - improving and ex­ tending the EMC results. The resur­ rection of ten-year-old empty tar­ get data by Arie Bodek and com­ pany proved, once and for all time, what a precision tool this detector is. This versatile cyclops has since become a primary player in the burgeoning field of high energy nuclear physics that grew up in the wake of the EMC effect. A rigid, focusing spectrometer that can be pivoted from one angle to another in minutes without altering its optical properties, the 8 GeV device is unique in physics. Its ability to separate one form factor or structure function from another with high accuracy insures that a steady stream of experimenters will want to use it in the near fu­ ture. The recent precision mea­ surement of the ratio of scattering contributions due to longitudinally and transversely polarized photons reported at Berkeley in 1986 and again this summer, at Uppsala and at Hamburg, is surely only the first in a long sequence of similar sep­ aration experiments. When correlated electron-posi­ tron peaks turned up in 1985 at Darmstadt (GSI), raising eyebrows throughout nuclear and particle physics, this trusty old detector the puzzling GSI phenomenon was old soldiers are still churning out was pressed into service once not due to an elementary axion. important data twenty years later again, in a hasty search for a puta­ So the 8 GeV spectrometer is solid testament to the wisdom tive 1.8 MeV axion that some note­ seems to have an industrious old and farsightedness of their de­ worthy theorists had suggested age ahead of it, with plenty of signers. as the source. By rolling the 8 GeV work to do at the busy new inter­ When I take friends now to visit spectrometer into the line of the face between nuclear and particle End Station A, I often caution them beam, physicists from Rochester, physics. No doubt worthy employ­ that they are standing on hallowed American University, Caltech, Fer­ ment will be found there for its ground. We walk among the dusty milab, Massachusetts, and SLAC 20 GeV companion, too. What old hodoscopes and particle detec­ were able to measure the flux of other detectors, in all of high ener­ tors littered about the cluttered high energy positrons emerging gy physics, could boast such a floor, remnants of the retinas that behind a short beam dump inserted long and productive lifetime? Few once sat inside these tired old eyes into the electron beam upstream other machines have had as great and caught the first faint glimpse of End Station A. The lack of any an impact upon the way we do of quarks. I try to tell them what excess gave definitive proof that physics today. The fact that these revolutionary changes these aging

10 CERN Courier, October 1987 The same detectors almost two decades later, still soldiering on.

hulks have wrought in the minds of men and women - in the way we now perceive our entire Uni­ verse. Occasionally a bird flies past the massive open doors and into this vast concrete cavern, diving and screeching above, as if laugh­ ing at my futile efforts. How is it possible to be so sen­ timental about mere artifacts - lifeless configurations of concrete and steel, lead, plastic, and glass? How can a scientist be so unscien­ tific? Were he alive today, a fa­ mous American general, hero of World War II, would probably understand my feelings for these aged veterans. 'Old soldiers never die,' he said when relieved of his command during the Korean War; 'They just fade away.'

Peter Fowler recounted the pion discovery at the Bristol 40th anniversary meeting. The pion's pioneers Fowler, whose association with Bristol stretches over a comparable span, is a grandson of Rutherford. His father, Ralph In 1946, a band of intrepid physi­ England from Brazil by Occhialini, Fowler, a key theorist at the Cavendish cists took a batch of a new kind also recently arrived from that Laboratory in Cambridge, was married to Rutherford's only daughter Eileen. of photographic emulsion up the country at the instigation of Pic du Midi in the French Pyrenees P. M. S. Blackett, with whom he to expose them to cosmic rays. had worked in the Cavendish Lab­ After analysing the results at Bris­ oratory in the early 1930s devel­ tol, C. M. G. Lattes, H. Muirhead, oping the counter-controlled cloud G. P. S. Occhialini and C. F. Powell chamber technique. Muirhead was were able to announce early the a young research student who had following year that they had seen graduated at Bristol in 1946 while the long-awaited pi meson, or pion, Powell, then a Reader in Physics postulated by Yukawa in 1935 as in the Wills Laboratory, had spent the carrier of the strong nuclear many years perfecting the nuclear force. This landmark discovery emulsion technique. quickly revitalized particle physics What these workers observed after all its wartime upheavals. in new llford nuclear research emul­ At the end of July, the H. H. sions exposed to the cosmic radi­ Wills Physics Laboratory at Bristol ation at mountain altitudes were was the scene of an unusual and clear-cut examples of the decay memorable international conference of a heavy meson (about three to mark the 40th anniversary of hundred times heavier than an elec­ this discovery. tron) to a lighter meson (about two 40 years ago, Lattes was a hundred times the electron mass) young research worker brought to of unique energy - a two-body

CERN Courier, October 1987