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Particle century - fifty years ago, in 1947, half a century after the discovery of the electron, Don Perkins, working at London's Imperial College, was the first to observe a clear example of what appeared to be the nuclear capture of a meson, producing a nuclear disintegration. On 17 April, Perkins gave a presentation at CERN on one hundred years of elementary particles. One half of this particle century is spanned by his own contributions.

More than anything else, the 1947 discovered electron-positron pair discoveries made physicists realize production, a key prediction of that the subnuclear world was more Dirac's ideas. complex than had been suspected by Cloud chambers played a major looking at everyday nuclei. The role in cosmic ray studies in the discovery of the pion and its following years, leading to the discov­ subsequent decay highlighted the ery of the 'mesotron' in 1937, origi­ role of the muon (discovered by nally identified as the nuclear force Anderson and Neddermeyer in carrier postulated by Hideki Yukawa 1936), while the Vs were the first in 1935. However, several difficulties examples of 'strange' particles soon arose with this hypothesis, even containing a third type of quark. though pictures of its decay to an The following article by Owen Lock, electron, as postulated by Yukawa to formerly of Bristol, Manchester, explain beta-decay, were observed in Birmingham and CERN, recalls the cloud chamber pictures in 1940. In pion discovery. Another article later particular, the mesotron appeared to this year will cover the discovery of have a very weak nuclear interaction V-particles. with matter, conclusively demonstrated by the counter experiments of Marcello Conversi, Ettore Pancini and Oreste Piccioni in Rome from 1943-1947. A possible explanation of these Half a century ago - difficulties had been put forward in trace their parentage back to 1897. Japan in 1942 and 1943 by Yasutaka Blackett was awarded the Nobel the pion pioneers Tanikawa and by Shoichi Sakata and Physics Prize in 1948 and Powell in Takeshi Inoue, who suggested a two- 1950. These discoveries were to be hile the classic discoveries of meson hypothesis with a Yukawa- the last major particle physics WThomson and Rutherford type meson decaying to a weakly revelations of a war-torn Europe. The opened successive doors to interacting mesotron. Because of the next European milestone - the subatomic and nuclear physics, war their ideas were not published in discovery of the neutral current in particle physics may be said to have English until 1946 and 1947, the 1973 - had to await the establishment started with the discovery of the journals in question not reaching the of CERN. positron in cosmic rays by Carl USA until the end of 1947. Sighting the pion and unravelling its Anderson at Pasadena in 1932, Unaware of the Japanese work, decay liberated physics from more verifying Paul Dirac's almost Robert Marshak had put forward a than a decade of dilemma, and the simultaneous prediction of its similar hypothesis in June 1947, at a pion looked full of promise. Perhaps existence. conference of American theoreticians this new particle held the key to the Anderson used a cloud chamber, on Shelter Island (off Long Island), mysterious forces which held the expanded at random, in a high and which he published later that nucleus together. However this hope, magnetic field. At the same time, year with Hans Bethe. None of the cherished since the time of Yukawa, Patrick Blackett at Cambridge was scientists at the conference knew was not to be fulfilled, and the joined by an inventive young Italian, that such two-meson decay events significance of the pion as a particle Giuseppe Occhialini, sent by a had already been observed some has diminished as our understanding master of counter coincidence weeks earlier by Cecil Powell and his of nuclear forces in terms of a deeper techniques, Bruno Rossi, then in collaborators in Bristol, using the layer, quarks, has advanced. If the Florence, to learn about cloud then little known photographic pion does play a special role, it is chambers. Very soon Blackett and emulsion technique, but which in because it is the lightest strongly Occhialini had built a counter- Powell's hands became a powerful interacting particle. controlled chamber with which they research tool.

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In the immediate post-World War II years, cosmic ray experiments using photographic emulsions at Bristol made historic discoveries. The picture shows Cecil Powell (standing) with M.G.K. Men on in the emulsion laboratory.

Powell had been a research student no longer work on the project. In­ under C.T.R. Wilson at the stead, he joined Powell in Bristol, Cavendish Laboratory in Cambridge, becoming a driving force behind the before joining the H.H. Wills Physics development of the new emulsion Laboratory, (also known as the technique. He was joined by one of Royal Fort), at Bristol University in his research students, Cesare Lattes, 1928 as an assistant to the Director, towards the end of 1946. Arthur Tyndall. They worked together Photographic manufacturers llford on the mobility of ions in gases until were soon able to supply 'Nuclear 1935 when Powell became interested Research Emulsions' and in autumn in nuclear physics, inspired by the 1946 Donald Perkins, then at discoveries in Rutherford's Imperial College, London, exposed Cavendish Laboratory. Together with some at 9,100 m in an RAF a young lecturer, Geoffrey Fertel, he aeroplane, while Occhialini took embarked on the construction of a several dozen plates to the Pic du 750 keV Cockcroft-Walton Midi at 2,867 m in the French accelerator, which they brought in to Pyrenees. At that time access to the operation in 1939. Pic was by a rough track in summer The original intention was to study and by ski in winter, a small low energy neutron scattering using a telepherique only being brought into Wilson cloud chamber. However, in service in the summer of 1952, but 1938 the theoretician Walter Heitler World War II interrupted the work, but Occhialini had been a mountain (then in Bristol) mentioned to Powell with the existing emulsions Powell guide in Brazil. that in 1937 two Viennese physicists, was able to show that for scattering Examination of the emulsions in Marietta Blau and Herta Wambacher, studies they gave results superior to Bristol and in London revealed, as had exposed photographic emulsions cloud chambers, as well as being Powell later wrote, "a whole new for five months at 2,300 m in the much faster. world. It was as if, suddenly, we had Austrian Alps and had seen the Blackett (who had been a broken into a walled orchard, where tracks of low energy protons as well contemporary of Powell in the protected trees flourished and all as 'stars' or nuclear disintegrations, Cavendish Laboratory) then played a kinds of exotic fruits had ripened in probably caused by cosmic rays. decisive role through his influence great profusion". This new world Heitler commented that the method with the Ministry of Supply of the became a subject of intensive was so simple that 'even a 1945 UK Labour Government. He investigation. Occhialini has well theoretician might be able also to do was largely responsible for the described the atmosphere at Bristol:- if. This intrigued Powell, and Heitler setting up of two panels, one to plan "Unshaved, sometimes I fear un­ travelled to Switzerland with a batch accelerator building in the United washed, working seven days of the of llford half-tone emulsions, 70 Kingdom (which he chaired) and one week till two, sometimes four in the microns thick, and exposed them on to encourage the development of morning, brewing inordinately strong the Jungfraujoch at 3,500 m. In a sensitive emulsions (chaired by coffee at all hours, running, shouting, letter to 'Nature' in August 1939, Joseph Rotblat, recently awarded the quarrelling and laughing, we were they were able to confirm the Nobel Peace Prize for his Pugwash watched with humorous sympathy by observations of Blau and work). the war-worn native denizens of the Wambacher. Towards the end of the war, Royal Fort". . . . "It was a reality of The half-tone emulsions could only Blackett had invited his erstwhile intense, arduous and continuous record the tracks of low energy collaborator Occhialini, then in Brazil, work, of deep excitement and incred­ protons and alpha particles and to join the British team working with ibly fulfilled dreams. It was the reality Powell realised that to do useful work the Americans on the atomic bomb. of discovery....". it was necessary to increase their Occhialini arrived in the United Perkins was the first to observe a sensitivity by increasing the Kingdom in mid-1945, only to learn clear example of what appeared to concentration of silver bromide. that, as a foreign national, he could be the nuclear capture of a meson in

CERN Courier, June 1997 3 Physics monitor

A classic Bristol pion. The track of the positively-charged pion produced in the interaction 'star' (top left) has been cut in two to facilitate presentation. Bottom right, the pion eventually decays into a muon, which after some 600 microns itself subsequently decays, producing an electron. This full decay chain was recorded in electron-sensitive emulsion, available from 1948, even more sensitive than the specially-developed nuclear research emulsions in which the pion was discovered in 1947.

station at 5,200 m. Arthur Tyndall recommended that Lattes should fly BOAC to Rio de Janeiro. Lattes preferred to take the Brazilian airline Varig, which had a new plane, the Super Constellation, thereby avoiding a disaster when the British plane crashed in Dakar and all on board were killed. Examination of the plates from Bolivia quickly yielded ten more two- meson decays in which the secondary particle came to rest in the emulsion. The constant range of around 600 microns of the secondary meson in all cases led Lattes, Occhialini and Powell, in their

Mosaic of phoio-micrographs uf o nuclear explosion accompanied by the ejection of a 7r+-particle. The track of the ir+-particle October 1947 paper in 'Nature ', to is given in two ports which should join at the point " a \ . The ^-particle shows the fxansmutation TT -*• y. -*• e. postulate a two-body decay of the primary meson, which they called n or pion, to a secondary meson, JLI or the emulsion and producing a nu­ cian, Max Roberts, who later worked muon, and one neutral particle. clear disintegration. Measurements at CERN for many years. In this Subsequent mass measurements on of the multiple scattering as a func­ event the secondary meson ended in twenty events gave the pion and tion of residual range indicated a the emulsion, with a range of 610 muon masses as 260+30 and mass between 100 and 300 times microns. 205±20 times that of the electron that of the electron. Perkins' observa­ The two events gave such convinc­ respectively, while the lifetime of the tions, published in January 1947, ing evidence for a two-meson decay pion was estimated to be some 10~8s. were confirmed by Occhialini and chain that Lattes, Muirhead, Present-day values are 273.31 and Powell, who published details of six Occhialini and Powell published their 206.76 electron masses respectively such events only two weeks later. findings in 'Nature' in the issue of 24 and2.6x10"8 s. Mesons were easily distinguished May, 1947. Commenting on the The number of mesons coming to from protons in the emulsion be­ problems surrounding the rest in the emulsion and causing a cause of their much larger scattering identification of the cosmic ray disintegration was found to be and by their variation of grain density mesotron with the Yukawa nuclear approximately equal to the number of with range. force meson, they wrote:- "Since our pions decaying to muons. It was, Yet another exotic fruit followed. In observations indicate a new mode of therefore, postulated that the latter the spring of 1947 one of Powell's decay of mesons, it is possible that represented the decay of positively- team of microscope observers, they may contribute to a solution of charged pions and the former the Marietta Kurz, found a meson these difficulties". nuclear capture of negatively- stopping and giving rise to a second More evidence was needed to charged pions. Clearly the pions meson, which left the emulsion when justify such a radical conclusion. For were the particles postulated by nearly at the end of its range. Powell some time no more two-meson Yukawa. This led to the conclusion and a young Bristol graduate, Hugh events were found in the Pic du Midi that most of the mesons observed at Muirhead, were the first physicists to emulsions and it was decided to sea level are penetrating muons look at the event, which they make exposures at much higher arising from the decay in flight of immediately recognised as being two altitudes. Lattes proposed going to pions created in nuclear related mesons. Within a few days a Mount Chacaltaya in the Bolivian disintegrations higher up in the similar event was found by Irene Andes, near the capital La Paz, atmosphere. Roberts, the wife of the group techni­ where there was a meteorological Powell was awarded the 1950

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Nobel Physics Prize for his develop­ ode ray experiments, said that ment of the emulsion technique and cathode rays had to be carried by for the discovery of the pion; negatively-charged particles, Electron centenary Occhialini was awarded the 1979 assuming they had unit charge, Wolf Prize (shared with George carrying only a small fraction of the Many events are being organized Uhlenbeck) for his contribution both mass of a hydrogen atom. Walter to mark the centenary of the to the pion discovery and to that of Kaufman in Berlin was puzzled by discovery of the electron by J.J. pair production with Blackett, who the result that the charge/mass ratio Thomson at Cambridge. Consult obtained the 1948 Nobel Physics of the cathode ray particles appeared http://www. ioppublishing. com/ Prize. to be the same, no matter what gas Physics/Electron. However for a was used. memorable guided tour through electron history, try the website By Owen Lock Thomson also measured this prepared by the American charge/mass ratio, and, like Institute of Physics' Center for Wiechert, suspected that it was so the History of Physics, http:// small because the cathode ray www. aip. org/history/electron particles themselves were small. This includes a recording of Unlike Wiechert, Thomson had the One hundred years Thomson's venerable yet temerity to state: 'Thus ... we have in exultant voice from the ago..... the cathode rays matter in a new soundtrack of the film, Atomic state, a state in which the subdivision Physics, copyright © J. Arthur of matter is carried very much Rank Organization, 1948 - "Could Marking a full century of subatomic further...'. anything at first sight seem more physics, an occasional series of Two years later, Thomson had impractical than a body which is CERN Courier article looks back to measured the charge on what would so small that its mass is an what was happening exactly one soon be universally called electrons insignificant fraction of the mass hundred years ago. (he preferred 'corpuscles') and had of an atom of hydrogen? - which also investigated their photoelectric itself is so small that a crowd of role. As several other researchers these atoms equal in number to ll over the world, this year is were to do in the ensuing century, the population of the whole world A being celebrated as the Thomson took the baton of discovery would be too small to have been centenary of the discovery of the and ran with it. detected by any means then electron by J.J. Thomson at the Emboldened by his electron known to science." Thomson died Cavendish Laboratory, Cambridge, in discovery, he also proposed in 1897 in 1940, three years after the 1897. At a time when sovereignty that his electric corpuscles were a death of his famous pupil, interests in science were much to the component of all atoms, the first step Rutherford, who was 15 years his fore, it is surprising that this in what was to become his famous junior. discovery has earned such universal 'plum-pudding' model of the atom recognition. It was not as if the which was not overthrown until electron came out of the blue. Rutherford's epic discovery of the The saga is described in Abraham nucleus some fifteen years down the Pais' masterpiece 'Inward Bound' line. working on the 'Becquerel rays', (Oxford University Press). Despite Thomson directed the Cavendish discovered in Paris the previous having been investigated for many research with a firm hand, and the year, and soon to become better years, cathode rays were still direction of Cavendish research had known as radioactivity. controversial. Were they rays or been profoundly altered by Rontgen's Elsewhere in physics, two topics particles? In 1895 Jean Perrin in 1895 discovery of X-rays. In 1896 were much discussed, both of them France had revealed that they Thomson had published, with his in connection with empirical formulas carried negative electric charge. student Ernest Rutherford, a classic whose deep meaning would not In 1897, Emil Wiechert in paper on gas ionization by X-rays. become apparent for some time. In Konigsberg, after carrying out cath- For his part, in 1897 Rutherford was 1885 the Swiss scientist Jakob

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