Spin at Lausanne

Part of the apparatus used at the Argonne ZGS to measure polar/zed proton-proton cross-sections. One year after the closedown of the ZGS, these results are still a talking point wherever spin physics is discussed.

(Photo Argonne)

From 25 September to 1 October, some 1 50 spin enthusiasts gathered in Lausanne for the 1980 Inter­ national Symposium on High Energy Physics with Polarized Beams and Polarized Targets. The programme was densely packed, covering phy­ sics interests with spin as well as the accelerator and target techniques **'hich make spin physics possible.

The physics

While major physics conferences these days are dominated by the promise of the 'new ortho- doxy'-standard electroweak theory plus quantum chromodynamics pro­ viding a basis for the understanding of all basic interactions (except grav­ ity) in terms of quark-lepton interac­ tions - this was far from being the case at Lausanne. Spin physics has given indications of new phenomena which are not readily explained in terms of existing ideas. Using high energy non-polar­ ized proton beams on non-polarized ^uclear targets, the polarization of *e produced lambda hyperons shows that spin is not just a detail confined to low energy interac­ tions. At Lausanne, Oliver Overseth pre­ sented latest polarization results from the long-standing Michigan/ Minnesota/Rutgers/Wisconsin col­ laboration at Fermilab. This now pro­ vides polarization measurements of charged, as well as neutral hyperons, tions, then something simple must date mechanisms have been pro­ and preliminary data shows that the be happening! posed which could explain why sin- polarization of positively-charged These hyperon results are sup- gle quarks coming from this sea sigma particles is opposite to that of ported by data at other energies would have a definite spin state, the lambda and cascade particles. from Brookhaven, the CERN ISR and The same Fermilab experimental Any hyperon polarization is puzz­ Serpukhov. One promising sugges- programme has provided indications ling if it is assumed that 400 GeV tion is that the hyperon polarization of hyperon magnetic moments. Final proton-nucleus collisions must be so comes directly from the strange results, covering a wider range of complex that any spin effects are quarks picked up from the sea of hyperons, are still awaited, but the averaged out. However Overseth virtual quark-antiquark pairs sur- data amassed so far at Fermilab and turns this argument around: if polar­ rounding the target nucléons. This from other experiments (such as the ization is observed in these reac- sea is itself unpolarized, but candi- HYBUC bubble chamber at CERN) give results which are not easily he for one was ready to give up The present state of the art was explained in terms of any conven­ QCD. reviewed at the Symposium by tional quark model. As well as the effects seen in T.O. Niinikoski, a specialist in the More unexplained results come hadron experiments, there is now a use of extremely low temperatures from spin studies using polarized lot of interest in the physics that can in polarized targets at CERN, where beams and/or polarized targets, be done using polarized electron polarized target research had been such as those carried out at the beams. The major results obtained at led for many years by M. Borghini. Argonne ZGS before its closure last SLAC on polarization effects in deep Among the new polarizable mater­ year. (There was a lot of discussion inelastic scattering showed up the ials which hold out the promise of on the possibility of continuing this delicate interference between weak higher polarization (and/or ha> valuable work at Argonne at other and electromagnetic interactions, more easily polarizable nuclei) at machines and at higher energies.) and could be the herald of a new type lithium deuteride, pursued particu­ Another open book is the spectro­ of physics experiment. The discus­ larly at Saclay, where polarizations scopy of dibaryon resonances, re­ sions at Lausanne mirrored this of over 70 per cent have been ported at Lausanne from a number of growing interest in polarized elec­ achieved, and ammonia, with which different experiments. With all this tron beams and the experiments 90 per cent polarization has been evidence, specialists are adamant which could be done with them at achieved, used at Bonn, Stanford that spin is an essential considera­ new machines such as LEP and and CERN. The techniques of frozen tion, important in all energy re­ HERA. spin targets, where the target mater­ gimes. The Lausanne meeting provided a ial can be polarized and then trans­ Quantum chromodynamics, hand­ very welcome opportunity for spin ferred, retaining its polarization, to a cuffed to perturbation theory, has specialists to meet and exchange location providing greater detector had little to say about the kinemati- views. It was easy to get the impres­ access, seem to be well mastered. cal areas where these spin effects sion that it also gave them a chance Performance of polarized sources occur and about spin in general, to vent some of their frustrations. has also advanced steadily. Polar­ although at Lausanne Stanley Although some unexplained spin ized negative hydrogen ions (re­ Brodsky of S LAC proposed how the effects are now several years old, viewed by W. Haeberli) are of partic­ spin of quark and gluon interactions some people were of the view that ular interest for multiturn injection in could lead to selection rules. the message is not getting across. proton accelerators. The technique The Lausanne meeting contained As it has been voiced in high (energy) using atomic beams is under deve its own mini-conference-a round- circles recently when referring to opment, for example, at Bonn, Dub- table discussion initially entitled 'Is some of these recent spin re­ na, Zurich and Argonne (for use at spin physics worthwhile?' - with sults - 'to believe that high energy the Brookhaven AGS following clo­ theory, experiment and machine physics is now understood is just sure of the ZGS) and is yielding specialists taking part. However the plain foolish !' negative ion beams of several |iA. participants were so convinced that The ionization method developed at spin physics is worthwhile that the The technology CERN, in collaboration with ANAC of first resolution was to change the New Zealand (achieving the record title of the discussion. The provision of the necessary current of 1 10 |iA of polarized pro­ The ensuing physics discussion experimental conditions to study tons) is being applied successfully at soon turned to the merits of quan­ high energy spin physics began with Zurich to negative ions. A new tum chromodynamics as a serious the work on polarized targets which method, proposed by Haeberli, theory of strong interactions. Elliott became feasible following develop­ where polarized neutral atoms col­ Leader challenged his fellow theo­ ment of the dynamic polarization lect an electron on passing through a rists to propose the 'cleanest' tests method in the early 60s and mastery paramagnetic vapour such as so­ of QCD. This invitation was directly of the technologies of high magnetic dium, is also giving promising results taken up by Stan Brodsky who was fields and low temperatures. The at Wisconsin and at KEK, Japan. of the view that QCD predictions for ideas came from A. Abragam at A team at CERN, with the assist­ certain exclusive channels were Saclay and were first pursued there ance of a Lausanne / Michigan / extremely clean, and that if these and at the Berkeley Laboratory under Rockefeller collaboration, has fas­ predictions turned out to be wrong, CD. Jefferies. tened onto the atomic beam polar- alone and, with the closure of the Polarization ZGS, it has now been passed to in electron rings Brookhaven, KEK and Fermilab. While Argonne pushed polarized One of the main talking points effects were recorded two protons, polarized electrons were at the Lausanne Symposium years later. Since 1973, Stan­ investigated mainly by Stanford, was the likelihood of achieving ford has also pursued the po­ using the SPEAR storage ring. The polarization in electron rings larization monitoring technique main concern now is whether polar­ at high energy. The realization by backscattering of a laser ization will be retained in the higher that synchrotron radiation beam, proposed by D. Fry- energy machines. A full review of the could result in polarization in berger and D. Prescott (the problem was given by J. Buon. The an electron storage ring came technique had earlier been happy news from the PETRA stor­ from A. A. Sokolov and suggested by V.N. Baier and age ring at DESY (see July/August I.M. Ternov in 1964. Experi­ V. Khoze in the Soviet Union). issue, page 196) was reported by ments were proposed on The laser polarimeter was R. Rossmanith. While polarization is SPEAR in 1971, following re­ operated on SPEAR in 1977 seen with a single beam, it is absent ports of polarization measure­ and a version for the PEP sto­ with two beams, so it is obvious that ments from Orsay and Novo­ rage ring is now under con­ some beam-beam conditions spoil sibirsk, and large polarization struction. the polarization. The hunt for stable polarization colliding beam con­ ized proton source configuration to aluminium arsenide cathodes and ditions will be pursued. Alex Chao, propose a polarized jet target as an other semiconductor materials. leaning on the experience with internal target for an accelerator pro­ Charlie Prescott covered the interest SPEAR, showed the calculated con­ viding a density of over 1012 atoms in polarized beam physics with the ditions in which polarization is per cm3 (reported by W. Kubischta). proposed Single Pass Collider at expected in PEP. Looking further into L Dick, in discussing the physics SLAC. the future, Bryan Montague has which would be possible with such a It is particularly in the area of been studying the problems of target, talked of future plans for a polarized beams in accelerators and retaining polarized beams in LEP and very high density target (over 1015 storage rings that a great deal of new of spin rotation in the colliding beam jjtoms per cm3) which could allow information and interest has welled regions. measurements with precision of up in recent years. This was reflected The accelerator physics behind 10~7 and make it possible to see at the Symposium by the presence in these calculations on the degree of such refined effects as the neutral force, for the first time at one of polarization which can be attained current interference with the strong these polarization meetings, of lead­ led to much discussion at the Sym­ interaction. A new range of polariza­ ing accelerator physicists. posium. A favourite candidate to tion physics with high precision will Polarized proton beams in acceler­ help retain polarization is the Siber­ also be possible on the LEAR storage ators were covered recently in the ian Snake, a magnetic field inversion ring at CERN (reported by K. Kilian), CERN COURIER (see May issue, technique so-called because it was given the high intensities of the low page 104), but in passing them by it invented at Novosibirsk. The full energy antiproton beams. is still worth acknowledging the extent of its abilities requires further Polarized electron sources have achievement of GeV range polarized detailed study, but it is already clear been a speciality at Stanford, where beams at the Argonne Zero Gradient that two Snakes would be needed in a Yale team installed the first source, Synchrotron. It was an Argonne / an electron storage ring. known as PEGGY, in 1974. The Michigan / CERN group who first Covering a wide and ambitious latest work, led by Charlie Sinclair, pushed this work theoretically and programme, the interdisciplinary as­ has achieved emission currents of experimentally from 1974. There pect of the Lausanne meeting was 60 A from a gallium arsenide photo- was interest at that time in polarized striking, and it was stimulating to see cathode. Typical emerging beams at beams in the CERN PS, but this the high level of interaction between 120 Hz have 40 per cent polariza­ development was halted to give particle physicists, condensed mat­ tion and 0.5 A currents. Improve­ priority to other projects. Argonne ter specialists and the accelerator ments are being sought with gallium thus carried the polarization flag fraternity.