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CERN Hypernuclei with Sigma Particles

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CERN Hypernuclei with Sigma Particles The first observation of hyper nuclei containing sigma particles. Spectra from a Heidelberg/Sacla y/Strasbourg collaboration at CERN using 720 MeV negative kaons on a beryllium target show two sharp peaks on the left due to the well-known hypernuc/ei containing lambda particles, while the smaller peaks in the centre are attributed to sigma hypernuc/ei. The two peaks in each case arise from different neutron spin states in the target nucleon. The bottom scale shows the respective binding energies. bunch for the production of further positrons. The intense bunches emerge at 50 GeV and a magnet bends them around opposite arms of a ring (about 300 m radius) of alter­ nating gradient focusing magnets so that they collide head-on after being focused to a radius around 1 |im. The particles are then rejected. The anticipated luminosity with such a system is 1030 per cm2 per s and the standard model then pre­ dicts a ratio of Z production to muon pair production of approximately 5000 with a total event rate of 1 50 per hour. There are a host of accelerator physics questions to be answered before the construction of such a project could be confronted with confidence. The Department of En­ ergy has been asked for money to finance preliminary engineering and design. CERN Hypernuclei with sigma particles Experiments at the 28 GeV Proton Synchrotron have created artificial nuclei (hypernuclei) containing sigma particles. Hypernuclei are formed when the usual nucleons in atomic nuclei are transformed into heavier particles (hyperons) by bombarding targets with low energy kaon beams. decay through strangeness conserv­ their study possible. The study of these artificial nuclei ing strong interactions into a nu­ Recent experiments at CERN by a reveals useful new information cleon and a kaon. Heidelberg / Saclay / Strasbourg about particle interactions and com­ The heavier sigma is also stable as collaboration exposed beryllium-9 plements the knowledge gained a single particle in strong interac­ and carbon-12 targets to a from scattering experiments. tions, however sigmas in nuclear 720 MeV separated beam of nega­ Previously, the only known hyper­ matter should be able to decay tive kaons. nuclei were those containing the through the strong reaction sigma The experimenters searched for neutral lambda particle, which at + nucleon -> lambda + nucleon. For strangeness exchange reactions 1115 MeV is the lightest hyperon. this reason it was first thought that where the kaons react with target The lambda is thus only slightly hypernuclei containing sigmas nucleons and, after having transfer­ heavier than the nucleon and cannot would not live long enough to make red their strangeness to target CERN Courier, December 1979 405 Aerial view of the Berkeley site with the location of the proposed relativistic heavy ion machine superimposed. The numbered buildings refer to existing facilities which could be used with the new machine. (Photo LBL) nucleons to produce sigmas or lamb­ das, emerge as pions. One spectrometer measured the momentum of the incident kaons, while the specially-designed SPES 11 spectrometer built by Saclay ana­ lysed the emergent pions. The large momentum acceptance of this in­ strument allowed pions coming from sigma and lambda production to be measured in the same spectrum. In addition, a scintillation counter surrounding the target was used to detect the fragmentation of hyper­ nuclei and the decay of lambdas. The sigma hypernuclei signals were found 77 MeV above the usual lambda hyperon levels, correspond­ ing to the mass difference between lambdas and neutral sigmas. The production of sigma hypernuclei was a quarter that of lambda hypernuclei, as expected from the relative proba­ bilities of the different possible strangeness-exchange reactions. Their width was measured at less than 8 MeV, a surprisingly narrow those available from the Bevalac (up signal in view of the instability of BERKELEY to 20 GeV per nucleon for heavy sigma particles in nuclear matter. The VENUS project ions), colliding ion beams of up to Further studies will now have to 20 GeV per nucleon in each beam. establish whether this narrow signal With several years of experience in The machine incorporates two is a peculiarity with light nuclei, or operating the Bevalac complex for superconducting accelerator/stor­ whether it is also found with heavier relativistic heavy ion physics behind age rings, about 200 m in diameter, nuclei. them, the accelerator physicists and in a tunnel on the Berkeley site. It will The results with beryllium-9 also experimentalists at Berkeley have use the SuperHILAC as injector. show a small peak which might prepared a project which would Because of the slope of the site, half correspond to hypernuclei contain­ enable them to pursue this field of the tunnel would be bored into the ing negatively-charged sigmas. The research with increased vigour hillside. position of this candidate peak beyond the late 1980s. The project The operation sequence for the full suggests that the interaction be­ is known as VENUS, for Variable energy beam would be to inject ions tween sigma particles and nuclei Energy Nuclear Synchrotron. into one ring at about 8.5 MeV/A differs from that between lambdas The aim has been to design a very and accelerate to 1 GeV/A. Further and nuclei, but that the sigma- versatile machine with the following stripping can then take place with­ nucleus interaction appears to be the major characteristics: intense ion out loss and the ions would be trans­ same for neutral and negatively- beams from protons to uranium, low ferred to the second ring for contin­ charged sigmas. energy (40 MeV per nucleon) capa­ ued acceleration to 20 GeV/A. The Further experiments with higher bility so as to 'overlap' with other beams could then be used for fixed levels of sigma hypernucleus pro­ machines, much higher intensities in target physics. However for beams duction could require more intense the intermediate energy range than of energy less than 8 GeV/A strip­ low energy kaon beams than exist at those currently available from the ping is not necessary and the second present. Bevalac, peak energies well above ring can be used to 'stretch' the 406 CERN Courier, December 1979 .
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