Lambda-Lambda and cascade hypernuclei

For S=-2 systems, the current beam intensities on the 2 GeV/c AGS beamline are sufficient to make a qualitative advance in our knowledge of and hypernuclei, which at present consists of a handful of questionable emulsion events. One key question is whether discrete quasiparticle states exist in nuclei. Except for the A=4 case, recent investigations at the AGS have not indicated a corresponding series of narrow states for the . However, the case is much more favorable, since the phase space available for the strong decay is much smaller than for . The binding energies of hypernuclear states would fix the strength of the interaction, the widths of the states the strength of the interaction, and the binding energies of hypernuclei the strength of the interaction. This basic information would provide an excellent test of quark exchange aspects of the strong force, since long range exchange is essentially absent in the S=-2 sector. This line of research also provides a gateway to the study of multiply strange hadronic or quark matter. This is of crucial importance for studies of the role of strangeness in the equation of state at high-density, as probed in the cores of neutron stars and high-energy heavy ion collisions at RHIC, for instance.

We propose to study nuclear hyperons with the reaction . This would be a ``discovery'' experiment, as there exist only a few events of a ``bound'' seen in emulsion to date. In these emulsion experiments, it is not possible to clearly identify the final state of the hypernucleus, which will be straightforward in a magnetic spectrometer experiment if narrow bound states exist. From the data, we can determine a binding energy and width for the bound . This is an important constraint on the Y-N interaction, which cannot be studied directly by N scattering. The depth of the nuclear potential is important also for the existence of strange hadronic matter (SHM), as SHM will not be bound if the depth of the N potential is less than about 15 MeV.

A large new spectrometer is needed for this experiment. The spectrometer (referred to here as the S2S) will have a solid angle of about 100 msr, and a resolution of 1 MeV when used on the D-6 line at the AGS. Several options are being considered. One of these is to construct a higher momentum version of the SKS spectrometer built by INS for KEK, which has already achieved 2 MeV resolution at 1.1 GeV/c and 100 msr solid angle.

We assume that the design, funding, and construction of the S2S spectrometer will take approximately 3 years. Thus this experiment could receive first beam in FY-2000. The AGS delivers 6x10 protons per pulse, and it is reasonable to use 2x10 protons per pulse on the target for D-6. With this proton intensity, we would then have a rate of 5x10 K per second at 1.8 GeV/c. The resulting rate for K from cascade hypernuclei from a 5 gm/cm**2 carbon target is about 2 K per hour for a state with 100 nb/sr cross section. Thus 1000 counts could be obtained in a 500 hour run, a rate which is similar to S=-1 hypernuclei produced in the reaction. With 1000 counts in a single state, it is feasible to consider experiments with detection of decay products.

The S2S spectrometer will be useful for several other experiments including direct production of hypernuclei, the study of the decay of nuclei into hypernuclei using the CDS, and observation of radiative transitions from doubly-strange systems.

The 2 GeV/c separated kaon line (D6) at the AGS was developed for two ongoing searches for the doubly-strange H particle, in the reactions and . The new work being initiated on the properties of S=-2 nuclear systems also bears directly on the existence of the H particle, since hypernuclear ground states would be expected to decay to H plus a nuclear core if .