New Physics Opportunities  Hadron Structure  Quark and Gluon Densities

Strangeness

The study of strangeness will be addressed with hadronic as well as with electromagnetic probes. In both cases selected spin observables will provide a way to reveal interesting small amplitudes through their interferences with the dominant ones. During the next decade, the proton beam of COSY (Jülich), the secondary kaon beam of DA$\Phi$NE (Frascati) as well as electron and photon beams of CEBAF (Newport News), GRAAL (Grenoble) and ELSA (Bonn) will allow the low energy sector of this field to be covered. At the beginning of next century, a new hadronic facility at Tsukuba in Japan (JHP) will make accessible its high energy sector. In the 1960s a comprehensive study of kaon-nucleon scattering, as well as kaon production in pion-nucleon collisions, led to a fair determination of the corresponding partial wave amplitudes. On the contrary, the data set available in kaon photo- and electroproduction channels is meagre and precludes a meaningful determination of the corresponding multipoles. In the next five years, the combined analysis of the data obtained with the intense, monochromatic and polarised beams of real photons at CEBAF, GRAAL and ELSA will make possible such an amplitude decomposition in the resonance region ( $\sqrt{s}-m\leq 1$ GeV). Above the resonance region (up to $E_{\gamma }\simeq 6$ GeV), Jefferson Lab will enter the regime where cross-sections are dominated by hard scattering on constituent quarks around 90 $^{\circ }$ and large transverse momentum. The determination of the kaon electromagnetic form factor will be carried out at Jefferson Lab, up to $Q^{2}\simeq 4$ GeV². Our knowledge of the free hyperon-nucleon (YN) interaction relies only on old bubble chamber data. The statistics are low and the range of the hyperons prevents access to low energy scatterings. It is remarkable that the only determination of the YN scattering lengths comes from the analysis of the energy spectrum of hypernuclei. To overcome this difficulty hyperons must be produced in nuclear reactions, in kinematics which enhance YN rescattering in the final state. For instance, the spectrum of the kaons emitted in the reaction $pp\rightarrowK^{+}\Lambda p$ has been measured at SATURNE. Its high energy part, which corresponds to a very small relative energy between the emitted proton and kaon, exhibits an enhancement due to the strong YN interaction in S waves. A cusp, due to the strong coupling between the $\Lambda N$ and $\Sigma N$ channels, has been observed at the $\Sigma$ production threshold. These measurements will be pursued at COSY with clever techniques pioneered at LEAR in the study of the $\overline{p} p\rightarrow \overline{Y}Y$ ( $Y=\Lambda ,\;\Sigma$) reaction close to threshold. The study of spin correlations between two polarised colliding protons will disentangle YN rescattering in the ³S₁ and ¹S₀ states. At Jefferson Lab a complete mapping of the cross-section of the reaction $D(\gamma ,K^{+}\Lambda )n$ will be achieved with the new large acceptance detector CLAS. The measurement of the polarisation of the emitted $\Lambda$ , as well as the use of polarised photons, will determine in details the nature of YN interaction. Strangeness can also be hidden in hadronic matter. For instance the $s\overline{s}$ component dominates the wave function of the $\phi$ meson, which decays mainly into a pair of kaons. To a lesser extent, the wave function of the $\eta$ and $\eta ^{\prime }$ mesons contains a small $s\overline{s}$ component. There are also speculations that the ground state of nucleons and nuclei may contain a sizeable strange component, which may be at the origin of evasions from the OZI selection rule as well as new parity violating effects. One of the major achievements is the large violations to the OZI rule observed at LEAR by the Obelix and Crystal Barrel Collaborations. Production of $\phi$ following the annihilation of $\bar{p}$ and $\bar{n}$ was particularly abundant in channels with a $\pi$ or a $\gamma$ in the final states, nearly two orders of magnitude larger than predicted by the simple OZI rule. Among the different explanations for such an effect, the one based on the presence of strange quark-antiquark pair ($s\bar{s}$) in the nucleon seems the most likely. At moderate momentum transfer, a possible $s\overline{s}$ component may be knocked out by a virtual photon and eventually couple to the $\phi$ meson. It can be revealed though interference with the dominant diffractive background as well as through amplitudes which do not conserve helicity.