Narrow Results

Motivated by the recent surprising discovery of two new meson states with quark content but unexpectedly low masses and narrow total decay widths, we work out, in a nonrelativistic potential-model approach developed already some two decades ago, the predictions for the energy levels of the corresponding charm–anti-strange quark bound states. In spite of the fact that this simple quark model reproduces the mass spectrum of the previously observed hadrons remarkably well, we are led to the conclusion that, without considerable modifications, both the new states do not fit into this framework.

The BaBar experiment has discovered an unexpected new narrow state, in the inclusive invariant mass distribution from e⁺e^- annihilation data at energies near 10.6 GeV. The same experiment has also shown evidence for structure in the 2.46 GeVc² region in the mass spectrum. These discoveries have triggered a search in several experiments for new states coupled to the meson which have confirmed the existence of together with both in inclusive e⁺e^- annihilation and in B-decays. These two new states are difficult to explain in terms of potential models.

The roles of the COSY-TOF and COSY-ANKE spectrometers in the measurement of strange meson production are briefly reviewed, mainly in connection with new results on the pp → K⁺pΛ, pp → K⁺pΣ⁰ and pp → K⁺nΣ⁺ reactions.

Strangeness production by both hadronic and leptonic projectiles with beam energies of up to a few GeV is reviewed. The focus is on the production of strangeness using proton and ion beams, as well as the photo- and electroproduction of strangeness, as observed at modern facilities. The elementary production of K^± and ϕ mesons as well as Λ, Σ hyperons on the nucleon is described. Based on these results, the production of strange mesons and strange baryons on nuclear targets, as well as the creation of light hypernuclei is discussed, emphasizing the influence of the nuclear medium.

Remarks on strange quarks in the nucleon.

COMPASS is a multi-purpose fixed-target experiment at CERN aimed at studying the structure and spectrum of hadrons. It has collected the so far world’s largest data set on diffractive production of the K⁻π⁻π⁺ decay, which in principle gives access to all kaon states. We performed an elaborate partial-wave analysis, using model-selection techniques to select the wave set based on a large systematically constructed pool of allowed partial waves. The partial- wave decomposition reveals signals in the mass region of well-known states, such as K₁(1270) and K₁(1400). In addition, we observe potential signals from excited states, such as K₁(1650).