η (and π) production and decay at Saturne

The search for rare (and forbidden) decay modes of light mesons like μ, π, K and η provides stringent tests of symmetries and conservation laws; they may also lead to hints of new interactions and particles. But while μ, π and K mesons are abundantly produced in various laboratories and their decay modes studied at the level of branching ratios of 10-12 and even lower, the data on η decays are scarce and very unprecise because until now η mesons were produced with low flux, essentially through the π- p → n η reaction. Recent experiments at Saturne have shown a remarkably high production rate of η mesons in the pd →3He η reaction, with a very low level of background. This opens the possibility to study rare decay modes of the η with an η flux several orders of magnitude higher than in any previous experiment. Another advantage is that the η source is tagged. Pions are also produced abundantly in the dp → 3He π0 reaction, however we shall stress the η case since it is an original and rather unique possibility of the Saturne synchrotron. It is worth to recall here that the η meson is isoscalar while π is isovector, so that their decay modes bring complementary informations. In the first paragraph we give the main features of the source of tagged η for the Saturne synchrotron, then we discuss the Physics involved in the study of various decay modes of the η meson into charged particles and the last paragraph deals with the feasibility of such experiments