Diffractive Dissociation into Three-Pion Final States at COMPASS

Understand

In order to study diffractive dissociation reactions, COMPASS has taken data with a 190 GeV/$c$ pion beam impinging on a liquid hydrogen target in 2008 and 2009.

• At squared four-momentum transfers to the target t' between 0.1 GeV$^2/c^2$ and 1.0 GeV$^2/c^2$ the number of events with three pions in the final state is about an order of magnitude larger than that acquired by any previous experiment.
• In COMPASS, the three-pion final state can be studied in the two channels $\pi^-\pi^-\pi^+$ and $\pi^-\pi^0\pi^0$.
• The large data sample in particular for the $\pi^-\pi^-\pi^+$ channel allows to find even small signals at the sub-percent level.