Partial-Wave Analysis of the Centrally Produced $\pi^+ \pi^-$ System in pp Reactions at COMPASS

COMPASS is a fixed-target experiment at CERN SPS which investigates the structure and spectroscopy of hadrons. During nine weeks in 2008 and 2009, a 190 GeV/c proton beam impinging on a liquid hydrogen target was used in order to study the production of exotic mesons and glueball candidates at central rapidities. As no bias on the production mechanism was introduced by the trigger system, the contribution from diffractive dissociation of the beam proton poses a challenge. We select a centrally produced sample by kinematic cuts and introduce a model to describe the data in terms of partial waves. Preliminary fits are presented, which are consistent with results from previous experiments. Particular attention is paid to the ambiguities in the amplitude analysis of the two-pseudoscalar final state.