Diffractive Dissociation into π[sup −]π[sup −]π[sup +] Final States at COMPASS

Abstract

QCD predicts gluonic excitations like hybrids to contribute to the meson spectrum in addition to qq̄ pair configurations. The most promising way to identify such states is the search for JPC quantum number combinations which are forbidden in the constituent quark model. The fixed target COMPASS experiment at CERN offers the opportunity to search for such states in the light quark sector with an unprecedented statistics.Diffractive reactions of 190 GeV/c pions on a lead target were studied by COMPASS during a pilot run in 2004. A Partial Wave Analysis (PWA) of the π−π−π+ final state with 42 waves including acceptance corrections through a phase‐space Monte Carlo simulation of the spectrometer was performed. The exotic π1(1600) meson with quantum numbers JPC = 1−+ has been clearly established in the rho‐pi decay channel with a mass of 1660±10(stat) MeV/c2 and a width of 269±21(stat) MeV/c2. The improved detector performance in 2008 allows us to study this channel with significantly higher statistics. First results of the ongoing analysis of the 2008 data taking period, using a 190 GeV/c pion beam on a liquid hydrogen target are presented in this paper.