Recent Results on Light-Meson Spectroscopy from COMPASS

Abstract

The main goal of the spectroscopy program at COMPASS is to explore the light-meson spectrum below about 2 GeV/c 2 in diffractive production. Our flagship channel is the decay into three charged pions: p + π − → π − π − π + + p recoil , for which COMPASS has acquired the so far world's largest dataset of roughly 50 M exclusive events using an 190 GeV/c π − beam. Based on this dataset, we performed an extensive partial-wave analysis. In order to extract the resonance parameters of the π J and a J states that appear in the π − π − π + system, we performed the so far largest resonance-model fit, using Breit-Wigner resonances and non-resonant contributions. This method in combination with the high statistical precision of our measurement allows us to study ground and excited states. We have found an evidence of the a 1 (1640) and a 2 (1700) in our data, which are the first excitations of the a 1 (1260) and a 2 (1320), respectively. The relative strength of the excited states with respect to the corresponding ground state is larger in the f 2 (1270) π decay mode compared to the ρ(770) π decay mode. We also study the spectrum of π 2 states in our data. Therefore, we simultaneously describe four J PC = 2 −+ waves in the resonancemodel fit by using three π 2 resonances, the π 2 (1670), the π 2 (1880), and the π 2 (2005). Within the limits of our model, we can conclude that the π 2 (2005) is required to describe all four 2 −+ waves properly.