Light-Meson Spectroscopy at COMPASS
by
NB2/158
RUB
Abstract:
The COMPASS experiment at the CERN SPS is a multi-purpose fixed-target experiment designed to study the strong interaction. Using a $190$ GeV/c hadron beam, COMPASS has recorded the world’s largest data set of diffractive scattering reactions. These data provide a unique opportunity to study the excitation spectrum of non-strange and strange light mesons with masses up to about $2.5$ GeV/c${}^2$, as well as to search for exotic states beyond the constituent quark model. This is achieved through comprehensive partial-wave analyses of various final states, in which the data are decomposed into amplitudes with well-defined quantum numbers, allowing for the extraction of resonance parameters of the contributing mesonic states.
In this talk, recent experimental results from the light isovector sector are
presented. Results on the exotic $\pi_1(1600)$, the lightest hybrid-meson candidate with $J^{PC} = 1^{−+}$, are highlighted. This state has been observed at COMPASS in all predicted dominant decay channels, including $\eta′\pi^-$ and $f_1(1285)\pi^-$ (both in the $\pi^-\pi^-\pi^+\eta$ final state), as well as $b_1(1235)\pi^-$ (observed in the $\omega\pi^-\pi^0$ final state). The $K^0_SK^−$ final state allows for a precise determination of excited $a_J$ states with even spin $J$ and enables an exclusive study of these mesons at high masses, a hitherto unexplored region. The analysis of the $K^−\pi^-\pi^+$ final state is further presented, representing the most comprehensive measurement of the strange-meson spectrum to date and providing first evidence for a supernumerary $J^P = 0^−$ state.
In addition, the analysis of non-resonant double-Regge exchange processes
using $\eta(′)\pi$ data in the high-mass region is discussed. This constitutes the first event-based likelihood fit to the full COMPASS data set in this regime and improves the understanding of non-resonant production mechanisms in light meson spectroscopy.