Light-Meson Spectroscopy with COMPASS

Abstract

Despite decades of research, we still lack a detailed quantitative understanding of the way quantum chromodynamics (QCD) generates the spectrum of hadrons. Precise experimental studies of the hadron excitation spectrum and the dynamics of hadrons help to improve models and to test effective theories and lattice QCD simulations. In addition, QCD seems to allow hadrons beyond the three-quark and quark-antiquark configurations of the constituent-quark model. These so-called exotic hadrons contain additional constituent (anti)quarks or excited gluonic fields that contribute to the quantum numbers of the hadron. The COMPASS experiment at CERN is studying the excitation spectrum of light mesons composed of up, down, and strange quarks. The excited mesons are produced via the strong interaction by scattering a 190 GeV/c pion beam off proton or nuclear targets. On heavy nuclear targets, in addition the electromagnetic interaction contributes in the form of quasi-real photon exchange at very low four-momentum transfer squared. COMPASS has performed the most comprehensive analyses to date of isovector resonances decaying into multi-particle final states. In this review, we give a general introduction into scattering theory and the employed partial-wave analysis techniques. We also describe novel methods developed for the high-precision COMPASS data. The COMPASS results are summarized and compared to previous measurements. In addition, we discuss possible signals for exotic mesons and conclude that COMPASS data provide solid evidence for the existence of the manifestly exotic ${\pi }_1(1600)$, which has quantum numbers forbidden for a quark-model state, and of the $a_1(1420)$, which does not fit into the quark-model spectrum. By isolating the contributions from quasi-real photon exchange, COMPASS has measured the radiative widths of the $a_2(1320)$ and, for the first time, that of the ${\pi }_2(1670)$.

175 pages, 83 figures, invited review article; typos corrected in Table 1 (quark charges) and in Eq.(10) (v2)

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Cited by in corpus (29)

• An updated review of the new hadron states
• 50 Years of Quantum Chromodynamics
• Masses of positive- and negative-parity hadron ground-states, including those with heavy quarks
• Novel approaches in Hadron Spectroscopy
• Triangle Singularity as the Origin of the $a_1(1420)$
• $S$- and $P$-wave fully-strange tetraquark states from QCD sum rules
• QCD axial anomaly enhances the $\eta {\eta }^{\prime }$ decay of the hybrid candidate ${\eta }_1(1855)$
• Bethe-Salpeter kernel and properties of strange-quark mesons
• Decay behaviors of the fully-bottom and fully-charm tetraquark states
• Central exclusive diffractive production of axial-vector $f_1(1285)$ and $f_1(1420)$ mesons in proton-proton collisions
• Photo- and electro-production of narrow exotic states: from light quarks to charm and up to bottom
• Constructing the $J^{P(C)}=1^{-(+)}$ light flavor hybrid nonet with the newly observed ${\eta }_1(1855)$
• Analysis of rescattering effects in $3\pi$ final states
• ${\pi }^{-}p\to {\eta }^{(\prime )}{\pi }^{-}p$ in the double-Regge region
• Light double-gluon hybrid states from QCD sum rules
• Hadronic molecular states with the quark contents $bc\bar{s}\bar{q}$, $b\bar{c}s\bar{q}$, and $b\bar{c}\bar{s}q$
• Effects of Final State Interactions on Landau Singularities
• Quark-antiquark states of the lightest scalar mesons within the Nambu-Jona-Lasinio model with flavor-dependent coupling constants
• Molecular pentaquark states with open charm and bottom flavors
• Fully-strange tetraquark states with the exotic quantum numbers $J^{PC}=0^{+-}$ and $2^{+-}$
• Light double-gluon hybrid states with the exotic quantum numbers $J^{PC}=1^{-+}$ and $3^{-+}$
• Simple relativistic quark models
• Analysis of the light $J^P=3^{-}$ mesons in QCD sum rules
• Second-order spin hydrodynamics from Zubarev's nonequilibrium statistical operator formalism
• Double-gluon charmonium hybrid states with various (exotic) quantum numbers
• Nature of $X(2370)$
• Emergence of the $\pi (1300)$ Resonance from Lattice QCD
• Strangeonium spectrum with the screening effects and interpretation of $h_1(1911)$ and $X(2300)$ observed by BESIII
• Strange-Meson Spectroscopy -- from COMPASS to AMBER