# The Hadron Spectrum Collaboration

The Hadron Spectrum Collaboration (hadspec) makes use of the numerical approach to Quantum Chromodynamics known as lattice QCD to study the spectrum of hadrons. An international collaboration of scientists in Europe, the US and India, we aim to understand how quarks and gluons confine themselves within the strongly interacting mesons and baryons observed in nature.

# News

### Hadspec calculates three-pion scattering amplitude

For the first time in lattice QCD, a three-hadron scattering amplitude has been determined using a general workflow that does not make use of model assumptions or a perturbative expansion. Focusing on the maximum isospin three-pion channel ($\pi^+ \pi^+ \pi^+ \to \pi^+ \pi^+ \pi^+$) the calculation uses a relativistic finite-volume formalism to relate lattice energies to the physical scattering amplitude. The work has been published in Physical Review Letters as an Editor’s Suggestion.

### Hadspec studies decays of a $1^{-+}$ hybrid meson

For the first time in lattice QCD, a calculation has shown the presence of an exotic $1^{-+}$ state appearing as an unstable resonance. The result shows that the longstanding model-based proposal that such a state would couple more strongly to the $\pi b_1$ final-state than the lower-lying $\pi \eta, \pi \eta'$ and $\pi \rho$ final-states is confirmed. Possible implications for the recently observed $\pi_1$ experimental candidate state are discussed. The calculation appears in a preprint posted to the arXiv, and has been accepted for publication in Physical Review D.

### Antoni Woss wins PANDA Theory PhD Prize

Hadspec member Antoni Woss has been awarded the PANDA Theory PhD Prize for his PhD thesis, "The scattering of spinning hadrons from lattice QCD". The award was announced by the spokesman of the Panda Collaboration, Klaus Peters from GSI, at the most recent Online Panda Collaboration meeting. The Panda Collaboration has awarded the Theory PhD Prize for the second time to honor the best theory dissertation written in connection with the Panda Experiment.

### New approach for obtaining coupled-channel amplitudes from finite-volume spectra published

PRD 101, 114505 (2020)

We present a method for efficiently finding solutions of Luescher’s quantization condition, the equation which relates two-particle scattering amplitudes to the discrete spectrum of states in a periodic spatial volume of finite extent such as that present in lattice QCD. The approach proposed is based on an eigenvalue decomposition in the space of coupled-channels and partial-waves, which proves to have several desirable and simplifying features that are of great benefit when considering problems beyond simple elastic scattering of spinless particles.