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

Scalar and tensor charmonium resonances computed from QCD
Using lattice QCD, scalar and tensor charmonium resonances have been determined from coupledchannel scattering amplitudes. Unstable charmonium resonances are readily observed at modern hadron physics experiments but often produce puzzling features. This new work demonstrates that by using QCD, and constraints between scattering amplitudes such as unitarity, features observed in different decay modes can be unified into a single description of a complex resonance pole coupled to multiple channels. This work has been published in Physical Review Letters and Physical Review D as Editors’ Suggestions.


Hadspec calculates threepion scattering amplitude
For the first time in lattice QCD, a threehadron 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 threepion channel ($\pi^+ \pi^+ \pi^+ \to \pi^+ \pi^+ \pi^+$) the calculation uses a relativistic finitevolume formalism to relate lattice energies to the physical scattering amplitude. The work has been published in Physical Review Letters as an Editors’ 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 modelbased
proposal that such a state would couple more strongly to the $\pi
b_1$ finalstate than the lowerlying $\pi \eta, \pi \eta'$ and $\pi
\rho$ finalstates 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 coupledchannel amplitudes from
finitevolume spectra published
PRD 101, 114505 (2020)
We present a method for efficiently finding solutions of Luescher’s
quantization condition, the equation which relates twoparticle
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 coupledchannels and partialwaves,
which proves to have several desirable and simplifying features that
are of great benefit when considering problems beyond simple elastic
scattering of spinless particles.

