The a1(1420) in a unitary coupled-channel three-body approach
4 affiliations
- 1Universität Bonn
- 2The George Washington University
- 3Theory Center, Thomas Jefferson National Accelerator Facility
- 4University of Bern
Abstract
A bstract : An enhancement in the three-pion energy at around √s s ≈ 1.42 GeV with a ₁ quantum numbers was observed at the COMPASS experiment. This was later attributed to the triangle singularity mechanism involving an on-shell K ∗ (892), K and K̅ K ¯ intermediate states. The alignment of the decay K with the spectator K̅ K ¯ produces an f ₀ (980), resulting in a kinematic enhancement, which is classically explained by the Landau equations. However, this one-loop process forms only part of a non-diagonal transition in a much larger coupled-channel framework. This study demonstrates the feasibility of embedding one-loop triangle-singularity calculations into a unitary three-body amplitude allowing one to consistently incorporate final-state interactions and their potentially substantial effect. For this, up to P -wave isobars and all sub-channel isospins are combined in a nine-channel production amplitude that is fitted to COMPASS lineshapes at different momentum transfers. The fitted amplitude reproduces the narrow enhancement in the ( πf ₀ ) P channel near √s s ≈ 1 . 42 GeV. This implies that the triangle singularity mechanism sufficiently explains the observed enhancement, and an additional genuine a ₁ (1420) pole is not required. Incidentally, the parameters of the ground state axial vector resonance (the a ₁ (1260)) are also extracted from that data.
Versions
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BibTeX
@article{sakthivasan2026a11420,
title = {{The a1(1420) in a unitary coupled-channel three-body approach}},
author = {Ajay S. Sakthivasan and Yuchuan Feng and Michael Döring and Maxim Mai},
journal = {Journal of High Energy Physics},
year = {2026},
month = oct,
doi = {10.1007/jhep10(2026)045},
url = {https://link.springer.com/10.1007/JHEP10(2026)045},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - The a1(1420) in a unitary coupled-channel three-body approach
AU - Ajay S. Sakthivasan
AU - Yuchuan Feng
AU - Michael Döring
AU - Maxim Mai
PY - 2026
DA - 2026/10/05
JO - Journal of High Energy Physics
DO - 10.1007/jhep10(2026)045
UR - https://link.springer.com/10.1007/JHEP10(2026)045
AB - A bstract : An enhancement in the three-pion energy at around √s s ≈ 1.42 GeV with a ₁ quantum numbers was observed at the COMPASS experiment. This was later attributed to the triangle singularity mechanism involving an on-shell K ∗ (892), K and K̅ K ¯ intermediate states. The alignment of the decay K with the spectator K̅ K ¯ produces an f ₀ (980), resulting in a kinematic enhancement, which is classically explained by the Landau equations. However, this one-loop process forms only part of a non-diagonal transition in a much larger coupled-channel framework. This study demonstrates the feasibility of embedding one-loop triangle-singularity calculations into a unitary three-body amplitude allowing one to consistently incorporate final-state interactions and their potentially substantial effect. For this, up to P -wave isobars and all sub-channel isospins are combined in a nine-channel production amplitude that is fitted to COMPASS lineshapes at different momentum transfers. The fitted amplitude reproduces the narrow enhancement in the ( πf ₀ ) P channel near √s s ≈ 1 . 42 GeV. This implies that the triangle singularity mechanism sufficiently explains the observed enhancement, and an additional genuine a ₁ (1420) pole is not required. Incidentally, the parameters of the ground state axial vector resonance (the a ₁ (1260)) are also extracted from that data.
SN - 1029-8479
ER -