Triangle Singularity as the Origin of the a1 (1420)

Alexeev, G. D.; Alexeev, M. G.; Amoroso, A.; Andrieux, V.; Anosov, V.; Antoshkin, A.; Augsten, K.; Augustyniak, W.; Azevedo, C. D. R.; Badelek, B.; Balestra, F.; Ball, M.; Barth, J.; Beck, R.; Bedfer, Y.; Berenguer Antequera, J.; Bernhard, J.; Bodlak, M.; Bradamante, F.; Bressan, A.; Burtsev, V. E.; Chang, W. -C.; Chatterjee, C.; Chiosso, M.; Chumakov, A. G.; Chung, S. -U.; Cicuttin, A.; Correia, P. M. M.; Crespo, M. L.; D'Ago, D.; Dalla Torre, S.; Dasgupta, S. S.; Dasgupta, S.; Denisenko, I.; Denisov, O. Y.; Donskov, S. V.; Doshita, N.; Dreisbach, C.; Dunnweber, W.; Dusaev, R. R.; Efremov, A.; Eversheim, P. D.; Faccioli, P.; Faessler, M.; Finger, M.; Finger, M.; Fischer, H.; Franco, C.; Friedrich, J. M.; Frolov, V.; Gautheron, F.; Gavrichtchouk, O. P.; Gerassimov, S.; Giarra, J.; Gnesi, I.; Gorzellik, M.; Grasso, A.; Gridin, A.; Grosse Perdekamp, M.; Grube, B.; Guskov, A.; Von Harrach, D.; Heitz, R.; Herrmann, F.; Horikawa, N.; D'Hose, N.; Hsieh, C. -Y.; Huber, S.; Ishimoto, S.; Ivanov, A.; Iwata, T.; Jandek, M.; Jary, V.; Joosten, R.; Jorg, P.; Kabuss, E.; Kaspar, F.; Kerbizi, A.; Ketzer, B.; Khaustov, G. V.; Khokhlov, Y. A.; Kisselev, Y.; Klein, F.; Koivuniemi, J. H.; Kolosov, V. N.; Kondo Horikawa, K.; Konorov, I.; Konstantinov, V. F.; Kotzinian, A. M.; Kouznetsov, O. M.; Koval, A.; Kral, Z.; Krinner, F.; Kulinich, Y.; Kunne, F.; Kurek, K.; Kurjata, R. P.; Kveton, A.; Lavickova, K.; Levorato, S.; Lian, Y. -S.; Lichtenstadt, J.; Lin, P. -J.; Longo, R.; Lyubovitskij, V. E.; Maggiora, A.; Magnon, A.; Makins, N.; Makke, N.; Mallot, G. K.; Maltsev, A.; Mamon, S. A.; Marianski, B.; Martin, A.; Marzec, J.; Matousek, J.; Matsuda, T.; Mattson, G.; Meshcheryakov, G. V.; Meyer, M.; Meyer, W.; Mikhailov, Y. V.; Mikhasenko, M.; Mitrofanov, E.; Mitrofanov, N.; Miyachi, Y.; Moretti, A.; Nagaytsev, A.; Naim, C.; Neyret, D.; Novy, J.; Nowak, W. -D.; Nukazuka, G.; Nunes, A. S.; Olshevsky, A. G.; Ostrick, M.; Panzieri, D.; Parsamyan, B.; Paul, S.; Pekeler, H.; Peng, J. -C.; Pesek, M.; Peshekhonov, D. V.; Peskova, M.; Pierre, N.; Platchkov, S.; Pochodzalla, J.; Polyakov, V. A.; Pretz, J.; Quaresma, M.; Quintans, C.; Reicherz, G.; Riedl, C.; Rudnicki, T.; Ryabchikov, D. I.; Rybnikov, A.; Rychter, A.; Samoylenko, V. D.; Sandacz, A.; Sarkar, S.; Savin, I. A.; Sbrizzai, G.; Schmieden, H.; Selyunin, A.; Sinha, L.; Slunecka, M.; Smolik, J.; Srnka, A.; Steffen, D.; Stolarski, M.; Subrt, O.; Sulc, M.; Suzuki, H.; Sznajder, P.; Tessaro, S.; Tessarotto, F.; Thiel, A.; Tomsa, J.; Tosello, F.; Townsend, A.; Tskhay, V.; Uhl, S.; Vasilishin, B. I.; Vauth, A.; Veit, B. M.; Veloso, J.; Ventura, B.; Vidon, A.; Virius, M.; Wagner, M.; Wallner, S.; Zaremba, K.; Zavada, P.; Zavertyaev, M.; Zemko, M.; Zemlyanichkina, E.; Zhao, Y.; Ziembicki, M.

doi:10.1103/PhysRevLett.127.082501

The COMPASS Collaboration experiment recently discovered a new isovector resonancelike signal with axial-vector quantum numbers, the a1(1420), decaying to f0(980)π. With a mass too close to and a width smaller than the axial-vector ground state a1(1260), it was immediately interpreted as a new light exotic meson, similar to the X, Y, Z states in the hidden-charm sector. We show that a resonancelike signal fully matching the experimental data is produced by the decay of the a1(1260) resonance into K∗(→Kπ)K̄ and subsequent rescattering through a triangle singularity into the coupled f0(980)π channel. The amplitude for this process is calculated using a new approach based on dispersion relations. The triangle-singularity model is fitted to the partial-wave data of the COMPASS experiment. Despite having fewer parameters, this fit shows a slightly better quality than the one using a resonance hypothesis and thus eliminates the need for an additional resonance in order to describe the data. We thereby demonstrate for the first time in the light-meson sector that a resonancelike structure in the experimental data can be described by rescattering through a triangle singularity, providing evidence for a genuine three-body effect.