Observation of a threshold cusp at the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="normal">Λ</mml:mi><mml:mi>η</mml:mi></mml:math> threshold in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi><mml:msup><mml:mi>K</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:math> mass spectrum with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msubsup><mml:mi mathvariant="normal">Λ</mml:mi><mml:mi>c</mml:mi><mml:mo>+</mml:mo></mml:msubsup><mml:mo stretchy="false">→</mml:mo><mml:mi>p</mml:mi><mml:msup><mml:mi>K</mml:mi><mml:mo>−</mml:mo></mml:msup><mml:msup><mml:mi>π</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:math> decays
S. B. Yang, K. Tanida, J. K. Ahn, I. Adachi, H. Aihara, D. M. Asner, H. Atmacan, V. Aulchenko, T. Aushev, R. Ayad, V. Babu, P. K. Behera, K. Belous, J. V. Bennett, M. Bessner, V. Bhardwaj, B. Bhuyan, D. Bodrov, J. Borah, A. Bożek, M. Bračko, P. Branchini, T. E. Browder, A. Budano, M. Campajola, D. Červenkov, M.-C. Chang, P. Chang, B. G. Cheon, K. Chilikin, H.-E. Cho, K. Cho, S.-J. Cho, S.-K. Choi, Y. Choi, S. Choudhury, D. Cinabro, S. Das, G. De Pietro, R. Dhamija, F. Di Capua, Z. Doležal, T. V. Dong, D. Dossett, D. Epifanov, B. G. Fulsom, R. Garg, V. Gaur, A. Garmash, A. Giri, P. Goldenzweig, E. Graziani, T. Gu, Y. Guan, K. Gudkova, C. Hadjivasiliou, K. Hayasaka, H. Hayashii, W.-S. Hou, C.-L. Hsu, T. Iijima, K. Inami, N. Ipsita, A. Ishikawa, R. Itoh, M. Iwasaki, Y. Iwasaki, W. W. Jacobs, E.-J. Jang, Q. P. Ji, S. Jia, Y. Jin, K. K. Joo, K. H. Kang, G. Karyan, T. Kawasaki, H. Kichimi, C. Kiesling, C. H. Kim, D. Y. Kim, K.-H. Kim, Y. J. Kim, Y.-K. Kim, H. Kindo, P. Kodyš, T. Konno, A. Korobov, S. Korpar, E. Kovalenko, P. Križan, P. Krokovny, M. Kumar, R. Kumar, K. Kumara, Y.-J. Kwon, T. Lam, J. S. Lange, M. Laurenza, J. Y. Lee, S. C. Lee
Abstract
We observe a narrow peaking structure in the $p{K}^{\ensuremath{-}}$ invariant-mass spectrum near the $\mathrm{\ensuremath{\Lambda}}\ensuremath{\eta}$ threshold. The peak is clearly seen in 1.5 million events of ${\mathrm{\ensuremath{\Lambda}}}_{c}^{+}\ensuremath{\rightarrow}p{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ decay using the $980\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ data sample collected by the Belle detector at the KEKB asymmetric-energy ${e}^{+}{e}^{\ensuremath{-}}$ collider. We try two approaches to explain this structure: as a new resonance and as a cusp at the $\mathrm{\ensuremath{\Lambda}}\ensuremath{\eta}$ threshold. The best fit is obtained with a coherent sum of a Flatt\'e function and a constant background amplitude with the reduced ${\ensuremath{\chi}}^{2}$ of $257/243$ ($p=0.25$), while the fits to Breit-Wigner functions are unfavored by more than $7\ensuremath{\sigma}$. The best fit explains the structure as a cusp at the $\mathrm{\ensuremath{\Lambda}}\ensuremath{\eta}$ threshold, and the obtained parameters are consistent with the known properties of $\mathrm{\ensuremath{\Lambda}}(1670)$. The observation gives the first identification of a threshold cusp in hadrons from the spectrum shape.