First Measurement of Chiral Dynamics in π^{-}γ→π^{-}π^{-}π^{+}

Adolph, C.; Alekseev, M.; Alexakhin, V.; Alexandrov, Y. u.; Alexeev, G.; Amoroso, A.; Antonov, A.; Austregesilo, A.; Badełek, B.; Balestra, F.; Barth, J.; Baum, G.; Bedfer, Y.; Bernhard, J.; Bertini, R.; Bettinelli, M.; Bicker, K.; Birsa, Renato; Bisplinghoff, J.; Bordalo, P.; Bradamante, Franco; Braun, C.; Bravar, A.; Bressan, Andrea; Burtin, E.; Chaberny, D.; Chiosso, M.; Chung, S.; Cicuttin, A.; Crespo, M.; DALLA TORRE, Silvia; Das, S.; Dasgupta, S.; Denisov, O.; Dhara, L.; Donskov, S.; Doshita, N.; Duic, Venicio; Dünnweber, W.; Dziewiecki, M.; Efremov, A.; Elia, Carmine; Eversheim, P.; Eyrich, W.; Faessler, M.; Ferrero, A.; Filin, A.; Finger, M.; Finger, M.; Fischer, H.; Franco, C.; du Fresne von Hohenesche, N.; Friedrich, J.; Garfagnini, R.; Gautheron, F.; Gavrichtchouk, O.; Gazda, R.; Gerassimov, S.; Geyer, R.; Giorgi, Marcello; Gnesi, I.; Gobbo, B.; Goertz, S.; Grabmüller, S.; Grasso, A.; Grube, B.; Gushterski, R.; Guskov, A.; Haas, F.; von Harrach, D.; Hasegawa, T.; Heinsius, F.; Herrmann, F.; Hess, C.; Hinterberger, F.; Horikawa, N.; Höppner, C. h.; D’Hose, N.; Huber, S.; Ishimoto, S.; Ivanov, O.; Ivanshin, Y. u.; Iwata, T.; Jahn, R.; Jasinski, P.; Jegou, G.; Joosten, R.; Kabuss, E.; Kang, D.; Ketzer, B.; Khaustov, G.; Khokhlov, Y. u.; Kisselev, Y. u.; Klein, F.; Klimaszewski, K.; Koblitz, S.; Koivuniemi, J.; Kolosov, V.; Kondo, K.; Königsmann, K.; Konorov, I.; Konstantinov, V.; Korzenev, A.; Kotzinian, A.; Kouznetsov, O.; Krämer, M.; Kroumchtein, Z.; Kunne, F.; Kurek, K.; Lauser, L.; Lednev, A.; Lehmann, A.; Levorato, Stefano; Lichtenstadt, J.; Maggiora, A.; Magnon, A.; Makke, Nour; Mallot, G.; Mann, A.; Marchand, C.; Martin, Anna; Marzec, J.; Massmann, F.; Matsuda, T.; Meyer, W.; Michigami, T.; Mikhailov, Y. u.; Moinester, M.; Morreale, A.; Mutter, A.; Nagaytsev, A.; Nagel, T.; Nerling, F.; Neubert, S.; Neyret, D.; Nikolaenko, V.; Nowak, W. D.; Nunes, A.; Olshevsky, A.; Ostrick, M.; Padee, A.; Panknin, R.; Panzieri, D.; Parsamyan, B.; Paul, S.; Perevalova, E.; Pesaro, Giulia; Peshekhonov, D.; Piragino, G.; Platchkov, S.; Pochodzalla, J.; Polak, J.; Polyakov, V.; Pontecorvo, G.; Pretz, J.; Quintans, C.; Rajotte, J. F.; Ramos, S.; Rapatsky, V.; Reicherz, G.; Richter, A.; Rocco, Elena; Rondio, E.; Rossiyskaya, N.; Ryabchikov, D.; Samoylenko, V.; Sandacz, A.; Sapozhnikov, M.; Sarkar, S.; Savin, I.; Sbrizzai, Giulio; Schiavon, Paolo; Schill, C.; Schlüter, T.; Schmitt, L.; Schönning, K.; Schopferer, S.; Schröder, W.; Shevchenko, O.; Siebert, H. W.; Silva, L.; Sinha, L.; Sissakian, A.; Slunecka, M.; Smirnov, G.; Sosio, S.; Sozzi, Federica; Srnka, A.; Stolarski, M.; Sulc, M.; Sulej, R.; Sznajder, P.; Takekawa, S.; Ter Wolbeek, J.; Tessaro, Susanna; Tessarotto, F.; Teufel, A.; Tkatchev, L.; Uhl, S.; Uman, I.; Vandenbroucke, M.; Virius, M.; Vlassov, N.; Windmolders, R.; Wiślicki, W.; Wollny, H.; Zaremba, K.; Zavertyaev, M.; Zemlyanichkina, E.; Ziembicki, M.; Zhuravlev, N.; Zvyagin, A.

doi:10.1103/PhysRevLett.108.192001

The COMPASS Collaboration at CERN has investigated the π-γ→π-π-π+ reaction at center-of-momentum energy below five pion masses, √s<5mπ, embedded in the Primakoff reaction of 190 GeV pions impinging on a lead target. Exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, t′<0.001 GeV2/c2. Using partial-wave analysis techniques, the scattering intensity of Coulomb production described in terms of chiral dynamics and its dependence on the 3π-invariant mass m3π=√s were extracted. The absolute cross section was determined in seven bins of √s with an overall precision of 20%. At leading order, the result is found to be in good agreement with the prediction of chiral perturbation theory over the whole energy range investigated.