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arXiv · 1403.2644

Measurement of radiative widths of $a_2(1320)$ and $\pi_2(1670)$

C. Adolph·R. Akhunzyanov·M. G. Alekseev·G. D. Alexeev·A. Amoroso·V. Andrieux·V. Anosov·A. Austregesilo·B. Badelek·F. Balestra·J. Barth·G. Baum·R. Beck·Y. Bedfer·A. Berlin·J. Bernhard·K. Bicker·J. Bieling·R. Birsa·J. Bisplinghoff·M. Bodlak·M. Boer·P. Bordalo·F. Bradamante·C. Braun·A. Bressan·M. Buechele·E. Burtin·L. Capozza·M. Chiosso·S. U. Chung·A. Cicuttin·M. L. Crespo·Q. Curiel·S. Dalla Torre·S. S. Dasgupta·S. Dasgupta·O. Yu. Denisov·S. V. Donskov·N. Doshita·V. Duic·W. Duennweber·M. Dziewiecki·A. Efremov·C. Elia·P. D. Eversheim·W. Eyrich·M. Faessler·A. Ferrero·A. Filin·M. Finger·M. Finger~jr.·H. Fischer·C. Franco·N. du~Fresne~von~Hohenesche·J. M. Friedrich·V. Frolov·F. Gautheron·O. P. Gavrichtchouk·S. Gerassimov·R. Geyer·I. Gnesi·B. Gobbo·S. Goertz·M. Gorzellik·S. Grabmueller·A. Grasso·B. Grube·A. Guskov·T. Guthoerl·F. Haas·D. von Harrach·D. Hahne·R. Hashimoto·F. H. Heinsius·F. Herrmann·F. Hinterberger·Ch. Hoeppner·N. Horikawa·N. d'Hose·S. Huber·S. Ishimoto·A. Ivanov·Yu. Ivanshin·T. Iwata·R. Jahn·V. Jary·P. Jasinski·P. Joerg·R. Joosten·E. Kabuss·B. Ketzer·G. V. Khaustov·Yu. A. Khokhlov·Yu. Kisselev·F. Klein·K. Klimaszewski·J. H. Koivuniemi·V. N. Kolosov·K. Kondo
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Abstract

The COMPASS Collaboration at CERN has investigated the reaction $\pi^- \gamma \rightarrow \pi^-\pi^-\pi^+$ embedded in the Primakoff reaction of $190~\textrm{GeV}$ pions scattering in the Coulomb field of a lead target, $\pi^- \text{Pb} \rightarrow \pi^-\pi^-\pi^+ \text{Pb}$. Exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at momentum transfer below $0.001~(\text{GeV}/c)^2$. Using a partial-wave analysis the amplitudes and relative phases of the $a_2(1320)$ and $\pi_2(1670)$ mesons have been extracted, and the Coulomb and the diffractive contributions have been disentangled. Measuring absolute production cross sections we have determined the radiative width of the $a_2(1320)$ to be $\Gamma_0(a_2(1320) \rightarrow \pi\gamma) = (358 \pm 6_{\textrm{stat}} \pm 42_{\textrm{syst}})~\textrm{keV}$. As the first measurement, $\Gamma_0(\pi_2(1670) \rightarrow \pi\gamma) = (181 \pm 11_{\textrm{stat}} \pm 27_{\textrm{syst}})~\textrm{keV} \cdot (\textrm{BR}^{\textrm{PDG}}_{f_2 \pi}/\textrm{BR}_{f_2 \pi})$ is obtained for the radiative width of the $\pi_2(1670)$, where in this analysis the branching ratio $\textrm{BR}^{\textrm{PDG}}_{f_2 \pi}=0.56$ has been used. We compare these values to previous measurements and theoretical predictions.

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C. Adolph, R. Akhunzyanov, M. G. Alekseev, G. D. Alexeev, A. Amoroso, V. Andrieux, V. Anosov, A. Austregesilo, B. Badelek, F. Balestra, J. Barth, G. Baum, R. Beck, Y. Bedfer, A. Berlin, J. Bernhard, K. Bicker, J. Bieling, R. Birsa, J. Bisplinghoff, M. Bodlak, M. Boer, P. Bordalo, F. Bradamante, C. Braun, A. Bressan, M. Buechele, E. Burtin, L. Capozza, M. Chiosso, S. U. Chung, A. Cicuttin, M. L. Crespo, Q. Curiel, S. Dalla Torre, S. S. Dasgupta, S. Dasgupta, O. Yu. Denisov, S. V. Donskov, N. Doshita, V. Duic, W. Duennweber, M. Dziewiecki, A. Efremov, C. Elia, P. D. Eversheim, W. Eyrich, M. Faessler, A. Ferrero, A. Filin, M. Finger, M. Finger~jr., H. Fischer, C. Franco, N. du~Fresne~von~Hohenesche, J. M. Friedrich, V. Frolov, F. Gautheron, O. P. Gavrichtchouk, S. Gerassimov, R. Geyer, I. Gnesi, B. Gobbo, S. Goertz, M. Gorzellik, S. Grabmueller, A. Grasso, B. Grube, A. Guskov, T. Guthoerl, F. Haas, D. von Harrach, D. Hahne, R. Hashimoto, F. H. Heinsius, F. Herrmann, F. Hinterberger, Ch. Hoeppner, N. Horikawa, N. d'Hose, S. Huber, S. Ishimoto, A. Ivanov, Yu. Ivanshin, T. Iwata, R. Jahn, V. Jary, P. Jasinski, P. Joerg, R. Joosten, E. Kabuss, B. Ketzer, G. V. Khaustov, Yu. A. Khokhlov, Yu. Kisselev, F. Klein, K. Klimaszewski, J. H. Koivuniemi, V. N. Kolosov, K. Kondo. 2014-03-11. Measurement of radiative widths of $a_2(1320)$ and $\pi_2(1670)$. https://doi.org/10.1140/epja/i2014-14079-8

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