Determination of the η-transition form factor in the γp→pη→pγe+e− reaction

H. Berghaeuser, V. Metag, A. Starostin, P. Aguar Bartolome, L. K. Akasoy, J. R. M. Annand, H. J. Arends, K. Bantawa, R. Beck, V. Bekrenev, A. Braghieri, D. Branford, W. J. Briscoe, J. Brudvik, S. Cherepnya, B. T. Demissie, E. J. Downie, P. Drexler, M. Dieterle, L. V. Fil'kovA. Fix, D. I. Glazier, E. Heid, D. Hornidge, O. Jahn, T. C. Jude, V. L. Kashevarov, A. Knezevic, R. Kondratiev, M. Korolija, M. Kotulla, A. Koulbardis, S. Kruglov, B. Krusche, B. Lemmer, V. Lisin, K. Livingston, I. J. D. MacGregor, Y. Maghrbi, D. M. Manley, M. Martinez-Fabregate, J. C. McGeorge, E. F. McNicoll, D. Mekterovic, S. Micanovic, A. Mushkarenkov, B. M. K. Nefkens, A. Nikolaev, R. Novotny, M. Ostrick, R. O. Owens, P. Pedroni, F. Pheron, A. Polonski, J. Robinson, S. N. Prakhov, G. Rosner, T. Rostomyan, S. Schumann, D. Sober, I. M. Suarez, I. Supek, C. M. Tarbert, M. Thiel, A. Thomas, M. Unverzagt, D. P. Watts, D. Werthmueller, I. Zamboni, F. Zehr

Research output: Contribution to journalArticlepeer-review


The Dalitz decay eta -> gamma e(+)e(-) has been measured using the combined Crystal Ball and TAPS photon detector setup at the electron accelerator MAMI-C. Compared to the most recent transition form-factor measurement in the e(+)e(-) channel, statistics have been improved by one order of magnitude. The e(+)e(-)invariant mass distribution shows a deviation from the QED prediction for a point-like particle, which can be described by a form-factor. Using the usual monopole transition form-factor parameterization, F(m(2)) = (1 - m(2)/Lambda(2))(-1), a value of Lambda(-2) = (1.92 +/- 0.35(stat) +/- 0.13(syst)) GeV-2 has been determined. This value is in good agreement with a recent measurement of the eta Dalitz decay in the mu(+)mu(-) channel and with recent form-factor calculations. An improved value of the branching ratio BR(eta -> gamma e(+)e(-)) = (6.6 +/- 0.4(stat) +/- 0.4(syst)) . 10(-3) has been determined. (C) 2011 Elsevier B.V. All rights reserved.

Original languageEnglish
Pages (from-to)562-567
Number of pages6
JournalPhysics Letters B
Issue number5
Publication statusPublished - 27 Jul 2011


  • Meson decays
  • Transition form factor

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