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arXiv · 2512.03431

New high-statistics measurement of the $\pi^0 \to e^+e^-\gamma$ Dalitz decay at the Mainz Microtron

S. Prakhov·L. Heijkenskjöld·S. Abt·P. Achenbach·P. Adlarson·F. Afzal·Z. Ahmed·K. Altangerel·J. R. M. Annand·M. Bashkanov·R. Beck·M. Biroth·N. S. Borisov·A. Braghieri·W. J. Briscoe·F. Cividini·C. Collicott·S. Costanza·A. Denig·M. Dieterle·A. S. Dolzhikov·E. J. Downie·P. Drexler·S. Fegan·S. Gardner·D. Ghosal·D. I. Glazier·I. Gorodnov·W. Gradl·M. Günther·G. M. Gurevich·D. Hornidge·G. M. Huber·A. Käser·V. L. Kashevarov·S. J. D. Kay·M. Korolija·B. Krusche·A. Lazarev·K. Livingston·S. Lutterer·I. J. D. MacGregor·D. M. Manley·P. P. Martel·R. Miskimen·M. Mocanu·E. Mornacchi·C. Mullen·A. Neganov·A. Neiser·M. Ostrick·P. B. Otte·D. Paudyal·P. Pedroni·A. Powell·E. Rickert·T. Rostomyan·V. Sokhoyan·K. Spieker·O. Steffen·I. I. Strakovsky·Th. Strub·I. Supek·M. Thiel·A. Thomas·Yu. A. Usov·S. Wagner·D. P. Watts·D. Werthmüller·J. Wettig·M. Wolfes·N. Zachariou
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Abstract

The Dalitz decay $\pi^0 \to e^+e^-\gamma$ has been measured with the highest statistical accuracy obtained so far in the $\gamma p\to \pi^0 p$ reaction with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value of the slope parameter for the $\pi^0$ electromagnetic transition form factor, $a_\pi=0.0315\pm 0.0026_{\mathrm{stat}}\pm 0.0010_{\mathrm{syst}}$, is obtained from the analysis of $2.4\times10^6$ $\pi^0 \to e^+e^-\gamma$ observed decays. Within experimental uncertainties, it is in agreement with existing measurements and theoretical calculations, with its own uncertainty being smaller than previous results based on the analysis of $\pi^0\to e^+e^-\gamma$ decays.

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S. Prakhov, L. Heijkenskjöld, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, Z. Ahmed, K. Altangerel, J. R. M. Annand, M. Bashkanov, R. Beck, M. Biroth, N. S. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, M. Dieterle, A. S. Dolzhikov, E. J. Downie, P. Drexler, S. Fegan, S. Gardner, D. Ghosal, D. I. Glazier, I. Gorodnov, W. Gradl, M. Günther, G. M. Gurevich, D. Hornidge, G. M. Huber, A. Käser, V. L. Kashevarov, S. J. D. Kay, M. Korolija, B. Krusche, A. Lazarev, K. Livingston, S. Lutterer, I. J. D. MacGregor, D. M. Manley, P. P. Martel, R. Miskimen, M. Mocanu, E. Mornacchi, C. Mullen, A. Neganov, A. Neiser, M. Ostrick, P. B. Otte, D. Paudyal, P. Pedroni, A. Powell, E. Rickert, T. Rostomyan, V. Sokhoyan, K. Spieker, O. Steffen, I. I. Strakovsky, Th. Strub, I. Supek, M. Thiel, A. Thomas, Yu. A. Usov, S. Wagner, D. P. Watts, D. Werthmüller, J. Wettig, M. Wolfes, N. Zachariou. 2025-12-03. New high-statistics measurement of the $\pi^0 \to e^+e^-\gamma$ Dalitz decay at the Mainz Microtron. https://doi.org/10.1103/9tk1-2dxp

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