First observation and branching fraction and decay parameter measurements of the weak radiative decay $\Xi^0 \to \Lambda e^{+}e^{-}$

Batley, J.R.; Cheze, J.B.; Goudzovski, E.; Cundy, D.; Cartiglia, N.; Lazzeroni, C.; Mazzucato, E.; Calvetti, M.; Sozzi, M.; Maier, A.; De Beer, M.; Dibon, H.; Scarpa, M.; Hristov, P.; Fonseca Martin, T.; Kleinknecht, K.; Ruggiero, G.; Lubrano, P.; Cheshkov, C.; Kubischta, W.; Costantini, F.; Giudici, S.; Gouge, G.; Monnier, E.; Fiorini, L.; Dalpiaz, P.; Behler, M.; Goy Lopez, S.; Veltri, M.; Gatignon, L.; Pepe, M.; Peyaud, B.; Gonidec, A.; Michetti, A.; Doble, N.; Norton, A.; Walker, A.; Mannelli, I.; Martini, M.; Valdata, M.; Grafstrom, P.; Iacopini, E.; Debu, P.; Eppard, K.; Anzivino, G.; Winhart, A.; Lamanna, G.; Holder, M.; Marchetto, F.; Koch, U.; Wotton, S.A.; Wahl, H.; Winston, R.; Sacco, R.; Mikulec, I.; Marel, G.; Frabetti, P.L.; Imbergamo, E.; Markytan, M.; Peters, A.; Vallage, B.; Piccini, M.; Masetti, L.; Wanke, R.; Slater, M.W.; Kalmus, G.E.; Stoynev, S.; Swallow, E.; Panzer-Steindel, B.; Litov, L.; Zinchenko, A.; Munday, D.J.; Nappi, A.; Eppard, M.; Moosbrugger, U.; Collazuol, G.; Pierazzini, G.; Marouelli, P.; Hirstius, A.; Wislicki, W.; Petrucci, F.; Savrie, M.; Velasco, M.; Biino, C.; Gianoli, A.; Patel, M.; Ceccucci, A.; Bocquet, G.; Baldini, W.; Neuhofer, G.; Pastrone, N.; Cenci, P.; Cerri, C.; Jeitler, M.; Falaleev, V.; Menichetti, Ezio A.; Petrucci, M.C.; Lenti, M.; Kekelidze, V.; Morales Morales, C.; Clemencic, M.; Potrebenikov, Yu.; Dabrowski, A.; Fantechi, R.; Bizzeti, A.; Madigozhin, D.; Molokanova, N.; Arcidiacono, R.; Rubin, P.; Widhalm, L.; Ziolkowski, M.

doi:10.1016/j.physletb.2007.04.066

Article
Report number	hep-ex/0703023 ; CERN-PH-EP-2007-004 ; CERN-PH-EP-2007-004
Title	First observation and branching fraction and decay parameter measurements of the weak radiative decay $\Xi^0 \to \Lambda e^{+}e^{-}$
Author(s)	Batley, J.R. (Cambridge U.) ; Kalmus, G.E. (Cambridge U.) ; Lazzeroni, C. (Cambridge U.) ; Munday, D.J. (Cambridge U.) ; Patel, M. (Cambridge U.) ; Slater, M.W. (Cambridge U.) ; Wotton, S.A. (Cambridge U.) ; Arcidiacono, R. (CERN) ; Bocquet, G. (CERN) ; Ceccucci, A. (CERN) ; Cundy, D. (CERN) ; Doble, N. (CERN) ; Falaleev, V. (CERN) ; Gatignon, L. (CERN) ; Gonidec, A. (CERN) ; Grafstrom, P. (CERN) ; Kubischta, W. (CERN) ; Mikulec, I. (CERN) ; Norton, A. (CERN) ; Panzer-Steindel, B. (CERN) ; Rubin, P. (CERN) ; Wahl, H. (CERN) ; Goudzovski, E. (Dubna, JINR) ; Hristov, P. (Dubna, JINR) ; Kekelidze, V. (Dubna, JINR) ; Litov, L. (Dubna, JINR) ; Madigozhin, D. (Dubna, JINR) ; Molokanova, N. (Dubna, JINR) ; Potrebenikov, Yu. (Dubna, JINR) ; Stoynev, S. (Dubna, JINR) ; Zinchenko, A. (Dubna, JINR) ; Monnier, E. (Chicago U.) ; Swallow, E. (Chicago U.) ; Winston, R. (Chicago U.) ; Sacco, R. (Edinburgh U.) ; Walker, A. (Edinburgh U.) ; Baldini, W. (Ferrara U. ; INFN, Ferrara) ; Dalpiaz, P. (Ferrara U. ; INFN, Ferrara) ; Frabetti, P.L. (Ferrara U. ; INFN, Ferrara) ; Gianoli, A. (Ferrara U. ; INFN, Ferrara) ; Martini, M. (Ferrara U. ; INFN, Ferrara) ; Petrucci, F. (Ferrara U. ; INFN, Ferrara) ; Savrie, M. (Ferrara U. ; INFN, Ferrara) ; Scarpa, M. (Ferrara U. ; INFN, Ferrara) ; Bizzeti, A. (Florence U. ; INFN, Florence) ; Calvetti, M. (Florence U. ; INFN, Florence) ; Collazuol, G. (Florence U. ; INFN, Florence) ; Iacopini, E. (Florence U. ; INFN, Florence) ; Lenti, M. (Florence U. ; INFN, Florence) ; Ruggiero, G. (Florence U. ; INFN, Florence) ; Veltri, M. (Florence U. ; INFN, Florence) ; Behler, M. (Mainz U., Inst. Phys.) ; Eppard, K. (Mainz U., Inst. Phys.) ; Eppard, M. (Mainz U., Inst. Phys.) ; Hirstius, A. (Mainz U., Inst. Phys.) ; Kleinknecht, K. (Mainz U., Inst. Phys.) ; Koch, U. (Mainz U., Inst. Phys.) ; Marouelli, P. (Mainz U., Inst. Phys.) ; Masetti, L. (Mainz U., Inst. Phys.) ; Moosbrugger, U. (Mainz U., Inst. Phys.) ; Morales Morales, C. (Mainz U., Inst. Phys.) ; Peters, A. (Mainz U., Inst. Phys.) ; Wanke, R. (Mainz U., Inst. Phys.) ; Winhart, A. (Mainz U., Inst. Phys.) ; Dabrowski, A. (Northwestern U.) ; Fonseca Martin, T. (Northwestern U.) ; Velasco, M. (Northwestern U.) ; Anzivino, G. (Perugia U. ; INFN, Perugia) ; Cenci, P. (Perugia U. ; INFN, Perugia) ; Imbergamo, E. (Perugia U. ; INFN, Perugia) ; Lamanna, G. (Perugia U. ; INFN, Perugia) ; Lubrano, P. (Perugia U. ; INFN, Perugia) ; Michetti, A. (Perugia U. ; INFN, Perugia) ; Nappi, A. (Perugia U. ; INFN, Perugia) ; Pepe, M. (Perugia U. ; INFN, Perugia) ; Petrucci, M.C. (Perugia U. ; INFN, Perugia) ; Piccini, M. (Perugia U. ; INFN, Perugia) ; Valdata, M. (Perugia U. ; INFN, Perugia) ; Cerri, C. (Pisa, Scuola Normale Superiore ; INFN, Pisa) ; Costantini, F. (Pisa, Scuola Normale Superiore ; INFN, Pisa) ; Fantechi, R. (Pisa, Scuola Normale Superiore ; INFN, Pisa) ; Fiorini, L. (Pisa, Scuola Normale Superiore ; INFN, Pisa) ; Giudici, S. (Pisa, Scuola Normale Superiore ; INFN, Pisa) ; Mannelli, I. (Pisa, Scuola Normale Superiore ; INFN, Pisa) ; Pierazzini, G. (Pisa, Scuola Normale Superiore ; INFN, Pisa) ; Sozzi, M. (Pisa, Scuola Normale Superiore ; INFN, Pisa) ; Cheshkov, C. (DSM, DAPNIA, Saclay) ; Cheze, J.B. (DSM, DAPNIA, Saclay) ; De Beer, M. (DSM, DAPNIA, Saclay) ; Debu, P. (DSM, DAPNIA, Saclay) ; Gouge, G. (DSM, DAPNIA, Saclay) ; Marel, G. (DSM, DAPNIA, Saclay) ; Mazzucato, E. (DSM, DAPNIA, Saclay) ; Peyaud, B. (DSM, DAPNIA, Saclay) ; Vallage, B. (DSM, DAPNIA, Saclay) ; Holder, M. (Siegen U.) ; Maier, A. (Siegen U.) ; Ziolkowski, M. (Siegen U.) ; Biino, C. (Turin U. ; INFN, Turin) ; Cartiglia, N. (Turin U. ; INFN, Turin) ; Clemencic, M. (Turin U. ; INFN, Turin) ; Goy Lopez, S. (Turin U. ; INFN, Turin) ; Marchetto, F. (Turin U. ; INFN, Turin) ; Menichetti, Ezio A. (Turin U. ; INFN, Turin) ; Pastrone, N. (Turin U. ; INFN, Turin) ; Wislicki, W. (Warsaw, Inst. Nucl. Studies) ; Dibon, H. (Vienna, OAW) ; Jeitler, M. (Vienna, OAW) ; Markytan, M. (Vienna, OAW) ; Neuhofer, G. (Vienna, OAW) ; Widhalm, L. (Vienna, OAW)
Affiliation	(Cavendish Laboratory University of Cambridge, UK) ; (CERN) ; (Joint Institute for Nuclear Research Dubna, Russian Federation) ; (Enrico Fermi Institute, The University of Chicago, Chicago) ; (Department of Physics and Astronomy University of Edinburgh, JCMB King's Buildings, Mayfield Road, Edinburgh) ; (Dipartimento di Fisica dell'Universita e Sezione dell'INFN di Ferrara, Italy) ; (Dipartimento di Fisica dell'Universita e Sezione dell'INFN di Firenze, Italy) ; (Institut fur Physik Universitat Mainz, Germany) ; (Department of Physics and Astronomy Northwestern University, Evanston) ; (Dipartimento di Fisica dell'Universita e Sezione dell'INFN di Perugia, Italy) ; (Dipartimento di Fisica, Scuola Normale Superiore e Sezione dell'INFN di Pisa, Italy) ; (DSM/DAPNIA-CEASaclay Gif-sur-Yvette, France) ; (Fachbereich Physik Universitat Siegen, Germany) ; (Dipartimento di Fisica Sperimentale dell'Universita e Sezione dell'INFN di Torino, Italy) ; (Soltan Institute for Nuclear Studies, Laboratory for High Energy Physics, Warsaw, Poland) ; (Osterreichische Akademie der Wissenschaften, Institut fur Hochenergiephysik, Wien, Austria)
Publication	2007
Imprint	21 Feb 2007
Number of pages	20
Note	20 pages, 5 figures, 4 tables; revised: 19 pages, 4 figures, 4 tables, after reviewers' comments: 1 figure removed, 1 figure corrected, minor editorial changes; to be published in Phys. Lett. B
In:	Phys. Lett. B 650 (2007) 1-8
DOI	10.1016/j.physletb.2007.04.066
Subject category	Particle Physics - Experiment
Accelerator/Facility, Experiment	CERN SPS ; NA48
Abstract	The weak radiative decay $\Xi^0 \rightarrow \Lambda e^{+} e^{-}$ has been detected for the first time. We find 412 candidates in the signal region, with an estimated background of $15 \pm 5$ events. We determine the branching fraction $\cal{B}(\Xi^0 \rightarrow \Lambda e^{+} e^{-}) = [7.6 \pm 0.4_{stat} \pm 0.4_{syst} \pm 0.2_{norm}] \times 10^{-6}$, consistent with an internal bremsstrahlung process, and the decay asymmetry parameter $\alpha_{\Xi \Lambda ee} = -0.8 \pm 0.2$, consistent with that of $\Xi^{0} \rightarrow \Lambda \gamma$. The charge conjugate reaction $\overline{\Xi^{0}} \rightarrow \overline{\Lambda} e^{+}e^{-}$ has also been observed.
Copyright/License	Elsevier B.V.

$COG versus $m_{\Lambda ee}$ after all other selection criteria were imposed. The three hatched boxes are side-band regions. The signal region is the open box at low COG around $m_{\Xi^0}$. The side-band regions at high mass-low COG and high COG were used to estimate accidental and combinatoric backgrounds in the signal region. All three side-band regions were used in the subtraction of background under the decay-angle distribution (see text).$ $noimg: Sources of expected background events.: Quantities that entered into $\Xi^0$ flux calculations.$ $$m_{\Lambda ee}$ after all selection criteria. Arrows indicate signal region. Stacked in various hatchings (see legend) are the estimated sources of background.$ Show more plots

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