Eur. Phys. J. C (2010) 68: 75-87
https://doi.org/10.1140/epjc/s10052-010-1349-8

Regular Article - Experimental Physics

Measurement of the direct emission and interference terms and search for CP violation in the decay K ^±→π ^± π ⁰ γ

¹ Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, UK
² CERN, 1211, Genève 23, Switzerland
³ Joint Institute for Nuclear Research, Dubna, 141980, Moscow region, Russia
⁴ The Enrico Fermi Institute, The University of Chicago, Chicago, IL, 60126, USA
⁵ Department of Physics and Astronomy, University of Edinburgh, JCMB King’s Buildings, Mayfield Road, Edinburgh, EH9 3JZ, UK
⁶ Dipartimento di Fisica dell’Università e Sezione dell’INFN di Ferrara, 44100, Ferrara, Italy
⁷ Sezione dell’INFN di Firenze, 50125, Firenze, Italy
⁸ Dipartimento di Fisica dell’Università e Sezione dell’INFN di Firenze, 50125, Firenze, Italy
⁹ Institut für Physik, Universität Mainz, 55099, Mainz, Germany
¹⁰ Department of Physics and Astronomy, Northwestern University, Evanston, IL, 60208, USA
¹¹ Sezione dell’INFN di Perugia, 06100, Perugia, Italy
¹² Dipartimento di Fisica dell’Università e Sezione dell’INFN di Perugia, 06100, Perugia, Italy
¹³ Sezione dell’INFN di Pisa, 56100, Pisa, Italy
¹⁴ Scuola Normale Superiore e Sezione dell’INFN di Pisa, 56100, Pisa, Italy
¹⁵ Dipartimento di Fisica dell’Università e Sezione dell’INFN di Pisa, 56100, Pisa, Italy
¹⁶ DSM/IRFU, CEA Saclay, 91191, Gif-sur-Yvette, France
¹⁷ Fachbereich Physik, Universität Siegen, 57068, Siegen, Germany
¹⁸ Dipartimento di Fisica Sperimentale dell’Università e Sezione dell’INFN di Torino, 10125, Torino, Italy
¹⁹ Österreichische Akademie der Wissenschaften, Institut für Hochenergiephysik, 10560, Wien, Austria
²⁰ University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
²¹ Università di Roma “La Sapienza” e Sezione dell’INFN di Roma, 00185, Roma, Italy
²² Istituto di Cosmogeofisica del CNR di Torino, 10133, Torino, Italy
²³ Scuola Normale Superiore, 56100, Pisa, Italy
²⁴ Faculty of Physics, University of Sofia “St. Kl. Ohridski”, 5 J. Bourchier Blvd., 1164, Sofia, Bulgaria
²⁵ Northwestern University, 2145 Sheridan Road, Evanston, IL, 60208, USA
²⁶ Centre de Physique des Particules de Marseille, IN2P3-CNRS, Université de la Méditerranée, Marseille, France
²⁷ Department of Physics and Astronomy, George Mason University, Fairfax, VA, 22030, USA
²⁸ Dipartimento di Fisica, Università di Modena e Reggio Emilia, 41100, Modena, Italy
²⁹ Istituto di Fisica, Università di Urbino, 61029, Urbino, Italy
³⁰ Physikalisches Institut, Universität Bonn, 53115, Bonn, Germany
³¹ SLAC, Stanford University, Menlo Park, CA, 94025, USA
³² Royal Holloway, University of London, Egham Hill, Egham, TW20 0EX, UK
³³ Laboratori Nazionali di Frascati, via E. Fermi, 40, 00044, Frascati (Rome), Italy
³⁴ Institut de Física d’Altes Energies, UAB, 08193, Bellaterra (Barcelona), Spain
³⁵ School of Physics Belfield, University College Dublin, Dublin 4, Ireland
³⁶ Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, 28040, Madrid, Spain

^* e-mail: mauro.raggi@lnf.infn.it

Received: 2 April 2010
Revised: 19 May 2010
Published online: 15 June 2010

Abstract

We report on the measurement of the direct emission (DE) and interference (INT) terms of the K ^±→π ^± π ⁰ γ decay by the NA48/2 experiment at the CERN SPS. From the data collected during 2003 and 2004 about 600k such decay candidates have been selected. The relative amounts of DE and INT with respect to the internal bremsstrahlung (IB) contribution have been measured in the range  MeV: where is the kinetic energy of the charged pion in the kaon rest frame. This is the first observation of an interference term in K ^±→π ^± π ⁰ γ decays, thus allowing the normalised electric and magnetic amplitudes to be measured, giving X _E =(−24±6) GeV⁻⁴, and X _M =(254±9) GeV⁻⁴.

In addition, a limit on the CP violating asymmetry in the K ⁺ and K ⁻ branching ratios for this channel has been determined to be less than 1.5×10⁻³ at 90% confidence level.