Search for T-violating transverse muon polarization in the K⁺-->pi⁰µ⁺nu decay

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M. Abe,^1,3 M. Aliev,² V. Anisimovsky,² M. Aoki,^3,4 Y. Asano,¹ T. Baker,⁵ M. Blecher,⁶ P. Depommier,⁷ M. Hasinoff,⁸ K. Horie,^3,4 Y. Igarashi,³ J. Imazato,³ A. P. Ivashkin,² M. M. Khabibullin,² A. N. Khotjantsev,² Yu. G. Kudenko,² Y. Kuno,⁴ K. S. Lee,⁹ A. Levchenko,² G. Y. Lim,^9,3 J. A. Macdonald,¹⁰ O. V. Mineev,² N. Okorokova,² C. Rangacharyulu,⁵ S. Shimizu,⁴ Y.-H. Shin,¹¹ Y.-M. Shin,^5,10 K. S. Sim,⁹ N. Yershov,² and T. Yokoi³

(KEK-E246 Collaboration)

¹Institute of Applied Physics, University of Tsukuba, Ibaraki 305-0006, Japan
²Institute for Nuclear Research, Russian Academy of Sciences, Moscow 117312, Russia
³IPNS, High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan
⁴Department of Physics, Osaka University, Osaka 560-0043, Japan
⁵Department of Physics, University of Saskatchewan, Saskatoon, Canada S7N 5E2
⁶Department of Physics, Virginia Polytechnic Institute and State University, Virginia 24061-0435, USA
⁷Laboratoire de Physique Nucléaire, Université de Montréal, Montréal, Québec, Canada H3C 3J7
⁸Department of Physics and Astronomy, University of British Columbia, Vancouver, Canada V6T 1Z1
⁹Department of Physics, Korea University, Seoul 136-701, Korea
¹⁰TRIUMF, Vancouver, British Columbia, Canada V6T 2A3
¹¹Department of Physics, Yonsei University, Seoul 120-749, Korea

Received 24 March 2006; published 26 April 2006

Atthe 12-GeV proton synchrotron of KEK, an experiment (KEK-PS-E246) wasperformed to measure the transverse muon polarization (P_T) in K⁺-->pi⁰µ⁺nudecays as a direct search of violation of time reversalinvariance (T-violation). P_T is a very sensitive probe of CPviolation beyond the standard model. A stopped positive kaon beamwas used. An elaborate high-acceptance detector, in conjunction with a12-sector superconducting toroidal spectrometer, was utilized. The stopped kaon methodprovided several advantages in performing a high-precision experiment. The decaywas identified by detecting a µ⁺ with the spectrometer anda pi⁰ with a CsI(Tl) calorimeter surrounding the target astwo photons as well as one photon with high energy.For the P_T measurement, pi⁰ events in the forward (fwd)and backward (bwd) regions in the calorimeter were relevant. Systematicerrors were greatly suppressed by exploiting the rotational and fwd-bwdsymmetry of the system. Polarization measurement was done in termsof asymmetry measurement of decay positrons of stopped muons inthe polarimeter. A longitudinal field method was applied for theP_T polarization component. In the data analysis a novel techniqueof the two-analysis method was employed. Two independent analyses wereperformed following their own policy and criteria and good eventswere selected. Then, the two analyses were combined. A dataquality check was carefully done by looking at the nullasymmetry A₀, the sensitivity function A_N, the kinematical attenuation coefficient,and the decay-plane distributions. The asymmetry measurement for finite-size muonstoppers was performed "differentially" using the position information by thechamber in front of the polarimeter. From the measurements during1996–2000, we accumulated 11.8×10⁶ good events after the two-analysis combination,and deduced P_T=-0.0017±0.0023(stat)±0.0011(syst), corresponding to the T-violating parameter Imxi=-0.0053±0.0071(stat)±0.0036(syst). Fromthese results the upper limits of |P_T|<0.0050 (90% C.L.) and|Imxi|<0.016 (90% C.L.) were deduced. Systematic errors were carefully studiedand estimated. Almost all of the errors were canceled bythe 12-sector summation and/or fwd-bwd subtraction, and the total systematicerror was concluded to be as a half of thestatistical error. The present result set constraints on model parametersof several theoretical models. For the three-Higgs-doublet model, e.g., alimit |Im(alpha₁gamma₁^*)|<544(M_H₁/GeV)² was obtained. Implications for several other models arediscussed.

URL:	http://link.aps.org/doi/10.1103/PhysRevD.73.072005
DOI:	10.1103/PhysRevD.73.072005
PACS:	11.30.Er; 12.60.Fr; 13.20.Eb 11.30.Er Charge conjugation, parity, time reversal, and other discrete symmetries in particles and fields 12.60.Fr Extensions of electroweak Higgs sector 13.20.Eb Leptonic/semileptonic decays of K mesons YEAR: 2006
KEYWORDS:	T invariance, kaon decay, semileptonic decays, CP invariance, standard model, Higgs model, unified gauge models

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