We report the recent results on rare kaon decays from the NA48/2 experiment.
The precise measurement of direct photon emission (DE) in the decay K+- -> pi+-
pi0 gamma and its interference (INT), with the INT amplitude being observed for
the first time, has been finalized. This study is based on the full NA48/2 data
set with about 600k reconstructed K+- -> pi+- pi0 gamma decays which is factor
of 30 larger than for previous experiments. Samples of about 7200 reconstructed
K+- -> pi+- e+ e-, and more than 3000 K+- -> pi+- mu+ mu- events, with very
small background contamination, have been collected. The latter is exceeding
the total existing statistics by a factor of five. A precise measurement of the
branching fractions and the form factors of the rare decays K+- -> pi+- l+ l-
has been performed using different theoretical models. The CP violating
asymmetry between K+ and K- in this channel is also measured.Comment: Proceedings to the EPS-HEP09 conference, 4 pages, 2 figure

Marinova, Evelina

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arXiv

Evelina MARINOVA

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Precision measurements of rare kaon decays

We report the latest results on rare kaon decays from the NA48/2 experiment. Samples of about 7200 reconstructed K+- -> pi+- e+ e-, and more than 3000 K+- -> pi+- mu+ mu- events, with very small background contamination, have been collected. The latter is exceeding the total existing statistics by a factor of five. A precise measurement of the branching fractions and the form factors of the rare decays K+- -> pi+- l+ l- has been performed using different theoretical models. The precise measurement of direct photon emission (DE) in the decay K+- -> pi+- pi0 gamma and its interference (INT), with the INT amplitude being observed for the first time, has been finalized. This study is based on the full NA48/2 data set with about 600k reconstructed K+- -> pi+- pi0 gamma decays which is factor of 30 larger than for previous experiments. We report the results on the CP violating asymmetry between K+ and K- obtained from rare kaon decaysWe report the recent results on rare kaon decays from the NA48/2 experiment. The precise measurement of direct photon emission (DE) in the decay K+- -> pi+- pi0 gamma and its interference (INT), with the INT amplitude being observed for the first time, has been finalized. This study is based on the full NA48/2 data set with about 600k reconstructed K+- -> pi+- pi0 gamma decays which is factor of 30 larger than for previous experiments. Samples of about 7200 reconstructed K+- -> pi+- e+ e-, and more than 3000 K+- -> pi+- mu+ mu- events, with very small background contamination, have been collected. The latter is exceeding the total existing statistics by a factor of five. A precise measurement of the branching fractions and the form factors of the rare decays K+- -> pi+- l+ l- has been performed using different theoretical models. The CP violating asymmetry between K+ and K- in this channel is also measured

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arXiv:0909.2486v2  [hep-ex]  15 Sep 2009 Precision measurements of rare kaon decaysEvelina MARINOVA†INFN Sezione di PerugiaE-mail: Evelina.Marinova@cern.chWe report the recent results on rare kaon decays from the NA48/2 experiment. The precise mea-surement of direct photon emission (DE) in the decay K ! pi pi0γ and its interference (INT),with the INT amplitude being observed for the first time, has been finalized. This study is basedon the full NA48/2 data set with about 600k reconstructed K ! pi pi0γ decays which is factorof 30 larger than for previous experiments. Samples of about 7200 reconstructed K ! pi e+ e  ,and more than 3000 K ! pi µ+ µ  events, with very small background contamination, havebeen collected. The latter is exceeding the total existing statistics by a factor of five. A precisemeasurement of the branching fractions and the form factors of the rare decays K ! pi e+ e has been performed using different theoretical models. The CP violating asymmetry between K+and K  in this channel is also measured.European Physical Society Europhysics Conference on High Energy Physics, EPS-HEP 2009,July 16 - 22 2009Krakow, PolandSpeaker.†On behalf of NA48/2 collaborationc Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/Precision measurements of rare kaon decays Evelina MARINOVAThe NA48/2 experiment at the CERN-SPS, designed primarily for charge asymmetry measure-ments [1], uses simultaneous K+ and K  beams and has collected large samples of rare decays.Detailed information about the main detector and its performance can be found elsewhere [2].K ! pi pi0γ decaysThe properties of the K ! pi pi0γ decays are described in terms of the Lorentz invariant Wwhich is given by W 2 = (PK  Pγ )(Ppi  Pγ )=(mK  mpi)2 [3, 4], where PK;Pγ ;Ppi are the particles mo-menta in the kaon rest frame. Both Inner bremsstrahlung (IB) and Direct emission (DE) contributeto the amplitude of the decay. DE can occur either through electric or through magnetic dipoletransition. The electric dipole transition can interfere with the IB term giving rise to an interfer-ence term with CP violating contributions. The previous measurements of IB and DE, reported inreference [5], are performed in the region 55 to 90 MeV in terms of kinetic energy of the pion inthe kaon rest frame, Tpi . The decay rate is written asdΓdW =dΓIBdW1+ 2cos( φ + δ 11   δ 20)m2pim2KXEW 2 + m4pim4K(jXEj2 + jXMj2)W 4: (1)The NA48/2 measurement is made in an enlarged region of T pi , (0 - 80 MeV). After applying a cuton W (0.2 - 0.9), about 600 000 events have been reconstructed, with a background estimation ofabout 1% of DE component (see Fig. 1 (a)). The mistagging probability for the radiated gammais as low as one per mille. As the different contributions have a different W dependence, IB,INT and DE distributions can be well separated. The main technique used to extract INT andDE contributions is based on Poissonian Maximum Likelihood fit. The relative contribution ofevery source is calculated in such a way that the difference between the number of events and thetotal number of simulated events in every bin is minimal (see Fig. 1 (b), (c)). A cross-check wasperformed using a polynomial fit.(a) (b) (c)Figure 1: (a) Reconstructed mass of the kaon; (b) Contribution of the different components to the decayrate,their sum and the data distribution; (c) The fit residuals distribution.The final results for the relative fraction of DE and INT with respect to IB are Frac(DE)Tpi (0  80)MeV =(3:32 0:15stat  0:14sys) 10  2, Frac(INT)T pi (0  80)MeV = (  2:35 0:35stat  0:39sys) 10  2. Inorder to compare the result for DE, INT was assumed to be 0, a polynomial fit was used, and T pi isin the range (55 - 90) MeV. The obtained result BR(DE)Tpi (55  90)MeV = (2:3 0:05stat  0:077sys)10  6 disagrees with the previous measurements [5] but the bad χ2 of the fit indicates that INT = 02Precision measurements of rare kaon decays Evelina MARINOVAis a wrong assumption. The simultaneous measurement of INT and DE allows a measurement ofthe electric and the magnetic contributions to DE. The results are XE = (  24 4stat  4sys)GeV  4,XM = (254 11stat  11sys)GeV  4.K ! pi e+ e  decaysThe properties of the rare decays K ! pi‘+‘  involving FCNC processes are describedin terms of z = M2‘‘=M2K . The decay rate is given by dΓ=dz  ρ(z)W(z)where ρ(z) is a phasespace factor. There are four theoretical models predicting the form factors W(z) (see Tab. 1).Each model is characterized by two free parameters which determine a model dependent branchingfraction. The measurements are done normalising to the K ! pi pi0Dalitz which has the samecharged particles in the final state. Due to the similarities between the signal and normalisationchannel, systematics due to particle identification, trigger efficiencies, etc., is kept very low. Intotal 7253 events have been reconstructed with a background estimation of 1% (see Fig. 2(a)). Thekinematical region in z is restricted to be larger than 0.8 (corresponding to Mee > 140 MeV/c2)in order to suppress the background from the abundant normalisation channel. Fits to all modelshave been performed and have a reasonable quality (see Fig. 2(b).) The results are reported inTab. 1. The combined result for a model dependent branching fraction is BR = (3:11 0:04stat 0:05syst  0:08ext  0:07model) 10  7 [9], in agreement with [10] and the predictions of [11]. A firstModel Form-factor Parameter Results with Combined errorW(z)= GFM2Kjf0j(1+ λ z) λ 2:32 0:18Model 1 Linear jf0j 0:531 0:016BR1 10  7 3:05 0:10W(z)= GFM2KWpol+ + W pipi+ (z) a+   0:578 0:016Model 2 ChPT at NLO [6] b+   0:779 0:066BR2 10  7 3:14 0:10W(w˜;β;z) w˜ 0:057 0:007Model 3 Combined framework ChPT and β 0:531 0:016Large-Nc QCD [7] BR3 10  7 3:13 0:10W(z) W(Ma;Mρ;z) Ma 0:974 0:035Model 4 The ChPT parametrization involving the Mρ 0:716 0:014resonances a and ρ contribution [8] BR4 10  7 3:18 0:10n/a restricted region in Mee > 140 MeV/c2 BRmi  10  7 2:28 0:08Table 1: Results of fits to the four models, and the model-independent branching ratio BRmi(z > 0:08):measurement of the direct CP violating asymmetry of K+ and K  decay rates in the full kinematicrange was obtained by performing BR measurements separately for K+ and K  and neglecting thecorrelated uncertainties:∆(Kpiee)= (BR+   BR )=(BR+ + BR )= (  2:2 1:5stat  0:3syst)%, where only the uncorrelatedsystematic errors have been taken into account. The result is compatible with no CP violation.However its precision is far from the theoretical SM expectation [6] of j∆(Kpiee)j 10  5 and fromthe theoretical SUSY expectation of j∆(Kpiee)j 10  3 [12].3Precision measurements of rare kaon decays Evelina MARINOVAK ! pi µ+ µ  decaysAbout 3100 events of the decay K ! pi µ+ µ  have been reconstructed from the full datasample. This sample is four times larger than the existing world statistics [5]. The main source ofbackground is the abundant K ! 3pi decay (with subsequent pi ! µ decay or with 2 misiden-tified pi). The background is studied both with data and MC simulation and is at the order of fewpercent and well under control.(a) (b) (c)2), GeV/c-e+e±piM(0.45 0.46 0.47 0.48 0.49 0.5 0.51 0.52 0.53Events0100200300400500600700z0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.52/dz, GeV/cΓd0102030405060-2410×(1)(2)(3)(4)Model (1):Model (2):Model (3):Model (4):/ndf=22.7/192χ/ndf=32.1/192χ/ndf=27.7/192χ/ndf=36.9/192χ2), GeV/c-µ+µ±piM(0.47 0.48 0.49 0.5 0.51 0.52Events050100150200250300350400450500Figure 2: (a) The invariant mass of the kaon, reconstructed from K ! pi e+ e  decays; (b) The computed(dΓ=dz)i and the results of the χ2 of the fits corresponding to the considered models; (c) The invariant massof the kaon, reconstructed from K ! pi µ+ µ  decays.ConclusionsIn conclusion, precise studies of several rare kaon decays measurements by the NA48/2collaboration were presented. The achieved precisions are similar to or better than the bestprevious ones. Due to the simultaneous K+ and K  beams, the CP violating charge asymmetry inK ! pi e+ e  was measured for the first time.References[1] J.R. Batley, et al., Eur. Phys. J. C 52 (2007) 875.[2] V. Fanti, et al., Nucl. Instrum. Methods A 574 (2007) 433.[3] N. Christ, Phys. Rev. 159, 1292 (1967).[4] G. D’Ambrosio, M. Miragliuolo and P. Santorelli, The Daphne Physics Handbook, Eds. L. Maiani, G.Pancheri and N. Paver (1992).[5] C. Amsler et al. [Particle Data Group], Phys. Lett. B 667, 1 (2008)[6] G. D’Ambrosio, et al., JHEP 9808 (1998) 4.[7] S. Friot, D. Greynat, E. de Rafael, Phys. Lett. B 595 (2004) 301.[8] A.Z. Dubnickova, et al., Phys. Part. Nucl. Lett. 5 (2008) 76, hep-ph/0611175.[9] J.R. Batley, et al., Phys. Lett. B 677 (2009), 246.[10] P. Bloch, et al., Phys. Lett. B 56 (1975) 201. C. Alliegro, et al., Phys. Rev. Lett. 68 (1992) 278.R. Appel, et al., Phys. Rev. Lett. 83 (1999) 4482.[11] C. Bruno and J. Prades, Z. Phys. C57 (1993) 585; J. Prades, PoS(KAON)022, arXiv:0707.1789.[12] A. Messina, Phys. Lett. B 538 (2002) 130.; G. D’Ambrosio, D.-N. Gao, JHEP 0207 (2002) 068.4

Precision measurements of rare kaon decays

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