Recent results on the kaon rare decays measured in NA48 experiment at CERN
are presented.Comment: 5 pages, 7 figure

Cuhadar-Donszelmann, Tulay

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Recent results on the kaon rare decays measured in NA48 experiment at CERN are presented.Recent results on the kaon rare decays measured in NA48 experiment at CERN are presented

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NA48: Rare Decay ResultsT. Cuhadar-Do¨nszelmannOn behalf of the NA48 CollaborationaCERN, Div. EP, CH-1211,Geneva 23, SwitzerlandRecent results on the kaon rare decays of the KS ! γγ, KS,L ! +−e+e−, KS ! 0e+e−measured in NA48 experiment at CERN are presented in this paper.1 The experimentThe NA48 experiment is designed to measure direct CP violation in two-pion decays and tosearch for rare decays of neutral kaons using simultaneous and almost collinear KS and KLbeams. The KL beam is produced by 450 GeV/c protons from SPS hitting a beryllium targetan angle of 2.4 mrad. The proton intensity is 1:5  1012 per SPS pulse and these pulses are2.4 s long with a repetition period of 14.4 s. Non-interacting protons are directed to the KStarget, which is positioned  120 m downstream of the KL beam line, to produce the KS beam.Anticounter at the end of the nal collimator(AKS) denes the beginning of the dual volumeof  40 m long. A set of seven scintillator counters(AKL) surround the decay region and vetoesevents outside the detector acceptance. The energy and the position of the electromagneticshowers of electrons and photons as the products of the kaon decays are detected in the liquidkrypton calorimeter(LKR), with an energy resolution of (E)=E ’ 0:100=E0:032=pE0:005(E in GeV). The momenta of charged particles are measured in a spectrometer consisting of adipole magnet and four drift chambers(DCH). The spectrometer has a momentum resolution of(p)=p ’ [0:009p  0:5]% (p in GeV). A hodoscope placed in front of the calorimeter is used tomeasure the event time with resolution of 150 ps. An iron-scintillator hadron calorimeter(HAC)aCagliari-Cambridge-CERN-Dubna-Edinburgh-Ferrara-Firenze-Mainz-Orsay-Perugia-Pisa-Saclay-Siegen-Torino-Vienna-Warsawis used to measure the energies of the hadrons and the muons are identied by an muon vetosystem. The detailed description of the experiment can be found elsewhere 1.The results presented here are based on data taken in 1998 and 1999 During two days in1999, data was recorded with a factor  200 higher beam intensity than the usual KS beam,resulting in 2:3 108 KS decays.2 Decays of KS ! γγIn the framework of the chiral perturbation theory(PT), the contributions of one loop of chargedpions and kaons to KS ! γγ decays are nite and give an unambigious prediction for branchingratio, 2:25 10−6 with less than 10% uncertainty 2. Thus, a precise determination of branchingratio provides a good test of PT. The CERN experiment NA31 3 measured a value compatiblewith theory: (2:4 0:9)  10−6.Two-gamma decays of short-lived kaons are reconstructed from KS high intensity data takenin 1999. KS ! γγ candidates are selected by requiring events with  2 clusters. The pairs ofthese clusters must be within 5 ns. No other cluster with energy > 1:5 GeV within 3ns aroundthe event time must be present. The energy of each cluster must lie between 3 < Ei < 100 GeVand the sum of cluster energies, the kaon energy, must be between 60 and 170 GeV. The centerof gravity(cog) measured for two clusters cog =√(∑i Eixi)2 + (∑i Eiyi)2=∑i Ei, must be lessthan 7cm. Events with some activity in the AKS, AKL and DCH are rejected. Energy depositedin the HAC must not exceed 3 GeV in a time window of 15ns around the event time. Afterthe initial event selection, the longitudinal vertex position of the event is calculated byzvertex = zLKR −√∑i;j;i>j EiEj[(xi − xj)2 + (yi − yj)2]mK; (1)where zLKR is the longitudinal position of the liquid krypton calorimeter from the AKS, xi; yiare the transverse position of the clusters at the LKR, and mK is the kaon mass. The study ofKS ! γγ is done in a limited region, −2 < zvertex < 5 m in order to minimize the main sourceof background which is KS ! 00. After the above cuts, 450 K ! γγ events are obtained.The zvertex distribution of those events are shown in Figure 1(left).11010 210 310 4-1500 -1000 -500 0 500 1000 1500zvertex[cm]Number of events / 100 cm KS→pi0pi0 K→γγγγ background from KS→pi0pi0 (MC)fit region050100150200250300350400-400 -200 0 200 400 600 800 1000 1200zvertex[cm]Number of events / 100 cm KS → γγ ( fitted ) KL → γγ K → γγ fit regionFigure 1: (left) The zvertex distribution for data and MC. The open circles represent KS ! 00 events whileclosed squares show K ! γγ. The solid line shows the γγ background from MC simulated KS ! 00 decays.The peak at z = −1250 cm is due to  mesons produced in the AKS. (right) The zvertex distribution obtained frommaximum likelihood method for fitted KS ! γγ events (dotted area) and KL ! γγ component (hatched area).The dots show the data and the arrows show the fitted region.The main background to KS ! γγ decays are KS ! 00 decays where two photons arenot detected. The maximum invariant mass of the γγ from 00 events is 458 MeV resulting ina 9 m shift in the zvertex. This background is minimized by requiring the zvertex cut describedabove. 108 KS ! 00 events are simulated to estimate this contribution. A second backgroundto KS ! γγ decays arises from KL ! γγ decays because of the KL flux produced at thetarget. This background is estimated by measuring KL flux from KS ! 00 decays. Theremaining background originated from  ! n0 is estimated by comparing cog distributionfor those events with signal events.Using a binned maximum likelihood method, 149  21 KS ! γγ events are estimated inthe signal region and the branching ratio of KS ! γγ is measured to be 4BR(KS ! γγ) = (2:58  0:36(stat) 0:22(sys))  10−6: (2)The result of the likelihood method is shown in Figure 1(right). The main systematic errorsources are due to uncertainty of BR(KL ! γγ)(5%), the selection cuts (4%), background(5%), acceptance (2%), and the trigger eciency (2%).From this measurement, the ratio of the decay widths of KS ! γγ to KL ! γγ iscomputed to beR = Γ(KS ! γγ)Γ(KL ! γγ) = 2:53  0:35(stat)  0:22(sys):3 Decays of KL ! +−e+e−The study of KL ! +−e+e− decays5;6 provides a way to observe CP violation in the neutralkaon system. The two main contributions to this decay are the CP conserving direct emissionassociated with a magnetic dipole transition (M1) and the CP violating innner bremsstrahlung.The interference between these two components produces CP violating asymmetry (AL  14%)in the angle  between normals to the +− and e+e− planes in the kaon center of mass system.The asymmetry and the branching ratio is determined from the combined data taken in 1998and 1999. KL ! +−e+e− events are selected by requiring two positive and two negativeintime tracks. The electron and pion identications are based on the E=p measurement, whereE is measured in the LKR and p is measured in the spectrometer.DataMCMC KL→pi+pi-pi0DDouble Ke3 evts[GeV]DataMCFigure 2: For combined 1998 and 1999 data: (left) Measured Mpi+pi−e+e− invariant mass for KL ! +−e+e−,(right) After acceptance correction, the  distribution from which asymmetry is obtained.The main background to KL ! +−e+e− decays is due to the KL ! +−0D where 0Ddecays into e+e−γ, with a lost γ. They are eliminated using a variable p020 , p020 = ((m2K −m20 −m2)2 − 4m20m2 − 4(p2?)m2K)=4(m2 + (p2?)). For KL ! +−0D events p020 is greaterthan zero. The second background source is due to the KL ! +−γ, where γ converts into ae+e− pair. The rejection is done by demanding 2 cm seperation between electron tracks in therst drift chamber.The invariant mass m+−e+e− distribution for KL ! +−e+e− is shown in Figure 2(left).Using KTEV results 7 on a1=a2 = −0:72  0:03 and ~gM1 = 1:35+0:20−0:17, the preliminary mea-surement on branching ratio of KL ! +−e+e−, based on 1337 reconstructed events, isBR(KL ! +−e+e−) = (3:1  0:1(stat)  0:2(sys))  10−7: The asymmetry is obtained fromthe  distribution corrected for acceptance (Figure 2(right)). The preliminary result on theasymmetry is given by AL = (13:9 2:7(stat) 2:0(sys))% which is in good agreement with thetheoretical prediction.4 Decays of KS ! +−e+e−The main contribution to the KS ! +−e+e− decay 5;6 arises from CP conserving innerbremsstrahlung and as a result no asymmetry in the  distribution is expected.The events selection is done in a similar way as in KL ! +−e+e−. The main backgroundarising from KL ! +−0D is eliminated by requiring the event to be tagged as coming fromthe KS beam. The rst observation of this decay is based on the data taken in 1998 8. Based onthe 56 KS ! +−e+e− events, the value obtained for the branching ratio of KS ! +−e+e−is BR(KS ! +−e+e−) = (4:50:7(stat)0:4(sys))10−5 : This value translates into the innerbremsstrahlung component of KL ! +−e+e−, BR(KIBL ! +−e+e−) = (1:40:2)10−7.The asymmetry, AS , and branching ratio of KS ! +−e+e− are measured by using com-bined 1998 and 1999 data including the 2-day KS high intensity data. Figure 3 shows the invari-ant mass M+−e+e− and  distributions for these data. Based on 921 KS ! +−e+e− events,the preliminary result for the asymetry is given by AS = (−0:2  3:4(stat)  1:4(sys))% whichis consistent with zero. The value obtained for the branching ratio is BR(KS ! +−e+e−) =(4:30:2(stat)0:3(sys))10−5 , and the inner bremsstrahlung component of KL ! +−e+e−,BR(KIBL ! +−e+e−) = (1:3  0:1))  10−7.KS→pi+pi-e+e-KL→pi+pi-pi0|→e+e-(γ) ↓ ↓[GeV]DataMCFigure 3: For combined 1998 and 1999 data: (left) Measured Mpi+pi−e+e− invariant mass for KS ! +−e+e−,(right) After acceptance correction, the  distribution from which asymmetry is obtained.5 Decays of KS ! 0e+e−The study of KS ! 0e+e− decay9 is important to improve the limit on the indirect CP violat-ing term in KL ! 0e+e−. The branching ratio is given by: BR(KS ! 0e+e−)  5:210−9a2Swhere aS is the strength of the indirect CP violating component in KL ! 0e+e−. The presentlimit on the branching ratio comes from the NA31 experiment 10, BR(KS ! 0e+e−) < 1:1 10−6 at 90% condence level. A more precise measurement of branching ratio of KS ! 0e+e−will provide better limits on the indirect CP violating contribution in KL ! 0e+e−.The KS ! 0e+e− search is based on the KS high intensity data taken in 1999. The eventsare selected by requiring at least four cluster in the LKR, two tracks and one vertex. Then thegroup of four clusters has to pass additional kinematic cuts such as the invariant mass of γγmust be within 2.5 MeV with respect to the nominal 0 mass, and the invariant mass of eeγγmust lie within 10 MeV with respect to nominal kaon mass.The main background KS ! 0e+e− comes from KS ! 0D0D, where both 0D decaysinto e+e−γ, and one electron and one positron are lost. This background is rejected by requiringinvariant mass dierence jmeγ−m0j to be greater than 30 MeV. Another source of backgroundarises from KS ! 00D, where 0D decays into e+e−γ with a lost γ. Due to the origin ofthe electrons, their invariant mass cannot exceed pion mass. The remaining background inKS ! 0e+e− are negligible.Figure 4 shows the invariant mass distribution of e+e− pair for events passing the above cuts.The events below mee < 165 MeV is in good agreement with the background KS ! 00Devents. No events are observed mee above 165 MeV. Using KS ! 00D as a normalizationchannel, at 90% condence level upper limit on the branching ratio is obtained,BR(KS ! 0e+e−) < 1:4  10−7: (3)020004000600080001000012000140000 0.05 0.1 0.15 0.2 0.25 0.3 0.35Mee (GeV)EntriesControl region Signal region   Data MC- pi0pi0DMC- pi0e+e-Figure 4: Invariant mass distribution of e+e− for data (dots) and for 00D monte-carlo events (solid line). Theexpected distribution for KS ! 0e+e− events is shown by the shaded area.References1. V. Fanti et al., Phys. Lett. B 465, 335 (1999),N. Doble et al., Nucl. Instrum. Methods B 119, 181 (1996).2. J. Kambor, B.R. Holstein, Phys. Rev. D 49, 2346 (1994).3. G.D. Barr et al., Phys. Lett. B 351, 579 (1995).4. A. Lai et al., Phys. Lett. B 493, 29 (2000).5. L.M. Sehgal, M. Wanninger, Phys. Rev. D 46, 1035 (1992),Erratum: Phys. Rev. D 46, 5209 (1992).6. P. Heiliger, L.M. Sehgal, Phys. Rev. D 48, 4146 (1993),Erratum: Phys. Rev. D 60, 079902 (1999).7. A. Alavi-Harati et at., Phys. Rev. Lett. 84, 408 (2000).8. A. Lai et al., Phys. Lett. B 496, 137 (2000).9. G. D’Ambrosio, G. Ecker, G. Isidori and J.Portoles, JHEP 08, 004 (1998).10. G. Barr et al., Phys. Lett. B 304, 381 (1993).

NA48: Rare Decay Results

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