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Results on Rare Decays on Neutral Kaons from the NA31 Experiment At MZ-ETAP/95-1 January 199 IN STITUT FUR PHYSIK JOHANNES GUTENBERG-UNIVERSITAT MAINZ RESULTS ON RARE DECAYS OF NEUTRAL KAONS FROM THE NA31 EXPERIMENT AT CERN Konrad Kleinknecht 7/ Insriruthir Physik, Johannes Gutenberg-Universitat Mainz 55099 Mainz, Germany Talk given at Second Int. Workshop on Heavy Quark Physics Q E October ,1994 § University of Virginia, Charlottesville, Virginia, USA {U2 E ai 5% §§ E3 OCR Output C """ + **fF7 {W - {LIP? _ *{ ?X= _ _ i is axm>:¤;* ” 1c\¤;H`I!LErS` if V .¢. johcnnes Q`}utenf:e:g iiupfcrsficf) non Ebetctl - 1 RESULTS ON RARE DECAYS OF NEUTRAL KAONS FROM THE NA31 EXPERIMENT AT CERN Konrad Kleinknecht Institut fiir Physik, Jahannes Gutenberg-Universitat Mainz 55099 Mainz, Germany ABSTRACT Experimental data from the neutral Kaon decay experiment NA31 at CERN have been analyzed. Results include the first observation of the mode KL ——> rtoyy with a branching ratio of (1.710.3) 10-9 and an invariant yy mass spectrum favouring chiral perturbation theory, a measurement of 2021 events of the Dalitz decay KL —> Ye+e· yielding a branching ratio of (9.1i0.3f8§)10and·‘6 of 8 events of the double Dalitz decay, KL —> e+e·e+e·, with a branching ratio of (1O.4·:3.7i1.1)10·8. Searches are also reported for the decay Kg —> 1t°e+e· (BR<1.1x10·9 at 90% C.L.), KL —> 1t°1t9y (BR<5.6x1O·9 at 90% C.L.) and KL ——> 3Y (BR<2.8x10·/ at 90% C.L.). Invited talk at the Heavy Quarks 94 conference, Charlottesville, University of Virgina, October 1994 OCR Output - 2 1 . Introduction The experiment NA31 was originally designed for measuring the CP violating decays KL —>·1:01:0 and KL —> 1:+1:i The experiment resulted in the first evidence for direct CP violation with the experimental result for the parameter e'/e = (2.30$0.65) x10·3 [1]. Concurrently with the 21: decay modes, other rare decays were registered in this detector. This report describes the experimental results on some of these decay modes. 2. The NA31 neutral Kaon detector This detector is able to measure the neutral and charged two-pion and other decay modes of neutral K mesons. While 1:01:0 decays are detected through four electromagnetic showers and 7C+7lZ“ decays through two charged tracks, for the final states discussed here it is necessary to detect electromagnetic showers (1t077, 1:01:07, 37) or charged particles (e+e·e+e·) or both photons and charged particles (7e+e—, 1t0e+e‘). The beam and detector for the experiment have been described in detail in ref. [2]. Long lived kaons are produced with an average momentum of 100 GeV/c by 450 GeV/c protons striking a beryllium target located 244m upstram of the electromagnetic calorimeter. Charged particles are removed by a sweeping magnet, and a neutral beam is selected by collimation to $0.2 mrad. The final collimator, with diameter 6 cm, is located 124 rn upstream of the electromagnetic calorimeter. It is followed by a 95 m long evacuated region. The neutral beam is further transported in an evacuated tube of diameter 20 cm through a central hole in the detectors. Decays occurring downstream of the final collimator over a region of 50 m in length are detected in a setup of wire chambers and calorimeters with outer diameters of 2.5 m typically. The relevant parts of the detector ordered in downstream direction - are: four ring-shaped anti—counters surrounding the decay region, and placed between 60 and 120 m from the final collimator, to veto events with photons missing the calorimeter; two wire chambers, spaced 23 m apart, each consisting of four planes, with a resolution of $0.5 mm in each projection; OCR Output - 3 a transition radiation dcmccior (TRD) for electron /pi0¤ discximination composed of four modules, cach with a plastic foil radiator and a wire chamber filled with a Xenon (30%)- Helium (55%)-Methane (15%) mixture [3I; scintillator hodoscopes triggering on charged and neutral decay modes; an electromagnetic lead/liquid argon calorimeter (LAC) with 1.25 cm wide strips measuring the energies of photons and electrons with a resolution of 7.5%/ and their impact points with a transverse spatial resolution of $0.5 mm in each projection; an iron/scintillator sandwich calorimeter which measures the energy of charged pions; two planes of scintillators behind an absorber of a thickness equivalent to 3 m of iron, which detect penetrating muons. The wire chambers are used to measure the directions of charged particles and to reconstruct the position of the decay vertex, while the liquid argon calorimeter measures energies and impact positions of electrons and photons. The hadron calorimeter and the transition radiation detector are used to reject events containing charged pions. 3 . Event reduction and reconstruction The trigger requires a minimum total energy of 35 GeV and at least two energy clusters in each projection in the liquid argon calorimeter. For decay modes with charged tracks, in addition more than six hits in the four planes of the first wire chamber are required. ln the off-line preselection of candidates, the following criteria are used: more than 45 GeV energy deposited in the liquid argon calorimeter, less than 6 GeV energy seen in the hadron calorimeter, and - for decay modes with charged tracks — at least one spacepoint in each wire chamber. These events were kinematically reconstnrcted. A complete reconstruction of the tracks - if any — in the chambers is performed. Independently, narrow energy clusters indicating electromagnetic ` i . showers are found from the pulse heights in the calorimeter. Electron tracks are reconstructed by linking space points from the two wire chambers with electromagnetic showers. For events which have only photons in the final state, the longitudinal position ZK of the decay vertex is calculated from the energy and transverse position of the electromagnetic showers in the LAC under the assumption that the invariant mass is the nominal kaon mass ("neutral vertex"). For events with two or four electron tracks, this longitudinal OCR Output - 4 position can also be calculated from the tracks ("charged vertex"). For two tracks, this is calculated from the minimum distance of approach of the two tracks, or in events with four tracks this is obtained by requiring the sum of the squared transverse distances d2 from the four tracks to the transverse vertex position to be a minimum: dz : g our - mz + on — yvv i=1 Gtiz where xt] and yu are the coordinates of track i at the longitudinal vertex position zv, and Gu ist the resolution in the transverse distance from track i to the vertex. For each decay mode, different fiducial cuts are made on these vertex positions, depending on the presence of background. Common kinematical fiducial cuts require: i) the total energy deposited in the LAC to be between 50 and 190 GeV (60 to 170 GeV for 1t0yy), ii) the individual electromagnetic shower energy to be between 3 and 100 GeV (for ye+e‘, e+e‘e+e", and rc°e+e· decays modes) or between 5 and 100 GeV (for now, rc91t°y, and 37), iii) the energy barycentre in the electromagnetic calorimeter to be within 5 cm from the beam axis (7 cm for Dalitz decays ye+e‘ and e+e‘e+e·, and 9 cm for Kg —> 1t°e+e‘). The last requirement rejects events with large missing transverse energy. 4. Decay KL .-) ypoyy The decay KL-> now is of considerable theoretical interest. Firstly, it may be used to estimate the CP conserving contribution to the decay KL —> rtOe+e" [4-6] via two—photon intermediate states. The decay KL -—> 1t°e+e— is a candidate for detecting direct CP violation. If the amplitudes for KL ——> now are small enough, then the decay KL ——> 1t°e+e· is dominated by CP violating contributions. This experiment provides a measurement of the dominant J=0, and a limit on the J=2 two-photon contribution to the KL -> now decay. Secondly, the decay KL —> Jtoyy may be used to test the chiral perturbation theory, and some semi-empirical models describing kaon decays. Lowest order chiral expansion [7] and models involving rt+1t· intermediate states in addition to the 1:0 [8] predict a branching ratio around 7x10"’ and J=0 for the photon pair. The yy invariant mass spectrum extends from about 2mT; to the kinematical limit. Higher order chiral expansion and other calculations including vector (or scalar) meson intermediate states [9] predict a OCR Output - 5 branching ratio between 0.7xl0and‘9 3x10depending‘6 on the contribution at low mw, for which the photon pair should have J=2. This decay was originally observed by NA31 [10]; the present analysis supports this result with more than twice the KL flux. Events with four electromagnetic showers are required for this experiment. The main problem is then the rejection of unwanted background from rt°rtO and 37tO decays. The four showers are grouped into two pairs. One pair is required to have a mass near the 1tO mass (between 125 and 145 MeV), assuming the "neutral vertex" ZK reconstructed from all four showers and the kaon mass. The other pair should not have the 1t° mass. The rejection of 1t°rt° states is done by requiring the quantity Tg ‘)_+_ mkl2 mi]() +MeV mkl 8‘ 2m1tMeV to be above 5 for all possible pairings (ij) and (kl) of the four showers. lf, in addition, events with similar energies in the same quadrant of the LAC are removed, 99.94% of the selected rt92t9 events are removed.
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