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U. E. R Institut National De De Physique Nucléaire Université Paris-Sud Et De Physique Des Particules LAL 92-26 Gestion INIS May 1992 Doe. enreg. Ie N* TFW : 0L Destination : I,t+D,D NEW RESULTS ON DIRECT CP VIOLATION FROM THE NA31 EXPERIMENT Olivier PERDEREAU For the NA31 collaboration CERN, Edinburgh. Main/., Orsay, Pisa and Sicg cn Talk given at the XXVIlth Rencontres de Moriond Uectroweak Interactions and Unified Theories" Les Arcs, Savoie, March 15-22,1992 U. E. R Institut National de de Physique Nucléaire Université Paris-Sud et de Physique des Particules Bâtiment 200 - 91405 ORSAY Cedex LAL 92-26 May 1992 New results on direct CP violation from the NA31 experiment O. Perdereau Laboratoire de l'Accélérateur Linéaire, IN2P3-CNRS et Université de Paris-Sud, F-91405 Orsay Cedex, France. For the NA31 collaboration : CERN, Edinburgh, Mainz, Orsay, Pisa and Siegen Abstract The NA31 experiment has published the first evidence for direct CP violation by measuring a non-zero value for the X(e'/e) parameter. Further data-taking periods took place in 1988 and 1989, producing two datasets with statistics comparable to the original one. This paper presents the final result of the anal- ysis of tbe 1988 dataset together with a preliminary result of the analysis of the 1989 one. The combined, preliminary, measurement of NA31 is R(e'/e) = (2.3 ± 0.7 ) 10~3, thus confirming our initial measurement and also in agree- ment with the standard model. Introduction Since its discovery, 28 years ago, the non respect of the discrete CP symetry, so-called "CP violation", has up to now been observed only in neutral Kaon decays. It was firstly understood as a consequence of a mixing of the Weak Interaction eigenstates in the K^ and Kg "physical" states. This is the "indirect" CP violation, parametrized by e. In addition, the possibility of a direct decay of a CP eigenstate into a state of the opposite CP eigenvalue has been investigated. Both are incorporated in the Stan- dard Model framework, when three families are assumed. However the fundamental mecanism of CP violation remains unknown. The parameter describing direct CP violation is SR(e'/e) . Its theoretical evalua- tion is a difficult task. It involves the Cabbibo-Kobayashi-Maskawa mixing matrix parameters (angles and phase), quark masses and hadronic matrix elements. Among those some are known with large errors, whereas others are unknown, such as the top quark mass m4. The latest evaluations lead to positive values for 5R(e'/e) 5 of the order of a few 10~~3 [I]. The most recent experimental results come from two experiments. The CERN experiment NA31 found ft(e'/e) = (3.3 ± 1.1) x 10 "3 using data collected in 198P [2]. The FNAL experiment E731 published result, based on 20% of their dataset, is 3 SR(e'/e) = (-0.4 ± 1.4 ± 0.6) x 10~ [3]. In addition, a preliminary result for the whole dataset has also been presented recently : 5R(e'/e) = (0.6 ± 0.58 ± 0.18) x 10-* [4]. 1 The NA31 experiment If direct CP violation exists, the characteristics of the CP-violatkig decays of K\ are different from the CP-conserving decays of the Kg . To study this, we compare the decay rates of neutral kaons into two pions by forming the double ratio : -» TT0TT0) V ^ Any deviation of Tl from unity reveals the presence of direct CP violation. This quantity is related to 5î(e'/e) as follows : •R = 1 - 6 x »(e'/£) (2) 1.1 The beams To measure the 4 decay amplitudes, we expose the detectors alternately to a K^ and a K$ beam, detecting the charged and neutral decay modes simultaneously. Details on the beam and detectors can be found elsewhere [5]. K°L are produced by a proton beam from the CERN SPS (450 GeV) hitting a berylium target about 245 m upstream the detectors. During the first 120 m, charged particles are swept away magnetically then the beam passes through a three-stage coJlimation scheme, designed to minimize K$ regeneration. The experiments counts kaon decays in a fiducial region (between 125 and 75 m upstream the detectors). The K% production is different than the K^ one. A lower energy (360 GeV) SPS proton beam is transported to a target in the fiducial region, followed by a sweeping magnet and a collimator equiped with an anticounter defining the begining of the decay region. These parts are sitting on a train which spans the whole fiducial region. Due to the large value of the K1I live time and their mean energy (100 GeV), K° decays distribute flatly in the fiducial region (JCT % 3km). Kg decay very close to the production point (7CT w 5m). With the mobile Kg target we are able to produce a Kg distribution very similar to the K)' one. The experiment is run about 36 hours in A-" mode, and then 12 hours in K" mode. Calibration runs are taken at intervals. 1.2 Detectors The main parts of the detector are, going downstream : • Two Wire Chambers, spaced by 23 m within a Helium tank. Each give a resolution per space-point of 750 /xm . • A large area Transition Radiation Detector (TRD), added for the 1988 run [6]. It consists in 4 radiators of polypropylene foils (20 fim) spaced by 650 fim, each followed by an Argon Wire Chamber to detect X-rays photons produced in the radiator. • A Liquid Argon Calorimeter (LAC) (2.5 m x 2.5 m, 25 X0), made by a sandwich of lead and liquid argon, readout by projective strips. The resolution for electrons (photons) is cr(E)/E = 7.5%/\/^- Due to the fine granularity (1536 strips in total) a position resolution of .5 mm per projection is achieved. Thus photons are separated if their distance is greater than 1.5 cm in projection. Some dead gaps were repaired before the 1989 run, improving the calorimeter uniformity. • A Hadron Calorimeter (HAC), constituting in scintillator strips separated by 2.5 cm of iron, which measures pion showers with a resolution of 65%/vE. For triggering purposes, veto counters surround this apparatus to remove events when a particle misses the detectors. In addition, muon anticounters, sandwiched by iron walls, have been installed downstream the HAC to veto events with a muon. 1.3 Charged events selection The trigger for 7T+ TT~ is started by coincidence signals from a scintillator hodoscope in front of the LAC. No veto signal, hits in the first wire chamber, loose cuts on calorimetric energy, including a cut on longitudinal shower développement (to reject electrons from neu ) are then required. Events are then filtered online : events with detected photons (ir+ir~n° ).or missing transverse momentum are rejected, and energy cuts are imposed after a rough calibration. Offline, events are fully reconstructed if 2 tracks can be found (2 hits per wire chamber). The tracks must reconstruct a decay vertex, i.e. approach each other to less than 2.5 cm, and be far enough from the beam axis (18 cm). The kaon energy EK is computed from the track opening angle. We cut on EK (between 60 and 180 GeV) and on the reconstructed kaon mass. 1.4 Neutral events selection The neutral trigger is initiated by a scintillator hodoscope between the two halves of the LAC. We also require no veto signal, less than 5 energy clusters in each view of the calorimeter (to reject 7r°7r°7r0 ), a total energy above threshold and less than 2 spacepoints in the first wire chamber. An online filter is also applied, requiring cuts on the total energy (roughly calibrated), center of gravity and decay vertex, computed assuming the kaon mass. Offline reconstruction looks for photon showers. We require 4 photons, well sep- arated (more than 5 cm), with an energy between 3 and 100 GeV. The distance between the center of gravity of the energy and the beam axis must be less than 10 cm (to cut on missing transverse energy). The total energy must be between 60 and 180 GeV, and no hit found in the first wire chamber. Assuming the kaon invariant mass we compute the vertex decay position. The photon pairs invariant masses are then computed from this vertex decay position. 2 Data analysis 2.1 Overview of the method The dataset is sampled in time periods of about 3 days, including K^ a.nd Kg beam running time. Each of these are divided in energy and vertex bins (12 bins from 60 to 180 GeV and 39 bins of 1.2 m). The calorimeter response is absolutely calibrated by fitting the position of the K3 anticounter from decay vertex position distributions (thus imposing the same energy scale for 7r°7r° and TT+7r~ decays). The double ratio Tl if computed in each bin, thus reducing the acceptance differ- ences (between K^ and K% ) and cancelling the spectra difference (between n+n~ and 7r"7r° ). The subtraction of the remaining backgrounds, presented below, is also per- formed in each bin. Finally, the (small) residual diferences between the two beams are corrected by Monte Carlo, and the effect of accidentals evaluated. 2.2 Charged background in K°s A —> p7r~ is a possible background for Kg —* ir+n~ when the proton is assumed to be a pion. Under this hypothesis, the decay is reconstructed in a very asymétrie way. We cut on the ratio of the two tracks energies (between .4 and 2.50). The remaining A background is estimated to be negligible, less than .01 % . 2.3 Charged background in K\ The main backgrounds in K^ —> Tr+Tr" come from the CP conserving modes { + - 0 TTfii/ , TT TTTr ) whose branching ratios are by far dominant.
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