NA48/2 studies of rare decays

The first observation of about 2000 candidates, with a background contamination below 3%, of the rare decay K± → π±π0e+e−is reported by the NA48/2 experiment. The preliminary branching ratio in the full kinematic region is obtained to be: B(K± → π±π0e+e−) = (4.06 ± 0.17) · 10−6 by analyzing the data collected in 2003. A sample of 4.687 × 10 K± → π±π0 D, decay candidates with a negligible background contamination collected in 2003–04 is analyzed to search for the dark photon (A′) via the decay chain K± → π±π0, π → γA′, A′ → e+e−. No signal is observed, and preliminary limits in the plane dark photon mixing parameter e versus its mass mA′ are reported. 1 The NA48/2 experiment The NA48/2 experiment at the CERN SPS collected a large sample of charged kaon (K±) decays during its 2003-04 data taking period. The NA48/2 beam line has been designed to deliver simultaneous narrow momentum band K+ and K− beams originating from the collision of the primary 400 GeV/c protons extracted from the CERN SPS on a beryllium target. Secondary beams with central momenta of (60± 3) GeV/c (r.m.s.) following a common beam axis were used. The beam kaons decayed in a fiducial decay volume contained in a 114 m long cylindrical vacuum tank. The momenta of charged decay products were measured in a magnetic spectrometer, housed in a tank filled with helium placed after the decay volume. The spectrometer comprised four drift chambers (DCHs) and a dipole magnet. A plastic scintillator hodoscope (CHOD) producing fast trigger signals and providing precise time measurements of charged particles was placed after the spectrometer. Further downstream was a liquid krypton electromagnetic calorimeter (LKr), an almost homogeneous ionization chamber with an active volume of 7 m3 of liquid krypton, 27X0 deep, segmented transversally into 13248 projective ∼2×2 cm2 cells and with no longitudinal segmentation. An iron/scintillator hadronic calorimeter and muon detectors were located further downstream. A dedicated two-level trigger was used to collect three track decays with a very high efficiency. A detailed description of the detector can be found in [1]. 2 First observation of K± → π±π0e+e−decay The K± → π±π0e+e−decay proceeds through virtual photon exchange which undergoes internal conversion into electron-positron pair, i.e. K± → π±π0γ∗→ π±π0e+e−. The γ∗ is produced ∗for the NA48/2 Collaboration †corresponding author: mauro.raggi@lnf.infn.it 1 ar X iv :1 50 8. 01 30 7v 1 [ he pex ] 6 A ug 2 01 5 by two different mechanisms: Inner Bremsstrahlung (IB), where the γ∗ is emitted by one of the charged mesons in the initial or final state and Direct Emission (DE) when γ∗ is radiated off at the weak vertex of the intermediate state. As a consequence the differential decay width consists of three terms: the dominant long-distance IB contribution (pure electric part E), the DE component (electric E and magnetic M parts) and the interference between them[2]. The interference term collects the different contributions, IBE, IBM and EM. For this reason the π±π0e+e−decay offers interesting short and long distance parity violating observables. In the K± → π±π0γ mode the interference consists only of the IBE term[3], because the remaining (EM) interferences are Pviolating, but cancel out upon angular integration. There are few theoretical publications related to the K± → π±π0e+e−[2][4][5]. Recently authors of [2] where able to predict, on the basis of the NA48/2 measurement of the magnetic and electric terms in K± → π±π0γ [6], the branching ratio of the single components. No experimental observation has so far been reported. 2.1 Selection and background estimates K± → π±π0e+e−event candidates are reconstructed from three charged tracks and two photons, forming neutral pion, pointing to a common vertex in the fiducial decay volume. Particle identification is based on the energy deposition in LKr (E) associated or not to a charged track momentum (p) measured in the spectrometer. The charged track is identified as electron/positron if its E/p ratio is greater than 0.85, and as a charged pion if the E/p ratio is lower than 0.85. Two isolated energy clusters without associated track in the LKr are identified as the two candidates photons from the π0 decay. Their invariant mass is required to be within ±10 MeV/c2 from the nominal PDG[7] π0 mass. The reconstructed invariant mass of the π±π0e+e−system is required to be within ±10 MeV/c2 from the nominal PDG[7] K± mass. Two main sources of background are contribution to the signal final state: K± → π±π0π0 D (K3πD) when one of the photon is lost, and K± → π±π0 D(γ) (K2πD), where π 0 D denotes the π 0 Dalitz decay π0 → e+e−γ. The suppression of the K3πD background events is obtained by requiring the squared invariant mass of the π+π0 system to be greater than 120 MeV2/c4, exploiting the presence of three particles with almost the same mass in the final state. In order to reject K2πD background contamination both the invariant masses Meeγ1,2 are required to be more than 7 MeV/c2 away from the nominal mass of the neutral pion. Analyzing the 2003 data, a sample of 1916 signal candidates has been selected with a background contamination below 3%. In particular MC simulation predicts a contribution of (26±5.1) candidates form K2πD and (30±5.5)from K3πD events. The normalization mode (K2πD) is recorded concurrently with the signal mode , using the same trigger logic. A common event reconstruction is considered as much as possible aiming to cancel of systematic effects such as particle identification and trigger inefficiencies. The selection of the normalization mode K2πD uses the same set of requirements as the signal selection except for the π0-reconstruction and background suppression parts. The neutral pion is reconstructed by requiring only one γ-candidate cluster and computing its invariant mass with the electron and positron pair. The only background source for the normalization channel is the Kμ3D mode (K ± → μ+νπ0 D). In the whole 2003 data sample 6.715 million K2πD candidates are selected with a background contamination smaller than 0.1%. 2.2 Branching ratio measurement The total Branching Ratio of K± → π±π0e+e−is obtained using the expression: B(K± → π±π0e+e−) = NS −NB NN AN N AS S B(N) (1)