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Recent results on kaon rare decays from the NA48/2 experiment

Recent results on kaon rare decays from the NA48/2 experiment. High Energy Physics in the LHC Era December 11-15 2006, Valparaiso – Chile. Massimiliano Fiorini (Università degli Studi di Ferrara – INFN Ferrara). on behalf of the NA48/2 Collaboration :

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Recent results on kaon rare decays from the NA48/2 experiment

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  1. Recent results on kaon rare decays from the NA48/2 experiment High Energy Physics in the LHC Era December 11-15 2006, Valparaiso – Chile Massimiliano Fiorini (Università degli Studi di Ferrara – INFN Ferrara) on behalf of the NA48/2 Collaboration: Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna

  2. Outline • The NA48/2 experiment at CERN SPS • The K± π±π0γ decay (Kπ2γ) • Matrix element contributions • Experimental status • NA48/2 preliminary measurement of Direct Emission and Interference Terms • The K± π0 π0e±ν decay (Ke400) • Introduction and events selection • NA48/2 preliminary measurement of Branching Ratio and Form Factors • Conclusions

  3. Width ~ 5 mm K+/K-~ 1.8 K+ 54 60 66 PK spectra, 603GeV/c BM K− p/p ~ 1 % x,y ~ 100 m The NA48/2 simultaneous beams SPS protons @ 400 GeV/c Simultaneus, unseparated, focused beams

  4. NA48/2 detector • Spectrometer: • LKR calorimeter • Hodo, AKL • MUV, HAC • Kabes

  5. NA48/2 data taking periods • 2003 run: ~ 50 days • 2004 run: ~ 60 days • Total statistics in 2 years: • K  π+π-π±: > 3·109 • K  π0π0π± : > 1·108 • Rare K± decays: BR’s down to 10–9can be measured • >200 TB of data recorded A view of the NA48/2 beam line Primary goal: Search for CP-violating charge asymmetries in K±  3decays (see Dmitri Madigozhin talk on Friday Plenary Session)

  6. K±→π±π0γdecay

  7. IB DE DE IB IB INT DE Gamma production mechanism Γ± depends on 2 variables (W e T*π), reduced to only W by integration With this parametrization the ratio data/MC(IB) has the form 1+αW2+βW4 P*K = 4-momentum of the K±P*π= 4-momentum of the π±P*γ= 4-momentum of the radiative γ Lorentz invariant variable:

  8. W distributions for IB,DE,INT IB and DE are well separated in W Inner Bremsstrahlung(IB) : (2.75±0.15)×10-4 PDG 2006 (55<T*π<90 MeV)Direct Emission (DE) : (4.4±0.8)×10-6 PDG 2006(55<T*π<90 MeV)Interference (INT): not yet measured

  9. Kπ2γ amplitudes • Two amplitudes: • Inner Bremsstrahlung (IB) • Calculable: QED corrections to K± π±π0 • Suppressed by ΔI=1/2 • Direct Emission (DE) • Insight on weak vertex structure • Electric contribution (E) comes from L4 ChPT lagrangian and loops L2 (non predictable) • Magnetic contribution (M) has contributions from reducible chiral anomaly (WZW calculable) and also direct contributions (non predictable) • Interference (INT) possible between IB and electric part of DE • Measuring at the same time DE and INT gives measurement of both M and E • In addition CPV could appear in INT • Present experimental results seem to suggest a M dominated DE

  10. BNL E787 KEK E470 Experimental measurements All the measurements have been performed: • in the T*π region 55-90 MeV to avoid π±π0π0 and π±π0 BG • assuming INT term = 0 History of DE Branching ratio Interference measurements: NA48/2 has analyzed > 5 times more events

  11. What’s new in NA48/2 measurement • In-flight Kaon decays • Both K+ and K- in the beam (possibility to check CP violation) • Very high statistics (220K π±π0γ candidates of which 124K used in the fit ) • Enlarged T*π region in the low energy part (0 < T*π< 80 MeV) • Negligible background contribution < 1% of the DE component • Good W resolution mainly in the high statistic region • More bins in the fit to enhance sensitivity to INT • Order ‰ γ mistagging probability for IB, DE and INT • Fit with free interference term

  12. Enlarged T*p region T*π(IB) T*π(INT) T*π(DE) • Using standard region 55 < T*π< 90 MeV is a safe choice for background rejection (π±π0π0 and π±π0) • But the region below 55 MeV is the most interesting for DE and INT measurements • The presented measurement is performed in the region 0 < T*π< 80 MeV to improve statistics and sensitivity to DE Standard region Standard region Standard region

  13. 200K events K±→π±π0γselection • Track Selection • # tracks = 1. • Pπ+ > 10 GeV • E/p < 0.85 • No muon hits • 0 MeV < T*π+ < 80 MeV • Gamma selection • Nγ= 3. (well separated in time LKr clusters) • Minimum γ energy > 3 GeV (>5 for the fit) • Gamma tagging optimization • CHA and NEU vertex compatibility • Only one compatible NEU vertex • BG rejection cuts • COG < 2 cm • Overlapping γ cuts • |MK-MKPDG| < 10 MeV

  14. LKr γ2 γ3 ZV(NEU) ZV(CHA) γ1 ZV(2-3) ZV(1-3) Reconstruction strategy We can get two independent determination of the K decay vertex: - The charged vertex ZV(CHA) using the K and π flight directions (DCH) - The neutral vertex ZV(NEU)imposing π0 mass to gamma couples (LKr) For the neutral vertex we have 3 values: ZV(12), ZV(23), ZV(13) We evaluate the kaon mass for all of them and then choose the vertex giving the best kaon mass. Once the neutral vertex has been chosen we also know what the radiated γ is.

  15. Main BG sources • Physical BG rejection: • For π±π0 we can rely on the cut in T*π< 80 MeV, MK and COG cuts • For π±π0π0 we have released the T*π cut, but we can anyway reach required rejection with kaon mass cut (missing γ) and overlapping γ cut (fused γ) • Accidental BG rejection: mainly π±π0, Ke3(π0e±ν) and Kμ3(π0μ±ν) • Clean beam, very good time, space, and mass resolutions.

  16. Fused g rejection:overlapping g cut Fused gamma events are very dangerous BG source: - MK and COG cuts automatically satisfied! - Releasing the T*π cut they can give sizable contribution NA48 LKr is very good to reject them: - very high granularity (2×2 cm) cells - very good resolution in Z vertex Zv(π01) Multi step algorithm looped over clusters: Zv(π02) Split 1 out of the 3 clusters in two γ of energies:εγ1=xECLεγ2=(1-x)ECLnow we got 4 γ’s and we start reconstructing the event as a π± π0 π0. Evaluate the ZV(x) pairing the gammas and extract x imposing that:Zv(π01)= Zv(π02)same K decay vertex: the 2π0 came from the same K! Put x back in the Zv(π02) to get the real ZV(neu)If the |Zv(CHA)-ZV(neu)| < 400 cm the γ are really fused and so we discard the event

  17. BG rejection performance • All physical background can be explained in terms of the π±π0π0 events only. • Very small contribution from accidentals K± π±π0γ K± π±π0π0 Selected region 220Kevents total background < 1% DE component

  18. ■DE▼IB The mistagged g events: a self BG • Mistagging problem: • mistagged gamma events behave like BG because they can induce fake shapes in the W distribution • they tend to populate the region of high W simulating DE events • must keep mistagging probability as small as possible • Simply demanding compatibility between zvc and best zvn gives 2.5% mistagging probability • Solution: Reject events with a second solution for neutral vertex close to best one • Require |zvn (second)-zvn (best)| Dzvn> xx cm The mistagging probability has been evaluated in MC as a function of the mistagging cut to be 1.2‰at 400 cm

  19. The K± π±π0  decay. Trigger L1 requires nx>2 or ny>2 • L1 trigger • Require one track and LKr information (peaks) compatible with at least 3 clusters • This introduces an energy dependence  distortion of W distribution • Correction found using all 3  events (K± π± π0π0with  lost) and applied to Monte Carlo • L2 trigger(rejects K± π±π0) • Using DCH information and assuming 60 GeV kaon along z axis on-line processors compute a sort of missing mass of the K-π system • Cut events whose missing mass is compatible with π0. Equivalent to T*π < 90 MeV cut • To avoid edge resolution effects require T*π < 80 MeV in analysis

  20. W(Data)/W(IBMC) IB dominatedregion Fitting region Data MC comparison The radiated γ energy (for the IB part of the spectrum) is very well reproduced The IB dominated part of the data W spectrum is very well reproduced by MC Data MC for Eγ (W<0.5) Eγmin cut

  21. Fitting algorithm To get the fractions of IB(α), DE(β), INT(γ), from data we use an extended maximum likelihood approach: The fit has been performed in 14 bins in W, between 0.2-0.9, with a minimum γ energy of 5 GeV, using a data sample of124K events. To get the fractions of DE and INT the raw parameter are corrected for different acceptances

  22. Systematic uncertainties Many systematic checks have been performed using both data and MC • Trigger efficiency dominates • In 2004 both L1 and L2 triggers have been modified in order to reduce systematics

  23. Fit results NA48/2 Preliminary First measurement in the region 0 < T*π< 80 MeV of the DE term with free INT term. First evidence of a non zero INT term • The error on the results is still dominated by statistics and we could profit of 2004 data (and the remaining fraction of 2003 data) to reduce statistical uncertainties. • High correlation (ρ=-0.92)

  24. INT=0 fit: just for comparison For comparison with other experiments, we have also extracted the fraction of DE, with the INT term fixed to 0 in the region 55-90 MeV. A likelihood fit using IB and DE MC only has been performed in the region 0 < T*π< 80 MeV. Extrapolating to the 55-90 MeV region using MC we get: The analysis of fit residuals shows a bad χ2 Description in term of IB+DE only is unable to reproduce W data spectrum.

  25. K±→ π0π0e±νdecay

  26. Introduction to Ke400 • Ke4 decays • form factors are good constraints on ChPT Lagrangian parameters • clean sources of π-π pairs at low energy (extraction of scattering lengths) • theoretically related to each other by isospin arguments • Ke400 is the simplest because decay kinematics is described by only one form factor (2 identical π0’s in final state  no P wave) • Best measurement so far by KEK-E470 (stopped kaon decays) based on 216 events • Measured branching ratio (2.29  0.33) × 10-5

  27. Signal selection • Ke400 Signal Topology: • 1 charged track • 2 π0’s (reconstructed from 4γ’s in LKr) • 1 electron • some missing energy and pT (neutrino) • Analysis cuts: • ≥ 4 good γ clusters in LKr • 2 good π0’s with similar vertex positions • Cut on E/p > 0.95 and on shower width for e/π separation • Assume 60 GeV/c Kaon momentum traveling on z axis for neutrino reconstruction • Elliptical cut in plane mK (hypothetical π±π0π0 mass) and pT in order to eliminate K± π±π0π0 decays

  28. Background estimate • Background: main sources • ππ0π0 decay + π misidentified as e (dominant) • π0eνγ (Ke3γ) decay + accidental γ • In total 9642 selected events (2003 data only) with the following BG: • 260 ± 94 from K±→π±π0π0 • 16 ± 2 accidental BG form Ke3(γ) Total contamination: 3%

  29. Branching Ratio Measurement Using only a fraction of 2003 data (9642 signal events with 276  94 background events) and ππ0π0 as normalization channel, the Branching Ratio of the Ke400 decay has been measured: NA48/2 Preliminary The result has been crosschecked using also Ke3 as normalization channel leading to consistent result. The statistical error can be further reduced using the 2004 data set (28K events)

  30. The Ke400 form factors In this case we have only 1 form factor F (2 identical p0): The fit has been performed using both 2003 and 2004data (~38K events) NA48/2 Preliminary No sensitivity to fe reached → fe set to 0.Under this assumption we get: Those values are consistent with the ones measured by NA48/2 in “charged” Ke4  see detailed presentation by Dmitri Madigozhin

  31. Conclusions • NA48/2 has performed the first measurement of the DE and INT terms in the region 0 < T*π< 80 MeV for the decay K±→π±π0γ: • The results seem to indicate the presence of a negative, non vanishing, interference and therefore a non negligible contribution of E terms to the DE • An improved measurement of the Ke400 BR has been achieved, to be compared with recent published value: • In addition Form Factors are measured, consistent with the charged Ke4 measurement • The results can be further improved using the remaining fraction of 2003 data and the full 2004 statistics NA48/2 Preliminary NA48/2 Preliminary

  32. SPARE SLIDES

  33. MBX1TR-P LVL2 trigger Aim to reject K±→π±π0events and get K±→π±π0π0.It’s based on the online computation of Mfake: Only events with MFAKE < 475 MeV, are collected by the trigger

  34. Ke4 neutral decays : formalism Branching fraction and Form factors measurements: Two identical π0 (no P wave)  only ONE form factor Fs Sπ/4m2 π0 Se/4m2π0 background

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