1. Recent Results From NA48/2 Experiment @ CERN-SPS Simone Bifani University of Turin – Experimental Physics Department INFN – Turin on Behalf of the NA48/2 Collaboration Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna VI Latin American Symposium on High Energy Physics Puerto Vallarta (Mexico), November 1 st -8 th 2006

2. VI SILAFAE Simone Bifani 2 Outline ›NA48/2 Experimental Setup ›CP Violating Charge Asymmetry: »“Charged” Mode: K ± -> π ± π + π - »“Neutral” Mode: K ± -> π ± π 0 π 0 ›“Cusp” Effect in K ± -> π ± π 0 π 0 Decay ›Rare Decay: K ± -> π ± π 0 γ

3. NA48/2 Experimental Setup

4. VI SILAFAE Simone Bifani 4 Some History NA48 (1997-2000): Direct CP-Violation in neutral K >Re(ε’/ε) = (14.7 ± 2.2)·10 -4 NA48/1 (2002): Rare K S decays >BR(K S -> π 0 e + e - ) = (5.8 +2.8 -2.3 ± 0.8)·10 -9 >BR(K S -> π 0 μ + μ - ) = (2.8 +1.5 -1.2 ± 0.2)·10 -9 NA48/2 (2003-2004): Direct CP-Violation in charged K …and many other results on kaon and hyperon decays

5. VI SILAFAE Simone Bifani 5 Simultaneous Beam Beams coincide within ~1mm all along 114m decay volume Simultaneous K + and K - beams: large charge symmetrization of experimental conditions 2-3M K/spill (  /K ~ 10)  decay products stay in pipe Flux ratio: K + /K – ~ 1.8 P K = 60±3 GeV/c 54 60 66 Width ~ 5 mm K + /K - ~ 1 mm Second achromat: Cleaning Beam spectrometer δP K /P K = 0.7% δ x,y ~ 100 μm ~7  10 11 p/spil 400 GeV/c Front-end achromat: Momentum selection Quadrupole, Quadruplet: Focusing  sweeping

6. VI SILAFAE Simone Bifani 6 Detector Beam pipe Magnetic spectrometer (4 DCHs): >4 view / DCH -> high efficiency >σ P /P = 1.0% + 0.044%·P [GeV/c] Hodoscope: >Fast trigger >σ t = 150ps Electromagnetic calorimeter (LKr): >High granularity, quasi-homogeneous >σ E /E = 3.2%/√E + 9%/E + 0.42% [GeV] Hadron calorimeter, muon veto and photon vetoes Trigger: >Fast hardware trigger (L1): hodoscope & DCHs multiplicity >Level 2 trigger (L2): on-line processing of DCHs & LKr information

7. VI SILAFAE Simone Bifani 7 Data Taking A view of the NA48/2 beam line Run periods: >2003: ~ 50 days >2004: ~ 60 days Total statistics in 2 years: >K ± -> π ± π + π - : ~ 4·10 9 >K ± -> π ± π 0 π 0 : ~ 1·10 8 -> >200 TB of data recorded Rare K ± decays can be measured down to BR ~ 10 –9

8. CP Violating Charge Asymmetry

9. VI SILAFAE Simone Bifani 9 CP-Violation History Major milestones in CP-Violation history: >1964: Indirect CP-Violation in K 0 (J.H. Christenson, J.W. Cronin, V.L. Fitch and R. Turlay) >1988, 1999: Direct CP-Violation in K 0 (NA31, E731, NA48, KTeV) >2001: Indirect CP-Violation in B 0 (BaBar, Belle) >2004: Direct CP-Violation in B 0 (Belle, BaBar) Look for CP-Violation in K ± (no mixing -> only Direct CPV is possible) Look for CP-Violation in K ± (no mixing -> only Direct CPV is possible)

10. VI SILAFAE Simone Bifani 10 Introduction (I) Kinematics: s i = (P K - P πi ) 2, i = 1,2,3 (3 = π odd ) s 0 = (s 1 + s 2 + s 3 ) / 3 u = (s 3 - s 0 ) / m π 2 v = (s 2 - s 1 ) / m π 2 Kaon rest frame: u = 2m K ∙ (m K /3 - E odd ) / m π 2 v = 2m K ∙ (E 1 - E 2 ) / m π 2 Matrix element: |M(u,v)| 2 ~ 1 + gu + hu 2 + kv 2 Direct CP violating quantity: slope asymmetry A g = (g + - g - ) / (g + + g - ) ≠ 0 The best two K ± decay modes: ›BR(K ± -> π ± π + π - ) = 5.57% “Charged” ›BR(K ± -> π ± π 0 π 0 ) = 1.73% “Neutral” π 1even π 3odd π 2even K±K± “Charged” mode g = -0.2154 ± 0.0035 |h|, |k| ~ 10 -2 u v

11. VI SILAFAE Simone Bifani 11 Introduction (II) Standard Model theoretical prediction in the range 10 -6 ÷5·10 -5 Models Beyond the SM predict enhancement of the A g value Experimental results before NA48/2: “Charged” mode: A g = (22±15 stat ±37 syst )·10 -4 (HyperCP - 54·10 6 evt.) “Neutral” mode: A g = (2±19)·10 -4 (TNF - 620·10 3 evt.) 10 -5 10 -4 10 -3 10 -2 SMSM SUSYSUSY NewPhysicsNewPhysics Ford et al. (1970) “Charged” HyperCP prelim. (2000) “Charged” TNF (2005) “Neutral” NA48/2 proposal “Charged” “Neutral” Smith et al. (1975) “Neutral” 10 -6 |A g |

12. VI SILAFAE Simone Bifani 12 Introduction (III) What’s new in NA48/2 measurement? ›Simultaneous K + and K – beams, superimposed in space, with momentum spectra (60±3) GeV/c ›Equalize K + and K – acceptances by frequently alternating polarities of relevant magnets ›Detect asymmetry exclusively considering slopes of ratios of normalized u distributions

13. VI SILAFAE Simone Bifani 13 If K + and K – acceptances are equal for the same u and v value, any difference between the experimental distributions would be a sign of Direct CP-Violation. Integrated over v, A g can be extracted from a fit to the ratio R(u) using the PDG value for g: ›The normalization is a free parameter in the fit and Δg does not depend on it ›For the “charged” mode a fit with linear function is suitable due to smallness of the slope g ›u calculation: »“Charged” mode: only the magnetic spectrometer is used »“Neutral” mode: only the calorimeter is used Measurement Strategy Δg = g + - g - << 1 R(u) == n~ n 1 + N + (u) N - (u) 1 + g + ·u + h·u 2 +… 1 + g - ·u + h·u 2 +… Δg·u 1 + g∙u + h∙u 2 -> A g = Δg/2g

14. VI SILAFAE Simone Bifani 14 Acceptance (I) Magnetic fields present in both beam line and spectrometer: this leads to residual charge asymmetry of the setup SuperSample (SS) data taking strategy: >Beam line polarity (A) reversed on weekly basis >Spectrometer magnet polarity (B) reversed on a more frequent basis (~daily in 2003, ~3 hours in 2004) The whole 2003+2004 data taking is subdivided in 9 SS in which all the field configurations are present Example: data taking from August 6 th to September 7 th, 2003 SuperSample 1 SuperSample 2 SuperSample 3 12 subsamples 4 subsamples Week 1 Week 2 Week 3 Week 4 Week 5 B+B– B+ B– B+ B– B+ B– A+ A– A+ A– A+ A–

15. VI SILAFAE Simone Bifani 15 Acceptance (II) >In each ratio the odd pions are deflected towards the same side of the detector (left-right asymmetry) >In each ratio the event at the numerator and denominator are collected in subsequent period of data taking (global time variations) N+(A-B+)N+(A-B+) RDS =RDS = N-(A-B-)N-(A-B-) N+(A+B-)N+(A+B-) N-(A+B+)N-(A+B+) RUJ =RUJ = N+(A+B+)N+(A+B+) N-(A+B-)N-(A+B-) R US = N+(A-B-)N+(A-B-) N-(A-B+)N-(A-B+) R DJ = R indices: ›U/D: beam line polarity ›S/J: π odd direction after the spectrometer magnet field z x y Up Down B+B+ B-B- K+K-K+K- A-A- A+A+ Saleve Jura π - π +

16. VI SILAFAE Simone Bifani 16 ›Double ratio: cancellation of global time instabilities (rate effects, analyzing magnet polarity inversion) ›Double ratio: cancellation of local beam line biases effects (slight differences in beam shapes and momentum spectra) ›Quadruple ratio: both previous cancellations + left-right detector asymmetry cancellation R S = R US × R DS R J = R UJ × R DJ f 2 (u) = n 2 ∙(1+Δg S  u) 2 f 2 (u) = n 2 ∙(1+Δg J  u) 2 R = R US × R UJ × R DS × R DJ f 4 (u) = n 4 ∙(1+ Δg  u) 4 The method is independent of K + /K – flux ratio and relative sizes of the samples (important: simultaneous beams) R U = R US × R UJ R D = R DS × R DJ f 2 (u) = n 2 ∙(1+Δg U  u) 2 f 2 (u) = n 2 ∙(1+Δg D  u) 2 Acceptance (III)

17. “Charged” Mode: K ± -> π ± π + π -

18. VI SILAFAE Simone Bifani 18 Event Selection (I) Main requirements (simplicity, charge symmetry): >Identification of the best 3-track vertex >z vertex > -18 m (downstream the last collimator) >Track times: |t i – t j | probability of event pile-up ~ 10 –4 >P t < 0.3 GeV/c (suppression of background decays) >|m 3π - m K | < 9 MeV/c 2 (5 times the resolution) K e4 background MC K 3π MC K 3π + π -> μν decay MC K -> 3πγ Data m 3π [GeV/c 2 ] Time difference for tracks pairs Events t i -t j [ns] z vertex [m] z-coordinate of the decay vertex Events

19. VI SILAFAE Simone Bifani 19 In the 2003+2004 data sample 3.11·10 9 K ± have been selected: Event Selection (II) π -> μν σ m = 1.7 MeV/c 2 Events K + : 2.00·10 9 events m 3π [GeV/c 2 ] K - : 1.11·10 9 events π -> μν Events m 3π [GeV/c 2 ] odd pion in beam pipe even pion in beam pipe

20. VI SILAFAE Simone Bifani 20 Monte-Carlo Simulation Example of Data/MC agreement: mean beam positions @ DCH1 K+K+ KK Run Number Due to acceptance cancellations, the analysis does not rely on Monte-Carlo to calculate acceptance. Still Monte-Carlo is used to study systematic effects. NA48/2 MC properties: ›Based on GEANT ›Full detector geometry and material description ›Local DCH inefficiencies ›Variations of beam geometry and DCH alignment Simulated statistics similar to experimental one

21. VI SILAFAE Simone Bifani 21 SS0 SS1 SS2 SS3 SS4 SS5 SS6 SS7 SS8 Δg Fit In SuperSamples

22. VI SILAFAE Simone Bifani 22 RunSuperSample Δg·10 4 Χ 2 of the R 4 (u) fit 2003 0-0.8±1.830/26 1-0.5±1.824/26 2-1.4±2.028/26 31.0±3.319/26 2004 4-2.0±2.218/26 54.4±2.620/26 65.0±2.226/26 71.5±2.110/26 80.4±2.323/26 Combined0.6±0.7 Results In SuperSamples

23. VI SILAFAE Simone Bifani 23 Systematics Systematic effectEffect on Δg·10 4 Spectrometer alignment±0.1 Momentum scale±0.1 Acceptance and beam geometry±0.2 Pion decay±0.4 Accidental activity (pile-up)±0.2 Resolution effects±0.3 Total systematic uncertainty±0.6 L1 trigger: uncertainty only±0.3 L2 trigger: correction-0.1±0.3 Total trigger correction-0.1±0.4 Systematic & trigger uncertainty±0.7 Raw Δg0.7±0.7 Δg corrected for L2 inefficiency0.6±0.7

24. VI SILAFAE Simone Bifani 24 Results ›A factor ~20 better precision than the previous measurements ›Uncertainties dominated by those of statistical nature ›Design goal reached. There is still some room to improve the systematic uncertainty ›Result compatible with the Standard Model predictions Based on the full 2003+2004 data sample Ford et al. (1970) HyperCP (2000) Preliminary 2003 Final 2003 Current 2003+2004 NA48/2 (results superseding each other) A g  10 4 Measurements of A g Final 2003 result published: PLB634 (2006) 474-482 Δg = (0.6 ± 0.7 stat ± 0.4 trig ± 0.6 syst )·10 -4 Δg = (0.6 ± 1.0)·10 -4 A g = (-1.3 ± 1.5 stat ± 0.9 trig ± 1.4 syst )·10 -4 A g = (-1.3 ± 2.3)·10 -4 Δg = (0.6 ± 0.7 stat ± 0.4 trig ± 0.6 syst )·10 -4 Δg = (0.6 ± 1.0)·10 -4 A g = (-1.3 ± 1.5 stat ± 0.9 trig ± 1.4 syst )·10 -4 A g = (-1.3 ± 2.3)·10 -4 Preliminary

25. “Neutral” Mode: K ± -> π ± π 0 π 0

26. VI SILAFAE Simone Bifani 26 Introduction M 00 2 - s 0 mπ2mπ2 u = s 0 = (s 1 + s 2 + s 3 ) / 3 u |v| odd pion in beam pipe Statistical precision in A g similar to “charged” mode: ›Ratio of “neutral” to “charged” statistics: N 0 /N ± ~ 1/30 (91·10 6 K ± have been selected in the 2003+2004 data sample) ›Ratio of slopes: |g 0 /g ± | ~ 1/3 ›More favourable Dalitz-plot distribution (gain factor ~1.5) For u calculation only the energy of the two neutral pions in laboratory frame is used (only calorimeter information) σ m =0.9 MeV/c 2 Events m 3π [GeV/c 2 ] π -> μν

27. VI SILAFAE Simone Bifani 27 Results In SuperSamples RunSuperSample Δg·10 4 2003 04.3±3.8 1+20.5±5.0 3-2.0±8.2 2004 55.6±6.8 64.7±5.1 73.5±5.6 8-1.4±5.8 Combined2.7±2.0

28. VI SILAFAE Simone Bifani 28 Results Based on the full 2003+2004 data sample Final 2003 result published: PLB638 (2006) 22-29 Δg = (2.7 ± 2.0 stat ± 1.2 syst ± 0.3 ext )·10 -4 Δg = (2.7 ± 2.4)·10 -4 A g = (2.1 ± 1.6 stat ± 1.0 syst ± 0.2 ext )·10 -4 A g = (2.1 ± 1.9)·10 -4 Δg = (2.7 ± 2.0 stat ± 1.2 syst ± 0.3 ext )·10 -4 Δg = (2.7 ± 2.4)·10 -4 A g = (2.1 ± 1.6 stat ± 1.0 syst ± 0.2 ext )·10 -4 A g = (2.1 ± 1.9)·10 -4 ›A factor ~10 better precision than the previous measurements ›The errors are dominated by statistics ›Design goal reached. Further improvements of the analysis are possible ›Result compatible with the Standard Model predictions A g  10 4 Measurements of A g Preliminary Final 2003 NA48/2 (results superseding each other) Current 2003+2004Smith et al. (1975) TNF (2005)

29. “Cusp” Effect in K ± -> π ± π 0 π 0 Decay

30. VI SILAFAE Simone Bifani 30 A “Cusp” ›From K ± -> π ± π 0 π 0 decay we observed an anomaly in the M 00 2 invariant mass distribution in the region around M 00 2 = (2m π+ ) 2 = 0.07792 GeV 2 ›This anomaly has been interpreted as a final state charge exchange scattering process of K ± -> π ± π + π - (π + π - -> π 0 π 0 ) ›The parameter a 0 -a 2 (difference between the S-wave ππ scattering lengths in the isospin I=0 and I=2 states) can be precisely measured using this sudden anomaly (“cusp”)

31. VI SILAFAE Simone Bifani 31 Reconstruction: ›At least 4 clusters 15 cm away from any track and 10 cm away from other clusters ›Select γ pairs with smallest distance between vertices ›M 00 2 computed using average vertex of two π 0 Standard Dalitz plot parameterization shows deficit in data before “cusp”: Event Selection cusp Whole region: c 2 /ndf=9225/149! Above cusp: c 2 /ndf=133/110 Standard parametrization M 00 2 [(GeV/c 2 ) 2 ] Events / 0.00015 [(GeV/c 2 ) 2 ]

32. VI SILAFAE Simone Bifani 32 Instrumental Effects (I) Good resolution and linear acceptance near the “cusp” region: σ ~ 0.5 MeV/c 2 @ M 00 = 2m π+ cusp Resolution (MC) cusp Acceptance (MC) M 00 2 [(GeV/c 2 ) 2 ]

33. VI SILAFAE Simone Bifani 33 Instrumental Effects (II) Data-MC comparisons above and below “cusp”: Event deficit is a real effect a/b ratios: Data (dot) vs. MC (full) Data distributions across the “cusp” agree with MC predictions without “cusp” I+/I-I+/I- E γ [GeV] min r γ [cm]max r γ [cm] min d γγ [cm]min d γ-track [cm]

34. VI SILAFAE Simone Bifani 34 Re-scattering model: two amplitudes contribute to K ± -> π ± π 0 π 0 ›M 0 : Direct emission ›M 1 : Charge exchange in final state of K ± -> π ± π + π - (π + π - -> π 0 π 0 ) The singularity in the invariant mass spectrum at π + π - threshold is mainly caused by the destructive interference of M 0 and M 1 The effect is present below the threshold and not above it (re-scattering model at one-loop (N. Cabibbo: PRL 93 (2004) 121801) ) Interpretation (I) M(K ± -> π ± π 0 π 0 ) = M 0 + M 1 CEDE

35. VI SILAFAE Simone Bifani 35 Interpretation (II) ›More complete formulation of the model including all re-scattering processes at one-loop and two-loop level (N. Cabibbo and G. Isidori: JHEP 0503 (2005) 21) has been used to extract NA48/2 results ›Experimental work in progress on an effective field theory model (CGKR: hep-ph/0604084) valid in whole decay region

36. VI SILAFAE Simone Bifani 36 Try fitting different theoretical models to M 00 2 distribution and evaluate: ›Fitting up to 0.097 (GeV/c 2 ) ›5 fitting parameters: norm, g 0, h’, a 0 -a 2 and a 2 ›For final results pionium set to theoretical expectation and 7 bins around the “cusp” excluded from the fit in order to reduce sensitivity to Coulomb corrections Results (I) One loop Х 2 /ndf=420/148 One+two loop Х 2 /ndf=155/146 Pionium Х 2 /ndf=149/145 Exclude 7 bins around cusp Х 2 /ndf=145/139 M 00 2 [(GeV/c 2 ) 2 ] Δ Δ Δ Δ

37. VI SILAFAE Simone Bifani 37 Systematic effects: acceptance determination, trigger efficiency and fitting interval Predictions in ChPT (PLB 488 (2000) 261) : ›(a 0 -a 2 )·m π+ = 0.265 ± 0.004 ›a 2 = -0.0444 ± 0.0010 Fit imposing ChPT constraint between a 0 and a 2 (PRL 86 (2001) 5008) Results (II) g 0 = 0.645 ± 0.004 stat ± 0.009 syst h’ = -0.047 ± 0.012 stat ± 0.011 syst (a 0 -a 2 )·m π+ = 0.268 ± 0.010 stat ± 0.004 syst ± 0.013 ext a 2 = -0.041 ± 0.022 stat ± 0.014 syst g 0 = 0.645 ± 0.004 stat ± 0.009 syst h’ = -0.047 ± 0.012 stat ± 0.011 syst (a 0 -a 2 )·m π+ = 0.268 ± 0.010 stat ± 0.004 syst ± 0.013 ext a 2 = -0.041 ± 0.022 stat ± 0.014 syst a 0 = 0.220 ± 0.006 stat ± 0.004 syst ± 0.011 ext (a 0 -a 2 )·m π+ = 0.264 ± 0.006 stat ± 0.004 syst ± 0.013 ext a 0 = 0.220 ± 0.006 stat ± 0.004 syst ± 0.011 ext (a 0 -a 2 )·m π+ = 0.264 ± 0.006 stat ± 0.004 syst ± 0.013 ext Based on partial sample of 2003 data 2003 results published: PLB 633 (2006) 173-182

38. VI SILAFAE Simone Bifani 38 Results (III) A factor ~2 better precision than the previous measurement Measurements of (a 0 -a 2 )·m π+ (a 0 -a 2 )·m π+ a) b) c) d) a)NA48 resultPLB 633 (2006) b)DIRAC resultPRL 619 (2005) c)G.Colangelo et al.NPB 603 (2001) d)J.R.Pelaez et al.PRD 71 (2005)

39. VI SILAFAE Simone Bifani 39 Change of Dalitz variables, from (s 3, s 2 -s 1 ) to (s 3, cosθ). Define θ as angle between π ± and π 0 in π 0 π 0 COM: g 0 and h’ change No change in (a 0 -a 2 )·m π+ and a 2 Results (IV) k’ = 0.0097 ± 0.0003 stat ± 0.0008 syst Fitting new Dalitz plot above “cusp” finds evidence for k’ > 0 term Based on a partial sample of 2003 data Data/MC comparison for different k’ values cosθ Events Preliminary

40. Rare Decay: K ± -> π ± π 0 γ

41. VI SILAFAE Simone Bifani 41 Rare Decays Statistics usually at least one order of magnitude above previous experiments. Several channels not yet observed ›K ± -> π + π - e ± ν ›K ± -> π 0 π 0 e ± ν ›K ± -> π + π - μ ± ν ›K ± -> π ± π 0 γ ›K ± -> π ± γγ ›K ± -> π ± e + e - γ ›K ± -> π ± π 0 γγ ›K ± -> π ± e + e - K e4 Kμ4Kμ4

42. VI SILAFAE Simone Bifani 42 Two amplitudes: ›Inner Bremsstrahlung (IB) ›Direct Emission (DE) Two type of contributions: >Electric (j=l±1) dipole (E 1 ) >Magnetic (j=l) dipole (M 1 ) Electric contributions come from L 4 CHPT Lagrangian, loops L 2 and are dominated by the IB term Magnetic contributions are dominated by Chiral Anomaly DE shows up only at order O(p 4 ) in ChPT: is generated both by E and M contributions. Present experimental results seem to suggest a M dominated DE Interference (INT) is possible between IB and electric part of DE: ›Measuring at the same time DE and INT gives measurement of M and E ›CP-Violation could appear in INT Introduction (I) DEIB PDG (55 MeV < T* π < 90 MeV) IB: DE: INT: (2.75 ± 0.15)·10 -4 (4.4 ± 0.8)·10 -6 not yet measured

43. VI SILAFAE Simone Bifani 43 Introduction (II) IB from K ± -> π ± π 0 INT sensitive to electric dipole DE sensitive to electric & magnetic dipole W2W2 1W4W4 P * K = 4 momentum of the K ± P * π = 4 momentum of the π ± P * γ = 4 momentum of the radiative γ WW W

44. VI SILAFAE Simone Bifani 44 Introduction (III) Interference found to be compatible with 0: -> Set INT = 0 and fit only DE (all measurements have been performed in the T * π region 55÷90 MeV to avoid K ± -> π ± π 0 π 0 background) BNL E787 KEK E470 Recent history of BR(DE) 4.7 3.5 3.2 3.8 0.0 1.0 2.0 3.0 4.0 5.0 6.0 199920002001200220032004200520062007 year BR(DE) 10 -6 BNL E787 KEK E470

45. VI SILAFAE Simone Bifani 45 Introduction (IV) What’s new in NA48/2 measurement? >Simultaneous K + and K - beams -> check for CP-Violation >Enlarged T* π region in the low energy part (0÷80 MeV) >Negligible background contribution (<1% of the DE component) >γ miss-tagging probability ~ ‰ for IB, DE and INT

46. VI SILAFAE Simone Bifani 46 Event Selection (I) Event selection: >Select 1 track and any number of clusters >Require 3 γs with E γ > 3 GeV outside 35 cm radius from π @ LKr γs and 10 cm away from other clusters >Charged vertex (z c ): calculate the K decay point as the position where the π ± track intersects the beam line >Selecting the γ pairing for the π 0 : »Three combinations are possible (choosing the wrong combination for the π 0 -> choosing the wrong odd γ (miss- tagging) -> distorts W) »Two possible methods used: select the combination giving the best π 0 or K ± invariant mass >Neutral vertex (z n ): from imposing π 0 mass to γ pairs; must be in agreement with charged vertex (within 400 cm)

47. VI SILAFAE Simone Bifani 47 Event Selection (II) P miss-tagging < 1.2 ‰ @ Δz n cut = 400 cm P miss-tagging < 1.2 ‰ @ Δz n cut = 400 cm Miss-tagged events move to large W: this could induce a DE component if difference between Data-MC ›Demanding the charged vertex compatible with the best neutral vertex gives P miss-tagging ~ 2.5% ›Rejecting events with a second solution for neutral vertex close to the best one, |z n second - z n best | < Δz n cut, reduces the P miss-tagging

48. VI SILAFAE Simone Bifani 48 Background (I) DecayBRBackground mechanism K ± -> π ± π 0 (21.13 ± 0.14) %1 accidental γ or hadronic extra cluster K ± -> π ± π 0 π 0 (1.76 ± 0.04) %1 missing or 2 overlapped γs K ± -> π 0 e ± ν(4.87 ± 0.06) %1 accidental γ and e misidentified as a π K ± -> π 0 μ ± ν(3.27 ± 0.06) %1 accidental γ and μ misidentified as a π K ± -> π 0 e ± ν(γ) (2.66 ± 0.2)·10 −4 e misidentified as a π K ± -> π 0 μ ± ν(γ)(2.4 ± 0.85)·10 −5 μ misidentified as a π Backgrounds can be rejected using particle ID, COG, mass and time cuts

49. VI SILAFAE Simone Bifani 49 Background (II) After all cuts the background estimation is π ± π 0 π 0 ›For every of the three γs in the event assume that its energy E i is really the overlap of 2 γs of energies E’ = x·E i and E’’ = (1-x)·E i ›Solve for sharing fraction (x) imposing that the two π 0 must come from the same vertex ›Reject event if any of the reconstructed π 0 vertex is compatible with charged vertex (within 400 cm) In addition need to use MUV detector to avoid miss-reconstruction of track momentum due to π -> μν decay in flight Cut on overlapping γs (allows avoiding T* π > 55 MeV): K ± -> π ± π 0 π 0 K ± -> π ± π 0 γ m K [GeV/c 2 ]

50. VI SILAFAE Simone Bifani 50 Data/MC Comparison In the 2003 data sample (~30% of the whole statistics) 220·10 3 K ± have been selected: ›After trigger efficiency correction good agreement between Data and MC for E γ, in particular for E γ > 5 GeV (used for final result) ›The ratio W(Data)/W(MC IB ) is in good agreement for IB dominated region and clearly shows DE E γ [GeV] W(Data)/W(MC IB ) Fitting region IB dominated region W

51. VI SILAFAE Simone Bifani 51 Systematic effectEffect on DEEffect on INT Miss-tagging-±0.2 Energy scale+0.09-0.21 Resolutions difference< 0.05< 0.1 LKr non linearity< 0.05 BG contributions< 0.05 Fitting procedure0.020.19 L1 trigger±0.17±0.43 L2 trigger±0.17±0.52 Total±0.25±0.73 Systematic effects dominated by the trigger (both L1 and L2 have been modified in 2004) Systematic effects dominated by the trigger (both L1 and L2 have been modified in 2004) Systematic checks have been performed using both Data and MC Systematics

52. VI SILAFAE Simone Bifani 52 Results (I) Use extended Maximum Likelihood for 0.2 < W < 0.9 to fit in the region 0 MeV < T* π < 80 MeV (based on 124·10 3 events) -> First evidence of Interference between Inner Bremsstrahlung and Direct Emission amplitudes Frac(DE) = (3.35 ± 0.35 stat ± 0.25 syst ) % Frac(INT) = (-2.67 ± 0.81 stat ± 0.73 syst ) % Frac(DE) = (3.35 ± 0.35 stat ± 0.25 syst ) % Frac(INT) = (-2.67 ± 0.81 stat ± 0.73 syst ) % Based on a partial sample of 2003 data ρ = -0.92 Frac(DE) Frac(INT) Preliminary

53. VI SILAFAE Simone Bifani 53 Results (II) Frac(DE) = (0.85 ± 0.05 stat ± 0.02 syst ) % Setting INT = 0 for comparison, fitting between 0 MeV < T* π < 80 MeV and extrapolating to 55÷90 MeV Fraction of DE(INT=0) 0.00 0.50 1.00 1.50 2.00 2.50 199920002001200220032004200520062007 year % DE BNL E787 KEK E470 NA48/2 A description in term of IB and DE only is unable to reproduce the W data spectrum A description in term of IB and DE only is unable to reproduce the W data spectrum The analysis of fit residuals shows a bad Χ 2

54. VI SILAFAE Simone Bifani 54 Summary (I) ›The preliminary result on the Direct CP violating charge asymmetry in K ± -> π ± π + π - based on the 2003+2004 data sample (whole statistics) is: A g = (-1.3 ± 1.5 stat ± 0.9 trig ± 1.4 syst )·10 -4 = (-1.3 ± 2.3)·10 -4 ›The preliminary result for A g in K ± -> π ± π 0 π 0 based on the 2003+2004 data sample (whole statistics) is: A g = (2.1 ± 1.6 stat ± 1.0 syst ± 0.2 ext )·10 -4 = (2.1 ± 1.9)·10 -4 ›Both results have ~10 times better precision than the previous measurements ›The errors are dominated by statistics

55. VI SILAFAE Simone Bifani 55 Summary (II) ›A new “cusp” structure in K ± -> π ± π 0 π 0 was observed (ππ final state charge exchange process of K ± -> π ± π + π - ) which provides a new method for the extraction of the ππ scattering lengths: (a 0 -a 2 )·m π+ = 0.268 ± 0.010 stat ± 0.004 syst ± 0.013 theor ›The measurement is based on a 2003 data sample and agrees both with another independent measurement and with the theoretical predictions ›Parameter a 2 directly measured for the first time even though with low accuracy: a 2 = -0.041 ± 0.022 stat ± 0.014 syst

56. VI SILAFAE Simone Bifani 56 Summary (III) ›The first measurement of Direct Emission and Interference terms in K ± -> π ± π 0 γ based on a 2003 data sample (~30% of the whole statistics) has been performed in the region 0 MeV < T* π < 80 MeV: Frac(DE) = (3.35 ± 0.35 stat ± 0.25 syst ) % Frac(INT) = (-2.67 ± 0.81 stat ± 0.73 syst ) % ›A first evidence of a negative Interference has been found and therefore a non negligible contribution of electric term to Direct Emission amplitude

57. Spares

58. CP Violating Charge Asymmetry

59. VI SILAFAE Simone Bifani 59 Time variations of spectrometer geometry do not cancel in the result. Alignment is fine-tuned by scaling π ± momenta (charge-asymmetrically) to equalize the reconstructed average K +, K - masses Sensitivity to DCH4 horizontal shift: |ΔM/Δx| ~ 1.5 KeV/μm The imperfect inversion of spectrometer field cancels in double ratio. Momentum scale adjusted anyway by constraining average reconstructed 3π masses to the PDG value Sensitivity to 10 -3 error on field integral: ΔM ~ 100 KeV/c 2 Transverse alignment Magnetic field Systematics - Spectrometer Maximum equivalent transverse shift: ~200  m @DCH1 or ~120  m @DCH2 or ~280  m @DCH4 Much more stable alignment in 2004 Max. effect in 2004 Subsample ΔM3 π [KeV/c 2 ]

60. VI SILAFAE Simone Bifani 60 › Acceptance largely defined by central hole edge (R ~ 10cm) › Geometry variations, non-perfect superposition: asymmetric acceptance › Additional acceptance cut defined by a “virtual pipe” (R = 11.5cm) centered on averaged reconstructed beam position as a function of charge, time and K momentum -> statistics loss: 12% Beam widths: ~ 1 cm Beam movements: ~ 2 mm Sample beam profile at DCH1 0 0.4 0.8-0.4 -0.8 0 0.4 0.8 -0.4 -0.8 x [cm] Y [cm] “Virtual pipe” also corrects for the differences between the upper and lower beam paths Y [cm] x [cm] 2mm [Special treatment of permanent magnetic fields effect on measured beam positions] Systematics - Beam Geometry

61. VI SILAFAE Simone Bifani 61 Only charge-asymmetric trigger inefficiency dependent on u can bias the result Trigger efficiencies measured using control data samples triggered by downscaled low bias triggers Statistical errors due to limited sizes of the control samples are propagated into the result 0.4 0.2 0.6 0.8 1.0 1.2 1.4 1.6 Inefficiency x10 3 L2 inefficiency vs u (normal conditions) 1.0 1.50.50.0-0.5 -1.5 u 0.4 0.2 0.6 0.8 1.0 1.2 1.4 1.6 Inefficiency % L2 inefficiency vs time (2003) Beginning of 2003 run: L2 algorithm tuning Max. inefficiency in 2004 Systematics - Trigger L2 trigger time-varying inefficiency (local DCH inefficiencies, tuning) 1–ε = 0.06% ÷ 1.5% u-dependent correction applied L1 trigger small and stable inefficiency 1–ε ~ 0.9·10 -3 no correction

62. VI SILAFAE Simone Bifani 62 Field map in decay volume (y projection) Decay volume: z coordinate 0.4 0 -0.4 -0.8 -1.2 [Gauss] Residual effects of stray magnetic fields (magnetized vacuum tank, earth field) minimized by explicit field map correction Further systematic effects studied: ›Accuracy of beam tracking, variations of beam widths ›Bias due to resolution in u ›Sensitivity to fitting interval and method ›Coupling of π -> μν decays to other effects ›Effects due to event pile-up ›π + /π - interactions with the material Other Systematics 0.5MeV/c 2 No magnetic field correction Magnetic field corrected

63. VI SILAFAE Simone Bifani 63 Time-Stability & Control Quantities 2003 (results consistent) 2004R LR (u)=R S /R J R UD (u)=R U /R D Control of setup time-variable biasesControl of differences of the two beam paths Monte-Carlo (reproduces apparatus asymmetries) Physics asymmetry Control quantities canceling in the result quadruple ratio components rearranged (smallness demonstrates 2 nd order effects negligible)

64. “Cusp” Effect in K ± -> π ± π 0 π 0 Decay

65. VI SILAFAE Simone Bifani 65 a 0 (UB) = 0.256 ± 0.008 stat ± 0.007 syst ± 0.018 th -> a 2 = -0.031 ± 0.015 stat ± 0.015 syst ± 0.019 th a 0 (UB) = 0.256 ± 0.008 stat ± 0.007 syst ± 0.018 th -> a 2 = -0.031 ± 0.015 stat ± 0.015 syst ± 0.019 th Preliminary Results From K ± e4 g 0 = 0.645 ± 0.004 stat ± 0.009 syst h’ = -0.047 ± 0.012 stat ± 0.011 syst (a 0 -a 2 )·m π+ = 0.268 ± 0.010 stat ± 0.004 syst ± 0.013 ext a 2 = -0.041 ± 0.022 stat ± 0.014 syst g 0 = 0.645 ± 0.004 stat ± 0.009 syst h’ = -0.047 ± 0.012 stat ± 0.011 syst (a 0 -a 2 )·m π+ = 0.268 ± 0.010 stat ± 0.004 syst ± 0.013 ext a 2 = -0.041 ± 0.022 stat ± 0.014 syst Predictions in ChPT (PLB 488 (2000) 261) : ›a 0 = 0.220 ± 0.005 ›a 2 = -0.0444 ± 0.0010 ›(a 0 -a 2 )·m π+ = 0.265 ± 0.004 “Cusp”

66. Rare Decay: K ± -> π ± π 0 γ

67. VI SILAFAE Simone Bifani 67 L2 trigger: ›Using DCH information and assuming 60 GeV K along z axis on-line processors compute a sort of missing mass of the K-π system ›Cut events with T* π > 90 MeV (to keep away of edge resolution effects require T* π < 80 MeV in analysis) Trigger L1 trigger: ›Require 1 track and LKr information (peaks) compatible with at least 3 clusters ›This introduces an energy dependence -> distortion of W distribution ›Correction found using all 3γs events (K ± -> π ± π 0 π 0 with γ lost) and applied to MC L1 requires n x >2 or n y >2

68. VI SILAFAE Simone Bifani 68 Overlapping γs Overlapping γs would give right K mass For any of the 3 γs with energies E 1, E 2, E 3 do the following: ›Assume that the γ with energy E 1 is really the overlap of 2 γ s of energies E’=x·E 1 and E’’=(1-x)·E 1 ›Suppose E’ comes from a π 0 1 together with cluster 2 and E’’ was coming from π 0 2 with cluster 3. Then the vertices would be: z π01 = √(Dist 12 ·E’·E 2 )/m π0 = √(Dist 12 ·x·E 1 ·E 2 )/m π0 z π02 = √(Dist 13 ·E’’·E 3 )/m π0 = √(Dist 13 ·(1-x)·E 1 ·E 3 )/m π0 ›As z π01 = z π02 we can solve for x ›We reject the event if |z π01 - z c | < 500 cm Thanks to this cut we can extend the fit region to 0 MeV < T* π < 80 MeV keeping the needed rejection Thanks to this cut we can extend the fit region to 0 MeV < T* π < 80 MeV keeping the needed rejection