Evgueni Goudzovski (JINR, Dubna) CERN particle physics seminar 1 st March 2005

1. The first results on direct CP-violation asymmetry measurement in K± π±π+π–decaysfrom the NA48/2 experiment Evgueni Goudzovski (JINR, Dubna) CERN particle physics seminar 1st March 2005 on behalf of the NA48 Collaboration: Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna

2. Overview • Direct CP-violation in charged K3π decays; • NA48/2 beam, detector and data-taking; • NA48/2 method of CP-violation measurement; • Preliminary result on 2003 data.

3.  Direct CP-violation in K3 BR(K±±+)=5.57%; BR(K±±00)=1.73%. • Matrix element: • Kinematic variables |M(u,v)|2 ~ 1 + gu + hu2+ kv2 Lorentz-invariants u = (s3-s0)/m2; v = (s2-s1)/m2; si = (PK-Pi)2, i=1,2,3 (3=odd ); s0 = (s1+s2+s3)/3. • Measured quantity sensitive to direct CP violation: Centre of mass frame u = 2mK∙(mK/3-Eodd)/m2; v = 2mK∙(E1-E2)/m2. Ag = (g+-g-)/(g++g-)≠0

4. Previous measurements of Ag • ”Charged” mode K±3π±: • Ford et al. (1970) at BNL:Ag=(-7.0±5.3)∙10-3; Statistics: 3.2M K±; • HyperCP prelim. (2000) at FNAL:Ag=(2.2±1.5±3.7)∙10-3; Statistics: 390M K+, 1.6M K-; Systematics due to knowledge of magnetic fields; Published as PhD thesis W.-S.Choong LBNL-47014 Berkeley 2000; • ”Neutral” mode K±π±π0π0: • Smith et al. (1975) at CERN-PS:Ag=(1.9±12.3)∙10-3; Statistics: 28000 K±; • ISTRA+ (2004) at IHEP Protvino:Ag=(0.2±1.9)∙10-3; Statistics: 0.52M K±;

5. Theoretical expectations Theoretical calculations: L.Maiani et al. G.D’Ambrosio G.Isidori G.Martinelli E.Shabalin A.Bel’kov I.Scimemi Conclusions: • Ag ~ 10-5:perfectly compatible with SM; • 3∙10-5<Ag<5∙10-5:compatible with SM, but inbad agreement with ε’/ε; • Ag>5∙10-5:SUSY /New Physics • Asymmetry in decay widthsexpected to be smaller thanin Dalitz-plot slopes(SM: ~10-6…10-7). Experimental limits by 2005: 10-2 Ford et al. (1970) |Ag| HyperCP prelim. (2000) ISTRA+ (2004) “neutral” mode 10-3 NA48/2 proposal 10-4 New physics 10-5 SUSY SM 10-6

6. NA48/2 goals and method • NA48/2 goals: • Measure slope asymmetries in both “charged” and “neutral” modes with an accuracy δAg<2∙10-4. • Statistics required for this measurement: >2∙109 in “charged” mode and >108in “neutral” mode. • NA48/2 method: • Two simultaneous K+ and K- beams, superimposed in space, with narrow momentum spectra; • Detect asymmetry exclusively considering slopes of ratios of normalized U distributions; • Equalize averaged K+ and K–acceptances by frequently alternating polarities of relevant magnets.

7. 54 60 66 PK spectra, 603GeV/c BM z 10 cm 200 250 m 1cm 50 100 NA48/2 experiment magnet K+ K+ focusing beams K ~71011 ppp K Second achromat • Cleaning • Beam spectrometer Front-end achromat Quadrupole quadruplet Beams coincide within <1mm all along 114m decay volume • Momentum • selection • Focusing •  sweeping vacuum tank He tank + spectrometer not to scale

8. PK spectra, 603GeV/c 54 60 66 NA48/2 narrow-band beams: simultaneous, coaxial, focused  Pion decay products stay in beam pipe...

9. KAon BEam Spectrometer (KABES) • 3 MICROMEGA gas chamber stations • measure charged beam displacement • within the magnetic field of the • second achromat • Measurement of kaon momentum: • Reconstruct K3π with a lost pion; • Redundancy in K3πanalysis; • Resolve Ke4reconstruction ambiguity. Not used yet for K±3± analysis

10. NA48 detector • Main detector components: • Magnetic spectrometer (4 DCHs): • redundancy  high efficiency; • Δp/p = 1.0% + 0.044%*p [GeV/c] • Hodoscope • fast trigger; • precise time measurement (150ps) • Liquid Krypton EM calorimeter (LKr) • High granularity, quasi-homogenious; • ΔE/E = 3.2%/√E + 9%/E + 0.42% [GeV]; • electron/pion discrimination. • Hadron calorimeter, photon vetos, • muon veto counters Beam pipe

11. L2 trigger for Kdata-taking • 3-track trigger • K±3π± [“charged” mode] • Ke4, events with Dalitz decay π0e+e– • 4-leptonic: K±e+e–eν, K±e+e–μν, K±μ+μ–eν, K±μ+μ–μν • 1-track missing mass trigger: (PK-Pπ)2>mπ02 • K±π0π0π± [“neutral” mode] • K±π±π0, K±π± • Suppress K±π±π0 • Control triggers (downscaled) • Trigger efficiency measurements; • Many 1-track channels: K±e±ν, e±ν, π0e±ν, etc…

12. Data taking: finished • 2003run:~ 50 days • (only 2003 data are presented) • 2004run: ~ 60 days • Total statistics in 2 years: • K - + : ~4·109 • K 0 0 : ~2·108 • ~ 200 TB of data recorded

13. K3π selection  • Simple selection: • Ghost track rejection; • Select 3-track vertex with smallest 2; • Track time consistency cuts; • Decay vertex within the decay volume; • Transverse momentum cut; • -1<U<1; • Only magnetic spectrometer involved • No muon veto, EM calo, hadron calo; • No significant background for this channel.

14. Selected statistics 2003 M=1.7 MeV/c2 Data-taking 2003: 1.61x109 events selected |V| even pion in beam pipe  K+ : 1.03x109 events odd pion in beam pipe  K: 0.58x109 events U

15. Measurement strategy |M±(u,v)|2 ~ 1 + g±u + hu2+ kv2 • PDG: • Kπ+π-π±: g = -0.2154±0.0035 • Kπ0π0π±: g = 0.652±0.031 • |h|, |k| << |g| • Project onto U axis • Neglect asymmetry in quadratic slopes h and k If acceptance is equal for K+ and K–, R(u) = N+(u)/N–(u) ≈ ≈n∙(1+g+u)/(1+g–u) ≈ ≈n∙(1+2gAgu) Ag can be extracted from a linear fit to the ratio R(u) Ag=g/2g

16. Acceptance cancellation • Supersample data taking strategy: • achromat polarity (A) was reversed on weekly basis; • analyzing magnet polarity (B) was reversed on daily basis. Example: Data taking from August 6 to September 7, 2003 Achromat – B+ B- B+ B- B+ B- Week 1 Supersample 1 12 subsamples B+ Achromat + B+ B- B+ B- B- Week 2 B- B- Achromat – B+ B- B+ B+ Week 3 Supersample 2 12 subsamples Achromat+ B+ B+ B- B+ B- B- Week 4 B- Achromat – B+ Week 5 Supersample 3 4 subsamples Achromat+ B+ B-

17. Supersamples 2003 Statistics selected for Ag measurement, million events

18. N(A+B+K+) RUS= N(A+B-K-) N(A+B-K+) RUJ= N(A+B+K-) N(A-B+K+) RDS= N(A-B-K-) N(A-B-K+) RDJ= N(A-B+K-) Acceptance cancellation within supersample Detector left-right asymmetry cancels in 4 ratios of K+ over K U-spectra: Y (same kaon deviation direction in numerator and denominator) X Achromats: K+ Up B+ Jura Z B Saleve Achromats: K+Down • Indeces of R’s correspond to • beamline polarity (U/D); • direction of kaon deviation in spectrometer (S/J).

19. More cancellations (1) 1.Charge asymmetrydue to permanent magnetic fields (Earth, tank magnetization) cancels integrating over azimuthal angle; 2. Cancellation ofeffects due to coupling of charge asymmetrywith left-right asymmetry, and effects of global time instabilities: RU = RUS*RUJ fit with f(u)=n∙(1+2AUu) fit with f(u)=n∙(1+2ADu) RD = RDS*RDJ 3. Cancellation ofbeam geometry differenceeffects: fit with f(u)=n∙(1+2ASu) RS = RUS*RDS RJ = RUJ*RDJ fit with f(u)=n∙(1+2AJu)

20. More cancellations (2) 4.Maximum cancellation: fit with f(u)=n∙(1+4∙u) R = RUS*RUJ*RDS*RDJ Normalization Slope difference The result is sensitive only to time variation of left-right asymmetries of experimental conditions with a time-scale of ~1 subsample • Relation between and asymmetry Ag: Δ = 2g∙Ag ≈ -0.43∙Ag • Goal of the experiment:δAg<2∙10-4corresponds to δ<0.9∙10-4

21. Beam spectra difference effect (an example of cancellation) Achromat reversal reverses K+ and K– beam spectra K+ Systematic differences of K+ and K–acceptance due to beam spectra mostly cancel in AU*AD Systematic check: Reweighting K+events so as to equalise momentum spectra leads to negligible effect Δ=0.03x10-4 K– SS 3 Supersample 2 Supersample 1

22. Monte-Carlo simulation Due to cancellations, theanalysis does not rely on Monte-Carlo. MC is used for systematics study. • Based on GEANT; • Full detector geometry and material description; • Local DCH inefficiencies simulated; • Beam geometry and DCH alignment variations are followed; • Simulated statistics similar to experimental one. Example of data/MC agreement: Mean beams’ positions @DCH1

23. Systematic effects (1) (1)Time variations of beam geometry • Beams’ non-perfect superposition, and width differences coupled to acceptance selection; (2)Time variations of spectrometer geometry • Mainly variations of chambers’ transverse alignment; (3)Variation of spectrometer magnetic field • Magnet current can not be inverted with accuracy better than 10-3;

24. Systematic effects (2) (4)Time variations of trigger efficiency • Sensitivity to time-dependent local inefficiencies of DCHs and hodoscope; (5)Stray magnetic fields in the decay region • Earth field can not be inverted; (6)Coupling ofπ±±decay to another systematic effects; (7)Time variations of accidental rate.

25. (1): Beam geometry • Geometrical acceptance is largely defined by pions undetected in the beam pipe; • Beams’ geometry is variable, beams’ superposition is not perfect … asymmetric acceptance; • Time-dependent cuts following beams’ movement are introduced. pion in beam pipe “Virtual pipe” cut: Cylinder following mean beam position measured by the spectrometer |Rπi-<RK>|>R0 at DCH1, DCH4 <RK> = f (kaon sign, time, momentum)

26. Beams’ movement in time Beam profile @DCH1 DCH1 Y, cm Y, cm 0.8 0.4 K K+ 0 -0.4 X, cm X, cm -0.8 DCH4 Y, cm -0.8 -0.4 0.8 0.4 0 4cm relative shift due to analyzing magnet field X, cm • Typical scales: • Beam width: ~5mm; • Beam movement in time:~2mm. X, cm

27. Momentum dependence of beams DCH4 • Upper and lower beam lines have slightly different geometric properties; • Effect partially cancels by achromat inversion; • Effect is corrected by “virtual pipe”. A+KJ, A-KS Y, cm 55 GeV/c 65 GeV/c 65 GeV/c 55 GeV/c X, cm A+KJ, A-KS Split by analyzing magnet field Y, cm Beam pipe 65 GeV/c 55 GeV/c 65 GeV/c 55 GeV/c Beam positions for various momenta X, cm

28. Alternative to the “virtual pipe”[not included into the analysis yet] Use kaon momentum measured by the beam spectrometer (KABES): • Need only two detected pions to reconstruct event kinematics; • Low sensitivity to beam pipe acceptance; • Possible add events with only 2 tracks in acceptance (+60% events); • Gain events in high-u region: higher sensitivity to Ag

29. Residual systematics The following sources were studied: • Time binning used to follow mean beam position; • Systematic shifts in measurement of mean beam position (pre-selection cuts, fitting limits); • Variation of beam widths in time; • Effect of Earth magnetic field on beam position. Conclusion: • Residual effect is <0.5·10-4.

30. (2): Spectrometer alignment • The observables AS and AJ are sensitive to time variations in spectrometer alignment; the effect does not cancel in ASJ. • Spectrometer transverse alignment is fine-tuned for each subsample by imposing the K+ and K- to have the same reconstructed mean invariant mass. • Sensitivity to DCH4 horizontal shift: M/x  1.5 keV/m. Maximum equivalent horizontal shift: ~200m @DCH1 or ~120m @DCH2 or ~280m @DCH4.

31. (3): Spectrometer magnetic field • The observables AS and AJ are sensitive to non-perfect inversion of spectrometer magnetic field; however, the effect mostly cancels in ASJ. • Effective momentum scale is adjusted for each subsample by imposing reconstructed mean invariant kaon mass to be equal to PDG mass. • Sensitivity: M  100 keV for typical inversion precision on magnetic field integral of I/I  10-3.

32. Spectrometer: residual systematics The following sources were studied: • Size of time binning used to follow calibration time-dependence; • Systematic shift in kaon mass measurement (pre-selection cuts, fitting limits); • Inhomogeneity of alignment correction; • Misalignment hypothesis: effect of correction to drift chambers’ positions before/after magnet. Conclusion: • residual effect is <0.1·10-4.

33. (4): Trigger efficiency • Trigger chain for K±3π±: • L1: at least 2 hodoscope hits; • L2: 3-track trigger (online vertex reconstruction); • Only the geometry-related parts of inefficiency considered (rate-dependent parts maximally separated). 10-3 cut cut L1 inefficiency stable: ~ 0.7x10-3 L2 inefficiency varies: 0.2% … 1.8% 3x10-3 cut cut

34. Trigger efficiency systematics • L1 inefficiency: small and stable in time • No correction is applied to the result; • Uncertainty on Δ due to statistical error of inefficiency Δ = 0.4x10-4; • L2 inefficiency: fluctuates in time,depends on local DCH inefficiency • Correction applied to U-spectrumin each subsample. • Resulting uncertainty due to statistical error: Δ = 0.8x10-4.

35. 10-4 P kick(stray field) P kick(spectrometer) (5): Stray magnetic fields • Field map was directly measured and used invertex reconstruction: only residual systematics remain (Δ<10-5); • Coupling of stray field with the “virtual pipe” acceptance cut: drift of pions and kaon trajectories in stray field. Relatively large effects (Δ~10-4) in AS, AJcancel in ASJ. No magnetic field correction Field map in decay volume: Y projection 100T Magnetic field corrected for

36. (6,7): π±±decay, accidentals x10-4 x10-4 Data Monte-Carlo 20 20 0: Nominal data; 1: No decay before DCH1; 2: No decay before DCH2; 3: No decay before DCH4. 15 15 10 10 5 5 0 0 systematic error due to±±decay: =0.4x10-4 (conservative estimation, limited by MC statistics) -5 -5 systematic error due toaccidental activity: =0.3x10-4 (conservative estimation) -10 -10 -15 -15 -20 -20

37. Other second-order effects • Interaction of pions with the detector material • <0.5x10-5 • Pion charge mis-identification • <<10-5 Other possible sources found to have negligible systematic effects on asymmetry:

38. Example of fits: supersample 1 2=37.5/38 AUS=(2.4±4.6)x10-4 AUJ=(-0.2±4.6)x10-4 2=50.5/38 U U 2=35.6/38 ADS=(8.2±4.4)x10-4 2=34.8/38 ADJ=(-1.0±4.4)x10-4 U U 2=38.1/38 =(2.3±2.2)x10-4 U

39. Asymmetry fits in supersamples SS2: =(-3.1±2.5)x10-4 2=29.5/38 SS0: =(0.6±2.4)x10-4 2=39.7/38 U U SS3: =(-2.9±3.9)x10-4 SS1: =(2.3±2.2)x10-4 2=38.1/38 2=32.9/38 U U

40. Stability of the result g x10-4 g x10-4 80 40 60 30 40 20 20 10 0 0 -20 -10 -40 -20 -60 -30 -80 -40

41. Time-stability: g vs supersample x10-4 20 Monte-Carlo describes effects of left-right asymmetries and differences of beamline properties 15 10 5 0 -5 -10 -15 -20

42. Result & systematics Result: Δx104 L2 trigger systematics included

43. Preliminary result: 2003 data Slope difference: Δg = (-0.2±1.0stat.±0.9stat.(trig.)±0.9syst.)x10-4 = (-0.2±1.7)x10-4 Charge asymmetry: Ag = (0.5±2.4stat.±2.1stat.(trig.)±2.1syst.)x10-4 = (0.5±3.8)x10-4 • Systematic errors are conservative for preliminary result; • 2004 data: • Statistics greater than in 2003 (~2x109 events); • Expect smaller systematic effects (more frequent polarity alternation, better beam steering).

44. Comparison with other results Ford et al. (1970) 10-2 HyperCP prelim. (2000) |Ag| 10-3 NA48/2 prelim.: run 2003 NA48/2 goal: runs 2003-04 10-4 New physics 10-5 SUSY SM 10-6

45. “Neutral” mode analysis K±±00 • U reconstructed with only LKr calorimeter; • Statistics analyzed: 28.0x106events in 1 month of 2003 (SS1-3); • Statistical error with analyzed data: Ag(stat)=2.2x10-4; • Extrapolation to 2003+2004 data: Ag(stat)=1.3x10-4. |v| M=1.1 MeV/c2 g fit for SS1 2=45.3/50 =4.8x10-4 u u m(3), GeV/c2

46. Conclusions • Preliminary NA48/2 2003 result on CP-violating charge asymmetry in K± π±π+π–:Ag = (0.5±2.4stat.±2.1stat.(trig.)±2.1syst.)x10-4 • There is a room to decrease trigger efficiency error and systematic error; • 2004 statistics are another 2x109 events, and expected to have smaller systematic effects; • Ultimate goal: reach the precision of Ag=2x10-4.Experimental precision is better than the theoretical one.

47. Spare slides

48. Theoretical predictions on Ag

49. Spectrometer calibration • Rough alignment made by interpolation between muon alignment runs [3 muon runs in 2003]; • Fine-tuning of momenta: P’ = P∙(1+β)∙(1+qbP) • P0 – measured momentum; • P – corrected momentum; • q – track charge; • b – magnetic field sign. • Time-dependent corrections: • β– for magnetic field integral; • – for spectrometer misalignment.

50. Systematics: U calculation & fitting The following sources were studied: • Variations of fitting limits cut; • Use of various U definitions with different resolution properties. Conclusion: • Effect is <0.5·10-4. Cinematic fitting routine is being implemented