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Search for direct CP-violation in K ±  p ± p + p – decays by NA48/2

Search for direct CP-violation in K ±  p ± p + p – decays by NA48/2. Ivan Mikulec HEPHY Vienna, Austria On behalf of the NA48/2 collaboration: Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna

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Search for direct CP-violation in K ±  p ± p + p – decays by NA48/2

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  1. Search for direct CP-violationin K± p±p+p–decays by NA48/2 Ivan Mikulec HEPHY Vienna, Austria On behalf of the NA48/2 collaboration: Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna XXXXth Rencontres de Moriond EWI&UT La Thuile March 5-12 2005

  2. Overview • Direct CP violation in K±→3p decays • NA48/2 experimental setup • Measurement principle • Systematic effects • Preliminary result in K± p±p+p– decay mode • Outlook for K±±00analysis • Conclusions I. Mikulec: Search for direct CP-violation in K± by NA48

  3. Brief history of CP violation 1964 - CP violation in K0 (Cronin, Christenson, Fitch, Turlay) 1993-99 - Direct CP violation in K0 (NA31, NA48, KTeV) 2001 - CP violation in B0 (Babar, Belle) 2004 - Direct CP violation in B0 (Belle, Babar) CP violation and especially direct: A window to physics beyond SM Complementary observables in Kaons: e’/e↔Ag↔rare decays • Look for direct CP violation in K± ! (only direct CPV in K±possible – no mixing) I. Mikulec: Search for direct CP-violation in K± by NA48

  4. Direct CP-violation observable Ag p1even K±→3p matrix element: Kπ+π-π±: g = -0.2154 Kπ0π0π±: g = 0.652 |h|, |k| << |g| p3odd K± p2even Dalitz variables K+-K- asymmetry in g: if Direct CP violation i=3 odd pion Kπ+π-π± I. Mikulec: Search for direct CP-violation in K± by NA48

  5. Experimental and theoretical status Experimental results Ford et al. (1970) Asymmetry in decay widths AG expected to be smaller than in Dalitz-plot slopes Ag (SM: ~10-7…10-6). 10-2 HyperCP prelim. (2000) TNF prelim. (2002) “neutral” mode |Ag| Theory NA48/2 proposal 10-3 SM estimates of Ag vary within an order of magnitude (few 10-6…8x10-5). “neutral” 10-4 “charged” Models beyond SM predict substantial enhancements partially within the reach of NA48/2. (theoretical analyses are by far not exhaustive by now) 10-5 10-6 SM New physics SUSY I. Mikulec: Search for direct CP-violation in K± by NA48

  6. Goals and method • Primary NA48/2 goals: • Measure slope asymmetries in “charged” and “neutral” modes with precisions δAg<2.2x10-4, and δAg0<3.5x10-4, respectively. • Statistics required for this measurement: >2x109 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 slopesof ratios of normalized U distributions; • Equalise K+ and K–acceptances by frequently alternating polarities of relevant magnets. I. Mikulec: Search for direct CP-violation in K± by NA48

  7. 54 60 66 PK spectra, 603GeV/c 10 cm 200 250 m 1cm 50 100 Experimental setup Analysing magnet K+ K+ beam pipe 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 I. Mikulec: Search for direct CP-violation in K± by NA48

  8. Data taking 2003run: ~ 50 days 2004run: ~ 60 days Total statistics in 2 years: K +: ~4x109 K 00: ~2x108 ~ 200 TB of data recorded First result based on 2003K± p±p+p– sample will be presented here I. Mikulec: Search for direct CP-violation in K± by NA48

  9. Accepted statistics M=1.7 MeV/c2 Data-taking 2003: 1.61x109K± p±p+p–events Events |V| even pion in beam pipe  Invariant ppp mass odd pion in beam pipe K+ : 1.03 x109 events K: 0.58 x109 events K+/K–≈ 1.8 U I. Mikulec: Search for direct CP-violation in K± by NA48

  10. Principle of the experiment • Build u-distributions of K+ and K-: N+(u),N-(u) • Make a ratio of these distributions: R(u) • Fit a linear function to this ratio: normalised slope ≈ Dg e.g. uncertainty δAg<2.2∙10-4corresponds to δg<0.9∙10-4 But, achromat and spectrometer magnet  unavoidable sources of apparatus asymmetry! I. Mikulec: Search for direct CP-violation in K± by NA48

  11. Four ratios Y X Achromats: K+ Up Jura (left) B+ Z B Saleve (right) Achromats: K+Down • Indeces of R’s correspond to • beamline polarity (U/D) • kaon deviation in spectrometer mag. field (S/J). • Supersample data taking strategy: • achromat polarity (A) was reversed on weekly basis; • spectrometer magnet polarity (B) was reversed on daily basis. 1 Supersample ~ 2 weeks  2003 data 4 supersamples I. Mikulec: Search for direct CP-violation in K± by NA48

  12. Quadruple ratio R = RUSRUJRDSRDJ~ 1+4Dg·u 3-fold cancellation of systematic biases: 1) Global time-variable biases (K+,K- simultaneously recorded) 2) Beam line biases (K+ beam up / K- beamup etc.) 3) Detector asymmetries (K+ toward Saleve / K- toward Saleve etc.) In addition, acceptance is defined respecting azimuthal symmetry 4) Effects of permanent stray fields(earth, vacuum tank magnetisation) cancel The result is sensitive only to time variation of asymmetries in experimental conditions with a characteristic time smaller than corresponding field-alternation period (beam-week, detector-day) I. Mikulec: Search for direct CP-violation in K± by NA48

  13. Monte Carlo simulation • Still MC is used to study systematics. MC features: • Based on GEANT; • Full detector geometry and material description; • Local DCH inefficiencies simulated; • Variations of beam geometry and DCH alignment are followed; • Simulated statistics similar to experimental one. Due to acceptance cancellations, theanalysis does not rely on Monte-Carlo to calculate acceptance Example ofdata/MCagreement: mean beam positions @DCH1 K+data Kdata K+MC KMC K+ K I. Mikulec: Search for direct CP-violation in K± by NA48

  14. Beam systematics Time variations of beam geometry Acceptance largely defined by central beam hole edge (~10 cm radius) Acceptance cut defined by (larger) “virtual pipe” centered on averaged beam positions as a function of charge, time and K momentum Sample beam profile at DCH1 x,y-beam vs. K momentum 0.8 Y, cm Y, cm 0.4 5mm 0 2mm -0.4 Beam movements:~ 2 mm Beam widths: ~ 5 mm -0.8 -0.8 -0.4 0.8 0.4 0 X, cm X, cm I. Mikulec: Search for direct CP-violation in K± by NA48

  15. Spectrometer systematics Time variations of spectrometer geometry - Alignment is fine tuned by forcing mean reconstructed invariant ppp masses to be equal for K+ and K- E.g. sensitivity to DCH4 horizontal shift: M/x  1.5 keV/m DMppp Momentum scale variation due to limited control of spectrometer magnet current (10-3) cancels due to simultaneous beams In addition, it is adjusted by forcing mean reconstructed invariant ppp masses to PDG value of MK+ Maximum equivalent horizontal shift: ~200m @DCH1 or ~120m @DCH2 or ~280m @DCH4 I. Mikulec: Search for direct CP-violation in K± by NA48

  16. Trigger systematics • Inefficiencies measured using control data from low bias triggers. • Rate-dependent parts of trigger inefficiencies assumed to be symmetric. L2 trigger(online vertex reconstruction):time-varying inefficiency (local DCH inefficiencies)1-e ≈ 0.2% to 1.8%flat in u within measurement precision u-dependent correction applied L1 trigger(2 hodoscope hits):stable and small inefficiency 1-e ≈ 0.7·10-3 (no correction) L2 inefficiency 3x10-3 cut cut statistical uncertainty from control sample Possibility to use MC studied I. Mikulec: Search for direct CP-violation in K± by NA48

  17. Other systematics Field map in decay volume: Y projection Residual effects of stray magnetic fields(magnetised vacuum tank, earth field) minimised by explicit field map correction Decay volume: Z coordinate No magnetic field correction Further systematic effects studied: • Bias due to resolution in u calculation • Sensitivity to fitting interval and method • Effects connected to p→mn decay • Effects due to event pile-up • p+/p-interactions in material • Track chargemisidentification Magnetic field corrected for I. Mikulec: Search for direct CP-violation in K± by NA48

  18. Fit linearity – four supersamples SS2: g=(-3.1±2.5)x10-4 2=29.5/38 SS0: g=(0.6±2.4)x10-4 2=39.7/38 U U SS3: g=(-2.9±3.9)x10-4 2=32.9/38 SS1: g=(2.3±2.2)x10-4 2=38.1/38 U U I. Mikulec: Search for direct CP-violation in K± by NA48

  19. Time stability Quadruple ratio with K(up)/K(down) K(right)/K(left) By regrouping the components in quadruple ratio – check residual detector and beam line asymmetries (~few 10-4) – they cancel safely in Dg fits K(+)/K(-) Dg 4 supersamples give consistent results control of detector asymmetry control of beam line asymmetry MC can reproduce these apparatus asymmetries I. Mikulec: Search for direct CP-violation in K± by NA48

  20. Systematics summary and result Combined preliminary result: in Δgx104 units (3 independent analyses) Including L2 trigger correction I. Mikulec: Search for direct CP-violation in K± by NA48

  21. Preliminary resultK±±+-(2003 data) Δg =(-0.2±1.0stat.±0.9stat.(trig.)±0.9syst.)x10-4 Δg =(-0.2±1.7)x10-4 Converted to asymmetry: Ag = (0.5±2.4stat.±2.1stat.(trig.)±2.1syst.)x10-4 Ag = (0.5±3.8)x10-4 • This is a preliminary result with conservative estimate of • systematic uncertainties • Extrapolated statistical uncertainty 2003+2004: Ag=1.6x10-4 • Expect smaller systematic effects in 2004 data • (due to more frequent polarity alternation, better L2 performance). I. Mikulec: Search for direct CP-violation in K± by NA48

  22. Comparison K±±+- Ford et al. (1970) 10-2 HyperCP prelim. (2000) This preliminary result is already an order of magnitude better than previous experiments |Ag| 10-3 NA48/2 prelim.: 2003 data NA48/2 goal: 2003-04 data 10-4 10-5 10-6 New physics SUSY SM I. Mikulec: Search for direct CP-violation in K± by NA48

  23. K±±00 analysis • ∙ u can be reconstructed with LKr calorimeter only • ∙ Statistics analyzed: 28 × 106 events(1 month of 2003) • ∙ Statistical error with analyzed data: Ag(stat)= 2.2 × 10−4 • ∙ Extrapolation to 2003+2004 data: Ag(stat)= 1.3 × 10−4 • ∙ Similar statistical precision as in “charged” mode • ∙ Possibly larger systematic errors v 3 Dalitz-plot 2 Statistical precision in Ag0 similar to “charged” mode: - Ratio of “neutral” to “charged” statistics: N0/N±~1/20 (√=1/4.5) - Ratio of slopes: |g0/g±|3 - More favourable Dalitz-plot distribution (gain factor f~1.5) 1 0 -1 -2 1.0 0.5 0 -0.5 -1.0 -1.5 u I. Mikulec: Search for direct CP-violation in K± by NA48

  24. Observation of pp scattering effect in K→3p decays Charge exchange process +00not negligible under 2mp threshold, destructive interference generates a cusp in the Dalitz plot,not seen earlier by lower precision experiments 30M events MC (no rescattering) Great potential for a new accurate measurement of ππ scattering lengths from this data New preliminary result at Moriond QCD! Data K±±00 4mπ+2 4mπ+2 1 bin = 0.00015 GeV2 M(00) GeV/c 2 I. Mikulec: Search for direct CP-violation in K± by NA48

  25. Conclusions • Preliminary NA48/2 result (2003 data) on direct CP−violating charge asymmetry in K± p±p+p– decays isAg = (0.5 ± 2.4stat. ± 2.1stat.(trig.) ± 2.1syst.)×10-4 • >10 times better precision than previous measurements • Further room to decrease systematic uncertainties • 2004 datacontain another 2×109K± p±p+p– events, with higher quality • Neutral mode asymmetry: complementary, comparable sensitivity • Design goaliswithin reach in both decay modes • Other interesting results will follow (pp scattering lengths, other CP asymmetries, rare decays) I. Mikulec: Search for direct CP-violation in K± by NA48

  26. Spare slides I. Mikulec: Search for direct CP-violation in K± by NA48

  27. Theoretical predictions of Ag I. Mikulec: Search for direct CP-violation in K± by NA48

  28. Experimental results so far • ”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: 41.8M K+, 12.4M 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:Ag0=(1.9±12.3)∙10-3; Statistics: 28000 K±; • TNF (2004) at IHEP Protvino:Ag0=(0.2±1.9)∙10-3; Statistics: 0.52M K±. I. Mikulec: Search for direct CP-violation in K± by NA48

  29. 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]. • Hadron calorimeter, muon veto counters, photon vetoes. Beam pipe I. Mikulec: Search for direct CP-violation in K± by NA48

  30. KAon Beam Spectrometer (KABES) • 3 MICROMEGA gas chamber stations • measure beam particle • charge (prob. mis-ID~10-2); • momentum (p/p=0.7%); • position in the 2nd achromat (x,y100m). • Measurement of kaon momentum: • Reconstruct K3π with a lost pion; • Redundancy in K3πanalysis; • Resolve Ke4reconstruction ambiguity. Not used yet for K±3± analysis I. Mikulec: Search for direct CP-violation in K± by NA48

  31. Break down of K±±+-statistics Statistics selected for Ag measurement, events x106 I. Mikulec: Search for direct CP-violation in K± by NA48

  32. Cancellation of beam spectra Achromat reversal reverses K+ and K– beam spectra K+ Systematic differences of K+ and K–acceptance due to beam spectra mostly cancel in RU*RD Systematic check: Reweighting K+events so as to equalise momentum spectra leads to negligible effect Δg=0.03x10-4 K– SS 3 Supersample 2 Supersample 1 I. Mikulec: Search for direct CP-violation in K± by NA48

  33. Fits to the “cusp” effect in K±±00 One-loop exchange: 2 = 463/149 Cabibbo: PRL 93 (2004) 121801 One and two loops: 2 = 159/147 Standard Dalitz-plot parametrization Cabibbo, Isidori: hep-ph/0502130 2 = 133/139 for M(00)>80 MeV/c2 Incl. + atoms: 2 = 144/146 M(00), GeV/c2 M(00), GeV/c2 I. Mikulec: Search for direct CP-violation in K± by NA48

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