Results on direct CP Violation in K ± 3π decays from the NA48/2 experiment at CERN

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1 Crimea 2005 Giuseppina Anzivino 1 Results on direct CP Violation in K ± 3π decays from the NA48/2 experiment at CERN Giuseppina Anzivino University of Perugia and INFN On behalf of the NA48/2 collaboration: Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna New Trends in High Energy Physics Yalta, September 2005

2 Crimea 2005 Giuseppina Anzivino 2 Outline Direct CP violation in K ± 3π decays NA48/2 experimental setup Measurement principle Systematic effects Preliminary result in K ± π ± π + π decay Outlook for K ± π ± π 0 π 0 analysis Neutral mode Cusp effect Conclusions

3 Brief history of CP violation CP violation in K 0 (Cronin, Christenson, Fitch, Turlay) Direct CP violation in K 0 (NA31, NA48, KTeV) CP violation in B 0 mixing (Babar, Belle) Direct CP violation in B 0 (Belle, Babar) Direct CP Violation, also known as CPV in decay amplitudes, is the most straightforward CP effect Hard to detect experimentally Hard to connect to the parameters of the underlying fundamental theory (i.e. SM) But it is a crucial window to physics beyond SM because possible non-sm enhancements to heavy quark loops are just at the core of DCPV processes Crimea 2005 Giuseppina Anzivino 3

4 Crimea 2005 Giuseppina Anzivino 4 Direct CPV in K 0 π πdecays Final NA48 result ε /ε = ( 14.7 ± 2.2 ) x 10-4

5 Crimea 2005 Giuseppina Anzivino 5 CP violation in K 3π decays ± Potentially large statistics Simple selection Low background No absolute K flux measurement Compare only Dalitz plot shapes Complementary observables in Kaons: ε /ε A g rare decays Look for direct CP violation in K ± (only direct CPV in K ± possible no mixing)

6 Crimea 2005 Giuseppina Anzivino 6 Direct CP violation observable A g u v π 1even π 2even = = s s 3 s 0 2 mπ s mπ π 3odd K ± M(u,v) 2 ~ 1 + gu + hu 2 + kv 2 s i = (PK p πi) 1 s0 = s 3 i 2 K ± π ± π + π BR = 5.57% ; g = ± K ± π ± π 0 π 0 BR = 1.73% ; g = ± h, k << g A g = g g g g = Δg 2g Δg = 0.43 i=3 odd pion A g 0 A g 5 x10-5 compatible with SM A g 1 x10-4 SUSY/new physics Direct CP violation

7 Experimental and theoretical status A g Smith et al. (1975) 10-2 Ford et al. (1970) HyperCP prelim. (2000) charged TNF prelim. (2002) NA48/2 proposal neutral Experimental results Theory SM estimates of A g vary within an order of magnitude (few x10-5 ) SM SUSY physics New Models beyond SM predict substantial enhancements partially within the reach of NA48/2. (theoretical analyses are by far not exhaustive by now) Crimea 2005 Giuseppina Anzivino 7

8 Crimea 2005 Giuseppina Anzivino 8 Primary NA48/2 goals: Goals and method Measure slope asymmetries in charged and neutral modes with precisions δa g <2.2x10-4, and δa g0 <3.5x10-4, respectively. Statistics required for this measurement: >2x10 9 charged mode and >10 8 in 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; Equalise averaged K + and K acceptances by frequently alternating the polarities of the relevant magnets. in

9 Experimental setup P K spectra, 60±3 GeV/c Analysing magnet K + K + beam pipe ~ ppp K focusing beams K 1cm Front-end achromat Momentum selection Quadrupole quadruplet Focusing μ sweeping Second achromat Cleaning Beam spectrometer Beams coincide within ~1mm all along 114m decay volume vacuum tank He tank 10 cm + spectrometer not to scale m Crimea 2005 Giuseppina Anzivino 9

10 Crimea 2005 Giuseppina Anzivino 10 The NA48 detector Main detector components: Magnetic spectrometer (4 DCHs): 4 views: redundancy efficiency σ(p)/p = 1.0% % p [GeV/c] Hodoscope: fast trigger and precise time measurement (150ps) Liquid Krypton e.m. calorimeter: High granularity, quasi homogeneous σ(e)/e = 3.2%/ E + 9%/E % [GeV] e/π discrimination Hadron calorimeter, photon vetos, muon veto counters

11 Crimea 2005 Giuseppina Anzivino 11

12 Crimea 2005 Giuseppina Anzivino 12 Data taking: completed 2003 run: ~ 50 days 2004 run: ~ 60 days Total statistics in 2 years: K ± π + π π ± : ~ K ± π 0 π 0 π ± : ~ ~ 200 TB of data recorded The result based on 2003 K ± π ± π + π sample will be presented here

13 Accepted statistics Data-taking 2003: 1.61x10 9 K ± π ± π + π events Events σ M =1.7 MeV/c 2 V even pion in beam pipe π μν Invariant πππ mass odd pion in beam pipe K + : 1.03 x10 9 events K : 0.58 x10 9 events K + /K 1.8 Crimea 2005 Giuseppina Anzivino 13 U

14 Crimea 2005 Giuseppina Anzivino 14 Method to extract A g Build u projections of the Dalitz plot for K + and K - : N + (u),n - (u) Make the ratio of these two distributions: R(u) Fit a linear function to this ratio to extract Δg = g+ - g- R( u) N N ( u) ( u) g g + + = u u 1+ Δgu Δg A g = 2g This holds only if the acceptance for K + and K - is the same δa g < δδg < The magnetic fields (achromat and spectrometer) are intrinsic sources of charge asymmetry in the detector!!!

15 Crimea 2005 Giuseppina Anzivino 15 In real life Instrumental asymmetries Detector acceptance asymmetries Time variation of detector response Charge-dependent beam optics Time variation of beams properties Spurious magnetic fields Charge-asymmetric interactions

16 Crimea 2005 Giuseppina Anzivino 16 Strategy of data taking Beam line (achromat) polarity (A) reversed on weekly basis Spectrometer magnet polarity (B) reversed on daily basis Example: August 6 to September 7, 2003 Week 1 Week 2 Achromat Achromat + B+ B- B+ B- B+ B+ B- B- B+ B+ B- B- Supersample 1 12 subsamples Week 3 Week 4 Achromat Achromat+ B+ B+ B- B- B+ B- B+ B+ B- B+ B- B- Supersample 2 12 subsamples Week 5 Achromat Achromat+ B+ B+ B- B- Supersample 3 4 subsamples

17 Crimea 2005 Giuseppina Anzivino 17 Acceptance cancellation Detector left-right asymmetry cancels in 4 ratios of K + /K distributions: (same deviation by spectrometer in numerator and denominator) R R R R US UJ DS DJ = = = = N( A+ B+ K+ ) N( A+ B K ) N( A+ B K+ ) N( A+ B+ K ) N( A B+ K+ ) N( A B K ) N( A B K + ) N( A B+ K ) Beam line: K + Up K + K Beam line: K + Down Y B Spectrometer field X B+ Saleve Indexes correspond to beamlinepolarity (U / D) direction of kaon deviation in spectrometer (S / J) Jura Z

18 Crimea 2005 Giuseppina Anzivino 18 Quadruple ratio R = R US R UJ R DS R DJ ~ 1+4Δg u 3-fold cancellation of systematic biases: 1) Global time-variable biases (K +,K - simultaneously recorded) 2) Beam line biases (K + beam up / K - beam up etc.) 3) Detector asymmetries (K + toward Saleve / K - toward Saleve etc.) 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 (beamweek, detector-day)

19 Crimea 2005 Giuseppina Anzivino 19 Monte Carlo simulation Due to acceptance cancellations, the analysis does not rely on Monte-Carlo to calculate acceptance Example of data/mc agreement: mean beam 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. K + K K + data K data K + MC K MC

20 Time variations of beam geometry Beam systematics Acceptance largely defined by central beam hole edge. Acceptance cut defined by (larger) virtual pipe centered on averaged beam positions as a function of charge, time and K momentum Effects due to beam movements and not perfect overlap corrected Y, cm 0.8 Sample beam profile at DCH1 Y, cm mm 2mm Beam widths: ~ 5 mm Beam movements: ~ 2 mm X, cm X, cm Crimea 2005 Giuseppina Anzivino 20

21 Crimea 2005 Giuseppina Anzivino 21 Spectrometer systematics Time variations of spectrometer geometry - Alignment is fine tuned by forcing mean reconstructed invariant πππ masses to be equal for K + and K - ΔM πππ Maximum equivalent horizontal shift: or or E.g. sensitivity to DCH4 horizontal shift: ΔM/Δx 1.5 kev/μm 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 πππ masses to PDG value of M K+

22 Crimea 2005 Giuseppina Anzivino 22 Trigger systematics L1 trigger (2 hodoscope hits): stable and small inefficiency: 1-e , charge-symmetric, flat in u NO CORRECTION NEEDED L2 trigger (online vertex reconstruction on DCH data): time-varying inefficiency (local DCH inefficiencies) 1-e 0.2% to 1.8%, flat in u within measurement precision u-dependent CORRECTION APPLIED L2 inefficiency 3x10-3 L2correction δ gx10 4 SS0 0.5±1.8 SS1 1.4±1.0 SS2-0.2±1.2 cut cut SS3-4.5±1.9 statistical uncertainty from control sample

23 Crimea 2005 Giuseppina Anzivino 23 Other systematics Residual effects of stray magnetic fields (magnetised vacuum tank, earth field) minimised by explicit field map correction Further systematic effects studied Bias due to resolution in u calculation Sensitivity to fitting interval and method Effects connected to π μν decay Effects due to event pile-up π + /π - interactions in material Track charge misidentification Magnetic field corrected for No magnetic field correction

24 Systematics summary and result Conservative estimations of systematic errors Acceptance and beam geometry Effect on x Spectrometer alignment 0.1 Analyzing magnet field 0.1 π ± μν decay 0.4 U calculation and fitting 0.5 Pile-up 0.3 Syst. errors of statistical nature Trigger efficiency: L2 0.8 Trigger efficiency: L1 0.4 Total systematic error 1.3 Combined preliminary result: in Δg x 10 4 units (3 independent analyses) Including L2 trigger correction Raw Corrected for L2 eff SS0 0.0± ±2.4 SS1 0.9± ±2.2 SS2-2.8± ±2.5 SS3 2.0± ±3.9 Total -0.2± ±1.3 χ 2 2.2/3 3.2/3 Crimea 2005 Giuseppina Anzivino 24

25 Crimea 2005 Giuseppina Anzivino 25 Result stability Δg x Δg x

26 Preliminary result (2003 data) slope difference g =(-0.2±1.0 stat. ±0.9 stat.(trig.) ±0.9 syst. )x10-4 g =(-0.2 ± 1.7) x 10-4 charge asymmetry A g = (0.5±2.4 stat. ±2.1 stat.(trig.) ±2.1 syst. )x10-4 A g = (0.5 ± 3.8) x 10-4 This is a preliminary result with conservative estimate of systematic uncertainties Extrapolated statistical uncertainty : δa g =1.6x10-4 Expect smaller systematic effects in 2004 data (due to more frequent polarity alternation, better L2 performance). Crimea 2005 Giuseppina Anzivino 26

27 Comparison K ± π ± π + π - A g 10-2 Ford et al. (1970) HyperCP prelim. (2000) NA48/2 prelim.: 2003 data NA48/2 goal: data This preliminary result is already an order of magnitude better than previous experiments 10-6 SM SUSY New physics Crimea 2005 Giuseppina Anzivino 27

28 K ± π ± π 0 π 0 analysis neutral mode wrt charged v 3 Dalitz-plot u Same stategy of analysis Only the Lkr used to define u Totally different systematics Statistical precision in A g0 similar Ratio of neutral to charged statistics: N 0 /N ± ~1/20( =1/4.5) Ratio of slopes: g 0 /g ± 3 More favourable Dalitz-plot distribution (gain factor f~1.5) Crimea 2005 Giuseppina Anzivino 28

29 Crimea 2005 Giuseppina Anzivino 29 Status of analysis Statistics analyzed: events Statistical error with analyzed data: δa g (stat)= Extrapolation to data ( ) δa g (stat)= Events σ M =1.1 MeV/c M(3π), GeV/c 2 Preliminary results will be announced soon

30 Crimea 2005 Giuseppina Anzivino 30 Observation of ππ scattering effect in K 3π decays Thanks to the large statistics in the neutral mode we can see (for the first time) the contribution of the charge exchange process π + π π 0 π 0 in the K ± π 0 π 0 π ± decay. This effect stimulated some theoretical work 30M events MC (no rescattering) Data K ± π ± π 0 π 0 4m π+ 2 4m π+ 2 M 2 (π 0 π 0 ) (GeV/c 2 ) 2 1 bin = GeV 2

31 (a 0 -a 2 ) determination in K ± π ± π 0 π 0 N. Cabibbo, hep-ph/ Phys. Rev. Lett. 93, (2004) one loop calculation N. Cabibbo and G. Isidori, hep-ph/ Phys. Rev. Lett. 93, (2005) two loops Crimea 2005 Giuseppina Anzivino 31

32 Crimea 2005 Giuseppina Anzivino 32 Fits to the cusp effect in K ± π ± π 0 π 0 Δ=(data-fit)/data One-loop exchange: χ 2 = 463/149 Cabibbo: PRL 93 (2004) Standard Dalitz-plot parametrization χ 2 = 133/139 for M(π 0 π 0 )>80 MeV/c 2 One and two loops: χ 2 = 159/147 Cabibbo, Isidori: hep-ph/ M 2 (π 0 π 0 ), (GeV/c 2 ) 2 The best fit obtained with two loops adding a small amount of pionium to improve the χ 2 Incl. π + π atoms: χ 2 = 144/146 M 2 (π 0 π 0 ), (GeV/c 2 ) 2

33 Crimea 2005 Giuseppina Anzivino 33 Preliminary result the pionium contribution has been fixed to the prediction: Z.K. Silagadze, hep-ph/ K + K π + + π + pionium + π + π (a 0 a 2 )m + has low sensitivity to pionium (a 0 a 2 )m + = ± 0.007(stat) ± 0.014(syst) ± 0.014(theor) In agreement with theory (a 0 a 2 )m + = ± (Colangelo 2001)

34 Crimea 2005 Giuseppina Anzivino 34 Conclusions Preliminary NA48/2 result (only 2003 data) on direct CP violating charge asymmetry in K ± π ± π + π decays is A g = (0.5 ± 2.4 stat. ±2.1 stat.(trig.) ±2.1 syst. ) 10-4 x 10 times better precision than previous measurements Further room to decrease systematic error (trigger efficiency) 2004 data contains another 2x10 9 K ± π ± π + π events, possibly with higher quality Design goal within reach K ± π ± π o π o neutral asymmetry: complementary, comparable sensitivity Cusp effect: determination of the ππ scattering lengths A lot of other interesting results coming (other CP asymmetries, rare decays)

35 SPARE SLIDES Crimea 2005 Giuseppina Anzivino 35

36 Crimea 2005 Giuseppina Anzivino 36 BR(K ± π ± π + π )=5.57% charged Kinematic variables Lorentz-invariants u = (s 3 -s 0 )/m π2 ; v = (s 2 -s 1 )/m π2 ; s i = (P K -P πi ) 2, i=1,2,3 (3=odd π); s 0 = (s 1 +s 2 +s 3 )/3. Centre of mass frame u = 2m K (m K /3-E odd )/m π2 ; v = 2m K (E 1 -E 2 )/m π 2. ± K 3π decays BR(K ± π ± π 0 π 0 )=1.73% neutral Matrix element parameterized in terms of slopes M(u,v) 2 ~ 1 + gu + hu 2 + kv 2 Measured quantity sensitive to direct CP violation: Slope asymmetry: A g = (g + -g - ) / (g + + g - )

37 Crimea 2005 Giuseppina Anzivino 37 NA48/2 narrow-band beams simultaneous, coaxial, focused Beam Positive Negative Primary proton momentum 400 GeV/c Duty cycle 5.2 s / 16.8 s Protons on target per cycle Production angle Beam acceptance ±0.36 mrad Beam momentum (60 ± 3) GeV/c Beamline length 102 m p / p per cycle (10 6 ) π + / π per cycle (10 6 ) K + / K per cycle (10 6 ) Decay region 115 m P K spectra, 60±3 GeV/c Pion decay products stay in beam pipe...

38 Theoretical predictions of A g L.Maiani, N.Paver 95 (2.3±0.6)x10-6 A. Bel kov 95 <4x10-4 Standard Model SUSY New physics G.D Ambrosio, G.Isidori 98 <10-5 E.Shabalin 01 <3x10-5 E.Gamiz, J.Prades, I.Scimemi 03 (-2.4±1.2)x10-5 E.Shabalin 05 (La Thuile 05) <8x10-5 G.D Ambrosio, G.Isidori, ~10-4 G.Martinelli E.Shabalin 98 [Weinberg model of ~4x10-4 extended Higgs doublet] I.Scimemi 04 >3x10-5 Crimea 2005 Giuseppina Anzivino 38

39 Crimea 2005 Giuseppina Anzivino 39 More cancellations (1) Double ratio cancellation of global time instabilities (rate effects, simultaneous beams): R U = R US R UJ R D = R DS R DJ R(u)=n (1+2 Δg U u) R(u)=n (1+2 Δg D u) (2) Double ratio cancellation of beam geometry difference effects: R S = R US R DS R(u)=n (1+2 Δg S u) R J = R UJ R DJ R(u)=n (1+2 Δg J u) (3) Fit with quadruple ratio: R = R US R UJ R DS R DJ R(u)=n (1+4 Δg u) Normalization Slope difference The fit result is sensitive only to time variation of asymmetries in experimental conditions on a time-scale of ~ 1 subsample

40 Crimea 2005 Giuseppina Anzivino 40 Break down of K ± π ± π + π - statistics Statistics selected for A g measurement, events x10 6 Dates Subsample s Achromat A+ Achromat A K + K K + K Total 54 Total events selected

41 Crimea 2005 Giuseppina Anzivino 41 Invariant πππ mass σ M =1.7 MeV/c 2 π μν π μν K + : 1.03x10 9 events No significant K : 0.58x10 9 events background

42 Crimea 2005 Giuseppina Anzivino 42 Fit linearity four supersamples SS0: Δg=(0.6±2.4)x10-4 χ 2 =39.7/38 SS2: Δg=( g=(-3.1±2.5)x10-4 χ 2 =29.5/38 SS1: Δg=(2.3±2.2)x102.2)x10-4 U χ 2 =38.1/38 SS3: Δg=( g=(-2.9±3.9)x10-4 U χ 2 =32.9/38 U U

43 Crimea 2005 Giuseppina Anzivino 43 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 R U *R D Systematic check: Reweighting K + events so as to equalise momentum spectra leads to negligible effect δδg=0.03x10-4 K Supersample 1 Supersample 2 SS 3

44 Theoretical predictions Crimea 2005 Giuseppina Anzivino 44

45 Experimental status Crimea 2005 Giuseppina Anzivino 45