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A direct search for K s  3 0 decay at KLOE

M. Martini, INFN Frascati, for the KLOE collaboration. 7-11 June 2004. INFN, LNF. A direct search for K s  3 0 decay at KLOE. K S 3 p 0. This decay has never been observed but is allowed by SM [CP violating; CP(K S )=+1, CP(3 p 0 )=-1]. From theoretical point of view:.

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A direct search for K s  3 0 decay at KLOE

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  1. M. Martini, INFN Frascati, for the KLOE collaboration. 7-11 June 2004 INFN, LNF A direct search for Ks 3 0 decay at KLOE

  2. KS3p0 • This decay has never been observed but is allowed by SM • [CP violating; CP(KS)=+1, CP(3p0)=-1]. • From theoretical point of view: All KLOE statistics  0.9 signal events produced (e=100%) Current published limit: BR(KS3p0) < 1.4x10-5 (SND, PL B459 1999) • The observation of this decay would confirm CP violation in KS • system. • Uncertainty on KS3p0 amplitude limits precision on CPT test • (Bell-Steinberger relation).

  3. KS3p0 KLCRASH b0.22 • KS tagging: from KL interactions on EMC KLCRASH(e~30%) This algorithm searches KL which interact directly on EMC. KLCRASH calorimetric clusters are selected using energy and b*. • High bkg rejection needed: • major expected background: KS2p0 (BR31%) + 2 fake photons either from • shower fragments or accidental overlap with clusters from machine • background.

  4. -- KS3p0 (MC) -- MC BKG  DATA • A kinematic fit is applied on the • Ks side requiring the conservation of • 4-momentum (NDOF=11). •  c2FIT Photons counting and Kinematic fit DATA=450pb-1 (2001+2002); MC =540pb-1 (all available statistics) • Preselected signal sample (KLCRASH and 6 photons): 39728 events • Normalization Sample (KLCRASH • and 4 photons): 23.5x106 events c2FIT< 30 is not enough (2/3 of bkg rejected)! Other discriminating variable have to be used: (c22p, c23p)

  5. The c22p KLCRASH + 4 photons  DATA -- MC The c22p is built selecting 4 out of 6 clusters which satisfies better the kinematics of KS  2p0 • The parameters used are: • mass distribution • opening angle between pions in • KS C.M. frame • 4-momentum conservation The calibration is done using KS2p0sample.

  6. The c23p  KS3p0 MC  DATA MCbg The c23p is based only on the 3 “best reconstructed” pion masses

  7. c23p c23p DATA-MC comparison • We define a signal box and 5 control boxes.

  8. c23p DATA-MC comparison • A small fraction of the background (3%) is constituted by fake KLCRASH • (KSp+p-) + KL3p0. Our MC well reproduces such fakes.

  9. Analysis summary • To increase bkg rejection we use • these analysis cuts: • Track veto: • to reject events with tracks • coming from IP. • DECUT: • After finding the four photons satisfying the kinematics • of KS2p0, the residual energy of the KS is measured: • E(KS) = EKs- Eg • 2 bodies decay  E(KS)~ 0 MeV • 3 bodies decayE(KS)~ Mp Rejecting events with: DE(KS)<10 MeV

  10. c22p all c22p>40 • TRKveto 14<c22p<40 c22p<14 • DECUT + c2FIT Analysis summary • No Cuts

  11. Candidates • We have optimized our analysis cuts looking at MC • c22p • c23p • c2FIT • DECUT c22p  DATA  MC c23p • e = (22.6±0.8)% • We count 4 candidates with • 3.2±1.4±0.2 • expected by MC • N3p ≤ 5.76@90% CL • Folding the proper bkg • uncertainty.

  12. Upper Limit determination • Using this upper limit we can calculate some parameters • directly related to CP and CPT test. Using PDG values and our limit: • We derive from our analysis: This improves of a factor ~70 the previous limit.

  13. Related parameters • Our limitcan be compared with NA48 • - interference term between KSKL in the • same final state (proportional to Re(h000) • and Im(h000)). KLOE versusNA48 (hep-ph/0310152) SND 90% CL • Assuming CPT invariance and ignoring • transition into I=3 final state: • Re(h000)=Re(e). • We can calculate the contribution of • 3p0 decay at Im(d) (Im(d)000) using • unitarity relation (eS,L=e±d):

  14. Related parameters • Since these are 2 opposite values, • we obtain the limits: Im(d)000 becoming negligible on the determination of Im(d) f000(rad) This value is based only on unitarity and increases the precision on Im(d) of  2.5 • The most conservative approach is to assign a phase toa000 • which gives the minimum and maximum values of Im(d)000:

  15. Conclusion • A direct search for KS3p0 decay has been performed • using the whole statistics collected at KLOE during 2001- • 2002 data taking. • We set an upper limit on the branching ratio to • 2.1x10-7 @ 90% C.L. • This reduce of a factor ~70 the previous limit. • This limit makes completely negligible the contribution of • this decay on Im(d) reducing of a factor~2.5 the • uncertainty on Im(d)

  16. Additional information

  17. The c23p  DATA  MCbg Due to DAFNE bkg which satisfy the trigger. Reduced by our Splash Filter: rmean > 70 cm Emean > 70 MeV “SPLASH” events Good agreement DATA-MC in all the regions  DATA MCbg  DATA MCbg  KS3p0 MC c2FIT < 30 Thec23pis based only on the 3 reconstructed pion masses

  18. DATA-MC comparison DATA ? FAKEKCRA MCbg In the data, a new category of BKG events (not simulated by the standard Kcrash MC) appears. The simulation takes into consideration only KL decaying after a cilinder bigger than DCH and smears the KL MC direction with the Kcrash resolution observed in data.

  19. The Kcrash fake background • A small fraction of the background (3%) is constituted by • a fake Kcrash and a KL 3p0 decay. • The fake KLCRASH is generated mostly by KSp+p- event, • where the pions, spiralizing around the pipe, interact on • the quadrupoles creating late clusters in the event. • We have add the simulation of these events in the MC and • calibrated their quantity.

  20. DATA-MC “fakes” comparison • Outside the • KLCRASHb* • windows DATA are • dominated by fakes • In the central band • there is also a small • percentage of • KLCRASH.  DATA -- MC Fake  DATA -- MC Fake EKLCRA EKLCRA EKLCRA b* Comparing events with tracks from IP. b*  MC fakes only (i.e. no Kcrash)  DATA dominated by fakes

  21. The agreement between DATA and MC is better than 10% in all the regions. c22p Region Nev (Data) Nev (MC) SBOX 337  18 309.5  16.3 UP Cup UP 493  22 479.8  21.2 DOWN 382  19 426.5  14.4 CSbox 5502  74 5595.1  51.0 CSox Cup 11249  106 11744.1  74.0 SBOX Cdown 24396  156 23844.9  103.8 DOWN Cdown c23p DATA-MC comparison

  22. Monte Carlo calibration -- DATA -- MC -- KCRAMC -- FAKEKCRA Normalized with the full sample Ng=6 (A) (B) Mean KCRAMC 0.446 0.447 0.447 FAKEKCRA 1.002 0.895 0.949 c23p < 3.5 FIT data using shapes from MC: ID(DATA) = a1 ID(MC)+ a2 ID(FAKES) Two different regions used: c23p < 3.5 (A) c23p < 8.8 (B) After removing SBOX

  23. DATA-MC comparison (I) c23p all c23p > 3.5  DATA -- MC  DATA -- MC c23p < 3.5 c23p< 8.8  DATA -- MC  DATA -- MC Normalization with the mean values of the coefficients

  24. DATA-MC comparison (II) c22p all c22p > 40  DATA -- MC  DATA -- MC 14<c23p<40 c23p< 14  DATA -- MC  DATA -- MC MC KS 3p0, BR=1.4x10-5 Normalization with the mean values of the coefficients

  25. The track veto MC SignalDATAMC bkg c22p all c22p > 40 14<c23p<40 c23p< 14 All events with tracks from IP are rejected rPCA < 4 cm ; |zPCA| < 10 cm MC signal, BR(KS3p0)=1.4x10-5

  26. The topological DECUT -- MCBG no 2fitcut  MCBG 2fit < 30  MCBG 2fit + TRKveto  MCSIG DECUT < 10 MeV After finding the four photons satisfying the kinematics of KS2p0, the residual energy of the KS is measured: E(Ks) = Eks- Eg • Summarizing: • c2FIT<30 • TRKveto • Sbox

  27. DATA/MC evaluated in Signal and control boxes varying the cut on the kin. Fit: • c2FIT = 500, 400, 300, 200, 100 • No dependence observed as a function of this cut Syst.error on BKG Syst1 =  9% (Calibration) Syst2 =  10% (Data/MC) Systot =  14 % Analysis efficiency and bkg systematics Sample of 10 Kevents KS 3p0 e3p = ( 21.30.7stat 0.3syst )

  28. Optimization of UL • We have optimized the UL by MC looking at 4 different • parameters: • c2FIT • c22p (both sides) • c23p • DECUT • The best choice of the cuts is the following (CUTMAX): • c2FIT < 31 • 14 < c22p < 60 • c23p < 3.7 • DECUT > 37 MeV • Optimization is done by minimizing the expected 90% CL • upper limit (ref. J.F.Grivaz, F. Le Diberder LAL 92-37) • All is done varying cuts on MC only.

  29. NA48 interference function K0 K0 Dilution factors KL decay KS decay Interference KL-KS

  30. Related parameters • The more conservative approach is to give ata000a phase which • maximizeIm(D)000: • Because there are 2 opposite values, we obtain a range for • |Im(D)|000@ 90% C.L.: Im(D)000 KLOE Im(D)000 CPLEAR Im(D)pln This value is obtained using only unitarity, and increases the precision on CPT test. Im(D)00 PDG 03 Im(D)+- x10-5

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