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Status report on analysis of BR( K S  p + p - p 0 )

Status report on analysis of BR( K S  p + p - p 0 ). A. Antonelli, M. Moulson, Second KLOE Physics Workshop, Otranto, 10-12 June 2002. Notes on the decay K S  p + p - p 0. Branching ratio not well measured at present: CPLEAR ’97 2.5 + 1.2 - 1.0 + 0.5 - 0.6 × 10 - 7

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Status report on analysis of BR( K S  p + p - p 0 )

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  1. Status report on analysis ofBR(KS  p+p-p0) A. Antonelli, M. Moulson, Second KLOE Physics Workshop, Otranto, 10-12 June 2002

  2. Notes on the decay KSp+p-p0 • Branching ratio not well measured at present: • CPLEAR ’97 2.5+1.2-1.0+0.5-0.6×10-7 • E621 ’96 4.8+2.2-1.6  1.1×10-7 • PDG Avg. 3.2+1.2-1.0×10-7 • Phenomenology & cPT 2.4  0.7 × 10-7 • Part of amplitude is CP violating: • L odd CP(p+p-p0)I=0,2 = +1 Centrifugal barrier • L even CP(p+p-p0)I=1,3 = -1 Violates CP • h+-0 = A[KS  (p+p-p0)CP-]/A[KL  p+p-p0] • CP– component extracted by integration over Dalitz plot, traditionally in interference measurements • BR[KS  (p+p-p0)I=1] ~ BR[KS  p0p0p0] ~ 10-9 • Unlikely to be observed at KLOE

  3. General analysis considerations • 0.34 events produced per pb-1: • 58 events in 170 pb-1 with 100% efficiency • Background rejection paramount: • KL reconstruction needed for signal ID • Use both KL crashes and vertices in DC • No reliable KS tag using DC vertices available: • New EVCL algorithm to include KS  p+p-p0 events in ksl stream starting from KS • Symmetric treatment of KL crash/vertex • Not a tagged measurement in the usual KLOE sense • Acceptable to rely on MC for most efficiencies: • KL crash studied in KS  p+p- • KS  p+p- additionally useful for normalization

  4. Efficiency evaluation • MC does a miserable job at simulating KL crash: • Estimate KL crash efficiency from KS  p+p- events in data Useful mainly for estimating sensitivity For the actual BR measurement: Fundamental assumption: KL crash detection probability cancels from ratio Detection probability for KL vertex more likely to depend on KS decay mode, but ratio of efficiencies more amenable to simulation

  5. Data and MC samples • Data: All 2002 dk0 DST’s as of 5 Jun 2002 • Ldt, VLAB: 125.7 pb-1 • 46.0M total KL tags • Useful control sample (downscaled 50×) • MC kspppmp0, dedicated production • Signal 100K events • MC all_phid, 10 M events • Background Runs 81-160 (2.4 M events, DBV-8) • Runs 161-439 (7.6 M events, DBV-13) • 1.42M total KL tags, effective Ldt  3.9 pb-1 • Dedicated Ntuples made from DST’s and MC files • DST Processing rate: ~0.6 pb-1/CPU hour • Volume of MC + data: 2.2 GB total

  6. Control sample: KS  p+p- • Unique vertex at origin, zero net charge: • rxy < 5 cm, -20 cm < z < 20 cm • M - MK < 10 MeV, P - PK < 5 MeV • One track (or daughter of recognized kink) must analytically extrapolate to cluster: • d < 30 cm, E > 50 MeV • Associated cluster used to set event t0 • s(KL tag): • 366 nb, using Ldatarec • 436 nb, using sf = 3.1 mb, eKLtag(MC) = 60.7%

  7. Event classification algorithm • Unique vertex at origin, zero net charge: • rxy < 5 cm, -20 cm < z < 20 cm • Two prompt neutral clusters: D(t-R/c) < min(5s, 2ns) • For each pair of clusters (p0 candidate): • Close kinematics using vertex, mKS, mp0, and cluster directions • Set t0 using clusters • Search for KL crash in 20 cone • If no KL crash, search for KL vertex in 20 cone • Veto vertices using tracks in KS vertex tree in post-reconstruction analysis • If more than one p0 candidate, correct candidate identified on basis of best alignment of KL direction and tagging momentum

  8. Reconstruction of p+p- vertex e = 36.3% Track momenta are low and anticorrelated Vertex reconstruction efficiency dominated by acceptance

  9. Reconstruction of p0 clusters E > 25 MeV e = 73.3% • Distributions are for clusters corresponding to MC truth, as reconstructed • Cut efficiencies shown evaluated in cascade • Expect overall p0 efficiency e  70% • Find e = 59.9% • Bug in tag! • Cut on Dt instead of D(t-R/c) r > 60 cm e = 96.6% E1 + E2 >100 MeV <250 MeV e = 99.98% cos(q12) > 75 e = 98.6 %

  10. Simulation of KL crash: b* KS  p+p-, MC KS  p+p-, Data b* b* Stop decays? b* spectra for MC and data completely different: Evaluate KL crash efficiency from KS  p+p-events in data

  11. Simulation of KL crash: b* Upper cut on b* Lower cut on b* Relative efficiency: Simulation of KL crash same for both KS decays t0 correct in each case Overall efficiency: 0.2275 0.2

  12. Simulation of KL crash: qLS Relative efficiency: cos-1 0.99 8.1° Significant influence from reconstruction of KS vertex qLS qLS Overall efficiency: Relative efficiency: Extent/position of KL crash in MC not as bad as timing qLS

  13. Preselection efficiency: KL crash

  14. FV for KL vertex analysis • Fiducial volume for KL vertex: 23.2% • selects region in which reconstruction efficiency high and constant • plays important role in eliminating backgrounds (mainly K+K-) eMC(KL vertex | KS  p+p-p0) eMC(KL vertex | KS  p+p-p0) 30 cm < rxy < 150 cm -100 cm < z < 100 cm

  15. Preselection efficiency: KL vertex Vertex efficiencies include acceptance and BR for KL decays with vertices

  16. Principal backgrounds Emphasis on KL crash sample for background elimination studies Cuts applied to KLvertex sample (almost) as an afterthought Crash background studied by using vertex sample and/or relaxing b*, qLS cuts • Composition of MC background in KL crash events: • 26 K+K- events • Both K+, K- both reconstructed and make vertex • One K or its daughter makes an unassociated crash cluster • Other K decays to pp0, 2 g’s fake p0 at origin (no prompt g’s in event) • One K reconstructed, other decays to pp0 before DC • 44 KS  p0p0 events • Conversions on DC wall: vertex forms “fish” and tracks cross at origin • Dalitz decays: p0  e+e-g

  17. Background elimination Signal KS p0p0 K+K- Angle between p+ and p-: cos q+- Angle between gg in p0 rest frame: cos y Cuts made: |cos q+-| < 0.85 cos y < -0.85 • Signal • 2646  2252 Crash • 3545  2922 Vertex • Background • 70  11 Crash • 439  69 Vertex

  18. Background elimination Signal (crash & vertex) Background to vertex Background to crash DE2 = (E2 – E2close)/s(E2) DE1 = (E1 – E1close)/s(E1) Cuts made: DE12 + DE22 < (2.5)2 • Signal • 2252  2103 Crash • 2922  2731 Vertex • Background • 11  3 Crash • 69  28 Vertex • q+- and y cuts applied

  19. Status of analysis Mmiss Mmiss KL crash KL vertex Efficiency: 3.2% Events expected: 1.4 Events found: 11 Events in MC bkg: 0 Efficiency: 1.5% Events expected: 0.6 Events found: 114 Events in MC bkg: 6

  20. Summary and outlook • Bug in EVCL tag a disappointment, but: • KL crash sample included in ksl stream by KL crash tag • KL vertex sample in 2001 data useable for study, at least • Only ~10% loss in efficiency • Presumably amenable to simulation • Fixed version of tag ready for DBV-14 • 2002 data taken so far may be reprocessed at some point anyway

  21. Summary and outlook • Many additional handles on background: • KS  p0p0 Prompt cluster multiplicity • Eliminate p+p- candidates that cross on DC wall • K+K- More work on analytical expansion of p+p- vertex tree • Use ADC information • Much additional work on physics backgrounds to do • Existing MC samples of K+K- • Dedicated MC generation of KS  p0p0 with p0  e+e-g and/or conversions • Work on noise and machine background not yet started • Prospects seem good for eliminating background entirely

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