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July 2001 Snowmass
A New Measurement of from KTeV
• Introduction• The KTeV Detector Analysis of 1997 Data• Update of Previous Result• Conclusions
E. Blucher, Chicago
The KTeV Collaboration: Arizona, Chicago, Colorado, Elmhurst, Fermilab, Osaka, Rice, Rutgers, UCLA, UCSD, Virginia, Wisconsin
1
/
/61
/Re0000
sL
sL
KK
KK
/ 0 direct CP violation
KL~ Kodd + Keven
“Direct”in decayprocess
“Indirect” from asymmetric mixing
Indirect vs. Direct CP Violation
00K K
Standard Model Prediction: Re() ~ (0-30)10-4
KTeV Datataking
• First result (PRL 83, 22 (99)) used from 1996 and from first 18 days of 1997 E832 run (1997a).• Current analysis is based on remaining 1997 data -- ~3 larger sample than first result.• 1999 run 1996 + 1997 with better systematics
1997 Reconstructed Mass Distributions(before background subtraction)
~1.6 MeV
~1.5 MeV
Vacuum beam (KL) Reg. Beam ("KS") 00 + 00 )stat
(106) (106) (106) (106) (10-4)1996 2.6 0.9 4.5 1.4 3.01997 8.6 2.5 14.9 4.1 1.796+97 11.2 3.4 19.4 5.6 1.5
Backgrounds
• Misidentified kaon decays– For K+-: KLe, KL
– For K00: KL000
• Scattered K events– From regenerator and final collimator
Main classes of background:
Vac
Reg
Vac00
Reg00
Misidentified K: Kl 0.090 0.003 K
0.11 ~0Scattered K: regenerator 0.073 0.25 1.13 collimator 0.010 0.010 0.12 0.09Total 0.100 0.087 0.48 1.23
Background levels (in %)
Center-of-energy for K Events
• KS+ distribution used to model scattering background in K .
2tp
• Improvements in procedure revealed a mistake in the background estimation for the published result: Re() = 1.7.
Yield after Background Subtraction
Vacuum Beam Reg. Beam
K 8,593,988 14,903,532
K00 2,489,537 4,130,392
Raw double ratio:
8593988 /149035320.96
2489537 / 4130392R
(no acceptance correction)
KL “KS”
Acceptance
Detailed Monte Carlo simulation based on measured detector geometry and response. Includes:• Accidental overlays• Full trigger simulation (L1,L2,L3)• For K00:
Geant-based shower library for CsI (showers cover 0.6750.675 m2)Detail photon veto simulation
• For K+-:Detailed drift chamber simulationMagnetic field mapCsI pion shower library
High statistics decay modes (e.g., K e,K 30) are used to check MC simulation.
Calorimeter Energy Scale
Final energy scaleadjustment based on regenerator edge.
Energy scale depends on PK.
Applying an energy-independentscale shifts Re() by -0.5 compared to nominal method.
Check E scale with different modes:K, K*KS,hadronic 2 production,K20 Dalitz, 30
Cross Checks of Energy Scale
Systematic Uncertainties for 1997
Uncertainty in (10
)Source of uncertainty
from
from
0.62
0.16--
0.280.250.20
0.330.150.370.53
0.16
1.370.660.380.371.06
0.480.08
--0.26
Data collection: Trigger (L1/L2/L3)Event recon., backgrounds: Energy scale Calorimeter nonlinearity Detector calibration alignment Analysis cut variations Background subtractionDetector acceptance: Limiting apertures Detector resolution Drift chamber simulation Z dependence of acceptanceKaon flux, physics parameters: Reg-beam attenuation m, s
Regeneration phase, screening
0.190.240.31
TOTAL 2.32
Calculating
0 0 0 0
0 0 0 0
N(Vac ) N(Reg )/
1 Acc(Vac ) Acc(Reg )Re( / ) 1 ,
N(Vac ) N(Reg )6/
Acc(Vac ) Acc(Reg )
Naively,
but regenerator beam is not purely KS.
KL - KS Interference Downstream of Regenerator
KTeV Preliminary Results: 6 1
14
00
(5262 7.7(exp) 13(ext)) 10
(8967.1 3.5(exp) 4(ext)) 10
44.11 0.72 (stat) 1.1 (syst)
0.41 0.22 (stat) 0.53 (syst)
S
m s
s
Geometry-only Monte Carlo
KLKL+
• Using acceptance correction from MC with perfect detector resolution (only geometry) shifts Re( by 12 compared to full MC. • Correcting for observed data/MC z slope reduces shift to ~2 .
Cross-check using reweighting method
• Provides check of Monte Carlo method• Statistically less significant than Monte Carlo method ().
Reweight KL decays to reg. beam distribution.
Reweighting Method Result
Based on preliminary study of correlation ofsystematic errors, difference between standard method and reweighting method is:
Re() = (1.5 2.1 (stat) 3 (syst))
Improvements in Analysis
• CsI Calibration• Drift Chamber calibration and
alignment• Neutral backgrounds• Apertures• Attenuation m, S
KTeV Results
• 1997 (independent from published result)
Re() = (19.8 1.7 (stat) 2.3 (syst)
0.6 (MC stat)) 10-4
• Updated 1996/1997a
Re() = (23.2 3.0 (stat) 3.2 (syst)
0.7 (MC stat)) 10-4
• Combined 1996+1997 Result
Re() = (20.7 1.5 (stat) 2.4 (syst)
0.5 (MC stat)) 10-4
= (20.7 2.8) 10-4
Conclusions
• KTeV results from 1996+1997 data:
Re() = (20.7 1.5 (stat) 2.4 (syst)
0.5 (MC stat)) 10-4
= (20.7 2.8) 10-4
New measurements of m, S, +, and • New world average:
Re() = (17.2 ) • Full KTeV data sample (96+97+99) will
reduce the statistical error on to
~ 1 10-4
significant work will be required to
reduce systematic error to similar level• Theory improvement needed to take full
advantage of this precision.
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