# Presentation: 005212

## Measurement of CP Violation in the KL Charge Asymmetry by KTeV

- The KTeV Collaboration
- Arizona, UCLA, UCSD, Chicago, Colorado, Elmhurst, Fermilab, Osaka, Rice, Rutgers, Virginia, Wisconsin
**University of Chicago**
** ****High Energy Physics Seminar **
- Hogan Nguyen
-  Fermilab
- April 23, 2001

## Asymmetric mixing:         “Explained” by a non-zero value for ImVtd  (or in the Wolfenstein parametrization)

## KL  e Charge Asymmetry

## KL Interferometry

-  Completely independent way to examine KL-mixing
-  Can be used to untangle effects in 

- In the 2 system, we are sensitive to a combination of CP and CPT violating effects from both the mixing part L and the decay amplitudes to 

## Relationship between  andL

- 

-  2amplitudes are sensitive to effects in both mixing and decay.
-  The charge asymmetry can be used to untangle these effects.

##  L Contribution from New Physics

- If there are CPT violating effects in Ke3 decays or the S = Q  rule is violated, then:
- L = 2 Re (  L – Y – X)
- Y = CPT Violation in  S =  Q  amplitude
- X= CPT Violation in  S =Q amplitude
- Comparisons with the 2 system would be ambiguous.   However, if we see any deviations, it would be quite exciting !

## S = Q Rule

- S = Q Rule

-  S = Q true in 1st order EW processes.

-  S = Q can be violated in 2nd order processes
-  Suppressed by O(10-7)   [Dib, Guberina, Luke  91]

## Measurements of KL l ChargeAsymmetry

- PDG 2000 Average:    3270 ± 120 ppm
- Current Best Measurement:  3408 ± 174 ppm
- CERN-Heidelberg 74  based on 34 Million Ke3s

-  26 years since the last measurement of L, and there has been tremendous advances in experimental techniques.
-  The measurement requires a careful control of systematics, beyond mere improvement in  statistics.
-  KTeV is a state-of-the-art experiment designed to precision studies of CP violation and rare kaon decays.
- We report here a preliminary result on L based on 298 million KL  e decays collected by E832 during the 1997 fixed-target run.

## KTeV Detector  (E832 Configuration)

- Overview of Strategy
- Since each of the two beams are not centered with respect to the detector, the detector has a significant geometrical asymmetry.
-     Rely on combining data collected with opposite magnet polarities in order to cancel this effect
-    Some biases are not related to acceptance, and so must be studied using other techniques.

##                          Overview of Analysis Requirements

- 2 track events from the vacuum beam uniquely consistent with KL  e kinematics  electrons/positrons  E/P > 0.925.  pe > 5 GeV.pions  E/P < 0.925.  pGeV.Tracks are in the fiducial area of the CsI  KL propertime†  > 10.5 S from the target298 Million KL  e (Ke3) events accepted
- † Low momentum solution used.

## 8 Configurations of Ke3 Decays 

-  eor e+
-  east or west  KL beam
-  + 411  or  – 411 MeV analysis magnet polarity
- Each configuration has a partner that has an identical geometrical acceptance:
- e+ East   +411 MeV    e  East  411 MeV
- e+  West  +411 MeV    e West  411 MeV
- e+  East   411 MeV    e  East  +411 MeV
- e+  West  411 MeV    e West  +411 MeV
- However, a given configuration and its partner do not necessarily have the same beam flux

- e+

- e

- 

- +

- CsI

## Illuminations of e± at the CsI

## 8 Configurations of Ke3 decays (cont)

- A single ratio is formed to cancel the acceptances, the beam fluxes, and beam optics.

*K*

*N*

*e*

*Br*

*west*

*e*

*A*

*west*

*K*

*N*

*e*

*Br*

*west*

*e*

*A*

*west*

*K*

*N*

*e*

*Br*

*east*

*e*

*A*

*east*

*K*

*N*

*e*

*Br*

*east*

*e*

*A*

*east*

*K*

*N*

*e*

*Br*

*east*

*e*

*A*

*east*

*K*

*N*

*e*

*Br*

*east*

*e*

*A*

*east*

*K*

*N*

*e*

*Br*

*L*

*L*

*L*

*L*

*L*

*L*

*L*

*L*

*L*

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

- Must Check if the Acceptances Really Cancel !

- Fluxes Cancel

## Systematic Effects and Corrections

-  Configuration acceptances do not cancel exactly
-  Particles and anti-particles behave differently in matter
-  Regenerative scattering by the beam
-  Backgrounds

- For each cut in analysis, measure:
- f +    inefficiency (background)  to e+  configuration
- f    inefficiency (background)  to e configuration
-  Correction to    =   ( f  f ) / 2

## Acceptance Checks of Configuration Pairs

- Illumination comparisons between configuration pairs.
- Differences consistent with small imperfections in the magnetic field reversal:
-  BY  component due to Earth’s magnetic field
-  small flaw in BX  component of magnet
- -3.1 ± 1.6 ppm correction for these  effects

- Slope due to Earth’s Magnetic Field

- Horizontal Illumination of Tracks at CsI

## Acceptance Checks of Configuration Pairs

- Vertical Illumination of Tracks at CsI

- small flaw in reversal of BX  component of magnet

## Detector Geometrical Bias 

- Combining data of opposite polarity is needed to cancel the detector’s geometrical bias.
- Bias shows up as a large difference between polarity settings:
- (411 MeV)  (+411 MeV) = 2192 ± 116 ppm
- Related to the complicated acceptance profile of the inner apertures  ( beam holes in the trigger scintillator and CsI).

## Detector Geometrical Bias (cont.) 

- Simulation reproduces the detector asymmetry:
- (411 MeV)  (+411 MeV) = 1889 ± 178 ppm
-  within 1.4  of the data
-  simulation recovers L input value

## Detector Geometrical Bias (cont.) 

## Particle/antiparticle differences in matter 

-   e+ edifferences:
-   e+ annihilation
-   -ray production differences between Bhabha (e+e) and  Moller (e e) scattering
-     differences:
-    Isospin dependence in nuclear interactions
-    Detector has an unequal number of protons and  neutrons
- These effects bias the reconstruction of track kinematics and particle identification.

- p versus  p total cross section (mb) vs p

- † PDG 2000 Compilation

## Particle/antiparticle differences in matter

- e± biases:   geant simulation/data
- ± biases:  measure directly with data
- In comparison to previous measurements, our biases will be smaller due to:
-  a vacuum decay tank
-  an ultra-thin magnetic spectrometer
-  higher momentum spectra
-  TRD in E799 data, crucial for understanding the p.i.d. biases in rest of detector

## KTeV Detector Material

- Detector                          material               proton excess        _element                           (mg/cm__2__)                  (mg/cm__2__)_
- Vacuum Window                   47.0                           2.7
- Wire Drift Chambers             78.0                           4.2
- Helium Bags                        483.8                                 0.0
- Trigger Scintillators           1131.5                                      83.8
- CsI                                       226.5 •103               36.9 •103
- Pb Absorbers                        113.5 •103              23.7 •103
- Fe Absorbers                      3148.0 •103                   216.8 •103
-     e+ ebiases depend on the magnetic spectrometer material.
-   biases depend on the proton excess and deficit.

## and Response in CsI 

- and Response in CsI
-  The requirement of E/P < 0.925 rejects more than

## and Response in CsI (cont.)

-  Measured the E/P shape of pions using KL  
- <f + > = 588414 ppm         <f > = 619614 ppm       Correction = 156 10 ppm

## ± lost due to interactions in front of CsI 

- ± lost due to interactions in front of CsI

- Hadronic interactions upstream of CsI,  most likely  in the trigger scintillators, causing failures in the matching of tracks to clusters.

## ± lost due to interactions in front of CsI   (cont)

- ± lost due to interactions in front of CsI   (cont)

- f + = 4245 ±14 ppm
- f  = 4352 ±14 ppm
- Correction  =  54± 10 ppm

##  Punch Through Correction

- To reduce the trigger rate, the trigger required no activity in muon veto.
- This vetoes pion decay-in-flight and punch through, which could cause a bias.

- decay and punch through sample

##  Punch Through Correction

- Trigger vetoes mainly decay-in-flight
- (expect no bias to delta)
-   punch through losses are smaller, but can in priciple can cause a bias

-  veto probability

## e+ and e Response in the CsI 

- Analysis requires e  E/P > 0.925
- Though no bias is expected, we use the data to limit any possible effect.

- Correction = 19  18  ppm
- Consistent with no bias

## e+ and e Response in the CsI  (cont)

- TRD used to remove pion background
- correction (E799) = 3  66 ppm

- e+ and e Response in the CsI  (cont)

## rays correction

- Correction of  ppm   to account for  Moller/Bhabha scattering differences

- -ray effect on kinematics

- Moller (ee)   vs  Bhabha (e+e)  scattering

## e+ and  absorption in trigger scintillator

- e+ and  absorption in trigger scintillator

- e+

- 

- 1 cm thick trigger scintillator

- 

- 

- n

- 0

- Trigger loss due to reduced energy deposit in scintillator

-  e+ annihilation loss estimated using Geant
-  11 ± 1 ppm
-  p  n  loss estimated from Inagaki et. al.   (NIM A359 - 1995)
-  2 ± 1 ppm

## Residual Production Target KS-KL Interference Correction

- Analysis removes events with propertime S since they have large KS-KL interference effects.
- Correction of 12  1 ppm  assigned to account for residual interference and propertime misreconstructions.

- † Low  solution used

## Summary of Corrections

- difference in CsI                     156  10
- lost in trigger scintillator            54 
- lost in spectrometer                      3   3
- punch through                            34  40
- ee difference in CsI                         19  18
- ray production                               8.5  4.3
- e+ annihilation in spectrometer            11  1
- background               0.5  0.7
- Target/absorber interference              12  1
- KL scattering in final collimator        1.2  2.3
- KL scattering in regenerator                   0  0
- _B-field reversal mismatch                 ____3.1 ____ 1.6_
- Sum                                                   97   46  (ppm)

## Corrections for Cern-Heidelberg 74

- Interference in He decay volume        62  12
- absorption in H2 Cerenkov             61 
- punch through                            169   41
- ray production                                  20
- e+ annihilation in spectrometer            28  1
- background                                         5  2
- Beam interaction in decay volume        2  15
- Accidentals                                             8  5
- KL regeneration                                  17  5
- _Cerenkov pmt pulse variation              ____1 ____  2 _
- Sum                                                    50  (ppm)

## Slide 36

## Preliminary Result for KL  e Charge Asymmetry

- 298 Million KL  e collected in 1997 run of KTeV-E832
- Raw  L          =   3417 ± 58  ppm
- Correction   =97 ± 46 ppm
-   L   = 3320 ± 58(stat) ± 46(sys)  ppm
-         =  3320 ± 74(comb)  ppm
- 2.4 x more accurate than the previous best result  (CERN-Heidelberg 1974)
- Excellent agreement with all previous measurements

- New Average:  3305 ± 63 ppm    (2 =  4.2/6 d.o.f.)

- Preliminary Result for KL  e Charge Asymmetry

## Other Systematic Checks

- Consistency Within Data Set

## Other Systematic Checks (cont)

- Dependence on e  and  momentum

## Comparison to KL  and limit on CPT Violation 

-        _Parameter       PDG2000 averages_
-                    2276  17  ppm
-          |  17 ppm
-  0.5 °
-  1.0 °
-       2Re( 37 ppm
-       2Re( 60 ppm
- L                                  3305  63 ppm
-                              (PDG avg and this result)

- Re(a)  = 2/3 Re Re–ReL
-            ppm   (assuming S=Q)

- 

- Comparison to KL  and limit on CPT Violation

## Slide 41