1. CP Violation and Mixing in Charm Meson Decays from BABAR
Chunhui Chen
Iowa State University
On behalf of Babar Collaboration
The 4th International Workshop on Charm Physics
Beijing, China
Oct. 21st – 24th 2010

2. Outline of the Talk Introduction Mixing and CP Violation in D0 decays
D0 K+π-, K+π-π0,
D0 KSπ+π-,KSK+K-,
Mixing in D0 Lifetime Difference
Direct CP violation in D Meson decays
D+ KSπ+ (New result)
CP Violation in Triple Product
Summary

3. Mixing and CP Violation
Flavor oscillation in neutral D meson systems:
Mass eigenstates ≠ Flavor eigenstates
Mixing parameters to study flavor oscillation
CP violation and mixing small in SM: x,y ~ 10-2, CPV < 10-3
Sensitive to new physics effects: |x|>>|y| or significant CPV

4. Mixing & CPV in WS D0 Decays
“f”:
Mix (D0D0)
(D0f)
Tag flavor at production using: D*+D0π+
RS D0 decay: D0K-π+,K-π+π0,K-π+π-π+
Doubly Cabbibo suppressed WS decays
WS decay rate (no CP violation, 2nd order of in x and y)
Ambiguity from strong phase diff (δf) between RS & DCS decays
If CP violation allowed: x’ and y’ different for D0 and D0 decay

5. Mixing & CPV Results in WS Decays
Based on 384fb-1 data
Significant evidence of mixing
D0K+π-:3.9σ, 1st evidence of mixing in BaBar PRL98:211802(2007)
D0K+π-π0: 3.2σ, PRL103:211801(2009)
Time-dependent Amplitude analysis
Phase dependent on the position of DP position
DP phase from model but an unknown phase δKππ
No evidence of CP violation

6. Mixing and CPV in D0KS0+-/KS0K+K-
Similar to D0K+π-π0 time-dependent amplitude analysis:
Self-conjugate final state (sum of CP-even/odd eigenstates)
“Unknown” overall strong phase “δKππ” is zero
So xD and yD can be determined directly
Dalitz plot describe by decay amplitude A(S+,S-)
S+=m2(KSh+), S-=m2(KSh-)
If no direct CPV:
Can also determine |q/p| and Arg(q/p)
Method pioneered by CLEO: PRD72:012001(2005)
Tag initial flavor the D*+D0π+
D0KS0+-: 540K events (98.5% purity)
D0KS0K+K-: 80K events (99.2% purity)
468.5fb-1 data
Published in PRL105:081803(2010)

7. Mixing and CPV in D0KS0+-/KS0K+K-
D0KS0K+K-
No CP violation:
x=(0.16±0.23±0.12±0.08)%, y=(0.57±0.20±0.13±0.07)%
The 3rd error is Dalitz model uncertainty
Allow CP violation (cross check):
x+=(0.00±0.33)%, y+=(0.55±0.27)%
x-=(0.33±0.33)%, y-=(0.59±0.28)%
Most precise
Measurement
to date

8. Mixing in D0 Lifetime Ratio Analysis
In absence of CP violation
Lifetime diff. between CP-even/odd final state leads y
K-π+: Assume mixed CP, τ mean of CP-even/odd state
h+h-=K+K-, π+π-: CP-even state
Fit t to exponential with resolution (KK not same as Kπ) K KK
± 1.00 fs
± 0.38 fs
yCP = [1.16  0.22 (stat.)  0.18 (syst.)]%
(4.1  evidence, tagged + untagged )
Phys.Rev.D80:071103,2009
384 fb-1 data

9. Direct CPV in D Meson Decays
Direct CP violation is expected small in SM for D decays:
Significant non-zero CPV indicate NP
2005
2010

10. Direct CP Violation in D+KS0+ Decay
A CP asymmetry ±0.006% in SM from neutral Kaon CPV
Possible enhancement or cancellation due to NP
Large decay branching fraction: ~ 1 million signal yield
Expect small statistical uncertainty ~ 0.1%
Possible to see non-zero SM CP asymmetry
Important reference/SM candle for future direct CPV search
Need control systematic uncertainty to ≤ 0.1%
Charged track reconstruction asymmetry
A major systematic uncertainty in all direct CPV analysis
Very difficult to measure directly in data
Only possible for few decay & limited application
D0K+K-,π+π- (control sample D0K-π+): very low pT track
τ decay sample: track pT unknown ……
A new method to measure charge track reco asymmetry in data:
Possible application for “all” direct CPV search

11. Signal Reconstruction of D+KS0+
Data sample: 470fb-1
Blind analysis:
Event selection, sys error fixed before seeing actual result
KS reconstructed from two charged tracks and combined with
another track to form a D+ candidate
KS decay length significance ≥ 3
p*(D+)≥2GeV:
Keep most D+ from ccbar with reasonable combinatorial bg
Some D+ from B decays, they are signals as well
Pion candidate (3rd track):
pT>400MeV within acceptance
Kaon, proton and electron PID veto
Reduce track selection asymmetries
Motivated by measurement of tracking reco asymmetry
See later slides for details
Final selection optimized using Boost Decision Tree algorithm
τ(D+), Lxy(D+), p*(D+), p(KS), pT(KS), p(π+), pT(π+)

12. Signal Distribution Signal PDF: Triple Gaussian with 2 equal means
BaBar
Preliminary
BaBar
Preliminary
Signal PDF: Triple Gaussian with 2 equal means
Reflection background: DS+KS0K+, small, fixed from MC
Combinatorial background: 2nd order polynomials

13. Track “Selection” Asymmetry
Goal: Correct any charge asymmetries from detector effects
New method developed:
ϒ(4S) produced at rest and decay almost isotropically
Expect equal number of positively and negatively charged
tracks for a given direction and momentum
Observed difference caused by tracking selection
New method developed:
ϒ(4S) produced at rest and decay almost isotropically
Expect equal number of positively and negatively charged
tracks for a given direction and momentum
Observed difference caused by tracking selection
Identical track selection using in analysis
pT>400MeV, reduce tracking asymmetry effect
PID veto: select pion tracks
The measured “selection asymmetry” include 2 effects
Asymmetry from tracking reconstruction
Asymmetry from PID (can be measured independently)

14. Measured Track “Selection” Asymmetry
Based on 8.5fb-1 on peak and 9.5fb-1 off peak data
Ratio of detection efficiency for negatively charged and positively charged pions is produced as a function of pion momentum and polar angle
Correction applied to negative D candidates according to their “third track” momentum
Small correction dominated by statistical fluctuation

15. CP Asymmetry Extraction
After removing the charge asymmetry by detector effects, the measured asymmetry can be written as:
Forward-backward asymmetry AFB may induce faked ACP
Different acceptance in forward and backward regions
Exact analytical expression of AFB is unknown
AFB is an odd function of the D+ polar angle Θ in CMS
Each pairs of symmetric cosΘ bins produces one ACP value
Combined to give the final value by a χ2 minimization

16. Simultaneous Fits
Simultaneous binned fit to extract D+ and D- signal yield in each
cosΘ bin
All PDF parameters (78) in each bin left float in the fit
Calculate A in each cosΘ to extract ACP and AFB
Fit validate by toyMC and full simulation: no bias
BaBar
Preliminary
BaBar
Preliminary

17. Fit Result New ACP=(-0.39±0.13)% Statistical error only BaBar BaBar
Preliminary
BaBar
Preliminary

18. Systematic Uncertainties
Bias due to small contamination of kaon, electron and muon
Asymmetry due to kaon PID requirement (measured in data)
Estimated to be +0.05%
Correct central value by 0.05%
Assign 0.05% as the sys error
Statistical error from charge asymmetry measurement
0.05%, can be reduced using large sample
Neutral Kaon regeneration in detector
0.05%, similar to Belle: [arxiv: ]
Other contributions: 0.011%
Signal and background PDF
cosΘ binning
Statistical of MC sample
Total systematic uncertainty: 0.1%

19. Final Result ACP=[-0.44±0.13(stat)±0.10(sys)]% (BaBar 470fb-1)
ACP=[-0.71±0.19(stat)±0.20(sys)]% (Belle 673fb-1)
2005
2010

20. CP Violation with T-Odd Correlation
Form T-odd correlation and difference of asymmetries
Look for T-violation assuming CPT invariance (Bigi hep-ph/ )
BaBar: D0K+K-+-, 470fb-1 data, PRD (2010)
If T violation:
Consistent with no CP violation

21. Conclusion
Studies of charm mixing and CP violation have been very active in Babar
Summary of results from BaBar
A new result of CPV measurement in D+ KSπ+
ACP=[-0.44±0.13(stat)±0.10(sys)]%
A new method to measure charge particle detection
asymmetry from data
General application to direct CPV in D decays
Expect more exciting analyses/results from BaBar in the future

22. Backup

23. The PEP-II B Factory (SLAC)
e- (9.0GeV) x e+ (3.1 GeV) ECM = 10.58GeV; bg = 0.56
Positron
(3.1 GeV)
Electron
(9 GeV)

24. BaBar Detector e+ (3.1GeV) e- (9GeV)
Detail see: Nucl.Instrum.Meth.A479:1-116,2002
ElectroMagnetic Calorimeter
6580 CsI(Tl) crystals
1.5T solenoid
e+ (3.1GeV)
Detector of Internally Reflected Cherenkov light (PID)
144 quartz bars
11000 PMTs
Drift Chamber
40 layers
e- (9GeV)
Silicon Vertex Tracker
5 layers, double sided strips
Instrumented Flux Return
Iron / Resistive Plate Chambers or Limited Streamer Tubes (muon / neutral hadrons)
Trigger
L1 ~2KHz, L3 120Hz
Trigger eff. ~98%
Collaboration founded in 1993
Detector commissioned in 1999
80 institutes, 11 countries, ~600 physicists
High luminosity offer great challenges to our detector

25. Collected Data Sample PEP-II Peak lum: 12.0691033cm-2sec-1
Deliver for BaBar >553fb-1

26. Mixing & CPV Results in D0K+π-