1. Charm Mixing, CPV and Rare D0 decays at BaBar
William S. Lockman
Representing the
Collaboration
Introduction
Lifetime difference measurement
Mixing in Wrong sign D0→K+p- and D0→K+p-p0 decays
Time-integrated CPV measurements
Radiative D0 decays
Summary
PANIC 2008: International Conference on Particles and Nuclei November 9-14, Eilat, Israel

2. Introduction
Mixing and CPV in the D0 system were discussed over 30 years ago
A. Pais and Treiman, Phys. Rev. D12, 2744 (1975)
But evidence for D0 mixing only recently observed:
Of all the neutral mesons, the D system exhibits the least mixing
New Physics signature: CPV
BABAR: PRL (2007)
D0K decay time analysis
3.9
BELLE: PRL (2007)
D0KKvs K lifetime difference analysis
3.2
BELLE: PRL (2007)
D0Ks time dependent amplitude analysis
2.2
CDF: PRL 100, (2008)
3.8
BABAR: PRD 78, R (2008) 3
BABAR: arXiv:0807, 4544 (2008)
D0K0 time dependent amplitude analysis
3.1
all mixing results combined by HFAG:
~10
short distance DC=2 suppression:
D mixing loop involves d-type quarks
b quark loop suppressed:
s and d quark loops: GIM suppressed
Mass difference ampl. < O(10-5)
Long distance amplitudes predominant but hard to quantify
Recent estimates: |x| ≤ 1%, |y| ≤ 1% consistent with current observations pp KK Kp
...
A. Petrov, Int.J.Mod.Phys.A21:5686 (2006).
System: x: y:
K0 (1956)
0.95
0.99
Bd (1987)
0.78 ≈0
Bs (2006) 26
0.15
D0 (2007)
0.0098
0.0075
E. Golowich, J. Hewett, S. Pakvasa, A. Petrov, Phys. Rev. D (2007)
W. Lockman
Nov. 9-14, 2008

3. Flavor state Mixing and CPV
Flavor eigenstates can mix through weak interaction:
Physical eigenstates D1 and D2 ≠ flavor eigenstates
In the limit of CP conservation: D1 = CP D2 = CP
If weak interaction splits the masses or widths of physical eigenstates, flavor state mixing will occur
Two parameters characterizing mixing:
where M1,2 and G1,2 are the masses and widths of physical states, resp.
The mixing rate is RM = (x2+y2)/2
CPV in mixing is characterized by the asymmetry where + (-) indicates an initial D0 (D0)
Schroedinger eqn governs time evolution
(off diagonal M and  elements determine mixing)
W. Lockman
Nov. 9-14, 2008

4. Recent Measurements Mixing measurements: D0K+K-, p+p- D0K+p-
D0K(*)-l+n
D0K+p- p0
D0Ksp+p-
D0KsK+K-
Quantum Correlations
Search for time integrated CPV:
D0p+p-p0, K+K-p0
D+K+K-p+
New Result
Not today
BaBar
BELLE
CLEO-c
CDF
W. Lockman
Nov. 9-14, 2008

5. Lifetime difference measurements
Using D*+→p+D0 decays, we measure the lifetimes of
CP-mixed D0K Cabibbo-favored (CF) decays and
CP-even D0KK andp+p- singly Cabibbo-suppressed SCSdecays
This allows a estimation of by
where and + (-) indicates an initial D0 (D0)
We also measure the CP asymmetry:
Relation to mixing parameters:
where AM is the mixing rate asymmetry and f characterizes CPV in the interference between mixing and decay
In the limit of CP conservation, yCP = y and DY=0
W. Lockman
Nov. 9-14, 2008

6. Decay time fits to determine (yCP, Y)
=409.3±0.7 fs
=401.3±2.5 fs
=404.5±2.5 fs
=407.6±3.7 fs
=407.3±3.8 fs
K and KK lifetimes differ!
W. Lockman
Nov. 9-14, 2008

7. Lifetime Difference Results
3.2  evidence - no CPV
PRL (2007) 540 fb-1
3.0  evidence - no CPV
PRD (R) (2008) fb-1
HFAG World Average: yCP = (1.072 ± )%
arXiv 0808:1297 (2008)
Combining 384 /fb tagged and 91 /fb untagged (BaBar):
yCP = (1.03 ± 0.33(stat.) ± 0.19(syst.))%
W. Lockman
Nov. 9-14, 2008

8. Mixing in “Wrong Sign” Decays (D0→K+p-)
Two types of WS Decays:
Doubly Cabbibo-supressed (DCS)
Mixing followed by Cabibbo-Favored (CF) decay
Two ways to reach same final state  interference!
D 0 f
D 0 D 0
D 0f
D 0 f
Discriminate between DCS and Mixing decays by their proper time evolution
(assuming CP-conservation and |x|«1, |y|«1) :
DCS decay
Interference between DCS and mixing
Mixing
K : strong phase difference between CF and DCS decay amplitudes
W. Lockman
Nov. 9-14, 2008

9. Observations of Mixing in D0→K+p-
Evidence for mixing from BaBar (3.9s) and confirmation by CDF (3.8s)
3.9 
384 fb-1 PRL 98, (2007)
1.5 fb-1 PRL 100, (2008)
3.8 
Two completely different experiments (BaBar and CDF) yield nearly identical results:
W. Lockman
Nov. 9-14, 2008

10. Mixing in WS D0  K+-0 Decays
Analysis formally similar to to wrong sign D0  K+- analysis but now mixing depends on position in Dalitz plot.
Final state can be reached in two ways, yielding sensitivity to mixing by through analysis of the time dependent WS decay rate (|x|,|y|<<1):
The measured mixing parameters are:
384 fb-1 – new result: arXiv:0807, 4544 [hep-ex], submitted to PRL
D 0 f
D 0 D 0
D 0f
D 0 f
DCS
Interference
Mixing
where = phase difference between DCS D0→rK+ and CF D0→rK+
reference amplitudes (and cannot be determined in this analysis)
W. Lockman
Nov. 9-14, 2008

11. Mixing in WS D0  K+-0 Decays
RS Dalitz plot
~660K ev.
WS Dalitz plot
3K events
signal box: <Dm< GeV/c2
1.8495<mKpp< GeV/c2
RS signal purity: 99% WS signal purity: 50%
Find CF amplitude from time-integrated fit to RS Dalitz plot
isobar model expansion
Use this in time-dependent fit to WS plot to determine and mixing parameters.
Results:
No evidence for CPV
Main systematics:
Dalitz plot model
Event selection criteria
Signal and background yields
W. Lockman
Nov. 9-14, 2008

12. Time integrated CPV
Two diagrams (tree and penguin) in SCS decays can lead to CPV
Measured asymmetry includes direct and indirect terms
SM predictions for ACP are tiny: O(0.001% %)
observation of ACP at ~0.1% level would indicate NP
Whereas previous measurements of ACP had uncertainties of ~(1-10)%, recent improvements in controlling experimental systematics have led to reduced errors ~( )% on ACP
F. Bucella et al., Phys. Rev. D51, 3478 (1995)
S. Bianco et al., Riv. Nuovo Cim. 26N7, 1(2003)
S. Bianco, F.L. Fabbri, D. Benson, and I. Bigi, Riv., Nuovo Cim. 26N7, 1 (2003).
A.A. Petrov, Phys. Rev. D69, (2004)
Y. Grossman, A.L. Kagan, and Y. Nir, Phys. Rev. D75, (2007)
W. Lockman
Nov. 9-14, 2008

13. Experimental Procedure
Measure the time integrated CP asymmetries
Relative ps+ and ps- tracking efficiencies not equal
Use D0→K-p+ tagged and untagged data to determine this
Due to Z/g interference and radiative corrections D0 and D0 are produced with a forward backward asymmetry in C.M. polar angle q*
compute the D0 -D0 flavor asymmetry vs cos in the center of mass
extract Acp and Afb by constructing even and odd functions of cos
W. Lockman
Nov. 9-14, 2008

14. Time integrated CPV in D0 KK, 
BaBar data sample 384 fb-1
PRL (2008)
No evidence for CP violation in either mode
2-3x improvement on 2006 world average errors:
W. Lockman
Nov. 9-14, 2008

15. Search for CPV in D0 KK0, 0
phase space integrated asymmetries:
Phys. Rev. D (2008)
No evidence of CP violation in either decay mode.
No significant difference between modes
Used technique described earlier to correct for tracking asymmetries
W. Lockman
Nov. 9-14, 2008

16. New HFAG Average for ICHEP08 http://www. slac. stanford
arXiv:
No-mixing point excluded at 9.8σ
No-CPV point still allowed at 1σ
W. Lockman
Nov. 9-14, 2008

17. Radiative D0→fg and K*g Decays
D0→fg Cabibbo suppressed, D0→K*0g Cabibbo favored radiative D0 decays dominated by long range processes
Results:
Phys. Rev. D78, (2008)
Vector Meson Dominance
Mode:
Theoretical BF (×10-5):
D0→fg
D0→K*0g
7-80
pole diagrams:
o = weak transition
P = pseudoscalar meson
Using world average
B(D0→K-p+)=(3.89±0.05)%:
new
W. Lockman
Nov. 9-14, 2008

18. Summary
After 30 years of searching for it, the collective evidence for D0 mixing is becoming compelling
The no-mixing point is excluded at ~10s, including systematic uncertainties
However, no single measurement exceeds 5s
BaBar will be adding more measurements soon
Average values of the mixing parameters are x~1 %, y~0.8 %
compatible with the upper range of standard model predictions
No evidence for CPV at the current experimental sensitivity (~0.25 %)
systematic uncertainties are likely to diminish as more B-factory data is analyzed
Theoretical predictions in accord with measured branching fractions
W. Lockman
Nov. 9-14, 2008

19. Extra
W. Lockman
Nov. 9-14, 2008

20. BaBar Generic Mixing Analysis
Identify the D0 flavor at production
using the decays
select events around the expected
The charge of the soft pion determines the flavor of the D0
Identify the D0 flavor at decay
using the charge of the Kaon
Vertexing with beam spot constraint
determines decay time,
and decay time error,
D0 decay vertex
Beam spot:
x ~ 100 m,
y ~ 6 m
right-sign (RS)
wrong-sign (WS)
D0 production
vertex
W. Lockman
Nov. 9-14, 2008