1. D0 Mixing and CP Violation from Belle
Eric White
U. of Cincinnati
CHARM 2010, Oct IHEP Beijing, China
Eric White – University of Cincinnati
1/18/2019

2. Introduction Review of Charm Mixing and CP Violation Formalism
Charm Mixing Results (WS, time-dep., …)
CP Violation in Time-integrated Decays (KSh±, KSP0, … )
Conclusion
Eric White – University of Cincinnati
1/18/2019

3. KEKB and Belle Detector
Largest integrated luminosity in the world, ~1020 fb-1
Electron-positron beam, proceeds through:
e+e- → Υ(4S) → bb
e+e- → γ → cc
σ(bb) = 1.1 nb
σ(cc) = 1.3 nb
~1.3 x 109 cc pairs
B-factories serve as good charm factories! _ _ _ _ _
Eric White – University of Cincinnati
1/18/2019

4. Mixing Formalism
Mixing often expressed in terms of mass and width differences of eigenstates: _
Evolution of D0-D0 system described by time-dependent Schrodinger equation
and mixing parameters: _
Mixing occurs when flavor eigenstates (D0, D0) produced in decays are not the same as mass eigenstates (D1, D2).
Time-dependent expression for D0 decay rate is given by:
Eric White – University of Cincinnati
1/18/2019

5. Contributions to x and y
Standard Model predictions for x and y
Contributions from SU(3) breaking and hadronic intermediate states give:
x, y < 10-2
Mixing strongly suppressed in charmed mesons by CKM and GIM mechanism, calculable short-distance effects give:
Difficult to calculate, BUT measurements of x and y can still provide useful constraints on New Physics models
|x|, |y| < 10-5
Birdman, Shipsey, Ann.Rev.Nucl.Part.Sci.53,431
Golowich,Hewett,Pakvasa,Petrov arXiv: v2
Eric White – University of Cincinnati
1/18/2019

6. Common experimental observables
CP Violation in Charm
CP violation arises as a single complex phase η in the CKM mixing matrix
CP violation in SM is CKM suppressed, ~O(10-3)
Observation of O(1%) CP violation would be a sign for New Physics
CP-violating effects are classified by three parameters:
1) CP violation in decay:
2) CP violation in mixing:
3) CP violation in interference:
Common experimental observables
Eric White – University of Cincinnati
1/18/2019

7. Decays to CP Eigenstates
In terms of mixing parameters:
Limit no CP violation, yCP = y and AΓ = 0
Eric White – University of Cincinnati
1/18/2019

8. Reconstructing D Mesons
Tagging the flavor of neutral D meson
Proper decay time determination
Reconstruct decay chain:
D*+  D0 π+
Beam point is used to constrain vertex fit:
Flavor of D0 is tagged by slow pion
Background suppression using
mass difference
D mesons from b  c are eliminated
with pD*cms > 2.5 GeV/c2
Uncertainty t typically between
1/6D and 1/2D
Eric White – University of Cincinnati
1/18/2019

9. D-Mixing in D0 → K+K-, π+π-
Proper decay time was fitted for each sample
[540 fb-1, ~130K K+K- events, ~57K π+π- events]
Staric et al. (Belle), PRL98, (2007) =
yCP = (1.31 ± 0.32 ± 0.25)%
Measured to 3.2σ significance, first evidence of mixing
AΓ = (0.01 ± 0.30 ± 0.15)%
No CPV observed to .4% sensitivity
Eric White – University of Cincinnati
1/18/2019

10. D-Mixing in D0 → KSK+K- yCP = (0.11 ± 0.61 ± 0.52)%
Measure lifetime differences between CP-even and CP-odd eigenstates
Zupanc et al. (Belle), Phys.Rev.D80: (2009)
fON and fOFF regions fit with 8-resonance Dalitz model
72K signal events in ON region, 62K in OFF
yCP = (0.11 ± 0.61 ± 0.52)%
Purity: 97%
Purity: 91%
Consistent with previous measurement of yCP
Eric White – University of Cincinnati
1/18/2019

11. Mixing in WS Decay D0 → K+π-
WS rate include DCS, interference, and mixing
RD = DCS/CF rate
x' = x cosδ + y sinδ
y' = y cosδ – x sinδ
(δ = strong phase)
Eric White – University of Cincinnati
1/18/2019

12. Mixing in WS Decay D0 → K+π-
Belle [400 fb-1]
Zhang et al. (Belle), Phys.Rev.Lett.96, (2006) _
D0 and D0 samples fit separately:
y‘ = ( )10-3
x’2 = ( )10-3
No-mixing point (0,0) corresponds to C.L. 3.9%
Eric White – University of Cincinnati
1/18/2019

13. Mixing in Time-dependent Dalitz Analyses
Dalitz plots provide unique environment for determining strong phases between D0 and D0 decays in self-conjugate final states.
Contributions include: _ _
CF: D0 → K*-π+
DCS: D0 → K*+π-
CP: D0 → KSρ0
Decay model is parameterized using Dalitz variables
The time-dep. decay rate allows simultaneous determination of x and y
Eric White – University of Cincinnati
1/18/2019

14. Mixing in Time-dependent D0→KSπ+π-
Zhang et al. Phys.Rev.Lett.99: (2007)
Belle [540 fb-1]
Allowing for CPV
Assuming no CP violation:
No CP violation observed
Eric White – University of Cincinnati
1/18/2019

15. CP Violation in Time-integrated Decays
Standard Model predicts O(0.1%) direct CP violation in Singly Cabibbo-Suppressed (SCS) decays
Grossman et al. arXiv:hep-ph/ v1
Examples: D+→KSπ+, D+→KSK+
Must distinguish possible CP asymmetry from reconstruction asymmetries, such as detector effects and production asymmetry
Includes ( ± 0.006)% CP violation due to neutral kaon
ACP = CP asymmetry, independent of kinematics
AFB = Production asymmetry, due to γ*/Z interference
Aε = Reconstruction efficiency asymmetry (plab, cosθlab)
Deviations in ACP greater than 0.1% could signify New Physics
Eric White – University of Cincinnati
1/18/2019

16. CP Violation in D+→KSπ+
Ko et al. (2010), PRL 104,
673 fb-1
Measured in bins of
(plab, cosθlab, cosθD*CMS)
1) For D+→KSπ+ measure
Arec = AFB + Aε+ + ACP
2) For DS+→π+ measure
Arec = AFB + Aε+
3) Subtract measured asymmetries
ACP(D+→KSπ+) = (-0.71 ± 0.19 ± 0.20)%
Results consistent with -0.33% due to neutral kaon
Eric White – University of Cincinnati
1/18/2019

17. CP Violation in D(s)+→KSK+
Ko et al. (2010), PRL 104,
673 fb-1
ACP(D+→KSK+) = (-0.16 ± 0.58 ± 0.25)%
ACP(DS+→KSK+) = (0.12 ± 0.36 ± 0.22)%
Consistent with -0.33% due to neutral kaon
Most precise measurement of SCS/CF:
R(D+) =  
R(DS+) =  
673 fb-1
Dashed line: leading-order prediction for AFB
Eric White – University of Cincinnati
1/18/2019

18. Results consistent with -0.33% due to neutral kaon
CP Violation in D0→KSP0
PRELIMINARY
Events reconstructed from tagged D* sample
791 fb-1
ACP(D0→KSπ0) = (-0.28 ± 0.19 ± 0.10)%
ACP(D0→KSη) = (-0.54 ± 0.51 ± 0.13)%*
ACP(D0→KSη’) = (-0.90 ± 0.67 ± 0.15)%*
*World’s first measurement
Results consistent with -0.33% due to neutral kaon
Eric White – University of Cincinnati
1/18/2019

19. ΔACP between D+→φπ+ and Ds+→φπ+
PRELIMINARY
Event reconstruction as follows:
ΔACP defined as ACP(D+→φπ+) - ACP(Ds+→φπ+)
850 fb-1
ΔACP = (0.62 ± 0.30 ± 0.15)%
No significant difference found in production asymmetry of D+ and Ds+
Eric White – University of Cincinnati
1/18/2019

20. Prospects with Belle’s Full Data Set
Belle now has about ~1 ab-1 for mixing measurements
Updating previous mixing analyses and working on new measurements
Expect significant improvement in sensitivity to mixing and CP violation
Additionally, expect 20-40% increase in signal yields due to new reprocessing
E.g. signal yield for D0→KSπ+π- analysis increased from 534K events to ~1.25M
Eric White – University of Cincinnati
1/18/2019

21. Conclusion Wide program of D-mixing and CP violation underway at Belle
Measurements have provided compelling evidence for D0-D0 mixing
No evidence of CP violation to 0.3% level of sensitivity
First ACP measure measurements for D0→KSη and D0→KSη’
Expect significant improvement
with full dataset, new analyses
underway
Best D-mixing and CP violation
measurements to come?
Eric White – University of Cincinnati
1/18/2019

22. Backup
Eric White – University of Cincinnati
1/18/2019