1. Representing the BaBar Collaboration
D0-D0 Mixing at BaBar
D0-D0 Mixing at BaBar
Jon Coleman
SLAC
Representing the BaBar Collaboration

2. B-factories are charm factories
B-factories are excellent laboratories for charm physics:
at a lumi 1.2x1034 cm-2s-1 produce ~ evts/sec
Main topics in charm physics at B-factories:
Charm meson and baryon spectroscopy
Dalitz plot analyses
Rare charm decays
D0 mixing and CP violation searches
July 8, 2008
SLAC Annual Program Review

3. SLAC Annual Program Review
D0-D0 Mixing Formalism
Mixing occurs if the weak interaction splits the masses or widths of the mass eigenstates: well established phenomenon in neutral K, Bd, Bs systems.
Mixing parameters x & y are expressed in terms of the mass and width differences of the mass eigenstates
July 8, 2008
SLAC Annual Program Review

4. SLAC Annual Program Review
Introduction
Mixing among the neutral meson flavor eigenstates provides important information about
electroweak interactions, including CP violation
the CKM matrix
mixing loop virtual constituents
D0 system exhibits the smallest mixing
Short distance Standard Model (SM) suppression:
D mixing loop involves down-type quarks
b quark loop suppressed:
s and d quark loops GIM suppressed
mass difference amplitude O(10-5) or less
long distance mixing amplitudes predominant but hard to quantify
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
A. Petrov, HEP-PH/
July 8, 2008
SLAC Annual Program Review

5. 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 improves mKp , Dm, flight length and hence proper decay time t, and its uncertainty st
Right-sign (RS) decay
Beam spot:
x ~ 100 m
y ~ 7 m,
D0 decay vertex
D0 production
vertex
D0  K-p+ - right-sign (RS)
D0  K+p- - wrong-sign (WS) +
July 8, 2008
SLAC Annual Program Review

6. Time dependent WS decay rate
Two types of WS Decays:
Doubly Cabibbo-supressed (DCS)
Mixing followed by Cabibbo-Favored (CF) decay
Two ways to reach same final state  interference possible!
Use time dependence to separate DCS and mixing:
mix
(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
July 8, 2008
SLAC Annual Program Review

7. SLAC Annual Program Review
D0K Fit Procedure
Unbinned maximum likelihood fit performed in stages
Fit m(K) and m distribution:
Separate signal from background in subsequent decay time fits
Fit RS decay time distribution:
Determine D0 lifetime and decay time resolution function R(t)
Fit WS decay time distribution:
Use D0 lifetime and decay time resolution function from RS fit
Fit WS signal to
Compare fits with and without mixing to determine significance
Fit D0 and D0 samples separately to search for CP violation
All parameters are determined by fits to data, not from MC
July 8, 2008
SLAC Annual Program Review

8. SLAC Annual Program Review
m(Kp) & Dm Fit Results
July 8, 2008
SLAC Annual Program Review

9. SLAC Annual Program Review
2007 D0Kresults
D0K+-
PRL 98, (2007)
3.9σ signal
2007 D0Kdecay time analysis: 384 fb-1
First evidence for mixing: 3.9(including systematic uncertainties)
x'2, y' consistent with previous BaBar result PRL (2003)
Mainly y' (consistent with ycp if K ~ 0)
No CP violation
Results confirmed by CDF
July 8, 2008
SLAC Annual Program Review

10. SLAC Annual Program Review
D0K CDF Measurement
Evidence for mixing at 3.8s
PRL 100, (2008)
Fitted signal
(12.7  0.3)K
Different Analysis
Different Production Environment
Confirmation of BaBar mixing result
Nearly identical results!
July 8, 2008
SLAC Annual Program Review

11. 2008 WS data D0Kdecay time fit
fit results still blinded
more data: 470 fb-1
reprocessed to improve tracking efficiency
eliminate s backgrounds from e- sources
improved signal PDF
improved systematics
prior to unblinding:
investigate systematics
perform cross checks
planned time table:
initial results ICHEP 2008
published results end 2008
Can we attain a 5 mixing signal in a single measurement?
UPDATE is BLINDED
July 8, 2008
SLAC Annual Program Review

12. D0-D0 Mixing in Lifetime Ratio of D0K+K, + vs D0K+
D0 mixing and CP violation alter decay time distribution of CP eigenstates with effective lifetimes thh:
Measured quantities
Mixing and CP
observables
yCP=y and DY=0 if CP conserved.
(yCP=0 and DY=0 if no mixing)
July 8, 2008
SLAC Annual Program Review

13. SLAC Annual Program Review
Mass Projections
Mass Projections (m GeV/c):
high signal purities ( < m < GeV/c2)
minimize effect of broken charm/combinatorial backgrounds in signal box
July 8, 2008
SLAC Annual Program Review

14. 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
July 8, 2008
SLAC Annual Program Review

15. SLAC Annual Program Review
yCP, Y results
Tagged results accepted for publication in PRD-RC (2008)
Tagged results from 384 fb-1:
combining with previous BaBar measurement of yCP obtained from an untagged sample of D0KKdecays:
results agree with those from BELLE
No evidence for CPV 3
BELLE, PRL 98, (2007) 540 fb-1
July 8, 2008
SLAC Annual Program Review

16. The Untagged Analysis: from 384 fb-1
No D* flavor tag used:
Higher background to signal than in the tagged analysis, but 4x the statistics
compared to tagged analysis, expect smaller statistical, larger systematic errors
No measurement of CP violating quantity, Y
Construct the untagged sample to be disjoint from the tagged sample
allows tagged and untagged results to be “trivially” combined KK K
UPDATE is BLINDED
July 8, 2008
SLAC Annual Program Review

17. SLAC Annual Program Review
Summary
No-mixing point excluded at > 6.7σ
No-CPV point still allowed at 1σ
BaBar was first to show evidence for D0 mixing
Result (y`~1%, x`2 ~0 )
independently confirmed by CDF in the same channel
Striving to improve our sensitivity to mixing through a new measurement of ycp employing an untagged analysis on a disjoint dataset and an updated measurement of x`2, y` in D0Kp
No evidence for CPV at current experimental sensitivity
July 8, 2008
SLAC Annual Program Review

18. SLAC Annual Program Review
The scorecard
Mixing analyses
D0K+p-
D0K+K-, p+p-
D0K(*)-l+n
D0K+p- p0
D0Ksp+p-
D0p+p-p0
D0 K+p- p+p-
Search for CP violation: time integrated
D0p+p-p0, K+K-p0
D+ K+K-p+
= results soon!
July 8, 2008
SLAC Annual Program Review

19. SLAC Annual Program Review
Extra material
July 8, 2008
SLAC Annual Program Review

20. Mixing between Flavor States
Flavor eigenstates can mix through weak interaction:
Physical eigenstates D1 and D2 ≠ flavor eigenstates
If weak interaction splits the masses or widths of physical eigenstates, flavor state mixing will occur, as seen from the time evolution:
mixing parameters:
Schroedinger eqn governs time evolution
(off diagonal M and  elements determine mixing)
D1 = CP
D2 = CP
In the limit of CP conservation:
July 8, 2008
SLAC Annual Program Review

21. RWS vs. decay-time slices
If mixing is present, it should be evident in an RWS rate that increases with decay-time.
Perform the RWS fit in five time bins with similar RS statistics.
Cross-over occurs at t ¼ 0.5 psec as in residuals plot.
No-mixing fit
RWS fits
Dashed line: standard RWS fit (2=24).
Solid, red line: independent RWS fits to each time bin (2 = 1.5).
July 8, 2008
SLAC Annual Program Review

22. 2007 Kpi systematics, validations
Systematics: variations in
Functional forms of PDFs
Fit parameters
Event selection
Computed using full difference w.r.t. original value
Results are expressed in units of the statistical error
Validations and cross-checks
Alternate fit (RWS in time bins)
Fit RS data for mixing
x’2 = (−0.01±0.01)x10-3
y’ = (0.26±0.24)x10-3
Fit generic MC for mixing
x’2 = (−0.02±0.18)x10-3
y’ = (2.2±3.0)x10-3
Fit toy MCs generated with various values of mixing
Reproduces generated values
Validation of proper frequentist coverage in contour construction
Uses 100,000 MC toy simulations
Systematic source RD y’
x’2
PDF:
0.59s
0.45s
0.40s
Selection criteria:
0.24s
0.55s
0.57s
Quadrature total:
0.63s
0.71s
0.70s
July 8, 2008
SLAC Annual Program Review

23. SLAC Annual Program Review
yCP, Y systematics
Systematic uncertainties (%):
Variations:
Signal: PDF shape, polar angle dependent resolution offset, signal interval
Charm backgrounds: yields and charm lifetime
Combinatorial backgrounds: yields, shape and sideband region
Selection: t criterion, treatment of multiple candidates
Detector: Alignment and energy loss
Tagged results limited by statistical errors (not systematics)
July 8, 2008
SLAC Annual Program Review