D 0 -D 0 Mixing at BaBar

1. D0-D0 Mixing at BaBar Abe Seiden University of California at Santa Cruz for The BaBar Collaboration Charm 2007 August, 2007

2. Status of Mixing Studies • Mixing among the lightest neutral mesons of each flavor has traditionally provided important information on the electroweak interactions, the CKM matrix, and the possible virtual constituents that can lead to mixing. • Among the long-lived mesons, the D meson system exhibits the smallest mixing phenomena. • The B-factories have now accumulated sufficient luminosity to observe mixing in the D system and we can expect to see more detailed results as more luminosity is accumulated and additional channels sensitive to mixing are analyzed.

3. BaBar Charm Factory: 1.3 million Charm events per fb-1 • BaBar integrated luminosity ~384 fb-1 (Runs 1-5) used for evidence for mixing result I will present. Corresponds to about 0.5 billion charm events produced. Present BaBar integrated luminosity is approximately 500 fb-1. BaBar Detector BaBar is a high acceptance general purpose detector providing excellent tracking, vertexing, particle ID, and neutrals detection.

4. Mixing Measurables The propagation eigenstates, including the electroweak interactions are: Propagation parameters for the two states are given by: With the observable oscillations determined by the scaled parameters: . In the case of CP conservation the two D eigenstates are the CP even and odd combinations. I will choose D1 to be the CP even state. The sign choice for the mass and width difference varies among papers, I will use the choice above.

5. Assuming CP conservation, small mixing parameters, and an initial state tagged as a D0, we can write the time dependence to first order • in x and y: Projecting this onto a final state f gives to first order the amplitude for finding f: • This leads to a number of ways to measure the effect of mixing, for example: • Wrong sign semileptonic decays. Here Af is zero and we measure directly the quantity, after integrating over decay times: • RM = (x2 + y2)/2 • Limits using this measurement however, are not yet sensitive enough to get down to the 10-4 level for RM. Using 334 fb-1 of data, electron decays only, and a double tag technique, BaBar measures RM = 0.4x10-4, with a 68% confidence interval (-5.6, 7.4)x10-4. • 2) Cabibbo favored, right sign (RS) hadronic decays (for example K-p+).These are used to measure the average lifetime, with the correction

6. from the term involving x and y usually ignored (provides a correction on O(10-3)). 3) Singly suppressed decays (for example K+K- or p+p-). In this case tagging the initial state isn’t necessary. For CP even final states: Af =Af. This provides the most direct way to measure y. With tagging we can also check for CP violation, by looking at the value of y for each tag type. BaBar will be updating this measurement with the full statistics later this year. The initial measurement was based on 91 fb-1 and gave the result y = 0.8, with statistical and systematic errors each about 0.4, consistent with the published Belle measurement. 4) Doubly suppressed and mixed, wrong sign (WS) decays (for example K+p-). Mixing leads to an exponential term multiplied by both a linear and a quadratic term in t. The quadratic term has a universal form depending on RM . For any point in the decay phase space the decay rate is given by: Here y ’ = y cosd – x sind, where d is a strong phase difference between the Cabibbo favored and Doubly suppressed amplitudes. For the K+p-decay there is just the one phase.

7. For multibody decays the phase d varies over the phase space and the term proportional to t will involve a sum with different phases if we add all events in a given channel. BaBar has analyzed the decay channel K+p-p0, with a mass cut that selects mostly K+r-decays, the largest channel for the Cabibbo allowed amplitude arising from mixing. Based on 230 fb-1, BaBar measures: The parameter a allows for the phase variation over the region summed over. Better would be a fit to the full Dalitz plot. This, however, requires a model for all the resonant and smooth components that contribute to the given channel, which may introduce uncertainties. BaBar is working on such a fit; will be based on approximately 1500 signal events. Another important 3-body channel is the KSp+p- decay channel. This contains: CP-even, CP-odd, and mixed-CP resonances. Must get relative amounts of CP-odd and CP-even contributions correct (including smooth components) to get the correct lifetime difference. Provides the possibility to measure x. BaBar also working on this channel, Belle has published their results.

8. Expectations for Mixing Parameters Final general comments: In the Standard Model y and x are due to long-distance effects. They may be comparable in value but this depends on physics that is difficult to model. Also, the sign of x/y provides an important measurement. Long-distance effects control how complete the SU(3) cancellation is, which would make the parameters vanish in the symmetry limit. Depends on SU(3) violations in matrix elements and phase space. One might expect the x and y parameters to be in the range O(10-3 to 10-2). Thus the present data are consistent with the Standard Model. Searches for CP violation are important goals of the B-factories, since observation at a non-neglible level would signify new physics. I will turn now to the strongestEvidence for D-Mixing from BaBar, using the Kp final state. (PRL 98, 211802 (2007))

9. Event Selection: Kp Decay of the D • Beam-constrained vertex fits of K, p, ptag tracks as shown in figure. ptag charge gives D flavor at production. • Require fit probability > 0.001 • D0 selection • CMS p* > 2.5 GeV/c • K, p particle identification • DCH hits > 11 • 1.81 < M(Kp) < 1.92 GeV/c2 • decay time error < 0.5 ps • -2 < decay time < 4 ps • ptag • CMS p* < 0.45 GeV/c • lab p > 0.1 GeV/c • SVT hits > 5 y beam spot interaction point x • 0.14 < DM < 0.16 GeV/c2, where DM = M(Kpptag) – M(Kp). • Select candidate with best vertex fit probability for multiple D*+ candidates sharing tracks

10. RS(top)/WS(bottom) Datasets After Event Selection Integrated Luminosity Approximately 384 fb-1 x103 BaBar Data BaBar Data 1,229,000 RS candidates events/1 MeV/c2 events/0.1 MeV/c2 BaBar Data BaBar Data 64,000 WS candidates

11. Analysis Strategy • Blind analysis of D*+→ D0(→Kp) p+tag • Event selection and fitting methodology determined before looking at the mixing results. • Unbinned maximum likelihood fit to the data using four variables per event. • First, correlated fit to the M(Kp), DM = M(Kpptag) – M(Kp) distributions (two of the variables) to establish shapes for different components (signal and backgrounds) of the two dimensional distribution. High-statistics RS and WS data samples fit simultaneously. These shapes used in later time dependent fits. • Fit RS proper-time distribution in the four variables, where the two additional variables are the event-by-event lifetime and its error. Establishes proper-time resolution function for signal and backgrounds. • The WS data are fit using the RS resolution functions. • Several WS proper time fits are performed. • no mixing • mixing, no CP violation • mixing, CP violation • Monte Carlo used to search for systematics and validate statistical significance of results.

12. RS/WS M(Kp), DM Distributions BaBar Data • Fit RS/WS M(Kp), DM distributions with signal and three background PDFs, • correlation between M and M in signal events taken into account in PDF. • Signal: peaks in M(Kp), DM • True D0 combined with random ptag: peaks in M(Kp) only • Misreconstructed D0: peaks in DM only • Purely combinatoric: non-peaking in either variable

13. Simultaneous Fit to RS/WS Data BaBar Data RS signal: 1,141,500±1200 Events. BaBar Data BaBar Data WS signal: 4030±90 Events. BaBar Data

14. Proper Time Analysis • Use M(Kp) and DM PDF shapes from mass fits • Fit RS decay time and error distribution to determine signal lifetime and resolution model Signal, background D0 PDF: exponential convolved with resolution function , which is the sum of three gaussians with widths proportional to the event-by-event lifetime errors. Random combinatoric PDF: sum of two gaussians, one of which has a power-law tail. • Fix WS resolution and DCS lifetime from RS fit Signal PDF: theoretical mixed lifetime distribution, which is proportional to: (RD + RD½y’ (Gt) + (x’ 2+y’ 2)(Gt)2/4) e-Gt , convolved with the resolution model from RS fit. RD is the ratio of WS to RS D0 decays. With CP violation fit separately for D and D. • Search for and quantify systematic errors by looking at results after: Variations in functional form of signal and background PDFs. Variations in the fit parameters. Variations in the event selection. Adding a small non-zero mean in the proper-time signal resolutionPDF.

15. RS Decay Time Fit BaBar Data The D0 lifetime is consistent with the Particle Data Group value, within the statistical and systematic errors of the measurement. Plot selection: 1.843<m<1.883 GeV/c2 0.1445<m< 0.1465 GeV/c2

16. WS Mixing Fit: No CP Violation • Varied fit parameters • Mixing parameters • Fit class normalizations • Combinatoric shape BaBar Data Mixing minus No mixing PDF Data minus No mixing PDF BaBar Data Plot selection: 1.843<m<1.883 GeV/c2 0.1445<m< 0.1465 GeV/c2

17. Mixing Contours: No CP Violation • y’, x’2 contours computed by change in log likelihood • Best-fit point is in non-physical region x’2 < 0, but one-sigma contour is in physical region • correlation: -0.95 BaBar Data • Accounting for systematic errors, the no-mixing point is at the 3.9-sigma contour RD: (3.030.160.06) x 10-3 x’2: (-0.220.300.21) x 10-3 y’: (9.74.43.1) x 10-3

18. M(Kp), DM Fits in Decay Time Bins • Kinematic fit done independently in five decay time bins • RWS independent of any assumptions on resolution model BaBar Data c2 for mixing fit is 1.5; for the no mixing fit (RWS = .353%) c2 is 24.0

19. Time Dependence of Mixed Final States: CP Violation • If CP is not conserved, the time distribution for D0 and D0 differ • Define CP violating observables • Direct CP violation in DCS Decay • CP violation in mixing • CP violation in interference between decay and mixing: • Rewrite time dependence to explictly include asymmetries

20. Final Results for Kp Analysis

21. Conclusions Assuming CP conservation and including systematic effects, BaBar finds a mixing signal at the 3.9 sigma confidence level in the Kp final state. The parameters describing the WS/RS branching ratio and mixing are: RD: (3.030.160.06) x 10-3 x’2: (-0.220.300.21) x 10-3 y’: (9.74.43.1) x 10-3 No evidence is seen for CP violation. Analyses in progress, along with the results of other experiments, should allow significant progress in reducing the errors on the parameters describing mixing.