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D0-D0 Mixing at BaBar

Charm 2007 August, 2007

Abe SeidenUniversity of California at

Santa Cruz

for

The BaBar Collaboration

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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.

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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.

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The propagation eigenstates, including the electroweak interactions are:

002,1 DqDpD 1

22 qp

1 2M M M 11 22 1 2

, 2

Mx y

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.

Mixing Measurables

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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:

1) 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 KThese are used to measure the average lifetime, with the correction

( / 2 )( ( ) )2

i m tff y ix t e

00 ( / 2 )D(t) = (D D ( ) ) .2

i m ty ix t e

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from the term involving x and y usually ignored (provides a correction on O(10-3)).

3) Singly suppressed decays (for example KK or ). 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). 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 cos – x sin, where is a strong phase difference between the Cabibbo favored and Doubly suppressed amplitudes. For the K

decay there is just the one phase.

22 2 ( )

( A A A ' A )2

tf ff f M

ty t R e

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For multibody decays the phase 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, with a mass cut that selects mostly K decays, the largest channel for the Cabibbo allowed amplitude arising from mixing. Based on 230 fb-1, BaBar measures:

The parameter 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 KS 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.

0.6 0.180.8 0.14' ( 1.2 0.2)%, (0.023 0.004)%My R

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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 strongest Evidence for D-Mixing from BaBar, using the K final state.

(PRL 98, 211802 (2007))

Expectations for Mixing Parameters

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Event Selection: KDecay of the D• Beam-constrained vertex fits

of K, , tag tracks as shown in figure. tag charge gives D flavor at production.

– Require fit probability > 0.001

• D0 selection– CMS p* > 2.5 GeV/c– K, particle identification– DCH hits > 11– 1.81 < M(K) < 1.92 GeV/c2 – decay time error < 0.5 ps– -2 < decay time < 4 ps

• tag

– CMS p* < 0.45 GeV/c– lab p > 0.1 GeV/c– SVT hits > 5

beam spot interaction pointx

y

• 0.14 < M < 0.16 GeV/c2,

where M = M(Ktag) – M(K).

• Select candidate with best vertex fit probability for multiple D*+ candidates sharing tracks

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even

ts/0

.1 M

eV

/c2e

ven

ts/1

MeV

/c2

64,000WS candidates

1,229,000 RS candidates

mK

mK

m

m

x103

RS(top)/WS(bottom) Datasets After Event Selection Integrated Luminosity Approximately 384 fb-1

BaBar Data

BaBar Data

BaBar Data BaBar Data

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Analysis Strategy• Blind analysis of D*+ → D0(→K)

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(K), M = M(Ktag) – M(K) 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.

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RS/WS M(K), M Distributions

Fit RS/WS M(K), M distributions with signal and three background PDFs,correlation between M and M in signal events taken into account in PDF. Signal: peaks in M(K), M

True D0 combined with random tag: peaks in M(K) only Misreconstructed D0: peaks in M only Purely combinatoric: non-peaking in either variable

BaBar Data

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Simultaneous Fit to RS/WS Data

RS signal: 1,141,500±1200

Events.

WS signal: 4030±90Events.

BaBar Data

BaBar Data

BaBar Data

BaBar Data

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Proper Time Analysis• Use M(K) and M 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 + RD½ y’ (t) + (x’ 2+y’ 2)(t)2/4) et ,

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 resolution PDF.

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RS Decay Time Fit

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

BaBar Data

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WS Mixing Fit: No CP Violation

• Varied fit parameters– Mixing parameters– Fit class

normalizations– Combinatoric shape

Plot selection:

1.843<m<1.883 GeV/c2

0.1445<m< 0.1465 GeV/c2

Data minus No mixing PDF

Mixing minus No mixing PDF

BaBar Data

BaBar Data

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Mixing Contours: No CP Violation

• Accounting for systematic errors, the no-mixing point is at the 3.9-sigma contour

• 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

RD: (3.030.160.06) x 10-3

x’2: (-0.220.300.21) x 10-

3

y’: (9.74.43.1) x 10-3

BaBar Data

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M(K), M Fits in Decay Time Bins

• Kinematic fit done independently in five decay time bins• RWS independent of any assumptions on resolution model

2 for mixing fit is 1.5; for the no mixing fit (RWS = .353%) 2 is 24.0

BaBar Data

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Time Dependence of Mixed Final States: CP Violation

• If CP is not conserved, the time distribution for D0 and D0 differ

2

2221

)(4

)(sincos)(

tyx

q

ptR

q

pxyR

e

tDDt

WS

1pq

DD RR• Direct CP violation in DCS Decay

• CP violation in mixing

• CP violation in interference between decay and mixing:

1cos

2

22

441

1sincos

11

11

1

1)(t

yx

A

Atxy

AA

AARR

A

A

e

t

M

M

MD

MDDD

D

Dt

WS

• Rewrite time dependence to explictly include asymmetries

MA1

• Define CP violating observables

MDMD

MDMDMD RR

RRA

,,

,,,

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Final Results for K Analysis

Fit type Parameter Fit results 3(/10 )

No CP viol. or mixing DR 3.53 ± 0.08 ± 0.04

No CP violation DR 3.03 ± 0.16 ± 0.10

2x -0.22 ± 0.30 ± 0.21 y 9.7 ± 4.4 ± 3.1

CP violation allowed DR 3.03 ± 0.16 ± 0.10

DA -21± 52 ± 15

2x -0.24 ± 0.43 ± 0.30 y 9.8 ± 6.4 ± 4.5

2x -0.20 ± 0.41 ± 0.29 y 9.6 ± 6.1 ± 4.3

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Conclusions

RD: (3.030.160.06) x 10-3

x’2: (-0.220.300.21) x 10-3

y’: (9.74.43.1) x 10-3

Assuming CP conservation and including systematic effects, BaBar finds a mixing signal at the 3.9 sigma confidence level in the K final state. The parameters describing the WS/RS branching ratio and mixing are:

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.