Recent Results on D 0 - D 0 Mixing from BaBar

Recent Results on D0-D0 Mixing from BaBar

William Lockman

for the BaBar Collaboration

Lepton-Photon 2007, Daegu, S. Korea

August 13, 2007 2LP07 - William LockmanQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

Topics from BaBar Experiment

• Introduction

• D0-D0 Mixing in Lifetime Ratio of D0K+K, + vs D0K+

• Search for CP Violation in D0K+K and D0

to be submitted to PRL

• D0-D0 Mixing in the Decay D0K

• Conclusion


Charm meson mixing

Why would observation of charm mixing be interesting?It would complete the picture of quark mixing already seen in the K, B, and Bs

systems.K — 1956

Bd — 1987

Bs — 2006

It would provide new information about processes with down-type quarks in the mixing loop diagram.

It would be a significant step toward observation of CP violation in the charm sector.

It could indicate new physics.


Current Evidence for D0-D0 mixing

3.9σ signal

PRL 98,211802

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x'2 = (9.7 ± 4.4 ± 3.1)×10−3

y' = (−0.22 ± 0.30 ± 0.21)×10−3

QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.

D0K

BELLE PRL 98, 211803

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yCP = (1.31± 0.32 ± 0.25)%

3.2σ signal

Combined

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x = (8.7−3.4+3.0 )×10−3

y = (6.6 ± 2.1)×10−3

δKπ = 0.33−0.29+0.26

5.7σ signal

x (%)

arXiv:0704.1000

BELLE

D0Ks

y (%)

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x = (0.80 ± 0.29 ± 0.17)%

y = (0.33± 0.24 ± 0.15)%

2.4σ signal

D0KK,


Flavor States Mixing

• Flavor eigenstates can mix through weak interaction:

• Mass eigenstates:

• Flavor state time evolution:

Mixing if either or nonzero


• Short-distance contributions from mixing box diagrams in the Standard Model are expected to be small :

– b quark is CKM-suppressed– s and d quarks are GIM suppressed– mainly contributes to the mass difference x ≈ O(10-5) or less

• Long-distance contributions dominate but hard to estimate precisely– expect |y| ≤ 0.01 – |x| ~ 0.1 - 1|y|

Standard Model Predictions

A. Petrov, HEP-PH/0611361


BABAR Charm Factory: 1.3 million Charm events per fb-1

Integrated luminosity ~384 fb-1 used for mixing results presented here ~500M cc events

BaBar is a large acceptance general purpose detector providing excellent tracking,vertexing, particle ID andneutrals detection


BaBar Generic Mixing Analysis

Identify the D0 flavor at productionusing the decays

– select events around the expected

– The charge of the soft pion determines the flavor of the D0

Identify the D0 flavor at decayusing the charge of the Kaon

Vertexing with beam spot constraintdetermines decay time, and decay time error,

right-sign (RS)

wrong-sign (WS)

Beam spot: x ¼ 100 m, y ¼ 6 m

D0 decay vertex

D0 productionvertex


D0-D0 Mixing in Lifetime Ratio of D0K+K, + vs D0K+

D0K: CP-mixed D0(t) KK, : CPeven

Determine the quantities

If CP is conserved in mixing and decay, but violated in the interference

between them, these quantities are related to the mixing parameters

CPV in interference of mixing and

decay:


Previous lifetime ratio results

BaBar, PRL 91, 162001(2002) 91 fb-1

BELLE, PRL 98, 211803 (2007) 540 fb-1


BaBar (yCP, Y) analysis overview

• Select D*D0 D0K KK decays from 384 fb-1

• Event selection chosen to minimize backgrounds

• systematics affecting signal mostly cancel• background systematics don’t cancel

between modes

• Unbinned likelihood fit to (t,t) to obtain hh

• signal resolution determined from fitting data• Backgrounds taken from MC and sidebands

• Determine ycp and Y from lifetimes

signal box

sidebandD0K


Decay time fits to determine (yCP, Y)

€

D0 → K +K −

€

D 0 → K +K −

€

D0 → π +π −

€

D 0 → π +π −€

t (ps)

€

D0 → K −π + +c.c.

=409.3±0.7 fs

€

t (ps)

€

t (ps)

€

t (ps)

€

t (ps)

=401.3±2.5 fs =404.5±2.5 fs

=407.6±3.7 fs =407.3±3.8 fsK and KK lifetimes differ!


BaBar (yCP, Y) results

Tagged results from 384 fb-1:

Result in good agreement with BELLE measurement


BaBar (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


Search for CPV in D0 KK,

Two amplitudes with different strong & weak phases needed to observe CPV (in SM from tree and penguins)

1 32 2 *

1 2 1 2 1

2 21

2

*2Im ( )( ) ( )10

( ) ( ) 2Re ( )CP

sinf fA

f f A A A A co

A A

s δδ

δδ −Γ −Γ

= = <Γ +Γ + + −

−

strong phase difference2 weak amplitudes

with phase difference

s

u

e.g., D0 → K+K- :

s

uW+

uW+

s

ss

K+

K-

K-

K+

c

uD0

D0

u

Only SCS decays probe penguins

c

u

Standard model predictions for direct CPV asymmetries in these modes: O(0.001% - 0.01%)

F. Bucella et al., Phys. Rev. D51, 3478 (1995)S. Bianco et al., Riv. Nuovo Cim. 26N7, 1(2003)



Measure the time integrated CP asymmetries

Experimental procedure:– fit m,m distributions to determine raw signal weights– Determine relative D0/D0 soft pion tagging efficiency using D0K data

greatly reduces systematic uncertainties

– correct for forward-backward asymmetries in eecc production– extract aCP



No evidence for CPV in either mode

KK

€

aCPKK

€

aCPππ


Mixing in D0K

Time dependent WS rate :

where

and

Two types of WS Decays:

– Doubly Cabbibo-supressed (DCS)

– Mixing followed by Cabibbo-Favored (CF) decay

Two ways to reach same final state interference! mix

δK : strong phase difference between CF and DCS decay amplitudes

DCS

Interference

Mixing


RS and WS (mK, m) fits

Determine signal and background yields in subsequent Dalitz analyses.

signal mis-tagged D0

mis-reconstructed D0

combinatoric

signal box yields:

signal andsidebandregionsm

m

m

m


D0K RS Dalitz fit

€

K −π +

€

K −π 0

€

+ 0

Time-integrated analysis to determine CF amplitudes,


D0(t)K WS Dalitz fit results

signal mis-tagged D0

mis-reconstructed D0

+combinatoric

€

K −π +

€

K −π 0

€

+ 0

€

t

Through t-dependence, distinguish DCS amplitudes from the CF amplitudes arising from mixing.


Mixing parameter contours and results

Results are consistent with no mixing at 0.8%, including systematics

y’’

stat.+syst.

68%95%

99%99.9%

x’’

+ no-mix

x best fit


BaBar D0-D0 Mixing Summary

Presented more evidence for D0-D0 mixing from BaBar experiment:– D0 K to D0 KK, lifetimes:

– D0K time-dependent Dalitz analysis:

In D0 KK, decays, – no evidence for CP violation

– no evidence for CP violation in mixing:

No mixing excluded at


Backup Slides


K backup


Time evolution of WS D0K decays

Two types of WS Decays:

– Doubly Cabbibo-supressed (DCS)

– Mixing followed by Cabibbo-Favored (CF) decay

Two ways to reach same final state interference!

mix

DCS decay Interference between DCS and mixing Mixing

Discriminate between DCS and Mixing decays by their proper time evolution(assuming CP-conservation and |x|«1, |y|«1) :

δK : strong phase difference between CF and DCS decay amplitudes


D0K Fit Procedure

Unbinned maximum likelihood fit performed in stagesFit 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

In this analysis, all parameters are determined by fitting data, not MC


RS and WS mK ,m Distributions

Separate signal from background by fitting over the full range shown in the plots1.81 GeV/c2 < mK < 1.92 GeV/c2 and 0.14 GeV/c2 < m < 0.16 GeV/c2

For displaying decay time fits, integrate over a signal box1.843 GeV/c2 < mK < 1.883 GeV/c2 and 0.1445 GeV/c2 < m < 0.1465 GeV/c2

Selected RS data Selected WS data


RS Proper Time Fit

plot selection:1.843<m<1.883 GeV/c2

0.1445<m< .1465 /GeV c2

RS decay time, signal regionD0 lifetime and resolution functionfitted in RS sample

Consistent with PDG

Systematics dominated bysignal resolution function


Δm - m(Kπ) Fit Results

RS signal:1,141,500±1200combinations

RS signal:1,141,500±1200combinations

WS signal:4,030±90

combinations

RS

WS

RS

WS


Wrong-sign mK , m fit

The mK , m fit determines the WS branching ratio RWS

BABAR (384 fb-1): RWS = (0.353 § 0.008 § 0.004)% (PRL 98,211802 (2007))BELLE (400 fb-1): RWS = (0.377 § 0.008 § 0.005)% (PRL 96, 151801 (2006))

4,030 § 90 WS signal events


WS Fit with Mixing

•Fit results allowing mixing:RD: (3.03±0.16±0.10)x10-3 x’2: (-0.22±0.30±0.21)x10-3

y’: (9.7±4.4±3.1)x10-3

data - no mix PDF mix - no mix PDF

WS mixing fit projection in signal region1.843 GeV/c2 < m < 1.883 GeV/c2

0.1445 GeV/c2 < m < 0.1465 GeV/c2


• Fit D0 and D0 samples together assuming no CP violation

• y, x’2 contours computed bychange in log likelihood

– Best fit point in non-physical region

1 contour extends into physical region

– correlation: -0.95• Accounting for systematic

errors, no-mixing point is atthe 3.9 contour

Best fit , ’Best fit x 2

≥ + : No mixing(,)

1 – CL =3.17 x 10-1 (1)4.55 x 10-2 (2)2.70 x 10-3 (3)6.33 x 10-5 (4)5.73 x 10-7 (5)

RD: (3.03 0.16 0.10) x 10-3

x’2: (-0.22 0.30 0.21) x 10-3

y’: (9.7 4.4 3.1) x 10-3

Mixing contours

Evidence for D0-D0 mixing!


Allowing for CP ViolationFit D0 (+) and D0 (-) samples separately

CP violation if any (+) parameter differs from corresponding (-)

x’+2: (-0.24±0.43±0.30)x10-3

y’+: (9.8±6.4±4.5)x10-3

x’-2: (-0.20±0.41±0.29)x10-3

y’-: (9.6±6.1±4.3)x10-3

RD=(0.303±0.016±0.010)%AD=(-2.1±5.2±1.5)%

No evidence for CP violation


BABAR 2

BABAR 3

BABAR 1

400 fb-1 PRL 96,151801

no-mixing excluded at 2

stat. only

BELLE 2 statistical

BaBaR/BELLE D0!K comparison

Results consistent within 2


Systematics: variations in Functional forms of PDFsFit parametersEvent selection

Computed using full difference with original value

Results are expressed in units of the statistical error

Validations and cross-checksAlternate 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 mixingx’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 valuesValidation of proper frequentist

coverage in contour constructionUses 100,000 MC toy

simulations

Systematic source

RD y’ x’2

PDF: 0.59 0.45 0.40

Selection criteria:

0.24 0.55 0.57

Quadrature total: 0.63 0.71 0.70

Systematics, Validations


Lifetime ratio backup


D0(t) KK, • Using the D*D0 D0K KK decays from 384 fb-

determine the quantities

and

• CP violating quantities:

• Lifetimes with CP violation:

• If CP is conserved in mixing:

where

CPV in mixing:

CPV in interference of mixing and

decay:


Event Categories


BELLE Ratio Measurement

Also evidence for D0-D0 mixing

BELLE PRL 98, 211803

€

yCP = (1.31± 0.32 ± 0.25)%

AΓ = (0.01± 0.30 ± 0.15)%

3.2σ signal


Mass Projections

• Mass Projections (144 m 146 GeV/c2):

• Signal Purities (1.8495 < m < 1.8795 GeV/c2):


Lifetime difference Cross Checks

Performed several cross checks to ensure unbiased fit results– Fits to generic and signal MC

– Fits with independent resolution functions

– Subdivided fit results into different running periods, D0 lab angles (cos, phi, psi=angle between D0 decay plane and bending plane)

• use high statistics Kpi untagged data sample

Conclusions– No hidden differences between the modes observed which could bias the

mixing parameters, except in the polar angle variation where a small difference in mixing parameters was observed. This is accounted for in the Signal systematic.


Direct CPV backup


Direct CPV Results: aFB

KK

There is a significant FB asymmetry


Direct CPV cross Validations


D0 KK, : CPV in Decay

Soft pion tagging efficiency determined using CF decay

Yields:

•no-tag D0K sample: determines the efficiency D0K relative to D0K

•tagged K sample: determines the slow pion efficiency D0K relative to D0K

•Slow pion efficiency correction is then applied to D0 and D0KK


Production asymmetries and CPV • Forward-backward asymmetries in cc production

• Interference in e−e cc as mediated by either a virtual photon or a virtual Z0. • Higher-order QED box- and Bremsstrahlung-diagram interference effects• Both effects are antisymmetric in cos, the polar angle of the D0 CMS momentum• Direct CPV is symmetric in this variable• Construct symmetric (aCP) and antisymmetric (aFB) combinations of the yield

asymmetries versus cos


Direct CPV Systematic Variations


D0(t)K WS Dalitz fit

• The WS signal contains both DCS and CF amplitudes.• The CF amplitudes are determined in the RS fit and fixed in the WS fit• The total time dependent WS PDF is

where yields are determined from the (m,m) fit• The Dalitz and time distributions for mis-tag events are taken from the

RS Dalitz model and RS time distributions• The term is determined by a (m,m,t) interpolation to the

signal box from the sideband regions


HFAG Rmix world average


D0(t)K Systematics/Checks• Systematics:

• Checks:– extensive Toy MC studies comparing generated and fitted mixing

parameters. No bias seen with high statistics toy samples

Recent Results on D 0 - D 0 Mixing from BaBar

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