D0-D0 Mixing at BABAR

Amir RahimiThe Ohio State University

For BABAR Collaboration

A. Rahimi Charm2006 2

Outline

– Introduction to mixing – Motivation for using this mode – Mixing formalism in a multibody decay– Lifetime fit and mixing results

D0-D0 Mixing with 0 0D K

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Introduction

• The mass eigenstates and have different masses and lifetimes

• Signs of new physics:– Observation of CP violation

–

Time integrated mixing rate:

1D 2D

2 2

2M

x yR

x y

001,2

2 1 2 1

2 1 2 1

2 ,

D p D q D

m mx y

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Decay Mode

• Obtain a pure sample by reconstructing:

• In cc-bar events – Obtain the flavor of D0/D0bar from charge of slow

– Right-sign Cabibo-favored (CF) D0K-+0 used for normalization

– Wrong-sign D0K+-0 has contributions from doubly Cabibo-suppressed (DCS) decays and CF mixed decays

– Separate signal from background by fitting to m(K0) and

M = m(K0s) – m(K0)

* 0 0 0,sD D D K

0 0D K

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Branching Ratio

• In non-leptonic search for D mixing, DCS obscure the signs of mixing:– Consider

– Belle

– BABAR

– Compare to the standard decay D0K+-

0 0

0 0

( )

( )

D KR

D K

0.0130.009(0.229 0.015( ) ( ))%R stat syst

0

0

( )(0.362 0.029)%

( )

D K

D K

(0.214 0.008( ) 0.008( ))%R stat syst preliminary

hep-ex/0507071

0 0D K

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Resonance Contributions

• The resonance amplitudes are different for DCS and CF- there is more sensitivity to mixing– In D0 K-+0 the main

resonance is K- +

– In D0 K+-0 the main resonance is K*+ -

u

u

u

uu

u

s

d

+, +

-, -

s

c

c

d

ss

W +

W +

CF

DCS

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Event-level tagging

• To do a Dalitz analysis need to reduce the large peaking background in DCS– Real D0’s with uncorrelated

slow pions

• Use an event-level tag• Require a second tag in the

opposite event hemisphere– Use K+, ±

s, e±, and ±, in the other side of the event

– Provides consistency check on

tagK+

tag

0

+tag

D0

D0-tag

interaction point

beamspot

-tag

e-tag

K-

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Event-Level Tagging

• Using an event-level tag significantly reduces background – Use K+, ±

s, e±, and ±, in the other side of the event

– Never done before in this type of analysis

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Event-Level Tagging

• With this second tag, we can now look at the resonance contributions

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Resonance Contributions

• Event-level tagged• Prominent K* peak in DCS Mode

D0 K- + 0 D0 K+ - 0

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Resonance Contributions

• Event-level tagged• Prominent peak in CF Mode

D0 K- + 0 D0 K+ - 0

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Selection of Phase-Space Regions

• Based on inspection of the Dalitz plots, we exclude events in the regions:– 850 < m(K) < 950 MeV/c2

– 850 < m) < 950 MeV/c2

WS (3.8 ± 0.36) x 102 (3.79 ±0.36) x 102

RS (2.518 ±0.006) x 105 (2.512 ±0.006) x 105

WS (7.5 ± 0.5) x 102 (8.1 ±0.5) x 102

RS (3.648 ± 0.007) x 105 (3.646 ± 0.006) x 105

(a)

(b)

D0 Cand. D0 Cand.

(a) entire allowed phase-space region

(b) selected phase-space region for mixing analysis

Preliminary

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2 22( )

( ) ( )( ) 4

0 1

WSD D

RS

t x yR y R t t

t

Decay Time with Mixing

• At any particular point in phase space (Dalitz Plot):

• Integrating over an arbitrary region of phase space:

2 22( )

( ) ( )( ) 4

WSD D

RS

t x yR y R t t

t

is the suppression factor) cos sin

cos sin

y y x

x y x

2 2 2 2x y x y

cos sin

cos sin

y y x

x y x

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1

1

2 2 2 2

0 0 0 0

0 0 0 0

( cos sin )

( ) ( )

( ) ( ) / ( )

( ) ( ) / ( )

py y x

q

px y x y

q

for D K D K

for D K D K

Consider CP Violation

• We account for possible CP violation by fitting D0

and D0 separately and making the substitutions:

( and are the suppression factors)

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PDF Fit to Decay Times

• Data after a statistical background subtraction

Decay times in a signal region

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Preliminary Mixing Results

CP Conserved0.0180.014

0.0260.022

(0.023 0.004)%

(0.164 0.012)%

M

D

R

R

0.0060.0080.012 0.002y

• RM < 0.054% upper limit at 95% confidence level (determined using logL )

• Consistent with no mixing at 4.5% confidence level (determined using a frequentist method)

Contours determined using logL levels

hep-ex/0605046

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Preliminary Mixing Results

CP Violation Allowed

0.0060.007

0.0020.005

cos 0.012

sin 0.003

y

x

1.91.02.2p q

• Contours determined using logL levels

0.0220.007(0.010 )%MR

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Summary and Outlook

• Performed the first analysis of D0 K-+0

• Uncovered the DCS Dalitz plot• Time-dependent Dalitz plot analysis of this mode is

underway• Additional BABAR mixing results coming up soon:

– Semi-leptonic mixing using doubly-tag analysis– D0 K-

– D0 K-

• An observation of D mixing may be on the horizon

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Charm Mixing in The Standard Model

• Box Diagram SM Charm Mixing is expected to be very low

• Long distance SM predictions accommodate higher rates

d, s, bV*ci Vui

Vuj V*cj

D0 D0

c

u

W

d, s, b

W

c

u

SM Mixing: box diagram

SM Mixing: a long-range contribution

D0

D0

c

u

c

u

u

W+

d

d

u

u

d

d

u

W-

(Plot courtesy of A. Petrov, hep/ph 0311271)

: x=M/: y=/2

mix

ing

rate

= |a

mpl

itude

|2

SM Mixing Predictions

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Fit to the CF Events

• ML Fit and data projected in signal regions

0.145 < M < 0.146 GeV/c2 1.85 < m(Kpp0)< 1.88 GeV/c2

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Upper Limit on RM

• We use logL to set an upper limit– Behavior near zero

consistent with a frequentist method

– Straight forward to compare with other experiments