New Results from Babar: Evidence for D 0 -D 0 Mixing In D 0 → K - p +

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New Results from Babar:

Evidence for D0-D0 Mixing In D0 → K+

Moriond EW March 13, 2007

Kevin FloodUniversity of Wisconsin

for

The Babar Collaboration

2

Outline

• Charm mixing formalism

• SM and New Physics predictions

• Mixing analysis methodology

• Results and systematics

• Summary

Moriond EW March 13, 2007Kevin Flood

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Time Dependence of Mixed Final States: No CP Violation

• Mixing implies that the physical states are not pure flavor states

002,1 DqDpD 1

22 qp,

• Charm mixing values typically quoted using scaled parameters x, y

2

22 yxRM

12 MMM

1221

12

• For |x|, |y| << 1, time-dependence of a hadronic final state with mixing and DCS amplitudes

2

22

4t

yxtRyR

t

tDD

RS

WS

in the limit of no CP violation, and where

KK yxx sincos KK xyy sincos ,with Kp being the relative strong phase between DCS and mixing amplitudes

• Time-integrated mixing rate

2 , y

Mx


4

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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• Possible enhancements to mixing due to new particles and interactions in new physics models

• Most new physics predictions for x– Extended Higgs, tree-level FCNC– Fourth generation down-type quarks– Supersymmetry: gluinos, squarks– Lepto-quarks

• Large possible SM contributions to mixing require observation of either a CP-violating signal or | x | >> | y | to establish presence of NP

c

u

u

c

H0

FCNC

c

u

u

c

Charm Mixing Predictions

• Box diagram SM charm mixing rate naively expected to be very low (RM~10-10) (Datta & Kumbhakar)

• Z.Phys. C27, 515 (1985)

– CKM suppression → |VubV*cb|2

– GIM suppression → (m2s-m2

d)/m2W

– Di-penguin mixing, RM~10-10 (Petrov)• Phys. Rev. D 56, 1685 (1997)

• Enhanced rate SM calculations generally due to long-distance y contributions:

• Recent SM predictions can accom-modate high mixing rate (Falk et al.)

– x,y ≈ sin2 C x [SU(3) breaking]2 ~1%• y: Phys.Rev. D 65, 054034 (2002)• x: Phys.Rev. D 69, 114021 (2004)

Standard Model New Physics

qq~ ~

g~

g~

supersymmetry


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Babar Detector and Dataset

• Babar integrated luminosity ~390 fb-1 (Runs 1-5)

• Peak instantaneous luminosity ~1.2 x 1034 cm-1 s-1

Runs 1-4Run 5

Babar Detector


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Mixing Analysis Strategy


• Blind analysis of D*+ → D0(→K) tag

–All event selection, fitting methodology determined before looking at the data

• Four-dimensional unbinned maximum LH fit– First fit M(K), M = M(Ktag) – M(K) [correlated fit]– Fix results of first fit, then fit decay time and errors

• High-statistics RS dataset gives WS signal PDFs–No MC dependence, all PDFs obtained from data

• Fit WS proper time distribution to distinguish DCS and mixing contributions

–Use M(K) and M to separate backgrounds from signal

• Several WS proper time fits–no mixing–mixing, no CP violation–mixing, CP violation

Kevin Flood

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Event Selection


• Beam-constrained simul-taneous fit of K, , tag tracks

– fit probability > 0.001– decay time error < 0.5 ps– -2 < decay time < 4 ps

• D0 selection– CMS p* > 2.5 GeV/c– K, particle identification– DCH hits > 12– 1.81 < M(K) < 1.92 GeV/c2

• 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

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

Kevin Flood

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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/WS Datasets After Event Selection


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RS/WS M(K), M Distributions


Correlation between m and m in signal events taken into account in PDF

Kevin Flood

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Signal and Background Kinematic Fit Categories


• Fit RS/WS M(K), M distributions with signal and three background PDFs

• RS categories– Signal: peaks in M(K), M– True D0 combined with random tag: peaks in M(K)– Mis-reconstructed D0: peaks in M

• Semileptonic D0 decays; singly mis-identified D0 → , KK

– Purely combinatoric, non-peaking

• WS categories– Signal: peaks in M(K), M– True D0 combined with random tag: peaks in M(K)– Mis-reconstructed D0: peaks in M

• Doubly mis-identified D0 → K

• Singly mis-identified D0 → , KK

– Purely combinatoric, non-peaking

Kevin Flood

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Simultaneous Fit to RS/WS Data


RS signal: 1,141,500±1200

combinations

WS signal: 4030±90

combinations

Kevin Flood

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Proper Time Analysis


• Fix M(K) and M PDF shapes

• Fit RS decay time, error distribution to determine signal lifetime and resolution model

– Signal, background D0 PDF: exponential with sum of three gaussians resolution model fit using per-event lifetime errors

– Random combinatoric PDF: gaussian + Crystal Ball function

• Fix WS resolution and DCS lifetime from RS fit– Signal PDF: theoretical mixed lifetime convoluted with

resolution model from RS fit– Background D0 PDF: shares RS PDF– Random combinatoric PDF: gaussian + Crystal Ball function

separate from RS fit

Kevin Flood

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RS Decay Time Fit


• Varied fit parameters– Fit class normalizations– D0 lifetime– Resolution model– Combinatoric shape

• D0 lifetime is con-sistent within total (stat+sys) error with PDG value

plot selection:

1.843<m<1.883 GeV/c2

0.1445<m< 0.1465 GeV/c2

Kevin Flood

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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 – No mixing PDF

Mixing – No mixing PDF

Kevin Flood

16Moriond EW March 13, 2007

Systematics

• Sources– Variations in functional form of signal and background PDFs– Variations in the fit parameters– Variations in the event selection

• Single parameter systematic estimates from difference between parameter value from fits with and without variation, expressed in units of the statistical error

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

Kevin Flood

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Mixing Contours: No CP Violation


• Accounting for sys-tematic errors, the no-mixing point is at the 4-sigma contour

best fit best fit, x’2 ≥ 0 X (0,0)

RD: (3.030.160.06) x 10-3 x’2: (-0.220.300.20) x 10-3

y’: (9.74.42.9) x 10-3

• 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: -0.94

Kevin Flood

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Mixing Contours: CP Violation


• Fit D0 and D0 samples for mixing separately

x'2+ = -0.024+-0.043%y'+ = 0.982+0.637%

x'2– = -0.020+-0.041%y’– = 0.963+0.614%

No evidence for CP violation

Kevin Flood

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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

Kevin Flood

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Validation: RS Mixing Fit


• Perform mixing fit to RS data– No mixing signal expected

• y’ = (2.6 ± 2.4) x 10-4

• x’2 = (9.2 ± 10.6) x 10-6

• No mixing signal found– -2 (logLHmix – logLHno-mix) = 1.4

Kevin Flood

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Summary


• Assuming CP conservation and including systematic effects, we find a charm mixing signal at the 4 sigma confidence level

– y’ = (9.7 ± 4.4 ± 2.7) x 10-3

– x’2 = (-0.22 ± 0.30 ± 0.20) x 10-3

• No evidence for CP violation• Results consistent with previous analyses

– Babar K, 2003: (-56 < y’ < 39) x 10-3 , x’ < 11 x 10-3 (95% CL)– Belle K, 2006: (-28 < y’ < 21) x 10-3 , x’ < 3.6 x 10-3 (95% CL)– Assuming K ~ 0, can also compare with Babar and Belle

measurements of y using decays to two-body CP eigenstates• Belle, 2003: y = (11.5 ± 6.9 ± 3.8) x 10-3

• Babar, 2003: y = (9 ± 4 ± 5) x 10-3

• New Babar charm mixing result in semileptonic mixing will be discussed in charm mixing review by Marco Staric

– complementary technique to K, but not as sensitive and no significant signal observed

Kevin Flood

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Additional Slides


23Moriond EW March 13, 2007Kevin Flood

New Results from Babar: Evidence for D 0 -D 0 Mixing In D 0 → K - p +

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