CHARM MIXING and lifetimes

on behalf of the BaBar Collaboration

XXXVIIth Rencontres de Moriond March 11th, 2002

at

Search for lifetime differences in D0 decays

New BaBar limits on D0 mixing parameter y:

- method and event selection

- lifetime extraction, systematics and results

Alexis POMPILI Università and I.N.F.N. Bari

2

MIXING PARAMETERS:

2, y

Mx

( flavour

eigenstates) 00 |,| DDMass eigenstates: 000

2,1||| DqDpD

Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

INTRODUCTION: x, y

masses: M1 , M2 & widths: 1 , 2

In the SM :

310|y|,|x| …but …New Physics can significantly enhance x

FSI and SU(3) breaking can enhance y

Current sensitivy @ level of 10-2 (few 10-3 with 0.5ab –1 @ Belle & BaBar )

3 types of experiments are sensitive to a combination of x & y :

Wrong-sign searches :

LIFETIME DIFFERENCE searches

HADRONIC

Semileptonic

21

21

21

2/)(

MMM

3Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

Lifetime difference search strategy

Rate asymmetry for neutral D decays into CP+(even) and CP (odd) eigenstates :

1)(

)π(1

)mixequal(ˆ

)(ˆ

)(ˆ)(ˆ)(ˆ)(ˆ

y0

0

KKD

KD

CP

CP

CPCP

CPCP

ττ

CP

KK+ (+) is CP+ eigenstate & K+ assumed an equal mixture of CP+, CP

φsin x 1)q/p(21φcosyy 2CP

NO CP yy CP

1|/| pq0φsin CP in interference mixing-decay

CP in mixing

000

2,1||

2

1| DDD

Extracting the rates by fitting to pure exp. the time-dependent rates:

seigenstateare CP

4Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

D0 proper time reconstruction method

π-π,,π;π; 00

s** KKKDDDXD --cc-ee Reconstructed DECAY

CHAIN TAG 3 independent

samples 1) allows flavor-tagging

2) reduces BKG

m = m(h+hs) -

m(h+h)

D* is primary vertex : use BS constraint

Refitting technique with BS constraint

D in

xym9000μzμm120xμm5:BS σσσ y

pointdecay locate toSB toback points)1 *0 Dp

D

point this torefitted pion π slow)2 s

s0* πrefitted theand D theexingbuilt vert )3 D

improves m resolution

TRTRTRTRTR

TR0

00

0

0 ˆ)(, :eproper tim * DDDdec

D

decD

pddlcp

lmtD

DECAY LENGTH

Beamspot

μm65resolution

μm130TR declμm5

5Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

D0 candidates selection criteria

REJECTION of D* from B decays:

GOOD VERTEX FIT for BOTH D0 and D* vertices :

GOOD QUALITY TRACKS & VERTEXING RESOLUTION (at least 6 SVThits for s)

cGeVpD

/5.2Y(4S)0

%1)χ( 2FIT

P

cc ONLY

PARTICLE IDENTIFICATION applied on BOTH D0 DAUGHTERS

K : tight likelihood selection : likelihood selection & muon veto

REJECTION of combinatorial BKG due to low p pions : helicity-cut onCMS D

0D-πθcos

m-cut :

hitswith[out]DCHπs forpeak fit around window]3[2 MeV

( pion contamination: <3% for p<3GeV/c )

This analysis uses 57.8 fb-1 (2000+2001 data) & selects events from continuum

ccIt uses also simulated (G4) data : Tracking [DCH+SVT], vertexing [SVT], PID [DIRC+(DCH+SVT)]

are crucial

samples& generic 30 1 signalqqfb

6Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

m(h+h

)m=m(h+hs)-

m(h+h)

K+

+

K+

Selected D0 candidates after m-cut

K-+

K-+

000,158

500,16

350,8

%5.99BS

S

%1.97BS

S

%4.92BS

S

7Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

Unbinned maximum likelihood fit

)parametersδt;,t,(where 11imiPiPiL

[1Psig(mi

)]

Psig(mi

) SIGNAL

BKG

proper-time RESOLUTION

(1-f2)(1-f3) + f2(1-f3) + f3

S1 S2 3

f0f4

4 fixed

(1-f0) + f0(1-f4)

NO-flying BKG

flying BKG

BUT with ...

for tails

SBKG S1

SBKG S2

3

BKG

D

8Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

K+ +K+

D0 candidate signal probability in the lifetime fit

Portionassigned to BKG

LOW MASS TAIL

BKG constrained in data

“sidebands”candidatesas part of likelihood fit

K-

+K-

+

9Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

Proper time distributions and lifetime fit results

K+ K+ +

Points : DATA |White histogram : FIT RESULT |Gray histogram : BKG fit portion

10Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

Lifetime ratio systematics - I

D0 lifetime measurement is an HIGH PRECISION measurement [ statistical uncertainty for our data sample is about 1.3fs namely 3/1000 ]At this level of precision wide studies of systematics sources

(tracking, vertexing, alignment) are needed.

At this stage we do not present an absolute D0 lifetime value because theoverall systematic uncertainty (about 34fs) can be reduced with further work

Note: 1.5m average bias in vertex position translates in 3fs lifetime bias 1MeV mass shift due to momentum bias means 0.3fs lifetime bias

We checked, on a limited sub-sample, the full compatibility of the fit resultwith both our HF2001 check and PDG value

fs])stat.(2412[ fs]8.26.412[

Many systematics effects on lifetime cancel in the lifetime RATIO

11Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

Lifetime ratio systematics - II

y systematics uncertainties calculated using large signal MC samples

Simulated data has been modified by variation reflecting uncertainties in…- detector biases- event selection criteria- size and composition of BKG contribution- beamspot position and size

SVT internal alignment systematic checks done using

MC estimation shows about null bias within its statistical error which is taken as a fur-ther systematic uncertainty

prongsee 4 γγ

[zero lifetime control sample]

12Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

RESULTS: limits on y

)()syst.(2.14.1)stat.(7.10.1

)()syst.(6.07.0)stat.(2.16.1

ππ0

0

yD

KKD

PRELIMINARY

%).syst(6.07.0).stat(0.14.1 ][y

EPS-2001

13Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

CONCLUSIONS AND NEAR TERM PROSPECTS

Limits on x, y from hadronic wrong-sign analysis coming soon !

Our preliminary y measurement is consistent with zero but suggest a positive value not incompatible with FOCUS measurement

It is crucial to get a new measurement over LARGER DATA SAMPLES

It is important to get a result from D*untagged KK sample :the statistical uncertainty is <1% but still working on systematics

BACKUP TRANSPARENCIES (NOT SHOWN)

1+Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

Wrong-sign hadronic search strategy

Look for deviation from exp in the time evolution of neutral D wrong-sign decay :

)(t 2241 yxt ye

π)(π)( 00

DCSDCSKtDKtDRR

x’ y’

DCSD INTERFERENCE MIXING

HF9

: strong phase diff. between DCSD & CF

D*+ D0(cu) +

D0

MIXING

CFD K+

DCSDNOCP

Type of experiment sensitive to (xcos+ ysin) cos, (ycos-xsin) sin, x2 +y2

2+Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

Kaon ID performance

< Efficiency >

< Mis-ID >

%85

cGeVp /5.2,%2

cGeVp /5.3,%5

3+

Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

Low mass tail

MC study: possible explanation is internal bremsstrahlung

Simulation by randomly selecting 5% of D0 candidates for which one daughter’s momentum is corrected by

the factor [1-(E/mD)] where

energy Eis properly randomly generated.

4+Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond

MC check : lifetime pull

MC indicates very little or null bias The resolution model is found to fit reasonably well to the MC

(measured – true) proper time(meas. – true) pr. time /pr. time error