CHARM MIXING and lifetimes

CHARM MIXING and lifetimes

on behalf of the BaBar CollaborationXXXVIIth Rencontres de Moriond March 11th, 2002

at

Search for lifetime differences in D0 decaysNew 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, yMx

( flavour eigenstates)

00 |,| DDMass eigenstates: 0002,1

||| 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 xFSI 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

y 0

0

KKDKD

CP

CPCPCPCPCP

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


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 xy

m9000μzμm120xμm5: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

000

0 ˆ)(, :eproper tim * DDDdecD

decD pddlcpl

mtD

DECAY LENGTH

Beamspot

μm65resolution

μm130TR declμm5


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 D0D-π

θcos

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


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


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


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-

+


Proper time distributions and lifetime fit results

K+ K+ +

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


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


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]


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


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 yeπ)(π)( 00

DCSDCSKtDKtD RR

x’ y’

DCSD INTERFERENCE MIXING

HF9

: strong phase diff. between DCSD & CF

D*+ D0(cu) +

D0

MIXINGCFD K+

DCSDNOCP

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


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.


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

CHARM MIXING and lifetimes

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