Roberto Covarelli (Università degli Studi di Perugia & INFN Frascati)

22/5/2003 ROBERTO COVARELLI

PARTIAL RECONSTRUCTION OF B0D*Ds

(*) AND EXTRACTION OF

BR(Ds ) AT THE BaBar EXPERIMENT

Roberto Covarelli

(Università degli Studi di Perugia & INFN Frascati)

Frascati Spring School, 22/5/2003


Physical motivations …• …for measuring BR(B0 D*Ds

(*))– They provide a test of factorization at high q2 values (q2

= MDs(*)2)

– They are quite poorly known:

BR(B0 D*-Ds+) = (1.11±0.33)% (PDG 2002)

BR(B0 D*-Ds*+) = (1.8±0.6)%

• …for measuring BR(Ds )– All Ds branching fractions are measured as relative to

BR(Ds )

– Latest measurement (CLEO, 1995) shows a 25% relative uncertainty:

BR(Ds+

+) = (3.6±0.9)%– Most b- or c-physics analyses where a Ds or a Ds* are

reconstructed rely on this uncertain value


Partial reconstruction• We refer to partial reconstruction of a B candidate

when all but one of its decay products (the “missing” candidate) are fully reconstructed. For instance:

• Disadvantage: no constraints can be applied on MB or EB in signal extraction worse signal purity

• Advantage: you don’t need to take into account the missing candidate reconstruction efficiency

statistics increased by a large factor (often ≈ 10)

B0 D* +

D0

The fast and slow pion arereconstructed (blue circle), while no attempt is made to reconstruct theD0 (red box).


The B0 D*Ds(*) decay

Partial reconstruction technique can be exploited in two ways:

• Fully reconstructed Ds(*)

• Slow pion from D* missing candidate = D0

or

• Fully reconstructed D*

• Soft photon from Ds*

missing candidate = Ds

B0 D* + Ds(*)

D0 (Ds )

B0 D* + Ds*

D0 Ds


Signal extraction variable:the missing mass

• Assume that the reconstructed candidates come from a B0 D*Ds

(*) decay, the mass of the missing candidate, or missing mass, can be evaluated:

• Signal extraction is achieved by requiring it to stay inside a M(D0) (or M(Ds)) mass window.

2πDB

2πDbeammiss )ppp()EEE(M (*)

s(*)s

2γDB

2γDbeammiss )ppp()EEE(M **

Method I:Ds

(*) –

Method II:D* –


Extraction of BR(B0 D*Ds(*))

• Let ND0 (NDs) be the signal yield from the missing mass calculation. BR(B0 D*Ds

(*)) can be promptly extracted:

• If we use Method II, BR(B0 D*Ds(*)) will no

longer depend on Ds branching fractions, but on D0 ones, which are known with smaller uncertainties.

reduced systematics from BR’s

BRεγ)Dπ)BR(DDD(BR

N

N1

)DDB(BRi

(*)is

*s

0*D

BB

(*)s

*0 0

BRεγ)DD(BRπ)DD(BR

N

N1

)DDB(BRj

*js

*s

0*

D

BB

*s

*0 s

Method I:Ds

(*) –

Method II:D* –


Extraction of BR(Ds )• Let’s now consider both partial reco methods

applied on the same data sample. Two independent estimates of BR(B0 D*Ds*) are obtained, one assuming D0 decay BR’s, the other assuming Ds BR’s.

• Dividing (2) by (1), BR(Ds ) can be extracted:

(1)

(2)

N

BRε

ε

Nφπ)D(BR

s

0

D

j*j

i*i

Ds

RRi = BRi / BR(D0 K)

BRεγ)Dπ)BR(DDD(BR

N

N1

)DDB(BRi

(*)is

*s

0*D

BB

(*)s

*0 0

BRεγ)DD(BRπ)DD(BR

N

N1

)DDB(BRj

*js

*s

0*

D

BB

*s

*0 s


1. Ds(*) – reconstruction:

candidate selection• Ds

selection:– Ds , KK

– Ds K0K, K0s

– Ds K*0K, K*0 K

– Cuts on kaon ID, invariant mass of intermediate states, helicity angles

– M(Ds) is required to be within 3of the signal distribution peak Mpeak (Ds) seen in the data

• For Ds* candidates only:– Photon selection : cuts on

E and E*, 0 veto

– |M – Mpeak| < 2.5M

M(Ds)

M(Ds) peak

M

20.8 fb-1 data



signal extraction

Ds

Ds*

• Observe the missing mass distributions

• The Mmiss distribution is fitted with a Gaussian + a threshold background function

fB = f (Mmiss – M0). The yield in 20.8 fb-1 data (≈ 22.7 x 106 BB pairs) is shown in figureKinematical end point

M0 = MD* - M

ND0 = 3700 ± 230

ND0 = 1493 ± 95

20.8 fb-1 data



final results• Branching fractions:

BR(B0 D*-Ds+) = (1.03±0.14stat.±0.13syst.±0.26)%

BR(B0 D*-Ds*+) = (1.97±0.15stat.±0.30syst.±0.49)%

• The longitudinal polarization fraction in B0 D*Ds* (pseudoscalar vector + vector decay) can also be measured:

L/(51.9 ± 5.0stat.±2.8syst.)%

25% systematicsfrom theuncertainty onBR(Ds )


2. D* – reconstruction: D* candidate selection

• 4 D0 decay modes:– D0 K– D0 K0

– D0 K

– D0 K0s, K0

s

• Cuts on:– vertex fit probabilities– |Q(D*) – Qpeak(D*)| < 2 MeV/c2

(Q(D*) = M(D*) – M(D0) – M())

– |M(D0) – Mpeak(D0)| < 2.5M(D0)

M(D0) in MC

Q(D*) in MC


2. D* – reconstruction: candidate selection

• Kinematic cuts and 0 veto are not enough to reject the substantial background from low energy particles. More stringent requirements needed:– cuts on shower shape

variables – best photon to be chosen on

the basis of a likelihood ratio that combines both kinematical and cluster shape variables

E* in MC

Bkgd (solid)

Signal (dashed)


2. D* – reconstruction: signal extraction

(MonteCarlo)• Fit the missing mass

distribution with a Gaussian + ARGUS background function

(fit yields in MC

shown in the figure)

BR(B0 D*-Ds*+) = (1.5±0.1)% (in MC) No bias

(1.4% in MC Production)

2

2

b

x1a

2

2

bx

1Cx)x( ef


2. D* – reconstruction: signal extraction (data)

• The Mmiss plot on data (20.25 fb-1) is shown

• The background shape and signal resolution are well reproduced by MonteCarlo

but• Before we apply the fitting procedure, we need cross-checks in order to validate this result


2. D* – reconstruction: cross - checks

• Data/MC agreement Kolmogorov test for

binned distributions

• Other cross-checks: “flipped” sample (the reconstructed D* flight direction is inverted to obtain a pure combinatoric background sample)

• Peaking backgrounds, systematics from detector inefficiency and other sources still under evaluation

M(D0)

MC (B0) MC (B±) MC (qq) data

Missingmass

normal flipped


Expected error on BR(Ds )

• Let’s put ourselves in the most favourable conditions (full BaBar statistics, just one D0 and Ds reconstruction mode, so that the systematics from branching fractions totally cancel)

• The relative uncertainty on BR(B0 D*-Ds*+) (Method I) is:

6.6%stat. 9.4%syst. = 11.5%

• and on BR(B0 D*-Ds*+) (Method II):

6.7%stat. 10%syst. = 11.6%

• we estimate a 16% maximum uncertainty on BR(Ds) (was 25% in the most recently published measurement)

Statistics from MonteCarlo Rough estimate


Summary• The branching fractions BR(B0 D*-Ds

+) and

BR(B0 D*-Ds*+), as well as the fraction of longitudinal polarization in B0 D*-Ds*+, have been measured using a Ds

(*) – partial reco technique. The results are:

BR(B0 D*-Ds+) = (1.03±0.14stat.±0.13syst.±0.26Ds )%

BR(B0 D*-Ds*+) = (1.97±0.15stat.±0.30syst.±0.49Ds )%

L/= (51.9±5.0stat.±2.8syst.)%

• The complementary reconstruction, D* - , is being performed, in order to obtain a second, independent measurement of BR(B0 D*-Ds*+) and extract BR(Ds)

Roberto Covarelli (Università degli Studi di Perugia & INFN Frascati)

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