roberto covarelli (università degli studi di perugia & infn frascati)
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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
22/5/2003 ROBERTO COVARELLI
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
22/5/2003 ROBERTO COVARELLI
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).
22/5/2003 ROBERTO COVARELLI
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
22/5/2003 ROBERTO COVARELLI
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* –
22/5/2003 ROBERTO COVARELLI
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* –
22/5/2003 ROBERTO COVARELLI
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
22/5/2003 ROBERTO COVARELLI
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
22/5/2003 ROBERTO COVARELLI
1. Ds(*) – reconstruction:
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
22/5/2003 ROBERTO COVARELLI
1. Ds(*) – reconstruction:
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 )
22/5/2003 ROBERTO COVARELLI
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
22/5/2003 ROBERTO COVARELLI
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)
22/5/2003 ROBERTO COVARELLI
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
22/5/2003 ROBERTO COVARELLI
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
22/5/2003 ROBERTO COVARELLI
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
22/5/2003 ROBERTO COVARELLI
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
22/5/2003 ROBERTO COVARELLI
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)
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