B semileptonic decays Nov.28 – Dec.2, 2005 Saga-Yonsei Workshop Youngjoon Kwon (Yonsei Univ.) 1 B semileptonic decays from Belle n Introduction –weak interaction

The Title*
Introduction
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Considering W-exchange only,
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B semileptonic decays Nov.28 – Dec.2, 2005 Saga-Yonsei Workshop Youngjoon Kwon (Yonsei Univ.)
Lepton universality
Do all leptons and quarks carry the same unit of weak charge?
YES, for leptons and NO for quarks
First, let’s consider a muon decay
low energy process (q2 ~ mm << mW)
specified by the Fermi constant: GF (~ g2/mW2)
dimensional analysis
Lepton universality
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Lepton universality
Since
Universality of weak interactions?
Do all leptons and quarks carry the same unit of weak charge?
Yes, for leptons and no for quarks
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Universality of W.I. for quarks?
Consider the (semileptonic) weak decay
Assuming universality of weak decays of quarks,
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So, what are we going to do?
No universality for weak interaction?
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Cabibbo theory
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Cabibbo theory
What we have done is to change our mind about the charged current: “Cabibbo-favored” vs. “-suppressed”
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Cabibbo theory
(Ex) What is the relationship between the weak couplings for muon decay (Gm=GF) and nuclear b-decay (Gb) ?
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Leptons do not have strong interactions
Quarks & Leptons do not change its flavor when interacting with neutral gauge bosons
quarks do not change flavor under strong int.
leptons & quarks do not change flavor when interacting with g or Z0
leptons & quarks change flavor only when interacting with W, and only within its family
W
t
b'
W+
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Then how does b decay at all?
Note: b W - t but m(t) >> m(b)
For quarks,
weak interaction
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CKM is 3x3 and unitary
only 3 generations in the SM
CKM is almost 1, but not exactly
Vii 1, Vij 0 for i j
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Free parameters of the S.M.
17 free parameters in the electroweak interactions
3 lepton masses
6 quark masses
m(Higgs)
10 of these are related with “quark flavors”
i.e. we don’t know much about flavor sector in the SM after all these years of learning...
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Vud
Vcd
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Vcs
from semileptonic decay of D meson
unitarity constraint assuming only 3 generations gives a much tighter bound
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Vcb: from and HQET
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Vub: from
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Vtb: from t-quark decay, assuming only 3 generations
at the Tevatron collider at Fermi Lab, top quarks are produced mainly in pairs
Assuming one could obtain a pure sample of
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Vtd
Vtd
(s)
(s)
The Unitarity Trianlge
* other triangles are difficult to measure
16640.bin
Challenge of Vub
Contains complex phase
No golden mode
Final testing ground for CKM mechanism
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Issues in Vub measurement
More accessible than B leptonic
Inclusive and Exclusive
Limited phase space
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CKM Reach
(Table compiled by SBF E2 subgroup & E2 convenors)
SBF numbers based on BABAR experience need simulations to estimate signal
efficiencies & bkgds so that the comparison to BTeV/LHC-b is on an equal footing
[Snowmass 2001]
Vub from semileptonic B decays
Charmless semileptonic B decay
Inclusive BF – predicted by OPE & HQS
MX
pl
q2
Reconstruction of MX and q2 at U(4S)
Bopp
Bsig
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Vub from inclusive S.L. B
~10%
~80%
~15%
Cuts increase theoretical error due to extrapolation error
1) Lepton energy (End-point)
2) Leptonic invariant mass
3) Hadronic invariant mass
Shape Function region:
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Existing analyses on Mx (1)
bg
0
-2
2GeV2
Existing analyses on Mx (2)
Full-reconstruction method
=>
=>
Inclusive B Xu l n
with simulated annealing
Novel Xu recon. by Belle
n reconstruction by (E,p)miss
as belonging to signal B (Bsig) and associated B (Bopp)
S. Kirkpatrick et al., Science 220, No.4598 (1983)
good effi. w/ reasonable Mx resol.
both S/N & dMx are worse than full recon.
Belle’s new result: PRL 92, 101801(2004)
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Separation of two B meson decays
by “simulated annealing”
6 discriminants are used
p*B, E*B , cos q*B, Nch of the Bopp in CM
QB x Ql, MM2Xln (w/ E*B , MB of Bopp fixed to known values)
Using BXln MC, calculate PDFs for correct
and random Xln combinations
Calculate W with two likelihoods L(correct) and L(random)
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Simulated Annealing
If Wnew>Wold, keep new configuration with probability of exp{-1000(Wnew-Wold)/Wold}, otherwise keep old configuration
Repeat this process untill all particles are moved at least once
One iteration
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Selecting “particles”
Require |MM2|<1.5GeV2
select combination with smallest |MM2|
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Simulated Annealing – results
Event Selection
Data sample
Belle data at the energy asymmetric e+e- collider KEKB
78.1 fb-1 on resonance (85106 BB) taken at the U(4S)
8.8 fb-1 off resonance (taken at 60 MeV below the resonance)
Selection of the final candidate
W < 0.1
0.25 < p*B < 0.42GeV
- 2 < Ql • QB< +1
- 0.2 < MM2Xln < +0.4GeV2
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MX and q2 Resolutions
Mx and q2 resolution distributions for BXuln signal MC events
Simulated annealing results
s=0.34GeV
Tests of validity
38K D*ln
84K J/ X, J/ l+l-
l+or l- treated as n
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Backgrounds
8910 events observed
Continuum b.g. = 25148 events is estimated using off-resonance data.
BXcln (dominant) and other BB backgrounds are estimated by MC.
MC is tuned by 2D-2fitting the Mx vs q2 distributions of MC events
to those of the real data in the BXcln dominated region (Mx >1.8 GeV).
In the fit, fractions for B D(*)ln,D** ln, D(*)pln
and the total rate for other BB backgrounds are floated.
BB b.g. in the signal region: 7283 130(fit) 63(MC stat) events
Net signal : 1376 167(stat) events
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Results
DB (BXuln ) for (MX<1.7 GeV & q2> 8 GeV2 )
Signal MC: based on Fazio & Neubert (hep-ph/9905351)
Leading order in HQE with O(as) QCD correction
Including the shape function effect
mb=4.800.12 GeV, l1=-0.300.11 GeV2
(From CLEO data for E(g) spectrum in bsg and Mx moments in bcln)
Unfolding: Ntrue= NobsF (F 1+N2/N1-N3/N1)
Efficiency correction: DB=0.5Ntrue/(esignalreff)/(2NB)
Nobs =1376 167(stat)
DB (MX<1.7 GeV and q2> 8 GeV2)
= (7.370.89(stat)1.12(syst)0.55(b c) 0.24(b u) )10-4
N2
N1
N3
generated
observed
DB (BXuln ) for (MX<1.7 GeV & q2> 8 GeV2 )
Systematic error (15%)
Dominated by distortion of Mx and q2 distributions due to imperfect detector simulation
Model dependence
B Xcln 7.4%
Change the form factor parameters for Dln and D*ln within the errors
Change fraction of D1ln and D2ln in D**ln by 25%
B Xuln 3.4%
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Extraction of |Vub|
Conclusion
Using a novel technique “simulated annealing”
The result
with much more reliable theory error estimation
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Epilogue
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Variables to suppress b c l n
(10% phase sp.)
Inclusive B Xu l n with End-point
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Main features
Kinematic selection
pe > pcut
No requirement other than on the
signal electron
high statistics
bkg. sys. uncertainty
b-quark Shape Function
What is it?
How to get it?
Universal at leading order for all b light quark transitions
Use E(g) in B Xs g
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Three models for SF
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Event Selection
Hadronic events
Continuum suppression
Fox-Wolfram moment
|cos qth| < 0.75
Background estimated by
low-p region for check
1.5 < pe < pcut (GeV/c)
E(e) spectrum for B Xu e n
eff. corrected
DB – partial BF
Standard method (DFN, JHEP 9906, 017)
New method (BLNP, NP B699 (2004))
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Vub from DB
Inclusive B Xu l n with Full-Recon.
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4.2105 evts./253 fb-1
Signal selection
Lepton Selection
J/y, g, p0 veto
No other lepton
For b u
Total charge = 0
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Kinematic selections
(1) MX < 1.7 GeV
2-component fit to extract N(bu)
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(2) MX < 1.7 GeV & q2 > 8 GeV2
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(3) P+ < 0.66 GeV
First measurement using P+
Signal Yield & DB
Vub from DB
with an improved treatment of shape-function effects
+ weak annihilation effects estimated
Exclusive B Xu l n with S.L. tag
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Experimental Strategy
ICHEP04 preliminary
PRL 93, 131803 (2004)
Lum.
How well can we measure the q2 dist. (for p l n) ?
+ p l n / r l n
(preliminary)
low
middle
high
poor
mod.
good
Analysis Method
Kinematics of double semileptonic decay
Back-to-back correlation of the two B’s constrains their direction to the intersection of the 2 cones.
or
Signal side reconstruction
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Calibration with Bsig→D*ln Decays
Validity of the method for double semileptonic decay
detection has been tested with
The ratio Nobs/Nexpected = 0.89 0.08 is used to correct the MC efficiency for p l n and r l n detection.
M(K+p-p-) on the signal side
xB2 distribution
Signal Extraction
q2 distribution is extracted by fitting the (mX,x2) distrib. for three q2 intervals.
mX (GeV/c2)
Fitting components: p ln, r ln, other Xu ln, and BB
PDF’s are based on MC.
Constraint for extracted Br:
q2<8GeV2
Results
Vub from exclusive B Xu l n
140 fb-1
Belle Vub Summary

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B semileptonic decays Nov.28 – Dec.2, 2005 Saga-Yonsei Workshop Youngjoon Kwon (Yonsei Univ.) 1 B semileptonic decays from Belle n Introduction –weak interaction