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The First Evidence of Bt n from Belle
& Future Prospect
As a contribution to WG2 (neutrino modes)
Ref: K.Ikado’s talk at FPCP06
hep-ex/0604018
Toru Iijima & Koji Ikado
(Talk presented by T.I.)
Nagoya University
May 15, 2006
“Flavour in the LHC era” @ CERN
Bt n (within the SM)
 Proceed via W annihilation in the SM.
 Branching fraction is given by
 Provide information of fB|Vub|
– |Vub| from BXu l n
– Br(Btn)/Dmd
fB
|Vub| / |Vtd|
 Expected branching fraction
| Vub | (4.39  0.33) 103
HFAG [hep-ex/0603003]
f B  (0.216  0.022) GeV
cf) Lattice (d~10%)
Br ( B  tn )
 (1.59  0.40) 105
HPQCD [PRL95,212001(2005)]
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Bt X as a Probe to Charged Higgs
Charged Higgs contribution to B decays
 Leptonic: Bt n
mb tan  + mu cot 
mt tan 
b
u
Br(SM)
~ 9 x 10-5
H+/W+
t+
 Semileptonic: BD t n
mb tan  + mc cot 
c
b
H+/W
+
mt tan 
t+
nt
Br(SM)
~ 8 x 10-3
B
( B  Dt vt )
( B  Dv )
Decay amplitude  mb mt tan 2 
Tauonic decay is the most sensitive
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Search for Btn




B tn is important for both SM and BSM.
Purely leptonic  Theoretically very clean
More than two n’s  Experimentally very challenging.
Its detection is a milestone of B physics.
B factories
LEP
First Evidence !
April 2006
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Btn Analysis Concepts
 B decays with missing neutrinos lack the kinematic
constraints which are used to separate signal events
from backgrounds (Mbc and DE).
 Reconstruct the decay of the non-signal B (tagging), then
look for the signal decay in whatever is left over
More than 2 neutrinos
appear in B tn decay
Tagging side :
Fully reconstruct
hadronic modes
n
n
B-
Y(4S)
Signal side :
B+
Reconstruct particles
from t decay
p+
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Features with Fully Reconstructed B Tag
 Pros: Offline B meson Beam
– B momentum is known.
 Resolution of Mmiss2 can be
significantly improved.
– B-flavor/charge is known.
Mmiss2 for B-D0  n (MC)
w/o B momentum
 We can treat charged &
neutral B separately
 Large background reduction
with B momentum
 Cons: Low statistics
– Tagging efficiency : 0.2 - 0.3%
 Large lum. required !
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Fully Reconstructed Tag at Belle (447M BB)
B+  D(*)0 + p + /  + / a1+ / DS(*)+
Ds+
D0p 0 / D0
(*)
B D
0
+p /  / a / D
+
+
D 0p  / D p 0
+
1
(*)+
S
Ds+
D0  7 modes
D   6 modes
+
s
D  2 modes
~ 180 channels
used
 Beam constrained mass
N= 680k
eff.= 0.29%
purity = 57%
N = 412 k
eff.= 0.19%
purity = 52%
m ~ 5.28 GeV/c2
s~ 3 MeV/c2 due to
s(Ebeam)
Charged B
Neutral B
~10% for feed-across
between B+ and B0
Signal region : -0.08 < DE < 0.06 GeV, Mbc > 5.27 GeV/c2
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Signal Selection (1)
 t lepton is identified in the 5 decay modes.
81% of all t decay modes
 Signal selection criteria.
 Signal-side efficiency including t decay br.)
32.92 0.12%
 All selection criteria were optimized before examining
the signal region (blind analysis).
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Signal Selection (2)
 Extra neutral energy in calorimeter EECL
– Most powerful variable for separating signal and background
– Total calorimeter energy from the neutral clusters which are not
associated with the tag B
Minimum energy threshold


Barrel : 50 MeV
For(Back)ward endcap : 100(150) MeV
Zero or small value of EECL arising only
from beam background
Higher EECL due to additional neutral
clusters
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MC includes overlay of random trigger
data to reproduce beam backgrounds.
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Signal Selection (3)
 Extra neutral energy EECL Validation by double tagged
sample (control sample);
– Btag is fully reconstructed
– Bsig is semileptonic decays
B+ D(*)0 X+ (fully reconstruction)
B- D*0 l-n
D 0 p0
K- p+
K- p+ p- p+
B+ B-
494  18
B0B0
7.9  2.2
Total
502  18
Data
458
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Purity ~ 90%
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Background Estimation
MC : 94.2  8.0
Data : 96
MC : 23.3  4.7
Data : 21
MC : 89.6  8.0
Data : 93
MC : 18.5  4.1
Data : 21
MC : 41.3  6.2
Data : 43
Sideband Total
MC : 267  14
Data : 274
Large MC samples for e+e- BB, qq, Xuln, Xu tn, t+ t , and rare B
decays are used (including beam-background).
Majority come from BD(*) X l n (~90%) + Xu l n/rare (~10%).
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Result: Opening the Box !
 The signal regions are examined after finalizing all of
the selection criteria.
414 fb-1
# estimated background
and observed events in
the signal region
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Observe excess in signal region !
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Btn Candidate Event
B+ g D0 p+
K+ p- p+ pB- g t - n
e-nn
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Verification of the Signal (1)
 For events in the EECL signal region, distribution of event
selection variables other than EECL are verified.
 They are consistent with MC expectation for Btn
signal + background.
Btn signal
Background
Mbc
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Pmiss
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Verification of the Signal(2)
 About 30% of background have neutral cluster in the
KLM detector (KL candidates).
 The excess remains after requiring KL veto.
KL in coincidence.
KL in veto
EECL
EECL
 We do not use this cut in the result, to avoid introducing large
systematic error due to KL detection efficiency uncertainty.
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Fit Results
 The final results are deduced by unbinned likelihood fit
to the obtained EECL distributions.
Signal +
background
S : Significance with systematics
Btn
Background
Signal
+6.7
- 5.7
Signal shape : Gauss + exponential
Background shape : second-order polynomial
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Observe 21.2
events with
a significance of 4.2s
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Systematic Uncertainty
 Signal selection efficiencies
 Tag reconstruction efficiency : 10.5%
Difference of yields between data and MC in the B- D*0l-n
control sample
 Number of BB : 1%
 Signal yield :
+12%
-10%
– signal shape ambiguity estimated by varying the signal PDF
parameters
– BG shape : changing PDF
 Total systematic uncertainty
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+17%
-15%
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Btn Branching Fraction
 Branching fractions are calculated by
Extracted branching fraction for
each t decay mode
 All t decay modes combined
SM : B(Btn)=(1.59  0.40)×10-4
Result is consistent with SM
prediction within error
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fB Extraction
 Product of B meson decay constant fB and
CKM matrix element |Vub|
 Using |Vub| = (4.39  0.33)×10-3 from HFAG
14%
11% = 8%(exp.) + 8%(Vub)
fB = 0.216  0.022 GeV
[HPQCD, Phys. Rev. Lett. 95, 212001 (2005) ]
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Constraints on |Vub|/|Vtd|
 Constraint in the (,h)
plane from the Btn
branching fraction and
Dmd
Constraint for
DBr ( B  tn )  0
Improved measurement
will help.
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Constraints on Charged Higgs
B(B tn )SM  (1.59  0.40) 104
B
Br ( B  tn )  Br ( B  tn )SM  rH
rH
A
2s
tan  / mH
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B
A
95.5%C.L. exclusion boundaries
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Future Prospect (1)
 Br(Bt n) measurement:
Further accumulation of luminosity help to reduce both
statistical and systematic errors errors.
– Some of the major systematic errors come from limited
statistics of the control sample.
 |Vub| measurement:
< 5% in future is an realistic goal.
 fB from theory
~10% now  5% ?
Assumption in the following
plots
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Note:
Br  Vub
Lum.
DB(Btn)
exp
2
f B2
D|Vub|
414 fb-1
36%
7.5%
5 ab-1
10%
5.8%
50 ab-1
3%
4.4%
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Future Prospect (2)
95.5%C.L. exclusion boundaries
DfB(LQCD) = 5%
5ab
-1
rH
2s
tan  / mH
50ab
-1
If D|Vub| = 0 & DfB = 0
rH
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tan  / mH
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Future Prospect (3)
Charged Higgs Mass Reach
(95%CL @ tan=30)
1TeV
1000
Only exp. error
(DVub=0%, DfB=0%)
800
700
DVub=2.5%, DfB=2.5%
系列5
系列6
系列7
500
400
DVub=5%, DfB=5%
300
200
100
0
9.0
00
11
.00
0
13
.00
15 0
.00
0
17
.00
0
19
.00
0
21
.00
0
23
.00
25 0
.00
0
27
.00
0
29
.00
0
31
.00
33 0
.00
0
35
.00
0
37
.00
0
39
.00
0
41
.00
43 0
.00
0
45
.00
0
47
.00
0
49
.00
0
0
7.0
0
0
5.0
0
0
0
3.0
0
Mass
600
1.0
0
Mass Reach (GeV)
900
5
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10
20
30
Luminosity
40
50
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Luminsoity(ab-1)
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fD measurements
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Summary
 We have seen the evidence of B t n with
414fb-1 data at Belle.
– The first evidence of purely leptonic B decays.
– Branching fraction
– B decay constant
– Constraint on charged Higgs.
Probe up to ~200GeV at tan=30
 O(ab-1) data, together with improved fB and |Vub|, will
allow us to probe large tanb-mass space of charged
Higgs.
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Backup Slides
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Fit Result (2)
 Likelihood fit results for each t decay mode.
Signal +
background
Background
Signal
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Fit Results (3)
 Likelihood distributions for each t decay mode.
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Future Prospect (5ab-1)
95.5%C.L. exclusion boundaries
DfB(LQCD) = 10%
rH
2s
DfB(LQCD) = 5%
rH
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2s
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@ Flavour
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 / mHin the LHC era
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Full Reconstruction Method
 Fully reconstruct one of the B’s to tag
– B production
– B flavor/charge
– B momentum
B
e
(8GeV)
e+(3.5GeV)
Υ(4S)
p
B
Decays of interests
BXu l n,
BK n n
BDtn, tn
full (0.1~0.3%)
reconstruction
BDp etc.
Single B meson beam in offline !
Powerful tools for B decays w/ neutrinos
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Search for Charged Higgs
 BDtn (semileptonic decay)
mb tan  + mc(u ) cot 
( B  Dt vt )
( B  Dv )
Band width from formfactor uncertainty
c
b
B
mt tan 
H+/W+
t+
nt
• Full reconstruction tag
• Signal  large missing mass
• Expected at 5ab-1
Mode
Nsig
D0t + ( +ntn )nt 280
D0t + (h+nt )nt 620
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Nbkg dB/B
550
7.9%
3600
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Constraint to Charged Higgs
 Once branching fraction is measured, we can constrain R.
MW
R
tan 
MH
M.Tanaka,
Z.Phys. C67 (1995) 321
Form factor error
 can be determined experimentally
by B semiletonic decays
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R  11 at 5ab-1
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Sensitivity for Charged Higgs
Constraint from BXs 
BDtn
Btn
(present)
LHC
100fb-1
D(form-factor) can
be reduced with
the present
BDn data.
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