Rare B Decays with

1
Rare B Decays with Missing Energy

• Tom Browder (University of Hawaii)

Representing the Belle Collaboration
Will discuss experimental results from Belle on
B??? (BELLE-CONF-0671) and B?K??(BELLE-CONF-0627)
All results discussed here are preliminary.
2
Motivation for B???
Sensitivity to new physics from charged Higgs if
the B decay constant is known
Most stringent published limit
BF(B?? ?) lt 2.6 x 10-4 (BaBar)
B. Aubert et al., PRD 73, 057101 (2006)
3
Why measuring B??? is non-trivial
Most of the sensitivity is from tau modes with
1-prong
The experimental signature is rather difficult B
decays to a single charged track nothing
4
Belles sample of B tags (447 x 106 BB)
7 modes

• 180 channels reconstructed

6 modes
2 modes
Signal region -0.08 lt DE lt 0.06 GeV, Mbc gt 5.27
GeV/c2
N680 K Eff0.29 Purity 57
N412 K Eff0.19 Purity 52

• m 5.28 GeV/c2
• s 3 MeV/c2 from s(Ebeam)

10 feed-across between B and B0
Charged Bs
Neutral Bs
Beam constrained mass distns
5
Outline of B? ??experimental analysis

• Reconstruct one B (Btag) in a charged hadronic b
  ? c mode (remove tags decay products from
  consideration.)
• Little or no extra electromagnetic calorimeter
  energy (EECL) . Beam-related backgrounds modeled
  in MC using random trigger data runs.
• For B ?X n known EB, mB, small pB
• ? narrow missing mass distn. (mn0)
• Two missing neutrinos, large missing p (cut
  depends on ? decay mode 0.2 GeV-1.8 GeV)

6
Outline of experimental analysis (contd)

• The t lepton is identified in the 5 decay modes
• Signal-side efficiency including t decay BFs)
• All selection criteria were optimized before
  examining the signal region (a.k.a. blind
  analysis)
• Fit the extra energy distribution (EECL), the
  signal peaks near zero

81 of all t decays
15.81 ?0.05
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Consistency Check with B?D l?

• Extra neutral energy EECL Validation with double
  tagged sample (control sample)
• Btag is fully reconstructed
• Bsig is a semileptonic decay

Calibration data
B? D()0 X (fully reconstruction) B-? D0
l-n D0 p0 K- p
K- p p- p
BB- 494 ? 18
B0B0 7.9 ? 2.2
Total 502 ? 18
Data 458
Purity 90
Extra energy in the calorimeter
8
Example of a B?? ? candidate
Tag B?D0 ?, D0 ?K???
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Evidence for B? ?? (Belle)
Btag?D()p,r,a1,Ds() 680k tags, 55 pure. 5
t decay modes
447 ?106 B pairs
Find signal events from a fit to a
sample of 54 events. 4.6s stat. significance w/o
systematics,
After including systematics (dominated by bkg),
the significance decreases to 3.5s
MC studies show there is a small peaking bkg in
the ????0 ? and ?????0 ? modes.
Extra Calorimeter Energy
10
B??? yields broken down by ? decay mode
(stat sig only)
For the first 3 modes, the background is fitted
with a 2nd order polynomial plus a small Gaussian
peaking component.
11
Error in the efficiency calculation
Due to a coding error, the efficiency quoted in
the 1st Belle preliminary result was incorrect.
The data plots and event sample are unchanged.
However, fB and the branching fraction must be
changed.
This mistake was not detected when checking the
B?D l ? control sample or in the internal review
process.
New value
(Preliminary)
Previous value
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Direct experimental determination of fB

• Product of B meson decay constant fB and CKM
  matrix element Vub
• Using Vub (4.39 ? 0.33)10-3 from HFAG

( Belle)
15
14 12(exp.) 8(Vub)
fB 216 ? 22 MeV (an unquenched lattice
calc.) HPQCD, Phys. Rev. Lett. 95, 212001 (2005)

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Constraints on the charged Higgs mass
Assume fB and Vub are known, take the ratio
to the SM BF.
rH1.13?0.51
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Motivation for B?K?? (b?s with 2 neutrinos)
BSM New particles in the loop
Other weakly coupled particles light dark matter
SM BF(B?K ??) 1.3 x 10-5 (Buchalla, Hiller,
Isidori)
PRD 63, 014015
c.f. SM BF(B?K- ??) 4 x 10-6
Belle preliminary (275 x 106 B Bbar) BF(B?K-
??) lt3.6 x 10-5 to be updated soon
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B?K()?? are particularly interesting and
challenging modes (B??? is even a small
background)
The experimental signature is B?K Nothing
The nothing can also be light dark matter (mass
of order (1 GeV)) (see papers by M. Pospelov et
al.)
(But need to optimize pK cut)
C. Bird et al PRL 93 201803
.(T. Adams et al. PRL 87 041801A. Dedes et al.,
PRD 65 015001)
Direct dark-matter searches cannot see Mlt10 GeV
region
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Search for B?K?? (532 x 106 B Bbar pairs)
Result from a blind analysis.
BELLE-CONF-0627
(1.7s stat. significance)
Sideband 19
MC expectation 18.7?3.3
SM (Buchalla, Hiller, Isidori) 1.3 x 10-5
Extra Calorimeter Energy (GeV)
(at 90 C.L)
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Search for B?K?? (properties of candidates)
b ? c background
rare B background (x 15 data)
udsc background
combined background
Signal x 20
Data
KpInv. mass
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Search for B?K?? (properties of candidates)
b ? c background
rare B background (x 15 data set)
udsc background
Signal shape
combined background
Need more b?c MC (only 2 x data)
Data
P_K
K momentum distribution
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Event display for a B?K?? candidate due to an
identified background (B?K?)
Tag Side B ? D a1- D ? K- pp a1- ? ?0 p- ,
?0 ?pp-
p
K-
Missing mass 0
(Hard photon is lost in the barrel-endcap
calorimeter gap)
?
MC Expected bkg from this source 0.3 evts.
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Future Prospects B???
95.5C.L. exclusion boundaries
DfB(LQCD) 5
Extrapolations (T.Iijima)
Lum. DB(B?tn) exp DVub
414 fb-1 36 7.5
5 ab-1 10 5.8
50 ab-1 3 4.4
50ab -1
rH
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Future Prospects Other probes of charged Higgs

• Semileptonic B?D() t n

Decay amplitude
Expected BF(SM) 8 x 10-3
Multiple neutrinos, low momentum lepton (use
es), large bkg but still might be possible with
enough data.
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Some modes are very difficult at hadron colliders
MC extrapolation to 50 ab-1
5s
Observation of B g K n n
(compare to K????and KL g p0nn)
MC
Belle result on B??? shows that B to one prong
decays can be measured.
SM pred G. Buchalla, G. Hiller, G. Isidori (PRD
63 014015 )
Extra EM calorimeter energy
Super B LoI Fig.4.18
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Conclusions on Missing Energy Decays

• Evidence for B???and experimental determination
  of fB (preliminary result has been updated)
• Search for B?K ?? (UL is still a factor of 10
  above the SM range)
• Further dramatic progress (e.g. signals for
  B?K() ??) will require Super B Factory class
  luminosity.

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Backup Slides
25
Contributions to systematic error for B???
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Peaking Backgrounds in B???
Tau tagging mode
Tau tagging mode
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Fits to individual B?? ? decay modes (updated for
ICHEP06)
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Requirements in B??? analysis

• The t lepton is identified in the 5 decay modes.
• Signal selection criteria.
• Signal-side efficiency including t decay br.)
• All selection criteria were optimized before
  examining the signal region (blind analysis).

81 of all t decay modes
15.81 ?0.05
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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
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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 a large systematic error due to the
  uncertainty in KL detection efficiency.

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Selection Requirements for B?K ??
MC signal and bkg distributions,
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Tag Side B ? D a1- D ? K- pp a1- ? ?0 p- ,
?0 ?pp-
p
tagB
tagB
tagB
K-
tagB
tagB
tagB
?