Rare B Decays with “Missing Energy”

Tom Browder (University of Hawaii)

Will discuss experimental results from Belle on

Bν (BELLE-CONF-0671) and BK*νν(BELLE-CONF-0627)

Representing the Belle Collaboration

All results discussed here are preliminary.

decay constant

Motivation for B++ν

BF(B++ ν) < 2.6 x 10-4 (BaBar)B. Aubert et al., PRD 73, 057101 (2006)

Most stringent published limit:

Sensitivity to new physics from charged Higgs if the B decay constant is known

Why measuring νis non-trivial

(4S)B- B+

e+

e

B++, +e+e

B-X

The experimental signature is rather difficult: B decays to a single charged track + nothing

Most of the sensitivity is from tau modes with 1-prong

Belle’s sample of B tags (447 x 106 BB)

7 modes

6 modes0 (*) (*)1/ / / SB D a D

0 0/D D

(*)0 (*)1/ / / SB D a D

0 0 0/D D sD

sD

0D

D

sD 2 modes

Beam constrained mass distn’s

Signal region : -0.08 < E < 0.06 GeV, Mbc > 5.27 GeV/c2

~10% feed-across between B+ and B0

m ~ 5.28 GeV/c2

~ 3 MeV/c2 from 　　　　　　　 (Ebeam)

~ 180 channels reconstructed

Charged B’s Neutral B’s

N=680 K

Eff=0.29%

Purity =57%

N=412 K

Eff=0.19%

Purity =52%

• Reconstruct one B (Btag) in a charged hadronic b c mode (remove tag’s 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. (mn~0)

·Two missing neutrinos, large missing p (cut depends on decay mode 0.2 GeV-1.8 GeV)

Outline of B νexperimental analysis

Outline of experimental analysis (cont’d)• The lepton is identified in the 5 decay modes:

• Signal-side efficiency including 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 decays

15.81 0.05%

Consistency Check with BD* lν• Extra neutral energy EECL Validation with double

tagged sample (control sample);– Btag is fully reconstructed– Bsig is a semileptonic decay

BB++ D D(*)0(*)0 X X+ + (fully (fully reconstruction)reconstruction) BB-- D D*0 *0 l- 　　　　　　　　　　　　 DD00 0

　　　　　　 　　　　　　 KK-- ++ KK-- + + -- ++

B+B- 494 18B0B0 7.9 2.2Total 502 18Data 458

Purity ~ 90% Extra energy in the calorimeter

Calibration data

Example of a B ν candidate

Tag: BD0 ,

D0 K

Evidence for B+ ν (Belle)

Find signal events from a fit to a sample of 54 events.4.6 stat. significance w/o systematics,

447 106 B pairs BtagD(*)[,a1,Ds(*)] 680k tags, 55% pure. 5

decay modes

5.34.717.2

MC studies show there is a small peaking bkg in the 0 and 0 modes.

After including systematics (dominated by bkg), the significance decreases to 3.5σ

Extra Calorimeter Energy

B yields broken down by decay mode

For the first 3 modes, the background is fitted with a 2nd order polynomial plus a small Gaussian peaking component.

(stat sig only)

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 BD* l control sample or in the internal review process.

418.034.016.028.0 1006.1)(BF

B

Previous value

New value 439.056.046.049.0 10)79.1()(BF

B (Preliminary)

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

fB = 216 22 MeV (an unquenched lattice calc.)[HPQCD, Phys. Rev. Lett. 95, 212001 (2005) ]

15%

14% = 12%(exp.) + 8%(Vub)

36 3031 34229Bf MeV

( Belle)

1.6 1.1 41.4 1.3(10.1 ) 10B ubf V GeV

Constraints on the charged Higgs mass

rH=1.130.51

Assume fB and |Vub | are known, take the ratio to the SM BF.

22

2(1 tan )BH

H

mrm

Motivation for BK* (bs with 2 neutrinos)

SM: BF(BK* ) ~1.3 x 10-5 (Buchalla, Hiller, Isidori)

BSM: New particles in the loop

c.f. SM: BF(BK- ) ~4 x 10-6

PRD 63, 014015

[Belle preliminary (275 x 106 B Bbar) : BF(BK- ) <3.6 x 10-5] to be updated soon

Other weakly coupled particles: light dark matter

BK(*)νν are particularly interesting and challenging modes (Bν is even a small background)

The experimental signature is BK + 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)

DAMA NaI 3Region

CDMS 04

CDMS 05

Direct dark-matter searches cannot see M<10 GeV region

C. Bird et al PRL 93 201803

.(T. Adams et al. PRL 87 041801;A. Dedes et al., PRD 65 015001)

Search for BK* (532 x 106 B Bbar pairs)

0 *0 4( ) 3.4 10B B K (at 90% C.L)

Extra Calorimeter Energy (GeV)

3.12.64.7Yield

(1.7σ stat. significance)

Sideband = 19

MC expectation = 18.73.3

SM (Buchalla, Hiller, Isidori) 1.3 x 10-5

BELLE-CONF-0627

Result from a blind analysis.

Search for BK* (properties of candidates)

KπInv. mass

b c background

rare B background (x 15 data)

udsc background

Data

combined backgroundSignal x 20

P*_K*

b c background

rare B background (x 15 data set)

udsc background

Data

combined backgroundSignal shape

Search for BK* (properties of candidates)

K* momentum distribution

Need more bc MC (only 2 x data)

π+

K-

γ

Tag Side

B D+ a1-

D+ K- π+π+

a1- ρ0 π- , ρ0 π+π-

Event display for a BK* candidate due to an identified background (BK*γ)

(Hard photon is lost in the barrel-endcap calorimeter gap)

Missing mass ~ 0

MC: Expected bkg from this source ~0.3 evts.

20

Future Prospects: BfB(LQCD) = 5%

95.5%C.L. exclusion boundaries

tan / Hm

tan / Hm

rH

50ab -1 If |Vub| = 0 & fB = 0

Lum. B(B) exp |Vub|414 fb-1 36% 7.5%5 ab-1 10% 5.8%

50 ab-1 3% 4.4%

Extrapolations (T.Iijima)

21

Future Prospects: Other probes of charged Higgs

cb

H/W

tan cotb cm m

tanm

Decay amplitude2tanbm m

( )( )

B D vB

B D v

Expected BF(SM)~ 8 x 10-3

•Semileptonic: BD(*)

Multiple neutrinos, low momentum lepton (use e’s), large bkg but still might be possible with enough data.

Some modes are very difficult at hadron colliders

MC extrapolation to 50 ab1

Observation of B± K± 5

Super B LoI Fig.4.18

(compare to K++ννand KL 0

Extra EM calorimeter energy

Belle result on Bν shows that B to one prong decays can be measured.

MC

SM pred: G. Buchalla, G. Hiller,

G. Isidori (PRD 63 014015 )

Conclusions on “Missing Energy Decays”

• Evidence for Bνand experimental determination of fB (preliminary result has been updated)

• Search for BK* (UL is still a factor of 10 above the SM range)

• Further dramatic progress (e.g. signals for BK(*) νν) will require Super B Factory class luminosity.

439.056.046.049.0 10)79.1()(BF

B

0 *0 4( ) 3.4 10BF B K

Backup Slides

Contributions to systematic error for B

Peaking Backgrounds in B

Tau tagging mode Tau tagging mode

Fits to individual B decay modes (updated for ICHEP06)

Requirements in Bν analysis• The lepton is identified in the 5 decay modes.

• Signal selection criteria.

• Signal-side efficiency including decay br.)

• All selection criteria were optimized before examining the signal region (blind analysis).

　　

81% of all decay modes

15.81 0.05%

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 signal + background.

Mbc Pmiss

B signalBackground

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.

• We do not use this cut in the result, to avoid introducing a large systematic error due to the uncertainty in KL detection efficiency.

KL in coincidence. KL in veto

EECLEECL

Selection Requirements for BK*

MC signal and bkg distributions,

γ

K-

π+

tagB

tagB

tagB

tagB

tagB

tagB

Tag Side

B D+ a1-

D+ K- π+π+

a1- ρ0 π- , ρ0 π+π-