Radiative & Electroweak Penguin B decays

at Belle

Yutaro Sato

Nagoya University

(Belle Collaboration)

19, Jul. 2013

Flavor Changing Neutral Current (FCNC) process

• Forbidden at tree level and allowed via loop diagrams in the SM.

• New particles may enter in the loop and alter physical observables.

Introduction 2

b

l

s l

W

t

g/Z

t

W W b

l

s

l

n

b s W

t

g

Radiative penguin decay

( bsg ) Electroweak penguin decay

( bsl+l- )

t ~

c ~ b s b s t

H-

Contents

Radiative penguin B decay (bsg)

• Search for B0p𝛬 p-g

Electroweak penguin B decay (bsl+l-)

• Forward-backward Asymmetry in inclusive BXs l+l-

• Both (preliminary) results are based on the full U(4S) data sample at Belle

and shown for the first time.

3

Time [year]

inte

grat

e lu

min

oti

sy [

fb-1

]

U(4S), 711 fb-1,

U(5S)

U(nS), n=1,2,3

Off-reso. Scan

Belle

B0p𝜦 p-g

• In B+pLg decay, mass peak of the pL system near threshold has been observed.

– Br (B+p𝛬 g) = 2.45−0.38

+0.44 ± 0.22 × 10−6

M.-Z. Wang et al. (Belle Collaboration), Phys. Rev. D. 76, 052004 (2007).

• Following hierarchy has been observed

in bs and bc baryonic transition.

– Br (B+p𝛬 p+p-) > Br (B0

p𝛬 p-) > Br (B+p𝛬 )

– Br (B+p𝛬 cp

+p-) > Br (B0p𝛬 cp

-) > Br (B+p𝛬 c)

• To check the hierarchy in the bsg transition, B0p𝛬 p-g is searched.

– It is also useful to tune the parameters in JETSET (hadronization program).

4

Theory prediction

B+p𝜦 g

Analysis method

• Main backgrounds : continuum (e+e-qq; q=u, d, s, c) events

– Likelihood ratio is constructed from event topology variables (KSFW)

and B flight direction(cosqB).

• Signals are extracted from 2D fit of Mbc and DE.

–

–

5

: beam energy @ c.m. frame

: 4-momentum of reconstructed B @ c.m. frame

ee BB continuum

「spherical」 「jet-like」

Result of B0p𝜦 p-g

• Nsig = 9.5−10.7+9.5 (statistical significance = 0.9)

– Dominant systematic source : experimentally unmeasured rare B decays

• Upper limit (90% C.L.) : Br (B0pΛ p-g ) < 6.5×10-7

– < Br (B+p𝛬 g) = 2.45−0.38

+0.44 ± 0.22 × 10−6

The expected hierarchy is not observed.

6

Total Signal

Self-cross feed Combinatorial BG

Rare B BG (BpLg ) Rare B BG (BpLp0 )

B0p𝛬 r-

B0p𝛴 0r-

B0p𝛬 p-h

B+p𝛴 0p0

B+p𝛬 h

Select events in DE [-0.4 GeV, +0.3 GeV]

Select events in Mbc [5.24 GeV, 5.29 GeV]

Forward-backward asymmetry in inclusive BXsl+l-

The bsl+l- has rich set of observables.

Forward-backward Asymmetry (AFB)

• very sensitive to the new physics

• Sensitive to three Wilson coefficients (C7, C9, C10).

Inclusive measurement

• Less theoretical uncertainty

than exclusive BK(*) l+l-.

• Experimental error is comparable with

theoretical uncertainty in exclusive

measurements.

The inclusive measurement would be

more important.

7

@ dilepton rest frame

b l+

l- s

q

Forward event b l+

l- s

q

Backward event

dilepton invariant mass squared

AFB in exclusive B K*l+l-

• The semi-inclusive reconstruction (sum-of-exclusive) method is utilized

for inclusive bsl+l-.

– Xs l+l- is reconstructed from 36(=18×2) exclusive modes.

– 20(=10×2) exclusive modes are used for the measurement of AFB.

• For (anti-)B0, only the self-tagging modes with a K± are used.

• K4p states are not used due to low expected signal yields.

• The final states not used for the measurement of AFB are removed

after the best candidate selection to suppress cross-feed.

The fraction of all Xs decays covered by 20 final states is 50 %.

Semi-inclusive reconstruction method 8

Xs = K±/KS + up to four p (at most one p0)

[ K ] : K, KS

[ Kp ] : Kp, KSp, Kp0, KSp0

[ K2p ] : K2p, KS2p, Kpp0, KSpp0

[ K3p ] : K3p, KS3p, K2pp0, KS2pp0

[ K4p ] : K4p, KS4p, K3pp0, KS3pp0

l+l- = e+e- or m+m-

Analysis method

Backgrounds

• NeuroBayes (NB) neural network is employed for backgrounds suppression.

– Semileptonic B decays

• lepton vertex separation

• missing mass

• visible energy

– Continuum events

• event topology variables

NB inputs : 23 variables in total

• Three peaking Backgrounds are considered in the final fit.

9

b b

l-

nl

X

l+

nl

X’

_ _

b

c

l-

l+

nl

X

nl

_

p K

l l

p p

2. Double miss-PID

Miss-PID

1. charmonium

p K l l

Escape veto selection

J/y, y(2S)

3. Swapped miss-PID

p K l

l p

l

Miss-PID

J/y, y(2S)

Analysis method

Signal extraction

• Divide q2 into 4 bins

• Extract AFB from simultaneous fit of Mbc with

forward/backward events in electron/muon channels.

– Dependence of reconstruction efficiency to the q2 and cosq is taken into account.

– AFBraw is transformed to AFB by the linear scaling factor.

10

AFB = a ・ AFBraw

Rec. eff. Xs m+m-

1st

2n

d

3rd

4th

J/y

vet

o

y(2

S)

vet

o

Xs e+e-

1st

2n

d

3rd

4th

J/y

vet

o

y(2

S)

vet

o

• Dominant systematic sources

– transformation from AFBraw to AFB

– estimation of peaking backgrounds

Fit result in 2nd q2 bin 11

Total Signal

Cross-feed Non-peaking B.G.

Peaking B.G.

Xs e+e- , Forward

Xs m+m-, Forward

Xs e+e- , Backward

Xs m+m-, Backward

Nsigee = 30.1±9.2 (stat)

Nsigmm = 23.9±10.5 (stat)

AFB = 0.04±0.31(stat) ±0.05(syst)

Results

• Results are consistent with the SM.

– Low q2 region ( 1st bin ) : Consistent with the SM within 1.8s (6.6 C.L.).

– High q2 region (3rd and 4th bin) : Exclude C10*C9 > 0 with 2.3s (97.9% C.L.)

Result of AFB

12

● Data ○ SM (bsll )

Total signal yields Nsig

ee = 139.9 ± 18.6 (stat) Nsig

mm = 160.8 ± 20.0 (stat)

Summary

• Radiative and electroweak penguin B decays are sensitive for physics

beyond SM.

Search for B0p𝜦 p-g

– Upper limit (90% C.L.) : Br (B0pLp-g ) < 6.5×10-7

Forward-backward asymmetry in inclusive BXs l+l-

– low q2 region : consistent with the SM within 1.8s (6.6% C.L.)

– high q2 region : exclude C10*C9 > 0 with 2.3s (97.9% C.L.)

Can be used to constrain new physics and will play important role at

precise measurement of Wilson coefficients.

13

Backup Slides

14

Operator Product Expansion

15

Current-current

QCD penguin

Magnetic operator

Vector electroweak penguin

Axial-vector electroweak penguin

Correction function

Procedure for making the correction function

1. Generate MC samples,

varying three Wilson coefficients(C7,C9,C10).

2. Calculate the AFBraw using rec. eff..

16

Parameter Range

[ A7 ] +A7 SM, -A7

SM

[A9, A10] -200% ~ +200 %

(Ai is leading term of Ci.)

Xs m

+ m-

Xs e

+ e-

1st q2 bin 2nd q2 bin 4th q2 bin 6th q2 bin

Ideal line (AFB = AFBraw) Fitted line

AFBraw

AF

B

B.G. Suppression with Neural Network

• Neural network is employed for the background suppression

17

“Function” is determined to efficiently

separate signal and background

by the “training”.

Outputs

+1 : signal-like

-1 :background-like

23 parameters are input. 1. Distance between dilepton along z axis

2. Confidence level of reconstructed B-vertex

3. Flight direction of B meson

4. Energy difference DE

5. Missing mass

6. Visible energy

7. 17 event shape parameters based on Legendre polynomials

Fit result in 1st q2 bin 18

Total Signal

Cross-feed Non-peaking B.G.

Peaking B.G.

Xs e+e- , Forward Xs e

+e- , Backward

Xs m+m-, Forward Xs m

+m-, Backward

Nsigee = 45.7±10.9 (stat)

Nsigmm = 43.4±9.2 (stat)

AFB = 0.34±0.24(stat) ±0.02(syst)

Results

Fit result in 3rd q2 bin 19

Total Signal

Cross-feed Non-peaking B.G.

Peaking B.G.

Xs e+e- , Forward Xs e

+e- , Backward

Xs m+m-, Forward Xs m

+m-, Backward

Nsigee = 25.0±7.0 (stat)

Nsigmm = 30.7±9.9 (stat)

AFB = 0.28±0.21(stat) ±0.01(syst)

Results

Fit result in 4th q2 bin 20

Total Signal

Cross-feed Non-peaking B.G.

Peaking B.G.

Xs e+e- , Forward Xs e

+e- , Backward

Xs m+m-, Forward Xs m

+m-, Backward

Nsigee = 39.2±9.6 (stat)

Nsigmm = 62.9±10.4 (stat)

AFB = 0.28±0.15(stat) ±0.01(syst)

Results