koji ikado beauty 2006 rare b decays : b l, ll, ll nagoya university koji ikado the 11 th...

Koji Ikado Beauty 2006

Rare B Decays : Bl, ll, ll

Nagoya UniversityKoji Ikado

The 11th International Conference on B-Physicsat Hadron Machines (Beauty 2006)

Sep. 28, 2006


Leptonic decay proceeding through W boson annihilation in the Standard Model

Decay rate simply related to B meson decay constant fB and |Vub|

Helicity suppressed by lepton mass : is favored over e and

Clean experimental method of measuring B meson decay constant fB

Physics beyond the SM could enhance the branching fraction through the introduction of a charged Higgs boson

Bl

Introduction


d dl

l

l

l

B0l+ l-

Neutral B mesons decay to l+l- via box or penguin annihilation

Branching fractions are suppressed by lepton mass

Flavor violating channel (B0 e+ –, etc.) are forbidden in SM Some new-physics can enhance the branching fractions by orders of ma

gnitude

B(B0 e+e–) ~ 10-15

B(B0 +–) ~ 10-10

B(B0 ) ~ zero

Introduction (cont.)


B0l+ l- Radiative dilepton decays

Photon emission from the initial state relaxes the helicity suppression SM predictions

SM branching fraction much below experimental sensitivity Search for new physics

B(B0 e+e–) ~ 10-10

B(B0 +–) ~ 10-10

B(B0 –) ~ 9 ×10-10

Introduction (cont.)


BAnalysis Concepts@Belle B decays with missing neutrinos lack the kinematic constraints which

are used to separate signal events from backgrounds (Mbc and E)

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 decay

Tagging side : Fully reconstruct hadronic modes

Signal side :Reconstruct particles from decay

(4S)B- B+

e+

e

B++, +e+eB-X


Fully Reconstructed B’s with 449 M BB @ Belle

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

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

~10% for feed-across between B+ and B0

m ~ 5.28 GeV/c2

~ 3 MeV/c2 due to 　　　　　　　 (Ebeam)

~ 180 channels used

N= 680keff.= 0.29% purity = 57%

N= 680keff.= 0.29% purity = 57%

N = 412 keff.= 0.19% purity = 52%

N = 412 keff.= 0.19% purity = 52%

Charged B Neutral B


Signal Selection lepton is identified in the 5 decay modes

　　

All the selection criteria have been optimized to achieve the highest sensitivity

81% of all decay modes

Total efficiency with decay branching fraction : 15.81 0.05%


Signal Selection

Extra neutral energy in calorimeter EECL

　 - Most powerful variable for separating signal and background

　 -Total calorimeter(ECL) energy from the neutral clusters which are not associated with the tag B

Minimum energy threshold Barrel : 50 MeV Forward(Backward) endcap : 100(150) MeV

Zero or small value of EECL arising only from beam background

Higher EECL due to additional neutral clusters

B+B- is dominant in background


Extra Calorimeter Energy in Data

Observed events compared with background expected

After finalizing the signal selection criteria, the signal region is examined

Observe excess in signal region

Number of data in signal region

414 fb-1


Unbinned Likelihood fit to the obtained EECL distributions

Fit Results

Signal shape : Gauss + exponential

Background shape : Gauss + second-order

: Significance with systematics

Background yield is consistent with the

expectation from the MC simulation

Observe 17.2 events with a significance of 3.5

+5.3 - 4.7

Signal +

background

B

Signal

Background

(peaking background is included)


Branching Fraction

All decay modes combined

Product of B meson decay constant fB and CKM matrix element |Vub|

Using |Vub| = (4.38 0.33)×10-3 from HFAG

B() = (1.59 0.40)×10-4 (SM prediction)

(|Vub| = (4.38 0.33)×10-3 & fB = 0.216 0.022 GeV )

fB = 0.216 0.022 GeV (HPQCD) Phys. Rev. Lett. 95, 212001

(2005)


Constraints on Physics Parameters

95.5%C.L. exclusion boundaries

2

tan / Hm

rH

Constraint on Charged Higgs

Phys. Rev. D48, 2342 (1993)


BSearch @ Babar

Babar searches for in a sample of 324x106 BB events Reconstruct one B in a semileptonic final state BDlX

DK , K , K , Ks (X=, from D*0 is not explicitly reconstructed) Require lepton CM momentum > 0.8 GeV Require that -2 < cosB-D0l < 1

Parent B energy and momentum are determined from the beam energy Tagged B reconstruction efficiency ~0.7%

Discriminate signal from

background using Eextra

lepton is identified in the

4 decay modes


ResultsObserve a result consistent with zero signal at 1.3, set a limit on the

branching fraction and quote a central value

Calculate the product of fB and |Vub|

BSearch @ Babar (cont.)

Babar preliminary


e+


Search for Bl in a sample of 229x106 BB events Reconstruct one B in a fully

hadronic final state

Reconstruct B D(*)0 X

e+@Babar

Identify a monoenergetic electron or muon recoiling against Btag


e+@Babar

fb

Observe 0 (0) events in the signal box in electron (muon) events

Set upper limits on the branching fractions

Babar preliminary


Highly energetic lepton Companion B reconstructed

with the remaining particles

Update is coming soon

PlB rest (GeV)

e+@Belle

Belle preliminary

Belle preliminary


B0 l+l–


B0 l+l– (e+e–, +–, e+–) @Babar

Very straightforward : reconstruct mES and E Extremely clean monochromatic kinematics Strategy: define selection for sideband

and blind signal box in mES, E

Selection variables | cos T |: T is angle between thrust

axes of l+l– candidate and rest of eventmROE: invariant mass of rest of event

R2: norm’d 2nd Fox-Wolfram moment

Ntrk + ½N: measure of multiplicity

EEMC < 11 GeV, rejects QED

Signal MC (blue) vs. sideband data

accept accept

acceptaccept


B(B0 e+e–) < 6.1 × 10-8 (90%CL) B(B0 +–) < 8.3 × 10-8 (90%CL) B(B0 e+ –) < 18 × 10-8 (90%CL)

B0 l+l– (e+e–, +–, e+–) results

e+e– +–

e-+

Signal regions Events observed

• Limits:

Phys. Rev. Lett. 94, 221803 (2005)

B(B0 e+e–) < 1.9 × 10-7 (90%CL) B(B0 +–) < 1.6 × 10-7 (90%CL) B(B0 e+ –) < 1.7 × 10-7 (90%CL)

B(B0d) < 2.3×10-8 (90% CL)

Phys. Rev. D 68, 111101 (2003)

78 fb

780 pb

111 fb

BB pairs used: (122.5±1.0)×106


B0 +– @Babar

First limit on this channel Experimentally very challenging : 2-4 neutrinos in signal Constrain in particular leptoquark coupling and tan in SUSY

Analysis Reconstruct one B in a fully

hadronic final state B D(*) X

=>280k events In the event remainder, look for

two decays (l, , ) Kinematics of charged partilce

momenta and residual energy

are fed into a neutral network

to separate signal and BG

Observed events : 26319

Expected events : 28148

B(B0 +–) < 4.1 × 10-3 (90%CL)

Phys. Rev. Lett. 96, 241802 (2006)

210 fb

Data Control sample


B0 invisible) @Babar

Semileptonic tags : B0D(*)-l+ (D*- D0 ) Require nothing in recoil: no charged tracks, limited neutralsSignal obtained from ML fit to Eextra

Signal: 17 ± 9, background 19 + 10

Upper limitsSystematics

Additive: 7.4 events Multiplicative: 10.9%

Frequentist limit-setting procedure

– 8

B pairs used: (88.5±1.0)×106

B(B0 invisible) < 22 × 10-5 (90%CL)

Phys. Rev. Lett. 93, 091802 (2004)


B0l+ l-


Search for Bll in a sample of 324x106 BB events Reconstruct B candidates from two leptons and a photon

Leptons required to be 0.3 < mll < 4.9 (4.7) GeV for electrons (muons)

Reject backgrounds from J/, (2S) decay (leptons) or 0 decay(photon) Reject qq background using signal B kinematics and event shape in a Fishe

r discriminant

Background is determined by extrapolating the mES sideband into the signal box

B0l+ l-@Babar

esignal = 6.07 0.14% &

signal = 4.93 0.12%

Nebkg = 1.28 0.80 & N

bkg = 1.40 0.42

e+e- +-


Observe 0 (3) events in the signal box in electron (muon) events Set upper limits on the branching fractions

B0l+ l-@Babar

Babar preliminary

e+ e- + -

B(B0 ) < 4.7 × 10-5 (90%CL)

B(B0 e+e-) < 0.7 × 10-7 (90%CL)B(B0 ) < 3.4 × 10-7 (90%CL)

Phys. Rev. Lett. 93, 091802 (2004)


Summary

Performed searches for rare leptonic B decays Bl, ll, ll Belle found first evidence for B

- First direct measurement of the B meson decay constant fB

4


Backup Slides


KEKB & Belle

500 fb-1 Asymmetric-energy e+e- collider 8GeV – 3.5 GeV High Luminosity

L ~ 1.5 x 1034

Collected ~560 fb-1


Features with Fully Reconstructed B Tag

Merit : Offline B meson Beam B momentum is available

-Resolution of Mmiss2 can be

significantly improved.

-separate similar semileptonic decays

reduce background significantly B-flavor is also available

-We can treat charged & neutral

B separately Demerit : Low statistics

Efficiency : 0.2 - 0.3% However, we can rely on KEKB,

providing World record luminosity

w/o B momentum

with B momentum

Mmiss2 for B-D0 (MC)

low middle high

poor

mod.

good

S/N

Eff.

trad. recon.

Advanced recon.

D(*) l tag

Lum.

Full recon.


Signal Selection

Extra neutral energy EECL Validation

　 -Double tagged sample, Btag is fully reconstructed and Bsig is semileptonic mode

Validate with doubletagged events

BB++ D D(*)0(*)0 X X+ + (fully reconstruction)(fully reconstruction)

BB-- D D*0 *0 l 　　　　　　　　　　　　 DD00 0

　　　　　　　　　　　　 KK-- ++ KK-- + + -- ++

B+B- 494 18

B0B0 7.9 2.2

Total 502 18

Data 458

Purity ～ 90%


Background Estimation

MC : 23.3 4.7 Data : 21

MC : 94.2 8.0 Data : 96

Large MC samples for e+e- BB, qq, Xul, Xu , + - , and rare B decays are used (including beam-background)

Sideband Total

MC : 267 14 Data : 274

MC : 89.6 8.0 Data : 93

MC : 41.3 6.2 Data : 43

MC : 18.5 4.1 Data : 21

Good agreement between data and MC in sideband region Validity of background MC simulation


B Candidate Event

BB++ D D0 0 ++

KK++ - - ++ --

BB-- --

ee--


Band Decay Constant fB

Expected branching fraction

Currently, our best knowledge of fB comes from lattice QCD calculations with uncertainty of 10%

|Vub| = (4.38 0.33)×10-3 from HFAG (hep-ex/0603003)

fB = 0.216 0.022 GeV from lattice QCD

HPQCD result, Phys. Rev. Lett. 95, 212001 (2005)


Systematic Uncertainty

Signal selection efficiencies

Tag reconstruction efficiency : 10.5%

Difference of yields between data and MC in the BB-- D D*0*0ll- control sample

Number of BB : 1% Signal yield :

signal shape ambiguity estimated by varying the signal PDF parameters

BG shape : changing PDF Total systematic uncertainty

+22.5% - 25.7%

+25.5% - 28.4%


Constraints on Physics Parameters CKM parameters

-Constraint in the (,) plane from the branching fraction and md


Mbc and Pmis distributions

Fit Results


Constraints at Super-B

Br(B) measurement :Further accumulation of luminosity helps to reduce both statistical and systematic 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% (?)

Lum. B(B) exp |Vub|

414 fb-1 36% 7.5%

5 ab-1 10% 5.8%

50 ab-1 3% 4.4%Assumption in the following plots

2 2ub BBr V f


Constraints at Super-B (cont.)

fB(LQCD) = 5%95.5%C.L. exclusion boundaries

rH 2

rH

5ab -1

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

tan / Hm

koji ikado beauty 2006 rare b decays : b l, ll, ll nagoya university koji ikado the 11 th...

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