Title: Rare B Decays : B?ln, ll, llg
1Rare B Decays B?ln, ll, llg
Nagoya University Koji Ikado The 11th
International Conference on B-Physics at Hadron
Machines (Beauty 2006) Sep. 28, 2006
2Introduction
B?ln
- 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 tn is
favored over en and mn - 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
3B0?l l-
Introduction (cont.)
- Neutral B mesons decay to ll- via box or penguin
annihilation - Branching fractions are suppressed by lepton mass
- Flavor violating channel (B0 ? e m, etc.) are
forbidden in SM - Some new-physics can enhance the branching
fractions by orders of magnitude
B(B0 ? ee) 10-15 B(B0 ? mm) 10-10 B(B0
? n n ) zero
4B0?l l-g
Introduction (cont.)
- 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 ? eeg) 10-10B(B0 ? mmg) 10-10
B(B0 ? nng) 9 10-10
5B?tn
6B?tn Analysis Concepts_at_Belle
- 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
Signal side Reconstruct particles from t decay
7Fully Reconstructed Bs with 449 M BB _at_ Belle
7 modes
6 modes
2 modes
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)
10 for feed-across between B and
B0
Neutral B
Charged B
Signal region -0.08 lt DE lt 0.06 GeV, Mbc gt 5.27
GeV/c2
8Signal Selection
- t lepton is identified in the 5 decay modes
-
81 of all t decay modes
Total efficiency with t decay branching fraction
15.81 ?0.05
All the selection criteria have been optimized to
achieve the highest sensitivity
9Signal 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
- BB- is dominant in background
10Extra Calorimeter Energy in Data
- Observed events compared with background expected
-
-
414 fb-1
After finalizing the signal selection criteria,
the signal region is examined
Number of data in signal region
Observe excess in signal region
11 Fit Results
- Unbinned Likelihood fit to the obtained EECL
distributions
Signal background
S Significance with systematics
Background
Background yield is consistent with the
expectation from the MC simulation
(peaking background is included)
B?tn Signal
Observe 17.2 events with a significance of
3.5s
5.3 - 4.7
Signal shape Gauss exponential Background
shape Gauss second-order
12Branching Fraction
- All t 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(B?tn) (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)
13Constraints on Physics Parameters
- Constraint on Charged Higgs
Phys. Rev. D48, 2342 (1993)
rH
2s
95.5C.L. exclusion boundaries
14B?tn Search _at_ Babar
- Babar searches for in a sample of 324x106 BB
events - Reconstruct one B in a semileptonic final state
B?DlnX - D?K p, K p p p, K p p, Ks p p (Xg, p from D0
is not explicitly reconstructed) - Require lepton CM momentum gt 0.8 GeV
- Require that -2 lt cosqB-D0l lt 1
- Parent B energy and momentum are determined from
the beam energy - Tagged B reconstruction efficiency 0.7
- Discriminate signal from
- background using Eextra
- t lepton is identified in the
- 4 decay modes
15 B?tn Search _at_ Babar (cont.)
- Results
- Observe a result consistent with zero signal at
1.3s, set a limit on the branching fraction and
quote a central value - Calculate the product of fB and Vub
Babar preliminary
16B?en B?mn
17B?en B?mn_at_Babar
- Search for B?ln in a sample of 229x106 BB events
- Reconstruct one B in a fully hadronic final state
- Reconstruct B? D()0 X
-
Identify a monoenergetic electron or muon
recoiling against Btag
18B?en B?mn_at_Babar
- Observe 0 (0) events in the signal box in
electron (muon) events - Set upper limits on the branching fractions
208.7fb-1
Babar preliminary
19B?en B?mn_at_Belle
- Highly energetic lepton
- Companion B reconstructed with the remaining
particles
Belle preliminary
PlB rest (GeV)
Belle preliminary
Update is coming soon
20B0 ? l l
21B0 ? l l (ee, mm, em) _at_Babar
- Very straightforward reconstruct mES and DE
- Extremely clean monochromatic kinematics
- Strategy define selection for sideband
- and blind signal box in mES, DE
- Selection variables
- cos qT qT is angle between thrust
- axes of ll candidate and rest of event
- mROE invariant mass of rest of event
- R2 normd 2nd Fox-Wolfram moment
- Ntrk ½Ng measure of multiplicity
- EEMC lt 11 GeV, rejects QED
accept
accept
accept
accept
Signal MC (blue) vs. sideband data
22B0 ? l l (ee, mm, em) results
BB pairs used (122.51.0)106
Signal regions
Events observed
ee
mm
B(B0 ? ee) lt 6.1 10-8 (90CL) B(B0 ?
mm) lt 8.3 10-8 (90CL) B(B0 ? e m) lt 18
10-8 (90CL)
e-m
111 fb-1
Phys. Rev. Lett. 94, 221803 (2005)
B(B0 ? ee) lt 1.9 10-7 (90CL) B(B0 ?
mm) lt 1.6 10-7 (90CL) B(B0 ? e m) lt
1.7 10-7 (90CL)
78 fb-1
Phys. Rev. D 68, 111101 (2003)
B(B0d?mm-) lt 2.310-8 (90 CL)
780 pb-1
23B0 ? tt _at_Babar
- First limit on this channel
- Experimentally very challenging 2-4 neutrinos
in signal - Constrain in particular leptoquark coupling and
tanb in SUSY - Analysis
- Reconstruct one B in a fully
- hadronic final state B? D() X
- gt280k events
- In the event remainder, look for
- two t decays (t?lnn, pn, rn)
- Kinematics of charged partilce
- momenta and residual energy
- are fed into a neutral network
- to separate signal and BG
Control sample
Data
B(B0 ? tt) lt 4.1 10-3 (90CL)
Observed events 263?19 Expected events 281?48
Phys. Rev. Lett. 96, 241802 (2006)
210 fb-1
24B0 ? n n (invisible) _at_Babar
B pairs used (88.51.0)106
- Semileptonic tags B0?D()-ln (D- ?D0 p-)
- Require nothing in recoil no charged tracks,
limited neutrals - Signal obtained from ML fit to Eextra
- Signal 17 9,
- background 19 10
- Upper limits
- Systematics
- Additive 7.4 events
- Multiplicative 10.9
- Frequentist limit-setting procedure
nn
8
nng
B(B0 ? invisible) lt 22 10-5 (90CL)
Phys. Rev. Lett. 93, 091802 (2004)
25B0?l l-g
26B0?l l-g _at_Babar
- Search for B?llg in a sample of 324x106 BB events
- Reconstruct B candidates from two leptons and a
photon - Leptons required to be 0.3 lt mll lt 4.9 (4.7) GeV
for electrons (muons) - Reject backgrounds from J/y, y (2S) decay
(leptons) or p0 decay(photon) - Reject qq background using signal B kinematics
and event shape in a Fisher discriminant - Background is determined by extrapolating the mES
sideband into the signal box
eesignal 6.07 ?0.14 emsignal 4.93 ?0.12
Nebkg 1.28 ?0.80 Nmbkg 1.40 ?0.42
ee-g
mm-g
27B0?l l-g _at_Babar
e e-g
m m-g
- Observe 0 (3) events in the signal box in
electron (muon) events - Set upper limits on the branching fractions
B(B0 ? ee-g) lt 0.7 10-7 (90CL)B(B0 ?
mm-g ) lt 3.4 10-7 (90CL)
Babar preliminary
B(B0 ? n n g ) lt 4.7 10-5 (90CL)
Phys. Rev. Lett. 93, 091802 (2004)
28Summary
- Performed searches for rare leptonic B decays
B?ln, ll, llg - Belle found first evidence for B ?tn
- - First direct measurement of the B meson decay
constant fB
4
29Backup Slides
30 KEKB Belle
500 fb-1
- Asymmetric-energy ee- collider
- 8GeV 3.5 GeV
- High Luminosity
- L 1.5 x 1034
Collected 560 fb-1
31Features 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
Mmiss2 for B-?D0 m- n (MC)
w/o B momentum
with B momentum
S/N
Full recon.
D() l n tag
Advanced n recon.
Lum.
trad. n recon.
Eff.
32Signal Selection
- Extra neutral energy EECL Validation
- -Double tagged sample, Btag is fully
reconstructed and Bsig is semileptonic mode
Bg D()0 X (fully reconstruction) B- g 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
Validate with double tagged events
33Background Estimation
MC 41.3 ? 6.2 Data 43
MC 89.6 ? 8.0 Data 93
MC 94.2 ? 8.0 Data 96
MC 18.5 ? 4.1 Data 21
Sideband Total MC 267 ? 14 Data 274
MC 23.3 ? 4.7 Data 21
Large MC samples for ee- ?BB, qq, Xuln, Xu tn,
t t- , and rare B decays are used (including
beam-background)
Good agreement between data and MC in sideband
region ? Validity of background MC simulation
34B?tn Candidate Event
B g D0 p K p- p p- B- g t -
n e-nn
35B?tn and 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)
36Systematic Uncertainty
- Signal selection efficiencies
- Tag reconstruction efficiency 10.5
- Difference of yields between data and MC in
the B-? D0l-n 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
37Constraints on Physics Parameters
- CKM parameters
- -Constraint in the (r,h) plane from the B?tn
branching fraction and Dmd
38 Fit Results
- Mbc and Pmis distributions
39Constraints at Super-B
- Br(B?t n) 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
- lt 5 in future is an realistic goal
- fB from theory
- 10 now ? 5 (?)
Lum. DB(B?tn) exp DVub
414 fb-1 36 7.5
5 ab-1 10 5.8
50 ab-1 3 4.4
Assumption in the following plots
40Constraints at Super-B (cont.)
95.5C.L. exclusion boundaries
DfB(LQCD) 5
5ab -1
rH
2s
50ab -1
rH