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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
Koji Ikado Beauty 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
Koji Ikado Beauty 2006
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.)
Koji Ikado Beauty 2006
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.)
Koji Ikado Beauty 2006
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
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%
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
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)
Koji Ikado Beauty 2006
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)
Koji Ikado Beauty 2006
Constraints on Physics Parameters
95.5%C.L. exclusion boundaries
2
tan / Hm
rH
Constraint on Charged Higgs
Phys. Rev. D48, 2342 (1993)
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
e+
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
e+@Babar
fb
Observe 0 (0) events in the signal box in electron (muon) events
Set upper limits on the branching fractions
Babar preliminary
Koji Ikado Beauty 2006
Highly energetic lepton Companion B reconstructed
with the remaining particles
Update is coming soon
PlB rest (GeV)
e+@Belle
Belle preliminary
Belle preliminary
Koji Ikado Beauty 2006
B0 l+l–
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
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)
Koji Ikado Beauty 2006
B0l+ l-
Koji Ikado Beauty 2006
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- +-
Koji Ikado Beauty 2006
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)
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
Backup Slides
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
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.
Koji Ikado Beauty 2006
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%
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
B Candidate Event
BB++ D D0 0 ++
KK++ - - ++ --
BB-- --
ee--
Koji Ikado Beauty 2006
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)
Koji Ikado Beauty 2006
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%
Koji Ikado Beauty 2006
Constraints on Physics Parameters CKM parameters
-Constraint in the (,) plane from the branching fraction and md
Koji Ikado Beauty 2006
Mbc and Pmis distributions
Fit Results
Koji Ikado Beauty 2006
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
Koji Ikado Beauty 2006
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