Transcript 2 Topics in Rare B Decays (exp)

Rare B Decays with “Missing Energy”
Tom Browder (University of Hawaii)
Representing the Belle Collaboration
Will discuss experimental results
from Belle on
Bν (BELLE-CONF-0671) and
BK*νν(BELLE-CONF-0627)
All results discussed here are preliminary.
Motivation for B++ν
Sensitivity to new
physics from
charged Higgs if the
B decay constant is
known
Most stringent published limit:
BF(B++ ν) < 2.6 x 10-4 (BaBar)
B. Aubert et al., PRD 73, 057101 (2006)
B decay constant
Why measuring Bν is non-trivial
e+
BB-X
(4S)
n
B+
ne
n
Most of the
sensitivity is
from tau
modes with 1prong
B++n, +e+nen
The experimental signature is rather difficult:
B decays to a single charged track + nothing
Belle’s sample of B tags (447 x 106 BB)
B  D(*)0    /   / a1 / DS(*)
D0 0 / D0
Ds
B0  D(*)    /   / a1 / DS(*)
D 0  / D  0
Ds
D0  7 modes
D   6 modes
~ 180 channels
reconstructed
Ds  2 modes
Signal region : -0.08 < DE < 0.06 GeV, Mbc > 5.27 GeV/c2
N=680 K
N=412 K
Eff=0.29%
Eff=0.19%
Purity =57%
Purity =52%
m ~ 5.28 GeV/c2
s~ 3 MeV/c2 from
s(Ebeam)
Charged B’s
Neutral B’s
Beam constrained mass distn’s
~10% feed-across
between B+ and B0
Outline of B νexperimental analysis
• 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 experimental analysis (cont’d)
• The  lepton is identified in the 5 decay modes:
81% of all  decays
• Signal-side efficiency including  decay BFs)
15.81 0.05%
• 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
Consistency Check with BD* lν
• Extra neutral energy EECL Validation with double tagged
sample (control sample);
– Btag is fully reconstructed
– Bsig is a semileptonic decay
Calibration data
B+ D(*)0 X+ (fully reconstruction)
B- D*0 l-n
D 0 0
K- +
K- + - +
B+ B-
494  18
B0B0
7.9  2.2
Total
502  18
Data
458
Purity ~ 90%
Extra energy in
the calorimeter
Example of a B ν candidate
Tag: BD0 ,
D0 K
Evidence for B+ ν (Belle)
447 106 B pairs
BtagD(*)[,,a1,Ds(*)] 680k tags, 55% pure.
5  decay modes
Find 17.25.3
4.7 signal events from a fit
to a sample of 54 events.
4.6s stat. significance w/o
systematics,
After including systematics
(dominated by bkg), the
significance decreases to
3.5σ
Extra Calorimeter Energy
MC studies show there is a small peaking
bkg in the 0 n and 0 n modes.
Bn yields broken down by  decay mode
(stat sig
only)
For the first 3 modes, the background is fitted with a 2nd
order polynomial plus a small Gaussian peaking
component.
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 BD* l n
control sample or in the internal review process.
New value
Previous
value
0.39
4
BF( B  n )  (1.7900..56
)

10
490.46
0.18
4
BF(B  n )  1.0600..34

10
280.16
(Preliminary)
Direct experimental determination of fB
• Product of B meson decay constant fB and
CKM matrix element |Vub|
1.1
4
f B Vub  (10.11.6
)

10
GeV
1.41.3
•
Using |Vub| = (4.39  0.33)×10-3 from HFAG
 3630
 3134
f B  229
MeV
( Belle)
15% 14% = 12%(exp.) + 8%(Vub)
fB = 216  22 MeV (an unquenched lattice calc.)
[HPQCD, Phys. Rev. Lett. 95, 212001 (2005) ]
Constraints on the charged Higgs mass
Assume fB and |Vub |
are known, take the
ratio to the SM BF.
mB2
rH  (1  2 tan 2  )
mH
rH=1.130.51
Motivation for BK*nn (bs with 2 neutrinos)
BSM: New
particles in
the loop
Other weakly coupled
particles: light dark matter
SM: BF(BK* nn) ~1.3 x 10-5
(Buchalla, Hiller, Isidori)
PRD 63, 014015
c.f. SM: BF(BK- nn) ~4 x 10-6
[Belle preliminary (275 x 106 B Bbar) : BF(BK- nn) <3.6 x 10-5] to be updated soon
BK(*)νν are particularly interesting and challenging
modes (Bν is even a small background)
The experimental signature is BK + 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 3s
Region
C. Bird et al
PRL 93 201803
.(T. Adams et al. PRL 87
041801;A. Dedes et al.,
PRD 65 015001)
Direct dark-matter searches cannot see M<10 GeV region
Search for BK*nn (532 x 106 B Bbar pairs)
Result from a
blind analysis.
BELLE-CONF0627
Yield  4.7
3.1
2.6
(1.7σ stat.
significance)
Sideband = 19
MC expectation =
18.73.3
Extra Calorimeter Energy (GeV)
B( B0  K *0nn )  3.4 104
SM (Buchalla, Hiller,
Isidori) 1.3 x 10-5
(at 90% C.L)
Search for BK*nn (properties of candidates)
b  c background
rare B background (x 15 data)
udsc background
Signal x 20
combined background
Data
KπInv. mass
Search for BK*nn (properties of candidates)
b  c background
rare B background (x 15 data set)
udsc background
Need more bc
MC (only 2 x data)
Signal shape
combined background
Data
P*_K*
K* momentum distribution
Event display for a BK*nn candidate due to an
identified background (BK*γ)
Tag Side
B  D+ a1D+  K- π+π+
π+
a1-  ρ0 π- , ρ0 π+π-
K-
Missing mass ~ 0
γ
(Hard photon is lost in
the barrel-endcap calorimeter gap)
MC: Expected bkg from
this source ~0.3 evts.
Future Prospects: Bn
95.5%C.L. exclusion boundaries
DfB(LQCD) = 5%
Extrapolations (T.Iijima)
Lum.
DB(Bn)
exp
D|Vub|
414 fb-1
36%
7.5%
5 ab-1
10%
5.8%
50 ab-1
3%
4.4%
tan  / mH
50ab
If D|Vub| = 0 & DfB = 0
-1
rH
tan  / mH
20
Future Prospects:
Other probes of charged Higgs
•Semileptonic: BD(*)  n
mb tan   mc cot 
c
b
H/W
m tan 

Expected BF(SM)~
8 x 10-3
Decay amplitude
 mb m tan 
2

n
B
( B  D v )
( B  Dv )
Multiple neutrinos, low momentum
lepton (use e’s), large bkg but still
might be possible with enough
data.
21
Some modes are very difficult at hadron colliders
MC extrapolation to 50 ab1
5s
Observation of B± g K± n n
(compare to K++ννand KL g 0nn)
Belle result on Bν
shows that B to one prong
decays can be measured.
MC
SM pred: G. Buchalla, G. Hiller,
G. Isidori (PRD 63 014015 )
Extra EM calorimeter energy
Super B LoI Fig.4.18
Conclusions on “Missing Energy Decays”
• Evidence for Bνand experimental determination
of fB (preliminary result has been updated)
0.39
4
BF( B  n )  (1.7900..56
)

10
490.46
• Search for BK* nn (UL is still a factor of 10 above
the SM range)
BF ( B0  K *0nn )  3.4 104
• Further dramatic progress (e.g. signals for BK(*)
νν) will require Super B Factory class luminosity.
Backup Slides
Contributions to systematic error for Bn
Peaking Backgrounds in Bn
Tau tagging mode
Tau tagging mode
Fits to individual B n decay modes
(updated for ICHEP06)
Requirements in Bν analysis
• The  lepton is identified in the 5 decay modes.
81% of all  decay modes
• Signal selection criteria.
• Signal-side efficiency including  decay br.)
15.81 0.05%
• All selection criteria were optimized before examining
the signal region (blind analysis).
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n
Bn signal
signal + background.
Background
Mbc
Pmiss
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.
KL in coincidence.
KL in veto
EECL
EECL
• We do not use this cut in the result, to avoid introducing a large
systematic error due to the uncertainty in KL detection efficiency.
Selection Requirements for BK* nn
MC signal and
bkg distributions,
Tag Side
B  D+ a1D+  K- π+π+
a1-  ρ0 π- , ρ0 π+ππ+
tagB
tagB
K-
tagB
tagB
tagB
tagB
γ