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Leptonic B decays: a rare window to New Physics
Paul D. JacksonUniversita’ “La Sapienza” & INFN Rome
A sketch of what follows
Motivation of leptonic B studies
Outline experiments/detectors
Techniques/analyses and results
Future outlook and summaryPaul D. Jackson
Leptonic B decays
2
The Lure of LeptonsA simple example illustrates the full extent of the attraction:
+πudπ
+π
+μ+μ+πF++ τVfm
m
π
mmG=νμπBR
22
2
2
21
8
2
Pros Cons
Relatively free of hadronic uncertainties (encapsulated in “f
X”, the “decay
constant of meson X”) Recognizable experimental signature
Helicity suppression (m2l)
less of a con when new physics can enhance the rate over the SM
Undetectable final state particle
W+
d
u
+
(And, it scales easily to similar decays starting from a heavy, pseudoscalar meson...)
Paul D. Jackson
Leptonic B decays
4
Extract fB through leptonic decays: B+ → l+νl
Cleanest source (1% measurements for π+,K+ ) Heavy B+: low branching ratios to leptons
2
2
2122||2
8
2)(
BM
mmBBMubVBf
FGBSMBR
Measurement of fBMeasurement of fB
B+
B+ τ+ντ
(disfavours l=e, favours l=τ)Helicity Suppression(disfavours M = B, Bs, D)
CKM mixing
τ+
ν
B meson decay constant
W+
Paul D. Jackson
Leptonic B decays
+W+, H+
2
2
22tan1)(
H
BSM m
mBBRBBR
Sensitive to contributions from physics beyond SM
In two Higgs doublet model (type II) (W.Hou, Phys. Rev. D. 48, 2342 (1993))
Provide important constraints onProvide important constraints on
Can enhance or suppress the rate depends on tanCan enhance or suppress the rate depends on tan and m and mH H
for example: mfor example: mHH = 140 GeV, tan = 140 GeV, tan = 40, give ~1.6 enhancement = 40, give ~1.6 enhancement
Hm/tan
tantan is the ratio of vacuum expectation values for two Higgs doublets. is the ratio of vacuum expectation values for two Higgs doublets.
Beyond the Standard ModelBeyond the Standard Model
Paul D. Jackson
Leptonic B decays
6
The measured branching fraction yields an independent constraint on the plane.
Likewise, all other measurements excluding the BF (and f
B) can constrain
it using the theory relations.
CKM Fit CKM Fit Prediction:Prediction:
(post-ICHEP (post-ICHEP update)update)
(1.39(1.390.44)0.44)1010-4-4post-ICHEP 2006 updatepost-ICHEP 2006 update
),()(
fmeasΔm
measν+τ+BBR
d
theory
B+→τ+ν and the CKM Formalism
Paul D. Jackson
Leptonic B decays
7
ExperimentsBaBar
Belle
CLEO-c
CDF
Dzero
There are currently several active experiments which produce and study heavy flavour in large numbers (and several others will join the game soon!).
I will present results primarily from BaBar, making comparisons to Belle, CLEO-c, and the Tevatron where/if appropriate.
Paul D. Jackson
Leptonic B decays
BaBar detectorBaBar detector
PEP-II Storage RingsPEP-II Storage Rings
LinacLinac
SF Bay SF Bay
Plan view of SLAC
Paul D. Jackson
Leptonic B decays
10
Not just a B-factory….similar cross section for b, c, τ production
B Factory operations
Asymmetric collider operating at (4S) resonance (Ecms=10.58 GeV) with 3.1 GeV e+ and 9 GeV e-
B-mesons in lab have =0.56
(4 )e e S BB
B B production threshold
nbnbccnbBB 9.0)(3.1)(1.1)(
Leptonic B decays Paul D. Jackson
11
The BaBar “Collider-Scope”©
Cherenkov Detector (DIRC)
144 quartz barsK, separation
Electromagnetic Calorimeter6580 CsI crystals
e+ ID, and reco
Drift Chamber40 layers
Tracking + dE/dx
Instrumented Flux Return19 layers of RPCs and KL ID
Silicon Vertex Tracker
5 layers of double sided silicon strips
e+ [3.1 GeV]
e- [9 GeV]
Leptonic B decays
Limited streamer tubes
Completed during winter 2006
Paul D. Jackson
12
PEP-II Luminosity PerformanceBest Performance
PEPII peak Luminosity: 1.2x1034
cm-2 sec-1 Integrated Luminosity: in 24 hours: 891.2 pb-1
0
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Inte
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min
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fb-1
]
2007~0.55 ab-1
Doubled againData
“doubled”2008
~1 ab-1
390 fb-1 total data sample
Paul D. Jackson
Leptonic B decays
13
Analysis Techniques I
“Single B Beam” or “Tag B” Methods
Takes advantage of anticipated quark/anti-quark system Reconstruct one of the two bottom mesons in a well-defined final state, “tagging” the event as BB. Search in the recoiling particles (the “single B beam”) for evidence of the target rare decay
Beam Kinematics Mesons produced directly by the beams must satisfy collider-based kinematic constraints Define “Energy-Substituted Mass” (m
ES) and “Energy Difference” (E)
variables, characterizing such production:
22 BbeamES
pE=m beamBB
Ep+m=ΔE 22
Paul D. Jackson
Leptonic B decays
14
Analysis Techniques IINon-resonant Background Rejection
Light quark events typically more “jetty” Use a suite of event-shape variables, such as thrust angles, Fox-Wolfram moments, etc. as selection variables or in a multivariate (Fisher, NN) approach.
Signalbox(es)/Sideband(s)
Use discriminating variables in which signal “peaks” and bkgds are distributed (or also peak) Define signal-rich regions (signal box) and background-rich regions (sidebands) as control samples
upper sideband
lower sideband
middle sideband
“Blind” signalbox
s
s
b
b
Paul D. Jackson
Leptonic B decays
15
Expectations from Theory
An example Standard Model diagram
γ
Photon emission from the initial state relaxes the helicity suppression
Unlikely to observe SM with current datasets
Fully reconstructable final state
Standard Model New Physics Models
Supersymmetry: different models predict different mechanisms
R-parity violating models allow for tree-level FCNC, enhancing the rate at smaller tan
MSSM models predict ~tan
enhancement, allowing for up to 100x enhancement over SM
ℓ+
ℓ-
Paul D. Jackson
Leptonic B decays
17
Paul D. Jackson
Signal Selection
Construct B0 candidates from two leptons (electron or muon) and a photon
leptons and photon required to be well within the fiducial region of the detector (reduce ISR, higher-order QED backgrounds)
leptons required to meet at a common vertex, and 0.3 < m
ll < 4.9 (4.7) GeV/c2 for electrons
(muons)
Constrain the B candidates to be consistent with production at the Ύ(4S) using m
ES and ΔE:
We search for B0→ℓ+ℓ-γ in a sample of 324106 BB events
Fully reconstructing the signal B means that a “tag B” technique is unnecessary.
BABAR(MC)
18
Background Rejection
Reject non-resonant e+e-→qq (q=u,d,s,c) background using signal B kinematics and event shape information in a Fisher discriminant
Optimal selection criteria are determined using Monte Carlo simulations and minimizing the predicted upper limit (assuming no signal is observed).
Reject backgrounds from J/ψ, ψ(2S) decay (leptons) or 0 decay (photon)
Define a plane in mES
and ΔE in which to perform the final signal
extraction: mES
> 5.2 GeV/c2 and -0.5 < ΔE < 0.5
electrons
Paul D. Jackson
Leptonic B decays
εesignal = (6.07±0.14)% εμ
signal = (4.93±0.12)%
19
Background Estimate
Determine the mES
shape of backgrounds using the upper+lower sideband - extrapolate the middle sideband into the signal box:
electrons
muons
Data shown are from the upper+lower sidebands
nbkg
exp=1.28±0.80
nbkg
exp=1.40±0.42
electrons
muons
electrons
upper sideband
lower sideband
middle sideband
Paul D. Jackson
Leptonic B decays
20
ResultsALL RESULTS ARE
PRELIMINARY
We observe 0 (3) events in the signal box in electron (muon) events. We set frequentist upper limits on the branching fractions (including systematic uncertainties):
C.L. 90% theat 100.7 70 <γeeBBR +
C.L. 90% theat 103.4 70 <γμμBBR +
Systematic Uncertainties
e ( μ ) Photon Energy 1.6% (1.6%)
Particle ID 0.7% (1.3%)
B counting 1.1% (1.1%)
Charged ParticleReconstruction 0.94% (0.94%)
Total: 2.3% (2.5%)
muonselectrons
PRELIMINARY
Paul D. Jackson
hep-ex/0607058
Leptonic B decays
21
ResultsRESULTS ARE PRELIMINARY
CDF presented their latest result in the search for Bd at
ICHEP '06. Using vertexed muon pairs and a likelihood discriminant to search for B(s,d)→.
central-centralcentral-extension
Standard Model Prediction:
-9-+ 100.53.4 ±=μμBBRs
CDF observes 1 (2) events in the Bs (B
d)
channel (consistent with background). Upper limits are set:
BR(Bs) < 1.0×10-7 @ 95% CL < 8.0×10-8 @ 90% CL
BR(Bd) < 3.0×10-8 @ 95% CL < 2.3×10-8 @ 90% CL Currently world's best limits.
CDF public note 8176
Leptonic B decays Paul D. Jackson
22
B+→l+ (l=e,)
Presence of photon removes helicity suppression and hence universality of leptonic branching fractions is recovered (apart from phase space)
R =1/ΛB , related to the B light cone distribution amplitude SM BR(B→l) ~(1-5)x10-6 (Korchemsky, Pirjol and Yan Phys Rev D61, 114510, 2000)
Recent BaBar analysis based on 232M BB pairs Identify lepton and signal photon and perform and inclusive
reconstruction (i.e. 4-vector sum) of the other B Neutrino 4-vector obtained from missing momentum vector Veto 0 candidates and suppress continuum backgrounds with
event shape variablesLeptonic B decays Paul D. Jackson
28
B+→l+ (l=e,) results
Extract signal from maximum likelihood fit to mES and neutrino E-|p| yields in signal and sideband regions
Br(B→) < 5.2x10-6
Br(B→e) < 5.9x10-6
Br(B→l) < 5.0x10-6
(combined)Experimental sensitivity approaching Standard Model rate
PRELIMINARYPRELIMINARY
BB MC
Continuum MC
Leptonic B decays Paul D. Jackson
29
Multiple neutrinos in the final state lacks experimental constraints
Reconstruct B meson in the event with B→D0lνX. Compare remainder of event with signature for signal decay.
Techniques pioneered by BaBar!!
B-X (X=anything)Main decay modes e, , 0
(4S)B- B+
e+
e
B++, +e+e
B-D(*)0 lνX
B- D(*)0 l- (l = e, ) D0 (0/) K- + K- + 0 K- + - + K0
s + -
Choose best tag with P(Vtx)
Apply cuts to suppress bkgd
Analysis Strategy 383x106 BB pairs
Paul D. Jackson
Leptonic B decays
31
B++”by eye” Tagged signal MC event, where D0→Kπ
and there is a neutral pion from
D*0 decay. The tau decays to ππ0
BABAR
Event contains a well-reconstructed
“tag” B, here decaying into a
semileptonic final state
Signal B decay is low multiplicity and should comprise of the remaining well-
reconstructed charged particles
Events then characterized by “E
extra”,
the sum of neutral (and in cases, charged)
energy left over after all tag and signal sources
are accounted.
Paul D. Jackson
Leptonic B decays
32
Event Selection Starting from 383106 BB events, reconstruct tag B in a
semileptonic final state B→D0lνX
Tag B reconstruction efficiency:
D0→K-π+, K-π+π-π+, K-π+π0, K0sπ
+π-
X = γ, π0 from D*0 decay, which we do not explicitly reconstruct
Require lepton CM momentum > 0.8 GeV/cRequire that -2.0 < cosθ
B-D0l < 1.0, where
and we determine the parent B energy and momentum from the collider energy
We study the tag reconstruction efficiency in “double-tag” events, where both B mesons are reconstructed as B→D0lνX, and correct based on the data/MC comparison
3100.060.036.57 syst.±stat.±
Data are from double-tag events, showing the extra energy distribution.
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Leptonic B decays
33
Eextra variable Most sensitive signal selection variable Eextra = Etotal - ΣEtag – ΣEsignal
Use energy from all unassigned clusters and tracks
Background prefers higher values, signal peaks near zero
bump from missing π0/gamma from tagB
PRELIMINARY
PRELIMINARY
Signal MC
Eextra (GeV) Eextra (GeV)Leptonic B decays Paul D. Jackson
34
Overview of signal selectionWe select signal candidates as: τ+→e+νν, μ+νν, π+ν, ρ+ν
Background rejection constrain the event missing mass and signal candidate momentum veto events with reconstructed K0
L candidates or extra neutral pions
reject non-resonant background using a combination of thrust and minimum invariant mass information (R)
Paul D. Jackson
Leptonic B decays
35
Leptonic B decays Paul D. Jackson
Double tagging
Reconstruct both B’s as B→D0lnuX tagging mode (several 1k’s events) Study quantities NK0L, MMiss, D0Mass, in this control sample Compare data/MC for corrections and syst. Errs
Ratio Ndata/NMC ≈ 1, difference from unity correction factor The uncertainty in this method → systematic error Similar comparisons for other quantities like Eextra etc
This method is essential to the analysis
36
sideband
Signal efficiency (relative to the number of semileptonic tag Bs):
scaled to BR=110-4
Bkg. MC is scaled using data vs. MC bkg. prediction ratio
Sideband scaling and projection
The final discriminant is Eextra
, sum of all unused energy from charged and neutral particles. Final bkgd prediction by scaling of sideband in data using sideband/signal ratio from MC.
Paul D. Jackson
Leptonic B decays
37
New Results
Interpret results with a modified frequentist technique. A ratio of likelihoods, includes systematics as Gaussians convoluted with the likelihoods
ALL RESULTS ARE PRELIMINARY
Observe a result consistent with zero signal at 1.6σ, so we set a limit and quote a central value.
Calculate the product f
B|V
ub|
20.223.3 ±=Ns
“Naive” signal yield
4100.16.01.0
syst.±stat.±=ν+τ+BBR
GeV410x.))(5.0.)(0.28.2
0.7(|| syststat
ubV
Bf
CL 90% theat 4-1.8x10 )(B BR
Paul D. Jackson
hep-ex/0608019 (update to be submitted to PRD)
Mode Expected Observed
τ+→e+νν 44.3±5.2 59
τ+→μ+νν 39.8±4.4 43
τ+→π+ν 120.3±10.2 125
τ+→π+π0ν 17.3±3.3 18
All modes 221.7±12.7 245
Hadronic tag: Analysis strategy
ee--
DD**
ee++
BBrecoreco
BBrecoilrecoil
e,e,,,,,,a,a11
1 or 2 1 or 2
Fully reconstruct the tag side in hadronic B to D(*) decays and look in the recoil for signal events.
e, , 0,
2-3 neutrinos in the final state lacks experimental constraints
Tag B reco. Clean event Get B count for BF calculation:
BR(B) =ns
• Pre-selection Sample divided in 5 subsamples
(e,,,,a1 (80% BF)) Removes continuum background Used to perform data/MC
comparisonPaul D. Jackson
Leptonic B decays
39
B counting• Fit to mES to determine the total number of B mesons
• Non-peaking component yields on the mES sideband (mES<5.26GeV)
• Peaking component obtained by subtracting the extrapolated comb. bkg under peak: nB ~ 6.86 x 105.
Shapes of the non peaking components fixed from MC
Continuum fraction scaled from off-peak data by the luminosity ratio
cont.
B0
B+
signal
mES (GeV/c2)
Paul D. Jackson
Leptonic B decays
PRELIMINARY
40
Cut on topological and kinematical variables to reject backgrounds.
Missing momentum direction and magnitude # of charged tracks particles momenta Resonance mass (for the)
Cuts values determined by an iterative procedure aiming at the best s/√b.
Final selection cuts
Paul D. Jackson
Leptonic B decays
41
Discriminating Variables
Data
Combinatorial bkg
total bkg (peaking+comb.)
signal
PRELIMINARYPRELIMINARY
PRELIMINARY
Paul D. Jackson
Leptonic B decays
42
The Eextra variable (again) As with the SL tags most
discriminating variable is the (extra) energy in the calorimeter. Since we don’t expect extra
reconstructed objects for signal events.
Defined as sum of the cluster
energies not overlapping with the tag B. Cut on minimum cluster energy
choosen to remove noise Data/MC agreement affected by
the choice of the threshold
e
PRELIMINARY
PRELIMINARY
Paul D. Jackson
Leptonic B decays
43
Efficiency evaluation Determined from signal MC Efficiency table before the cut on Eextra
Cross feed between channels taken into account to calculate the total efficiency of each reconstruction mode
PRELIMINARY
Paul D. Jackson
Leptonic B decays
44
To be completed….
Paul D. Jackson
Leptonic B decays
Hadronic tagged analysis not quite completed but is complimentary to the S.L. tagged analysis and can be easily combined.
Should provide competitive measurement and good consistency check within expt.
Expect (hopeful) that both BaBar analysis will complete updates with our entire data set in the next few weeks/months.
Stay tuned…..
45
The final results are deduced by unbinned likelihood fit to the obtained EECL distributions.
Signal shape : Gaussian + exponentialBackground shape : second-order polynomial
+ Gaussian (peaking component)
Signal +
background
Background
B
Signal
Observe 17.2 events in the signal region. Significance decreased to 3.5 after including systematics
+5.3 - 4.7
: Statistical Significance
Belle ResultPRL 97 251802
(2006)Revised for ICHEP06
Leptonic B decays Paul D. Jackson
46
Measured branching fraction;
Product of B meson decay constant fB and CKM matrix element |Vub|
Using |Vub| = (4.39 0.33)×10-3 from HFAG
fB = 216 22 MeV
[HPQCD, Phys. Rev. Lett. 95, 212001 (2005) ]
15% 16% = 14%(exp.) + 8%(Vub)
+0.56 +0.46 -4-0.49 -0.51Br B 1.79 ×10
+1.6 +1.3 -4B ub -1.4 -1.4f V =10.1 ×10 GeV
+0.036 +0.034B -0.031 -0.0370.229f = GeV
Belle Result
Leptonic B decays Paul D. Jackson
47
Combined Results
Combining the central values of recent BaBar and Belle results.
Errors are still large, but…. There have been great
improvements by both collaborations
BF(B+→τ+ν) = 1.36±0.48 (BaBar+Belle combined)
PRELIMINARY
Paul D. Jackson
Leptonic B decays
48
Results in context of BSMInterpretting the results from B+→τ+ν using the type II 2HDM:
LEP, Direct Search(excluded at 95%CL)
BaBar+Belle
(excluded at 95%CL)
Predicted SM branching fraction taken from UTFit prediction.
W.S. Hou, Phys.Rev.D. Brief Report 48 (1993)
2342.
The excluded regions (coloured) are determined using the naive BaBar+Belle average for B+, the LEP direct search limit (>79.3 GeV)
Paul D. Jackson
Leptonic B decays
50
Physics Reach: Expectations
Expect each experiment's branching fraction uncertainty to go from ~0.65x10-4 to ~0.4x10-4 with the advent of 109 B+ mesons at the B factories (assuming no improvements)
BaBar also has a statistically independent hadronic-tagged analysis comparable to Belle's, which effectively “doubles” the number of Bs, bringing the BaBar error down to 0.48 (0.3) with the current (future 109) B+ sample.
B+:
109 B
2 BaBar + 1 Belle
analysis
LEP, Direct Search (excluded at 95%CL) LEP, Direct Search (excluded at 95%CL)
BaBar+Belle(excluded at 95%CL)
Paul D. Jackson
Leptonic B decays
51
Physics Reach: Future Br(B) measurement:
More luminosity help to reduce both stat. and syst. errors. Some of the syst. errors limited by statistics of the control
sample.
|Vub| measurement: < 5% in future is a 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%
My assumption fB(LQCD) = 5% 5ab-150ab-1
Br(B)/md to cancel fB ?G.Isidori&P.Paradisi, hep-ph/0605012
Leptonic B decays Paul D. Jackson
52
Leptonic decays of heavy mesons are
Experimentally interesting
Critical tests of the Standard Model
Potential gateways to new physics phenomena
Large datasets at many experiments will
Allow further reach in the rarest decays, making some of them accessible
Allow experimental limitations to be placed on new physics in time for LHC
Concluding remarks
Paul D. Jackson
Leptonic B decays
Standard Model
New Physics in
The flavour sector
1ab-1 @ B-factories…
Direct evidence from LHC
Super-flavour factory(?)
Art listings I
“The Calling of St. Matthew” (1599-1600)
Contarelli Chapel, San Luigi di Francesi, Roma
“The Incredulity of St. Thomas” (1601-1602)
Neues Palais, Potsdam
“Supper at Emmaus” (1601-1602)
National Gallery, London
“The Inspiration of St. Matthew” (1602)
Contarelli Chapel, San Luigi di Francesi, Roma
All works:
Caravaggio (1571-1610)
Paul D. Jackson
Leptonic B decays
Art listings II
“The Sacrifice of Isaac” (1601-1602)
Galleria degli Uffizi, Firenze.
“St. Francis in Ecstacy” (1595)
Wadsworth Atheneum. Hartford, Connecticut.
“The Calling of Saints Peter and Andrew” ??
Currently above platform 24 of Rome’s Termini Station.
Paul D. Jackson
Leptonic B decays
Limit Setting Procedure (LEP Higgs method, A. L. Read, J. Phys. G28, 2693 (2002) )
obsb
obsbs
b
bss N
N
LC
LCLC
..
....
Using a likelihood ratio estimator to combine different channels :
bL
bsLQ
channels ichannelsn iii n
i i
ni
b
i i
nii
bs
n
bebL
n
bsebsL
11 !,
!
itagBBi BBRNs .)( • Statistical and systematic uncertainties on expected backgrounds are included in the likelihood definition by convoluting with a Gaussian G(bi,bi), where bi is the expected background and bi is the uncertainty on background expectation.
),()()(ibiiiii bGbsLbsL
• Branching fraction upper limit calculated by running toy MC for different branching fraction hypothesis.
• The confidence level (C.L.) for certain signal hypothesis is computed as:
Leptonic B decays Paul D. Jackson
b
u
l+
W+
Calculable in Lattice QCD
– The radiation of a photon relieves the helicity suppression, at the cost of an additional 30-50% theoretical uncertainty (at least for the time being).
~
Mode Challenge SM prediction Current upper limit (90% C.L.)
B → Helicity suppression (4±2) 10-7 <6.6 x 10-6 BaBar PRL 92, 221803 ’04
B → Multi- final state (9±4) 10-5 <2.6 x 10-4 BaBar PRD 73, 057101 ‘06
The purely leptonic decay B+ l+l provides a theoretically clean means of testing a QCD calculation of a simple process:
But these decays are difficult to observe:
2
2
222
22
18
)(
B
llBBB
ubF
m
mmmf
VGlBB
Motivation
Paul D. Jackson
Leptonic B decays
B+→l+ (l=e,)
Theoretical predictions: B(B+ l+) ~ (1–4)×10-6
R, a ratio of moments of the spectator quark momentum. A measurement or even an upper limit on the branching fraction of B→lνγ would allow us to derive non-trivial constraints on R.
(Korchemsky, Pirjol, Yan, PRD 61 114510, 2000) Most recent results are from Belle (hep-ex/0408132,
unpublished) using 140 fb-1: B( B ee ) < 2.210-5 (90% C.L.) B( B ) < 2.310-5 (90% C.L.)
Radiative Leptonic Decays
Paul D. Jackson
Leptonic B decays
B+→l+ (l=e,)
Inclusive reconstruction: sum up all missing E, p in event Use 232M BB pairs in on-peak data (+ off-peak, MC) Blind analysis
Validate simulation with control samples and sideband fit before unblinding
Event selection criteria: Signal side: lepton, photon energies, angle, cos BY Recoil B side: total recoil energy and momentum Neutrino reconstruction: missing E – missing |p| Miscellaneous: Event shape, 0 veto Two-photon rejection: longitudinal momentum, etc.
Iterative cut optimization procedure Binned ML fit to extract signal count
Analysis scheme
Paul D. Jackson
Leptonic B decays
B+→l+ (l=e,)
Unscaled recoil Band LP
Vectorsum,flipped
Scaled neutrinoScaled recoil Band LP
Unscaled neutrino
Signal lepton(select highest CM E)
Signal photon(selecthighest CM E)
Vector sum
LP combination
Detected recoil particles(CM 3-vectors)
Vector sum
Unscaled recoil B candidate
Scale toexpectedmagnitude(~320 MeV)Scaled recoil B
candidate
Neutrino reconstruction
Paul D. Jackson
Leptonic B decays
B+→l+ (l=e,)
nuEP E – |scaled p| Beam-constrained neutrino energy
= E = Ebeam – ELP
Use scaled momentum for better res
mES calculated for the recoil B candidate Recoil E is not very useful: very poor resolution
0 veto: Combine the signal photon candidate with every other
photon candidate in the event and take the invariant mass combination closest to the 0 mass.
Important selection variables
no cuts applied
Paul D. Jackson
Leptonic B decays
Recoil B reconstruction After choosing signal lepton and photon, remaining
particles are assigned to the recoil B candidate Compute standard kinematic variables for this
inclusively reconstructed B Unlike a standard exclusive analysis, the loose reconstruction
reduces the power of these variables
Recoil variables: mES: Still has discriminatory power; useful for fit E:
Computed assuming charged tracks have pion mass and neutrals are all photons Tried PID in the calculation with little improvement
Resolution is not good
MES
E
0 2 GeV-4
5.3 GeV5
Signal MC, valid. sample, all cuts
Paul D. Jackson
Leptonic B decays
Final signal extraction – fit I
Signal extraction fit is a compromise between a cut-and-count and full-blown ML fit Too few off-peak events are expected for any reasonable PDF
Divide plane of recoil mES and nuEP into 1 signal and 3 sideband regions Exploit the differences in shape between signal, BB background, and continuum
Boundaries between regions optimized using toy-MC fits for best sensitivity
B3 B2
B1 S
Paul D. Jackson
Leptonic B decays
Final signal extraction - fit IIScaled region event counts, electron mode, valid. sample
nuEP mES
S
sigsig
B1
sigside
B2
sidesig
B3
sideside
Template shapes
Signal(3×10-6)
21.2 3.7 3.1 0.9
bul7-mode MC
41.1 15.7 52.2 33.7
Gen B MC
21.0 39.1 82.0 310.5
Cont MC*
14.1 67.8 10.5 117.4
0.0
50.0
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BFx10
Paul D. Jackson
Leptonic B decays
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