Leptonic B decays: a rare window to New Physics Paul D. Jackson Universita’ “La Sapienza” & INFN Rome

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

Motivation and Theory

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

The Experimental Environment

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

200

400

600

800

1000

1200

1400

1600

1800

2000

Jan-

05

Jul-0

5

Jan-

06

Jul-0

6

Jan-

07

Jul-0

7

Jan-

08

Jul-0

8

Jan-

09

Jul-0

9

Jan-

10

Jul-1

0

Inte

gra

ted

Lu

min

osi

ty [

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

Analyses:

B0→l+l-(γ)

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


22

Analyses:

B+→µ+ν

B+→l+ν hadronic tagged


B+→l+ν hadronic tagged


25

B+→l+ν Inclusive approach


26

Analyses:

B+→l+νγ

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


29

Analyses:

B+→τ+ν

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.

Paul D. Jackson

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


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


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


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

Future prospects and summary

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


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(?)

Backup slides and additional material

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


b→s and B→τν

Exploring complimentary regions of phase space


More BSM effects…..LFV Courtesy of G. Isidori


Limit Setting Procedure (LEP Higgs method, A. L. Read, J. Phys. G28, 2693 (2002) )

obsb

obsbs

QQ

QQ

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:


Courtesy of A. StocchiLeptonic B decays Paul D. Jackson

UT fit 2006 vs 2015 Courtesy of M. Ciuchini


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

100.0

150.0

200.0

250.0

300.0

0.0

50.0

100.0

150.0

200.0

250.0

300.0

0.0

50.0

100.0

150.0

200.0

250.0

300.0

0

50

100

150

200

250

300

BFx10

Paul D. Jackson

Leptonic B decays

Documents

Leptonic B decays: a rare window to New Physics Paul D. Jackson Universita’ “La Sapienza” & INFN Rome