Update on the Inclusive Measurement of the b s

Transition Rate Using a Lepton Tag Using Run I-V DataPhilip Bechtle (until 5/07)*, Rainer Bartoldus

SLAC

Colin Jessop, Kyle Knoepfel

Notre Dame University

Al Eisner, Bruce Schumm, Luke Winstrom

UC Santa Cruz

Minghui Lu

University of Oregon

John Walsh

University of Pisa

Students

* Now at DESY

Bruce SchummSCIPP6/07 BaBar Coll. Meeting

b s is a leading constraint on new Electroweak scale physics…

The SM transition is high order (two weak plus one EM vertex…

So new physics can enter at leading order

Direct searches (LEP)

B s constraints

MSSMConstraints

Extra DimensionsSUSY

b s also provides universal constraints on hadronic effects

Photon spectrum can be used to measure universal heavy quark parameters (largest uncertainty in |Vub| from inclusive measurement of b ul)

In addition to partial BF, we measure 1st and 2nd moments of the photon distribution

b motion

(J. Walsh)

1.9

Run1-2 Babar Fully Inclusive

BaBar 2006 inclusive result (Run I-II only):

B(B Xs ; 1.9 < EB < 2.7) = 3.67 0.29 0.34

0.29,

where errors are statistical, experimental, and model uncertainty, and E

B is the photon energy in the B rest frame.

Current Status of b s BF Measurements

Phys.Rev.Lett.97:171803,2006

To interpret the partial BF, one must extrapolate from E

B = 1.9 GeV (experimental lower limit) to E

B = 1.6 GeV (where theoretical calcul-ations are done). [We are not yet concerning ourselves with that step for Run 1-V analysis.]

BaBar Sum ofExclusive Modes

qq + ττ

BB

XSγ

Inclusive b s: little effect from long distance physics, but how do you eliminate backgrounds?

Continuum Bkgds:

• Shape variables (was Fisher discriminant; now Neural Net)

• Lepton tag indicates heavy flavor in “rest-of-the-

event” decay

(4S) Bkgds:

• Reconstruct (usually asym-metric) 0 and decays

• Calorimeter cluster shapes sup- press merged 0s, hadrons

Source: BAD 323, based on the 81 fb-1 Run I-II sample

What are the sources of B/Bbar background?

And then…

• Subtract off small remaining continuum using off-resonance (dominant statistical term)

• Develop independent estimates B/Bbar backgrounds and subtract them (critical step)

• Confirm B/Bbar estimates with control region

Theorists would love us to push below 1.9 GeV, but B/Bbar backgrounds intimidate…

After Selection Cuts

B/Bbar background control region BB Cont.

Signal

Sig. Region

Source: BAD 323, based on the 81 fb-1 Run I-II sample

Truth Match Parentage

Fraction of Total

1.5 < E* < 1.9

Fraction of Total

1.9 < E* < 2.7

Photon 0 0.573 0.666

Photon 0.171 0.156

Photon 0.037 0.021

Photon 0.011 0.008

Photon B 0.034 0.014

Photon J/ 0.005 0.008

Photon electron 0.093 0.047

Photon other 0.004 0.004

All Photon 0.928 0.924

0 Any 0.000 0.000

electron Any 0.048 0.037

neutron/antineutron Any 0.017 0.029

proton/antiproton Any 0.000 0.001

K0L Any 0.002 0.001

or K Any 0.002 0.002

none Any 0.002 0.006

other Any 0.000 0.000

All non-photon 0.072 0.076

All 1.000 1.000

Nominal B/BBarBackgroundSources

82% of B/Bbarbackground

Electron categories x2 larger than that of prior simulation (was 3.7% combined). This raises questions, in-cluding the modeling of bremsstrahlung

B/Bbar background contribution “guess”

(selection not yet finalized)

Constraining the 0 - Background with a Measurement of Inclusive Production

invariant mass

Fits done to both data and MC

MC Correction Factors

• Measure 0/ yields in on- and off-peak data and MC

• Determine MC correction factors in bins of E: Correction = [(On-peak data) – s*(off-peak data)]/[BB MC]

• Use corrected MC to predict background contribution

• Also need to know recon. efficiency. of background s

How Do We Reconstruct 0s and ’s?

• Begin with reconstructed high-energy (HE) with cms energy E*

• Search GoodPhotonsLoose list for potential sibling with the following minimum lab energy (E2,lab) requirement (from Run 1-2; not yet optimized for current analysis):

• Find potential sibling that, in combination with HE , has invariant mass M closest to the 0 () mass.

• Reject event if 115 < M < 155 (508 < M < 588) MeV for the best 0 () combination.

Reconstruct 0 E2,lab > 40 MeV for E* < 2.3 GeV E2,lab > 80 MeV for E

* > 2.3 GeV

Reconstruct E2,lab > 175 MeV for E* < 2.3 GeV E2,lab > 275 MeV for E

* > 2.3 GeV

From Run I-II Analysis; subject to further optimization for current Run I-V result

And with What Efficiency?If high-energy (HE) truth-matches to a 0 daughter, make succession of requirements on MC truth properties of other (low-energy) daughter

coslab

1

Require 2nd photon to be in fiducial volume

-.74 < coslab < .94

E*

2 Require 2nd photon to be above minimum energy cut

E*

3

Require 2nd photonto have a truth match

E*

Of remaining bkgd events, almost all make a good 0 candidate with the HE

Observations:

• Typically reconstruct only about ½ (depends on E

*) of background 0s

• 20% truth-matching inefficiency; only about 6% due to merged 0s. Could the rest be conversions?

must understand conversion effects to subtract background correctly (not appreciated before)

Material and the Inclusive Measurement of b s

Material enters into the measurement of b s in three substantial ways:

• Conversions HE efficiency,

• Conversions 0 reconstruction efficiency

• Bremsstrahlung electron fake rate

There are complications associated with esti-mating these effects. For example, a photon converting in the DIRC may or may not be reconstructed as the original photon, depending on its energy, the depth in the DIRC, etc.

This must be understood, in addition to the distribution of material in the detector and the brem/conversion cross-sections. Additional control samples may need to be developed and applied (“radiative bhabha” to understand bremsstrahlung?).

More clever rejection of 0 backgrounds? ( analysis used likelihood based on mass and E2,lab) try NN rejection

Sig

nal E

ffic

ienc

yS

igna

l Eff

icie

ncy

Background Efficiency

Ignoring E* information

Run I-II analysis performance

Using E* information

Variables considered:

M E*

E2,lab coslab

HE 2nd moment HE isolationHE Lat. Moment LE 2nd momentLE isolation LE Lat. Moment

M

E*

E2,lab

Most power in M, E2,lab (already in use) and E

* (dangerous). Will not pursue.

Continuum Suppression for Run I-V Analysis

Develop Neural Net to make most efficient use of shape variable information. Inputs include Fox-Wolfram moments, lepton tagging variables, energy-flow variables:

Two classes of NNs, separated by energy-flow approach:

• Energy cones (three variants) Two different cuts on NN output (standard and relaxed) One without lepton momentum

• Legendre moments plus momentum-tensor quantities (similar to sphericitiy tensor)

Prior (Run I-II) analysis used Fisher Discriminant composed only of shape variables

Note: At the end of the day, the continuum subtraction will be determined from the off-resonance data, not from

a-priori understanding of the NN efficiency

% of total Error

Statistitical

Systematic

Model

Run I-II Result (Phys.Rev.Lett.97:171803,2006 )

Br (BXs) = (3.67 0.29 0.34 0.29) x 10-4

Neural Net Selection: A Word About Run I-II Syst. Errors

Different b s models (b mass,Fermi motion)

E* [GeV]

E* [GeV]

Important: Run I-V optimi-zation must consider both statistical and systematic (especially model) error!

Selection efficiency vs. E* for Run I-IIselection

Econes I

• better statistical precision

• larger model error

Event-Shape NN Selection

Legendre Moments

• more stats in 0/ control sample

• reduced model error eff. slope = 1.5

eff. slope = 3.2

Eff vs. E

Consider both partial BF as well as moment calculations. All in all…

• None of the candidate NNs is clearly preferable

• Choose Legendre-moment-based NN in view of its modest dependence of signal efficiency on E

*

Other Backgrounds: AntineutronsWas 7.7% of the B/Bbar background for RUN I-II

Contribution can be constrained by looking at antiprotons. Must understand:

Production Rate

Two components: fragmentation and decay; have different isospin relations (p/n fraction) and different momentum spectra

Working with hadronics group (D. Muller) to sort out.

Signature in EMC

Use -bar sample (high momentum)[Develop dE/dX-identified sample (low momentum) ?]

ECAL Lateral Moment

Data

MC

Other Backgrounds: and ’

BAD 179 + private updates

BAD 163

: nominally 2.1% of B/Bbar background; d/dp* measured; use to correct rates in MC (correction factor “”)

/: nominally 0.8% of B/Bbar background; less well-constrained, but less of a contribution.

X’ = E’/Ebeam Range B(B /) Data B(B /) MC

0.1 = 0.39 (1.54 0.41) x 10-2 4.15 x 10-2

0.39 – 0.52 (1.00 0.33) x 10-3 5.63 x 10-4

Simulation estimates that HE backgrounds photons with B meson parents are twice as common than that of Run I-II simulation (1.4% vs. 0.7% of B/Bbar background) .

These gammas seem to be coming predominantly from SL decay; how well do we understand this number?

Why did it change in the MC simulation?

Other Backgrounds: B X

b s Outlook I

The lepton-tagged inclusive analysis is gelling…

• CM2 migration complete• Low-energy truth-matching work-around• Shape-variable selection (NN) finalized• 0 and production rates measured• 0 background rejection revisited• Several other selection cuts established (merged 0s …)

A number of “standard” things remain (treatment developed for Run 1-2)

• Anti-neutron rejection criteria• Final optimization• “Control region” test of B/Bbar background contribution• Estimation of most sources of systematic errors

An admirable goal would be Lepton/Photon – what kind of shape are we in?

However, some new considerations have arisen

• Brehmsstrahlung and conversions (material effects)Non-DST level study of conversion, brehm propertiesNew control samples (radiative Bhabha?)

• Understanding of direct B backgrounds.

Also, the loss of Philip Bechtle (to DESY) was a set-back, but students (Kyle, Luke) now coming up to speed on production code.

Initial preliminary results will include measurements of:

• Partial branching fraction (1.9 < E* < 2.7) further tighten

constraint on new physics• 1st and 2nd moments of photon energy distribution generic constraint on fermi motion of b quark

• ACP Independent probe for new physics (current: -.110.115.017)

We have our work cut out for us…

b s Outlook II