Update on the Inclusive Measurement of the b s

Transition Rate Using a Lepton Tag

Philip Bechtle (until 5/07)*, Rainer Bartoldus

SLAC

Colin Jessop, Kyle Knoepfel, Postdoc (TBD)

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

Run1-2 Babar Fully Inclusive

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

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

0.29,

where the errors are statistical, experimental uncertainty, and model error.

Current Status of b s Measurements

Phys.Rev.Lett.97:171803,2006

To interpret the partial BF, one must extrapolate from E

* = 1.9 GeV (experimental lower limit) to E

* = 1.6 GeV (where theoretical calcul-ations are done). We are not yet concerning ourselves with that step.

BaBar Sum ofExclusive Modes

qq + ττ

BB

XSγ

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

Continuum:

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

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

event” decay

(4S):

• Reconstruct (usually asym-metric) 0 and decays

• Calorimeter cluster shapes elim- inate merged 0s, hadrons

What are the sources of B/Bbar background?

And then…

• Subtract off small remaining continuum using off-resonance

• 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

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

B/BBarBackgroundSources (XXX Monte Carlo)

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 brehmsstrahlung

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

• Also need to know recon. eff. 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:

• 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

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 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 efficiency appears to be mostly conversions (only about 6% of background 0s are merged)

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

For low-energy photons that are not truth-matched…

Distance (m) between truth-matched HE and true low-energy sibling

Dis

tanc

e (m

) be

twee

n re

cons

truc

ted

HE

and

near

est c

lust

er “Merged” 0s (photons form single cluster)

Material and the Inclusive Measurement of b s

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

• Conversions (HE efficiency, 0 reconstruction efficiency)

• Brehmsstrahlung (electron fake rate)

There are complications associated with estimating 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 brehm/conversion cross-sections.

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.

% 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 error!

Selection efficiency vs. E* for Run I-IIselection

Four neural net algorithms under consideration:• 3 variants using Energy Cones• 1 uses Legendre Moments• Choose based on best uncertainty (including dominant systematics)

Econes I

• better statistical precision

• larger model error

Eff vs. E

Event-Shape NN Selection

Legendre Moments

• more stats in 0/ control sample

• reduced model error

Neural Net

Statistical

Error

Systematic

Error

Model

Error

Total

Error

ECone 3.6 2.7 4.3 6.2

Econe NoP

3.9 2.6 3.5 5.9

Econe

Relaxed

3.5 2.4 4.2 5.9

Legendre 4.3 1.9 3.8 6.1

Run1-2 7.9 9.2 7.9 14.5

Differences are relatively small choose Legendre NN for itssmall syst. and model errors

Expected Partial Branching Fraction Errors

(Only uncertainties dominant in Run I-II analysis included)

Other Backgrounds: AntineutronsNominally 2.9% of B/Bbar background

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 ECAL

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 (1.4% of B/Bbar background) than that of Run I-II simulation.

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

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 (on our to-do list from early on)

• 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