Fully Inclusive b->s Status Report UCSC: Eisner, Schmitz, Schumm Edinburgh: Tinslay Pisa: Bucci,...

Fully Inclusive b->sStatus Report

UCSC: Eisner, Schmitz, SchummEdinburgh: TinslayPisa: Bucci, Walsh

Royal Holloway: Brown, McMahonSLAC: Jessop, Ozcan

John WalshINFN-Pisa

OutlineI. IntroductionII. New FeaturesIII. BB Background

DeterminationIV. Plans/Conclusions

BaBar Italia – Aprile 2003Capri

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Physics of b->s

• Sensitive to New Physics due to internal loop.

• SM theoretical prediction on fairly solid ground:– Gambino and Misiak: 3.57 +-

0.30 x 10-4

• Very low expected CP violation

• Distribution of mXs (and hence E*) depends mb and p, Fermi motion of b quark inside B meson


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Key Points of Inclusive Analysis• Select high energy photons: the minimum photon energy

is an important aspect of the analysis– Lower energy threshold

• less model dependence • higher BB background higher systematic uncertainty.

• Backgrounds – few handles to suppress them compared to exclusive analyses– Continuum events

• Photon from initial state radiation, ISR 0 () decay photon

– BB events• () decay photon (90-95%)• hadronic fakes (5-10%)


4

Analysis Strategy

• Photon selection – similar to K* analysis• Background suppression

– shape cuts, lepton tags, 0/ vetoes

• Off-resonance data to subtract remaining continuum background safe, although sacrifice statistical precision

• Use control samples to study BB background:– dedicated 0/ skim – hadronic control sample– use to correct BB MC estimates


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• Branching ratio measurement (56 ifb on-peak):• B(bXs) = 3.88 +- 0.36 (stat) +- 0.37 (syst) + 0.43 – 0.23

(model)

• Systemics from ICHEP result– Photon Selection 3.7 % – Event Shape Cuts negligible– Lepton Tags 2.0 %– Normalization 1.1 %– BB background 7.8 %

• Rely on MC simulation to estimate the BB background• Use a control sample to establish how well MC simulation models data,

derive correction factors

ICHEP 2002 Result


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New Features for 2003

• Use full (up to 2002) data set: – 80 ifb On-resonant data– 9.6 ifb Off-resonant data

• Improve event-shape selection by combining several variables optimally into Fisher Discriminant

• Improved treatment of BB background: dedicated measurement of 0 and yields in BB events (Pisa group)

• Reduce minimum photon energy cut – 2 GeV is the goal

• Determination of photon spectrum as well as branching ratio


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Fisher Discriminant – Event Shape

• Have found it useful to combine several event shape variables into a Fisher Discriminant– R2’/R2 (R2’ is 2nd FW

moment with photon removed from calculation for ISR removal)

– 18 energy flow cones around photon direction

• Gives significant improvement in S2/(S+B’) compared to ICHEP analysis

• Introduces small energy-dependence in efficiency negligible increase in model dependence

Fisher > 0.575


8

Determining Background from BB

• ICHEP treatment – – use control sample of events that fail the 0/ vetoes– compare rates in data and MC as function of photon energy

– Drawbacks:• limited statistics

need to extrapolate from low energy bins

• mismatch in bsg and control sample p0/h energy due to inverting vetoes


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Improved BB Background Determination

• New / skim:– increase statistics with dedicated 0/ skim with lower

photon energy threshold (1 GeV instead of 1.5 Gev)

• Method:1. Select candidates using all the standard bs selection cuts,

except for 0/ vetoes2. Perform fits of the 0/ mass distributions in bins of 0/

energy, thus determining the background subtracted 0/ energy spectra.

3. Compare continuum subtracted data spectrum with MC BB spectrum and take ratio as “correction factors”, in bins of 0/ energy.


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Results – 0

• mass fits

• energy spectrum

• data/MC ratio

0 Energy (GeV)


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Results –

• mass fits

• energy spectrum

• data/MC ratio

Energy (GeV)


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Validation: Mass fits

• Compare fit yield with number of truth-matched candidates

• On MC we find that fit yields are systematically less than the number of “true” candidates. Typical yield about 85-90%.

• Find that using Novosibirsk does not help significantly, culprit is long tails in mass distribution

• Similar problem with fits• Is problem with fit or with

truth-matching?


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2 vs Ghit Truth-Matching

• Our original skim produced with 2 truth matching

• We compared with a small sample run with GHIT matching

• Qualitatively, it appears that the Ghit performance is significantly better

• Decided to reproduce 0/ skim with GHIT matching

All candidates

Truth-matched 0s

2 matching Ghit matching


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0 mass fits – Log scale

• The better truth-matching required a better fit function

• Fit function:– signal: double

gaussian with power-law low energy tail

– background: polynomial (of degree 1, 2 or 3)

• Use MC and high-statistics untagged sample to help determine some parameters

• Fit yields now within 3% of true value


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mass fits – Log scale

• Fit function:– signal: gaussian

with low and high energy power law tails

– background: polynomial (of degree 1, 2 or 3)

• Fewer energy bins due to low statistics

• Again fit yields within 2-3% of true value


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Yields from Low Statistics

• Sometimes it’s necessary to obtain a 0 yield from very low statistics. In this case, we estimate yield by simple counting in the signal region, with sideband subtraction.

• We are currently validating this technique. There seems to be a slight bias, which we are investigating.

Estimate - Truth

(Estimate – Truth)/error


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Next Steps

• Produce new BB background correction factors for 0 and as soon as the last skim jobs are complete

• Correct BB MC and perform checks on unblinded control regions:– 1.5 < E* < 1.9 GeV of standard sample (lower side-

band)– 1.0 < E* < 2.5 GeV of 0/ sample

• Extract photon energy spectrum• Unblind and move towards publication


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Conclusions

• The inclusive bs analysis is moving forward, although at a slower pace than was hoped

• An inclusive measurement of 0 and yields in BB events to a precision of a few percent has proved more difficult than originally envisioned, but we feel things are under control

• We have improved continuum background suppresion with a Fisher Discriminant and studyied the associated systematic effects

• We are converging towards a new result and hopefully it will be available soon.

Fully Inclusive b->s Status Report UCSC: Eisner, Schmitz, Schumm Edinburgh: Tinslay Pisa: Bucci,...

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