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Update on the Inclusive Measurement of the b  s Transition Rate Using a Lepton Tag

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

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Update on the Inclusive Measurement of the b  s Transition Rate Using a Lepton Tag

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  1. 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 Schumm SCIPP 6/07 BaBar Coll. Meeting

  2. Direct searches (LEP) 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 SUSY Extra Dimensions B  s constraints MSSM Constraints

  3. Current Status of b  s  Measurements 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. Phys.Rev.Lett.97:171803,2006 BaBar Sum of Exclusive Modes Run1-2 Babar Fully Inclusive 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.

  4. 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

  5. Sig. Region B/Bbar background control region BB Cont. Signal After Selection Cuts • 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… What are the sources of B/Bbar background?

  6. 82% of B/Bbar background B/BBar Background Sources (XXX Monte Carlo) Electron categories x2 larger than that of prior simulation (was 3.7% combined). This raises questions, in-cluding the modeling of brehmsstrahlung

  7. Constraining the 0  Background with a Measurement of Inclusive Production • Measure p0/h yields in on- and off-peak data and MC • Determine correction factors in bins of E(p0): Correction = [(On-peak data) – s*(off-peak data)]/[BB MC] • Also need to know recon. eff. of background p0s gg invariant mass MC Correction Factors Fits done to both data and MC

  8. 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.

  9. Require 2nd photon to be above minimum energy cut Require 2nd photon to be in fiducial volume -.74 < coslab < .94 1 2 3 E* coslab Of remaining events, almost all make a good 0 candidate with the HE  Require 2nd photon to have a truth match E* E* 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 • 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)

  10. For low-energy photons that are not truth-matched… “Merged” 0s (photons form single cluster) Distance (m) between reconstructed HE  and nearest cluster Distance (m) between truth-matched HE  and true low-energy  sibling

  11. 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.

  12. E2,lab M E* More clever rejection of 0 backgrounds? ( analysis used likelihood based on  mass and E2,lab)  try NN rejection Run I-II analysis performance Signal Efficiency Using E* information Variables considered: M E* E2,lab coslab HE  2nd moment HE  isolation HE  Lat. Moment LE  2nd moment LE  isolation LE  Lat. Moment Signal Efficiency Ignoring E* information Most power in M, E2,lab (already in use) and E* (dangerous). Will not pursue. Background Efficiency

  13. % of total Error Statistitical Model Systematic Neural Net Selection: A Word About Run I-II Syst. Errors Run I-II Result (Phys.Rev.Lett.97:171803,2006 ) Br (BXsg) = (3.67  0.29  0.34  0.29) x 10-4 Different b  s models (b mass, Fermi motion) E* [GeV] Selection efficiency vs. E* for Run I-II selection Important: Run I-V optimi-zation must consider both statistical and systematic error! E* [GeV]

  14. Event-Shape NN Selection Four neural net algorithms under consideration: • 3 variants using Energy Cones • 1 uses Legendre Moments • Choose based on best uncertainty (including dominant systematics) Eff vs. Eg Econes I • better statistical precision • larger model error Legendre Moments • more stats in p0/h control sample • reduced model error

  15. Expected Partial Branching Fraction Errors (Only uncertainties dominant in Run I-II analysis included) Differences are relatively small  choose Legendre NN for its small syst. and model errors

  16. Other Backgrounds: Antineutrons Nominally 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) Data MC ECAL Lateral Moment

  17. Other Backgrounds:  and ’ BAD 163 : nominally 2.1% of B/Bbar background; d/dp* measured; use to correct rates in MC (correction factor “”) BAD 179 + private updates /: nominally 0.8% of B/Bbar background; less well-constrained, but less of a contribution.

  18. Other Backgrounds: B   X 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?

  19. b  s Outlook I An admirable goal would be Lepton/Photon – what kind of shape are we in? • 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

  20. b  s Outlook II • However, some new considerations have arisen • Brehmsstrahlung and conversions (material effects) • Non-DST level study of conversion, brehm properties • New 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…

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