Update on the Inclusive Measurement of the b s Transition Rate Using a Lepton Tag Philip Bechtle...
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Transcript of Update on the Inclusive Measurement of the b s Transition Rate Using a Lepton Tag Philip Bechtle...
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