# Presentation: 746558

## Measurement of 		σ(Z+b)/σ(Z+j)

*Marc Buehler*
*University of Illinois at Chicago*
*for the DØ Collaboration*

## Tevatron Collider in Run II

- Colliding protons and antiprotons at              TeV
- 36 x 36 proton and antiproton bunches with 396 ns bunch crossing time
- Current peak instantaneous luminosity  1 x 1032cm-2s-1

## The DØ Detector

- New Tracking: Silicon and Fibers in 2T Solenoidal Magnetic Field
- New Central and Forward Preshower Detectors
- Liquid –Argon/Uranium Calorimeter
- Upgraded Muon System in 1.8T Toroidal Magnetic Field
- Upgraded DAQ & Trigger Systems: 50-60 Hz rate to tape

## DØ  Data Taking

- Delivered = 590 pb-1

- On tape = 470 pb-1

- Run I = 130pb-1 recorded
- Results presented here will be based on  180 pb-1 of data

## Z + Heavy Flavor Production

- Z + heavy flavor is background to Z+Higgs
- Z + single b-quark
- Probe of b-quark PDF
- b-quark PDF important for SUSY Higgs production (gbbh) and single top process (qgtq’)

- This analysis measures the ratio of Z+b and Z+j inclusive cross sections combining the dielectron and dimuon decay channels of the Z (many systematics cancel !)

## Analysis: Event Selection

- Dimuon Channel:
- Luminosity  180pb-1
- Reconstruct Z+- events
- 2 loose & isolated muons
- pT > 15 GeV and |η|<2.0
- Track Match & Opposite sign
- 65.0 < M < 115.0 GeV

- Dielectron Channel:
- Luminosity  189pb-1
- Reconstruct Ze+e- events
- 2 isolated EM clusters
- pT > 15 GeV and |η|<2.5
- At least one track matched
- 80.0 < Mee < 100.0 GeV

- Ze+e-

- Z+-

- DØ Run II
- Preliminary

- DØ Run II
- Preliminary

## Jets

- Jet ET > 20 GeV, || < 2.5
- Run II cone algorithm: R=0.5
- Taggable jets (=containing tracks for b-tagging) are required
- Event count:
- Z +  1 calorimeter jet:
- 2219 Ze+e- and 1754 Z+-
- Z +  1 taggable jet:
- 1658 Ze+e- and 1406 Z+-
- Z +  1 b-tagged jet:
- 27 Ze+e- and 22 Z+-

- Looking for jets in Z events and applying b-tagging

- ET of taggable jets (dimuon & dielectron channels combined)

## b-Tagging

- Dilepton Mass of b-tagged events

- Asymmetry in decay length significance is
- indicating production of heavy flavor:

- Applying Secondary Vertex b-tag:

- Ze+e-

- Z+-

- DØ Run II
- Preliminary

- DØ Run II
- Preliminary

## Disentangling light, c, b contributions

- After background is subtracted, two equations, one before and one after b-tagging is applied, allow one to determine the contributions from different flavors in the remaining events:
- N = # of events with b,c and light jets
-  = tagging efficiencies for different jet types
- t = taggabilities for different jet types
- Use light and b-tagging efficiency from data
- c-tagging efficiency from MC and scaled for data/MC difference in b-tagging
- NC = 1.69NB from theory (MCFM NLO calculation)

## Results

- Theory predicts 0.018*
- Large part of systematic error from tagging efficiency and background estimation
- Cross checks with different b-tagging algorithms
- Soft lepton tagger
- Impact parameter tagger

- ***J. M. Campbell, R. K. Ellis, F.Maltoni and S.Willenbrock, *****Phys. Rev. D69 (2004) 074021***

## Systematics

- b/c tagging efficiency (13.4%):
- Largest source of systematic uncertainty
- Jet Energy Scale (6.7%):
- Jet energies are fluctuated up and down by 1 standard deviation
- Background estimation (6.2%):
- Based on different background subtraction methods
- Mistag rate (3.3%)
- Taggability (2.8%):
- A difference observed in tb/tL using different MC samples is regarded as an uncertainty
- σ(Z+c)/σ(Z+b) (2.5%):
- Uncertainty from theory
- Muonic vs Hadronic jets (1.8%):
- Analysis does not differentiate between muonic and hadronic jets
- Tagging efficiencies were derived from muonic jets

## Summary and Outlook

- A first preliminary Run II measurement of the inclusive cross section ratio of σ(Z+b) to σ(Z+j) was performed and the ratio was found to be 0.023 ± 0.004(stat) ± 0.002(sys)
- Systematic errors are dominated by b-tagging efficiency estimation
- Stay tuned for more results at the winter conferences: cross section measurement of Z+b and Z+bb