Measurement of the b-jet Cross section in events with a Z Boson

Measurement of the b-jet Cross section in events with a Z Boson

We describe a new preliminary measurement of the b-jet cross section in events with a Z boson by the CDF collaboration using an integrated luminosity of 1.5 fb^-1. The jets must have a transverse energy E_T > 20 GeV and pseudorapidity |eta| < 1.5. Jets are measured with a cone algorithm with cone size 0.7. Events are selected for the decay channels Z -> ee and Z -> mumu. Jets containing a $b$ hadron are indentified by reconstructing a secondary vertex. The data are unfolded to the hadron level and compared to predictions from PYTHIA and NLO QCD. This is an update of a previously published measurement with 5 times more data.

Contacts: Beate Heinemann and Andrew Mehta
Data: Run II, 1.5 fb^-1
Documentation: previous publication

Results:

The measurement is the b-jet cross section and is defined to be proportional to the number of b-jets. The result is

sigma(Z+b-jet)=0.94 +- 0.15 (stat.) +- 0.15 (syst.) pb
sigma(Z+b-jet)/sigma(Z)=0.369 +-0.057 (stat.) +- 0.055 (syst.) %
sigma(Z+b-jet)/sigma(Z+jet)=2.35 +- 0.36 (stat.) +- 0.45 (syst.) %

The NLO QCD predictions are:

sigma(Z+b-jet)=0.51 pb x 1.09 = 0.56 pb
sigma(Z+b-jet)/sigma(Z)=0.21 % x 1.09 = 0.23%
sigma(Z+b-jet)/sigma(Z+jet)=1.88% x 0.94 = 1.77%

The correction factors quoted are to account for the corrections due to the underlying event and the hadronisation. The theoretical calculation was made using MCFM, CTEQ6M for the parton density unctions and M_Z+p_T² for the factorisation and renormalisation scales. The uncertainty on this prediction due to higher orders and parton distribution functions is about 15%.
PYTHIA predicts 0.35% for sigma(Z+b-jet)/sigma(Z) and 2.18% for sigma(Z+b-jet)/sigma(Z+jet)

Method:

The measurement is made by counting the number of jets with a secondary vertex and evaluating the fraction of b-jets among those by fitting the invariant mass of tracks at this secondary vertex. Only jets with p_T > 20 GeV and |eta| < 1.5 are considered. Pythia is used for unfolding this measurement to the hadron level. The details are described in the previous publication.

Plots and Tables:


Dilepton invariant mass for events with a tagged b-jet:
	EPS or GIF
Invariant mass of the tracks at the secondary vertex:
	EPS or GIF
Pseudorapidity of the tagged b-jet:
	EPS or GIF
Transverse Momentum of the tagged b-jet:
	EPS or GIF
Here is the Table of systematic uncertainties