2 The CDF Experiment at Fermilab Contents - Harvard University ...

50 Section 2: CDF Physics Analysis
GeV. This requirement is 95% ecient for tt events while removing most of the soft lepton
tags arising from Drell-Yan, Z or upsilon production which are quite isolated. Secondly, we
extended the Method II background calculation to the SLT tagged sample. In order to do
this correctly, we subtracted from the raw fake tag rate the fraction ( 10%) associated with
real heavy avor jets.
The third technique is called the \jet probability" tag. This algorithm uses the impact
parameter of a track contained in a jet to determine the probability the track originates from
the primary vertex, given the SVX detector resolution. The combination of the probabilities
of all tracks in a jet gives an overall probability for the jet, i.e., the jet probability or \JPB."
For heavy avor jets, this probability is expected to be small.
Our requirement on JPB is loose in the sense that the eciency for b-jets is about the
same as SECVTX, while about twice the number of c-jets is accepted. Since the overlap
of JPB and SECVTX is large, this new algorithm does not bring much new information in
terms of b-quark tagging. However, the loose JPB tags are more sensitive than SECVTX to
backgrounds from mistags and c-quark jets, a fact which we have exploited to study these
important backgrounds. From these studies we uncovered and corrected some inconsistencies
and mistakes in the standard Method II background calculations developed for SECVTX
tags. We also showed that the eciencies for SECVTX were underestimated by 25% relative
by studies of the low-P T inclusive electron sample. In contrast, the resolution functions
required by JPB are derived separately for data and Monte Carlo, leading to a relatively
reliable performance.
The problems with the SECVTX eciency led us to re-evaluate the rate of gluon splitting
to b b and cc pairs, important for estimating the amount of W + b b and W + cc background.
Before correcting SECVTX b-tagging eciency, we had found inconsistent results
from SECVTX and JPB. Applying the correction, however, the two algorithms gave identical
results. This lends credence to the SECVTX eciency correction we derived.
We developed a new technique for explaining and determining the amount of SECVTX
and JPB mistags using the characteristics of JPB. Our simple model successfully predicts
the rate and kinematical distributions of instrumental backgrounds with better than 10%
accuracy is superior to the method developed in the old analysis. It also allows for an
improved understanding of the heavy avor tagging and the mechanisms contributing to the
negative tagging rate.
Based on the new techniques and the extended background studies described above,