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

Section 2: W Mass 55
problem we are seeing may also have been present in Run Ia as the Z mass obtained
from these data is consistent with both the CERN result of 91:187 GeV as well as a
number 410 MeV lower.
A better momentum measurement is obtained by including a beam constraint in the
track it. This constraint tends to bias electrons which have radiated towards higher
P T , and if we are not simulating the beam constraint correctly, we may fail to correct
for this bias. From the impact parameter resolution, the width of the E/p distribution,
and correlation between E/p and impact parameter, we can see that the Monte Carlo
covariance matrix does not perfectly describe the data. We have tried perturbing
the covariance matrix, including tting to the data directly, but we have seen only
negligible changes in the E/p peak position.
Radiative decays W ! e modify the E/p distribution. If they are not simulated
correctly, asystematic error could result. We nd that internal Bremsstrahlung shifts
the E/p peak by :25%. If our generator is wrong by 100%, then this might
account for some of the eect we are seeing. Our default generator is PHOTOS,
but we have also tried using a Berends and Kleiss calculation [14], as well as a more
recent calculation by Baur, Keller, and Wackeroth [15]. We have also implemented
an algorithm by Laporta and Odorico [16], and all these calculations give the same
results. All these calculations, of course, are based on the same physics, nonetheless
the similarity of the results makes it unlikely that there is a bug in the generator code,
or our implementation of it.
Recently CDF has re-calculated the CTC calibration and alignment. While this improves
the P T resolution by 10%, the E/p peak position does not change.
The ALEPH and KTeV Collaborations have observed systematic dierences in tracking
electrons as compared to muons and pions. This is thought to come from the eects of
keV level photons emitted by the electron as it passes through the gas. The sensitivity
to these photons depends greatly on tracking algorithms. We modeled these eects
in detail and showed that they would be important only if they were ten or twenty
times larger. We also compared exhaustively the properties of the electron and muon
tracks from W and Z decays and found no important dierence. Finally, we compared
the masses for J= and obtained from electron tracks and, taking into account the
relatively large radiative corrections for electrons, observed good agreement with the