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

56 Section 2: CDF Physics Analysis
masses obtained with muon tracks. We concluded that for CDF electron and muon
tracks are essentially the same.
We have also checked a number of interesting possibilities, all of which turn out to
be insignicant. One is the Landau-Pomeranchuk-Migdal eect, which can suppress
soft Bremsstrahlung. SLAC has measured this eect for 25 GeV electrons, and the
amount of suppression shown by that experiment does not signicantly change the
E/p shape. We have also considered the possibility that the 400 of aligned silicon
crystals in the SVX may cause signicant energy loss for electrons traveling along an
axis of symmetry. The E/p distribution for electrons which do not pass through the
SVX looks the same as those which do, and we conclude that this is not a signicant
eect. The electron is being accelerated in a circle by the magnetic eld, and we are not
simulating the resulting synchrotron radiation. This is predicted to be only a few MeV
however, and can be safely neglected.
We will use the Z mass to set the energy scale, as this avoids nearly all systematic errors
related to tracking. Nevertheless, the contradiction with E/p remains and is interesting in
itself.
We have also completed a model of the neutrino measurement. The missing E T measurement
is dominated by the electron E T , but it also contains signicant contributions from
energy unassociated with the event, and also from the low energy particles which balance the
W transverse momentum. We write U ~ for the vector sum of all the transverse energy in the
event, excluding the electron energy E T . Instead of trying to model ~U from rst principles,
we produce an empirical model which is t from Z data.
The model treats U 1 and U 2 as independent, Gaussian random variables, where U 1 is ~U
projected parallel to the boson, and U 2 perpendicular. The mean of U 2 is assumed to be
zero, while the mean of U 1 varies with the boson P T . The variation of the mean of U 1 with
boson P T is t from the Z data. The widths of both variables are determined from the total
energy in the event, and are also allowed to vary slightly with the boson P T . Figure 29 shows
the U 1 and U 2 distributions of the data. For the gure, we have subtracted the predicted
value for the mean of U 1 and divided by our predicted resolutions. The evidence that our
model is reasonable is that both distributions correspond well to Gaussians centered on zero
with unit widths.
The P T distribution of Z's in the data is only known within the measurement resolution,