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

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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,
Section 2: W Mass 57 and we correct for the eect that this has on our model ts before we apply the model to the W Monte Carlo. Since the Z data sample is signicantly smaller than the W sample, we do not necessarily expect a perfect t to the W data. Nevertheless, we are able to perturb the model to describe both the W data and the Z data simultaneously. The uncertainty related to the modeling is 15 MeV on the W mass. This includes the uncertainty on the P T distribution for the W's. The backgrounds have also been estimated. The largest background is W ! ! e, and there is also a background contribution from QCD events where one jet is apparently electromagnetic, and the other is lost. Both the background and the QCD background occur at low M T { only 0:8% and 0:5% of the events in the high M T tting region are from these backgrounds, respectively. There is also a 0:1% background from Z events, where one of the electrons passes through a calorimeter crack. All these backgrounds are included in the ts, and they are small enough to have negligible eects on the results. The mass of the Z bosons both in the electron and muon channels are shown in Figure 30. The analysis is complete in all respects, and Gordon successfully defended his thesis in November 1998. The electron and muon results were presented at conferences this winter and will be submitted to Physical Review this summer. The nal uncertainties are listed in Table 6. The nal results for the W mass are in the electron channel, Mw = 80.473 0.113 GeV, in the muon channel, Mw = 80.465 0.143, and the combined measurement, muons plus electrons, Mw = 80.470 0.089.
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1 2 The CDF Experiment at Fermilab
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Section 2: CDF 3 of Run I data. Rec
Page 5 and 6: Section 2: CDF Hardware Projects 5
Page 7 and 8: Section 2: Central Outer Tracker 7
Page 21 and 22: Section 2: DAQ for the Silicon Vert
Page 27 and 28: Section 2: Central Muon Extension 2
Page 33: Section 2: Central Muon Extension 3
Page 36 and 37: 36 Section 2: CDF 2.3 Run I Physics
Page 38 and 39: 38 Section 2: CDF Physics Analysis
Page 93 and 94: Section 2: B Mixing and CP Violatio
Page 95 and 96: Section 2: B Mixing and CP Violatio
Page 97 and 98: Section 2: CDF 97 multijet trigger
Page 99 and 100: Section 2: CDF 99 The Jet Pseudora
Page 101 and 102: Section 2: CDF 101 while the proble
Page 103 and 104: Section 2: CDF 103 Talks at Confere
Page 105 and 106: Section 2: CDF 105 References [1] T
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