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

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54 Section 2: CDF Physics Analysis 2.3.3 The W Mass Measurement (Prof. Franklin & Dr. Gordon) The W mass is measured at CDF using both electron and muon decay channels. The Harvard group worked on the electron decay mode. W ! e decays are characterized by a high E T electromagnetic cluster and large 6E T , where the EM cluster has a high P T track pointing at it. Here E T refers to the calorimeter measurement and P T to the measurement from the central tracking chamber (CTC). The W mass is extracted from a t to the transverse mass distribution. Using the data (80 pb ,1 ) from Run IB, a statistical error of 70 MeV is obtained, which is half the error of the previous measurement. An equally large source of uncertainty arises from the determination of calorimeter energy scale. Z decays can be used to set the energy scale with an uncertainty of 0:1%. We can also tie the calorimeter scale to the CTC scale with the peak of the E/p distribution, where E is the calorimeter measurement, and p is the CTC measurement of the electron. Attothe E/p distribution is shown in Figure 28. Bremsstrahlung causes a shift in the peak and also creates a large high-end tail. The statistical error associated on the peak position is 0:04%, and there is an additional uncertainty from the amount of material inducing Bremsstrahlung of 0:04%. The corresponding error on M W is also 0:04%. Unfortunately, the energy scale as determined from the E/p distribution and from the Z mass dier by 0:45%, and this is a 4 deviation. Simply stated, if we set the energy scale using the E/p distribution, then we measure avalue for the Z mass which is 410 MeV too low. A great deal of eort has been expended looking for the solution to this problem. Among other things, we have checked the following: The E/p distribution from Z decays agrees with the distribution from W decays, which indicates that the problem is not a simple non-linearity in the energy scale between W and Z energy scales. Possible non-linearities were further constrained by obtaining the E/p distribution from decays ! e + e , . Extreme non-linearities would be required to explain the E/p anomaly, and are completely ruled out from this distribution. We have applied our Monte Carlo to analyze the Run Ia data and have reproduced the Run Ia results well, including both the Z mass and E/p distribution. This indicates that we have not introduced a bug by various changes in the simulation. Whatever
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
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1 2 The CDF Experiment at Fermilab
Page 3 and 4: 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
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Section 2: CDF 105 References [1] T
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