Measurement of the Z boson cross-section in - Harvard University ...

Chapter 8: Properties of Z bosons and measurement of Z → µµ cross-section 234
8.2 Cross-section measurement
From the analysis described in this thesis, using 331 nb −1 of data, we expect
120.1 ± 15.3 events and observe 109. The resulting total and fiducial Z/γ ∗ → µ + µ −
cross-sections are extracted using Eq. 6.2, and are reported in table 8.4 [95].
σZ/γ ∗ × BR(Z/γ∗ → µ + µ − ) 0.87 ± 0.08 (stat) ± 0.05 (sys) ± 0.10 (lum) nb.
σ fid
Z/γ ∗ × BR(Z/γ ∗ → µ + µ − ) 0.43 ± 0.04 (stat) ± 0.02 (sys) ± 0.05 (lum) nb.
Table 8.4: Measured values of the total and fiducial Z → µµ cross-section, including
statistical, systematic and luminosity errors.
We see that, even with the low statistics used in this measurement, the uncertain-
ties in the cross-section values are dominated by systematics. The 11% uncertainty
from the luminosity measurement comes primarily from the uncertainty in the beam
current measurement 3. With the latest Van der Meer calibration (as of October
14, 2010), the luminosity uncertainty is expected to decrease to 5-10% [69]. The ex-
perimental uncertainties will improve as we collect more data and use the tag-probe
approach to measure the various efficiencies.