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

Chapter 1: Introduction and Theoretical Overview 30
controls small-x gluon distribution, decreased by 35% as a result.
W charge asymmetry: The quantity AW = W + −W −
W + +W − depends on the PDFs
of valence u and d quarks at x ≈ 5 × 10 −3 . This is because, at leading order, the
expression reduces to:
AW =
uv − dv
uv + dv +ū + ¯ d
(1.7)
Because of the missing neutrino, ATLAS cannot directly measure the W charge
asymmetry, but rather the decay lepton charge asymmetry. Nevertheless, the sensi-
tivity to valence u and d quark distributions should remain 11 , and the measurement
will give information about the distributions in a region of x of which there is no cur-
rent knowledge. Indeed, a first measurement of this asymmetry is already underway
using the 2010 data.
Before we can measure the physics-induced lepton charge asymmetry, we must
ensure that we know detector-induced charge asymmetries very well. One way to
account for the latter asymmetry is to use data from runs in which the solenoid
and toroid polarities were alternately reversed. In this case, any charge bias from
geometrical acceptance and alignment errors should cancel out. D0, for example,
regularly reverses their magnet polarities, so that the magnets are in each polarity
about half of the time [100]. ATLAS decided against using this technique in the 2010
run.
Z + b-jet production: To measure the Z production cross-section to ≈1%
precision, the bottom quark PDF must be known to better than ≈20% [103]. The
11 The lepton charge asymmetry is a convolution of the W charge asymmetry and the V-A asymmetry
from the W decay. Assuming that W decay occurs via a pure V-A interaction, the lepton
charge asymmetry remains sensitive to PDFs.