Unformatted text preview: the isospin I3 , and the sine of the weak
mixing angle sin2 W . The differential cross section as a
function of the direction of the fermion resulting from the
Ã decay is given by 10
50 100 150 200 mT (GeV/c2) FIG. 6. The mT distributions used for the ÀW measurement.
(a) The CDF W ! channel. (b) The CDF W ! e channel.
(c) The D0 W ! e channel. d
¼ að1 þ cos2 Þ þ b cos
d cos (5) in which is the angle of the fermion from the Z=
measured relative to the incoming quark direction in the Z=
Ã TABLE IV. The ÀW measurements. For the result, the ﬁrst uncertainty is the statistical uncertainty
and the second is the systematic uncertainty.
Channel Yield Fit range (GeV=c2 ) ÀW (MeV) 2 =dof CDF W !
CDF W ! e
D0 W ! e 2619
3436 90 < mT < 200
90 < mT < 200 17=21
19=21 5272 100 < mT < 200 1948 Æ 67 Æ 71
2118 Æ 60 Æ 79
2032 Æ 45 Æ 57
2028 Æ 39 Æ 61 Rev. Mod. Phys., Vol. 84, No. 4, October–December 2012 75:2=75 Hobbs, Neubauer, and Willenbrock: Tests of the standard electroweak model at . . . 0.4
0 Z/ * + e e - MC band includes
Total -0.6 þ À À
þ þ À 40 60 100 2
10 200 300 600 2 M ee (GeV/c ) in which þ and À are the integrated cross sections for the
cases cos > 0 and cos < 0, respectively. The asymmetry
extracted experimentally is given by
AFB ¼ AFB corrected assuming SM
and using Z-quark couplings fits 0.6 Nþ À NÀ
Nþ þ NÀ FIG. 8 (color online). Dielectron forward-backward asymmetry as
a function of dielectron mass (CDF). (6) DØ 1.1 fb-1
= 10.6/14 χ2/d.o.f. in which Nþ and NÀ are the acceptance, efﬁciency,
and background corrected fermion yields in the forward
ðcos > 0Þ and backward ðcos < 0Þ directions, respectively.
Measuring the asymmetry rather than differential cross sections allows cancellation of many systematic uncertainties,
particularly those affecting the overall normalization.
Measurements of the asymmetry as a function of dielectron
mass have been made by both the CDF (Acosta et al., 2005a)
and D0 (Abazov et al., 2008a) using the dielectron ﬁnal state.
The CDF result uses a sample with Ldt ¼ 72 pbÀ1 , and the
D0 measurement uses Ldt ¼ 1:1 fbÀ1 . The selection criteria are similar to those for the Z cross section measurements
although a larger dielectron mass range was selected for the
asymmetry measurements. Two experimental issues of particular importance to these measurements are (1) controlling
asymmetries in either detector acceptance or selection efﬁciency as a function of dielectron mass and (2) limiting the
impact of electron charge misidentiﬁcation.
In Table V, the dielectron mass range and the predicted and
measured values of AFB for each mass bin from the D0
measurements are shown, and Figs. 8 and 9 show the 0.5 AFB A 1
0.8 AFB rest frame. The relative Z and Ã contributions to the cross
section vary as a function of the Z=
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