Measurements of the W boson mass and trilinear gauge boson couplings at the Tevatron Page: 4 of 9
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1 Measurement of the W Boson Mass
The mass of the W boson is a fundamental parameter of the standard model and is related
to the Fermi constant GF, the electromagnetic coupling constant aEM, the Z boson mass
mz, and Ar, which represents the effects of radiative corrections. GF, aEM, and mz are
all measured1 with high precision. In the standard model Ar depends on the top quark
and Higgs boson masses and in theories beyond the standard model it depends on the
particle spectrum of the new theory. Therefore, together with a measurement of the top
mass, a precise measurement of the W boson mass can be used to constrain the Higgs
mass in the standard model and to constrain theories beyond the standard model.
The recent measurement published by DO2 and the preliminary measurement from
CDF 3, both from Run Ib data (1994-95), are briefly described here. The measurements
are made using a fit to the observed transverse mass spectrum mT = 2p$T(1 - cosAO)
in W -* & events. The transverse mass spectrum is modeled using a Monte Carlo event
generator which incorporates a W boson production model and a detailed model of the
detector response, which is calibrated using collider data.
Calibration of the muon momentum scale is achieved in CDF by comparing the re-
constructed J/0 - + - mass (Fig. 1) to the world average. The error in the W boson
mass due to the momentum scale uncertainty is 6mw = 40 MeV/c2, while the momentum
resolution contributes 6mw = 25 MeV/c2.
In DO the electromagnetic calorimeter energy scale is determined from test beam
measurements and collider data. The observed energy EosS is parametrized as Eobs -
6 + aEtrue, and the constants 6 and a are determined from 7r - yy, J/0 -+ ee, and
Z -* ee events as shown in Fig. 2. The resulting values are a = 0.9533 + 0.0008, and
6 = (0.16+1) GeV, where the errors include the systematic uncertainty due to underlying
event corrections and non-linearity of the response at low ET. The contribution of the
energy scale uncertainty to the W boson mass error is 6mw = 70 MeV/c2. The energy
resolution contributes 6mw = 25 MeV/c2.
In both CDF and DO the response of the detector to the recoil system, (hadrons
recoiling against the W boson, interactions of the proton and antiproton spectator quarks,
and energy from multiple interactions), is calibrated using the transverse energy balance
in Z -* ee decays. The method employed by DO is illustrated in Fig. 3. The recoil
response R is defined by
IuT-4TI = R~qT
where UT is the transverse momentum of the recoil system, qT = grgT is the transverse
momentum of the Z boson. The LHS of this equation is the projection of the recoil
system transverse momentum along the Z boson transverse momentum vector, and for
an ideal detector R = 1. A detailed GEANT-based Monte Carlo simulation shows that
the response can be parametrized using two constants a and 3 (see Fig. 3), which are
determined using Z -+ ee data, yielding a = 0.693 + 0.060, and 3 = 0.040 + 0.021. The
resulting contribution to the W boson mass error is 6mw = 20 MeV/c2. The contribution
from the recoil resolution is 6mw = 25 MeV/c2.
Fits to the transverse momentum distributions are shown in Fig. 4 and Fig. 5. The
CDF data yield the result mw = 80.43 + 0.10 (stat) + 0.12 (syst) GeV/c2, and the DO
result is mw = 80.44 + 0.10 (stat) + 0.07 (syst) GeV/c2. Table 1 itemizes the sources of
uncertainty in the measurements.
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Ellison, John. Measurements of the W boson mass and trilinear gauge boson couplings at the Tevatron, article, June 1, 1998; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc627770/m1/4/: accessed April 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.