science.org SCIENCEBy Claudio Campagnari^1 and Martijn Mulders^2O
ver the past 60 years, the standard
model (SM) has established itself as
the most successful theory of matter
and fundamental interactions—to
date. The 2012 discovery of the Higgs
boson only added to the streak of tri-
umphs for the theory ( 1 , 2 ). However, the
SM is known to be incomplete and has no-
ticeable shortcomings, such as its inability
to account for dark matter in the universe
or to include gravity in a consistent fash-
ion. Physicists have looked for phenomena
that directly challenge the SM in the hope
of finding hints on what a more complete
theory may look like. Although no “new”
particle has yet been found, a few fissures
have recently been exposed in the SM by
precise measurements that are at odds with
the model’s predictions ( 3 , 4 ). On page 170
of this issue, the Collider Detector at
Fermilab (CDF) Collaboration ( 5 ) adds fur-
ther intrigue with its measurement of the
W boson mass.
The W boson, whose existence and de-
tailed properties were first predicted in the
1960s and confirmed at CERN in 1983, is a
key building block of the SM. It is a particle
that is associated with the weak force, which
is responsible for radioactive nuclear β de-
cay, and that plays a similar role as that of
the photon in the electromagnetic interac-
tion. Although the photon is massless, the
W boson is massive; it is about 80 times
the mass of a hydrogen nucleus. Within the
theoretical framework of the SM, the W bo-
son mass is a parameter, with a value that
is bounded by other observables such as the
electron charge and the masses of other par-
ticles, including the top quark and the Higgs
boson. A very accurate measurement of the
W boson mass can therefore provide a strin-
gent test of the self-consistency of the SM.
Over the past 30 years, there have been
ever more precise measurements of the W
boson mass, and the CDF Collaboration now
adds to these reports. Based on 10 years of
data recorded at the CDF, they report a W
boson mass with an impressive precision of
117 parts per million (ppm)—twice as pre-cise as the previous most accurate measure-
ment. Their measured W boson mass is in
direct contention with the SM because it
is heavier than the SM prediction by seven
standard deviations. This could be a signa-
ture for new interactions or new particles
that are either too massive to be produced
or too hard to detect at existing accelera-
tors. Nonetheless, such yet-to-be-known
particles and physical interactions could
alter the relationships between the various
observables through hidden interactions
with the W boson and cause the observed
deviation from SM predictions.
Effects on the W boson mass from pre-
viously undetected particles have been ob-
served before. Notably, the observations of
these effects were used to probe the masses
of the top quark and the Higgs boson before
their direct detection. After the observationand precise measurement of each discov-
ered particle, the web of SM predictions
was weaved with greater strength and ac-
curacy. With more and more precise mea-
surements of physical quantities—such as
cross sections, decay rates, and masses of
fundamental particles—fissures between
SM predictions and reality may have begun
to show. When not in agreement with the
theoretical predictions, such measurements
can provide a first glimpse of physics be-
yond the SM.
Because extraordinary claims require ex-
traordinary evidence, the claim by the CDF
Collaboration will require additional experi-
ments to provide an independent confirma-
tion. Scientists at the Large Hadron Collider
(LHC) have already collected samples of W
bosons that are larger than those available
at Fermilab and, in principle, could achieve
better precision. The Tevatron experiment
at Fermilab—DZero—may also get back in
the W boson mass-measuring race. The re-
sult from the CDF Collaboration provides
an impetus to improve the measurements
of other SM parameters that can help to
test and constrain the theory, such as thetop quark mass, the strong coupling con-
stant, and the Weinberg angle, named after
the late Steven Weinberg ( 6 , 7 ), a founding
father of the electroweak model that is cur-
rently being challenged.
The High-Luminosity LHC—an up-
graded version of the LHC at CERN that
will come online later in this decade—will
provide higher beam energy and collision
rates with updated and more powerful de-
tectors. The upgraded collider will offer
ample opportunity for more precise mea-
surements and for direct searches for new
particles. Particle physicists are also look-
ing forward to the next generation of ac-
celerators. Electron-positron colliders are
particularly well suited for carrying out
precision measurements. Several propos-
als for electron-positron colliders—such as
the International Linear Collider in Japan,
the Compact Linear Collider, the Future
Circular Collider (FCC-ee) at CERN, and the
Circular Electron Positron Collider in China
( 8 )—are under consideration in the ongoing
discussions for the future of particle phys-
ics. Among them, the FCC-ee would offer
the best prospects for an improved W boson
mass measurement, with a projected sensi-
tivity of 7 ppm ( 9 ), more than 10 times bet-
ter than the current best measurement.
Among possible theories that could ex-
plain the discrepancy with the SM predic-
tion is the theory of supersymmetry (SUSY),
which is an old favorite of particle physicists
because it provides a plausible explanation
for some of the SM’s unexplained properties
and forms a natural connection to deeper
level descriptions of the universe such as
string theory. However, none of the many
exotic particles predicted by SUSY have
been observed, despite extensive searches
at particle detectors around the world. The
surprisingly high value of the W boson mass
reported by the CDF Collaboration directly
challenges a fundamental element at the
heart of the SM, where both experimental
observables and theoretical predictions
were thought to have been firmly estab-
lished and well understood. The finding of
the CDF Collaboration offers an exciting
new perspective on the present understand-
ing of the most basic structures of matter
and forces in the universe. jREFERENCES AND NOTES
1. G. A a d et al., Phys. Lett. B 716 , 1 (2012).- S. Chatrchyan et al., Phys. Lett. B 716 , 30 (2012).
- B. Abi et al., Phys. Rev. Lett. 126 , 141801 (2021).
- R. Aaji et al., arxiv:2103.11769 [hep-ex] (2021).
- CDF Collaboration, Science 376 , 170 (2022).
- F. Wilczek, Nature 596 , 183 (2021).
7. J. Preskill, Science 373 , 1092 (2021). - J. de Blas et al., J. High Energy Phys. 2020 , 139 (2020).
- M. Benedikt, A. Blondel, P. Janot, M. Mangano, F.
Zimmermann, Nat. Phys. 16 , 402 (2020).
10.1126/science.abm0101PARTICLE PHYSICSAn upset to the standard model
Latest measurement of the W boson digs at the most
important theory in particle physics
INSIGHTS | PERSPECTIVES(^1) Department of Physics, University of California, Santa
Barbara, Santa Barbara, CA, USA.^2 CERN, Geneva,
Switzerland. Email: [email protected];
[email protected]
“This could be a
signature for new interactions
or new particles...”
136 8 APRIL 2022 • VOL 376 ISSUE 6589