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SCIENCE
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ET AL.
, PROG. THEOR. EXP. PHYS.
2020, 083C01 (2020 ; T. AALTONEN
ET AL.
, PHY. REV.
LETT.108, 151803 (2012 ; M. AABOUDET AL., EUR. PHYS. J.C78, 100 (2018 ; T. AALTONENET AL., SCIENCE, 376, 6589 (2022 SCIENCE science.org 8 APRIL 2022 • VOL 376 ISSUE 6589 125NEWS | IN DEPTHP
article physicists may have finally
poked a hole in their understanding
of the subatomic realm—which they
would relish. A new look at old data
suggests an ephemeral particle called
the W boson is heavier than predicted
by physicists’ “standard model” of particles
and forces. The discrepancy could hint at
particles not included in the 40-year-old the-
ory, says Doreen Wackeroth, a theorist at the
University at Buffalo who was not involved in
the work. “I’m very excited about the result!”
But the finding, reported today in
Science (p. 170), also clashes with
previous measurements, giving
some physicists pause. “All these
measurements claim to measure
the same quantity,” says Martin
Grünewald, an experimental phys-
icist at University College Dublin.
“Somebody must be, I will not say
wrong, but maybe made a mistake
or pushed the error evaluation
too aggressively.”
Vexingly successful, the stan-
dard model was completed in 2012,
when the world’s largest atom
smasher, the Large Hadron Collider
(LHC) at the European particle
physics laboratory CERN, discov-
ered its last missing piece, the long-
predicted Higgs boson. The the-
ory accounts for every particle
interaction seen so far, but it suf-
fers obvious deficiencies. It includes three
forces—electromagnetic, strong, and weak—
but leaves out gravity. It also contains no dark
matter, the invisible stuff that makes up 85%
of the universe’s matter.
Now that all the standard model par-
ticles are known, physicists can test the
theory’s internal consistency, because
each particle’s properties depend on those
of others. For example, the mass of the
W boson—which conveys the weak nu-
clear force just as the photon conveys the
electromagnetic force—depends on those of
the Higgs and a heavy but fleeting subatomic
particle called the top quark. So, from those
input measurements, physicists can predict
the W’s mass and look for a discrepancy with
the measured value.
The measurement is tricky. Created in a
high energy particle collision, a W quicklydecays into either an electron or its heavier
cousin, a particle called a muon, and an anti-
neutrino. The antineutrino cannot be de-
tected, so physicists must deduce its presence
by summing up the momenta and energies
of all the other particles spewing from each
collision and looking for events in which
something unseen seems to fly out the side of
the cylindrical detector. From the energy and
momentum of the decay particles, analyzed
statistically over many events, they can esti-
mate the W’s mass.
Now, one team says its reading conflicts
with the standard model prediction. The data
come from the Collider Detector at FermiNational Accelerator Laboratory (CDF), a
particle detector fed by the Tevatron collider,
which ran at Fermilab from 1984 until 2011.
After a decade of work, Ashutosh Kotwal, a
particle physicist at Duke University, and
his 397 CDF collaborators find the W boson
has a mass of 80,443.5 megaelectron volts—
86 times that of a proton. The measurement
differs from the predicted mass by seven
times the experimental uncertainty.
“What does it mean? That’s the next big
question,” Wackeroth says. Physicists have
spotted a couple of other small anomalies
that suggest the standard model may finally
be cracking, she says. For example, she notes
that the muon appears to be slightly more
magnetic than predicted (Science, 9 April
2021, p. 113).
However, earlier measurements of the
W’s mass generally agreed with the standardmodel (see chart, below). The new result
even contradicts the CDF’s previous result,
published in 2012, which was based on the
first quarter of the current data set, notes
Dmitri Denisov, a physicist at Brookhaven
National Laboratory who worked on D0, a
rival Tevatron detector. “That’s my first con-
cern,” he says.
But CDF researchers made several im-
provements in the analysis that account for
the difference, Kotwal says. “We are confi-
dent in the techniques we have used,” he
says. “It is a distinct possibility that there is
something new in nature that the standard
model does not capture.”
Physicists should soon get yet
another W boson mass measure-
ment. Scientists with the Compact
Muon Solenoid, a detector at the
LHC, hope to publish one early
next year, says Guillelmo Gomez-
Ceballos, a CMS physicist at the
Massachusetts Institute of Tech-
nology. He is also a CDF member,
and although he didn’t work on
the new study, he says, “I don’t
remember any analysis that has
been done with so much care.”
It may take years to reconcile
the measurements. But physi-
cists won’t be left rudderless in
the meantime. Since 1957, the
Particle Data Group (PDG) at
Lawrence Berkeley National Lab-
oratory (LBNL) has maintained
a compendium of particles and
arbitrated disputes over their measured
properties. The new W boson mass value
comes as the PDG is preparing its latest an-
nual update, says Michael Barnett, a retired
LBNL physicist who led the PDG from 1990
to 2015 and still works on it. “We’re going
to have to stop the presses, just like we did
when the Higgs was discovered,” he says.
For a parameter like the W boson’s mass,
the PDG averages the most current and re-
liable measurements. If they disagree far
beyond their uncertainties, the group ap-
plies a specific mathematical algorithm
that effectively widens the error bars to en-
compass the discordant individual results,
Barnett says. Ironically, even though the
CDF has now reported the single most
precise measurement of the W mass, the
official value will likely become even less
certain than before. jBy Adrian ChoPARTICLE PHYSICSNew estimate of W boson mass conflicts with prediction from “standard model”
Particle’s mass may tip the scales to new physics
80,300 80,350 80,400 80,450Standard model
prediction with
uncertainty ( )CDF 2012ATLAS 2014Official averageCollider Detector
at Fermilab (CDF)Mass (megaelectron volts)Weighty issue
A new measurement of the mass of a particle called the W boson disagrees
strongly with the theoretical prediction—and with previous measurements,
including one from the same group.