A new measurement of a fundamental particle called the W boson appears to
defy the standard model of particle physics, our current understanding of
how the basic building blocks of the universe interact. The result, which
was a decade in the making, will be heavily scrutinised, but if it holds
true, it could lead to entirely new theories of physics.
“It would be the biggest discovery since, well, since the start of the
standard model 60 years ago,” says Martijn Mulders at the CERN particle
physics laboratory near Geneva, Switzerland, who has written a commentary on
the result for the journal Science.
The standard model describes three distinct forces: electromagnetism, the
strong force and the weak force. Particles called bosons serve as mediators
for these forces between particles of matter. The weak force, which is
responsible for radioactive decay, uses the W boson as one of its
messengers.
The W boson is so central to the standard model that physicists have tried
to measure its mass with ever greater precision since it was first observed
in 1983. These measurements have all broadly agreed with each other, an
apparent confirmation of the standard model’s validity.
But we know that the standard model is wrong. It has no explanation for
gravity, dark matter and the absence of antimatter in our universe, so
physicists are constantly on the lookout for deviant measurements that could
lead to new theories.
Now, Ashutosh Kotwal at Duke University in North Carolina and his colleagues
have announced a new measurement for the W boson’s mass using data from the
Tevatron collider in Illinois, putting it at 80.4335 gigaelectronvolts.
The generally accepted mass for the W boson is 80.379 gigaelectronvolts, and
while the discrepancy may seem small, the new value is the most precise so
far, equivalent to measuring your body weight to within under 10 grams.
More importantly, its difference from the generally accepted value of the W
boson mass has a statistical significance of around 5 sigma, corresponding
to a probability of about 1 in 3.5 million that a pattern of data like this
would show up as a statistical fluke.
Physicists normally use 5 sigma as the level of significance to count
something as a “discovery”, but the difference between the new mass
measurement and that predicted by the standard model is even higher, at 7
sigma. This corresponds to around a 1 in 780 billion probability of seeing a
result like this by chance.
Kotwal and his team are aware that they are making an extraordinary claim
that could overturn physics as we know it, but he says they have done all
the tests they can think of to confirm it. A small amount of systematic
uncertainty – essentially potential errors within the experimental set-up –
remains, but now it is time for others to weigh in on the result, he says.
“We think the answer is holding up to our own scrutiny,” he says.
The team measured the boson’s mass by smashing beams of protons and
antiprotons together and analysing the particles produced in the collision.
The analysis was so complex that the result took more than a decade to
produce, after the Tevatron shut down in 2011, but its potential
implications are huge.
“If the W boson mass is deviating that much from the standard model
expectation, and if we understand all the [systematic] uncertainties, then
it’s a huge deal,” says Ulrik Egede at Monash University in Australia.
The “if” is the important point for many physicists who, while excited at
the result, are cautious about its divergence from previous measurements.
“We need first to understand the discrepancy between [this result] and all
other experiments before we think about explanations from physics beyond the
standard model,” says Matthias Schott at CERN, who worked on a previous W
boson measurement using data from the ATLAS experiment gathered at the Large
Hadron Collider (LHC) up to its shutdown in 2018.
Figuring out the source of the discrepancy is no easy task. W bosons quickly
decay into other particles, either an electron and an electron neutrino, or
a heavier muon and muon neutrino. Neutrinos are hard to detect, so Kotwal
and his team had to infer where they were from large amounts of data. “[W
boson masses] are recognised to be some of the experimentally most difficult
measurements to make,” says Egede.
The 2018 ATLAS measurement for the W boson mass is the most recent to date,
but it may also not be much help in solving the riddle. ATLAS used two beams
of protons, rather than a second one of antiprotons, making the results
harder to compare, says Kotwal.
If physicists can’t find a problem with Kotwal and his team’s work, then the
next step will be producing another measurement, which could come from three
experiments at the LHC. “It’s the only collider with a high enough energy to
create W bosons,” says Harry Cliff at the University of Cambridge. The LHC
is gearing up for a new run this year after being offline since 2018, but
Mulders says data collected for the CMS experiment during the previous run
could yield a new W boson measurement by next year.
If the result is borne out, it could join other unexplained anomalies like
those from the Muon g-2 experiment and discrepancies picked up at the LHC
relating to subatomic particles called bottom quarks, which might require
new theories of physics to explain. While there are no clear contenders for
such a theory at present, Kotwal says that some variants of supersymmetry,
which requires the existence of a whole new set of particles, might
accommodate the higher W boson mass.
Despite the result taking 10 years to produce, Kotwal says this is just the
beginning for understanding its significance as physicists around the world
get their hands on the data. “The science will be investigated and we will
continue to think about it,” he says.
Reference:
A. V. Kotwal, High precision measurement of the W-boson mass with the CDF II
detector, Science (2022).
DOI: 10.1126/science.abk1781
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Physics