The W boson measurement provides insight into existing physics and opens the door to new arenas.
A new
surprising result from digging in Fermilab archives could help
scientists uncover new laws of physics and solve mysteries such as the nature
of dark matter.
In a study appearing in the April 8 issue of the journal
Science, the most precise measurement yet of a key particle linked with the
weak force known as the W boson reveals it appears surprisingly heavier than
previously thought.
The weak force is one of the four
known fundamental forces of nature — the other three are gravity,
electromagnetism, and the strong force. It helps trigger the nuclear reactions
that keep the sun burning and also drives the decay of particles. Also known as
the weak interaction or the weak nuclear force, it is effective only to a
distance of about 10^-17 meters, or roughly 1 percent of the diameter of a
typical atomic nucleus. It gets its name from how it is much weaker than both
the strong force and electromagnetism, although all three are stronger than
gravity.
The weak force can help protons turn into neutrons and vice
versa. This helps initiate the nuclear fusion reactions that power stars, and
can also lead radioactive nuclei to decay. In addition, most subatomic
particles are unstable and decay via the weak force. The existence of the W
boson as an agent of the weak force was predicted in the 1960s and first
confirmed in 1983 at CERN, the largest particle physics lab in the world.
The W boson is a key building block of the Standard Model,
which is currently the best explanation for how all the known elementary
particles behave. However, the Standard Model has a number of major
shortcomings — for example, it currently cannot explain dark matter, a thus-far
invisible substance thought to make up roughly five-sixths of all the matter in
the universe. It also doesn’t account for gravity or the excess of matter over
antimatter in the universe. Any new discoveries at odds with the Standard Model's
predictions may lead the way to a better model beyond it.
The Tevatron at Fermilab, now defunct. Fermilab |
WHAT DID THE SCIENTISTS DO?
The researchers spent nearly a
decade examining data from the Tevatron particle accelerator at Fermilab, once
the world's largest particle accelerator. They analyzed huge amounts of data
from the now-defunct accelerator generated from 2002 to 2011 regarding about 4
million W boson candidates, around four times as many used in previous
Tevatron-based measurements of the particle's masss.
The Tevatron collided protons and antiprotons at nearly the
speed of light, creating flashes of energy that coalesced into particles that
in turn decayed into more stable particles. As the debris from these collisions
passed through Tevatron's detectors, scientists could deduce their energy, path,
and charge, helping them measure particles such as the W boson.
"We used mature techniques from previous analyses and
developed many new analysis techniques," study co-author Ashutosh Kotwal,
a particle physicist at Duke University who initiated and led the W boson mass
analyses, tells Inverse. "We implemented new ideas to use our data in
novel ways to calibrate our experimental apparatus much more precisely than in
the past.”
“We also incorporated new information about the colliding
proton's structure that the particle physics community has collected over the
last decade,” he adds. “The combination of a four times larger dataset, more
insightful methods and ideas of using our data, and new information about the
proton structure allowed us to improve the precision of this measurement
substantially."
A demonstration of W boson decay. CERN |
WHAT DID THEY FIND?
The new study estimated the W boson
had a mass of about 80.433 billion electronvolts, with a precision of 0.01
percent. This is twice as precise as the previous best measurement, and
corresponds to measuring the weight of an 800-pound gorilla to within 1.5
ounces.
"The precision of this very delicate and challenging
measurement is quite impressive," Martijn Mulders, an experimental
particle physicist at CERN who did not participate in this study, tells
Inverse.
However, this new estimate is roughly 76 million
electronvolts heavier than what the Standard Model predicts as the W boson's
mass — about 80.357 billion electronvolts. In comparison, the electron's mass
is about 0.5 million electronvolts, whereas the proton's mass is about 938
million electronvolts.
"The deviation from the Standard Model prediction is
very large," Claudio Campagnari, an experimental particle physicist at the
University of California at Santa Barbara who did not take part in this
research, tells Inverse. "If the result is confirmed by another
experiment, and if a careful re-examination of the theoretical calculation,
including the other experimental inputs that go into it, does not find any
problem, this would be solid evidence that there is new physics beyond what is
encoded into the Standard Model of particles and their interactions."
WHAT'S NEXT?
Since extraordinary claims require
extraordinary evidence, additional experiments are needed to confirm these new
findings. "The big question will be, 'Can anyone find anything
wrong?'" Mulders says.
Particle accelerators that might carry out such work include
the Large Hadron Collider at CERN, as well as proposed colliders such as the
International Linear Collider in Japan, the Compact Linear Collider, the Future
Circular Collider at CERN, and the Circular Electron Positron Collider in
China. The Future Circular Collider offers the best prospects for better W
boson mass measurements, with a projected sensitivity of 7 parts per million,
more than 10 times better than the current best measurement, Mulders and
Campagnari wrote in a commentary on the new study.
These measurements of the W boson's mass do require a very
precisely calibrated detector, and given how the calibration procedure is quite
complicated, something might have gone wrong there with Tevatron's results,
potentially explaining this deviation. "On the other hand, the people that
carried out this work are some of the world's experts in these type of
measurements," Campagnari says.
If this result is accurate, previous research might not have
measured the W boson's mass properly for a number of reasons. Perhaps the
particle accelerators used in prior work lacked the power to generate new
massive particles whose interactions with the W boson influence its mass, or
perhaps these other particles were very difficult to measure by other detectors
for other reasons, Mulders says.
"If confirmed, the difference between the measured
value and the Standard Model calculation of the W boson mass would have to be
due to a new mechanism in nature, a new fundamental principle we do not know
about," Kotwal says. "This could manifest as a new particle or
subatomic interaction and be discovered in running and future
experiments."
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