New Scientist - USA (2022-04-16)

8 | New Scientist | 16 April 2022

News

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

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.

The standard model describes

three 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.

Physicists have tried to find

the mass of the W boson 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 the model is

wrong. It has no explanation

for gravity, dark matter and the

absence of antimatter in our

universe, so physicists are on the

lookout for deviant measurements

that could lead to new theories.

Now, the Collider Detector at

Fermilab Collaboration of around

400 scientists has a new figure

for the W boson’s mass. Using

data from the Tevatron collider

at Fermilab in Illinois, it puts

it at 80.4335 gigaelectronvolts

(Science, doi.org/hpsb).

The generally accepted mass

is 80.379 gigaelectronvolts. While

the discrepancy may seem small,

the new value is the most precise so

far, equivalent to measuring your

body weight to within less than

10 grams. More importantly, its

difference from the accepted value

has a statistical significance of

around 5 sigma, corresponding to a

probability of about 1 in 3.5 million

that measurements 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 two

measurements is even higher,

at 7 sigma. This corresponds to

about a 1 in 780 billion probability

of seeing a result like this by

chance. Ashutosh Kotwal at Duke

University in North Carolina, who

led the collaboration’s analysis,

says its members have done all the

tests they can think of to confirm

their extraordinary result, and it

is now time for others to weigh in.

“We think the answer is holding

up to our own scrutiny,” he says.

The collaboration 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...

it’s a huge deal,” says Ulrik Egede

at Monash University in Australia.

That “if ” is important. Many

physicists are excited but cautious.

“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 2018.

Figuring out the source of the

discrepancy is tricky. 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 the collaboration

had to infer where they were from

large amounts of data.

Measure twice

The 2018 ATLAS measurement for

the W boson mass was 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 the collaboration’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 might require new theories

of physics to explain. 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 start 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. ❚

Particle physics

GR

AN

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R/A

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Shock result stuns physicists

The W boson is slightly heavier than we think it should be, according to data from Fermilab –

and that could overturn our understanding of the universe, reports Alex Wilkins

The Tevatron collider

at Fermilab in Illinois,

pictured in 1992

“ It would be the biggest

discovery since the

start of the standard

model 60 years ago”