14 | New Scientist | 7 May 2022
News
ON 7 April, researchers announced
results from the now-defunct
Tevatron collider in Illinois finding
that the mass of the W boson is
higher than the standard model
of particle physics predicts – a
shockingly solid measurement
that could demand new physics.
Ideas to explain it have already
begun flowing in earnest.
The W boson is one of the
particles that carries the weak
nuclear force in the same way that
a photon carries electromagnetic
force. It is also important to
interactions between neutrinos
and other subatomic particles.
Before the Tevatron measurement,
all of the modern observations
of its mass clustered around
80.379 gigaelectronvolts.
The new result puts it at
80.4335 gigaelectronvolts, which
is different from the previously
observed value with a statistical
significance of 5 sigma – meaning
that there is about a 1 in 3.5 million
chance of finding a value like this
by chance if the old one is actually
correct. A decade of data analysis
by the Tevatron team has made it
apparently one of the most secure
particle physics measurements in
modern memory.
The W boson mass anomaly
indicates that something about
the standard model is wrong or
incomplete. This is shocking in
one sense because the standard
model has been extraordinarily
accurate in its predictions thus far,
but it isn’t necessarily surprising
in that we already knew it was
incomplete – the model contains
no explanation of dark matter,
the prevalence of matter and
antimatter in the known universe
or even gravity.
Since the Tevatron results were
announced, particle physicists
have produced numerous papers
explaining how the standard
model could be adapted orParticle physicsSH
UTTE
RS
TOCK
/ANDREY^VPWhy is the W boson so heavy?
The particle’s unexpectedly high mass has sent physicists scrambling for explanations
that include a strange new Higgs particle and supersymmetry, reports Leah CraneThe mass of the W boson
could upend the standard
model of particle physics80.
Previously known mass of the
W boson in gigaelectronvolts80.
The latest mass measurement of
the W boson in gigaelectronvolts1 in 3.5m
Chance of getting a result like the
new W boson mass value by flukeexpanded to account for the
higher W boson mass. “What we
are finding is that it is very easy to
accommodate this anomaly – it’s
almost a bit surprising that this is
so easy to do,” says Ramona Gröber
at the University of Padua in
Italy. “In the past, with particle
anomalies, it has been far more
difficult to accommodate them.”
Many of those explanations
involve a strange or additional
Higgs boson, the fundamental
particle coupled to the Higgs field,
which provides other particles –
including the W boson – with
mass. “The most obvious
mechanism to justify a larger
mass of the W is either a non-
standard Higgs or a different
number of matter fields [similar to
the Higgs field], or a combination
of both,” says Francesco Sannino
at the University of Southern
Denmark. “There are a lot of
variations on the theme, but
these are the basic mechanisms.”
“Even such a simple change as
adding a second Higgs boson is
able to accommodate the valuethat we observed,” says Ashutosh
Kotwal at Duke University in
North Carolina, who was part
of the Tevatron team. “There
are grand schemes and there
are simple schemes, and the
ones I find the most intriguing are
the ones which are the simplest.”Possible new particles
The various additional Higgs
bosons that have been suggested
have a range of properties
different from the known Higgs.
Some carry electric charges,
while the regular Higgs boson
is neutral. Some are made up of
other, smaller particles – in some
models, these are known particles,
such as gluons making up what
is called a glueball Higgs – and
in others they are potential new
particles, such as so-called
techniquarks making up the
possible Technicolor Higgs.
In all of these models, the
new Higgs particles come
with additional fields that
lend extra mass to the W boson.
Many of them also aim to solve
other open questions in particle
physics. For example, a popular
model that accounts for the
W boson’s extra heft would add
a new type of particle called a
leptoquark that could explain
another major mystery in particle
physics – the muon g-2 anomaly,
which arises from the fact that
muons seem to rotate faster than
the standard model predicts.
One set of possible particles,
called supersymmetric
particles or “sparticles”,
would accommodate a more
massive W boson, the muon
g-2 anomaly and the mystery
of why fundamental particles
in general have the masses that
they do, in one fell swoop.
“We expect the new particles
this supersymmetry predicts to