15 January 2022 | New Scientist | 39Features Cover story
model. This made it very hard to make any
meaningful comparison with experimental
measurements – any discrepancy could be
down to the imprecision of the predictions.
“We realised that we hit a wall,” says Hiller.
Undeterred, she and her collaborator Frank
Krüger realised that if you look at how often
this decay occurred compared with a similar
decay that spits out electrons instead, the nasty
uncertainties from QCD cancelled out. The
ratio of the two decays could be predicted
very precisely – but would be sensitive only to
forces pulling on the electrons and muons with
differing strength. That was a long shot. All
known forces pull on the two particles equally,
and the assumption was that any undiscovered
forces would do so too, meaning Hiller and
Krüger’s ratio wouldn’t reveal anything new.
A decade later, collisions at CERN’s Large
Hadron Collider (LHC) began producing a
torrent of beauty quarks, which were recorded
and analysed by the LHCb experiment,
one of four large particle detectors on the
27-kilometre accelerator ring beneath the
French-Swiss border. Now, physicists could
really start to put these rarest decays under
the microscope. As they did so, intriguing
anomalies began to emerge.
The first came when early measurements
suggested that decays producing a strange
quark and two muons happened less often
than the standard model predicted. Then,
in 2013, the LHCb experiment released a new
measurement that analysed the angles that the
particles produced in these decays went flying
out at. This time, there were even stronger
hints of deviations from the standard model.
And yet there were still sufficient theoretical
uncertainties to leave room to quibble.
Could Hiller and Krüger’s ratio help? InDawn of a
new physics?
Hints from particle smash-ups at the Large Hadron Collider
are firming up. We could finally be looking at a new force of
nature and a deeper theory of reality, says physicist Harry Cliff
A
T HALF past six on the evening of
20 January 2021, amid the gloom of
a long winter lockdown, a small team
met on Zoom to share a moment they knew
might change physics forever. “I was literally
shaking,” says Mitesh Patel at Imperial College
London. He and his team were about to
“unblind” a long-awaited measurement from
the LHCb experiment at the CERN particle
physics laboratory near Geneva, Switzerland –
one that might, at long last, break the standard
model, our current best picture of nature’s
fundamental workings.
The measurement concerns subatomic
particles known as “beauty” or “bottom”
quarks. Over the past few years, their
behaviour has hinted at forces beyond our
established understanding. Now, with the
hints continuing to firm up, and more results
imminent, it’s crunch time. If these quarks are
acting as they appear to be, then we are not
only seeing the influence of an unknown force
of nature, but perhaps also the outline of a new,
unified theory of particles and forces.
That is a big if – but many particle
physicists are on tenterhooks, myself
included.“I’ve never seen som ething like this,”
says Gino Isidori, a theorist at the University
of Zurich, Switzerland. “I’ve never been so
excited in my life.”
For all its dazzling success in describing
the basic ingredients of our universe, the
standard model of particle physics has many
shortcomings. It can’t explain dark matter,
the invisible stuff that keeps galaxies from
flying apart, or dark energy, which seems to
be driving the accelerating expansion of the
universe. Nor can it tell us how matter survived
the big bang, rather than being annihilated by
an equal amount of antimatter. What’s more, >
it has several apparently arbitrary features that
beg deeper explanations. Clearly, the standard
model isn’t the whole picture. To complete it,
we need to break it.
The saga of the beauty quarks began in the
mid-2000s when Gudrun Hiller, a theoretical
physicist then at the University of Munich,
Germany, was panning for insights in a flood
of data from the Belle experiment in Japan
and the BaBar experiment in California.
These “B-factories” produced beauty quarks
by colliding electrons with their antiparticles,
positrons. The beauty quarks would live for an
instant – around 1.5 trillionths of a second, on
average – before decaying into other particles.A strange beauty
Hiller was particularly interested in an
extremely rare decay where a beauty quark
transforms into a strange quark, the third
heaviest of six types of quark (see “The
standard model: A new addition?”, page 40).
In doing so, it emits two oppositely charged
muons, heavier versions of electrons. Rare
decays such as these are very valuable, as they
could be strongly influenced by unknown
forces of nature, should they exist. The idea
is to make the most precise measurement
possible of such decays and compare them
with the most precise predictions theorists can
muster using the standard model. If the two
disagree, you have evidence for a new force.
The trouble was, theoretical predictions of
how often a beauty quark should transform
into a strange quark and two muons were
plagued by uncertainties from quantum
chromodynamics (QCD), the theory of
the strong force that governs how quarks
interact with one another within the standard