24 October 2020 | New Scientist | 27Hunting muons
Photographer Noemi Caraban
Gonzalez/CERN
THIS amazing instrument may be
the world’s heaviest camera-like
device. But instead of photons of
light, it records something far
more exotic: muons, produced
by the collisions of billions of
subatomic particles.
Weighing in at 14,000 tonnes,
the Compact Muon Solenoid
(CMS) detector is one of four big
experiments at the Large Hadron
Collider, based at the CERN particle
physics laboratory near Geneva,
Switzerland. The LHC is the most
powerful particle accelerator in
existence and can push protons
close to the speed of light.
The CMS sits at one of the LHC’s
collision points, and it builds a
picture of the particles produced
when protons collide. It specialises
in muons, negatively charged
particles similar to electrons but
more than 200 times heavier.
The solenoid magnet at its core
bends charged particles, helping
the CMS detect their charge and
momentum. Muons are very shy
of interacting with matter, so they
pass undetected through most
of the CMS. They are caught by
components around the detector’s
edge, providing extra information
that allows the complex particle
interactions occurring as protons
collide to be reconstructed.
One of the particles that these
collisions produce is the Higgs
boson, the particle that gives all
other fundamental particles mass
and which has a characteristic
decay into four muons.
In September, CERN finished
installing the CMS’s outermost
layer of muon detectors, letting it
pick up muons that scatter at an
angle of 10 degrees. CERN plans
to add hundreds of new detectors
to expand the range of muons
it can detect. ❚
Gege Li