Sky & Telescope - January 2021 - 18

Dark Matter

Though a WIMP would be at least the mass of a proton -
"massive" in particle physics terms - and an axion perhaps
one hundred billionths of the mass of the tiny electron
(or even much less), in many ways WIMPs and axions are
similar. Like WIMPs, axions are proposed to have formed in
the early universe and only very weakly interact with normal
matter. In addition, either one of them could potentially
explain dark matter in a way that forms a clean extension
to the Standard Model of particle physics, without having to
completely rewrite fundamental laws.
But many scientists think it is now time for WIMPs to exit
stage left and allow axions to step into the limelight.

An Omnipresent Particle?
Axions were first theorized in the
late 1970s independently by Steven
Weinberg and Frank Wilczek. They
are the consequence of an idea that
solved a problem - the strong CP
problem - in a part of the Standard
Model of particle physics known as
quantum chromodynamics (QCD).
Weinberg initially called the new
hypothetical particle the "Higglet,"
but it was Wilczek's suggestion of
"axion" - named after a laundry
detergent that had caught his eye at
the supermarket a few years before
- that stuck.
Theory predicts these QCD axions
pervade space like the cosmic microwave background, and that they

should very weakly interact with ordinary matter and be very
light. But, frustratingly, theory does not predict exact values.
However, a few years after QCD axions were first proposed, Pierre Sikivie (University of Florida) and others realized that there is a range of masses and interaction strengths
that would make them add up to the missing matter in the
universe. It is around this band of values that researchers currently focus most of their efforts.
To find a miniscule hypothetical particle that has no
electric charge and that rarely interacts with the universe we
see is a challenge, to put it mildly. Theory suggests one of the
few ways to catch an elusive axion red-handed is with magnetic fields, which can change axions into photons and back
again. In their photon form, researchers
can measure the light's frequency, which
relates directly to the axion mass.
Dark matter hunters pursue two broad
lines of investigation based on this idea:
building powerful magnets that induce
axion identity-swapping here on Earth,
WT ADMX Left: Physicists use the Axion Dark
Matter Experiment to search for axions by moving
two rods inside a cavity (cavity is gold chamber at
bottom of cutaway). The rods' placement determines the cavity's resonant frequency, which then
makes it sensitive to different wavelengths. Below:
ADMX physicists examine the experiment (top),
which normally is lowered into the bore of the superconducting magnet at their feet (bottom).

Axions are an attempt to solve the
strong CP problem, a strange dilemma in
which two fundamental nuclear forces
don't act the same way. The weak
nuclear force breaks what's called
charge-parity symmetry, which essentially
states that switching from matter to
antimatter will create the mirror image
of the original physical process. But
the strong nuclear force follows CP
symmetry, even when involving the
same kind of particles that violate
the symmetry in weak interactions.
The problem seems to be the value
of a constant in the strong force
equations. Physicists realized in the
1970s that if this constant is instead a
field permeating space, it could solve
the strong CP problem. The axion is
the particle of that hypothetical field.

18

JANUARY 2 021 * SK Y & TELESCOPE

ADMX CUTAWAY: ADMX COLL ABOR ATION; E X PERIMENT AND
PH YSICISTS: M A RY LE VIN / UNIV ERSIT Y OF WASHINGTON; A DM X
IN G ROUND: M A RK STONE / UNIV ERSIT Y OF WASHINGTON

The Strong CP Problem



Sky & Telescope - January 2021

Table of Contents for the Digital Edition of Sky & Telescope - January 2021

Contents
Sky & Telescope - January 2021 - Cover1
Sky & Telescope - January 2021 - Cover2
Sky & Telescope - January 2021 - 1
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