Dark Matter Research Paradigm Shift

• Past decade: astrophysicists dramatically shifted opinion about dark matter
• Most popular idea still particle-based, but different particle now favored
• Axion particles attracting most attention vs previous focus on WIMPs
• Graph shows publications about Axion particles increasing each year
• Trend not entirely new, but Axion research took off 10-15 years ago

WIMP Theory Decline

• LHC failed to find biggest Axion competitor: WIMPs (Weakly Interacting Massive Particles)
• WIMPs expected to have masses comparable to heaviest known particles (like Higgs boson)
• Should have appeared at Large Hadron Collider - didn't appear
• Doesn't prove non-existence, could be heavier requiring bigger collider
• WIMP appeal factors:
  • Came with ideas to unify standard model interactions
  • WIMP miracle: masses in LHC range + weak nuclear force strength = right early universe production amount
• No WIMPs detected anywhere: not at LHC, not in detection experiments
• Experiments resulted in constraints on WIMP masses/interactions
• Didn't rule out WIMPs but removed greatest appeal (WIMP miracle)
• WIMP papers peaked, best time behind them

Axion History and Revival

• Original introduction: 1970s physics
• Quickly ruled out <2 years after introduction
  • Neutron stars would emit many Axions
  • Would cool stars very quickly
  • Incompatible with observations
• 1981: new type introduced
  • Originally called "harmless Axion" or "invisible Axion"
  • Designed to be compatible with observations
  • Now often just called "the Axion"

Axion Properties vs WIMPs

• Like WIMPs: doesn't interact with light, goes through normal matter
• Could make up dark matter
• Key difference: extremely small masses vs WIMP large masses
• Basically impossible to detect in particle colliders
  • Go through detectors
  • Detection method: check for missing energy
  • Works for high-mass particles like WIMPs
  • Small-mass Axions: missing energy below measurement accuracy
  • Analogy: "finding invisible needle in $5 billion haystack"

Axion Advantages for Dark Matter

• Small masses make them appealing dark matter candidates
• Quantum uncertainty depends on wavelength
• Smaller mass = larger wavelength = larger uncertainty
• Very small masses: uncertainty extends through entire galaxies
• Solves WIMP distribution problem:
  • WIMP distribution has sharp peak in galaxy centers (incompatible with observations)
  • Axion large quantum uncertainty blurs out peak
  • "Same idea as photoshopping pimples"

Axion Formation Mechanism

• Don't fill universe same way as WIMPs
• WIMPs form clouds of freely floating particles
• Axions form condensate in early universe
• "More like puddles than clouds"
• Past concern: fine-tuning numbers to fit observations
• Physicists stopped worrying: "any solution better than none"

Axion Detection Methods

• Direct detection experiments use strong magnetic fields
• Should convert Axions into photons
• Some experiments running long time without findings
• Recent claims: Axions collect around rotating neutron stars (pulsars)
• Pulsars have strong magnetic fields
• Axions would leave characteristic signal in received radiation
• Could help pin down dark matter particle

Scientific Motivation Comparison

• Most astrophysicists: Axions next to WIMPs are best motivated dark matter candidates
• Existence consequence of other ideas:
  • WIMPs: unification of forces
  • Axions: solution to strong CP problem (keeps standard model parameter small)
• Author's opinion: neither motivation particularly good
• Analogy: Axion like spare IKEA part - not sure what it's for but could hold whole thing together