12.4 Beyond the Standard Model

The Standard Model of particle physics is a cornerstone of modern physics, but it's not the whole story. Scientists are exploring theories beyond it to explain unexplained phenomena and unify fundamental forces.

These theories, like supersymmetry and string theory, aim to fill gaps in our understanding. They tackle mysteries like dark matter, dark energy, and neutrino masses, pushing the boundaries of what we know about the universe.

Theories Beyond the Standard Model

Supersymmetry and String Theory

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• Supersymmetry proposes a symmetry between fermions and bosons
  • Predicts superpartner particles for each known particle
  • Addresses hierarchy problem in particle physics
  • Provides potential dark matter candidate particles
• String theory postulates fundamental particles as vibrating strings
  • Unifies quantum mechanics and general relativity
  • Requires extra spatial dimensions (10 or 11 total)
  • Different vibrational modes correspond to different particles
• M-theory emerges as a unifying framework for string theories
  • Incorporates 11 dimensions and includes supergravity
  • Proposes existence of multidimensional objects called branes

Grand Unified Theories (GUTs)

• GUTs aim to unify strong, weak, and electromagnetic forces
  • Predict convergence of coupling constants at high energies
  • Propose existence of X and Y bosons as force carriers
• SU(5) model serves as simplest GUT framework
  • Groups quarks and leptons into larger multiplets
  • Predicts proton decay with long but finite lifetime
• SO(10) model extends SU(5) to include right-handed neutrinos
  • Provides natural explanation for neutrino masses
  • Incorporates seesaw mechanism for neutrino mass generation

Unexplained Phenomena

Dark Matter and Dark Energy

• Dark matter accounts for ~27% of the universe's mass-energy content
  • Inferred from gravitational effects on visible matter
  • Candidates include WIMPs (Weakly Interacting Massive Particles)
  • Detection methods involve direct detection (underground experiments) and indirect detection (cosmic ray observations)
• Dark energy comprises ~68% of the universe's mass-energy content
  • Responsible for accelerating expansion of the universe
  • Possible explanations include cosmological constant or quintessence field
  • Studied through observations of Type Ia supernovae and cosmic microwave background

Neutrino Oscillations and Mass

• Neutrino oscillations describe flavor changes between neutrino types
  • Observed in solar, atmospheric, and reactor neutrinos
  • Implies non-zero neutrino masses, contradicting Standard Model predictions
  • Characterized by mixing angles (θ12, θ23, θ13) and mass-squared differences
• Neutrino mass hierarchy remains undetermined
  • Normal hierarchy: m1 < m2 << m3
  • Inverted hierarchy: m3 << m1 < m2
  • Absolute mass scale still unknown, constrained by cosmological observations

Predicted Effects

Proton Decay and Baryon Number Violation

• Proton decay predicted by many GUTs and some supersymmetric theories
  • Violates baryon number conservation
  • Typical decay modes include p → e+ + π0 and p → K+ + ν̄
  • Current experimental lower limit on proton lifetime exceeds 1034 years
• Neutron-antineutron oscillations serve as alternative baryon number violation process
  • Predicted by some theories beyond the Standard Model
  • Would indicate Majorana nature of neutrinos if observed

Neutrino Physics Beyond Oscillations

• Sterile neutrinos hypothesized as additional neutrino flavors
  • Do not interact via weak force, only through gravity
  • Could explain anomalies in short-baseline neutrino experiments
• Neutrinoless double beta decay searches probe Majorana nature of neutrinos
  • Would violate lepton number conservation if observed
  • Provides information on absolute neutrino mass scale

Dark Matter Detection and Characterization

• Direct detection experiments aim to observe WIMP-nucleon scattering
  • Use low-background detectors in underground laboratories (XENON, LUX)
  • Seasonal variation in event rate expected due to Earth's orbit
• Indirect detection searches for dark matter annihilation products
  • Focus on regions of high dark matter density (galactic center, dwarf galaxies)
  • Look for excess gamma rays, neutrinos, or antimatter in cosmic rays
• Collider searches attempt to produce dark matter particles
  • Look for missing energy signatures in particle collisions
  • Constrain properties of dark matter candidates through precision measurements