13.1 Unsolved problems in particle physics

Particle physics is full of mysteries waiting to be solved. From dark matter and energy to the matter-antimatter imbalance, scientists are grappling with big questions about the universe's fundamental nature.

These unsolved problems push the boundaries of our understanding. They're driving research into new particles, forces, and theories that could revolutionize physics and our view of the cosmos.

Unsolved Problems in Particle Physics

Dark Matter and Dark Energy

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Top images from around the web for Dark Matter and Dark Energy

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  Astronomers Witness a Web of Dark Matter - Universe Today View original

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  Dark Energy, Dark Matter, and the Multiverse View original

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  Mayall Telescope Archives - Universe Today View original

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  Astronomers Witness a Web of Dark Matter - Universe Today View original

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• Dark matter comprises hypothetical matter not interacting with electromagnetic radiation but exerting gravitational effects on visible matter
  • Inferred from gravitational effects on visible matter and cosmic microwave background
  • Estimated to make up ~27% of the universe's mass-energy content
  • Candidates include Weakly Interacting Massive Particles (WIMPs) and axions
• Dark energy permeates all space, responsible for accelerating universe expansion
  • Makes up ~68% of the universe's mass-energy content
  • Nature and properties remain unknown, challenging particle physics and cosmology
  • Possible explanations include cosmological constant, quintessence, or modified gravity theories
• Strong CP problem addresses unexplained absence of CP violation in strong nuclear force
  • Led to proposed solutions such as axion particle
  • Axions could potentially explain both strong CP problem and serve as dark matter candidates

Grand Unification and Quantum Gravity

• Grand Unification Theories (GUTs) aim to unify electromagnetic, weak, and strong forces
  • Predict phenomena like proton decay and magnetic monopoles
  • Experimental evidence remains elusive (no observed proton decay or magnetic monopoles)
  • Examples include SU(5) theory and SO(10) theory
• Quantum gravity represents attempt to reconcile general relativity and quantum mechanics
  • Challenges include reconciling discrete nature of quantum mechanics with continuous spacetime of general relativity
  • Proposed approaches include string theory, loop quantum gravity, and causal set theory
  • Planck scale ($10^{-35}$ meters) where quantum gravity effects become significant

The Hierarchy Problem

Understanding the Hierarchy Problem

• Hierarchy problem addresses large discrepancy between weak force and gravity
  • Weak force operates at $10^{-18}$ meters, while gravity becomes significant at Planck scale ($10^{-35}$ meters)
  • Questions why Higgs boson mass ($125$ GeV) much lighter than Planck mass ($10^{19}$ GeV)
• Higgs boson mass sensitive to quantum corrections from virtual particles
  • Corrections should theoretically drive mass up to Planck scale
  • Observed mass much lower than expected, requiring extreme fine-tuning in Standard Model
• Fine-tuning considered unnatural by many physicists
  • Suggests potential new physics beyond Standard Model to explain hierarchy
  • Naturalness principle violated by required level of fine-tuning

Proposed Solutions and Significance

• Supersymmetry introduces superpartner particles to cancel out quantum corrections
  • Predicts existence of superpartners for all known particles (selectrons, squarks, gluinos)
  • No experimental evidence found yet at Large Hadron Collider
• Extra dimensions propose gravity operates in higher dimensions
  • Explains apparent weakness of gravity in our 4-dimensional spacetime
  • Models include Randall-Sundrum model and ADD model
• Multiverse hypothesis suggests our universe one of many with different physical constants
  • Anthropic principle used to explain observed values in our universe
• Significance of hierarchy problem guides research towards new fundamental principles
  • Potential discovery of new symmetries or particles
  • Could lead to revision of our understanding of naturalness in physics

Matter-Antimatter Asymmetry

Observed Asymmetry and Sakharov Conditions

• Matter-antimatter asymmetry observed dominance of matter over antimatter in visible universe
  • Contrary to expected equal amounts produced in Big Bang
  • Baryon-to-photon ratio estimated at $\eta \approx 6 \times 10^{-10}$
• Sakharov conditions outline necessary requirements for baryogenesis
  • Baryon number violation
  • C and CP violation
  • Departure from thermal equilibrium
• CP violation observed in weak interactions but insufficient to explain observed asymmetry
  • Observed in K and B meson systems
  • Jarlskog invariant quantifies CP violation in Standard Model ($J \approx 3 \times 10^{-5}$)

Research Directions and Theories

• Search for additional sources of CP violation key area of research
  • Investigation in lepton sector (neutrino oscillations)
  • Exploration of physics beyond Standard Model (supersymmetry, extra dimensions)
• Leptogenesis theories propose asymmetry originated in lepton sector
  • Asymmetry later transferred to baryon sector through sphaleron processes
  • Requires existence of heavy right-handed neutrinos
• Electroweak baryogenesis explores possibility of asymmetry generation during electroweak phase transition
  • Requires first-order phase transition and additional CP violation sources
  • Constrained by current Higgs boson mass measurements
• Implications for cosmology and particle physics intertwine
  • Understanding asymmetry could provide insights into early universe physics
  • May reveal new particles or interactions beyond Standard Model

Neutrinos in Particle Physics

Neutrino Properties and Oscillations

• Neutrinos interact only via weak nuclear force and gravity
  • Extremely difficult to detect due to low interaction cross-sections
  • Three flavors electron neutrino, muon neutrino, tau neutrino
• Neutrino oscillations change flavor as they propagate
  • Provides evidence for non-zero neutrino mass, contrary to original Standard Model
  • Described by PMNS matrix with mixing angles $\theta_{12}$, $\theta_{23}$, $\theta_{13}$
• Neutrino mass problem arises as Standard Model lacks mass generation mechanism
  • Possible mechanisms include seesaw mechanism, radiative mass generation
  • Absolute mass scale unknown, only upper limits and mass-squared differences determined
• Nature of neutrinos (Dirac or Majorana) remains open question
  • Neutrinoless double beta decay experiments aim to provide answer
  • If Majorana, neutrinos would be their own antiparticles

Research Frontiers and Implications

• Sterile neutrinos hypothetical particles not interacting via any fundamental forces except gravity
  • Could explain anomalies in short-baseline neutrino oscillation experiments
  • Potential dark matter candidates
• CP violation in neutrino sector actively investigated
  • Could provide insights into matter-antimatter asymmetry
  • Studied through long-baseline neutrino oscillation experiments (DUNE, Hyper-Kamiokande)
• Understanding neutrino properties implications for fundamental symmetries
  • Lepton number conservation
  • CP and CPT symmetry in lepton sector
• Neutrino physics connects to various areas of research
  • Cosmology (relic neutrino background, role in Big Bang nucleosynthesis)
  • Astrophysics (supernova explosions, neutron star cooling)
  • Particle physics beyond Standard Model (leptogenesis, grand unified theories)