8.2 CP violation in weak interactions

CP violation in weak interactions is a fascinating twist in particle physics. It shows that nature isn't perfectly symmetrical, which blew scientists' minds when they found out.

This asymmetry between matter and antimatter might explain why our universe exists at all. It's a key piece of the puzzle in understanding the fundamental laws of physics and the origins of everything.

CP Violation in Kaon Systems

Discovery and Implications

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• James Cronin and Val Fitch discovered CP violation in 1964 through neutral kaon decays
• Neutral kaon system comprises K0 and its antiparticle K̄0, mixing to form CP eigenstates KS (short-lived) and KL (long-lived)
• Long-lived neutral kaon state KL unexpectedly decayed into two pions, violating CP symmetry
  • Previously thought to decay only into three pions due to CP conservation
• CP violation in kaon decays exhibited small branching ratio (about 2 parts in 1000 for CP-violating decay mode)
• Discovery challenged understanding of fundamental symmetries in nature
  • Led to Nobel Prize for Cronin and Fitch in 1980
  • Profound implications for particle physics and cosmology

Neutral Kaon System Characteristics

• CP symmetry initially believed to be conserved in all physical processes, including weak interactions
• K0 and K̄0 mix to form CP eigenstates
  • KS (short-lived state)
  • KL (long-lived state)
• CP violation manifests in unexpected decay modes
  • KL occasionally decays into two pions, violating CP symmetry
• Observed CP violation smaller than anticipated
  • Branching ratio of approximately 0.2% for CP-violating decay mode
• Discovery opened new avenues for research in particle physics
  • Prompted investigations into CP violation in other particle systems (B mesons, D mesons)

Direct vs Indirect CP Violation

Types and Mechanisms

• CP violation occurs through two distinct mechanisms: direct and indirect
• Direct CP violation results from differing decay amplitudes for particle and antiparticle
  • Leads to different decay rates for CP-conjugate processes
  • Observed in various meson decay channels (K mesons, B mesons)
• Indirect CP violation (mixing-induced) arises from neutral meson mixing
  • Occurs in systems like K0-K̄0 and B0-B̄0
  • Results from interference between mixing and decay processes
• Neutral kaon system characterizes indirect CP violation with parameter ε
• Direct CP violation in kaon system described by parameter ε'
• Ratio ε'/ε measures relative strength of direct to indirect CP violation
  • Non-zero value experimentally confirmed, validating direct CP violation existence

Experimental Observations

• B-meson decays exhibit both direct and indirect CP violation
• Time-dependent decay rate asymmetries often used to study indirect CP violation in B mesons
• Experiments like BaBar, Belle, and LHCb have measured CP violation in various B meson decay channels
  • B0 → J/ψKS (indirect CP violation)
  • B0 → K+π- (direct CP violation)
• Kaon system experiments (NA48, KTeV) precisely measured ε'/ε ratio
  • Confirmed direct CP violation in kaon decays
• Ongoing experiments seek to measure CP violation in charm meson and baryon systems
  • LHCb observed CP violation in charm sector (2019)

CP Violation and Matter-Antimatter Asymmetry

Sakharov Conditions and Baryogenesis

• CP violation fulfills one of three Sakharov conditions for generating baryon asymmetry in early universe
  • Explains observed dominance of matter over antimatter
• Standard Model CP violation insufficient to account for observed matter-antimatter asymmetry
  • Suggests existence of additional CP violation sources beyond Standard Model
• Baryogenesis theories attempt to explain universe's matter dominance
  • Incorporate CP violation, baryon number violation, and departure from thermal equilibrium
• Connection between CP violation and matter-antimatter asymmetry underscores importance of CP violation studies
  • Crucial for understanding cosmology and early universe evolution

Beyond the Standard Model

• Leptogenesis mechanism generates baryon asymmetry through lepton number violation
  • Involves CP violation in neutrino sector
  • Active area of research in particle physics and cosmology
• Experimental searches for CP violation motivated by quest to understand matter-antimatter asymmetry origin
  • Investigations in neutrino oscillations
  • Searches for electric dipole moments of fundamental particles
• Theories beyond Standard Model (supersymmetry, extra dimensions) predict additional sources of CP violation
  • Could potentially explain observed matter-antimatter asymmetry
• Precision measurements of CP violation parameters constrain new physics models
  • Help guide theoretical developments in particle physics and cosmology

CP Violation in B-Meson Decays

Experimental Discoveries and Techniques

• BaBar and Belle experiments first observed CP violation in B-meson decays (2001)
  • Confirmed Standard Model predictions and CKM mechanism
• B-factory experiments utilized e+e- colliders to produce large numbers of B-B̄ meson pairs
  • Enabled precise measurements of time-dependent CP asymmetries
• sin(2β) determination from B0 → J/ψKS decays provided first clear evidence of CP violation outside kaon system
• Direct CP violation observed in various B-meson decay channels
  • B0 → K+π-
  • B+ → D0K+
• LHCb experiment at CERN made significant B-physics contributions
  • First observation of CP violation in Bs0 system
  • Discovery of CP violation in charm meson decays

Precision Measurements and Implications

• CP-violating parameter measurements in B-meson decays allow stringent tests of CKM matrix unitarity
  • Provide constraints on potential new physics beyond Standard Model
• Study of rare B-meson decays (B0 → K*0μ+μ-) offers complementary probes of CP violation
  • Sensitive to new physics effects in flavor-changing neutral currents
• Time-dependent CP asymmetry measurements in B0 → π+π- and Bs0 → K+K- decays
  • Test for presence of new physics in penguin-dominated b → d and b → s transitions
• Precision studies of B-meson CP violation contribute to understanding of matter-antimatter asymmetry
  • Constrain models of baryogenesis and leptogenesis
• Ongoing and future experiments (Belle II, LHCb upgrade) aim to improve precision of CP violation measurements
  • Search for deviations from Standard Model predictions
  • Probe higher energy scales for new physics effects