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The Mystery of Antimatter
Context:
A research, published in a recent preprint paper, suggests that a new particle species could help explain the imbalance between matter and antimatter, a phenomenon that has puzzled physicists for decades.
More on News:
The paper’s mechanism brings the Standard Model closer to satisfying one of the Sakharov conditions, but the other two conditions (baryon number violation and thermal equilibrium violation) remain challenges.
Recent Discoveries:
- Recent experiments at CERN have shed new light on antimatter. Scientists have created and trapped antihydrogen atoms, allowing them to study their properties.
- One key discovery is that antimatter responds to gravity in the same way as matter, falling downwards just like regular matter.
- This finding is crucial for understanding why the universe is matter-dominated.
What is Antimatter?
- Antimatter, theorised by Paul A.M. Dirac in 1928 and observed in cosmic rays by Carl Anderson in 1932, is composed of particles with the same mass but opposite charge compared to regular matter.
- For example, the antielectron (positron) has the same mass as an electron but carries a positive charge instead of a negative one.
- The Big Bang should have produced equal amounts of matter and antimatter, but for reasons still unknown, matter came to dominate, allowing galaxies, stars, and planets to form.
- While antiparticles are detected (e.g., antielectrons from potassium-40 decay, antiprotons and antielectrons from cosmic rays), antimatter is rare in the universe.
Matter-Antimatter Asymmetry:
- For every 1.7 billion proton-antiproton pairs in the early universe, there was one extra unpaired proton, leading to a matter-dominated universe.
- The reason for this imbalance is still unknown, but any theory must satisfy three conditions (Sakharov conditions).
The Sakharov Conditions:
- To explain this asymmetry, physicists refer to the Sakharov conditions, which include:
- CP violation (violation of symmetry between particles and antiparticles).
- Baryon number violation (protons and neutrons have a baryon number of 1, antiparticles -1).
- Out-of-equilibrium interactions (particle processes must occur at different rates in forward and backward directions).
- The Standard Model of particle physics, the best current theory to explain the world, falls short of satisfying all these conditions.
CP Symmetry Violation:
- In 1957, it was discovered that the weak force doesn’t conserve parity (P), but it does conserve combined charge and parity (CP).
- In 1964, CP symmetry violation was observed, but not always, which prompted further exploration by physicists James Cronin and Val Fitch.
- CP violation occurs in weak-force processes but is too small (around 1 in 1,000) to explain the large asymmetry.
- Kobayashi-Maskawa Model (1973): The model predicted that CP violation was unavoidable if there were at least three variants (generations) of each quark species (e.g., up quark, charm quark, and top quark). This model explains the weak CP violation observed in nature.
New mechanism for matter-antimatter asymmetry:
- A new paper points out a potential solution: if mesons (quark-antiquark pairs) decay into particles not contained in the Standard Model, this could control the matter-antimatter asymmetry.
- Mechanism for Evolving CP Violation: The new mechanism suggests that the fraction of meson decays into non-standard particles could be large in the early universe but decrease over time, allowing CP violation to play a more significant role in the past.
Implications and Future Research:
- While this research addresses one of the Sakharov conditions, challenges remain for the others. Further studies and experiments are needed to fully understand the mechanisms behind the dominance of matter in the universe.
- This discovery opens new avenues for exploring the fundamental nature of the cosmos and advancing our knowledge of particle physics.
