lpetrich
Contributor
Baryonic matter, to be more precise. Why is there any at all? Why doesn't the Universe consist only of photons, neutrinos, dark matter, and dark energy? I should add space-time to be complete, even if it is not usually thought of as a constituent of the Universe.
The problem is that the particles of baryonic matter -- electrons, protons, and neutrons, or looking inward, electrons, up quarks, and down quarks -- are particles that have distinct antiparticles. In the very early Universe, it was hot enough to produce pairs of ordinary particles and their antiparticles, and these ordinary particles and antiparticles would then run into each other, annihilating and producing photons and the like. As the Universe expanded, it cooled, eventually becoming too cold to make particle-antiparticle pairs. This did not stop annihilation, however, and most of the particles and their antiparticles disappeared. For every approximate billion antiparticles in the early Universe, there was the same approximate billion ordinary particles with one extra, and all that was left was the extra ones, giving the baryonic matter in the Universe.
Why did this asymmetry happen? It must have been some post-inflation effect, since inflation would have diluted a pre-inflation effect to unobservability. The first clue came from violation of some expected elementary-particle symmetries. There are three that are related to space-time: matter-antimatter (charge, C), reflection in space (parity, P), and reflection in time (time, T) symmetries. There is a very general theorem called the CPT theorem that requires some broadly plausible assumptions. It states that C, P, and T are conserved together, even if not necessarily individually.
The strong interaction, electromagnetism, and gravity all conserve C, P, and T separately, and particle physicists thought that that would be true of the weak interaction also. But they discovered in the 1950's that weak interactions violate parity, and in the early 1960's that weak interactions also violate charge and parity together (CP violation). Meaning that by the CPT theorem, they must violate time symmetry also.
In 1967, Soviet physicist Andrei Sakharov proposed three conditions for generating matter-antimatter asymmetry by elementary-particle ineractions:
The first CP-violation effects to be observed were in the decay of neutral kaons, down-strange combinations. More recently, CP-violation effects were observed in the decay of B mesons, bottom-up, bottom-down, and bottom-strange combinations.
World's Largest Atom Smasher May Have Just Found Evidence for Why Our Universe Exists noting Observation of $C\!P$ violation in charm decays - CERN Document Server and CP violation in charm decays with the LHCb experiment (21 March 2019) · Indico
These are about observation of time asymmetry in some decay modes of D0 mesons, charm-up combinations. The anticharm-ordinary-up version decays into some particles about 1/1000 faster than the ordinary-charm-antiup version.
This further adds to observations of CP violation, giving four kinds of decay where it has been observed. An important question is how well can the Standard Model account for these observations, since it has only one weak-interaction CP-violation parameter. But calculating thes rates requires some difficult QCD (strong-interaction) calculations, so it may be hard to get good numbers.
To continue with this sort of observation, we could get results for the other D mesons, charm-down and charm-strange, and we could get evidence of CP violation from the electric dipole moments of electrons, protons, and neutrons. For the neutron, the EDM now has an upper limit that is approaching what some Beyond-Standard-Model theories predict, even if still well above the Standard Model's prediction.
The problem is that the particles of baryonic matter -- electrons, protons, and neutrons, or looking inward, electrons, up quarks, and down quarks -- are particles that have distinct antiparticles. In the very early Universe, it was hot enough to produce pairs of ordinary particles and their antiparticles, and these ordinary particles and antiparticles would then run into each other, annihilating and producing photons and the like. As the Universe expanded, it cooled, eventually becoming too cold to make particle-antiparticle pairs. This did not stop annihilation, however, and most of the particles and their antiparticles disappeared. For every approximate billion antiparticles in the early Universe, there was the same approximate billion ordinary particles with one extra, and all that was left was the extra ones, giving the baryonic matter in the Universe.
Why did this asymmetry happen? It must have been some post-inflation effect, since inflation would have diluted a pre-inflation effect to unobservability. The first clue came from violation of some expected elementary-particle symmetries. There are three that are related to space-time: matter-antimatter (charge, C), reflection in space (parity, P), and reflection in time (time, T) symmetries. There is a very general theorem called the CPT theorem that requires some broadly plausible assumptions. It states that C, P, and T are conserved together, even if not necessarily individually.
The strong interaction, electromagnetism, and gravity all conserve C, P, and T separately, and particle physicists thought that that would be true of the weak interaction also. But they discovered in the 1950's that weak interactions violate parity, and in the early 1960's that weak interactions also violate charge and parity together (CP violation). Meaning that by the CPT theorem, they must violate time symmetry also.
In 1967, Soviet physicist Andrei Sakharov proposed three conditions for generating matter-antimatter asymmetry by elementary-particle ineractions:
- Baryon-number (B) violation
- C and CP violation
- Departure from thermal equilibrium
The first CP-violation effects to be observed were in the decay of neutral kaons, down-strange combinations. More recently, CP-violation effects were observed in the decay of B mesons, bottom-up, bottom-down, and bottom-strange combinations.
World's Largest Atom Smasher May Have Just Found Evidence for Why Our Universe Exists noting Observation of $C\!P$ violation in charm decays - CERN Document Server and CP violation in charm decays with the LHCb experiment (21 March 2019) · Indico
These are about observation of time asymmetry in some decay modes of D0 mesons, charm-up combinations. The anticharm-ordinary-up version decays into some particles about 1/1000 faster than the ordinary-charm-antiup version.
This further adds to observations of CP violation, giving four kinds of decay where it has been observed. An important question is how well can the Standard Model account for these observations, since it has only one weak-interaction CP-violation parameter. But calculating thes rates requires some difficult QCD (strong-interaction) calculations, so it may be hard to get good numbers.
To continue with this sort of observation, we could get results for the other D mesons, charm-down and charm-strange, and we could get evidence of CP violation from the electric dipole moments of electrons, protons, and neutrons. For the neutron, the EDM now has an upper limit that is approaching what some Beyond-Standard-Model theories predict, even if still well above the Standard Model's prediction.
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