Material against antimontalia

Linas Juozenas

Matter vs. Antimatter: The Imbalance That Allowed Matter to Dominate

One of the deepest mysteries of modern physics and cosmology is why our Universe consists almost entirely of matter, with very little antimatter. According to current understanding, matter and antimatter should have been created in almost equal amounts in the earliest moments after the Big Bang, and so they should have completely annihilated—but they didn't. A small excess of matter (about one part in a billion) survived to form galaxies, stars, planets, and eventually life as we know it. This apparent asymmetry of matter and antimatter is often referred to as the asymmetries of the baryonic Universe term and is closely associated with phenomena called CP violation and barigenesis.

In this article we will discuss:

A brief historical perspective on the discovery of antimatter.
The nature of the imbalance between matter and antimatter.
KP (charge and parity) symmetry and its violation.
Sakharov's conditions for barigenesis.
Hypotheses for the formation of matter-antimatter asymmetry have been proposed (e.g., electroweak barygenesis, leptogenesis).
Ongoing experiments and future directions.

By the end of the article, you will have a general understanding of why we believe there is more matter than antimatter in the Universe, and you will learn how science is trying to determine the exact mechanism that causes this cosmic imbalance.

1. Historical context: the discovery of antimatter

The concept of antimatter was first theoretically predicted by an English physicist Paul Dirac In 1928, Dirac formulated a set of equations (Dirac's equations) describing relativistic electrons. This equation unexpectedly yielded solutions corresponding to particles with positive energy and negative energy. The "negative energy" solutions were later interpreted as particles with the same mass as the electron but an electric charge of the opposite sign.

Discovery of the positron (1932): 1932 American physicist Karl Anderson experimentally confirmed the existence of antimatter by detecting a positron (the antiparticle of the electron) in the traces left by cosmic rays.
Antiproton and antineutron: The antiproton was discovered in 1955 by Emilio Segre and Owen Chamberlain, and the antineutron was discovered in 1956.

These discoveries reinforced the idea that for every type of particle in the Standard Model, there exists an antiparticle with opposite quantum numbers (e.g., electric charge, baryon number), but the same mass and spin.

2. The nature of the matter-antimatter imbalance

2.1 Uniform formation in the early Universe

At the time of the Big Bang, the Universe was extremely hot and dense, so the energy level was high enough for pairs of matter and antimatter particles to form. According to conventional wisdom, for every particle of matter that was created, a corresponding antiparticle should have been created. As the Universe expanded and cooled, these particles and antiparticles should have almost completely annihilated, converting mass into energy (mostly gamma-ray photons).

2.2 Remaining material

However, observations show that the Universe is mostly made of matter. The net disproportion is small, but it was the one that was decisive. This ratio can be quantified by looking at the ratio of the density of baryons (matter) to the density of photons in the Universe, often denoted η = (n_B - n_B) / n_γ. Cosmic Microwave Background (CMB) – obtained from missions such as COBE, WMAP and Planck – the data shows:

η ≈ 6 × 10⁻¹⁰.

This means that for every billion photons left over from the Big Bang, there is about one proton (or neutron) – but the key is that that one baryon outnumbered its corresponding antibaryon. The question arises: How did this tiny but fundamental asymmetry arise?

3. KP symmetry and its violation

3.1 Symmetries in physics

In particle physics K (charge conjugation) symmetry refers to the interchange of particles and their antiparticles. P (Parity) symmetry refers to spatial inverse reflection (to change the sign of the spatial coordinates). If a physical law remains unchanged under both K and P transformations (i.e. "if the image remains the same when particles are replaced by antiparticles and left and right are swapped"), we say that it holds KP symmetries.

3.2 Early detection of CP violation

It was initially thought that KP symmetry might be a fundamental property of nature, especially after the discovery of parity (P) violation alone in the 1950s. However, in 1964 James Cronin and Val Fitch found that neutral kaons (K⁰) breaks the KP symmetry when decaying (Cronin & Fitch, 1964 [1]). This revolutionary result showed that even KP can sometimes be broken in certain weak interaction processes.

3.3 KP violation in the Standard Model

In the Standard Model of Particle Physics, KP violation can arise from phases Kabibo-Kobayashi-Moscow (CKM) in a matrix that describes how quarks of different "flavors" transition into each other under the influence of the weak interaction. Later, in neutrino physics, another term for the mixing matrix appeared - Pontecorvo–Macio–Nakagawa–Sakata (PMNS) matrix, which may also contain CP-violating phases. However, the extent of CP violation observed so far in these sectors is too smallto explain the baryonic asymmetry of the Universe. Therefore, it is believed that there are additional sources of KP violation outside the Standard Model.

4. Sakharov's conditions for barigenesis

1967 Russian physicist Andrei Sakharov formulated three necessary conditions for the asymmetry of matter and antimatter to arise in the early Universe (Sacharov, 1967 [2]):

Baryon number violation: There must be interactions or processes that change the net baryon number B. If the baryon number is strictly conserved, the asymmetry of baryons and antibaryons cannot arise.
K and KP violation: Processes that separate matter and antimatter are necessary. If K and KP were perfectly symmetrical, any process that creates more baryons than antibaryons would have a mirror image that creates the same number of antibaryons, thus "cancelling" any excess.
Deviation from thermal equilibrium: In thermal equilibrium, the processes of particle creation and annihilation occur equally in both directions, thus preserving balance. Non-thermally balanced environments, such as the rapidly expanding and cooling Universe, allow certain processes to "capture" the asymmetry.

Any successful theory or mechanism of barygenesis must satisfy these three conditions to explain the observed imbalance between matter and antimatter.

5. Proposed mechanisms for the formation of matter-antimatter asymmetry

5.1 Electroweak barygenesis

Electroweak barygenesis suggests that the baryonic asymmetry arose around the time of the electroweak phase transition stage (~10⁻¹¹ sec. after the Big Bang). Key aspects:

Higgs field takes on a nonlinear vacuum value and thus spontaneously breaks electroweak symmetry.
Non-perturbative processes, called spheralons, can violate the total number of baryons and leptons (B+L), but preserve the difference between baryons and leptons (B−L).
The phase transition, if it were of first order (i.e., characterized by bubble formation), would create the necessary deviation from thermal equilibrium.
KP-violating interaction processes in the Higgs sector or during quark mixing would contribute to the matter-antimatter imbalance that occurs in the bubbles.

Unfortunately, in the current range of Standard Model parameters (especially with the discovery of the 125 GeV Higgs boson), it is unlikely that the electroweak phase transition stage was of first order. Furthermore, the KP violation provided by the CKM matrix is too small. Therefore, many theorists suggest outside the Standard Model existing physics – such as additional scalar fields – to make electroweak barygenesis more realistic.

5.2 DVT (GUT) barigenesis

Grand Unified Theories (GUT) seeks to unify the strong, weak and electromagnetic interactions at extremely high energy levels (~10¹⁶ GeV). In many DVT models heavy language bosons or Higgs bosons can mediate proton decay or other baryon number-violating processes. If these processes occur in a non-thermal environment in the early Universe, they could in principle generate baryon asymmetry. However, the KP violation in these DVT scenarios requires that they be sufficiently large, and the proton decay predicted by DVT has not yet been detected experimentally at the frequencies expected. This limits simpler models of DVT barygenesis.

5.3 Leptogenesis

Leptogenesis starts with the asymmetry of leptons and antileptons. This asymmetry of leptons later in spheralon processes during the electroweak period is partially converted into baryonic asymmetry, as these processes can convert leptons into baryons. One popular mechanism:

Seesaw mechanism: Heavy right-handed neutrinos (or other heavy leptons) are introduced.
These heavy neutrinos can decay through KP violation, creating lepton sector asymmetry.
Part of the interaction between spheralons is transformed by these leptonic asymmetries into baryonic asymmetries.

Leptogenesis is attractive because it links the origin of neutrino masses (observed in neutrino oscillations) to the cosmic matter-antimatter imbalance. Moreover, it does not have some of the constraints that hinder electroweak barygenesis, and is therefore often cited as one of the key building blocks of new theories of physics.

6. Ongoing experiments and future directions

6.1 High-energy accelerators

Accelerators such as Large Hadron Collider (LHC) – especially the experiment LHCb – may be sensitive to KP violation in the decays of various mesons (B, D, etc.). By measuring the extent of KP violation and comparing it to the predictions of the Standard Model, scientists hope to find discrepancies that could indicate new physics beyond the Standard Model.

LHCb: Specializes in precise studies of rare decays and KP violation in the b-quark sector.
Belle II (KEK in Japan) and already completed BaBar (SLAC) also studied KP violation in B-meson systems.

6.2 Neutrino experiments

Next-generation neutrino oscillation experiments, such as DUNE (Deep Underground Neutrino Experiment) USA and Hyper-Kamiokande in Japan, aims to measure the phase of the KP violation in the PMNS matrix with high precision. If neutrinos show a pronounced KP violation, it would further support the hypothesis of leptogenesis as a solution to the matter-antimatter imbalance.

6.3 Search for proton decay

If the GUT barygenesis scenarios are correct, proton decay could be an important source of clues. Experiments such as Super Kamiokande (and in the future Hyper-Kamiokande) imposes strict limits on the lifetime of the proton for different decay channels. Any discovery of proton decay would be extremely important, as it would provide serious clues about the violation of the baryon number at high energies.

6.4 Search for stocks

Although axions (hypothetical particles involved in solving the strong KP problem) are not directly related to barygenesis in the conventional sense, they could also play a role in the thermal history of the early Universe and determine possible matter-antimatter disproportions. Therefore, the search for axions remains an important part of solving the general puzzle of the Universe.

Conclusion

The cosmic dominance of matter over antimatter remains one of the fundamental open questions in physics. The Standard Model predicts some KP violation, but not enough to explain the observed magnitude of the asymmetry. This discrepancy suggests the need for new physics – either at higher energies (e.g., at the DVT scale), or by introducing additional particles and interactions that we have not yet discovered.

Although electroweak barygenesis, DVT barigenesis and leptogenesis are possible mechanisms, further experimental and theoretical analysis is necessary. High-precision experiments in accelerator physics, neutrino oscillation studies and rare decay studies, and astrophysical observations continue to test these theories. The answer to the question of why matter won over antimatter may not only expand our understanding of the origin of the Universe, but also reveal entirely new aspects of our reality.

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