In 2024, our understanding of the fundamental particles as outlined in the Standard Model remains limited, with eight significant questions still awaiting answers.

Throughout history, our comprehension of the universe has evolved dramatically. Initially, we believed in a purely classical and deterministic nature, but the discovery of quantum physics and the wave behavior of light revealed a more intricate reality. We once thought atoms were the basic building blocks of matter; however, we've since recognized that they are merely constructs made up of more fundamental particles. The existence of the Standard Model and the quantum field theories governing these particles leads us to question whether we truly grasp the entirety of the universe’s constituents.

As scientists pursue the search for new fundamental particles to address numerous perplexing mysteries, including:

• The enigma of dark matter
• The matter-antimatter imbalance
• The dark energy conundrum
• The hierarchy dilemma
• The origins of the universe

It is clear that the field of physics is still rich in unexplored territory, despite the multitude of known particles. Here are eight critical questions that highlight the gaps in our knowledge.

1.) Does the proton decay?

According to the Standard Model, protons are considered stable. While various decay pathways exist that conserve energy, momentum, and electric charge, they all violate baryon number conservation. Protons are the lightest stable baryons, and no known decay pathways exist for them. Experiments suggest that protons have a lifetime exceeding approximately 10³? years, which is vastly longer than the universe's age.

However, the mystery persists regarding how the universe achieved a matter-antimatter asymmetry. While the Standard Model allows for certain baryon number-violating interactions, if we can extend the proton's lifetime, we may eliminate many grand unified theories and potentially discover insights into baryogenesis as we enhance our detection capabilities.

2.) Can neutrinoless double beta decay occur?

Atomic nuclei, composed of protons and neutrons, can be stable or unstable. Unstable nuclei can undergo various decay processes, such as alpha decay, beta decay, and gamma decay. One notable process is double beta decay, where two neutrons decay simultaneously.

If neutrinos are their own antiparticles (Majorana particles), neutrinoless double beta decay might occur, resulting in the emission of only protons and electrons. This hypothetical process is still under investigation, as its discovery would indicate new physics beyond the Standard Model.

3.) Do glueballs exist?

Currently, the only known particles bound by the strong nuclear force are made up of quarks or antiquarks. Theoretical predictions suggest that glueballs, bound states of gluons, could exist. Recent observations of the X(2370) particle suggest it might correspond to the lightest glueball state, offering an exciting opportunity to probe the validity of quantum chromodynamics.

4.) Do any forces unify beyond the electroweak scale?

Currently, four fundamental forces are recognized: gravitational, strong nuclear, electromagnetic, and weak interactions. The electroweak theory proposes that at high energies, the electromagnetic and weak forces unify, suggesting the possibility of further unification with the strong force or even gravity at even higher energies. Discoveries related to grand unified theories could expand our understanding of fundamental interactions.

5.) How does the electroweak symmetry break?

Symmetry breaking can lead to the emergence of Goldstone bosons and various phase transitions. The electroweak symmetry breaking process could involve a first-order phase transition, indicating the potential existence of additional Higgs-like particles, which could have implications for our understanding of matter-antimatter asymmetry.

6.) How heavy is each species of neutrino?

Neutrinos were once thought to be massless, but current understanding acknowledges their mass and oscillation between different flavors. Experiments aim to measure the mass differences and total neutrino mass, yet the exact masses of the three neutrino types remain elusive.

7.) Can any Standard Model particles be “cracked open?”

While we have identified all Standard Model particles and their properties, we have yet to determine if they are truly fundamental or composed of smaller entities. There is a possibility that particles like quarks and leptons could be composite, but this remains an open question as we explore higher energy scales.

8.) Are inertial and gravitational mass always equal?

Experiments suggest a strong correlation between inertial and gravitational mass, supporting Einstein's equivalence principle. However, further testing is necessary to ascertain if this equivalence holds under all conditions, especially for exotic particles.

In conclusion, while much has been learned, many questions remain unanswered in particle physics. The search for deeper understanding continues as researchers explore the complexities of the universe, and we must be cautious in assuming we know all there is to know about fundamental particles.

Starts With A Bang is authored by Ethan Siegel, Ph.D., known for works including Beyond The Galaxy and Treknology. New titles, such as the Encyclopaedia Cosmologica, are forthcoming!