The Standard Model of Particle Physics is an incredibly successful theoretical framework that describes the fundamental particles and their interactions, incorporating the electromagnetic, weak, and strong nuclear forces. However, there are several reasons why it is considered incomplete:
Gravity: The Standard Model does not include gravity, which is one of the fundamental forces of nature. General relativity provides a description of gravity in terms of the curvature of spacetime, but it has not been successfully incorporated into the Standard Model. As a result, the Standard Model cannot account for the gravitational interactions between particles.
Dark Matter: Observational evidence suggests that there is a significant amount of matter in the universe that does not interact electromagnetically or strongly with normal matter. This "dark matter" has been inferred from its gravitational effects on visible matter and the large-scale structure of the universe. The Standard Model does not provide a particle candidate for dark matter or explain its properties.
Neutrino Masses: The Standard Model assumes that neutrinos are massless particles. However, experimental observations have confirmed that neutrinos do have non-zero masses, indicating the need for an extension of the Standard Model to accommodate this fact.
Matter-Antimatter Asymmetry: The observed universe contains an abundance of matter but very little antimatter. The Standard Model does not explain why there is an asymmetry between matter and antimatter or why matter prevailed in the early universe. This is known as the matter-antimatter asymmetry problem.
Hierarchy Problem: The Higgs boson, discovered in 2012, helps explain the origin of particle masses. However, it introduces a new problem known as the hierarchy problem. The Higgs mass is sensitive to quantum corrections, leading to a large discrepancy between the Higgs mass scale and the scale at which gravity becomes significant. This calls for an explanation of why the Higgs mass is so much lighter than the Planck scale.
These are some of the key reasons why the Standard Model is considered incomplete. Physicists are actively researching and developing theories beyond the Standard Model, such as supersymmetry, string theory, and various other extensions, in the quest for a more complete understanding of the fundamental laws of nature.