Are there particles we haven't discovered yet? | Sean M. Carroll

Key Concepts:

• Standard Model of Particle Physics: The prevailing theoretical framework describing the fundamental particles and forces.
• Up Quarks & Down Quarks: Elementary particles that are the constituents of protons and neutrons.
• Electrons: Fundamental subatomic particles with a negative electric charge, orbiting the nucleus of an atom.
• Strong Nuclear Force: The strongest of the four fundamental forces, responsible for binding quarks together to form hadrons (like protons and neutrons) and for holding atomic nuclei together.
• Electromagnetism: One of the four fundamental forces, responsible for interactions between electrically charged particles and for phenomena like light, electricity, and magnetism.
• Higgs Boson: A fundamental particle associated with the Higgs field, which is theorized to give mass to other fundamental particles.
• Gravity: One of the four fundamental forces, responsible for the attraction between objects with mass or energy.
• Dark Matter: A hypothetical form of matter that does not interact with electromagnetic radiation (and thus cannot be seen directly) but whose presence is inferred from its gravitational effects on visible matter.
• Particle Accelerators: Devices that use electromagnetic fields to propel charged particles to very high speeds and energies, often for collision experiments to study fundamental particles.
• Ordinary Matter: Matter composed of protons, neutrons, and electrons, which makes up all visible objects in the universe.

Composition of Ordinary Matter and Fundamental Forces

The vast majority of what constitutes "you" and all ordinary matter is explained by a limited set of fundamental ingredients and forces. Specifically, ordinary matter is made out of up quarks, down quarks, and electrons. These components are held together by the strong nuclear force (which binds quarks into protons and neutrons, and nuclei together) and electromagnetism (which binds electrons to nuclei and atoms into molecules). This understanding alone accounts for "99.99% of understanding you." While crucial for a complete picture, other details like particle masses, the Higgs boson, and gravity are also important, though gravity's direct role in holding a person together is less dominant than the nuclear and electromagnetic forces.

The Enigma of Dark Matter

Particle physicists frequently hypothesize the existence of new fields and particles yet to be discovered. The most compelling direct evidence for such unknown particles is the presence of dark matter in the universe. Dark matter is essential to explain the observed gravitational effects in galaxies and the universe, which cannot be accounted for by ordinary matter alone.

While we know "how much dark matter there is and where it is" based on its gravitational influence, "we don't know what kind of particle it is." A key characteristic of dark matter is that "it doesn't interact with you and your body" or ordinary matter. This non-interaction is inferred because if dark matter particles could interact with the constituents of ordinary matter (electrons, protons, and neutrons), they would be detectable and producible in laboratory experiments.

Particle Physics Methodology and Challenges in Discovery

Particle physicists employ a direct experimental approach to search for new particles: they "smash other particles together" in particle accelerators and meticulously observe the resulting products. This method is a cornerstone of particle physics research.

However, despite these extensive efforts, "we haven't found anything by doing that that is not already in the standard model of particle physics." There are two primary hypotheses for why new, unknown particles (like dark matter) have not yet been detected in accelerators:

1. High Energy/Mass Requirement: These particles might be "too energetic to make in our best particle accelerators" because they are "too massive," requiring an immense amount of energy to create.
2. Feeble Interaction: Alternatively, they might "only interact with ordinary matter so feebly, so weakly" that even if they are created in collisions, their interactions are too rare or subtle to be detected with current technologies.

Conclusion: The Enduring Nature of Ordinary Matter

Regardless of the ongoing search for new particles and fields, the fundamental composition of "you" and all ordinary matter remains consistent. It is made of electrons, protons, and neutrons (which are themselves composed of up and down quarks) interacting through the fundamental forces of electromagnetism, gravity, and the nuclear forces (strong and weak).