Particle Physics
Particle physics investigates the most fundamental constituents of matter. It goes beyond molecules and atoms, delving into what lies beneath: quarks, electrons, neutrinos, photons, and other elementary particles.
Elementary subatomic particles share a remarkable property: they are absolutely identical to one another.
For example, every electron in the universe is exactly the same - there are no large or small electrons, no old or new ones.
This fundamental symmetry sets particle physics apart from disciplines that study macroscopic objects.
Note. Unlike machines or biological cells, elementary particles have no internal structure and are indistinguishable from their identical counterparts. They are flawless by nature. This simplifies their study: once we understand the properties of a single electron, we understand all electrons. For instance, if we observe the decay of a muon, we can reproduce the process thousands of times in the lab, and the outcome will always be the same. This reproducibility allows physicists to uncover the physical laws governing these processes.
At subatomic scales, particles cannot be manipulated directly or touched with physical tools.
To study them, physicists rely on three main approaches:
- Scattering: particles are collided, and the resulting trajectories reveal structural information.
- Decay: the analysis of the byproducts when a particle spontaneously disintegrates.
- Bound states: composite systems such as protons and neutrons are studied to understand the interactions among their constituents.
Classical physics cannot adequately describe phenomena involving elementary particles.
While classical mechanics successfully explains gravity and forces between macroscopic objects, it falls short at the quantum scale.
To understand particle behavior, two foundational theories are essential:
- Quantum mechanics, which introduces probabilistic behavior and governs the dynamics of particles at very small scales.
- Special relativity, which describes how objects behave at speeds approaching the speed of light.
Because elementary particles are both incredibly small and often travel at near-light speeds, both theories are required simultaneously.
Their synthesis gave rise to quantum field theory, the theoretical framework at the heart of modern particle physics.
Quantum field theory describes the behavior of subatomic particles moving at relativistic speeds in extremely small domains.
The Standard Model (SM)
Today, all known fundamental interactions between particles - except gravity - are described by the Standard Model (SM).
This framework is built around a unifying principle known as gauge invariance.
Note. The Higgs boson, predicted by the Standard Model, was experimentally confirmed in 2012. Without it, particles would remain massless. Its discovery validated a key aspect of the theory.
The Standard Model: a triumph of modern physics - yet incomplete
Despite its extraordinary precision and consistency with virtually all experimental results, the Standard Model has well-known limitations.
It successfully describes three of the four fundamental forces - electromagnetic, weak, and strong - but it excludes gravity, offers no explanation for dark matter or dark energy, and doesn’t account for the origin of neutrino masses.
In this sense, the Standard Model resembles a detailed map that only covers part of the territory: it accurately portrays what we know, but leaves vast regions of the universe unexplored.
The goal of theoretical physics today is precisely this: to go beyond the Standard Model.
Many physicists believe the Standard Model is merely the low-energy limit of a deeper, more comprehensive framework - often referred to as beyond Standard Model physics or simply new physics.
History
In the 5th century BCE, Leucippus and Democritus first proposed that matter is made of indivisible particles called atomos, a Greek term meaning “uncuttable.”
At the time, the atom was not a physical entity, but a philosophical construct intended to explain change and permanence in nature. For nearly two millennia, the atom remained a purely speculative idea.
It wasn’t until the 19th century, with the rise of modern chemistry, that the atom was treated as a real, indivisible unit of matter.
The fall of indivisibility
In 1897, J.J. Thomson discovered the electron, a negatively charged particle present in all atoms.
His work with cathode rays revealed that atoms had internal structure and were not truly indivisible.
The electron was soon recognized as the carrier of electric current in metals.
The atomic nucleus and subatomic particles
In 1911, Ernest Rutherford’s gold foil experiment showed that most of an atom’s mass is concentrated in a tiny positive nucleus, with electrons orbiting around it.
The proton was identified as the particle responsible for the nucleus’s positive charge.
The discovery of the neutron
In 1932, James Chadwick discovered the neutron - an electrically neutral particle with mass - solving the problem of the atom’s missing mass.
For example, a helium nucleus contains 2 protons and 2 neutrons.
Antimatter and the rise of particle physics
In the 1930s, Carl Anderson observed a particle identical to the electron but with opposite charge: the positron.
This marked the first experimental detection of antimatter.
It was soon discovered that when a positron meets an electron, they annihilate, producing two gamma photons.
Soon afterward, scientists hypothesized the existence of antiprotons and antineutrons.
The quark hypothesis
By the 1960s, evidence began to emerge suggesting that protons and neutrons were not fundamental, but made of smaller particles called quarks.
Key evidence came from deep inelastic scattering experiments conducted at SLAC.
The birth of the Standard Model
In the 1970s, elementary particles were grouped into two broad categories:
- Fermions (matter): quarks and leptons (such as the electron and neutrino)
- Bosons (force carriers): photon (electromagnetic force), gluon (strong force), and W and Z bosons (weak force)
The Higgs boson
On July 4, 2012, the ATLAS and CMS experiments at CERN announced the discovery of the Higgs boson, with a mass of approximately 125 GeV/c².
This particle explains how other particles acquire mass via the Higgs mechanism.
Beyond the Standard Model
Despite its immense success, the Standard Model leaves many questions unanswered:
- It doesn’t incorporate gravity
- It doesn’t explain why there are three families of particles
- It fails to account for dark matter and dark energy
- It requires arbitrary parameters, such as particle masses, to be manually inserted
Today, various theoretical proposals aim to extend the Standard Model’s framework.
For instance, in supersymmetry (SUSY), every known particle has a corresponding "superpartner." The hypothetical "graviton" is proposed as the force carrier for gravity. String theory suggests that particles are actually vibrational modes of one-dimensional strings. Other models propose even more fundamental constituents, known as "preons," which may underlie quarks and leptons.
However, none of these speculative models have yet been confirmed by experimental evidence.
The ultimate structure of matter remains an open question. We do not yet know whether deeper layers of matter exist - or whether we are already looking at the fundamental building blocks of the universe.
The search continues.
Particle Detectors
Detecting particles relies on their interactions with the medium they travel through - be it vapor, gas, liquid, or another substance. Charged particles leave ionizing tracks, while neutral ones must be inferred indirectly through secondary interactions.
To identify elementary particles, physicists use dedicated systems known as particle detectors, which make it possible to observe and record the passage of subatomic particles.
Detection works by converting a particle’s interaction with a medium into a measurable physical signal - such as light, electric charge, or condensed vapor.
How do they work?
Most detectors operate on the principle of ionization: when a charged particle passes through a material, it interacts with atoms and knocks electrons out of their outer shells.
This process generates positive ions and free electrons, which can then be harnessed to produce a detectable signal.
For example, cloud chambers use supersaturated alcohol vapor as the medium. The ions formed by ionization act as condensation nuclei, creating visible trails of droplets. Bubble chambers rely on a superheated liquid (typically liquid hydrogen), where ionization triggers the formation of bubble tracks. In spark chambers, a gas-filled region between electrodes serves as the medium, and ionization initiates electrical discharges - visible as sparks - between the electrodes, outlining the particle’s path.
That said, ionization-based detectors have a fundamental limitation: they only respond to charged particles. Neutral particles go undetected unless they interact in other ways.
Neutral particles (such as neutrons and neutrinos) do not ionize the medium directly and therefore leave no trace in detectors that rely solely on ionization.
To detect them, physicists rely on indirect methods - looking for the byproducts of their interactions with other particles.
For example, neutrons can be detected via elastic collisions with light nuclei (like hydrogen), which produce recoil protons that are visible. Neutrinos are observed indirectly by analyzing secondary events such as muon decay or electron emission. In a bubble chamber, for instance, a neutrino interacting with a proton might produce a muon, which leaves a visible trail.
In addition to traditional techniques, modern detectors often feature integrated systems composed of multiple sub-detectors, each optimized for a specific measurement or type of particle.
These advanced setups combine various technologies to capture every aspect of a particle’s passage. The resulting signals are synchronized and processed by computers, which reconstruct the full event and map the particles’ trajectories.
Such integrated detectors can identify both charged particles - via direct ionization - and neutral particles - through the secondary effects they generate during interactions.
Sources of Elementary Particles
Several types of sources are used to study elementary particles:
- Protons and electrons
Protons can be readily obtained by ionizing a hydrogen atom (H), while electrons are typically produced through thermionic emission (by heating metals), the photoelectric effect, or field emission under strong electric fields. - Decay processes
In a decay process, a particle spontaneously disintegrates, emitting one or more elementary particles - such as photons or neutrinos - as part of the transformation. - Cosmic rays
Cosmic rays are natural high-energy sources of particles, consisting primarily of protons, but also including helium nuclei, heavier nuclei, electrons, positrons, and antiprotons. These particles originate from outer space and, upon interacting with Earth’s atmosphere, produce extensive showers of secondary particles - including muons, pions, and neutrinos. Despite their usefulness, cosmic rays are unpredictable and the flux reaching a particle detector is extremely low. - Nuclear sources
Nuclear reactors emit large quantities of electron neutrinos, as well as neutrons, and alpha, beta (electrons or positrons), and gamma (photons) radiation. These sources are particularly useful in controlled experiments involving weak interactions or neutron behavior. - Particle accelerators
Particle accelerators generate both primary particles (such as protons and electrons) and secondary particles (such as pions, muons, kaons, and antiprotons) through high-energy collisions - including head-on collisions at facilities like the LHC and LEP - or via lower-energy interactions like scattering. Scattering refers to the process in which two particles collide and interact, changing direction, exchanging energy, or producing new particles. Accelerators are especially valuable for investigating short-lived or rare particles under highly controlled and repeatable conditions. As such, they remain one of the most powerful tools available for probing the subatomic world.
