Particle physics
Particle physics, often called high-energy physics, delves into the most fundamental constituents of our universe — the tiny particles that make up everything around us, and the forces that govern their interactions. It’s a quest to understand the very building blocks of matter and energy, from the smallest flicker of light to the colossal forces holding stars together. Join us as we explore the invisible symphony that composes reality. The Standard Model is our most successful theory, classifying all known fundamental particles and the forces that bind them, acting as the universe's instruction manual. Matter particles, called fermions, and force-carrying particles, known as bosons, represent the two major families that construct and interact within our cosmos. Despite its triumphs, the Standard Model remains incomplete, leaving profound mysteries like gravity's role, dark matter, and dark energy as frontiers for future discovery.
AI Summary
Particle physics, often called high-energy physics, delves into the most fundamental constituents of our universe — the tiny particles that make up everything around us, and the forces that govern their interactions. It’s a quest to understand the very building blocks of matter and energy, from the smallest flicker of light to the colossal forces holding stars together. Join us as we explore the invisible symphony that composes reality.
- The Standard Model is our most successful theory, classifying all known fundamental particles and the forces that bind them, acting as the universe's instruction manual.
- Matter particles, called fermions, and force-carrying particles, known as bosons, represent the two major families that construct and interact within our cosmos.
- Despite its triumphs, the Standard Model remains incomplete, leaving profound mysteries like gravity's role, dark matter, and dark energy as frontiers for future discovery.
Unveiling the Universe's Deepest Secrets
Particle physics is essentially the ultimate detective story — scientists are trying to figure out what the universe is made of at its most basic level, and how all those tiny pieces interact. It's not just about what's inside atoms, but what's inside those things, and even what exists beyond ordinary matter. We call it high-energy physics because often, to peer into these minuscule realms, we need enormous amounts of energy.
At the heart of our current understanding is the Standard Model, a comprehensive theory that describes almost all known fundamental particles and forces. Think of it as the universe's instruction manual, explaining how everything from light to radioactivity works. It's been incredibly successful, but it's not the final answer.
The Two Families of Fundamental Particles
The Standard Model organizes all these fundamental particles into two main families: fermions and bosons. Fermions are the 'matter particles' — they're the building blocks, like quarks and electrons, that give substance to everything. Bosons, on the other hand, are the 'force-carrying particles' — they mediate interactions, allowing particles to 'talk' to each other, like photons carrying the electromagnetic force.
Let's start with the fermions that make up ordinary matter. These include 'up' and 'down' quarks, which combine to form protons and neutrons. We also have electrons and electron neutrinos. Fascinatingly, quarks are never found alone; they're always bound together by the strong force into composite particles called hadrons.
Hadrons come in two main types: baryons and mesons. Baryons are made of three quarks — like our familiar protons and neutrons, which form the nuclei of atoms. Mesons, however, are made of a quark and an antiquark. Most mesons are highly unstable, vanishing in mere fractions of a microsecond after being created, often in high-energy particle collisions.
Antimatter: The Mirror Universe
For almost every particle, there's an antimatter counterpart. An antiparticle has the same mass but opposite charge and other quantum numbers. For example, the electron, with its negative charge, has an antimatter twin called the positron, which carries a positive charge. If a particle meets its antiparticle, they annihilate each other in a burst of energy, converting their mass into other particles.
This process of annihilation perfectly illustrates Einstein's famous mass-energy equivalence. When matter and antimatter collide, their combined mass is converted directly into energy, often in the form of photons or other particles. It's a dramatic demonstration of how mass and energy are two sides of the same cosmic coin.
E = mc^{2}Some particles, like the photon, are their own antiparticles. The concept of antimatter isn't just theoretical; positrons, for instance, are routinely used in medical imaging techniques like PET scans.
Particles as Quantum Field Excitations
In the quantum world, particles aren't just tiny billiard balls; they're seen as excitations or ripples in invisible quantum fields that permeate all of space. Imagine a vast ocean: a particle is like a wave or a ripple traveling across its surface. These fields also dictate how particles interact with each other, forming the intricate dance of forces we observe.
The Interplay of Theory and Experiment
Particle physics thrives on a dynamic relationship between theory and experiment. Theorists develop mathematical models and make predictions, while experimentalists design and build powerful machines like particle accelerators to test those predictions. The discovery of the Higgs boson, for example, was first theorized decades before it was experimentally confirmed at the Large Hadron Collider in 2012, showcasing this powerful synergy.
A Brief History of the Infinitesimal
The idea that everything is made of fundamental, indivisible particles is ancient, dating back to Greek philosophers like Democritus. For centuries, the 'atom' — from the Greek 'atomos' meaning indivisible — was considered the ultimate building block. But the 19th and early 20th centuries began to chip away at this notion.
John Dalton's work on elements solidified the atomic theory, but soon after, discoveries like the electron revealed that atoms themselves weren't fundamental. Pioneering work in nuclear physics showed atoms had internal structure and could be split, leading to pivotal moments like Lise Meitner's explanation of nuclear fission. These discoveries, including Hans Bethe's work on nuclear fusion, ushered in a new era of understanding matter and energy.
The mid-20th century saw a dizzying array of new particles discovered in high-energy collisions, earning the period the nickname 'the particle zoo.' Physicists were overwhelmed by the sheer number of seemingly unrelated entities. This era also brought puzzling observations like CP violation, hinting at a fundamental asymmetry between matter and antimatter.
It wasn't until the formulation of the Standard Model in the 1970s that clarity emerged. This grand theory explained that most of the 'particle zoo' were not fundamental particles at all, but composite structures made from a much smaller set of truly elementary particles. This reclassification marked the dawn of modern particle physics as we know it.
Inside the Standard Model
The Standard Model is a beautifully elegant framework that describes the strong, weak, and electromagnetic fundamental interactions, all mediated by special particles called 'gauge bosons.' These include eight gluons for the strong force, the W and Z bosons for the weak force, and the photon for the electromagnetic force.
In total, the Standard Model accounts for 61 elementary particles. Beyond the force carriers, there are 24 fundamental fermions — 12 matter particles and their 12 corresponding antiparticles. And let's not forget the Higgs boson, which was the last piece of the Standard Model puzzle to be confirmed, giving mass to other particles via the Higgs mechanism.
The Unfinished Symphony
Despite its phenomenal success in explaining almost every experimental observation to date, the Standard Model isn't a 'theory of everything.' It has notable limitations. For instance, it doesn't incorporate gravity, nor does it explain the existence of dark matter and dark energy, which constitute the vast majority of the universe's mass and energy. The discovery of neutrino mass also presented the first experimental deviation from the original Standard Model.
A Closer Look at Subatomic Particles
Modern particle physics focuses intently on subatomic particles, those smaller than atoms. This includes not just electrons, protons, and neutrons, but also fleeting particles produced in high-energy events like photons, neutrinos, and muons. All observed particles and their interactions fit almost perfectly within the Standard Model's embrace.
These particles don't behave like tiny solid spheres; their dynamics are governed by quantum mechanics. They exhibit a fascinating 'wave-particle duality,' meaning they can act like particles in some experiments and like waves in others. In the language of physics, they are described by quantum state vectors within a Hilbert space, an abstract mathematical construct.
Type Generations Antiparticle Status Number of 'Colors' Total Quarks 3 (pairs) Yes 3 36 Leptons 3 (pairs) Yes None 12 Bosons (Force Carriers) Various Mostly Own Varies 12 Higgs Boson None Own None 1 Total known elementary particles: 61 Note: The 'Total' column for Quarks and Leptons includes their antiparticles and color variations where applicable, leading to 36 for quarks (3 generations x 2 types x 3 colors x 2 (particle/antiparticle)) and 12 for leptons (3 generations x 2 types x 2 (particle/antiparticle)). Bosons are counted based on specific types (8 gluons, 3 W/Z, 1 photon). This table reflects the 61 elementary particles of the Standard Model, including particles and antiparticles where distinct, and taking into account color charge for quarks and gluons. Some particles like the photon and gluon are their own antiparticles.
Quarks: The Colorfully Confined Builders
Ordinary matter, like you and me, is primarily built from the first generation of quarks (up and down) and leptons (electrons and electron neutrinos). Quarks carry a unique property called 'color charge'—arbitrarily named red, green, or blue—which governs their powerful interactions. Unlike electric charge, color charge is always 'neutralized' or 'confined' within composite particles; you can never find a free quark.
There are three generations of quarks: up/down, strange/charm, and top/bottom. Similarly, there are three generations of leptons: the electron and its neutrino, the muon and its neutrino, and the tau and its neutrino. While the first generation forms stable matter, the heavier second and third generations are unstable and quickly decay into lighter particles.
Bosons: The Universe's Communicators
Bosons are the force carriers, mediating the fundamental interactions that shape the cosmos. The photon carries the electromagnetic force, responsible for light, electricity, and magnetism. The W and Z bosons mediate the weak force, which drives radioactive decay and nuclear fusion in stars. The gluon carries the strong force, binding quarks together inside protons and neutrons.
Then there's the Higgs boson, a particle with zero spin that interacts with other particles to give them mass. This interaction occurs through the 'Higgs mechanism,' a field that permeates space. Particles that interact more strongly with the Higgs field acquire more mass, while particles like photons, which don't interact with it, remain massless.
Antiparticles and Their Notation
The universe doesn't just have particles; it has their antimatter twins. For instance, an electron (e-) has a negative charge, while its antiparticle, the positron (e+), has a positive charge. Particles with zero charge, like the electron neutrino (\nue), have antiparticles denoted with a bar over their symbol, like the electron antineutrino (\bar{\nu}e). When they meet, particle and antiparticle annihilate, transforming into pure energy.
The Intricate Dance of Hadrons
Protons and neutrons, the building blocks of atomic nuclei, are perfect examples of composite particles called baryons. A proton consists of two up quarks and one down quark, while a neutron has one up quark and two down quarks. These quarks are held together by gluons, exchanging 'color charges' in a complex interplay described by Quantum Chromodynamics, or QCD.
The rule for these composite particles is 'color confinement': the combined color charge of the quarks and gluons inside a hadron must always be neutral, or 'white.' This is why we never observe isolated quarks. Beyond protons and neutrons, physicists have discovered exotic hadrons, like tetraquarks (four quarks) and pentaquarks (five quarks), pushing the boundaries of our understanding of matter.
Hypothetical Particles: Beyond the Known
The Standard Model is brilliant, but incomplete. Physicists are constantly proposing hypothetical particles to address its shortcomings. Perhaps the most famous is the graviton, the theoretical force-carrying particle for gravity, which remains elusive. Other proposed particles include supersymmetric partners to known particles, which could help solve fundamental problems like the hierarchy problem, or axions, which might explain dark matter.
Exploring the Subatomic World: Labs and Theories
Experimental particle physics takes place in some of the most awe-inspiring laboratories on Earth. These facilities, like CERN on the Franco-Swiss border or Fermilab in the United States, house colossal particle accelerators. These machines smash particles together at nearly the speed of light, recreating conditions from the early universe to uncover new particles and observe their interactions.
For instance, CERN's Large Hadron Collider (LHC) is the world's most powerful particle accelerator, colliding protons and heavy ions to explore phenomena never before seen. Other labs, like Brookhaven National Laboratory with its Relativistic Heavy Ion Collider (RHIC), focus on colliding heavy ions to study the quark-gluon plasma, a state of matter thought to have existed just microseconds after the Big Bang.
Article
Particle physics
Particle physics or high-energy physics is the study of fundamental particles and forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the scale of protons and neutrons, while the study of combinations of protons and neutrons is called nuclear physics.
The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) and bosons (force-carrying particles). There are three generations of fermions, although ordinary matter is made only from the first fermion generation. The first generation consists of up and down quarks which form protons and neutrons, and electrons and electron neutrinos. The three fundamental interactions known to be mediated by bosons are electromagnetism, the weak interaction, and the strong interaction.
Quarks form hadrons, but cannot exist on their own. Hadrons that contain an odd number of quarks are called baryons and those that contain an even number are called mesons. Two baryons, the proton and the neutron, make up most of the mass of ordinary matter. Mesons are unstable and the longest-lived last for only a few hundredths of a microsecond. They occur after collisions between particles made of quarks, such as fast-moving protons and neutrons in cosmic rays. Mesons are also produced in cyclotrons or other particle accelerators.
Particles have corresponding antiparticles with the same mass but with opposite electric charges. For example, the antiparticle of the electron is the positron. The electron has a negative electric charge, the positron has a positive charge. These antiparticles can theoretically form a corresponding form of matter called antimatter. Some particles, such as the photon, are their own antiparticle.
These elementary particles are excitations of the quantum fields that also govern their interactions. The dominant theory explaining these fundamental particles and fields, along with their dynamics, is called the Standard Model. The reconciliation of gravity to the current particle physics theory is not solved; many theories have addressed this problem, such as loop quantum gravity, string theory and supersymmetry theory.
Experimental particle physics is the study of these particles in radioactive processes and in particle accelerators such as the Large Hadron Collider. Theoretical particle physics is the study of these particles in the context of cosmology and quantum theory. The two are closely interrelated: the Higgs boson was postulated theoretically before being confirmed by experiments.
History
Particle physics
The Geiger–Marsden experiments observed that a small fraction of the alpha particles experienced strong deflection when being struck by the gold foil.
The idea that all matter is fundamentally composed of elementary particles dates from at least the 6th century BC. In the 19th century, John Dalton, through his work on stoichiometry, concluded that each element of nature was composed of a single, unique type of particle. The word atom, after the Greek word atomos meaning "indivisible", has since then denoted the smallest particle of a chemical element, but physicists later discovered that atoms are not, in fact, the fundamental particles of nature, but are conglomerates of even smaller particles, such as the electron. The early 20th century explorations of nuclear physics and quantum physics led to proofs of nuclear fission in 1939 by Lise Meitner (based on experiments by Otto Hahn), and nuclear fusion by Hans Bethe in that same year; both discoveries also led to the development of nuclear weapons. Bethe's 1947 calculation of the Lamb shift is credited with having "opened the way to the modern era of particle physics".
Throughout the 1950s and 1960s, a bewildering variety of particles was found in collisions of particles from beams of increasingly high energy. It was referred to informally as the "particle zoo". Important discoveries such as the CP violation by James Cronin and Val Fitch brought new questions to matter-antimatter imbalance. After the formulation of the Standard Model during the 1970s, physicists clarified the origin of the particle zoo. The large number of particles was explained as combinations of a (relatively) small number of more fundamental particles and framed in the context of quantum field theories. This reclassification marked the beginning of modern particle physics.
Standard Model
Particle physics
The current state of the classification of all elementary particles is explained by the Standard Model, which gained widespread acceptance in the mid-1970s after experimental confirmation of the existence of quarks. It describes the strong, weak, and electromagnetic fundamental interactions, using mediating gauge bosons. The species of gauge bosons are eight gluons, W− , W+ and Z bosons, and the photon. The Standard Model also contains 24 fundamental fermions (12 particles and their associated anti-particles), which are the constituents of all matter. Finally, the Standard Model also predicted the existence of a type of boson known as the Higgs boson. On 4 July 2012, physicists with the Large Hadron Collider at CERN announced they had found a new particle that behaves similarly to what is expected from the Higgs boson.
The Standard Model, as currently formulated, has 61 elementary particles. Those elementary particles can combine to form composite particles, accounting for the hundreds of other species of particles that have been discovered since the 1960s. The Standard Model has been found to agree with almost all the experimental tests conducted to date. However, most particle physicists believe that it is an incomplete description of nature and that a more fundamental theory awaits discovery (See Theory of Everything). In recent years, measurements of neutrino mass have provided the first experimental deviations from the Standard Model, since neutrinos do not have mass in the Standard Model.
Subatomic particles
Particle physics
<table><thead><tr><th></th><th>Types</th><th>Generations</th><th>Antiparticle</th><th>Colours</th><th>Total</th></tr></thead><tbody><tr><td>Quarks</td><td>2</td><td>3</td><td>Pair</td><td>3</td><td>36</td></tr><tr><td>Leptons</td><td>Pair</td><td>None</td><td>12</td></tr><tr><td>Gluons</td><td>1</td><td>None</td><td>Own</td><td>8</td><td>8</td></tr><tr><td>Photon</td><td>Own</td><td>None</td><td>1</td></tr><tr><td>Z Boson</td><td>Own</td><td>1</td></tr><tr><td>W Boson</td><td>Pair</td><td>2</td></tr><tr><td>Higgs</td><td>Own</td><td>1</td></tr><tr><td>Total number of (known) elementary particles:</td><td>61</td></tr></tbody></table>
Modern particle physics research is focused on subatomic particles, including atomic constituents, such as electrons, protons, and neutrons (protons and neutrons are composite particles called baryons, made of quarks), that are produced by radioactive and scattering processes; such particles are photons, neutrinos, and muons, as well as a wide range of exotic particles. All particles and their interactions observed to date can be described almost entirely by the Standard Model.
Dynamics of particles are also governed by quantum mechanics; they exhibit wave–particle duality, displaying particle-like behaviour under certain experimental conditions and wave-like behaviour in others. In more technical terms, they are described by quantum state vectors in a Hilbert space, which is also treated in quantum field theory. Following the convention of particle physicists, the term elementary particles is applied to those particles that are, according to current understanding, presumed to be indivisible and not composed of other particles.
Quarks and leptons
A Feynman diagram of the β− decay, showing a neutron (n, udd) converted into a proton (p, udu). "u" and "d" are the up and down quarks, "e− " is the electron, and "ν e" is the electron antineutrino.
Ordinary matter is made from first-generation quarks (up, down) and leptons (electron, electron neutrino). Collectively, quarks and leptons are called fermions. They have a quantum spin of half-integers (−1/2, 1/2, 3/2, etc.) and obey the Pauli exclusion principle, where no two particles may occupy the same quantum state. Quarks have fractional elementary electric charge (−1/3 or 2/3) and leptons have whole-numbered electric charge (0 or -1). Quarks also have color charge, which is labeled arbitrarily with no correlation to actual light color as red, green and blue. Because the interactions between the quarks store energy which can convert to other particles when the quarks are far apart enough, quarks cannot be observed independently. This is called color confinement.
There are three known generations of quarks (up and down, strange and charm, top and bottom) and leptons (electron and its neutrino, muon and its neutrino, tau and its neutrino), with strong indirect evidence that a fourth generation of fermions does not exist.
Bosons
Bosons are the mediators or carriers of fundamental interactions, such as electromagnetism, the weak interaction, and the strong interaction. Electromagnetism is mediated by the photon, the quanta of light. The weak interaction is mediated by the W and Z bosons. The strong interaction is mediated by the gluon, which can link quarks together to form composite particles. Due to the aforementioned color confinement, gluons are never observed independently. The Higgs boson gives mass to the W and Z bosons via the Higgs mechanism – the gluon and photon are expected to be massless. All bosons have an integer quantum spin (0 and 1) and can have the same quantum state.
Antiparticles and color charge
Most aforementioned particles have corresponding antiparticles, which compose antimatter. Normal particles have positive lepton or baryon number, and antiparticles have these numbers negative. Most properties of corresponding antiparticles and particles are the same, with a few gets reversed; the electron's antiparticle, positron, has an opposite charge. To differentiate between antiparticles and particles, a plus or negative sign is added in superscript. For example, the electron and the positron are denoted e− and e+ , respectively. However, in the case that the particle has a charge of 0 (equal to that of the antiparticle), the antiparticle is denoted with a line above the symbol. As such, an electron neutrino is ν e, whereas its antineutrino is ν e. When a particle and an antiparticle interact with each other, they are annihilated and convert to other particles. Some particles, such as the photon or gluon, have no antiparticles.
Quarks and gluons additionally have color charges, which influences the strong interaction. Quark's color charges are called red, green and blue (though the particle itself have no physical color), and in antiquarks are called antired, antigreen and antiblue. The gluon can have eight color charges, which are the result of quarks' interactions to form composite particles (gauge symmetry SU(3)).
Composite
A proton consists of two up quarks and one down quark, linked together by gluons. The quarks' color charge are also visible.
The neutrons and protons in the atomic nuclei are baryons – the neutron is composed of two down quarks and one up quark, and the proton is composed of two up quarks and one down quark. A baryon is composed of three quarks, and a meson is composed of two quarks (one normal, one anti). Baryons and mesons are collectively called hadrons. Quarks inside hadrons are governed by the strong interaction, thus are subjected to quantum chromodynamics (color charges). The bounded quarks must have their color charge to be neutral, or "white" for analogy with mixing the primary colors. More exotic hadrons can have other types, arrangement or number of quarks (tetraquark, pentaquark).
An atom is made from protons, neutrons and electrons. By modifying the particles inside a normal atom, exotic atoms can be formed. A simple example would be the hydrogen-4.1, which has one of its electrons replaced with a muon.
Hypothetical
The graviton is a hypothetical particle that can mediate the gravitational interaction, but it has not been detected or completely reconciled with current theories. Many other hypothetical particles have been proposed to address the limitations of the Standard Model. Notably, supersymmetric particles aim to solve the hierarchy problem, axions address the strong CP problem, and various other particles are proposed to explain the origins of dark matter and dark energy.
Experimental laboratories
Particle physics
Fermi National Accelerator Laboratory, USA
The world's major particle physics laboratories are:
• Brookhaven National Laboratory (Long Island, New York, United States). Its main facility is the Relativistic Heavy Ion Collider (RHIC), which collides heavy ions such as gold ions and polarized protons. It is the world's first heavy ion collider, and the world's only polarized proton collider. • Budker Institute of Nuclear Physics (Novosibirsk, Russia). Its main projects are now the electron-positron colliders VEPP-2000, operated since 2006, and VEPP-4, started experiments in 1994. Earlier facilities include the first electron–electron beam–beam collider VEP-1, which conducted experiments from 1964 to 1968; the electron-positron colliders VEPP-2, operated from 1965 to 1974; and, its successor VEPP-2M, performed experiments from 1974 to 2000. • CERN (European Organization for Nuclear Research) (Franco-Swiss border, near Geneva, Switzerland). Its main project is now the Large Hadron Collider (LHC), which had its first beam circulation on 10 September 2008, and is now the world's most energetic collider of protons. It also became the most energetic collider of heavy ions after it began colliding lead ions. Earlier facilities include the Large Electron–Positron Collider (LEP), which was stopped on 2 November 2000 and then dismantled to give way for LHC; and the Super Proton Synchrotron, which is being reused as a pre-accelerator for the LHC and for fixed-target experiments. • DESY (Deutsches Elektronen-Synchrotron) (Hamburg, Germany). Its main facility was the Hadron Elektron Ring Anlage (HERA), which collided electrons and positrons with protons. The accelerator complex is now focused on the production of synchrotron radiation with PETRA III, FLASH and the European XFEL. • Fermi National Accelerator Laboratory (Fermilab) (Batavia, Illinois, United States). Its main facility until 2011 was the Tevatron, which collided protons and antiprotons and was the highest-energy particle collider on earth until the Large Hadron Collider surpassed it on 29 November 2009. • Institute of High Energy Physics (IHEP) (Beijing, China). IHEP manages a number of China's major particle physics facilities, including the Beijing Electron–Positron Collider II(BEPC II), the Beijing Spectrometer (BES), the Beijing Synchrotron Radiation Facility (BSRF), the International Cosmic-Ray Observatory at Yangbajing in Tibet, the Daya Bay Reactor Neutrino Experiment, the China Spallation Neutron Source, the Hard X-ray Modulation Telescope (HXMT), and the Accelerator-driven Sub-critical System (ADS) as well as the Jiangmen Underground Neutrino Observatory (JUNO). • KEK (Tsukuba, Japan). It is the home of a number of experiments such as the K2K experiment and its successor T2K experiment, a neutrino oscillation experiment and Belle II, an experiment measuring the CP violation of B mesons. • SLAC National Accelerator Laboratory (Menlo Park, California, United States). Its 2-mile-long linear particle accelerator began operating in 1962 and was the basis for numerous electron and positron collision experiments until 2008. Since then the linear accelerator is being used for the Linac Coherent Light Source X-ray laser as well as advanced accelerator design research. SLAC staff continue to participate in developing and building many particle detectors around the world.
Theory
Particle physics
Theoretical particle physics attempts to develop the models, theoretical framework, and mathematical tools to understand current experiments and make predictions for future experiments (see also theoretical physics). There are several major interrelated efforts being made in theoretical particle physics today.
One important branch attempts to better understand the Standard Model and its tests. Theorists make quantitative predictions of observables at collider and astronomical experiments, which along with experimental measurements is used to extract the parameters of the Standard Model with less uncertainty. This work probes the limits of the Standard Model and therefore expands scientific understanding of nature's building blocks. Those efforts are made challenging by the difficulty of calculating high precision quantities in quantum chromodynamics. Some theorists working in this area use the tools of perturbative quantum field theory and effective field theory, referring to themselves as phenomenologists. Others make use of lattice field theory and call themselves lattice theorists.
Another major effort is in model building where model builders develop ideas for what physics may lie beyond the Standard Model (at higher energies or smaller distances). This work is often motivated by the hierarchy problem and is constrained by existing experimental data. It may involve work on supersymmetry, alternatives to the Higgs mechanism, extra spatial dimensions (such as the Randall–Sundrum models), Preon theory, combinations of these, or other ideas. Vanishing-dimensions theory is a particle physics theory suggesting that systems with higher energy have a smaller number of dimensions.
A third major effort in theoretical particle physics is string theory. String theorists attempt to construct a unified description of quantum mechanics and general relativity by building a theory based on small strings, and branes rather than particles. If the theory is successful, it may be considered a "Theory of Everything", or "TOE".
There are other areas of work in theoretical particle physics ranging from particle cosmology to loop quantum gravity.
Practical applications
Particle physics
In principle, all physics (and practical applications developed therefrom) can be derived from the study of fundamental particles. In practice, even if "particle physics" is taken to mean only "high-energy atom smashers", many technologies have been developed during these pioneering investigations that later find wide uses in society. Particle accelerators are used to produce medical isotopes for research and treatment (for example, isotopes used in PET imaging), or used directly in external beam radiotherapy. The development of superconductors has been pushed forward by their use in particle physics. The World Wide Web and touchscreen technology were initially developed at CERN. Additional applications are found in medicine, national security, industry, computing, science, and workforce development, illustrating a long and growing list of beneficial practical applications with contributions from particle physics.
Future
Particle physics
Major efforts to look for physics beyond the Standard Model include the Future Circular Collider proposed for CERN and the Particle Physics Project Prioritization Panel (P5) in the US that will update the 2014 P5 study that recommended the Deep Underground Neutrino Experiment, among other experiments.