Subatomic particle
A subatomic particle is any particle smaller than an atom. Some of them can be cut into smaller pieces. Others, as far as anyone can tell, cannot. The proton and neutron, sitting inside every atomic nucleus, are each built from still smaller things called quarks. The electron is not. It appears to be one of nature's true building blocks, with nothing inside it at all. Light itself joins this strange catalogue. Experiments show that light can behave like a stream of particles, called photons, and also behave like a wave. That double life earned a name, wave-particle duality, and even a playful nickname for these objects, wavicles. The whole picture is held together by a framework physicists call the Standard Model. Yet ask the experts a simple question, what exactly is a particle, and the answers scatter. One definition calls a particle a collapsed wave function. Another calls it an excitation of a quantum field. A third reaches for pure mathematics, an irreducible representation of the Poincaré group. A fourth simply says a particle is an observed thing. Why would the people who study these objects most closely disagree about what they even are? How do you sort an invisible thing by what it is made of, by its mass, by whether it survives at all? And how did a century of experiments, from a single named electron to the Higgs boson, fill in this list one entry at a time?
A proton can be taken apart. Inside it sit two up quarks and one down quark, bound together. The neutron holds the same kinds of pieces in a different mix, two down quarks and one up quark. These are composite particles, made of more than one elementary particle locked together. An electron, by contrast, is elementary. It is not composed of other particles, and there is nothing smaller to find inside it. The Standard Model lists its elementary pieces in tidy families. There are six flavors of quarks, named up, down, strange, charm, bottom, and top. There are six leptons, the electron, the muon, the tau, and a neutrino paired with each of those three. Then come the force carriers, twelve gauge bosons in all. One is the photon of electromagnetism. Three are the W and Z bosons of the weak force. Eight are the gluons of the strong force. Standing apart is a single odd member, the Higgs boson. Every one of these has now been confirmed in an experiment. The list filled in slowly, with the top quark arriving in 1995, the tau neutrino in 2000, and the Higgs boson in 2012. Theories that reach beyond the Standard Model predict more, including an elementary particle of gravity called the graviton. As of 2026, none of those extra particles have been found.
The word hadron comes from Greek, introduced in 1962 by the physicist Lev Okun. It names the composite particles built from quarks bound by gluons, those holding five quarks or fewer counting antiquarks. Quarks refuse to appear on their own. A property called color confinement keeps them locked inside hadrons, never found singly, always clustered together. That rule splits the hadrons into two camps. Baryons hold an odd number of quarks, almost always three, and the proton and neutron, the two nucleons, are the famous examples. Mesons hold an even number, almost always one quark and one antiquark, and the pions and kaons are the ones most often named. Stability is rare here. Except for the proton and the neutron, every other hadron is unstable, decaying into other particles in microseconds or less. The two survivors do something the rest cannot. Protons and neutrons bind into an atomic nucleus, so a helium-4 nucleus is two protons and two neutrons together. A handful of strange exceptions break the quark pattern entirely. Positronium and muonium are composite particles that contain no quarks at all.
Spin sorts every particle into one of two great classes. A boson carries integer spin. A fermion carries odd half-integer spin, and the two behave in opposite ways, since fermions cannot overlap or combine while bosons can. In the Standard Model every elementary fermion has spin one half. Those fermions split again, into the quarks that carry color charge and feel the strong interaction, and the leptons that do not. The elementary bosons gather on the other side. The photon, the W and Z, and the gluons each carry spin one, while the Higgs boson stands alone as the only elementary particle with spin zero. Force carriers like photons and gluons behave unlike the particles with rest mass. They carry quanta of energy but have no rest mass and no fixed diameter. The W and Z bosons break that habit, holding relatively large rest masses of roughly 80 and 90. The hypothetical graviton would need spin two, though it sits outside the Standard Model. Some extensions, such as supersymmetry, predict particles with spin three halves, none discovered as of 2023. Composite particles inherit their spin from their parts, so baryons with three quarks land at spin one half or three halves and count as fermions, while mesons with two quarks take spin zero or one and count as bosons.
Mass and energy are two faces of one quantity, tied together by E = mc2, the energy of a particle at rest equal to its mass times the speed of light squared. The very names of the particle families began as a ranking by weight. Baryon means heavy. Meson means intermediate. Lepton means lightweight. The order mostly holds, baryons heavier than mesons, mesons heavier than leptons, but it bends. The tau, the heaviest lepton, outweighs the two lightest baryons, the nucleons. When these names were coined in the 1950s, they referred to masses and nothing more. The quark model changed their meaning. After it gained acceptance in the 1970s, baryons became composites of three quarks, mesons composites of a quark and an antiquark, and leptons the elementary fermions with no color charge. All composite particles are massive, and any particle carrying electric charge is massive too. The reverse case is just as strict. Every massless particle is elementary, including the photon and the gluon, though the gluon can never be pulled out and isolated on its own.
Most subatomic particles do not last. All leptons and all baryons decay, by either the strong force or the weak force, with the proton as the lone holdout. The muon and the tau, along with their antiparticles, fall apart through the weak force. The proton sits in a category of its own. Protons are not known to decay, yet whether they are truly stable remains unknown, because some important Grand Unified Theories actually require that they eventually do. Neutrinos play by different rules. Neutrinos and antineutrinos do not decay, though a related effect, neutrino oscillations, is thought to occur even in a vacuum. The electron and its antiparticle, the positron, are held stable in theory by charge conservation, safe unless some lighter particle carrying the elementary charge exists, which is thought unlikely. Charge itself draws a hard line through all of this. Every observable subatomic particle carries an electric charge that is a whole-number multiple of the elementary charge. Quarks break that rule, holding non-integer charges, but color confinement hides the violation, since inside any baryon or meson the quark charges add back up to a whole-number multiple of the elementary charge.
G. Johnstone Stoney named the electron in 1891, years before anyone had caught one. J. J. Thomson identified it in 1897, making it the first subatomic particle ever found, the minimum unit of electrical charge. From there the discoveries came in a long parade of names and dates. Ernest Rutherford ran through the story again and again, proving with Thomas Royds in 1907 that alpha particles are helium nuclei, work that won him the Nobel Prize for Chemistry in 1908, then identifying the proton around 1919. James Chadwick found the neutron in 1932, the same year Carl D. Anderson confirmed antiparticles that Paul Dirac had predicted in 1928. The theorists often led the way. Hideki Yukawa predicted the pion in 1935 to explain the nuclear force, and César Lattes, Giuseppe Occhialini, and Cecil Powell found it in 1947. Wolfgang Pauli proposed the neutrino in 1930, Enrico Fermi gave it its name, and Clyde Cowan and Frederick Reines detected it in 1956. Murray Gell-Mann and George Zweig proposed quarks in 1964. The roll call of confirmations stretched on, the tau found by Martin Lewis Perl in 1975, the W and Z bosons at CERN in 1983, the top quark at Fermilab in 1995, and the Higgs boson, predicted by Peter Higgs in 1964, confirmed at CERN in 2012.
Interactions between particles have been scrutinized for centuries, and a few simple laws underpin them all. The laws of conservation of energy and conservation of momentum allow calculations across scales that run from stars down to quarks. They are the basics of Newtonian mechanics, set out in Philosophiae Naturalis Principia Mathematica, first published in 1687. The uncertainty principle marks a sharper limit. Some properties of a particle, taken together, such as its position and its momentum at the same moment, simply cannot be measured exactly. Wave nature, meanwhile, reaches further than anyone first expected. Through the work of Albert Einstein, Satyendra Nath Bose, Louis de Broglie, and others, current theory holds that all particles have a wave nature, verified not only for elementary particles but for atoms and even molecules. In principle it applies to macroscopic objects too, though their wavelengths are far too small to detect. Several entries on the list of particles remain unsettled. The tetraquark and the pentaquark are proposed new classes of hadrons, with a candidate event labeled Zc(3900) in 2013 not yet confirmed. The graviton, the magnetic monopole that Paul Dirac proposed in 1931, and the predicted particles of supersymmetry all wait in the same condition, theorized and undiscovered, the next blank lines in a list that has been filling in for over a century.
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Common questions
What is a subatomic particle in physics?
A subatomic particle is a particle smaller than an atom. According to the Standard Model of particle physics, it can be either a composite particle made of other particles, such as a proton or neutron, or an elementary particle that is not composed of other particles, such as an electron.
What is the difference between composite and elementary subatomic particles?
A composite particle, such as a proton or a neutron, is composed of other particles bound together, while an elementary particle, such as an electron, is not composed of anything smaller. Protons and neutrons are built from quarks, but the electron is one of the Standard Model's elementary particles.
What is the difference between bosons and fermions?
Bosons have integer spin, while fermions have odd half-integer spin. Force carriers such as photons and gluons are bosons and lack rest mass, whereas fermions like quarks and leptons have rest mass and cannot overlap or combine.
What are hadrons and who named them?
Hadrons are composite particles containing five or fewer quarks bound together by gluons. The word hadron comes from Greek and was introduced in 1962 by the physicist Lev Okun, and hadrons divide into baryons with an odd number of quarks and mesons with an even number.
When was the Higgs boson discovered?
The Higgs boson was confirmed at CERN in 2012. It had been predicted by Peter Higgs in 1964 and is the only known elementary particle with spin zero.
What was the first subatomic particle to be identified?
The electron was the first subatomic particle to be identified, by J. J. Thomson in 1897. G. Johnstone Stoney had suggested the name electron in 1891, defining it as the minimum unit of electrical charge.
Which subatomic particles are stable and which decay?
Except for the proton and the neutron, nearly all hadrons are unstable and decay in microseconds or less. Protons are not known to decay, neutrinos and antineutrinos do not decay, and the electron and positron are theoretically stable due to charge conservation.