Hadron
Delving into the heart of matter reveals a bustling world of fundamental particles. Among them are hadrons, the composite particles that make up nearly all the visible mass in our universe, including the protons and neutrons within every atom. These tiny powerhouses are bound by the universe's strongest force, holding secrets to the very stability of existence and offering a window into extreme physics. Hadrons are composite subatomic particles, like miniature molecular systems, predominantly forming the protons and neutrons that give ordinary matter its mass. They are categorized into two main families—baryons (odd number of quarks, like protons) and mesons (even number of quarks, like pions)—based on their quark composition. The vast majority of a hadron's mass comes not from the individual quarks it contains, but from the immense binding energy of the strong nuclear force that holds them together.
AI Summary
Delving into the heart of matter reveals a bustling world of fundamental particles. Among them are hadrons, the composite particles that make up nearly all the visible mass in our universe, including the protons and neutrons within every atom. These tiny powerhouses are bound by the universe's strongest force, holding secrets to the very stability of existence and offering a window into extreme physics.
- Hadrons are composite subatomic particles, like miniature molecular systems, predominantly forming the protons and neutrons that give ordinary matter its mass.
- They are categorized into two main families—baryons (odd number of quarks, like protons) and mesons (even number of quarks, like pions)—based on their quark composition.
- The vast majority of a hadron's mass comes not from the individual quarks it contains, but from the immense binding energy of the strong nuclear force that holds them together.
The Building Blocks of Visible Matter
Imagine zooming in on an atom, past the swirling electrons, right into its dense nucleus. There, you'd find protons and neutrons — the true heavyweights of ordinary matter. These aren't elementary particles themselves; they're composite, meaning they're made of even smaller constituents called quarks, bound together by the universe's most potent grip: the strong nuclear force. We call these composite particles hadrons.
Hadrons, then, are like miniature, extremely dense molecules, but instead of the electromagnetic force, they're governed by the strong force. This force is so incredibly powerful that it confines quarks, preventing them from ever being observed in isolation. It's a fundamental principle of particle physics known as color confinement.
A Name Born at CERN
The term 'hadron' itself is relatively new to the scientific lexicon. It was coined in 1962 by physicist L. B. Okun during a plenary talk at the International Conference on High Energy Physics at CERN. He needed a clear, concise term to distinguish these 'strongly interacting particles' from 'leptons' (like electrons), which interact weakly.
Okun proposed 'hadron' from the Greek word 'ἁδρός' (hadrós), meaning 'large' or 'stout' — a fitting contrast to 'λεπτός' (leptós), meaning 'small' or 'light.' This new terminology proved to be incredibly convenient and quickly became standard in particle physics.
Two Main Families: Baryons and Mesons
Hadrons aren't all alike; they fall into two primary families based on the number of quarks they contain. Think of them as two distinct tribes in the subatomic world.
Baryons: The Heavyweights
First, we have baryons — the heavyweights. These hadrons are made of an odd number of quarks, typically three. The most famous baryons are, of course, the protons and neutrons that make up atomic nuclei.
Because they're made of an odd number of quarks, baryons are classified as fermions. Each quark carries a 'baryon number' of +1/3, meaning a baryon like a proton (three quarks) has a total baryon number of 1. Their antimatter counterparts, antibaryons, are made of antiquarks and have a baryon number of -1.
Mesons: The Messengers
Then there are mesons. These hadrons are typically composed of an even number of quarks — specifically, one quark and one antiquark. Think of them as messengers, often mediating forces within atomic nuclei.
With an even number of constituent particles, mesons are classified as bosons. Since a quark has a baryon number of +1/3 and an antiquark has -1/3, their baryon numbers cancel out, giving mesons a total baryon number of 0. Pions and kaons are common examples of mesons.
The Mystery of Mass
Here's a mind-bending fact: the tiny masses of the quarks inside a hadron only account for a small fraction of the hadron's total mass. So, where does the rest come from? It's all about energy!
According to Einstein's famous equation, mass and energy are interchangeable. Within a hadron, the strong nuclear force is incredibly intense, and the energy associated with this binding force — the gluons that mediate it — contributes the vast majority of the hadron's mass. The quarks are just along for the ride, so to speak.
E = mc^{2}It's a dynamic, ever-changing environment within a hadron. The strong force's energy is so great that it continually spawns fleeting pairs of virtual quarks and antiquarks, which pop into existence and then vanish. It's a quantum foam, with only a slight excess of 'valence' quarks determining the hadron's identity from the outside.
Colorless Particles
While quarks possess a property called 'color charge' (red, green, or blue), hadrons themselves must be 'colorless' or 'white.' This is due to color confinement, ensuring that only color-neutral combinations can exist freely.
For baryons, this means three quarks, each carrying a different 'color' charge (red + green + blue = white). For mesons, it's a quark of one color and an antiquark of its corresponding anti-color (e.g., red + anti-red = white).
Exotic Hadrons: Beyond the Standard
For decades, the simple three-quark baryon and quark-antiquark meson models were considered the norm. But the universe often has surprises in store! In recent years, scientists have discovered 'exotic' hadrons that challenge this conventional understanding.
One such discovery was the Z(4430)−, a tetraquark state — a meson made of four quarks! First identified by the Belle Collaboration in 2007 and confirmed by the LHCb collaboration in 2014, it opened the door to a new world of particle structures.
The LHCb collaboration also announced the discovery of two pentaquark states, P+c(4380) and P+c(4450), in 2015. These are baryons made of five quarks, further expanding our understanding of how quarks can combine. Other exotic candidates, like glueballs (made only of gluons) and hybrid mesons (quarks bound by excited gluons), are also being investigated.
Studying the Subatomic World
How do we learn about these incredibly tiny and fleeting particles? Scientists often collide hadrons, like protons, at extreme speeds in giant particle accelerators like CERN's Large Hadron Collider. By smashing them together, they create a shower of debris, allowing detectors to analyze the resulting particles and infer their properties.
Nature offers its own high-energy experiments too. When cosmic rays — high-energy particles from space — smash into gas particles in Earth's upper atmosphere, they produce hadrons like muons and pions, which then rain down on our planet.
Under extreme conditions of temperature and pressure, hadrons might even 'melt.' At very high energies, the strong interaction weakens — a phenomenon called asymptotic freedom. This means quarks and gluons are no longer confined within hadrons, forming a 'quark-gluon plasma,' a state of matter believed to have existed moments after the Big Bang.
The Curious Case of Stability
Almost all free hadrons are unstable. They decay rapidly into other particles. For instance, a free neutron has a half-life of about 10 minutes before it decays into a proton, an electron, and an antineutrino.
However, there's one incredibly important exception: the proton. Protons appear to be completely stable, or at least have an extraordinarily long lifespan — far longer than the age of the universe. This remarkable stability is crucial for the existence of all atoms, and thus, all matter as we know it.
Article
Hadron
A hadron is a composite subatomic particle. Every hadron must fall into one of the two fundamental classes of particle, bosons and fermions.
In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived from Ancient Greek ἁδρός (hadrós) 'stout, thick'. They are analogous to molecules, which are held together by the electric force. Most of the mass of ordinary matter comes from two hadrons: the proton and the neutron, while most of the mass of the protons and neutrons is in turn due to the binding energy of their constituent quarks, due to the strong force.
Hadrons are categorized into two broad families: baryons, made of an odd number of quarks (usually three), and mesons, made of an even number of quarks (usually two: one quark and one antiquark). Protons and neutrons (which make the majority of the mass of an atom) are examples of baryons; pions are an example of a meson. A tetraquark state (an exotic meson), named the Z(4430)−, was discovered in 2007 by the Belle Collaboration and confirmed as a resonance in 2014 by the LHCb collaboration. Two pentaquark states (exotic baryons), named P+ c(4380) and P+ c(4450), were discovered in 2015 by the LHCb collaboration. There are several other "Exotic" hadron candidates and other colour-singlet quark combinations that may also exist.
Almost all "free" hadrons and antihadrons (meaning, in isolation and not bound within an atomic nucleus) are believed to be unstable and eventually decay into other particles. The only known possible exception is free protons, which appear to be stable, or at least, take immense amounts of time to decay (order of 1034+ years). By way of comparison, free neutrons are the longest-lived unstable particle, and decay with a half-life of about 611 seconds, and have a mean lifetime of 879 seconds (see Free neutron decay).
Hadron physics is studied by colliding hadrons, e.g. protons, with each other or the nuclei of dense, heavy elements, such as lead (Pb) or gold (Au), and detecting the debris in the produced particle showers. A similar process occurs in the natural environment, in the extreme upper-atmosphere, where muons and mesons such as pions are produced by the collisions of cosmic rays with rarefied gas particles in the outer atmosphere.
Terminology and etymology
Hadron
The term "hadron" is a new Greek word introduced by L. B. Okun in a plenary talk at the 1962 International Conference on High Energy Physics at CERN. He opened his talk with the definition of a new category term:
Notwithstanding the fact that this report deals with weak interactions, we shall frequently have to speak of strongly interacting particles. These particles pose not only numerous scientific problems, but also a terminological problem. The point is that "strongly interacting particles" is a very clumsy term which does not yield itself to the formation of an adjective. For this reason, to take but one instance, decays into strongly interacting particles are called "non-leptonic". This definition is not exact because "non-leptonic" may also signify photonic. In this report I shall call strongly interacting particles "hadrons", and the corresponding decays "hadronic" (the Greek ἁδρός signifies "large", "massive", in contrast to λεπτός which means "small", "light"). I hope that this terminology will prove to be convenient. — L. B. Okun (1962)
Properties
Hadron
All types of hadrons have zero total color charge (three examples shown).
According to the quark model, the properties of hadrons are primarily determined by their so-called valence quarks. For example, a proton is composed of two up quarks (each with electric charge ++2/3 e, for a total of +4/3 e together) and one down quark (with electric charge −+1/3 e). Adding these together yields the proton charge of +1 e. Although quarks also carry color charge, hadrons must have zero total color charge because of a phenomenon called color confinement. That is, hadrons must be "colorless" or "white". The simplest ways for this to occur are with a quark of one color and an antiquark of the corresponding anticolor, or three quarks of different colors. Hadrons with the first arrangement are a type of meson, and those with the second arrangement are a type of baryon.
Massless virtual gluons compose the overwhelming majority of particles inside hadrons, as well as the major constituents of its mass (with the exception of the heavy charm and bottom quarks; the top quark vanishes before it has time to bind into a hadron). The strength of the strong-force gluons which bind the quarks together has sufficient energy (E) to have resonances composed of massive (m) quarks (E ≥ mc2). One outcome is that short-lived pairs of virtual quarks and antiquarks are continually forming and vanishing again inside a hadron. Because the virtual quarks are not stable wave packets (quanta), but an irregular and transient phenomenon, it is not meaningful to ask which quark is real and which virtual; only the small excess is apparent from the outside in the form of a hadron. Therefore, when a hadron or anti-hadron is stated to consist of (typically) two or three quarks, this technically refers to the constant excess of quarks versus antiquarks.
Like all subatomic particles, hadrons are assigned quantum numbers corresponding to the representations of the Poincaré group: JPC (m), where J is the spin quantum number, P the intrinsic parity (or P-parity), C the charge conjugation (or C-parity), and m is the particle's mass. Note that the mass of a hadron has very little to do with the mass of its valence quarks; rather, due to mass–energy equivalence, most of the mass comes from the large amount of energy associated with the strong interaction. Hadrons may also carry flavor quantum numbers such as isospin (G-parity), and strangeness. All quarks carry an additive, conserved quantum number called a baryon number (B), which is ++1/3 e for quarks and −+1/3 e for antiquarks. This means that baryons (composite particles made of three, five or a larger odd number of quarks) have B = 1 whereas mesons have B = 0.
Hadrons have excited states known as resonances. Each ground state hadron may have several excited states; several hundred different resonances have been observed in experiments. Resonances decay extremely quickly (within about 10−24 s) via the strong nuclear force.
In other phases of matter the hadrons may disappear. For example, at very high temperature and high pressure, unless there are sufficiently many flavors of quarks, the theory of quantum chromodynamics (QCD) predicts that quarks and gluons will no longer be confined within hadrons, "because the strength of the strong interaction diminishes with energy". This property, which is known as asymptotic freedom, has been experimentally confirmed in the energy range between 1 GeV (gigaelectronvolt) and 1 TeV (teraelectronvolt). All free hadrons except (possibly) the proton and antiproton are unstable.
Baryons
Hadron
Baryons are hadrons containing an odd number of valence quarks (at least 3). Most well-known baryons such as the proton and neutron have three valence quarks, but pentaquarks with five quarks—three quarks of different colors, and also one extra quark-antiquark pair—have also been proven to exist. Because baryons have an odd number of quarks, they are also all fermions, i.e., they have half-integer spin. As quarks possess baryon number B = 1/3, baryons have baryon number B = 1. Pentaquarks also have B = 1, since the extra quark's and antiquark's baryon numbers cancel.
Each type of baryon has a corresponding antiparticle (antibaryon) in which quarks are replaced by their corresponding antiquarks. For example, just as a proton is made of two up quarks and one down quark, its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark.
As of August 2015, there are two known pentaquarks, P+ c(4380) and P+ c(4450), both discovered in 2015 by the LHCb collaboration.
Mesons
Hadron
Mesons are hadrons containing an even number of valence quarks (at least two). Most well known mesons are composed of a quark–antiquark pair, but possible tetraquarks (four quarks) and hexaquarks (six quarks, comprising either a dibaryon or three quark–antiquark pairs) may have been discovered and are being investigated to confirm their nature. Several other hypothetical types of exotic meson may exist which do not fall within the quark model of classification. These include glueballs and hybrid mesons (mesons bound by excited gluons).
Because mesons have an even number of quarks, they are also all bosons, with integer spin, i.e., 0 ħ, +1 ħ, or −1 ħ. They have baryon number B = 1/3 − 1/3 = 0. Examples of mesons commonly produced in particle physics experiments include pions and kaons. Pions also play a role in holding atomic nuclei together via the residual strong force.