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. 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 protons and neutrons make up the majority of an atom's mass, most of that mass actually resides in the binding energy of their constituent quarks due to the strong force.
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. Experimental confirmations began decades later when a tetraquark state named the Z(4430) was discovered in 2007 by the Belle Collaboration. This exotic meson was confirmed as a resonance in 2014 by the LHCb collaboration. Two pentaquark states were discovered in 2015 by the same LHCb team. These discoveries expanded the known boundaries of what constitutes a standard hadron family.
Hadrons are categorized into two broad families based on their valence quark composition. Baryons contain an odd number of valence quarks, usually three. Most well-known baryons such as the proton and neutron have exactly three valence quarks. Because they possess an odd number of quarks, all baryons are fermions with half-integer spin. Mesons contain an even number of valence quarks, usually two: one quark and one antiquark. Since mesons have an even number of quarks, they are all bosons with integer spin. Examples of mesons commonly produced in particle physics experiments include pions and kaons.
Almost all free hadrons and antihadrons are believed to be unstable and eventually decay into other particles. Hadrons have excited states known as resonances that decay extremely quickly within about 10^-23 seconds via the strong nuclear force. Several hundred different resonances have been observed in experiments. Beyond standard configurations, exotic hadron candidates exist including tetraquarks and hexaquarks. A tetraquark state named the Z(4430) was discovered in 2007 by the Belle Collaboration. Two pentaquark states were discovered in 2015 by the LHCb collaboration. These multi-quark configurations challenge traditional classification models.
Massless virtual gluons compose the overwhelming majority of particles inside hadrons. They also form the major constituents of its mass except for 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 to create resonances composed of massive quarks. One outcome is that short-lived pairs of virtual quarks and antiquarks are continually forming and vanishing again inside a hadron. When a hadron or anti-hadron is stated to consist of two or three quarks, this technically refers to the constant excess of quarks versus antiquarks.
Hadron physics is studied by colliding hadrons with each other or the nuclei of dense, heavy elements such as lead or gold. Detectors observe the debris in the produced particle showers. A similar process occurs naturally in the extreme upper-atmosphere where muons and mesons such as pions are produced by collisions of cosmic rays with rarefied gas particles. Free neutrons decay with a half-life of about 611 seconds and have a mean lifetime of 879 seconds. This natural decay provides data on unstable particle behavior without requiring artificial accelerators.
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Common questions
What is a hadron in particle physics?
A hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Every hadron must fall into one of the two fundamental classes of particle, bosons and fermions.
When was the term hadron introduced to science?
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. Experimental confirmations began decades later when a tetraquark state named the Z(4430) was discovered in 2007 by the Belle Collaboration.
How are baryons and mesons classified within the hadron family?
Baryons contain an odd number of valence quarks, usually three, while mesons contain an even number of valence quarks, usually two: one quark and one antiquark. Because they possess an odd number of quarks, all baryons are fermions with half-integer spin, whereas mesons are all bosons with integer spin.
What happened to the Z(4430) tetraquark state after its discovery?
This exotic meson was confirmed as a resonance in 2014 by the LHCb collaboration. Two pentaquark states were discovered in 2015 by the same LHCb team.
Why do most free hadrons decay so quickly?
Hadrons have excited states known as resonances that decay extremely quickly within about 10^-23 seconds via the strong nuclear force. Almost all free hadrons and antihadrons are believed to be unstable and eventually decay into other particles.