A hadron (pronounced /hɑːdɹɒn/, from Greek ἁδρός, hadros, thick), in particle physics, is any strongly interacting composite subatomic particle. Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them A subatomic particle is an elementary or composite Particle smaller than an Atom. All hadrons are composed of quarks. In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. Hadrons are divided into two classes:

• Baryons, strongly interacting fermions such as a neutron or a proton, made up of three quarks. Baryons are the family of Subatomic particles with a Baryon number of 1 In Particle physics, fermions are particles which obey Fermi-Dirac statistics; they are named after Enrico Fermi.
• Mesons, strongly interacting bosons consisting of a quark and an antiquark. In Particle physics, a meson is a strongly interacting Boson &mdashthat is a Hadron with integer spin. In Particle physics, bosons are particles which obey Bose-Einstein statistics; they are named after Satyendra Nath Bose and Albert Einstein

Notice that mesons are composite bosons, but they are not composed of bosons (quarks are fermions).

Like all subatomic particles, hadrons have quantum numbers corresponding to the representations of the Poincaré group: J^PC(m), where J is the spin, P, the parity, C, the C parity, and m, the mass. A subatomic particle is an elementary or composite Particle smaller than an Atom. Quantum numbers describe values of conserved numbers in the dynamics of the Quantum system. In the mathematical field of Representation theory, group representations describe abstract groups in terms of Linear transformations of In Physics and Mathematics, the Poincaré group, named after Henri Poincaré, is the group of isometries of Minkowski spacetime In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin In Physics, C parity or charge parity is a Multiplicative quantum number of some particles that describes its behavior under a symmetry operation of In addition they may carry flavour quantum numbers such as isospin (or G parity), strangeness etc. In Particle physics, flavour or flavor (see spelling differences) is a Quantum number of Elementary particles related to their In Physics, and specifically Particle physics, isospin ( isotopic spin, isobaric spin) is a Quantum number related to the In Theoretical physics, G-parity is a Multiplicative quantum number that results from the generalization of C-parity to Multiplets of particles Moreover,

• Baryons always carry an additive conserved quantum number called baryon number (B). Baryons are the family of Subatomic particles with a Baryon number of 1 In Particle physics, the baryon number is an approximate conserved Quantum number of a system B=1 for nucleons (the proton and the neutron), which are part of the atomic nucleus. In Physics a nucleon is a collective name for two Baryons the Neutron and the Proton. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom
• Mesons have B=0. In Particle physics, a meson is a strongly interacting Boson &mdashthat is a Hadron with integer spin.

Most hadrons can be classified by the quark model which posits that all the quantum numbers are derived from those of the valence quarks (the quarks which form the hadron). In Physics, the quark model is a classification scheme for Hadrons in terms of their valence quarks, i In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. For instance, since each quark has B=1/3, each baryon, composed of three quarks, has B=1.

Excited baryon or meson states are known as resonances. In Physics, resonance is the tendency of a system to Oscillate at maximum Amplitude at certain frequencies, known as the system's Each ground state hadron may have many excited states, and hundreds have been observed in particle experiments. Resonances decay extremely quickly (within about 10⁻²⁴ s) via strong interactions.

Mesons which lie outside the quark model classification are called exotic mesons. Non- Quark model mesons include exotic mesons, which have Quantum numbers not possible for mesons in the Quark model; Glueballs These include glueballs, hybrid mesons and tetraquarks. In Particle physics, a glueball is a strongly interacting particle containing no valence Quarks It is composed entirely of Gluons Such a state In Particle physics a tetraquark is a hypothetical Meson composed of four valence Quarks. The only baryons which lie outside the quark model at present are the pentaquarks, but evidence for their existence is unclear as of 2006. A pentaquark is an hypothetical subatomic particle consisting of a group of five Quarks (compared to three quarks in normal Baryons and two in Mesons, or Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar.

All hadrons are single particle excitations of the basic theory of strong interactions, called quantum chromodynamics. In particle physics the strong interaction, or strong force, or color force, holds Quarks and Gluons together to form Protons and Quantum chromodynamics (abbreviated as QCD is a theory of the Strong interaction ( color force a Fundamental force describing the interactions of the Due to a property called confinement that this theory enjoys at energies below the QCD scale, these excitations are not quarks and gluons, which are the basic fields, but the hadrons which are composite, and carry no color charge. Color confinement, often called just confinement, is the Physics phenomenon that Color charged particles (such as Quarks cannot be isolated singularly In Physics, a coupling constant, usually denoted g, is a number that determines the strength of an Interaction. In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. Gluons ( Glue and the suffix -on) are Elementary particles that cause Quarks to interact and are indirectly responsible for the In Particle physics, color charge is a property of Quarks and Gluons which are related to their Strong interactions in the context of Quantum

In other phases of QCD matter the hadrons may disappear. Quark matter or QCD matter (see QCD) refers to any of a number of theorized phases of matter whose degrees of freedom include Quarks and Gluons For example, at very high temperature and high pressure, unless there are sufficiently many flavors of quarks, QCD predicts that quarks and gluons will interact weakly and in particular no longer be confined. This property, which is known as asymptotic freedom, has been experimentally confirmed at the energy scales between a GeV and a TeV. In Physics, asymptotic freedom is the property of some gauge theories in which the interaction between the particles such as Quarks, becomes arbitrarily

See also

• Large Hadron Collider (LHC)
• Subatomic particles: list of particles, leptons
• Quantum chromodynamics, quark model, confinement
• Quark star
• QCD matter

References and external links

• The Particle Data Group[1] maintains listings of properties of all known particles. A subatomic particle is an elementary or composite Particle smaller than an Atom. This is a list of the different types of particles known and hypothesized Leptons are a family of fundamental Subatomic particles comprising the Electron, the Muon, and the Tauon (or tau particle as well as their Quantum chromodynamics (abbreviated as QCD is a theory of the Strong interaction ( color force a Fundamental force describing the interactions of the In Physics, the quark model is a classification scheme for Hadrons in terms of their valence quarks, i Color confinement, often called just confinement, is the Physics phenomenon that Color charged particles (such as Quarks cannot be isolated singularly A quark star or strange star is a hypothetical type of Exotic star composed of Quark matter, or Strange matter. Quark matter or QCD matter (see QCD) refers to any of a number of theorized phases of matter whose degrees of freedom include Quarks and Gluons The Particle Data Group is an international collaboration of particle Physicists that compiles and reanalyzes published results related to the properties of particles

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