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A hadron (pronounced both as had-ron and as hay-dron[1]) is a composite particle made up of quarks bound together by the strong force. Although the defining feature is their binding by strong force interactions, they also may be subject to weak and electromagnetic forces.

All hadrons are colorless, or white, consisting of either a color/anticolor quark combination (the mesons) or a triple combination of quarks with all three quark colors: red, green and blue (the baryons).[2] Consequently, their external exertion of strong forces (also called color forces) is limited, and long-range color forces are not observed.[3]

## Types

The hadrons fall into two groups: the baryons with baryon number B=±1 and spin an odd multiple of 1/2, and mesons with B=0 and integer spin. The hadrons are classified further according to their strong isospin I and its component I3, and also by their strangeness S. The combination Y = B + S is called the strong hypercharge. Under strong interactions, B, I, I3, S are conserved, but conservation of S is violated by the weak force interactions.[4]

In general, hadrons are particles made up of quarks, antiquarks, and gluons. Their strong interactions are described by quantum chromodynamics.[1] The meson is a quark and an antiquark paired in various ways, the proton is made up of two up quarks and one down quark, and a neutron is made up of two down quarks and one up quark.[5][6] Some other examples appear in the tables below. Symbols u, d, s refer to up, down and strange quarks, and overlines indicate antiparticles. The unified atomic unit of mass is used, the Dalton, symbol u, with a value of 931.494 061(21) MeV.[7]

Some baryons
Particle name Symbol Quark
structure
Q (e) Baryon
number B
Strangeness S Spin Isospin I Mass (u) Lifetime (s)
Proton p ${\displaystyle uud}$ 1 1 0 1/2 1/2 1 Stable
Neutron n ${\displaystyle udd}$ 0 1 0 1/2 1/2 1 898
Lambda zero Λ0 ${\displaystyle uds}$ 0 1 -1 1/2 0 1.2 2.6 × 10-10
Omega minus Ω ${\displaystyle sss}$ -1 1 −3 3/2 0 1.8 0.8 × 10-10

Some mesons
Particle name Symbol Quark
structure
Q (e) Baryon
number B
Strangeness S Spin Isospin I Mass (u) Lifetime (s)
Pion zero π0 ${\displaystyle \left(u{\overline {u}}-d{\overline {d}}\right)/{\sqrt {2}}}$ 0 0 0 0 1 0.14 0.8 × 10-16
Pion plus π+ ${\displaystyle u{\overline {d}}}$ 1 0 0 0 1 0.14 2.6 × 10-8
Kaon plus K+ ${\displaystyle u{\overline {s}}}$ 1 0 1 0 1/2 0.52 1.2 × 10-8
Kaon zero K0 ${\displaystyle d{\overline {s}}}$ 0 0 1 0 1/2 0.53 8.9 × 10-11

The properties of many hadrons can be found in the listings maintained by the Particle Data Group.[8]

## References

1. Leonard Susskind (2005). The cosmic landscape: string theory and the illusion of intelligent design. Hachette Digital, Inc., p. 159. ISBN 0316155799.
2. Sadri Hassani (2010). From Atoms to Galaxies: A Conceptual Physics Approach to Scientific Awareness. CRC Press, p. 527. ISBN 143980849X.
3. Christopher G. Tully (2011). “Chapter 4: Hadrons”, Elementary Particle Physics in a Nutshell. Princeton University Press, pp. 124 ff. ISBN 0691131163.
4. Walter Greiner, Berndt Müller (2000). “§6.1 The world of hadrons”, Gauge theory of weak interactions. Springer, pp. 205 ff. ISBN 3540676724.
5. Don Lincoln (2004). Understanding the universe: from quarks to the cosmos. World Scientific, p. 115. ISBN 9812387056.
6. Helen R. Quinn, Yossi Nir (2010). The Mystery of the Missing Antimatter. Princeton University Press, p. 249. ISBN 1400835712.
7. Fundamental physical constants: atomic mass unit-electron volt relationship (1 u)c02. The NIST reference on constants, units, and uncertainty. NIST. Retrieved on 2011-09-04.
8. K Nakamura et al. (Cut-off date for this update was January 15, 2011). Particle listings. PDG: Particle Data Group. Retrieved on 2011-09-04.