Hadron

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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[edit]

See also the articles Quark and Standard Model

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[edit]