Hadron

A hadron (pronounced hay-dron, and based upon the Greek hadr=strong) is a composite particle subject to the strong nuclear force. Although the defining feature is their strong force interactions, they also may be subject to weak and electromagnetic forces.

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 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.

In general, hadrons are particles made up of quarks, antiquarks, and gluons. Their strong interactions are described by quantum chromodynamics. 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. Some other examples appear in the tables below.

Some baryons

Particle name	Symbol	Quark structure	Q (e)	Baryon number B	Strangeness S	Spin	Isospin I	Mass (relative)	Lifetime (s)
Proton	p	$uud$	1	1	0	1/2	1/2	1	Stable
Neutron	n	$udd$	0	1	0	1/2	1/2	1	898
Lambda zero	Λ0	$uds$	0	1	-1	1/2	0	1.2	2.6 × 10-10
Omega (hadron)	Ω−	$sss$	-1	1	−3	1/2	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 (relative)	Lifetime (s)
Pion zero	π0	$\left(u{\overline {u}}-d{\overline {d}}\right)/{\sqrt {2}}$	0	0	0	0	1	0.14	0.8 × 10-16
Pion plus	π+	$u{\overline {d}}$	1	0	0	0	1	0.14	2.6 × 10-8
Kaon plus	K+	$u{\overline {s}}$	1	0	1	0	1/2	0.52	1.2 × 10-8
Kaon zero	K0	$d{\overline {s}}$	0	0	1	0	1/2	0.53	8.9 × 10-11