Rho meson

Rho meson

Statistics	Bosonic
Family	Mesons
Interactions	Strong, Weak, Gravitational and Electromagnetic
Symbol	ρ+ , ρ0 , and ρ−
Antiparticle	ρ+ : ρ− ρ0 : self
Types	3
Mass	~770 MeV/c2
Mean lifetime	~4.5×10−24 s
Decays into	ρ± : π± + π0 ρ0 : π+ + π−
Electric charge	ρ± : ±1 e ρ0 : 0 e
Color charge	0
Spin	1
Isospin	ρ± : ±1 ρ0 : 0
Hypercharge	0
Parity	−1
C parity	−1

In particle physics, a rho meson is a short-lived hadronic particle that is an isospin triplet whose three states are denoted as
ρ⁺
,
ρ⁰
and
ρ⁻
. Along with pions and omega mesons, the rho meson carries the nuclear force within the atomic nucleus. After the pions and kaons, the rho mesons are the lightest strongly interacting particle, with a mass of 775.45±0.04 MeV for all three states.

The rho mesons have a very short lifetime and their decay width is about 145 MeV with the peculiar feature that the decay widths are not described by a Breit–Wigner form. The principal decay route of the rho mesons is to a pair of pions with a branching rate of 99.9%.

History

After several false starts, the ρ meson and the ω meson were discovered at Lawrence Berkeley Laboratory in 1961.

Composition

The rho mesons can be interpreted as a bound state of a quark and an anti-quark and is an excited version of the pion. Unlike the pion, the rho meson has spin j = 1 (a vector meson) and a much higher value of the mass. This mass difference between the pions and rho mesons is attributed to a large hyperfine interaction between the quark and anti-quark. The main objection with the De Rujula–Georgi–Glashow description is that it attributes the lightness of the pions as an accident rather than a result of chiral symmetry breaking.

The rho mesons can be thought of as the gauge bosons of a spontaneously broken gauge symmetry whose local character is emergent (arising from QCD); Note that this broken gauge symmetry (sometimes called hidden local symmetry) is distinct from the global chiral symmetry acting on the flavors. This was described by Howard Georgi in a paper titled "The Vector Limit of Chiral Symmetry" where he ascribed much of the literature of hidden local symmetry to a non-linear sigma model.

Rho mesons

Particle name	Particle symbol	Antiparticle symbol	Quark content	Mass (MeV/c2)	IG	JPC	S	C	B'	Mean lifetime (s)	Commonly decays to (>5% of decays)
Charged rho meson	ρ+ (770)	ρ− (770)	u d	775.11±0.34	1+	1−	0	0	0	(4.415±0.024)×10−24	π± + π0
Neutral rho meson	ρ0 (770)	Self	$\mathrm {\tfrac {u{\bar {u}}-d{\bar {d}}}{\sqrt {2}}} \,$	775.26±0.25	1+	1−−	0	0	0	(4.453±0.027)×10−24	π+ + π−