Hyperon

In particle physics, a hyperon is any baryon containing a strange quark, but no charm quarks or bottom quarks.

Properties and behavior of hyperons

A combination of three u, d or s-quarks with a total spin of 3/2 form the so-called baryon decuplet. The lower six are hyperons.

Being baryons, all hyperons are fermions. That is, they have half-integer spin and obey Fermi-Dirac statistics. They all interact via the strong nuclear force, making them types of hadron. They are composed of three light quarks, at least one of which is a strange quark, which makes them strange baryons. Hyperons decay weakly with non-conserved parity.

Types of hyperons

File:Hyperon.png

There are three Sigma hyperons,
Σ⁺
,
Σ⁰
and
Σ⁻
. They have rest energies of ~1190 MeV and lifetimes of ~1×10⁻¹⁰ s with the exception of
Σ⁰
whose lifetime is shorter than 10×10⁻¹⁴ s.

There is one Lambda hyperon,
Λ⁰
. It has a rest energy of 1115 MeV with a lifetime of 2.6×10⁻¹⁰ s.

There are two Xi hyperons,
Ξ⁰
and
Ξ⁻
. They have rest energies of 1315 MeV and 1320 MeV and lifetimes of 2.9×10⁻¹⁰ s and 1.6×10⁻¹⁰ s.

There is one Omega hyperon, the last discovered,
Ω⁻
, with a mass of 1670 MeV and a lifetime of 8.2×10⁻¹¹ s.

Hyperon decay

Since strangeness is conserved by the strong interactions, the ground-state hyperons cannot decay strongly. However, they do participate in strong interactions.

Λ decay

Λ⁰
→
p⁺
+
π⁻

Λ⁰
→
n⁰
+
π⁰

Λ⁰
may also decay on rare occurrences via these processes

Λ⁰
→
p⁺
+
e⁻
+
ν
_e

Λ⁰
→
p⁺
+
μ⁻
+
ν
_μ

Σ decay

Σ⁺
→
p⁺
+
π⁰

Σ⁺
→
n⁰
+
π⁺

Σ⁰
→
Λ⁰
+
γ

Σ⁻
→
n⁰
+
π⁻

Ξ decay

Ξ⁰
→
Λ⁰
+
π⁰

Ξ⁻
→
Λ⁰
+
π⁻

Ξ
particles are also known as "cascade" hyperons, since they go through a two-step cascading decay into a nucleon by first decaying to a
Λ⁰
and emitting a
π^±
.

Ω⁻ decay

The
Ω⁻
has a baryon number of +1 and hypercharge of -2 giving it strangeness of −3. It takes multiple flavor-changing weak decays for it to decay into a proton or neutron. Murray Gell-Mann's SU(3) model (sometimes called the Eightfold Way) predicted this hyperon's existence, mass and that it will only undergo weak decay processes.

Experimental evidence for its existence was discovered in 1964 at Brookhaven National Laboratory. Further examples of its formation and observation using particle accelerators confirmed the SU(3) model.

Ω⁻
→
Ξ⁰
+
π⁻

Ξ⁰
→
Λ⁰
+
π⁰

Λ⁰
→
p
+
π⁻

Hyperon research

The first research into hyperons happened in the 1950s, and spurred physicists on to the creation of an organized classification of particles. Today, research in this area is carried out on data taken at many facilities around the world, including CERN, Fermilab, SLAC, JLAB, Brookhaven National Laboratory, KEK, and others. Physics topics include searches for CP violation, measurements of spin, studies of excited states (commonly referred to as spectroscopy), and hunts for exotic states such as pentaquarks and dibaryons.

List of hyperons

Hyperons

Particle	Symbol	Makeup	Rest mass MeV/c²	Isospin I	Spin(Parity) JP	Q	S	C	B	Mean lifetime s	Commonly decays to
Lambda	Λ0	u d s	1 115.683(6)	0	1⁄2+	0	−1	0	0	2.60×10−10	p+ + π− or n0 + π0
Sigma	Σ+	u u s	1 189.37(0.7)	1	1⁄2+	+1	−1	0	0	(8.018±0.026)×10−11	p+ + π0 or n0 + π+
Sigma	Σ0	u d s	1 192.642(24)	1	1⁄2+	0	−1	0	0	(7.4±0.7)×10−20	Λ0 + γ
Sigma	Σ−	d d s	1 197.449(30)	1	1⁄2+	−1	−1	0	0	(1.479±0.011)×10−10	n0 + π−
Sigma resonance	Σ+ (1385)	u u s	1 382.8(4)	1	3⁄2+	+1	−1	0	0		Λ + π or Σ + π
Sigma resonance	Σ0 (1385)	u d s	1 383.7±1.0	1	3⁄2+	0	−1	0	0		Λ + π or Σ + π
Sigma resonance	Σ− (1385)	d d s	1 387.2(5)	1	3⁄2+	−1	−1	0	0		Λ + π or Σ + π
Xi	Ξ0	u s s	1 314.83(20)	1⁄2	1⁄2+	0	−2	0	0	(2.90±0.09)×10−10	Λ0 + π0
Xi	Ξ−	d s s	1 321.31(13)	1⁄2	1⁄2+	−1	−2	0	0	1.639	Λ0 + π−
Xi resonance	Ξ0 (1530)	u s s	1 531.80(32)	1⁄2	3⁄2+	0	−2	0	0		Ξ + Ξ
Xi resonance	Ξ− (1530)	d s s	1 535.0(6)	1⁄2	3⁄2+	−1	−2	0	0		Ξ + Ξ
Omega	Ω−	s s s	1 672.45(29)	0	3⁄2+	−1	−3	0	0	(8.21±0.11)×10−11	Λ0 + K− or Ξ0 + π− or Ξ− + π0

See also

• Delta baryon
• Hypernucleus
• Lambda baryon
• List of mesons
• List of particles
• Nucleon
• Omega baryon
• Physics portal
• Sigma baryon
• Timeline of particle discoveries
• Xi baryon

References

• Henry Semat, John R. Albright (1984). Introduction to atomic and nuclear physics. Chapman and Hall. ISBN 0-412-15670-9.