In
particle physics
Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...
, strangeness (symbol ''S'') is a
property
Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. Depending on the nature of the property, an owner of property may have the right to consume, alter, share, re ...
of
particles
In the physical sciences, a particle (or corpuscle in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, from s ...
, expressed as a
quantum number
In quantum physics and chemistry, quantum numbers are quantities that characterize the possible states of the system.
To fully specify the state of the electron in a hydrogen atom, four quantum numbers are needed. The traditional set of quantu ...
, for describing
decay of particles in
strong
Strong may refer to:
Education
* The Strong, an educational institution in Rochester, New York, United States
* Strong Hall (Lawrence, Kansas), an administrative hall of the University of Kansas
* Strong School, New Haven, Connecticut, United ...
and
electromagnetic interactions that occur in a short period of
time
Time is the continuous progression of existence that occurs in an apparently irreversible process, irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequ ...
. The strangeness of a particle is defined as:
where ''n''
represents the number of
strange quark
The strange quark or s quark (from its symbol, s) is the third lightest of all quarks, a type of elementary particle. Strange quarks are found in subatomic particles called hadrons. Examples of hadrons containing strange quarks include kaons (), ...
s () and ''n''
represents the number of
strange antiquarks (). Evaluation of
strangeness production has become an important tool in search, discovery, observation and interpretation of
quark–gluon plasma
Quark–gluon plasma (QGP or quark soup) is an interacting localized assembly of quarks and gluons at Thermodynamic equilibrium#Local and global equilibrium, thermal (local kinetic) and (close to) chemical (abundance) equilibrium. The word ''plasm ...
(QGP). Strangeness is an excited state of matter and its decay is governed by
CKM mixing.
The terms ''strange'' and ''strangeness'' predate the discovery of the quark, and were adopted after its discovery in order to preserve the continuity of the phrase: strangeness of particles as −1 and anti-particles as +1, per the original definition. For all the quark flavour quantum numbers (strangeness,
charm,
topness and
bottomness) the convention is that the flavour charge and the electric charge of a quark have the same sign. With this, any flavour carried by a charged
meson
In particle physics, a meson () is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, the ...
has the same sign as its charge.
Conservation

Strangeness was introduced by
Murray Gell-Mann
Murray Gell-Mann (; September 15, 1929 – May 24, 2019) was an American theoretical physicist who played a preeminent role in the development of the theory of elementary particles. Gell-Mann introduced the concept of quarks as the funda ...
,
Abraham Pais,
Tadao Nakano and
Kazuhiko Nishijima to explain the fact that certain particles, such as the
kaon
In particle physics, a kaon, also called a K meson and denoted , is any of a group of four mesons distinguished by a quantum number called strangeness. In the quark model they are understood to be bound states of a strange quark (or antiquark ...
s or the
hyperons and , were created easily in particle collisions, yet decayed much more slowly than expected for their large masses and large production
cross sections. Noting that collisions seemed to always produce pairs of these particles, it was postulated that a new conserved quantity, dubbed "strangeness", was preserved during their creation, but ''not'' conserved in their decay.
In our modern understanding, strangeness is conserved during the
strong
Strong may refer to:
Education
* The Strong, an educational institution in Rochester, New York, United States
* Strong Hall (Lawrence, Kansas), an administrative hall of the University of Kansas
* Strong School, New Haven, Connecticut, United ...
and the
electromagnetic interactions, but not during the
weak interaction
In nuclear physics and particle physics, the weak interaction, weak force or the weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, and gravitation. It is th ...
s. Consequently, the lightest particles containing a strange quark cannot decay by the strong interaction, and must instead decay via the much slower weak interaction. In most cases these decays change the value of the strangeness by one unit. This doesn't necessarily hold in second-order weak reactions, however, where there are mixes of and mesons. All in all, the amount of strangeness can change in a weak interaction reaction by +1, 0 or −1 (depending on the reaction).
For example, the interaction of a K
− meson with a proton is represented as:
Here strangeness is conserved and the interaction proceeds via the strong nuclear force.
[{{Cite web, url=https://www.nobelprize.org/prizes/physics/1968/alvarez/lecture/, title=The Nobel Prize in Physics 1968, website=NobelPrize.org, language=en-US, access-date=2020-03-15]
Nonetheless, in reactions like the decay of the positive kaon:
Since both pions have a strangeness of 0, this violates conservation of strangeness, meaning the reaction must go via the weak force.
See also
*
Strangeness and quark–gluon plasma
*
Strange particles
References
Physical quantities
Quarks
Flavour (particle physics)
Strange quark
Murray Gell-Mann