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Flavour mixing |
Inparticle physics,strangeness (symbolS)[1][2] is aproperty ofparticles, expressed as aquantum number, for describingdecay of particles instrong andelectromagnetic interactions that occur in a short period oftime. The strangeness of a particle is defined as:wheren
s
represents the number ofstrange quarks (
s
) andn
s
represents the number ofstrange antiquarks (
s
). Evaluation ofstrangeness production has become an important tool in search, discovery, observation and interpretation ofquark–gluon plasma (QGP).[3] Strangeness is an excited state of matter and its decay is governed byCKM mixing.
The termsstrange andstrangeness 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 andbottomness) 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 chargedmeson has the same sign as its charge.
Strangeness was introduced byMurray Gell-Mann,[4]Abraham Pais,[5][6]Tadao Nakano andKazuhiko Nishijima[7] to explain the fact that certain particles, such as thekaons or thehyperons
Σ
and
Λ
, were created easily in particle collisions, yet decayed much more slowly than expected for their large masses and large productioncross 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, butnot conserved in their decay.[8]
In our modern understanding, strangeness is conserved during thestrong and theelectromagnetic interactions, but not during theweak interactions. 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
K0
and
K0
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.[9]
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.[9]
pages 1188 (Mesons), 1716 ff (Baryons)
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