Kaon

Kaon
Composition
K+
:
u

s


K0
:
d

s
and
s

d


K
:
s

u
StatisticsBosonic
InteractionsStrong, weak, electromagnetic, gravitational
Symbol
K+
,
K0
,
K
Discovered1947
Types4
Mass
K±
: 493.677±0.013 MeV/c2

K0
: 497.648±0.022 MeV/c2
Electric charge
K±
: ±1 e

K0
: 0 e
Spin0
Strangeness
K+
: +1


K0
: ±1


K
: -1
The decay of a kaon (
K+
) into three pions (2 
π+
, 1 
π
) is a process that involves both weak and strong interactions.

Weak interactions : The strange antiquark (
s
) of the kaon transmutes into an up antiquark (
u
) by the emission of a boson; the
W+
boson subsequently decays into a down antiquark  (
d
) and an up quark (
u
).

Strong interactions: An up quark (
u
) emits a gluon (
g
) which decays into a down quark (
d
) and a down antiquark (
d
).

In particle physics, a kaon n/, also called a K meson and denoted
K
,[a] 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) and an up or down antiquark (or quark).

Kaons have proved to be a copious source of information on the nature of fundamental interactions since their discovery in cosmic rays in 1947. They were essential in establishing the foundations of the Standard Model of particle physics, such as the quark model of hadrons and the theory of quark mixing (the latter was acknowledged by a Nobel Prize in Physics in 2008). Kaons have played a distinguished role in our understanding of fundamental conservation laws: CP violation, a phenomenon generating the observed matter–antimatter asymmetry of the universe, was discovered in the kaon system in 1964 (which was acknowledged by a Nobel Prize in 1980). Moreover, direct CP violation was discovered in the kaon decays in the early 2000s by the NA48 experiment at CERN and the KTeV experiment at Fermilab.

Basic properties

The four kaons are :


  1. K
    , negatively charged (containing a strange quark and an up antiquark) has mass 493.677±0.013 MeV and mean lifetime (1.2380±0.0020)×10−8 s.

  2. K+
    (antiparticle of above) positively charged (containing an up quark and a strange antiquark) must (by CPT invariance) have mass and lifetime equal to that of
    K
    . Experimentally, the mass difference is 0.032±0.090 MeV, consistent with zero; the difference in lifetimes is (0.11±0.09)×10−8 s, also consistent with zero.

  3. K0
    , neutrally charged (containing a down quark and a strange antiquark) has mass 497.648±0.022 MeV. It has mean squared charge radius of −0.076±0.01 fm2.

  4. K0
    , neutrally charged (antiparticle of above) (containing a strange quark and a down antiquark) has the same mass.

As the quark model shows, assignments that the kaons form two doublets of isospin; that is, they belong to the fundamental representation of SU(2) called the 2. One doublet of strangeness +1 contains the
K+
and the
K0
. The antiparticles form the other doublet (of strangeness −1).

Properties of kaons
Particle name Particle
symbol
Antiparticle
symbol
Quark
content
Rest mass (MeV/c2) IG JPC S C B' Mean lifetime (s) Commonly decays to
(>5% of decays)
Kaon[1]
K+

K
493.677±0.016 12 0 1 0 0 (1.2380±0.0021)×10−8 + μ or
+ or
+ + or
+ + e
Kaon[2]
K0

K0
497.611±0.013 12 0 1 0 0 [§] [§]
K-Short[3]
K0
S
Self [†] 497.611±0.013[‡] 12 0 [⁎] 0 0 (8.954±0.004)×10−11 + or
+
K-Long[4]
K0
L
Self [†] 497.611±0.013[‡] 12 0 [⁎] 0 0 (5.116±0.021)×10−8 + + e or
+ + μ or
+ + or
+ +
Quark structure of the kaon plus (K⁺).

[⁎] See Notes on neutral kaons in the article List of mesons, and neutral kaon mixing, below.
[§]^ Strong eigenstate. No definite lifetime (see neutral kaon mixing).
[†]^ Weak eigenstate. Makeup is missing small CP–violating term (see neutral kaon mixing).
[‡]^ The mass of the
K0
L
and
K0
S
are given as that of the
K0
. However, it is known that a relatively minute difference between the masses of the
K0
L
and
K0
S
on the order of 3.5×10−6 eV/c2 exists.[4]

Although the
K0
and its antiparticle
K0
are usually produced via the strong force, they decay weakly. Thus, once created the two are better thought of as superpositions of two weak eigenstates which have vastly different lifetimes:

  • The long-lived neutral kaon is called the
    K
    L
    ("K-long"), decays primarily into three pions, and has a mean lifetime of 5.18×10−8 s.
  • The short-lived neutral kaon is called the
    K
    S
    ("K-short"), decays primarily into two pions, and has a mean lifetime 8.958×10−11 s.
    Quark structure of the kaon minus (K⁻).

(See discussion of neutral kaon mixing below.)

An experimental observation made in 1964 that K-longs rarely decay into two pions was the discovery of CP violation (see below).

Main decay modes for
K+
:

Quark structure of the neutral kaon (K⁰).
Results Mode Branching ratio

μ+

ν
μ
leptonic 63.55±0.11%

π+

π0
hadronic 20.66±0.08%

π+

π+

π
hadronic 5.59±0.04%

π+

π0

π0
hadronic 1.761±0.022%

π0

e+

ν
e
semileptonic 5.07±0.04%

π0

μ+

ν
μ
semileptonic 3.353±0.034%

Decay modes for the
K
are charge conjugates of the ones above.