K Interactions

In nuclear collisions of π and protons of high energy, Kaons are produced, both charged and neutral, with masses equal to 966 me. These particles can transform in a variety of modes (and much more) :

The first indication of the existence of the Kaons was found by Leprince-Ringuet and L’Héritier (1944) who observed a charged particle in a Wilson cloud chamber which collided with an electron. An analysis of the dynamics of the collision led them to the conclusion that the mass of the incident particle was ∼1000 me. Later in 1947, Rochester and Butler found two forked tracks that they interpreted them as the decay ‘in flight’ of two particles, each with mass about 1000 me, one charged and the other neutral (lambda particle). In 1948, Leprince-Ringuet observed in an emulsion a particle which reached the end of its range ; a nuclear disintegration occurred from which a π- of low range (200 μm, about 2,5 MeV) was emitted. The following years, the others mode of decay of Kaons was observed in emulsions.

K±→ π± + π++ π (branching ratio 5.6% )


Left picture : First observation (1949) of the reaction K+→ π+ + π++ π. Jungfraujoch under 10 cm of lead. The mass of the incoming particle (τ) is determined by measuring the grain-density and scattering along the track. A slow particle (a) emerges from the end of the range of the parent Kaon (τ) at A, and comes to rest at B where it produces a nuclear disintegration characteristic of a π particle. Tracks b and c are π+ with 33 and 31 MeV of kinetic energy. The πparticle (a) (as it makes a nuclear disintegration) has about 1 MeV of kinetic energy. 

Following this observation, it was more than a year and a half before a second example was found and by July 1953, the number had increased only to twelve. Although Kaons decaying into three charged π occurred rarely, it was easy to identify because of the following features of the tracks : when a kaon moves through a solid substance, it is commonly brought to rest before decaying. The three charged π into which it transforms are therefore emitted in directions which are co-planar. The directions of motions of theses particles and their velocities (which may be determined by the grain-density in the tracks) must be consistent with the assumption that if the secondary particles are of equal mass, their resultant momentum is zero. The Q value of this mode of decay is about 76 MeV.


Other examples of the reaction K±→ π± + π++ π

Above pictures.   A : Jungfraujoch under 30 cm of lead (1951) . B : Jungfraujoch in glacier (1951) . The parent particle undergo a large angle scattering near the centre of disintegration. C : Mt Rosa under 6 cm of aluminium (1952) . D : Balloon flight at 90.000 ft, no absorber (1952). The marked scattering of the Kaon at points near the centre of disintegration is an evidence that the particle was approaching the end of its range.

It was found in 1953 that almost all the Kaon, after being arrested in the emulsion are positive. K in this mode of disintegration, rarely decay at rest but most often decay in flight or is captured by nuclei (like negative muon & pion). This K capture process frequently leads to the emission of a π, and little energy appears in the form of ejected proton, α-particles and other evaporated nucleons (neutrons).

Bottom picture (1953) :  The Kaon reaches the end of its range at the point P and decays into three particles of which the initial directions of motion are co-planar. One of the three secondary particles stops at Q and emits a μ which in turn decays, at the point R, into an electron.

 First observation of the successive decay K → π → μ → β

Bottom picture (1953) :  This event provided the first evidence for the direct production of Kaon in nuclear disintegrations. The Kaon emerges from a nuclear encounter in the emulsion at the point O, and stops in the emulsion at the point P, its total range being ~ 7 mm.

Left picture : The observations were made in 1952 in a stack of 46 emulsions each 15 x 15 cm exposed at high altitude over Southern England. The number in the stack of the plate in which a particular event occurred, is shown ringed. The first observation was of the decay at A of a μ into an electron. On following back the muon, it was found to have been produced by the decay of a pion at B in the same plate, n°6. On following back the pion in turn, it was found to have originated in the decay of a Kaon at point C. This Kaon originated  in a nuclear disintegration at the point E, where 21 secondary particles are emitted. Theses particles from this disintegration were followed to the end of their range in the stack, or to their point of escape, but none of them showed any remarkable features. At the magnification employed in the photographs, the distance between the points E and C in the plate would be about 40 m in air. 

The energy of the three π particles formed by the decay of the Kaon were 12 ; 26 and 38 MeV. The total release of energy was therefore 76 MeV.

One of the π formed by the decay of the Kaon reach the point D in plate n°21, where it produced a nuclear disintegration. Among the secondary products of this disintegration there were four singly charged particle and a 8Li3 nucleus. The latter, on reaching the end of its range, suffered β-decay with the formation of a 8Be4 nucleus in an excited state. This nucleus then disintegrated spontaneously into two α-particles. The resulting ‘hammer’ tracks are due to the α-particles which recoiled from one another with equal and opposite momenta. The characteristics (see here, method n°2) of the disintegration produced by the π at D are such that the parent nucleus must have been one of the light elements in the emulsion, carbon, nitrogen or oxygen. Since the total charge of the secondary particles is at least (3+4)e=7e, the charge of the original nucleus must have been 8e, i.e oxygen (prior to the collision the total charge was -1(pion) + 8 =7, identical to the final state).


K+ π++ 2π0 (branching ratio 1.76% )

Bottom picture (1954) :  At A, a nuclear disintegration emits a K+ which reach the end of its range at O and decay into a π+which have about 41 MeV of kinetic energy. It then decay into a muon, which is heavily scattered at point Q, as a result of Coulomb forces in a collision with a nucleus of the emulsion (Ag, Br, C..). The muon then decay into a β particle.

Decay of a Kaon with the emission of a secondary pion


K μ + π0μ (branching ratio 3.3% )

Bottom picture (1954) : Measurements of scattering and grain-density show that the secondary particle was a muon ejected with an energy ~34 MeV. The track of the primary particle (Kaon) was 1,5 mm long.

K+→ π+ + π0 (branching ratio 21 % )