Strange particles

In December 1947, Rochester and Butler published the cloud

chamber photograph.

Cosmic ray particles enter from the upper left and strike the lead plate

producing a neutral particle which decayed into two charged secondaries 𝜋+ 𝑎𝑛𝑑 𝜋−

A new neutral particle with at least twice the mass of the pion was found. It is known as Kaon 𝐾0

𝐾0    →    𝜋+   +          𝜋−

In 1949, Brown and her collaborators showed the decay of charged Kaon as

𝐾+      →      𝜋+       +     𝜋+     +    𝜋−

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  ҧ 𝑠)  of the kaon transmutes into an up antiquark ത 𝑢) by the emission of a W boson; the W boson subsequently decays into a down antiquark ҧ 𝑑) and an up quark (

Strong interactions: An up quark (u) emits a gluon (g) which decays into a down quark (d) and a down antiquark ҧ 𝑑  The kaons behave in some respects like heavy pions, so

the meson family was extended to include them.

In due course, many more mesons were discovered the 𝜂(𝑒𝑡𝑎), the 𝜙(𝑃ℎ𝑖), the 𝜔(𝑂𝑚𝑒𝑔𝑎), the 𝜌(Rho), and so on.

In 1950, Anderson found one more heavier than the proton a particle called as lambda lambdaΛ

Λ      →    𝑝+      +     𝜋−
It suggests that the lambda belongs to the baryon family.

Why is the proton stable?

Why proton does not decay into a positron and a

photon?

𝑝+    →    𝑒+   +     𝛾                          or

 𝑝+    →     𝑒+    +       𝜋0

If such reactions have to go then all atoms would disintegrate. (This reaction violates conservation of lepton number 1953)

In 1938, Stackelberg proposed a law of conservation of Baryon number to account for the stability of protons.

Baryon number 𝐴=+1for all baryons ( 𝑝and 𝑛)  and  A=−1  for all anti baryons

The total baryon number is conserved in any physical process.

If baryon number is conserved in all physical processes,then the proton, being the lightest baryon, should not decay

Neutron beta decay is allowed

𝑛     →            𝑝+     +     𝑒−    +         $\overline {\nu }$

Baryon number consideration ::(1  = 1+ 0 + 0)

And also the first reaction confirming the existence of

antiproton as

𝑝     +     𝑝     →     𝑝     +     𝑝+      𝑝     +     $\OVERLINE {\p }$

Baryon number consideration ::(1+1=1+1+1−1)

In early studies of cosmic ray showers, it was found that certain particles, which have since been identified with  𝐾(Kaon) mesons and the Σ(Sigma)  and  Λ0(𝐿𝑎𝑚𝑏𝑑𝑎𝑛𝑒𝑢𝑡𝑟𝑎𝑙)baryons, were produced strongly

(that is, with large cross-sections of the order of millibarns

But their lifetimes are characteristic of weak interactions: 10−10 sec. These particles were always produced in pairs

K in association with  either  a Σ  or Λ0

All this was certainly puzzling and led to a suspicion that a new quantum number might be associated with such particles. When specific reactions, such as

𝜋−      +     𝑝   →     𝐾0      +      Λ0

were studied with the  Λ0   and 𝐾0  subsequently decaying as

Λ0     →         𝜋−     +     𝑝      and

 𝐾0      →       𝜋−     +     𝜋+

It was observed that the  Λ0    was always produced in association with a 𝐾0   and never with just a 𝜋0.

The    Λ0     was also observed to be produced in association with a 𝐾+,   but not with a  𝐾-

The puzzle of associated production was clarified by Murray Gell Mann and Abraham Pais, who proposed that these particles carried a new additive quantum number, which they called strangeness, which is conserved in strong production processes but violated

in weak decays. All the ordinary mesons and baryons (as well as the photon) were assumed to be non-strange (S=0)

Thus, in any strongly associated production reaction with the initial state having no strangeness, the total strangeness of the particles in the final state must also add up to zero. From the analysis of such reactions, it was deduced that the strangeness of the 𝐾 +  and  𝐾0  must be opposite to that of the Σ+, Σ0, Σ−, and Λ0.

In a pion proton interaction

𝜋−   +    𝑝    →     𝐾0      +      Λ0    
𝜋−   +    𝑝 →   𝐾+     +       Σ−     
𝜋−   +    𝑝  →    𝐾0   +    Σ0

Here the 𝐾’s carry strangeness 𝑆=+1 and  Σ  𝑎𝑛𝑑  Λ    carries Strangeness   𝑆=−1  and the other ordinary particles carry 𝑆=0

Strange Particles They are produced abundantly on a time scale of about 10−23sec, but they decay relatively slowly (typically about 10−10sec).

The strange particles are produced by the strong force (the same one that holds the nucleus together), but they decay by the weak force (the one that accounts for beta decay and all other neutrino processes

There exists a consistent assignment of strangeness numbers to all the hadrons (baryons and mesons) that account for the observed strong processes and “explains” why the others do not occur. (The leptons and the photon don’t experience strong forces at all, so strangeness does not apply to them.)

Willis Lamb’s Nobel Prize acceptance speech 1955

When the Nobel Prizes were first awarded in 1901, physicists knew something of just two objects which are now called “elementary particles”: the electron and proton.

 A deluge of other “elementary” particles appeared after 1930; neutron,

neutrino, 𝜋meson, 𝜇meson, heavier mesons, and various hyperons. In fact, “the finder of a new elementary particle used to be rewarded by a Nobel Prize, but such a discovery now ought to be punished by a $10,000 fine”. [Source: Les Prix Nobel 1955, The Nobel Foundation, Stockholm

The Eightfold Way ( 1961-1964)

In 1961, Murray Gell Mann introduced the so-called Eightfold Way

The Eightfold Way arranged the baryons and mesons into weird geometrical patterns, according to their charge and strangeness. The eight lightest

baryons fit into a hexagonal array, with two particles at the center

                                                                         THE meson  OCTATE

The baryon octate

the baryon decuplet