bei48482_FM

Strangeness

Introducing baryon and lepton numbers still left some loose ends in the world of

elementary particles. In particular, a number of particles were discovered that behaved

so unexpectedly that they were called “strange particles.” They were only created in

pairs, for instance, and decayed only in certain ways but not in others that were al-

lowed by existing conservation rules. To clarify the observations, M. Gell-Mann and,

independently, K. Nishijina introduced the strangeness numberS, whose assignments

for the particles of Table 13.3 are shown there.

Strangeness number Sis conserved in all processes mediated by the strong and

electromagnetic interactions. The multiple creation of particles with S0 is the result

of this conservation principle. An example is the result of this proton-proton collision:

pp→ 

0 K^0 p

S:0 0 1 10 0

On the other hand, Scan change in an event mediated by the weak interaction.

Decays that proceed via the weak interaction are relatively slow, a billion or more times

slower than decays that proceed via the strong interaction (such as those of resonance

particles). Even the weak interaction does not allow Sto change by more than1 in

a decay. Thus the baryon does not decay directly into a neutron since

→ n^0 

S:  200

but instead via the two steps

→ 

0  

0 → n^0 ^0

S:  2  10  100

A

remarkable theorem discovered early in this century by the German mathematician Emmy

Noether states that

Every conservation principle corresponds to a symmetry in nature.

What is meant by a “symmetry”? In general, a symmetry of a particular kind exists when a cer-

tain operation leaves something unchanged. A candle is symmetric about a vertical axis because

it can be rotated about that axis without changing in appearance or any other feature; it is also

symmetric with respect to reflection in a mirror.

The simplest symmetry operation is translation in space, which means that the laws of

physics do not depend on where we choose the origin of our coordinate system to be. Noe-

ther showed that the invariance of the description of nature to translations in space has as a

consequence the conservation of linear momentum. Another simple symmetry operation is

translation in time, which means that the laws of physics do not depend on when we choose

t 0 to be, and this invariance has as a consequence the conservation of energy. Invariance

under rotations in space, which means that the laws of physics do not depend on the orien-

tation of the coordinate system in which they are expressed, has as a consequence the

conservation of angular momentum.

Conservation of electric charge is related to gauge transformations, which are shifts in the

zeros of the scalar and vector electromagnetic potentials V and A. (As elaborated in

Symmetries and Conservation Principles

486 Chapter Thirteen

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