bei48482_FM

Σ−

Σ^0

Σ+

Λ^0

N np

Λ

Σ

Ξ Ξ−

Ξ^0

Mass

Figure 13.7Baryon supermultiplet whose members have spin ^32 and Figure 13.8Origin of the baryon supermultiplet shown in Fig. 13.5.

(except ), are short-lived resonance particles. The *and *

particles here are heavier and have different spins from the ones in

Table 13.3. The particle was predicted from this scheme.

Elementary Particles 489

Murray Gell-Mann (1929–) was

born in New York and entered Yale

University at fifteen. After obtain-

ing his Ph.D. from the Massachu-

setts Institute of Technology in

1951 he was at the Institute

for Advanced Study in Princeton

and at the University of Chicago

before joining the faculty of the

California Institute of Technology. In 1953 Gell-Mann intro-

duced strangeness number and its conservation in certain

interactions to help understand the properties of elementary

particles. In 1961 he formulated a method of classifying

elementary particles that enabled him to predict the 

particle, which was later discovered. Two years later Gell-Mann

came up with the idea of quarks, the ultimate entities from

which particles subject to the strong interaction are composed.

He received the Nobel Prize in Physics in 1969.

The members of each supermultiplet would all be the same in the absence of any

interactions, which are responsible for the differences that occur. Figure 13.8 shows

how this idea applies to the baryon supermultiplet of Fig. 13.5. The strong interaction

splits the basic baryon state into the four components , , , and N(for nucleon),

and the electromagnetic interaction further splits the , , and Ncomponents into

multiplets. Because the strong interaction is more powerful than the electromagnetic

one, the mass differences between multiplets are greater than those between members

of a multiplet. Thus there is only 1.3-MeV difference between the pand nmasses, but

176 MeV separates them from the mass.

13.5 QUARKS

The ultimate constituents of hadrons

An effort to explain why only certain hadron families, such as those shown in

Figs. 13.5, 13.6, and 13.7, occur but not others led Gell-Mann and, independently,

George Zweig to propose in 1963 that all baryons consist of three still more fundamental

Ξ∗^0

Σ∗− Σ∗^0

Ξ∗−

Ω−

∆− ∆^0 ∆+

− 1 Σ∗+

0

s

− 3

− 2

Q − 1 0 + 1 + 2

∆+ +

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