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Table 13.3 Some hadrons and their properties. The symbol Sstands for strangeness number, discussed in

Sec. 13.4. Antiparticles have strangeness numbers the negative of those shown.

Mass, Mean

Class Particle Symbol Antiparticle MeV/c^2 Life, s Spin S

  140 2.6 10 ^8

Mesons Pion ^0 Self 135 8.7 10 ^1700

  140 2.6 10 ^8

K K 494 1.2 10 ^8

Kaon KS^0 KKS^0  498 8.9 10 ^110  1

KL^0 K^0 L 498 5.2 10 ^8

Eta

^0 Self 549 5  10 ^1900

 Self 958 2.2 10 ^21

Baryons Nucleon Proton pp 938.3 Stable  1

Neutron nn 939.6 889 2 ^0

Lambda 

0 ^0  1116 2.6 10 ^10 ^12   1

  1189 8.0 10 ^11

Sigma ^0 ^0  1193 6  10 ^20 ^12   1

  1197 1.5 10 ^10

Xi ^0 ^0  1315 2.9 10 ^10

  1321 1.6 10 ^10

^12   2

Omega   1672 8.2 10 ^11 ^32   3



and muons in the earth’s atmosphere) indeed metamorphose to and from tau neutrinos. Further

experiments will no doubt provide a definitive answer to whether electron neutrinos also

undergo oscillations into another flavor or flavors. In the meantime the solar neutrino mystery

no longer seems so mysterious and it appears that neutrinos do have mass, settling a question

seventy years old.

13.3 HADRONS

Particles subject to the strong interaction

Unlike leptons, hadrons are subject to the strong interaction. Table 13.3 lists the hadrons

with the longest lifetimes against decay into other particles. Mesonsare bosons and

consist of a quark and an antiquark; about 140 types are known. The lightest meson

is the pion, with other meson masses ranging beyond the proton mass. Baryonsare

fermions and consist of three quarks; about 120 types are known. Of the hadrons listed,

the ^0 and ^0 are their own antiparticles. The charged pions differ in charge, so they

are antiparticles of each other, but have no other attributes that are different, so each

is both a particle and an antiparticle.

The lightest baryon is the proton, which is also the only hadron stable in free space.

Or apparently stable—current theories call for the proton to decay with a very long

lifetime, perhaps longer than the experimentally determined lower limit of 10^32 years.

Hence the ultimate stability of the proton is still an open question. (For comparison,

the age of the universe is a little over 10^10 years.) The neutron, although stable inside

a nucleus, beta-decays in free space into a proton, an electron, and an antineutrino

with a mean life of 14 min 49 s.

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