5.1. BASIC KNOWLEDGE OF PARTICLE PHYSICS 261
1.Decays. Particle decay means that a particle is spontaneously decomposed into several
other particles. The most remarkable example is theβ-decay, i.e., a neutronndecays to a
protonp, an electrone−and an anti-neutrinoνe:
(5.1.6) n→p+e−+νe.
Current list of discovered particle consists of hundreds ofnumbers. Except electrons
and protons, all massive particles will decay therefore they are finite lifetime. The massless
particles, such as the photon, the gluons and the neutrinos,do not decay, and have infinite
lifetime.
All decays obey the following laws and rules:
1) Decays are caused by the three fundamental forces: the electromagnetic, weak, strong
interactions, and, therefore, are mainly classified into two types: weak interaction de-
cays and strong interaction decays;2) Decays always take place from higher masses toward to lower masses. For example in
theβ-decay (5.1.6), the masses of particles on the right-hand side are smallerthan the
mass of the neutronn.3) Both strong and weak interaction decays have to obey some basic conservation laws,
as shown in the next subsection.Remark 5.1.The following transition
(5.1.7) p→n+e++νe
is often calledβ-decay as well. In fact, in Nature the process (5.1.7) cannot spontaneously
take place, and it always occurs under certain energetic excitation. Hence, (5.1.7) is an excited
scattering.
2.Scattering. If the decay is a spontaneous behavior, then scattering is a forced behavior
of particles under certain force actions, such as collisions and energetic excitations. The
transition (5.1.7) is a scattering, and the precise decay mechanism is writtenas
p+γ→n+e++νe.In general, a scattering reaction is written as
(5.1.8) A 1 +···+AN→B 1 +···BK,
whereAn( 1 ≤n≤N)are the initial particles, andBk( 1 ≤k≤K)are the final particles.
There are two forms of scatterings: the elastic and non-elastic scatterings, both of which
are caused by the three interactions: the electromagnetic,the weak and the strong interac-
tions. Collision is the most important experimental methodto detect new particles and new
phenomena. The scatterings (5.1.8) satisfy various conservation relations.