Class 12 > Unit # 28: Particle Physics > Hadrons, Quarks, and Beta Decay Mechanism

Physics Class 12 • Unit 28

Particle Physics — Lecture 3: Hadrons, Quarks, and Beta Decay Mechanism

1. Composite Fermions (Hadrons)

Definition: Composite subatomic particles such as protons, neutrons, alpha particles, etc., are composed of two or more elementary particles. All composite particles are massive. Those made of quarks are called hadrons. The two main categories of hadrons are baryons and mesons.

(a) Baryons

Baryons are hadrons that consist of three quarks. Protons and neutrons are the most well-known baryons.

• Proton: Consists of two up and one down quarks (uud). An up quark has +2/3 charge and a down quark has -1/3 charge.

Proton = uud = 2/3 + 2/3 - 1/3 = +1

• Neutron: Consists of one up and two down quarks (udd).

Neutron = udd = 2/3 - 1/3 - 1/3 = 0

(b) Mesons

Mesons are hadrons consisting of one quark and one anti-quark (e.g., pions π). They carry the strong nuclear force, binding protons and neutrons together.

2. Quarks and Beta Decay

• β^- Decay Process

In β^- decay, a neutron in an atomic nucleus transforms into a proton. A down quark emits a field particle called a W^- boson (weak force carrier) and transforms into an up quark. The W^- boson quickly decays into an electron and an antineutrino.

• β⁺ Decay Process

Electric charge is conserved during beta decay with the emission of an electron and an antineutrino. In β⁺ decay, a proton transforms into a neutron where an up quark emits a W⁺ boson and transforms into a down quark. The W⁺ boson quickly breaks up into a positron (anti-electron) and a neutrino.

3. The Weak Interaction Mechanism Trick

An up quark can change into a down quark by emitting a W⁺ weak particle. How is this possible?

Nature can create short-lived particles (particle-antiparticle pairs) out of nothing by borrowing energy permitted by the uncertainty principle. This principle allows energy level fluctuations at any given moment, enabling particles to briefly come into existence before annihilating each other.

Imagine a pair created near an up quark: a down quark and an anti-down quark (d̄). The anti-down quark has a charge of +1/3, making the net charge of the pair zero. When the down quark replaces the up quark, the up quark joins the remaining anti-down quark to create the W⁺ particle (2/3 + 1/3 = +1). The W⁺ boson subsequently decays into a positron and a neutrino.