# Quarks and Leptons

Particle interactions and conservation laws

The Particle Zoo

·

Baryons are always either 3 quarks or 3 anti-quarks

·      There is a further set of leptons called Tau particles that are not required for AS Physics

Table of Quarks

 Name Symbol Charge Strangeness Up u +2/3 0 Anti-up -2/3 0 Down d -1/3 0 Anti-down +1/3 0 Strange s -1/3 -1 Anti-strange +1/3 +1 Charm c +2/3 0 Anti-charm -2/3 0 Bottom b -1/3 0 Anti-bottom +1/3 0 Top t +2/3 0 Anti-top -2/3 0

Conservation Rules

·

You will be familiar with the concept of ‘conservation of energy’. This states that energy cannot be created or destroyed, just converted from one form to another. In a particle accelerator this would be used to calculate the rest energy of the new particles made:

·      Other conserved quantities

-   Charge, Q

Particles charge is normally indicated in the symbol – anti-particles have the opposite to their corresponding particle.

Total charge before      =      Total charge after

-   Lepton Number, L

Leptons e.g. electrons have L= +1, anti-leptons e.g. positrons have L= -1

Lepton number before =      Lepton number after

-   Baryon Number, B

Baryons e.g. protons have B= +1, anti-baryons e.g. anti-protons have B= -1

Quarks have B = +1/3, anti-quarks have B = -1/3

Baryon number before        =          Baryon number after

-   Strangeness, S

Is not conserved in weak interaction – only strong (where quark flavor is changing)

Strange quarks have (or particles containing them) have S = -1, anti-stange quarks (and particles containing them) have S = +1

Strangeness before      =      Strangeness after

Examples:

Are the following particle interactions possible?

1.

Q:

L:

B:

S:

2.

Q:

L:

B:

S:

3.

Q:

L:

B:

S:

4.

Q:

L:

B:

S: