A-Level Physics (AQA) – Application of conservation laws

1.9 - Application Of Conservation Laws

1Creation and decay of strange particles
2Conservation laws obeyed in particle interactions
3How conservation laws apply to strong and weak interactions

4 min

Strange particles

Strange particles have a quantum property called strangeness that is conserved in strong interactions.

Strangeness number:

Quarks have strangeness of -1, 0 or +1

Strange quarks are produced in the strong interaction and decay by the weak interaction.

To conserve strangeness, strange particles must be created or destroyed in pairs by strong interactions.

Worked example - Production of kaons

Kaons are produced via the strong interaction:

$P + P \to P + P + K^{+} + K^{-}$

The table below shows how strangeness, baryon number, lepton number and charge are conserved during the production of Kaons.

Property	Calculation
Strangeness (s)	0 + 0 $\to$ 0 + 0 + 1 + (-)1
Baryon number (B)	1 + 1 $\to$ 1 + 1 + 0 + 0
Lepton number (L)	0 + 0 $\to$ 0 + 0 + 0 + 0
Charge (Q)	1 + 1 $\to$ 1 + 1 +1 + (-)1

Conservation laws in particle interactions

In order for an interaction to be possible, some properties must be conserved.

The following must always be conserved:

Energy
Momentum
Charge
Baryon number
Lepton numbers

Strangeness is also conserved in strong interactions.

Decay of a strange particle

Strange particles decay via the weak interaction.

The strangeness can change by +1, 0, or -1 in weak interactions.

An example of weak decay is:

$K^{+} \to μ^{+} + ν_{μ}$

In this interaction the strangeness has changed from -1 to 0.

	K $^{+}$ $\to$ $μ^{+}$ + $ν_{μ}$
Baryon Number (B)	$0$ $\to$ 0 + 0
Lepton Number (L)	$0$ $\to$ -1 + 1
Strangeness (S)	-1 $\to$ 0 + 0
Charge (Q)	$1$ $\to$ 1 + 0

Property

Calculation

Strangeness (s)

0 + 0

\to

0 + 0 + 1 + (-)1

Baryon number (B)

1 + 1

\to

1 + 1 + 0 + 0

Lepton number (L)

0 + 0

\to

0 + 0 + 0 + 0

Charge (Q)

1 + 1

\to

1 + 1 +1 + (-)1

^{+}

\to

μ^{+}

ν_{μ}

Baryon Number (B)

0

\to

0 + 0

Lepton Number (L)

0

\to

-1 + 1

Strangeness (S)

-1

\to

0 + 0

Charge (Q)

1

\to

1 + 0