2.2 Classification of Particles

The baryon decuplet is a vision tool used by particle physicists to classify groups of particles called baryons.

Discuss the properties of baryons.

In the baryon decuplet, strangeness S is plotted on the horizontal axes and charge Q is plotted on the diagonal axes, as shown in Figure 1. Some information is missing.

                              Figure 1

Deduce the quark composition of the Ω^– baryon, using each axis to justify your answer.

Deduce the quark composition and an appropriate symbol for the missing baryon in Figure 1.

The discovery of the Higgs Boson marked a huge accomplishment for particle physicists. 

The Higgs Boson was hypothesised by Peter Higgs and his team in 1964 as a mechanism to explain why some particles have mass. It was discovered by a large collaborative effort at the CERN particle physics laboratory in 2012. 

Explain what is meant by the term ‘hypothesised’ and suggest why it took over forty years to discover the Higgs Boson.

The experimental results which hinted at the existence of a new particle caught the attention of media worldwide.

 While public excitement grew, the scientists involved in the experiment were cautious before confirming that the results proved the existence of a new particle. In fact, confirmation came many months after the first set of results were reported.

Discuss why it takes many months after results are first discovered to confirm the existence of a new particle.

Other exciting results from experiments in particle physics have contributed to the field’s considerable advances over the past 100 years.

 State and explain two reasons which demonstrate why collaboration between scientists and engineers is necessary in order for such advances to be made.

A young student of physics reads up about particles and anti-particles.

In their physics lesson, they excitedly tell their teacher how they learned that a proton has an anti-particle called an anti-proton, and the neutron has an anti-particle called an anti-neutron.

They go on to say that, since the neutron is neutrally charged, it is its own anti-particle.

Identify the student’s misconception and explain why they are incorrect.

Suggest another particle which is an example of being its own anti-particle and explain your reasoning.

Interactions between protons and neutrons can temporarily violate conservation laws.

 One such interaction is shown in Figure 1:

(i)         Identify the type of interaction shown in Figure 1. 

(ii)        By referencing the properties of the exchange particle, explain how it temporarily violates conservation laws.

The extent to which conservation laws can be violated is governed by Heisenberg’s Uncertainty Principle.

 Simply put, this principle says that the uncertainty in some quantity of energy ΔE multiplied by the uncertainty in a time interval Δt must always be constant. This can be written as:

              ΔE × Δt  = 

The range of the strong force is around 1 × 10^–15 m. 

 Assuming the strong force is mediated at near the speed of light, show that pions are about 200 times more massive than electrons.

You may use the energy-mass equation: 

               E = mc²

Table 1 gives information about some fundamental particles.

 Table 1

Particle	Quark structure	Charge	Strangeness	Baryon Number
				-1
sigma+ (∑+)		+1		+1
pion		-1	0

Complete the table by filling in the missing information. 

 Cosmic ray showers are a source of high-energy pions and kaons. 

State the group of particles that pions and kaons belong to and determine the charge on a kaon with a quark structure of .

Kaons have a lifetime of 1.24 × 10^–8 s whilst pions have a lifetime of 8.0 × 10^–17 s.

Give a reason why you would expect kaons to have a longer lifetime.

Another strange particle,  , decays in the following way:

State what interaction produces particle and what interaction is involved in its decay.    

Write an equation representing a kaon decay.

Table 1 shows three particles, two pions and one kaon.

                   Table 1

Particle	Quark combination

Complete Table 1 to show the correct quark combinations.

One of the lambda () baryons has a quark structure of .

Determine the following properties of the lambda baryon:           

• Charge
• Baryon number
• Lepton number
• Strangeness 

State the following properties of the anti-particle of the lambda baryon: 

• Charge
• Baryon number
• Lepton number
• Strangeness 

What distinguishes a lepton from other subatomic particles? 

Table 1 gives information about some leptons.

 Table 1

Lepton symbol	Name	Relative Charge	Lepton Number
		+1
	muon		+1
	antielectron neutrino
	muon neutrino	0

Complete Table 1 by filling in the missing information.

A negative muon () may decay into three leptons             

Write down an equation representing the muon decay.

Particle physics is a collaborative effort and there are have been considerable advances in this field over the past 100 years. 

 Explain why it is necessary for large teams of scientists and engineers to collaborate and validate new knowledge for these advances to occur.

What is the baryon number of an alpha (α) particle ? 

Particle X has the following characteristics shown in Table 1.

 Table 1

Using Table 1, determine the quark composition of particle X and through which type of interaction it will decay.

Which baryon will particle X eventually decay into? Give a reason for your answer.

Explain why a baryon with quark structure  cannot exist.