Particle physics

Complete the particle equation for this decay.

Explain whether the conservation laws of charge and baryon number are obeyed for this decay.

The table summarises the charges for three types of quark.

The sigma particle has one strange quark within its structure.

State why the neutron does not leave a track.

The initial momentum of the positive pion is 1.1 10^–19N s. The scale of the diagram is 1 cm represents 15 cm.

Determine the magnitude of the magnetic flux density acting perpendicularly to the tracks.

At the Facility for Rare Isotope Beams (FRIB) isotopes of elements that do not occur naturally can be produced.

FRIB contains a linear accelerator. The initial section of the accelerator is shown.

Explain why the tubes get progressively longer.

A uranium atom (²³⁸₉₂U) has 80 electrons removed. The uranium ion is then accelerated between two adjacent tubes until its kinetic energy is 200 MeV per nucleon.

Calculate the accelerating potential difference.

Show that the uranium ions will reach a velocity of about 65% of the speed of light.

When a uranium ion hits a target at this speed the ion is likely to break up and produce smaller elements. One particular isotope produced in this way is⁴⁰₁₂Mg. The naturally occurring common stable isotope of magnesium is²⁴₁₂Mg. Explain why the break-up of²³⁸₉₂U is likely to produce unusual isotopes such as⁴⁰₁₂Mg.

Particle accelerators produce beams of high speed charged particles which are used to investigate particle collisions. New particles are created during the collisions.

In the Large Hadron Collider (LHC), protons are maintained in a circular path by the force exerted on the protons due to a magnetic field.

The magnetic flux density required to maintain the protons in a circular path increases as the kinetic energy of the protons increases.

Explain why.

Calculate the magnetic flux density required to maintain the protons in the LHC when the protons are travelling at a speed of 3.0 10⁷m s^–1.

circumference of LHC = 27 km

A maximum of 13 600 GeV of energy is available to create a new particle.

Calculate the theoretical maximum mass, in kg, of the new particle.

*(b) Fifty years ago, fixed‑target collision experiments were carried out at the Stanford Linear Accelerator Centre (SLAC). In these experiments, high speed particles were made to collide with a stationary target.

Currently, colliding beam experiments are carried out at SLAC. In these experiments, high speed particles travelling in opposite directions are made to collide head‑on.

For a given total energy in the collision, colliding beam experiments allow particles to be created with a larger mass than could be created in a fixed‑target experiment.

Explain why colliding beam experiments allow a new particle with the larger mass to be created. You should consider how the conservation of energy and the conservation of momentum apply to each type of collision.

Beams of antiprotons are often used in particle physics experiments.

Show that the rest mass of an antiproton is about 900 MeV / c².

The decay of a positive muon produced a positron, an electron neutrino and a muon antineutrino. The diagram shows the tracks formed in a particle detector.

A muon belongs to a family of particles called leptons.

State two features that all particles in the lepton family have in common.

Write a nuclear equation for the decay of the muon (μ) described above.

Describe the role of the magnetic field in a particle detector.

Explain how the diagram gives evidence that a particle or particles, other than the positron, were produced in this decay.

A delta particle decays into a proton and a pion.

The diagram shows tracks in a particle detector formed when the delta particle decays.

State why it can be concluded from the diagram that the delta particle is neutral.

Deduce the charge on the pion.

Complete the particle equation for the decay of the delta (Δ⁰) particle.

State why the delta particle must be classified as a baryon based on the evidence of its decay.

Explain how the momentum of the proton compares with the momentum of the pion.

The mass of the delta particle is 1232 MeV/c².

Calculate the mass of the delta particle in kg.

The mass of the proton is 939 MeV/c² and the mass of the pion is 139 MeV/c².

Explain why the sum of the masses of the two particles after the decay is not equal to the mass of the delta particle.

A series of experiments was carried out in the 1970s to investigate the structure of protons using the linac at Stanford, USA.

*(a) Explain how an electron is accelerated in a linac.

The electron leaves the accelerator with a high energy.

Explain why electrons need high energies to investigate the structure of a proton.

An electron leaves the accelerator with a momentum of 20 GeV / c.

Explain, with reference to base units, why GeV / c can be used as a unit of momentum.

An electron with initial momentum 20 GeV / c collides with a stationary proton. After the collision the electron is deflected by an angle of 20° as shown and its momentum is 9.1 GeV / c. The momentum of the proton after the collision is 11.9 GeV / c.

Deduce whether the law of conservation of momentum is obeyed.

The collisions between electrons and the protons in these experiments are sometimes inelastic.

State what is meant by an inelastic collision.

A cosmic ray, consisting of a fast‑moving proton, collides with a proton within the nucleus of an atom in the upper atmosphere. Three particles, a proton, a neutron and a pion result from the collision.

Write a particle equation for this collision.

The table shows the properties of two quarks.

Give the quark structure for each of the particles produced by this collision.

The mass of a pion is 140 MeV / c².

Calculate the mass of the pion in kg.

The mass of a neutron is about the same as the mass of a proton. A student suggests that the minimum kinetic energy the cosmic ray proton would need to create the pion in this collision is 140 MeV.

Discuss whether this suggestion is correct. Your answer should include reference to the laws of conservation of momentum and conservation of energy.

At the beginning of the 20th century, Rutherford carried out large-angle alpha particle scattering experiments using gold (¹⁹⁷₇₉Au) foil. The vast majority of the alpha particles went straight through the foil whilst a few were deflected straight back.

Describe how the model of the atom changed, as a consequence of these experiments.

In one experiment the alpha particles had an initial energy of 7.7 MeV.

Calculate the distance of closest approach of the alpha particles to the nucleus of a gold atom. Assume that the gold nucleus remains at rest.

Rutherford also carried out the experiment with aluminium (₁₃²⁷Al) foil. The aluminium foil had the same thickness as the gold foil and the alpha particles had the same initial kinetic energy.

The following observations were made.

Observation 1: The fraction of alpha particles scattered at any particular angle for aluminium foil was always much less than for gold foil.

Observation 2: The alpha particles scattered from aluminium foil had less kinetic energy than the alpha particles scattered from gold foil.

Explain how these observations can be used to deduce how an aluminium nucleus compares to a gold nucleus.

The bubble chamber photograph shows tracks made by a proton and a pion. The proton and pion were both created by the decay of a lambda particle. No other particles were produced.

*(a) Explain how observations and measurements from the photograph can be used to establish information about the lambda particle.

The lambda particle consists of up, down and strange quarks.

Explain how the conservation of charge, baryon number and lepton number apply to the decay of the lambda particle.

Write an equation to represent the decay of the lambda (Λ) particle.

The rest mass of the lambda particle is 1115 MeV / c².

Calculate this mass in kg.

The rest mass of a proton is 940 MeV / c². The rest mass of a pion is 140 MeV / c².

The kinetic energy of the lambda particle just before decay is 4.95 GeV.

Calculate the total kinetic energy of the proton and pion in MeV.

A proton has a mass of 1.67 × 10⁻²⁷kg.

Which of the following shows the conversion of this mass to GeV/c² ?

A proton has a mass of 1.67 × 10⁻²⁷kg.

Which of the following shows the conversion of this mass to GeV/c² ?

A negatively charged pion decays into a muon and an antineutrino. The diagram shows tracks in a particle detector formed in such an event.

Deduce whether the antineutrino is charged, giving two reasons for your decision.

Write a particle equation to represent this decay.

According to the standard model, the pion and muon are classified within two different groups of particles.

State which group each particle belongs to and describe the two groups.

The momentum of the pion just before it decays is 9.1 × 10⁻²⁰N s.

Determine the magnetic flux density of the magnetic field which acts in the detector and state its direction.

Scale of diagram 1 cm represents 10 cm

Use a vector diagram to determine the momentum of the antineutrino.

The initial momentum of the muon is 1.59 × 10⁻¹⁹N s.

A negatively charged pion decays into a muon and an antineutrino. The diagram shows tracks in a particle detector formed in such an event.

Deduce whether the antineutrino is charged, giving two reasons for your decision.

Write a particle equation to represent this decay.

According to the standard model, the pion and muon are classified within two different groups of particles.

State which group each particle belongs to and describe the two groups.

The momentum of the pion just before it decays is 9.1 × 10⁻²⁰N s.

Determine the magnetic flux density of the magnetic field which acts in the detector and state its direction.

Scale of diagram 1 cm represents 10 cm

Use a vector diagram to determine the momentum of the antineutrino.

The initial momentum of the muon is 1.59 × 10⁻¹⁹N s.

An example of this decay is given by the equation

Which of the following particle equations is correct for the decay of a proton within a nucleus?

The discovery of the Higgs particle was an important contribution to our understanding of particle physics.

Describe the standard model for subatomic particles. You should identify the fundamental particles and the composition of the particles we can observe.

The mass of the Higgs particle is 2.2 10^–25 kg.

Calculate this mass in GeV/c².

The Higgs particle was discovered using the Large Hadron Collider (LHC) in 2012. Two beams of very high energy protons, moving in opposite directions, were made to collide.

Explain the need for such high energy collisions.

The beams of protons are contained within a ring of superconducting magnets.

Calculate the momentum of a proton in a beam.

magnetic field strength = 8.3 T circumference of the ring = 27 km

State the total momentum of the products of the collision between the two beams of protons.

The LHC accelerates protons until they gain energies of about 7 TeV.

A student used the equation E = to predict the energy of a proton in the beam, using the momentum calculated in (c)(ii), but found the energy was far higher than 7 TeV.

Explain why.

Our understanding of the atom has developed over time, from early models in which atoms were considered to be hard incompressible spheres, through to the nuclear model of the atom and the ladder model in which electrons exist in a discrete number of allowed energy states.

*(a) The model of atoms as hard incompressible spheres, moving rapidly and randomly, can be used to explain why gases exert a pressure.

Explain, using ideas of momentum, why the pressure exerted by a gas increases as the temperature of the gas increases.

The nuclear model of the atom was established following a series of experiments in which alpha particles were directed at thin gold foil.

An alpha particle approaching a gold nucleus,¹⁹⁷₇₉Au, head-on will be brought to rest and returned along its original path.

Calculate the minimum distance between the alpha particle and the nucleus for alpha particles of energy of 5.5 MeV.

It is observed that electrons, with energy of 5.5 keV, are diffracted as they pass through the thin gold foil.

Explain a conclusion about the electrons that can be made from this observation.

In the energy ladder model of the atom, electrons exist in a discrete number of allowed energy states. The collision of electrons with gold atoms may lead to the production of high frequency electromagnetic radiation.

Explain how high frequency electromagnetic radiation may be produced when electrons collide with atoms in a metal.

The neutral lambda Λ⁰ particle is a baryon of mass 1116 MeV/c² and contains one strange quark.

The table shows quarks and their relative charge.

State, with justification, the quark content of a Λ⁰ particle.

Calculate the mass of the Λ⁰ particle in kg.

A student suggests five ways a Λ⁰ particle might decay. These are

Deduce which of these decay processes are not possible.

Lambda particles were first detected in experiments which made use of cosmic rays entering the atmosphere. Cosmic rays are mainly high‑energy protons which have a mass less than that of a lambda particle.

Explain why a cosmic ray could lead to the creation of a lambda particle.

The Λ⁰ particle cannot be directly observed in particle experiments, however some of the decay products can.

Explain why the Λ⁰ particle cannot be directly observed but information about it can be obtained by studying its decay particles.