Particle physics masterclass

The LHC was designed to accelerate hadrons, like protons, at almost the speed of light. Visit 'How an accelerator works'⁶.

As these hadrons collide, they smash to bits, spit out their constituent particles and also create new particles. It is the energy of the colliding particles that is converted into new particles. Although most of the resulting particles do not last for long, they do leave tracks in the detectors incorporated into the accelerator. The tracks are stored as data in the detector’s computing grid. It is these tracks that can be interpreted. See 'How a detector works'⁷.

And this is what gets particle physicists excited using the Compact Muon Solenoid (CMS) and ATLAS (A Toroidal LHC Apparatus) detectors, ‘two of a kind, they are looking for whatever new particle they can find’. We will focus on ATLAS which was designed to cover the widest possible range of investigations. Its main objectives are to discover the Higgs boson (which it did on 4 July 2012), to confirm (or not) the principle of supersymmetry, to study matter and antimatter, the formation of dark matter and to search for extra dimensions and new phenomena.

Take the virtual tour of the ATLAS detector on CERN’s ATLAS⁸ page. A diagram of its structure allows you to see a cut section.

The ATLAS detector consists of a series of concentric cylinders mounted so that the beams collide in the centre. The particles resulting from the collision pass through the detectors in turn and either stop at one of them or continue on their way out of the detector.

The different layers allow us to identify the particles and measure their direction, charge, momentum, and energy. An example of how different particles interact with different layers of the detector is shown in the illustration ‘Interaction of particles with the ATLAS detector’.

In the ‘Analyse ATLAS data’ section below, you will see similar representations of the ATLAS detector when you prepare to analyse decay particles in proton-proton collision ‘events’.

The detector is located in the centre of the device and consists of three different sub-detectors, all designed to detect charged particles. The neutral particles (e.g. photons) pass through the detectors unnoticed. All charged particles interact with the detector, but exit with about the same energy and direction as before entering.

In the innermost part are high-resolution semiconductor detectors while the outer parts are gas detectors. All elements are mounted inside the central solenoid which creates a magnetic field with 2 Tesla intensity. The internal diameter of the detector is 1.15 m while its total length is 7m.

Calorimeters measure the energy of both charged and neutral particles. They consist of absorbers and active parts. The collision of particles with the absorbers creates a shower of secondary particles that are collected and recorded by the sensors. The energy of the initial particle in most cases is absorbed by the calorimeter. Because of this, the calorimeters are located outside the inner detector, so that the track of the charged particle is recorded before being absorbed by the calorimeter.

The ATLAS calorimeters consist of two different sections:

• The electromagnetic calorimeter (green area) in which the hadrons leave a small portion of their energy. In contrast, the electromagnetic calorimeter absorbs the full energy of e+, e- and photons.

• The hadronic calorimeter (orange area) measures the total energy of hadrons (like protons, neutrons).

Only muons (and neutrinos) have the ability to penetrate the detector first, and then the calorimeters, leaving little energy, and continue to the muon chambers.

The muon chambers consist of thousands of tubes which detect charged particles positioned in the magnetic field created by huge superconducting toroidal coils.

The muons are particles like electrons, but they are about 200 times heavier. They are the only charged particles that can pass through the calorimeters without losing very much and reach the muon chambers located in the outside layer of ATLAS. The muon chambers measure the momentum and the trajectory of muons that cross them with great precision.

The only particles that pass through ATLAS undetected are neutrinos.

Watch the video, ‘The LHC Magnets’ (56 secs) for more details.

ATLAS uses two types of magnets, solenoid and toroidal with eight coils each. The detector is in a strong magnetic field which bends the trajectories of charged particles. The magnetic field generated by four different magnets: three toroidal (one large central and two smaller pieces in front) and a solenoid. Positively and negatively charged particles are curved in opposite directions from the same magnetic field. The radius of curvature and direction of the particles is used to determine the momentum and the charge of particles.

You can see the large toroidal magnets being incorporated in this two minute video of the 'Construction of the ATLAS detector'.

Understanding the LHC experiments requires coming to grips with very small and very large quantities.

A detailed accessible description of the internal structure of a helium atom.

The 'Scale of the Universe 2'¹² (requires Flash) will provide you with useful size comparisons for many things in the universe.

The International System of units (SI) uses prefixes to specify the size of the quantity. See the SI units and prefixes table below.

International System (SI) units

Protons and neutrons are believed to have diameters of approximately 10^-15 m or 1 f (1 femto-metre), while the largest energy collision achieved to date in the LHC had an energy of 13 x 10¹² eV of 13 TeV (13 Tera electron volts).

Mass in physics is usually measured in kilograms, but because particle physicists deal with such small masses they use atomic mass units (u) which is defined as 1/12^th of the mass of a Carbon 12 atom.

1 u = 1.6605 x 10^-27

Einstein showed that mass was a form of energy. Using his equation E = mc² we can show that a gram of matter is equivalent to 9 thousand billion joules (90TJ) of energy and that an atomic mass unit is equivalent to 931.5MeV of energy.

1u = 931.5eV/c²

Therefore an electron can be said to have a mass of 9.10938356 x 10^-31 kg or 0.000548579909u or an energy equivalence of 0.5109989461 MeV.

An electric field can be shown by drawing lines of force that show the direction that a positive charge would experience a force if placed at each point.

The direction of the electric field at a point in space is determined by placing an imaginary positive ‘test charge’ at the point. The direction of the force on the positive test charge is the direction of the electric field at that point.

We can show the electric field by drawing lines of force showing the direction a positive charge would experience a force if placed at each point.

Watch 'Electric field definition' for more information.

The strength of the field at a point is defined as the force that a unit positive charge would experience if it was placed at the point:

We can use this relationship to show that the units of the electric field are newtons per coulomb (NC^-1). And we can rearrange this relationship to show that the force on a charge (q) place in an electric field of strength (E) is given by:

F = qE

We can make a simple charge accelerator by placing a voltage across parallel plates using two methods. See the diagram, right.

If the space between the plates is evacuated, a charge placed near the plate with the same polarity as the charge will be accelerated by the electric field towards the opposite plate.

View the video 'Accelerating charges'.

Because the mass of protons and ions are so small, electric fields can accelerate charges rapidly. By directing charges around a circular path, and repeatedly passing them through an accelerating electric field, they can be accelerated to velocities near the speed of light.

For non-relativistic velocities we can calculate the final velocity of the charge using either Newton’s second law or conservation of energy.

Magnetic fields are called ‘B’ fields and the strength of the field is measured in Tesla. The earth’s magnetic field has a strength of approximately 50μT; the LHC uses magnetic fields as high as 8T.

Magnetic fields are directed from the north pole to the south pole of a permanent magnet. See the diagram, right.

Charges only experience a force in a magnetic field if they are moving and some component of their velocity is perpendicular to the magnetic field. If the charges are stationary or moving parallel to the magnetic field, they do not experience a force.

When a charge moves perpendicular to a magnetic field the magnetic field exerts a force given by:

F = qvB

The direction of this force is perpendicular to the velocity of the charge in a direction given by the right hand palm rule.

A negative charge moving in a magnetic field would experience a force in the opposite direction to the force that a positive charge would experience. So, you can use your left hand to find the force direction when the moving charge is negative.

A detailed accessible description of uniform circular motion.

Because the magnetic force exerts a force that is perpendicular to the velocity of the charge the force acts as a centripetal force that will cause the charge to move in a circular path.

View the video 'Circular motion in a magnetic field'.

The radius of curvature of a charged particle moving perpendicular to a magnetic field is given by:

r = mv/qB

As the magnetic force exerts a centripetal force, we can use the centripetal force equation (F_c = mv²/r) to find the radius of curvature:

F_c = F_magnetic
mv²/r = qvB
Hence, r = mv/qB

Einstein proved that if all rest frames are equivalent and the speed of light is an absolute constant, then an observer would see:

• time run slow

• length shrink in the direction of motion

• mass increase in a frame moving with respect to the observer.

The amount that these quantities change can be calculated using Einstein’s equations of special relativity:

Time dilation formula: Length contraction formula: Mass dilation formula:	Key: t = time l = length m = mass v = relative speed c = the speed of light = 3 x 108 ms-1 subscript 0 (0)= the quantities in the rest (proper) frame subscript v (v) = the moving frame

Use the material in this section to learn about superconductors. Also research other uses of superconductors. Discuss the issues with superconductors, including its advantages and disadvantages of their use.

See Technology rubrics for any classroom²⁴ for guidelines for interactive tutorials and adapt the guidelines for screencasts and vodcasts for your podcast. (A podcast will require less time than a screencast or vodcast.) Aim for an Outstanding ‘Grade’.

The LHC was built to test the Standard Model and future models that will replace it. The predictions of the Standard Model, that have been tested, have all been verified.

Most physicists believe the model is still incomplete because it:

• requires 20 experimentally determined factors to make it work

• says nothing about dark matter and dark energy

• fails to include the force of gravity.

Quarks are charged particles that experience the strong nuclear force, the force that holds the nucleons together in the nucleus.

Particles made up of quarks are called hadrons:

• baryons are hadrons with three quarks. Protons (see diagram on right, with two ‘up’ quarks and one ‘down’ quark) and neutrons are examples of baryons

• mesons are hadrons consisting of quarks and antiquarks

Quarks interact via the strong force, which is so powerful that we never observe isolated quarks.

Leptons are particles that do not experience the strong force. Electrons and neutrinos are examples of leptons.

Watch 'Neutrinos – nature’s identity thieves?' to learn more about these particles.

Gauge bosons, such as photons, are called force mediating particles because they are the quanta of force fields (more on this later).

The Higgs boson was predicted by the Standard Model in the 1960s, but its existence was only confirmed by experiments at the LHC in 2012.

The Higgs particle is not a force mediating particle, but is still considered to be a boson as it is the quanta of a field that is responsible for giving particles their mass (inertia).

Watch a simple explanation of the Pauli Exclusion Principle in the 'Theory of Everything: What is Matter'.

Quarks and leptons are grouped together as fermions as they obey the Pauli Exclusion Principle (fermions cannot occupy the same quantum state).

Bosons do not obey the Pauli Exclusion Principle therefore can be placed in the same quantum state. This is why it’s possible to make lasers in which huge numbers of photons (bosonic particles) can be produced in the same quantum state.

Contrasting normal matter and dark matter

If you want to know more about dark matter watch 'Complex dark matter'.

Watch this video to learn more about gravity at the sub-atomic level. That is the quantum level.

Gravity at the microscopic quantum (minimum amount) scale

Watch this video on supersymmetry for more information.

Feynman diagrams consist of space and time axes.

Interactions occur where the lines intersect (here: red dot). They also mark places where charge, momentum and energy must be conserved. This red dot represents the muon decay into a W^- particle and a muon neutrino. The W^- particle is called a messenger particle.

You can understand the time sequence from the horizontal t axis. The spatial sequence can be understood from the vertical axis. [Cosmic ray muons (μ) have a lifetime of 1/500 of a millisecond before they decay.]

The second diagram shows a second interaction (second red dot), shows the creation of particles when the W^- decays into an electron (e^-) and an electron antineutrino ( ̅ν_e).

Leptons are straight lines with a little arrow in the middle of the line whereas the messenger particles of the weak (W, Z) and electromagnetic (γ) interactions are shown by wavy lines. Antileptons are always displayed with an arrow that points backward in time.

During beta minus decay, an electron is emitted from an unstable nucleus when a neutron inside the nucleus transforms into a proton. An electron anti-neutrino is also produced. In the diagram below the:

1. Second down quark (d) transforms into a W^- particle and an up quark (u).

2. W^- particle decays into an electron and anti-neutrino.

During beta plus decay, a positron is emitted from an unstable nucleus when a proton inside the nucleus transforms into a neutron. An electron neutrino is also produced. In the diagram below the:

1. Second up quark (u) transforms into a W⁺ particle and a down quark (d).

2. W⁺ particle decays into a positron and a neutrino.

The W particle (also called W boson) is the particle most commonly produced in proton-proton collisions in the LHC. They almost immediately decay.

The W particle is heavy (80.4 GeV/c²) and decays almost immediately after it is produced:

• ²/₃ of the time it decays to a quark-antiquark pair which appears as jets in the detector

• ¹/₃ of the time it decays to a lepton and a neutrino (the leptons, matter particles, can be an electron, a muon, or a tau

• before the tau can be detected, it decays as well.

There are four processes producing W particles.

Other background events may occur, such as creation of Z⁰ particles.

A: In the extreme conditions of supernova explosions. Z bosons are also thought to have been extremely common in the early universe.

In nature, charge is conserved. Since a Z boson is neutral the sum of the charges of its decay products must be zero charge. Therefore Z must decay into a particle and antiparticle pair.

The possible Z decays is divided between the groups of particles below according to conservation laws.

• 10% of the Z-decays, produce charged lepton-antilepton pairs. These include electron-antielectron (positron), muon-antimuon, and tau-antitau pairs all are equally probable.

• 20% of Z boson decays produce a neutrino-antineutrino pair. LHC detectors can’t detect neutrinos directly. So they are detected by the loss of some energy and/or momentum after the collision.

• 70% of Z decays produce a quark-antiquark pair. These appear as particle showers called “jets” in the detector.

The Z' boson is a heavy partner of the Z boson (Z⁰).

Some theories beyond the Standard Model predict the need for a new weak force. These theories predict that the production and decay of the Z' boson will be very similar to that of the ordinary Z boson.

Using a pencil and paper, try your hand at drawing a Feynman diagram for

• Z¹ boson decay to an electron (e^-) and a positron (e⁺)

• Z¹ boson decay to a muon (μ^-) and an antimuon (μ⁺)

Visit Z' boson³⁸ and select the hand-drawn Feynman diagrams on that page to check your Feynman diagrams.

A: Particles that ordinary matter is made of, and other particles that only existed just after the Big Bang.

Inside each proton you can find a ‘sea’ of quarks and gluons. We say that a proton consists of three quarks, but it also has a whole bunch of ‘sea’ or ‘virtual’ quarks and anti-quarks stemming from gluons.

The new particles are usually much heavier than the original colliding particles. All the energy put into the collision can come out as mass instead of energy.

E=mc²

Energy and momentum are conserved.

Researchers from the University of Witwatersrand, Johannesburg analysed CERN data and think they have seen 'Glimpses of the Madala boson: have we detected the dark Higgs?'⁴²

Read the article, and then discuss why scientists can have different interpretations of results.

This resource includes many equations and mathematical expressions that cannot be read by a screen reader. Some of these have been converted into images and have alt text that will be read as the correct wording by a screen reader but may present issues for someone with low vision who needs to increase the screen size hugely.

In the downloadable word documents the equations, where possible, have been created in such a way that they can be copied into STEMReader⁴⁴ and converted into Mathml which will then be read by the build in screen reader in STEMReader.

Students with a visual disability will need assistance to fully engage with this resource. For schools within the Department of Education NSW you may need to access the Itinerant support teachers (hearing or vision)⁴⁵ for your region through the ‘Ed services page⁴⁶’.

A virtual visit with ATLAS CERN is free. It is not free of commitment, if you wish to commit you will need to set up a Masterclass. This function is not yet availble.

Video conference opportunities with ATLAS CERN will be available soon through DART connections.

Its aim is to provide users with the opportunity to:

• Build their physics knowledge and understanding so that the Particle physics with the masters at ATLAS CERN can be a genuine case of particle physics masters honing the users already deep understanding. Thus providing significant and genuine experience with working scientists at the forefront of their field.

• Experience real scientific work by analysing real ATLAS data.

• Communicate with working scientists to improve users’ scientific data analysis and understand the physics principles that allow scientists to draw conclusions and new insights from the data.

• Meet curriculum outcomes in a meaningful way through participating in a cutting-edge real world context.

• Use LHC physics masterclass for project based learning. The teacher may wish to design a culminating task for this purpose. It could require the completion of the tasks, data analysis and Particle physics with the masters at ATLAS CERN visit contained within the resource. Such a culminating task could be a collaborative task. Collaboratus: resource model⁴⁷ can assist student groups to develop different collaborative products. Different groups of students could for example design and develop different types of products that demonstrate some aspect of their experience for a genuine purpose like science communication. Students could engage their creativity to develop ideas, decide on some, then team with other students interested the various ideas. The curriculum based tasks completed during the LHC physics masterclass could form the basis of a group’s vodcast for example or other communication format. What is Cooperative Learning: More than Just Working in Groups⁴⁸ may assist.

• Undertake the relevant sections of LHC physics masterclass during the year when relevant content or option is being covered. The Virtual Visit can be scheduled for a time when all curriculum content areas have been completed.

• Use a flipped classroom approach where students do the learning outside school in preparation for working during class on tasks that require this preparation and perhaps some coaching from the teacher. This idea can also be combined with idea 2.

• To revise and apply concepts covered earlier in the senior curriculum and develop students abilities to apply their knowledge to unfamiliar situations.

H1. evaluates how major advances in scientific understanding and technology have changed the direction or nature of scientific thinking

H2. analyses the ways in which models, theories and laws in physics have been tested and validated

H3. assesses the impact of particular advances in physics on the development of technologies

H4. assesses the impacts of applications of physics on society and the environment

H5. identifies possible future directions of physics research

H9. explains the effects of electric, magnetic and gravitational fields

• 9.2 Space

  • 4. Current and emerging understanding about time and space has been dependent upon earlier models of the transmission of light

  • Students

    • solve problems and analyse information using:

E = mc²

Time dilation formula: Length contraction formula: Mass dilation formula:	Key: t = time l = length m = mass v = relative speed c = the speed of light = 3 x 108 ms-1 subscript 0 (0)= the quantities in the rest (proper) frame subscript v (v) = the moving frame

• 9.4 From Ideas to Implementation

  • 1. Increased understandings of cathode rays led to the development of television

  • Students learn to

    • identify that moving charged particles in a magnetic field experience a force

    • identify that charged plates produce an electric field

    • describe quantitatively the force acting on a charge moving through a magnetic field

    • discuss qualitatively the electric field strength due to a point charge, positive and negative charges and oppositely charged parallel plates

  • Students

    • solve problem and analyse information using F=qE

  • 4. Investigations into the electrical properties of particular metals at different temperatures led to the identification of superconductivity and the exploration of possible applications

  • Students learn to

    • describe the occurrence in superconductors below their critical temperature of a population of electron pairs unaffected by electrical resistance

    • discuss the advantages of using superconductors and identify limitations to their use

H10. describes the nature of electromagnetic radiation and matter in terms of the particles

• 9.8 Option - Quanta to Quarks

  • 4. An understanding of the nucleus has led to large science projects and many applications

  • Students learn to

    • identify ways by which physicists continue to develop their understanding of matter, using accelerators as a probe to investigate the structure of matter

    • discuss the key features and components of the standard model of matter, including quarks and leptons

H12. evaluates ways in which accuracy and reliability could be improved in investigations

• 12.3 gather information from secondary sources by:

  • e) identifying practising male and female Australian scientists, and the areas in which they are currently working and information about their research

• 12.4 process information to:

  • b) identify and apply appropriate mathematical formulae and concepts

H14. assesses the validity of conclusions drawn from gathered data and information

• 14.1 analyse information to:

  • f) use models, including mathematical ones, to explain phenomena and/or make predictions

• 14.2 solve problems by:

  • c) using identified strategies to develop a range of possible solutions to a particular problem

H16. justifies positive values about and attitude towards both the living and non living components of the environment, ethical behaviour and a desire for critical evaluation of the consequences of the applications of science 

PH12-13 explains and analyses the electric and magnetic interactions due to charged particles and currents and evaluates their effect both qualitatively and quantitatively

• Subtopic: Charged Particles, Conductors and Electric and Magnetic Fields

• Inquiry question: What happens to stationary and moving charged particles when they interact with an electric or magnetic field?

• Students:

  • investigate and quantitatively derive and analyse the interaction between charged particles and uniform electric fields, including: (ACSPH083)

  • acceleration of charged particles by the electric field (F ⃗=ma ⃗ ,F ⃗=qE ⃗ )

  • model qualitatively and quantitatively the trajectories of charged particles in electric fields and compare them with the trajectories of projectiles in a gravitational field

  • analyse the interaction between charged particles and uniform magnetic fields, including: (ACSPH083)

  • acceleration, perpendicular to the field, of charged particles

  • the force on the charge (F ⃗=qv ⃗B ⃗sinθ)

  • compare the interaction of charged particles moving in magnetic fields to:

  • the interaction of charged particles with electric fields

  • other examples of uniform circular motion (ACSPH108)

PH12-14 describes and analyses evidence for the properties of light and evaluates the implications of this evidence for modern theories of physics in the contemporary world

• Subtopic: Light and special relativity

• Inquiry question: How does the behaviour of light affect concepts of time, space and matter?

• Students:

  • investigate the evidence, from Einstein’s thought experiments and subsequent experimental validation, for time dilation (t=t_0/√(1-v^2/c^2 )) and length contraction (l=l_0 √(1-v^2/c^2 )) and analyse quantitatively situations in which these are observed, for example:

  • evidence from particle accelerators

  • describe the consequences and applications of relativistic momentum with reference to:

    • p_v=mv/√(1-v^2/c^2 )

  • the limitation on the maximum velocity of a particle imposed by special relativity (ACSPH133)

  • Use Einstein’s mass–energy equivalence relationship (E=mc2) to calculate the energy released by processes in which mass is converted to energy, for example: (ACSPH134)

  • particle–antiparticle interactions, eg positron–electron annihilation

PH12-15 explains and analyses the evidence supporting the relationship between astronomical events and the nucleosynthesis of atoms and relates these to the development of the current model of the atom

• Subtopic: Deep inside the Atom

• Inquiry question: How is it known that human understanding of matter is still incomplete?

• Students:

  • analyse the evidence that suggests:

  • that protons and neutrons are not fundamental particles

  • the existence of subatomic particles other than protons, neutrons and electrons

  • investigate the Standard Model of matter, including:

  • quarks, and the quark composition hadrons

  • leptons

  • fundamental forces (ACSPH141, ACSPH142)

  • investigate the operation and role of particle accelerators in obtaining evidence that tests and/or validates aspects of theories, including the Standard Model of matter (ACSPH120, ACSPH121, ACSPH122, ACSPH146)

PH11/12-4 selects and processes appropriate qualitative and quantitative data and information using a range of appropriate media

• Students:

  • select qualitative and quantitative data and information and represent them using a range of formats, digital technologies and appropriate media (ACSPH004, ACSPH007, ACSPH064, ACSPH101)

  • apply quantitative processes where appropriate

PH11/12-5 analyses and evaluates primary and secondary data and information

• Students:

  • derive trends, patterns and relationships in data and information

  • assess the relevance, accuracy, validity and reliability of primary and secondary data and suggest improvements to investigations (ACSPH005)

PH11/12-6 solves scientific problems using primary and secondary data, critical thinking skills and scientific processes

• Students:

  • use modelling (including mathematical examples) to explain phenomena, make predictions and solve problems using evidence from primary and secondary sources (ACSPH006, ACSPH010)

  • use scientific evidence and critical thinking skills to solve problems

Ensure that students practice good citizenship by providing correct attribution and copyright information for images.

Useful links:

Digital Citizenship for schools⁵²

Creative Commons Australia⁵³

Students and copyright⁵⁴