What Is a Particle Accelerator?

A particle accelerator is a machine that increases the energy of charged particles and directs them into targets or into other particle beams.

Scientists use particle accelerators to investigate the structure of matter at extremely small scales.

Accelerators can be used to:

• study fundamental particles
• investigate nuclear structure
• create unstable particles
• test particle-physics theories
• search for previously unknown particles
• produce radioactive isotopes
• support medical and industrial applications

In particle physics, accelerators are especially important because higher particle energies allow scientists to investigate smaller distance scales and create heavier particles.

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Why Accelerate Particles?

There are two major reasons for giving particles very high energies.

1. Higher energy can create new particles

Einstein's mass-energy relationship tells us:

E = mc²

During a high-energy collision, some kinetic energy can be converted into the rest mass-energy of new particles.

Therefore:

higher collision energy → possibility of producing heavier particles

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2. Higher energy allows smaller structures to be investigated

Particles also behave as waves.

Their wavelength is related to momentum.

Very high-momentum particles have extremely short wavelengths.

Shorter wavelengths can probe smaller structures.

Therefore:

higher momentum → shorter wavelength → smaller distance scale

This is similar to using shorter-wavelength light to obtain greater detail with a microscope.

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How Are Charged Particles Accelerated?

Charged particles gain energy from electric fields.

An electric field exerts a force on a charged particle.

The force is:

F = qE

where:

F = electric force

q = particle charge

E = electric field strength

When a charged particle moves through a potential difference, its energy changes.

The electrical energy gained is:

ΔE = qV

where:

q = charge

V = potential difference

This is the basic principle behind particle acceleration.

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Example: Accelerating an Electron

An electron moving through a potential difference of:

1000 V

gains an energy of:

1000 eV

because the magnitude of the electron charge is one elementary charge.

Therefore:

1 electron through 1 volt → 1 eV

This is why the electronvolt is such a convenient unit in particle physics.

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What Is an Electronvolt?

An electronvolt (eV) is the amount of energy gained by a particle with one elementary charge moving through a potential difference of one volt.

Useful units include:

1 keV = 10³ eV

1 MeV = 10⁶ eV

1 GeV = 10⁹ eV

1 TeV = 10¹² eV

Modern research accelerators can reach energies measured in GeV and TeV.

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Linear Accelerators

A linear accelerator, often called a linac, accelerates particles along a straight path.

The particles pass through a sequence of accelerating regions.

Each region increases their energy.

A simplified sequence is:

particle source → accelerating sections → high-energy beam

Electric fields repeatedly push the charged particles forward.

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Radio-Frequency Cavities

Modern accelerators often use radio-frequency cavities, or RF cavities.

These cavities create rapidly changing electric fields.

The timing is arranged so that particles receive a forward push each time they pass through an accelerating region.

Repeated pushes gradually increase the particle energy.

We can think of it as repeatedly giving the particles carefully timed boosts.

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Circular Accelerators

Another major design uses a circular or nearly circular path.

Particles move around the accelerator many times.

Each time they pass through accelerating cavities, they gain more energy.

This allows particles to be accelerated repeatedly without requiring an impossibly long straight accelerator.

Two important types are:

• cyclotrons
• synchrotrons

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Magnetic Fields and Particle Motion

Electric fields primarily increase particle energy.

Magnetic fields are used mainly to:

• bend the path of charged particles
• steer the beam
• focus particles into a narrow beam

A moving charged particle experiences a magnetic force.

For motion perpendicular to a magnetic field:

F = qvB

where:

q = charge

v = particle speed

B = magnetic field strength

The magnetic force can make the particle follow a curved path.

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Why Strong Magnets Are Needed

As particles gain momentum, they become harder to bend.

The relationship for a charged particle moving perpendicular to a magnetic field can be written:

p = qBr

where:

p = momentum

q = charge

B = magnetic field strength

r = radius of curvature

To keep very high-momentum particles moving around a circular accelerator, scientists need:

• stronger magnetic fields
• larger accelerator rings
• or both

This is one reason high-energy accelerators can be enormous.

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Particle Beams

A particle accelerator does not usually accelerate just one particle.

Instead, particles are grouped into bunches.

These bunches travel through the accelerator at extremely high speeds.

In a collider, two beams may travel in opposite directions.

At selected locations, the beams are made to cross.

That produces high-energy collisions.

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Fixed-Target Accelerators

In a fixed-target experiment, a high-energy beam strikes a stationary target.

For example:

high-energy proton → stationary target

The collision can produce many particles.

Fixed-target experiments are useful for many types of nuclear and particle research.

However, some of the incoming energy must remain as motion of the final system because momentum must be conserved.

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Particle Colliders

In a particle collider, two beams travel toward each other.

For example:

proton → ← proton

If the two particles have equal and opposite momentum, the total momentum of the system can be approximately zero.

This means a much larger fraction of their energy is available for creating new particles.

Therefore, colliders are particularly useful when searching for very massive particles.

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Particle Detectors

Producing particles is only part of the experiment.

Scientists must also determine what was created.

A particle detector measures the products of particle collisions.

Different detector systems can measure:

• particle tracks
• electric charge
• momentum
• energy
• speed
• particle type

Modern particle detectors often contain several layers, with each layer performing a different job.

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Tracking Detectors

A tracking detector records the paths of charged particles.

Charged particles passing through detector material leave signals along their trajectories.

If the detector is placed in a magnetic field, charged particles follow curved paths.

The curvature helps determine:

• the sign of the particle's charge
• its momentum

Positive and negative particles curve in opposite directions.

Higher-momentum particles curve less strongly.

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Calorimeters

A calorimeter measures particle energy.

A particle enters the detector and interacts with the material, producing a cascade or shower of secondary particles.

The detector measures the energy deposited.

Different types of calorimeters are designed for different particles.

For example:

• electromagnetic calorimeters measure electrons and photons
• hadronic calorimeters measure particles such as protons, neutrons, and pions

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Muon Detectors

Muons can pass through large amounts of matter more easily than many other charged particles.

For this reason, muon detectors are often placed in the outer layers of a detector.

If a charged particle passes through the inner detector and calorimeters and continues into the outer muon system, it may be identified as a muon.

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Detecting Neutrinos

Neutrinos interact extremely weakly with matter.

They usually pass through particle detectors without leaving a direct signal.

Scientists therefore infer their presence from missing energy and momentum.

For example, if the measured particles do not account for all the momentum in an event, an undetected neutrino may have carried some away.

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Putting Detector Layers Together

A simplified multipurpose detector may contain:

collision point

↓

tracking detector

↓

electromagnetic calorimeter

↓

hadronic calorimeter

↓

muon detector

Each system contributes different information.

Scientists combine the measurements to identify particles.

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How Can a Detector Identify a Particle?

Suppose a collision produces an unknown charged particle.

Scientists may examine:

• how much its path curves
• which detector layers it reaches
• how much energy it deposits
• whether it produces an electromagnetic or hadronic shower
• how quickly it travels

Combining this information allows physicists to distinguish among particles such as:

• electrons
• muons
• protons
• pions
• photons

No single detector component usually provides the complete answer.

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Bubble Chambers and Early Particle Detection

Before modern electronic detectors, scientists often used bubble chambers.

A bubble chamber contained a superheated liquid.

Charged particles passing through the liquid created trails of bubbles.

Photographs of these tracks allowed physicists to study particle paths and interactions.

Curved tracks in magnetic fields revealed information about particle charge and momentum.

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Discoveries Made with Particle Accelerators

Particle accelerators have played a major role in the development of modern physics.

They have helped scientists discover or investigate many particles, including:

• new mesons and baryons
• antimatter particles
• quarks
• W and Z bosons
• the tau lepton
• the Higgs boson

Accelerators have also provided evidence supporting the Standard Model of particle physics.

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Discovering Quarks

Quarks cannot normally be observed as isolated particles because of quark confinement.

However, high-energy scattering experiments revealed that protons and neutrons contain smaller point-like constituents.

Experiments in which high-energy electrons were scattered from protons provided important evidence for internal structures later understood as quarks and gluons.

This was similar in principle to Rutherford's earlier use of scattering to investigate the atomic nucleus.

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W and Z Bosons

The W and Z bosons carry the weak interaction.

They were discovered in accelerator experiments at CERN in the early 1980s.

Their discovery strongly confirmed the electroweak theory, which connects electromagnetic and weak interactions within the Standard Model.

The particles are:

W⁺

W⁻

Z⁰

They are much more massive than particles such as electrons, so high-energy collisions were necessary to produce them.

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The Higgs Boson

One of the most famous accelerator discoveries is the Higgs boson.

The Higgs boson is associated with the Higgs field.

The Higgs mechanism helps explain why fundamental particles such as the W and Z bosons, quarks, and charged leptons have mass.

The Higgs boson was discovered in 2012 by experiments at the Large Hadron Collider.

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The Large Hadron Collider

The Large Hadron Collider, or LHC, is a very large particle accelerator at CERN.

It is located in a circular underground tunnel approximately:

27 km in circumference

near Geneva, on the border between Switzerland and France.

The LHC is primarily designed to collide high-energy proton beams.

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Why Is It Called the Large Hadron Collider?

The name describes what it does.

Large

It has a circumference of roughly 27 km.

Hadron

It accelerates hadrons, particularly protons.

Protons are baryons and therefore belong to the larger family of hadrons.

Collider

Two particle beams travel in opposite directions and are brought into collision.

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How the LHC Works

A simplified sequence is:

1. Protons are produced.

2. Smaller accelerators increase their energy.

3. The protons are injected into the LHC.

4. Two proton beams travel in opposite directions.

5. RF cavities increase their energy.

6. Powerful magnets steer and focus the beams.

7. The beams cross at detector locations.

8. Collision products are measured.

This entire process requires extremely precise control.

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Superconducting Magnets

The LHC uses powerful superconducting magnets.

When some materials are cooled to extremely low temperatures, they can conduct electrical current with essentially no resistance.

This allows very large currents to flow and produce strong magnetic fields.

These magnetic fields bend the high-energy proton beams around the accelerator ring.

The magnets are cooled using liquid helium systems to temperatures close to absolute zero.

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Major LHC Experiments

Several large experiments are located around the LHC.

Two of the best known are:

ATLAS

and

CMS

Both are general-purpose detectors designed to investigate many different types of particle interactions.

Other experiments specialize in particular areas of particle physics.

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ATLAS and CMS

ATLAS and CMS independently analyze proton-proton collisions.

Having two different experiments study similar physics is valuable.

If both experiments observe the same phenomenon independently, confidence in the result increases.

This was important in the discovery of the Higgs boson.

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Why Was the Higgs Discovery Important?

The Standard Model predicted the existence of the Higgs boson.

Before 2012, scientists had strong indirect evidence for the Higgs mechanism but had not observed the Higgs boson itself.

The LHC produced enough high-energy collisions for Higgs bosons to be created.

The Higgs bosons decayed almost immediately.

Scientists identified them by analyzing their decay products.

In July 2012, the ATLAS and CMS collaborations announced the discovery of a new particle consistent with the Higgs boson.

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Why Didn't Scientists Simply See the Higgs?

The Higgs boson is extremely short-lived.

It decays before it can travel through a detector as an observable particle.

Instead:

collision → Higgs boson → decay products

Scientists measure the decay products.

They then reconstruct the mass and properties of the original particle.

A concentration of events near a particular reconstructed mass provides evidence for the particle.

This is similar to solving a puzzle from the pieces left behind.

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What Else Does the LHC Study?

The LHC is not just a "Higgs machine."

Scientists use it to study:

• properties of the Higgs boson
• quarks and gluons
• top quarks
• matter-antimatter differences
• heavy-ion collisions
• conditions similar to the early universe
• possible physics beyond the Standard Model

Researchers also search for evidence of particles that might help explain dark matter or other unresolved questions in physics.

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Heavy-Ion Collisions

The LHC can also collide heavy atomic nuclei, such as lead nuclei.

At extremely high collision energies, these interactions can produce a state of matter known as a quark-gluon plasma.

Scientists believe the early universe contained similar conditions shortly after the Big Bang.

Studying this state helps researchers understand how quarks and gluons behave under extreme conditions.

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Why Are Particle Accelerators So Large?

Higher-energy particles have greater momentum.

That makes them harder to bend around a circular path.

From:

p = qBr

increasing momentum requires either:

• stronger magnetic field B
• larger radius r
• or both

There are practical limits to magnetic-field strength.

Therefore, increasing accelerator size can allow particles to reach higher energies.

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Why Can't Particles Be Accelerated Beyond the Speed of Light?

As a particle with mass gains energy, its speed approaches the speed of light.

However, it never reaches or exceeds:

c = 3.00 × 10⁸ m/s

Adding more energy continues to increase its total energy and momentum, but the increase in speed becomes extremely small.

This is why accelerator physicists focus on particle energy, not simply particle speed.

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Medical Uses of Particle Accelerators

Particle accelerators are not used only for fundamental physics.

They also have important medical applications.

Examples include:

• X-ray production
• radiation therapy
• proton therapy
• production of medical radioisotopes

Hospitals commonly use smaller linear accelerators for cancer treatment.

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Proton Therapy

In proton therapy, accelerated protons are directed toward a tumour.

Protons can deposit a large fraction of their energy near the end of their path.

This allows doctors to target some tumours while reducing radiation exposure to surrounding tissue.

Particle-accelerator technology therefore has direct medical applications.

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Industrial and Scientific Applications

Accelerators are also used for:

• materials research
• semiconductor manufacturing
• sterilization
• isotope production
• archaeological analysis
• studying the structure of materials
• synchrotron light production

This makes particle accelerators valuable far beyond particle physics.

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Example 1: Why Use Electric Fields?

A student says:

"Magnetic fields are what speed particles up in an accelerator."

This is generally incorrect.

A magnetic force acts perpendicular to the velocity of a charged particle, so it mainly changes the direction of motion.

Electric fields can do work on charged particles and increase their kinetic energy.

Therefore:

electric fields → accelerate

magnetic fields → steer and focus

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Example 2: Why Use a Collider?

Suppose two identical high-energy particles travel toward each other with equal and opposite momentum.

Their total momentum can be approximately:

0

If they collide, a large fraction of their total energy is available for creating new particles.

By contrast, if one strikes a stationary target, the products must carry substantial momentum away.

This is why high-energy research often uses colliders.

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Example 3: Detecting Charge

Suppose two charged particles pass through the same magnetic field.

One curves left.

The other curves right.

The particles must have opposite signs of electric charge, assuming they are travelling in the same initial direction.

The direction of curvature therefore gives information about charge.

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Example 4: Detecting Momentum

Two particles with equal charge travel through the same magnetic field.

Particle A curves strongly.

Particle B curves only slightly.

Since:

p = qBr

the particle with the larger radius has greater momentum.

Therefore:

Particle B has the greater momentum.

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Did You Know?

Particle accelerators do not only help us study extremely small objects.

Accelerator technology has produced tools used in medicine, industry, chemistry, biology, materials science, and engineering.

Only a relatively small fraction of the world's particle accelerators are dedicated to high-energy particle physics.

Many are used for practical research, manufacturing, and healthcare.

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Connecting the Ideas

Particle accelerators bring together many concepts from this course:

Charged particles

↓

Electric fields increase energy

↓

Magnetic fields steer and focus

↓

High-energy particle beams

↓

Collisions

↓

E = mc² allows new particles to be created

↓

Unstable particles decay

↓

Detectors measure the products

↓

Conservation laws help reconstruct the interaction

↓

New particles and physical laws can be investigated

The accelerator and detector therefore work together as one experimental system.

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Key Terms

Particle accelerator – A machine that increases the energy of charged particles.

Linear accelerator (linac) – An accelerator in which particles travel mainly along a straight path.

Synchrotron – A circular accelerator in which magnetic fields and accelerating systems are synchronized with the particles.

Electric field – A field that can exert a force and do work on charged particles.

Magnetic field – A field used to bend and focus moving charged particles.

Particle beam – A directed group of high-energy particles.

Collider – An accelerator designed to bring two particle beams into collision.

Fixed-target experiment – An experiment in which an accelerated beam strikes a stationary target.

Particle detector – Equipment used to measure particles produced in interactions.

Calorimeter – A detector designed to measure particle energy.

Tracking detector – A detector that records the paths of charged particles.

Large Hadron Collider (LHC) – A large particle collider at CERN used to study high-energy particle interactions.

Higgs boson – A particle associated with the Higgs field, discovered at the LHC in 2012.

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Key Takeaways

• Particle accelerators increase the energy of charged particles.
• Scientists use accelerators to investigate matter at extremely small scales and to create new particles.
• Electric fields increase particle energy.
• Magnetic fields steer and focus moving charged particles.
• Linear accelerators move particles mainly along straight paths, while circular accelerators allow repeated acceleration.
• High-energy collisions can convert kinetic energy into the mass-energy of new particles.
• Colliders allow more energy to be available for particle production than comparable fixed-target experiments.
• Particle detectors identify particles using information such as tracks, charge, momentum, and energy.
• Tracking detectors, calorimeters, and muon systems perform different roles.
• Neutrinos are often identified indirectly through missing energy and momentum.
• Particle accelerators have contributed to major discoveries, including the W and Z bosons and the Higgs boson.
• The Large Hadron Collider is one of the most important tools for testing the Standard Model and searching for new physics.
• Accelerator technology also has important applications in medicine, industry, and materials research.