Modern Particle Physics
2. Beyond the Standard Model
Learning outcomes
- I can identify questions that the Standard Model cannot answer.
- I can describe dark matter as an unresolved problem.
- I can explain why gravity is not included in the Standard Model.
- I can describe current areas of particle physics research.
- I can explain why scientific models continue to evolve.
The Standard Model Is Extremely Successful — But Incomplete
The Standard Model of particle physics is one of the most successful scientific models ever developed.
It describes:
- quarks
- leptons
- photons
- gluons
- W and Z bosons
- the Higgs boson
- the strong interaction
- the electromagnetic interaction
- the weak interaction
Its predictions have been confirmed by many experiments.
However, the Standard Model does not explain everything we observe in nature.
Important unanswered questions remain.
What Does "Beyond the Standard Model" Mean?
The phrase Beyond the Standard Model, often shortened to BSM, refers to theories and experiments that investigate physics not fully explained by the Standard Model.
Researchers are asking questions such as:
- What is dark matter?
- How can gravity be described quantum mechanically?
- Why do neutrinos have mass?
- Why is there more matter than antimatter?
- Why do fundamental particles have the masses they do?
- Are there additional particles or forces?
- Is the Higgs boson part of a larger theory?
- Are quarks and leptons truly fundamental?
These questions show that the Standard Model may be part of a deeper theory.
Major Problems the Standard Model Cannot Fully Explain
A useful summary is:
| Problem | Why It Matters |
|---|---|
| Dark matter | Most matter in the universe does not appear to be ordinary Standard Model matter |
| Gravity | No complete quantum description is included |
| Neutrino masses | Minimal Standard Model originally treats neutrinos as massless |
| Matter-antimatter asymmetry | Known physics does not fully explain why matter dominates |
| Dark energy | Cause of accelerated cosmic expansion is unknown |
| Hierarchy problem | Higgs mass appears unusually small compared with some fundamental energy scales |
| Strong CP problem | Strong interaction appears to preserve CP symmetry much more closely than expected |
Not every proposed solution is correct, but each problem motivates active research.
Dark Matter
One of the biggest mysteries is dark matter.
Astronomical observations suggest that galaxies and galaxy clusters contain much more mass than can be explained by visible matter alone.
This unseen component is called dark matter.
We do not directly see dark matter because it does not appear to interact strongly with electromagnetic radiation.
That means it does not:
- emit ordinary light
- reflect ordinary light
- absorb light strongly in the way normal matter does
Scientists infer its presence mainly from its gravitational effects.
Evidence for Dark Matter
Several lines of evidence point toward additional unseen mass.
These include:
- galaxy rotation curves
- motion of galaxies within clusters
- gravitational lensing
- structure formation in the universe
- measurements of the cosmic microwave background
Galaxy Rotation Curves
In a galaxy, stars orbit the galactic centre.
If most of the mass were concentrated only where we see stars and gas, stars farther from the centre should generally orbit more slowly.
Instead, many galaxies have relatively flat rotation curves.
This means outer stars are moving faster than expected from visible matter alone.
One explanation is that galaxies are surrounded by large halos of dark matter.
Gravitational Lensing
Mass bends spacetime and therefore bends the path of light.
This effect is called gravitational lensing.
By observing how light from distant galaxies is distorted, scientists can estimate the amount and distribution of mass in foreground objects.
In some galaxy clusters, the gravitational mass inferred from lensing is much greater than the visible matter alone.
This provides another important clue for dark matter.
What Could Dark Matter Be?
Scientists do not yet know.
Possible candidates include hypothetical particles such as:
- WIMPs
- axions
- sterile neutrinos
- other undiscovered particles
These possibilities remain under investigation.
No dark matter particle has yet been confirmed experimentally.
Could Dark Matter Be Ordinary Matter We Cannot See?
Some dark objects do exist, such as:
- black holes
- faint stars
- planets
- cold gas
However, observations indicate that these ordinary forms of matter cannot account for all of the dark matter inferred cosmologically.
Therefore, much dark matter is thought to be non-baryonic, meaning it is not mainly made from ordinary protons and neutrons.
Dark Matter Detection
Scientists use several approaches.
Direct Detection
Experiments look for possible dark matter particles interacting very weakly with detectors.
These experiments are often located deep underground to reduce background radiation.
Indirect Detection
Scientists search for particles or radiation that might be produced if dark matter particles annihilate or decay.
Collider Searches
Particle accelerators such as the Large Hadron Collider can search for collision events that may indicate invisible particles.
A possible signal would involve:
missing energy and momentum
suggesting that undetected particles carried energy away.
Gravity and the Standard Model
Gravity is another major unresolved problem.
The Standard Model describes three fundamental interactions:
- strong
- electromagnetic
- weak
Gravity is not included.
Gravity is instead described extremely successfully on large scales by Einstein's general theory of relativity.
General Relativity
General relativity describes gravity as the curvature of spacetime caused by mass-energy.
It works extremely well for:
- planetary motion
- stars
- galaxies
- black holes
- gravitational waves
- the expansion of the universe
However, general relativity is not a quantum theory.
The Problem
Particle physics is described using quantum field theory.
Gravity is described using general relativity.
Both theories work extremely well in their own domains.
The problem appears when both are needed at the same time.
Examples include extreme conditions such as:
- the very early universe
- the centre of black holes
- extremely high-energy, tiny-scale environments
A complete theory should somehow combine:
quantum mechanics + gravity
Scientists call this problem quantum gravity.
Why Can't We Simply Add Gravity?
The mathematical methods that work so well for the other Standard Model forces do not straightforwardly produce a complete, predictive quantum theory of gravity.
Attempts to treat gravity in the same way lead to mathematical difficulties at extremely high energies.
This does not mean general relativity is wrong.
It means that general relativity may be incomplete when quantum effects become important.
The Graviton
Some quantum gravity models predict a hypothetical particle called the graviton.
A graviton would be the quantum carrier associated with gravity.
It is usually predicted to have:
- zero electric charge
- zero rest mass
- spin 2
However:
no graviton has been experimentally detected.
The graviton is therefore not a confirmed Standard Model particle.
Neutrino Masses
Neutrinos provide another clue that our understanding is incomplete.
The original minimal Standard Model treated neutrinos as massless.
However, experiments show that neutrinos can change from one type into another.
This is called:
neutrino oscillation
Why Neutrino Oscillation Matters
Neutrino oscillation requires that at least some neutrino mass states have different, nonzero masses.
Therefore:
neutrinos are not exactly massless.
This means the simplest original version of the Standard Model must be extended.
Scientists still do not know:
- the absolute neutrino masses
- the exact mass ordering with complete certainty
- whether neutrinos are their own antiparticles
- the full origin of neutrino mass
Majorana Neutrinos
One possibility is that neutrinos could be Majorana particles.
A Majorana particle would be its own antiparticle.
If neutrinos are Majorana particles, it could have major consequences for particle physics and cosmology.
One important experimental search involves:
neutrinoless double beta decay
If observed, this would provide evidence that lepton number is not an exact conservation law and that neutrinos may be Majorana particles.
Matter-Antimatter Asymmetry
The observable universe contains much more matter than antimatter.
This is surprising.
Many particle processes can create matter and antimatter in approximately equal amounts.
So why does the universe contain:
- stars
- planets
- galaxies
- people
instead of equal amounts of matter and antimatter?
This problem is called the matter-antimatter asymmetry or baryon asymmetry.
CP Violation
One clue involves CP violation.
CP roughly combines:
- charge conjugation, which swaps particles with antiparticles
- parity, which reverses spatial coordinates
Some weak-interaction processes treat matter and antimatter slightly differently.
CP violation has been observed experimentally.
However, the amount known within the Standard Model appears insufficient to explain the enormous dominance of matter in the universe.
This suggests that additional physics may be involved.
Dark Energy
Dark matter and dark energy are not the same thing.
Dark matter helps explain gravitational effects associated with unseen mass.
Dark energy is the name given to whatever is causing the expansion of the universe to accelerate.
Dark energy appears to make up a large fraction of the total energy content of the universe.
Its fundamental nature remains unknown.
Dark Matter vs Dark Energy
| Dark Matter | Dark Energy |
|---|---|
| Behaves gravitationally like matter | Associated with accelerated cosmic expansion |
| Helps galaxies and clusters remain bound | Acts on very large cosmological scales |
| Clumps around structures | Appears relatively smooth |
| Particle nature unknown | Physical nature unknown |
Neither is fully explained by the Standard Model.
The Higgs and Unanswered Questions
The Higgs boson was a major success for the Standard Model.
However, its discovery also raised further questions.
For example:
- Why does the Higgs have its measured mass?
- Is there only one Higgs boson?
- Is the Higgs truly fundamental?
- Does the Higgs interact with dark matter?
- Could there be additional Higgs-like fields?
Scientists continue to measure the Higgs boson's properties with increasing precision.
The Hierarchy Problem
One theoretical puzzle is called the hierarchy problem.
The measured Higgs mass is much smaller than some enormous energy scales that may exist in physics.
Quantum corrections might be expected to push the Higgs mass toward much larger values unless some mechanism stabilizes it.
Possible explanations include:
- new symmetries
- new particles
- composite Higgs models
- extra dimensions
- other unknown physics
No single solution has yet been confirmed.
Supersymmetry
One proposed extension of the Standard Model is supersymmetry, often abbreviated:
SUSY
Supersymmetry proposes a relationship between fermions and bosons.
Each known particle would have a hypothetical partner.
Examples would include:
- electron → selectron
- quark → squark
- gluon → gluino
Some supersymmetric particles could potentially be dark matter candidates.
However, no supersymmetric partner particle has yet been confirmed.
Extra Dimensions
Some theories propose that there may be additional spatial dimensions beyond the three we experience directly.
These extra dimensions could be extremely small or otherwise difficult to detect.
Such models have been explored as possible ways to address questions involving:
- gravity
- particle masses
- unification of forces
No experimental evidence has yet confirmed extra spatial dimensions.
String Theory
String theory proposes that the most fundamental objects may not be point-like particles.
Instead, they could be tiny vibrating strings.
Different vibration patterns would correspond to different particles.
One appealing feature is that some versions naturally include a quantum description of gravity.
However, string theory has not yet received direct experimental confirmation.
Grand Unified Theories
Another major goal is to determine whether the fundamental interactions were once part of a single unified interaction.
At very high energies, electromagnetism and the weak interaction combine into the:
electroweak interaction
This suggests that further unification might be possible.
A Grand Unified Theory, or GUT, attempts to combine:
- strong interaction
- weak interaction
- electromagnetic interaction
into a single framework.
Gravity would still require further unification.
Proton Decay
Some grand unified theories predict that the proton may eventually decay.
For example, a hypothetical reaction might produce lighter particles.
If proton decay exists, its lifetime must be extremely long because ordinary matter is clearly very stable.
Large underground detectors search for possible proton-decay events.
So far, proton decay has not been confirmed.
Are Quarks and Leptons Truly Fundamental?
The Standard Model treats quarks and leptons as fundamental.
But scientists continue to ask whether they might themselves be made from smaller constituents.
Hypothetical substructures have sometimes been called:
preons
No convincing experimental evidence for quark or lepton substructure has been found.
Current experiments place increasingly strict limits on how small and fundamental these particles appear to be.
New Forces and New Particles
Physicists also search for possible additional interactions.
There might be:
- new gauge bosons
- weakly interacting particles
- hidden-sector particles
- additional scalar particles
- unknown forces
Some hypothetical particles are given names such as:
Z′ boson
This would represent an additional neutral gauge boson beyond the known Z⁰.
No such new force carrier has yet been confirmed.
Precision Measurements
Searching for new physics does not always require directly producing a new particle.
Scientists can also make extremely precise measurements of known particles.
They measure properties such as:
- masses
- lifetimes
- magnetic moments
- decay rates
- branching fractions
- interaction strengths
If measurements disagree with Standard Model predictions, that could provide evidence for new physics.
The Muon
The muon is a heavier relative of the electron.
Its magnetic properties can be measured very precisely.
Experiments compare the measured magnetic moment with theoretical predictions.
Any persistent, confirmed disagreement could point toward undiscovered particles or interactions.
This is an example of an indirect search for physics beyond the Standard Model.
Rare Particle Decays
Some particle decays are extremely rare.
The Standard Model predicts their probabilities very precisely.
Scientists look for:
- unexpected decay rates
- forbidden decays
- unusual particle combinations
A deviation could indicate new particles participating indirectly in the process.
Particle Colliders
Large colliders remain one of the major tools for exploring new physics.
At high energies, scientists can:
- create heavy particles
- study rare interactions
- test Standard Model predictions
- search for missing energy
- examine Higgs properties
- look for unexpected particles
Why Higher Energy Helps
Higher-energy collisions allow physicists to:
create heavier particles
because:
E = mc²
They also allow scientists to probe smaller distance scales.
A useful conceptual relationship is:
higher momentum → shorter wavelength → finer resolution
This is similar to using a shorter-wavelength microscope to see smaller structures.
But Energy Is Not Everything
New physics might not appear simply by increasing collision energy.
It could involve:
- extremely rare processes
- very weak interactions
- tiny deviations from expected values
Therefore, modern particle physics also focuses heavily on:
precision
as well as:
energy
Underground Experiments
Some of the most sensitive particle-physics experiments are built deep underground.
Rock above the detector absorbs much of the cosmic radiation that would otherwise create background signals.
Underground detectors are used to study:
- neutrinos
- dark matter
- proton decay
- rare nuclear processes
Cosmic-Ray Physics
Nature also produces particles with enormous energies.
Cosmic rays strike Earth's atmosphere and produce showers of secondary particles.
These natural high-energy events allow researchers to study particle interactions at energies sometimes beyond those directly produced in human-built accelerators.
Astroparticle Physics
Modern research increasingly connects:
- particle physics
- astronomy
- cosmology
This field is often called astroparticle physics.
Examples include studying:
- dark matter
- neutrinos
- cosmic rays
- black holes
- gravitational waves
- the early universe
The boundaries between nuclear physics, particle physics, and astrophysics are increasingly connected.
The Early Universe as a Physics Laboratory
Shortly after the Big Bang, temperatures and energies were enormously high.
Conditions may have existed that cannot easily be reproduced in laboratories today.
By studying the universe, scientists can therefore test ideas about:
- particle creation
- symmetry breaking
- matter-antimatter imbalance
- dark matter
- cosmic inflation
- phase transitions in fundamental fields
Why Scientific Models Change
A scientific model is not simply a guess.
A strong scientific model:
- explains observations
- makes testable predictions
- agrees with experiments
- can be revised when new evidence appears
The Standard Model remains extremely useful because it explains an enormous range of data.
Its limitations do not mean it is "wrong."
They mean it may be incomplete.
Newton, Einstein, and Scientific Progress
A useful comparison comes from gravity.
Newton's law of gravitation works extremely well for many problems.
Einstein's general relativity provides a deeper description.
This does not make Newton's model useless.
Instead:
new theory extends the older theory
and reproduces its successful predictions under appropriate conditions.
The same may eventually happen with the Standard Model.
Models Have Domains of Validity
A model can be extremely accurate within a certain range.
For example:
classical mechanics
works very well for everyday objects.
quantum mechanics
becomes essential at atomic scales.
Similarly:
general relativity
works extremely well for gravity.
the Standard Model
works extremely well for known particle interactions.
A deeper theory may eventually connect them.
Scientific Models Evolve Through Evidence
The process often follows:
observation
↓
model
↓
prediction
↓
experiment
↓
comparison with data
↓
model confirmed, refined, or replaced
This process is central to science.
Example 1: Dark Matter Reasoning
Suppose astronomers calculate that visible matter in a galaxy should produce an orbital speed of:
120 km/s
but stars are observed moving at:
220 km/s
far from the centre.
One possible conclusion is:
additional gravitational mass is present
Dark matter is one explanation for this discrepancy.
The observation does not directly reveal what dark matter is.
It reveals that our visible-matter model is incomplete.
Example 2: Gravity
Which of the four fundamental interactions is not included in the Standard Model?
Gravity
It is currently described by general relativity rather than the Standard Model.
Example 3: Neutrinos
Suppose neutrinos are observed changing flavour while travelling.
This tells us that:
the neutrino states have nonzero mass differences
Therefore, the simplest massless-neutrino version of the Standard Model is incomplete.
Example 4: New Particle Search
Suppose a collider collision begins with a known total momentum.
The detector measures all visible particles but finds substantial missing momentum.
One possibility is that:
an invisible particle left the detector
This does not automatically prove dark matter.
Scientists must first rule out:
- neutrinos
- detector effects
- incomplete measurements
- Standard Model backgrounds
Only repeated statistically significant evidence could support a new-particle interpretation.
Evidence Must Be Strong
Particle physics uses extremely strict statistical standards before announcing major discoveries.
Rare fluctuations can occur by chance.
Scientists therefore require:
- large datasets
- careful background analysis
- independent checks
- reproducible measurements
This is why scientific discoveries can take years.
A Map of Beyond-Standard-Model Questions
Standard Model
↓
What is missing?
↓
Gravity → quantum gravity
Dark matter → unknown particle or new physics
Neutrino mass → extended neutrino sector
Matter-antimatter imbalance → additional CP violation?
Higgs questions → new symmetry or fields?
Force unification → grand unified theories
Early universe → unknown high-energy physics
These questions guide much of modern fundamental physics.
Did You Know?
Some of the most important discoveries in physics came from tiny inconsistencies rather than completely unexpected observations.
Small disagreements between theory and experiment have historically led to major advances.
For this reason, physicists do not only search for spectacular new particles.
They also repeatedly measure familiar particles with extraordinary precision.
A tiny deviation could reveal an entirely new layer of physics.
Connecting the Ideas
The development of particle physics can be viewed as:
atoms
↓
electrons and nuclei
↓
protons and neutrons
↓
quarks and leptons
↓
Standard Model
↓
unanswered questions
↓
new experiments and theories
↓
possible deeper model
Scientific knowledge therefore develops rather than simply stopping once a successful model has been created.
Current Research Questions
Important areas of modern particle physics include:
- searching for dark matter particles
- determining neutrino masses
- testing whether neutrinos are Majorana particles
- searching for proton decay
- studying Higgs properties
- searching for additional Higgs-like particles
- looking for supersymmetric particles
- testing matter-antimatter differences
- measuring rare particle decays
- testing the Standard Model at higher precision
- investigating quantum gravity
- searching for additional forces
- studying the early universe
Not all of these ideas will turn out to be correct.
That is precisely why experiments are required.
Key Terms
Beyond the Standard Model (BSM) – Research into physics not fully explained by the Standard Model.
Dark matter – Unseen matter inferred mainly from gravitational effects.
Dark energy – The name given to the unknown cause associated with the accelerated expansion of the universe.
Quantum gravity – A proposed framework that would describe gravity using quantum principles.
Graviton – A hypothetical quantum particle associated with gravity.
Neutrino oscillation – The change of neutrinos between flavour states during propagation.
CP violation – A difference between certain processes involving matter and antimatter.
Baryon asymmetry – The observed dominance of matter over antimatter in the universe.
Supersymmetry – A proposed symmetry connecting fermions and bosons.
Grand Unified Theory – A theory attempting to combine the strong, weak, and electromagnetic interactions.
Majorana particle – A particle that is identical to its own antiparticle.
Precision measurement – A highly accurate measurement used to test theoretical predictions.
Key Takeaways
- The Standard Model is extremely successful but incomplete.
- It describes the strong, electromagnetic, and weak interactions, but not gravity.
- Dark matter is inferred from gravitational observations, but its particle nature remains unknown.
- Gravity is described by general relativity rather than the Standard Model.
- A complete theory of quantum gravity has not yet been experimentally established.
- Neutrino oscillations show that neutrinos have nonzero mass differences, requiring physics beyond the simplest Standard Model.
- The Standard Model does not fully explain why the universe contains much more matter than antimatter.
- Dark energy, the hierarchy problem, and the strong CP problem are additional major unanswered questions.
- Scientists search for new physics using colliders, underground detectors, astrophysical observations, rare decays, and extremely precise measurements.
- Proposed ideas such as supersymmetry, extra dimensions, grand unification, and string theory remain unconfirmed.
- New scientific theories must reproduce the successes of earlier models while also explaining observations the earlier models cannot.
- Scientific models continue to evolve because new evidence can reveal deeper levels of understanding.