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Particle Interactions & Conservation

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Edexcel IAL Physics — Revision Guide

Particle Interactions & Conservation

The big idea: Every particle interaction in the universe — whether it's a proton smashing into another proton, or a neutron quietly decaying — has to obey a strict "accounting" system. Charge, baryon number, and lepton number all have to balance perfectly before and after, or the interaction simply cannot happen, no matter how much energy is available.

Summary — What's In This Chapter

All matter is built from quarks and leptons — the fundamental building blocks of the Standard Model.
Hadrons (particles made of quarks) split into baryons (3 quarks) and mesons (quark + antiquark).
Leptons are fundamental — not made of anything smaller — and don't feel the strong force.
Every matter particle has an antimatter twin with identical mass but opposite charge.
Three quantities are always conserved in any interaction: charge (Q), baryon number (B), and lepton number (L).
You can use conservation laws to work out whether a proposed reaction is even allowed, or to identify a mystery particle in an equation.

1. The Standard Model — Quarks & Leptons

Let's start from the very bottom. If you kept splitting matter into smaller and smaller pieces, you'd eventually hit a wall — particles that aren't made of anything else. These are called fundamental particles, and the Standard Model sorts every one of them into two families: quarks and leptons.

Think of it like LEGO Quarks are like tiny LEGO bricks that only ever get used in combination — you never find a single quark sitting on its own. Leptons, on the other hand, are more like finished mini-figures — complete, independent, fundamental units that don't need to combine with anything else to exist.

The Three Generations

Both quarks and leptons come in three generations, with mass increasing as you go down the generations. Everyday matter (you, this screen, the air around you) is built entirely from Generation I particles — the up quark, down quark, electron, and electron neutrino. Generations II and III are heavier, unstable versions that only show up in high-energy events like cosmic rays or particle accelerators.

Generation	Quarks	Charge	Leptons	Charge
I	up (u), down (d)	+2/3e, −1/3e	electron (e⁻), electron neutrino (ν_e)	−1e, 0
II	charm (c), strange (s)	+2/3e, −1/3e	muon (μ⁻), muon neutrino (ν_μ)	−1e, 0
III	top (t), bottom (b)	+2/3e, −1/3e	tau (τ⁻), tau neutrino (ν_τ)	−1e, 0

Neat historical fact

Only the up, down, strange, and charm quarks were found experimentally at first. The symmetry of the Standard Model — the neat pattern of three matching generations — predicted that a top and bottom quark must also exist before anyone had seen them. Physicists went looking specifically because the pattern demanded it, and found them exactly as predicted. That's the power of a good theory: it tells you what to look for.

Hadrons: Baryons & Mesons

Quarks never fly solo. They're always bound together in combinations, and any particle made of quarks is called a hadron. Hadrons feel the strong nuclear force (which is, unsurprisingly, the force that holds quarks together). There are two types:

Baryon = 3 quarks (e.g. proton, neutron)

Meson = quark + antiquark (e.g. pion, kaon)

HADRONS / \ BARYONS MESONS (3 quarks) (quark + antiquark) e.g. p, n e.g. π⁺, K⁺

Why can't a baryon mix quarks and antiquarks? Every quark has a baryon number of +1/3, and every antiquark has a baryon number of −1/3. Baryons and mesons must always have a whole number (integer) overall baryon number — that's a hard rule of nature. Three quarks gives you 1/3 + 1/3 + 1/3 = 1 ✓. But something like an up, an anti-down, and a down would give 1/3 − 1/3 + 1/3 = 1/3, which isn't a whole number — so it simply doesn't exist as a baryon.

Quick Rule Baryons = quarks only (qqq) | Anti-baryons = antiquarks only (q̄q̄q̄) | Mesons = quark + antiquark (qq̄)

You can never mix quarks and antiquarks within a single baryon.

Leptons

Leptons are the "loners" of particle physics — genuinely fundamental, not built from anything smaller. Because they contain no quarks, they never feel the strong nuclear force. Instead they interact via the weak force, the electromagnetic force (if charged), or gravity.

The electron and muon are essentially the same kind of particle, just with wildly different masses — the muon is about 200 times heavier than the electron, yet carries exactly the same charge (−1e). Neutrinos are the featherweights of the particle world: no charge, and a mass so tiny it's often treated as zero.

Common mix-up

Students often assume quarks count as leptons because they're both "fundamental." They don't — quarks are their own separate family. "Fundamental" just means "not made of smaller pieces"; it doesn't automatically make something a lepton.

Photons

Photons are a bit different from everything above — they're not matter at all. They're the exchange particle ("carrier") for the electromagnetic force, meaning two charged particles repelling or attracting each other are, at a deeper level, described as constantly "throwing" photons back and forth. Photons are massless and uncharged, and you'll meet their energy formula elsewhere in the course:

Photon Energy E = hf = hc / λ

where h = Planck's constant, f = frequency, c = speed of light, λ = wavelength.

Practice Question 1

The baryon Δ⁺⁺ has a charge of +2. Given that quarks have charges of +2/3e (up), −1/3e (down), and −1/3e (strange), which quark combination gives this particle: uuu, cds̄, ūd, or c̄c̄c̄?

Practice Question 2

Explain why an anti-proton (p̄) must be made up entirely of antiquarks, and state its quark composition (a proton is uud).

2. Antimatter

Every single particle of matter has a "mirror twin" made of antimatter. The antimatter version is identical in every way except charge — same mass, same rest mass-energy, just the opposite sign of charge (and other quantum numbers, like baryon or lepton number, also flip sign).

Think of it like a photo negative A photo negative has exactly the same shape, the same detail, the same everything as the original photo — except every light area is dark and every dark area is light. Antimatter is like that: same "shape" (mass), but every charge is flipped.

Matter	Charge	Antimatter	Charge
Electron (e⁻)	−1	Positron (e⁺)	+1
Proton (p)	+1	Anti-proton (p̄)	−1
Neutron (n)	0	Anti-neutron (n̄)	0
Neutrino (ν)	0	Anti-neutrino (ν̄)	0

Notice the naming convention: apart from the electron/positron pair (which get completely different names for historical reasons), every antiparticle is just the matter particle's name with the prefix "anti-" and a bar drawn over its symbol.

Neutral particles are special

A truly neutral particle — like the photon — is its own antiparticle. There's nothing left to flip! Neutrons and neutrinos, however, still have distinct antiparticles even though their charge is zero, because other properties (like baryon number or lepton number) still flip sign.

Key Fact Mass(particle) = Mass(antiparticle)

Charge flips sign, but mass and rest mass-energy stay identical. A proton and antiproton weigh exactly the same.

Practice Question

A student says "the antiproton must be lighter than the proton because it has a negative charge." Explain why this statement is incorrect.

3. Conservation Laws in Particle Physics

Here's the heart of this whole chapter. Whenever particles interact — colliding, decaying, annihilating — the outcome isn't a free-for-all. Nature keeps strict "books" on certain quantities, and those books must balance perfectly on both sides of the equation. Three quantum numbers are always conserved:

Charge, Q

Baryon number, B

Lepton number, L

Think of it like a bank account Imagine charge, baryon number, and lepton number are three separate bank balances. No interaction is allowed to secretly create or destroy money from any of these accounts — whatever total you start with, you must end with. If a proposed "transaction" (interaction) would leave one account unbalanced, the bank (nature) simply refuses to process it. That interaction cannot happen.

Conservation of Charge, Q

This is the one you probably already trust instinctively — total electric charge before an interaction always equals total charge after.

Particle type	Charge Q
Proton	+1
Electron	−1
Up quark	+2/3
Down quark	−1/3
Photon / neutrino	0

Conservation of Baryon Number, B

Baryon number counts how many baryons (net) are involved. It works like this:

BARYON NUMBER, B / | \ B = +1 B = -1 B = 0 (baryons) (anti-baryons) (mesons, leptons, e.g. p,n e.g. p̄, n̄ photons — anything that isn't a baryon)

Every quark contributes +1/3 to baryon number, and every antiquark contributes −1/3. That's why 3 quarks (a baryon) always gives a whole number: +1. Mesons (quark + antiquark) always give exactly 0: +1/3 + (−1/3) = 0. Leptons and photons contain no quarks at all, so they're automatically B = 0.

Conservation of Lepton Number, L

Same idea, but for leptons. And importantly — each generation actually has its own separately conserved lepton number in the full theory (electron-lepton-number, muon-lepton-number, etc.), though at this level you'll mostly just track the total.

LEPTON NUMBER, L / | \ L = +1 L = -1 L = 0 (leptons) (anti-leptons) (baryons, mesons — e- , μ- e+, μ+ anything that νe , νμ ν̄e, ν̄μ isn't a lepton)

THE classic exam trap

Students very often assume the electron has lepton number −1 because its charge is negative. This is wrong. Charge and lepton number are totally separate quantities. The electron (matter particle) has L = +1. It's the positron (the antiparticle) that has L = −1. Don't let the negative charge trick you into flipping the wrong sign!

The Golden Rule Σ Q(before) = Σ Q(after) & Σ B(before) = Σ B(after) & Σ L(before) = Σ L(after)

If even ONE of these three totals doesn't match on both sides of a proposed interaction, the interaction is forbidden and cannot occur.

Worked Example — Checking Beta-Minus Decay

Beta-minus decay: n → p + e⁻ + ν̄ₑ

Baryon number check: Left side: neutron is a baryon, B = 1. Right side: proton is a baryon (B = 1), electron is not a baryon (B = 0), antineutrino is not a baryon (B = 0). Total right side = 1 + 0 + 0 = 1. ✓ Matches — baryon number is conserved.

Lepton number check: Left side: neutron isn't a lepton, L = 0. Right side: electron is a lepton (L = +1), antineutrino is an anti-lepton (L = −1). Total right side = +1 + (−1) = 0. ✓ Matches — lepton number is conserved.

Charge check: Left side: neutron, Q = 0. Right side: proton (+1) + electron (−1) + antineutrino (0) = 0. ✓ Matches.

All three balance, so this decay is allowed — which is exactly why we observe it happening in nature!

Practice Question 1

In the reaction n + μ⁺ → p + X, if lepton number must be conserved, is particle X a neutrino or an anti-neutrino?

Practice Question 2

Is the following reaction permitted? Justify your answer using baryon number.
p + p → p + p + π⁺ + p̄

4. Particle Interaction Equations

This section is really the "exam technique" section — how to actually apply everything above to decide whether a given interaction is allowed, or to find a missing/unknown particle.

Full particle interactions must obey conservation of:

Charge, Q
Baryon number, B
Lepton number, L
Energy (or mass-energy)
Momentum

At this level, exam questions almost always focus on Q, B, and L, since those are the quantities you can check just by counting particles — energy and momentum conservation usually get tested separately (in kinematics-style questions).

Your step-by-step method

1. Write down every particle on the left-hand side and every particle on the right-hand side.
2. For each conserved quantity (Q, then B, then L), add up the values on the left, then add up the values on the right.
3. Compare the two totals for each quantity.
4. If all of them match → the reaction is permitted. If even one fails → the reaction is forbidden.
5. If you're finding a missing particle, isolate the "missing" side of the equation and work out what value it needs — then match that to a known particle.

Worked Example — Checking an Antiproton Production Reaction

p + p → p + p + π⁺ + p̄

Charge: Left = 1 + 1 = 2. Right = 1 + 1 + 1 + (−1) = 2. ✓ Conserved.

Baryon number: Left = 1 + 1 = 2. Right = 1 + 1 + 0 + (−1) = 1. ✗ Not conserved!

Conclusion: Because baryon number fails to balance, this reaction is not permitted — even though charge alone looked perfectly fine. This is exactly why you must check every conserved quantity, not just the first one that comes to mind.

Worked Example — Using Quarks to Verify Charge in Beta Decay

Beta-minus decay happens at the quark level too: a down quark inside a neutron changes into an up quark, turning the neutron into a proton.

Left side (neutron = udd): +2/3 − 1/3 − 1/3 = 0.

Right side (proton = uud, plus electron, plus antineutrino): (+2/3 + 2/3 − 1/3) + (−1) + (0) = 1 − 1 + 0 = 0.

Both sides equal 0, so charge is conserved — confirmed all the way down at the quark level, not just at the "whole particle" level.

Watch out for strangeness

You'll notice the chapter mentions "apart from strangeness in weak interactions." Strangeness (a quantum number given to strange quarks) is conserved in strong and electromagnetic interactions, but can change in weak interactions. Don't panic if you meet a strangeness-violating equation involving the weak force — that's expected, not an error!

Practice Question

A proposed reaction is: e⁻ + p → n + X. Using conservation of charge and lepton number, identify what particle X must be.

What to Memorise

Hadron

Any particle made of quarks. Feels the strong nuclear force. Splits into baryons and mesons.

Baryon

3 quarks (qqq). B = +1. E.g. proton, neutron.

Meson

Quark + antiquark (qq̄). B = 0. E.g. pion, kaon.

Lepton

Fundamental particle, not made of quarks. Never feels the strong force. L = +1. E.g. electron, neutrino.

Antimatter

Identical mass to matter counterpart, opposite charge (and opposite B/L).

Photon

Exchange particle for the electromagnetic force. Zero mass, zero charge.

Charge, Q

Always conserved. Up quark = +2/3e, down quark = −1/3e, electron = −1e.

Baryon number, B

Baryons = +1, anti-baryons = −1, everything else = 0. Always conserved.

Lepton number, L

Leptons = +1, anti-leptons = −1, everything else = 0. Always conserved. (Electron has L=+1, NOT −1!)

Quark baryon number

Every quark = +1/3, every antiquark = −1/3. This is why baryons (3 quarks) always sum to a whole number.

Concepts Checklist

I can name the three generations of quarks and leptons, and state that mass increases down the generations.

I can explain the difference between a baryon and a meson, and give an example of each.

I understand why baryons can't be made from a mix of quarks and antiquarks.

I can explain why leptons never interact via the strong nuclear force.

I know that photons are the exchange particle for the electromagnetic force, with zero mass and charge.

I can state that antimatter particles have identical mass but opposite charge to their matter counterpart.

I know that a truly neutral particle (like a photon) is its own antiparticle.

I can state the three quantities always conserved in particle interactions: charge, baryon number, lepton number.

I can assign B and L values correctly to baryons, anti-baryons, leptons, anti-leptons, mesons, and photons.

I can check whether a proposed particle reaction is allowed by testing Q, B, and L on both sides.

I can identify an unknown particle in an equation using conservation laws.

I can verify conservation of charge in beta decay using the quark model (d → u quark change).

Exam Tips & Common Mistakes

Mistake #1

Giving the electron a lepton number of −1 because its charge is negative. Fix: charge and lepton number are unrelated. Electron L = +1, positron L = −1.

Mistake #2

Only checking ONE conserved quantity (usually charge) and declaring a reaction "allowed" — but forgetting to check baryon number and lepton number too. All three must balance.

Mistake #3

Assuming quarks count as leptons because both are "fundamental" particles. They are two completely separate families.

Mistake #4

Forgetting that antiparticles flip all their quantum numbers (charge, B, L) — not just charge. An antiproton has B = −1, not B = +1.

Examiner loves to see...

Clear working showing the totals on each side of the equation separately for each conserved quantity, followed by an explicit comparison and conclusion (e.g. "Left = +2, Right = +1, therefore not conserved, therefore not permitted"). Don't just state the final answer — show the arithmetic.

Good habit

Any unusual or "exotic" particle's quantum numbers (charge, baryon number, strangeness etc.) will always be given to you in the question if you're expected to use it — you don't need to memorise anything beyond the standard particles listed in this guide.

Fast method for quark combos

Rather than memorising every baryon's quark combination, just remember the quark charges (+2/3 for up-type, −1/3 for down-type) and work backward from the particle's known total charge until the numbers add up.

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Also in the full note

1. The Standard Model — Quarks & Leptons
Exam Tips & Common Mistakes
Hadrons: Baryons & Mesons

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