11.2 Fundamental particles

Learning objectives

11.2.1A quark is a fundamental particle and that there are six flavours (types) of• understand that a quark is a fundamental particle and that there are six flavours (types) of quark: up, down, strange, charm, top and bottom 11.2.2The charge of each flavour of quark and understand that its respective• recall and use the charge of each flavour of quark and understand that its respective antiquark has the opposite charge (no knowledge of any other properties of quarks is required)11.2.3That protons and neutrons are not fundamental particles and describe protons• recall that protons and neutrons are not fundamental particles and describe protons and neutrons in terms of their quark composition 11.2.4A hadron may be either a baryon (consisting of three quarks) or a meson• understand that a hadron may be either a baryon (consisting of three quarks) or a meson (consisting of one quark and one antiquark)11.2.5The changes to quark composition that take place during β– and β+ decay• describe the changes to quark composition that take place during β– and β+ decay 11.2.6That electrons and neutrinos are fundamental particles called leptons• recall that electrons and neutrinos are fundamental particles called leptons

A fundamental particle is not known to contain smaller constituent particles: it cannot be subdivided into a simpler internal structure. Quarks are fundamental particles.

Group for charge learning	Quark flavours
up-type	up (u), charm (c), top (t)
down-type	down (d), strange (s), bottom (b)

The six flavour names are up, down, strange, charm, top and bottom. Each also has a corresponding antiquark flavour, so there are six antiquark flavours.

Meson, baryon, proton and neutron are not quark flavours; they are composite hadrons. For this syllabus, learn quark flavour and charge only—no other quark properties are required.

Flavours	Quark charge	Corresponding antiquark charge
u, c, t	+(2/3)e	−(2/3)e
d, s, b	−(1/3)e	+(1/3)e

Replace every constituent by its signed charge and add. An antiquark keeps the flavour family but reverses the charge sign.

Proton uud: +(2/3)e +(2/3)e −(1/3)e = +e. Neutron udd: +(2/3)e −(1/3)e −(1/3)e = 0. Antiproton ūūd̄: −(2/3)e −(2/3)e +(1/3)e = −e.

Do not reverse flavour when forming an antiquark: anti-up is still the antiparticle of up, but its charge is −(2/3)e. Fractional constituent charges can sum to an integer hadron charge.

Nucleon	Quark composition	Charge sum
proton	uud	2/3 + 2/3 − 1/3 = +1
neutron	udd	2/3 − 1/3 − 1/3 = 0

Protons and neutrons are not fundamental because each is a composite baryon containing three quarks.

$For Z protons and N neutrons: number of u = 2Z + N, number of d = Z + 2N$

Tritium has Z = 1 and N = 2, so it contains 2(1)+2 = 4 up quarks and 1+2(2) = 5 down quarks. An alpha particle has two protons and two neutrons, giving 6 up and 6 down quarks.

A neutron is neutral because its fractional constituent charges sum to zero, not because it contains no charged quarks.

Hadron class	Constituent pattern	Examples
baryon	three quarks	proton uud, neutron udd
antibaryon	three antiquarks	antiproton ūūd̄
meson	one quark + one antiquark	pion

A hadron is a composite particle made from quarks. For the syllabus classification, hadrons are baryons or mesons; flavour combinations need not use identical quarks.

A neutral meson can pair a quark with its corresponding antiquark, such as uū: +(2/3)e −(2/3)e = 0. Charge validity must agree with the constituent signs.

Electrons, positrons, neutrinos and antineutrinos are leptons, not hadrons. A meson is not two baryons and does not contain two ordinary quarks.

In β− decay a down quark changes to an up quark, turning a neutron udd into a proton uud and emitting an electron and antineutrino; β+ reverses the quark change with a positron and neutrino.

Track the quark charge change and the emitted lepton charges together. The weak interaction mediates the flavour change.

A neutron’s d→u raises hadron charge by +e, balanced by the emitted electron’s −e in β− decay.

The beta electron is not an orbital electron released from the atom; it is created in the weak decay process.

Leptons are fundamental particles: they are not made from quarks. In this syllabus, recognise electrons and neutrinos and their antiparticles as leptons.

Lepton	Antiparticle	Electric charge
electron e⁻	positron e⁺	−e; antiparticle +e
electron neutrino νₑ	electron antineutrino ν̄ₑ	0; antiparticle 0

Fundamental leptons	Composite hadrons (not leptons)
electron, positron, neutrino, antineutrino	proton, neutron, baryon, meson

Beta-minus emission contains two leptons: an electron and an electron antineutrino. Beta-plus emission contains a positron and an electron neutrino.

A positron is a positively charged lepton, so it is false that every charged lepton has charge −e. A neutrino is not a neutral hadron or a photon.

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—Cambridge International AS & A Level Physics (9702)Read

11.2 Fundamental particles

Learning objectives

Quarks are fundamental particles with six flavours

Use quark and antiquark charges to find composite charge

Protons are uud and neutrons are udd

Hadrons are baryons or mesons according to constituent pattern

Beta decay changes quark flavour while conserving total charge and nucleon number

Electrons and neutrinos are fundamental leptons

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