AcademyParticles and Cosmology
Academy
Quarks and Gluons
Level 1 - Physics topic page in Particles and Cosmology.
Principle
Quarks carry fractional electric charge and colour charge. Gluons mediate the strong interaction, and only colour-neutral hadrons are observed.
Notation
\(u\)
up quark
\(d\)
down quark
\(s\)
strange quark
\(Q\)
electric charge in units of e
1
\(B\)
baryon number
1
\(S\)
strangeness
1
Method
Derivation 1: Quark charges build hadron charges
The light-quark charges explain proton, neutron, pion, and kaon charges.
Up quark
\[Q_u=+\frac23\]
Down and strange quarks
\[Q_d=Q_s=-\frac13\]
Derivation 2: Baryon number is shared among quarks
Each quark has baryon number \(+1/3\), while each antiquark has baryon number \(-1/3\).
Baryon
\[B=3\left(\frac13\right)=1\]
Meson
\[B=\frac13-\frac13=0\]
Derivation 3: Colour-neutral combinations
Observed hadrons combine colour charges so the total state is colour neutral.
Baryon colour
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Meson colour
\[c\bar c\]
Rules
Up charge
\[Q_u=+\frac23\]
Down and strange charge
\[Q_d=Q_s=-\frac13\]
Baryon number conservation
\[\sum B_i=\sum B_f\]
Examples
Question
Find the charge of \(udd\).
Answer
\[Q=+2/3-1/3-1/3=0\]
so \(udd\) is neutral.Checks
- Antiquarks have opposite charge and opposite baryon number to quarks.
- Mesons have baryon number zero.
- Baryons have baryon number \(+1\); antibaryons have \(-1\).
- Free isolated quarks are not seen because of confinement.