AQA Physics A Quarks and Leptons – Exam Style Worked Answers
This is the mark scheme for the Exam style questions from Nelson Thornes AQA Physics A Chapter 2 –
Quarks and Leptons P 28-29.
1 (a) (i) Strong Interaction (1)
(ii) Weak Interaction (1)
(iii) Strangeness is always conserved in the strong interaction (1) (3 Marks)
(b) (i) three quarks – Up, down and strange u,d,s (1). These are combined in any combination of three quarks to make different Baryons (1). E.g. uud – proton, udd – neutron, dds – Σ - or in pairs of quark and antiquark to make mesons (1) ud-bar – Pion uS-bar Kaon
(ii) meson are always quark-antiquark pairs(1). The only possible combinations of u,d and s and their anti particles are:
 up –anti up (u-bar)
 up –anti down (d-bar)
 up – anti strange (s-bar)
 down – anti up (u-bar charge = 0 (+2/3 -2/3) charge = +1 (+2/3 +1/3) charge = +1 (+2/3 +1/3) charge = -1 (-1/3 -2/3)
 down –anti down (d-bar) charge = 0 (-1/3 +1/3)
 down – anti strange (s-bar) charge = 0 (-1/3 +1/3)
 strange – anti up (u-bar charge = -1 (-1/3 -2/3)
 strange – anti down (d-bar) charge = 0 (-1/3 +1/3)
 strange – anti strange charge = 0 (-1/3 +1/3)
Of the total of 3 quarks and their anti quarks there are only 9 permitted combinations(1) of which 4 are charged(1). QED. (6 marks)
2. from these particles p, n-bar, ν e
, e + , µ , π 0
(i) hadrons p, n-bar, π 0 (1)
(ii) leptons ν e
, e + , µ - (1)
(iii) antiparticles n-bar, e + (1)
(iv) charged p, e + , µ (1) (4 Marks)
3. (a) (i) positron, neutron, neutrino (2)
(ii) electron, proton, negative muon (2) (4 Marks)
(b) (i) 𝜇
−
→ 𝑒
−
+ ν e
bar + ν
μ
(1)
(ii) Same Charge and Different mass. (2) (3 Marks)

4. (a) Neutron = udd (2 Marks)
(b) Neutron - baryon(1) and hadron(1) (2 marks)
5. (a) (i) Meson (1)
(ii) Q = -1 (-2/3 -1/3) (1)
(iii) B = 0 (+1/13 -1/3) (1) (3 Marks)
(b) 𝜇
+
→ 𝑒
+
+ ν e
Q +1 = +1 + 0 OK (1)
L e
0 = +1 + -1 OK
L
µ
+1 = 0 + 0 NOT OK
L +1 = +1 + +1 NOT OK (1)
B 0 = 0 + 0 OK (1) ( 3 Marks)
6. (i) 3 Quarks (1)
(ii) Sigma will decay by the weak interaction (1)
(iii) Proton (1) (3 Marks)
7. (a) Made of Quarks (1) (1 Mark)
(b) Baryons qqq (1) and antibaryons (3 antiquarks) (1) and mesons q – antiq pairs (1) (3 Marks)
(c) n + 𝜇 + → 𝑝 + ν
μ
bar
Q 0 + +1 = +1 + 0 OK (1)
L 0 + 1 = 0 + 1 OK (1)
B 1 + 0 = 1 + 0 OK (1) (3 Marks)
8. d ubar (3) Q = -1/3 + -2/3 (1) B= 1/3 – 1/3 Pions are only u and d (or their anti quark)(1) quark and antiquark (1) (3 Marks)
9.
𝑛 → 𝑝 +
0
−1 𝑒 + 𝛎 𝐞 𝒃𝒂𝒓
(a) 𝛎 𝐞 𝒃𝒂𝒓 antineutrino - symbol (1) anti particle bar (1) (2 Marks)
(b) Q 0 = +1 + -1 + 0 OK (1)
B +1 = +1 + 0 + 0 OK (1)
L 0 = 0 + 1 + -1 OK (1) (3 Marks)

(c) 938Mev KE min for each incoming proton (2) I mark for 2 * 938 =1876 Mev (1) (2 Marks)
10. (a) (i) antibaryon (1)
(ii) K 0 remember a meson is also a hadron (2) (3 Marks)
(b) (i) u sbar (1)
(ii) Weak interaction ( S is not conserved) (1)
(iii) 𝐾 + → 𝜇 − + ν
μ
𝑏𝑎𝑟 (1 mark for each particle) (2)
(iv) leptons (1)
(v) mass (1) (6 Marks)
11. (a) 𝐾
0
+ 𝑝 → 𝑛 + 𝜋
+
Q 0 + 1 = 0 + 1 OK (1)
B 0 + 1 = 1 + 0 OK (1)
L 0 + 0 = 0 + 0 OK (1) (3 Marks)
(b) K 0 d sbar (2)
π + u dbar (1)
P uud (1) (4 Marks)
Total marks 68.