Outline Solutions to Particle Physics Problem Sheet 1

Transcription

1 2010 Subatomic: Particle Physics 1 Outline Solutions to Particle Physics Problem Sheet 1 1. List all fundamental fermions in the Standard Model There are six letons and six quarks. Letons: e, ν e, µ, ν µ, τ, ν τ. Quarks: u, d, c, s, t, b. Plus their anti-article counterarts, i.e. 24 fermions in total. In fact, we should really list the quarks by their individual colour charges. There s red-u-quarks, u R, blue-u-quarks, u B and green-u-quarks, u G, and similarly for all of the other quarks flavours. Generation Quarks Letons I u R, u B, u G d R, d B, d G e ν e II c R, c B, c G s R, s B, s G µ ν µ III t R, t B, t G b R, b B, b G τ ν τ 2. What quantum numbers are associated with letons? Are they conserved in strong, weak and electromagnetic interactions? L e, L µ, L τ, total leton number L = L e + L µ + L τ and electric charge!) They are conserved in all interactions. 3. What quantum numbers are associated with quarks, and how are they defined? Are they conserved in strong, weak and electromagnetic interactions? Total quark number, N q = Nq) N q). U quark number N u Nu) Nū) Down quark number, N d Nd) N d) Strange quark number N s Ns) N s) Charm quark number, N c Nc) N c) Bottom quark number, N b Nb) N b) To quark number, N t Nt) N t) Total quark number and charge are conserved in all interactions. N u, N d, N s, N c, N b, N t are conserved in strong and electromagnetic interactions. They are not necessarily) conserved in weak interactions. 4. What are the charge and quark flavour quantum numbers for the ū, d and s quarks? What are the quantum numbers of the lambda anti-baryon, Λ 0, and of the antiroton,? Make sure you understand these in terms of quark content! Antiroton = ūū d) and Λ 0 = ū d s). article charge Qe) N d N u N s N c total quark number, N q ū 2/ d +1/ s +1/ Λ

2 2010 Subatomic: Particle Physics 2 5. Use the Pauli exclusion rincile to argue why in the ++ baryon which has total sin, S = 3/2 h and consists of three u quarks) all the quarks have a different colour charge. The Pauli exclusion rincile states that no two fermions in a multi-article state can have identical quantum numbers. The three quarks are fermions, so we have to aly the Pauli exclusion rincile to them. They all have identical u-quark number N u = +1), they all have identical sin e.g. S = 1/2 h, S z = 1/2 h, to make overall total sin of 3/2 h). Therefore there needs to be another quantum number which is different for the different quarks, and that s the colour charge quantum number. Therefore all the u quarks must carry a different colour charge quantum number: one red, one blue and one green. 6. Exlain why the decays µ e ν e ν µ and µ + e + ν e ν µ are allowed and why µ + e + γ and µ + e + e e + are forbidden. Leton family number conservation For all rocesses, L e, L µ & L τ are conserved quantum numbers, e.g. µ e ν e ν µ µ + e + e e + L e : ) + 0 = 0 OK! L e : ) + 1 = +1 forbidden L µ : = 1 OK! L µ : = 0 forbidden What about µ + e ν e ν µ? We don t even need to think about leton number here, this decay violates conservation of charge! 7. What is 1 fm in inverse GeV? How many seconds is 1 inverse GeV? I ve written this answer in quite a lot of detail, as this concet can be quite confusing. We need to write length in terms of inverse energy, and some as yet unknown) factors of c and h. We can do this by balancing dimensions: [L] = [E] 1 [ h] m [c] n = [E] 1 [E] m [T ] m [L] n [T ] n = [E] m 1 [T ] m n [L] n Where we have used the fact that h is measured in [E][T ] and c is measured in [L][T ] 1. The dimensions on both side of the equation have to balance. So we have to set n = 1 and m = 1. Therefore length is measured in GeV 1 hc. What we want to know is how many GeV 1 hc corresonds to 1 fm: 1 fm = l GeV 1 hc l = 1 fm/ hc) We have to use measurements of h and c in aroriate units: GeV s and fm/s. In lecture 2, age 5 there is an exression in the aroriate units: hc = 197 MeV fm = GeV fm Therefore l = 1/ Or 1 fm 5 GeV 1. To write time in units of inverse energy: [T ] = [E] 1 [ h] m [c] n = [E] 1 [E] m [T ] m [L] n [T ] n = [E] m 1 [T ] m n [L] n This time to make the dimensions balance m = 1, n = 0. Then: 1 s = t GeV 1 h t = 1 s/ h) h in aroriate units is = MeV s = GeV s. Therefore, t = 1/ = We can invert this to get the answer: 1 GeV 1 = s.

3 2010 Subatomic: Particle Physics 3 8. Write down the tyical lifetimes for articles that decay by: a) The strong force: to s b) The electromagnetic force: to s c) The weak force: to 10 3 s By looking at the liftetimes on the Particle Proerties sheet, which force is resonsible for the decay of 0, B +, ω 0? 0 lifetime is s, electromagnetic decay. B + lifetime is s, weak decay. ω 0 lifetime is s, strong decay. 9. The lifetime of the η 0 has not been measured directly. The total width of the η 0 has been measured to be Γη 0 ) = ± MeV. What is the lifetime of the η 0? What force is resonsible for its decay? The total width of any article is given by Γ = h/τ τη 0 ) = h/γη 0 ). Using h = MeV s, gives τη 0 ) = s. By looking at the lifetime we see that the strong force is resonsible for the decay of η 0. You could also do this roblem in natural units h = c = 1). Then τ = 1/Γ = 4.93 MeV 1. We can convert this back to seconds using the answer to question 3: 1 s = GeV 1 = MeV 1, therefore: τη 0 ) = 4.93/ = s. 10. What are the Centre-of-Momentum CM) energies of the following machines: LEP1: e + e collider, both beams 45.6 GeV LHC: collider, both beams 7 TeV HERA: e collider, E e = 30 GeV and E = 820 GeV. If HERA were a fixed target machine what energy would the electron require to give an equivalent CM energy? We write the four-momenta of the two beams as: a = E a /c, a ) and b = E b /c, b ). The four momenta is the thing you wrote in Dynamics and Relativity as a. The first thing to note is that if we square any four momenta, we get the mass of the article: 2 a = E a /c a a = m 2 ac 2 This is much easier if we use natural units, so set c = 1. Then we get a = E a, a ) and b = E b, b ), and 2 a = m 2 a. The centre-of-mass energy, s, is calculated from: s = E CoM a + b = 2 a + 2 b + 2 a b = m 2 a + m 2 b + 2E a E b a b ) = m 2 a + m 2 b + 2E a E b a b cos θ) 4E a E b Where we have assumed the beams collide head on: θ = 180, and the energies of the beam is much larger than the mass of the colliding articles: E a, E b m a, m b.

4 2010 Subatomic: Particle Physics 4 Here E e = 45.6 GeV, much larger than the mass of the electron, m e = 511 kev. Then s = 2E e = 91.2 GeV. The mass of the Z-boson, which was the whole oint of LEP!) At LHC, again we can ignore the roton mass: s = 2E = 14 TeV. At HERA s = = GeV 2 or s = 314 GeV/c. Note that at roton colliders not all this energy is in ractise available, since only a fraction of the roton momenta is carried by the quarks and gluons, which are the articles actually involved in the scattering. At a fixed target machine s = 2m E e electron and roton mass can be neglected) which imlies that E e = 52.5 TeV would be necessary to achieve the same CoMenergy. As a consequence colliding beam machines are the only viable means of reaching the highest energies. 11. The ++ baryon can be roduced as a resonance by aiming a ion beam onto a hydrogen target to roduce the reaction Calculate the energy and momentum of the ions in the ++ centre-ofmass frame. We need the masses of the articles, from the article roerties sheet: m = MeV/c 2, m = MeV/c 2 and m = 1232 MeV/c 2. In the centre-of-mass frame of the ++, it is at rest. Therefore its three momentum is 0, and therefore the energy of the is E = m c 2. We can write down the four-momenta for the ++, + and as: = m c, 0 ) = E /c, ) = E /c, ) Using four momentum conservation: = + = Squaring this we get: = + 2 Where we have used the key fact: m 2 c 2 = m 2 c 2 + m 2 c 2 2m c E /c = m 2 c 2. Rearranging we get: E /c 2 = 1 2m m 2 + m 2 m 2 ) E = MeV We can get the three momentum of the ion using E 2 = 2 c 2 + m 2 c 4 : = E /c m c = MeV/c From the measured total width Γ)=120 MeV calculate the lifetime of the ++. Using h = MeV s we obtain the ++ lifetime τ = h Γ = s.

5 2010 Subatomic: Particle Physics The B d meson has a mass of 5.28 GeV/c 2 and mean lifetime of 1.54 s. At the LEP B d mesons are roduced with an average energy of 32 GeV. Calculate the mean decay length of a B d meson. The average decay length in the lab frame is L = γβcτ B. We can calculate the Lorentz factors γβ from the average B d momentum: B = E B /c m B c = GeV/c This gives γβ = c/mc 2 = /mc = 31.56/5.28 = 5.98 Also, cτ B = = m Then the mean decay length is: L = γβcτ B = = m = 2.67mm, which is a measurable distance. 13. The cross section to make b-quarks at LEP with E CM = 91.2 GeV was σe + e b b) = 4.5 nb. How many e + e b b events were roduced with a integrated luminosity of Ldt = 100 b 1? N = σ Ldt = 4.5 nb 100 b 1 = 4.5 nb 100, 000nb 1 = 450, ,000 e + e b b events were roduced with 100 b 1.