Elementary Particle Physics

2. The KEKB accelerator collides 8 GeV electrons with 3.5 GeV positrons head on in the BELLE
experiment. Calculate the centre of mass energy of this system. [ 4]
3. The BaBar detector had a solenoid creating ad 1.5T. The PEPII accelerator was running at the
same energy as KEKB (see previous exercise). Assume that only a pair of B mesons (m3
5279 MeV) is created in the collision and then one of them decays in two muons. Assuming
also that all the decays happen in the plane perpendicular to the magnetic field, calculate the
radius of curvature of the muons in the BaBar detector. [ S]
4. The 2″ is an unstable hyperon with mass m = 1385 MeV and decay width F = 35 MeV, with
a branching ratio into the channel 2” -) 1121\0 of 88%. Is the decay of the 2″ strong or weak?
Explain. [ 5]
S. The LHC beam is not continuous. Instead, it has a bunch structure. Bunches of particles collide
every 25 ns at 14 TeV centre-of-mass energy with a luminosity (number of particles per second
per square centimetre) of 1034 cm‘zs’l. The total inelastic proton-proton cross section at 14
TeV is 70 mb.
(i) Compute the number of interactions occurring per second as well as every time two bunches
collide. These are called minimum bias events and are of almost no interest. [ 4]
(ii) The Higgs particle is produced at the LHC mainly through gluon fusion (gg -+ H) which
has a cross section of about 40 pb. One of the Higgs decay channels involves the Higgs going
into two photons (H -) W) and it has a probability (branching ratio) of 0.2 x 10’3. How many
minimum bias events are expected to be produced for every Higgs event observed via the two-
photon channel if one ignores detector effects? [ S]


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