4. Muons are elementary particles carrying the same charge and spin as electrons but having the much larger mass of 105.7 MeV/c². They are produced through high energy interactions in particle accelerators and in the upper atmosphere by cosmic rays. (b) The muon is an unstable particle, with a lifetime of 2.2 s. This suggests a cosmic ray muon would travel barely one kilometre from its point of production, at an altitude of 15 km, until it decays. Explain how it is yet possible for such muons to reach the surface of the Earth, and even penetrate into deep mines. [5 marks] (c) A high energy muon is decelerated to rest in an electric field. What is its energy when it reaches a speed of exactly one half the speed on light? [5 marks] (d) Draw a graph showing how the muon acceleration changes as a function of its decreasing energy. [5 marks]