2. ( 8 pts) At high energy, Ec m. mgc2, the cross-section for e+e -+ scattering due to electromagnetic interactions must have the form = C ha cb ap/sq, for some exponents a, b, p, q and overall dimensionless coefficient C, which turns out to equal 47/3. Here a e2/(4neohc) is the fine structure constant, and s (pe-+pe+)2 = E2 /c2. The power of a is the same as for elastic scattering due to Coulomb interactions. What is the exponent p? Justify your reasoning. Determine the other exponents using dimensional analysis. What is the value of the cross-section in barns (or nanobarns, etc.) when Ec = 50 GeV?