Solution

ID: past-exam-of-the-theoretical-physics-major-option-course-of-the-university-of-oxford/2018/1/d/solution

Use one common ultraviolet cutoff for all physical degrees of freedom, with positive sign for each bosonic degree of freedom and negative sign for each fermionic degree of freedom. For the mass-moment cancellation of vacuum energy divergences, define the weighted supertrace . The large-momentum expansion is
After multiplication by the radial measure , these three terms produce quartic, quadratic and logarithmic ultraviolet divergences. Therefore the necessary and sufficient cancellations for the displayed unsubtracted finite-species vacuum energy are
They respectively mean equal numbers of bosonic and fermionic physical degrees of freedom, equal sums of squared masses, and equal sums of fourth powers of masses. Once they hold, the remaining radial integrand falls as and is integrable at infinity. Nonnegative masses introduce no lower-end divergence.
Pairing every bosonic degree of freedom with a fermionic one of the same mass, as in an unbroken supersymmetric spectrum, is sufficient and cancels the entire expression. It is stronger than these three moment conditions. For a concrete non-paired example, take bosonic squared masses and fermionic squared masses . Both have three degrees of freedom, squared-mass sum and fourth-power-mass sum , but their spectra differ. If a vacuum energy counterterm is instead permitted, a renormalized finite answer need not satisfy them; that is a different prescription from finiteness of the source's bare sum.

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