  We construct a family of models for the evolution of energetic particles in
the starburst galaxy M82 and compare them to observations to test the
calorimeter assumption that all cosmic ray energy is radiated in the starburst
region. Assuming constant cosmic ray acceleration efficiency with Milky Way
parameters, we calculate the cosmic-ray proton and primary and secondary
electron/positron populations as a function of energy. Cosmic rays are injected
with Galactic energy distributions and electron-to-proton ratio via type II
supernovae at the observed rate of 0.07/yr. From the cosmic ray spectra, we
predict the radio synchrotron and \gamma-ray spectra. To more accurately model
the radio spectrum, we incorporate a multiphase interstellar medium in the
starburst region of M82. Our model interstellar medium is highly fragmented
with compact dense molecular clouds and dense photoionized gas, both embedded
in a hot, low density medium in overall pressure equilibrium. The spectra
predicted by this one-zone model are compared to the observed radio and
\gamma-ray spectra of M82. Chi-squared tests are used with radio and \gamma-ray
observations and a range of model predictions to find the best-fit parameters.
The best-fit model yields constraints on key parameters in the starburst zone
of M82, including a magnetic field strength of ~250 \mu G and a wind advection
speed in the range of 300-700 km/s. We find that M82 is a good electron
calorimeter but not an ideal cosmic-ray proton calorimeter and discuss the
implications of our results for the astrophysics of the far infrared-radio
correlation in starburst galaxies.
