Probing photon decay with the Sunyaev-Zel'dovich effect
  The fundamental properties of the photon have deep impact on the
astrophysical processes that involve it, like the inverse Compton scattering of
CMB photon by energetic electrons residing within galaxy cluster atmospheres,
usually referred to as the Sunyaev-Zel'dovich effect (SZE). We calculate the
combined constraints on the photon decay time and mass by studying the impact
of the modified CMB spectrum, as recently calculated (Heeck 2013), on the SZE
of galaxy clusters. We analyze the modifications of the SZE as produced by
photon decay effects. We study in details the frequency regimes where these
modifications are large and where the constraints derived from the SZE can be
stronger with respect to those already obtained from the CMB spectrum. We show
that the SZE can set limits on the photon decay time and mass, or on $E^* =
\frac{t_0}{\tau_\gamma}m_\gamma c^2$, that are stronger than those obtained
from the CMB: the main constraints come from the low frequency range $\nu
\approx 10-50$ GHz where the modified SZE $\Delta I_{mod}$ is larger than the
standard one $\Delta I$, with the difference $|(\Delta I_{mod} - \Delta I)|$
increasing with the frequency for increasing values of $E^*$; additional
constraints can be set in the range $120 - 180$ GHz where there is an increase
of the frequency position of the minimum of $\Delta I_{mod}$ with respect to
the standard one with increasing values of $E^*$. We demonstrated that the
effect of photon decay can be measured or constrained by the Square Kilometer
Array in the optimal range $\approx 10-30$ GHz setting limits of $E^* \leq 1.4
\times 10^{-9}$ eV and $5 \times 10^{-10}$ eV for 30 and 260 hour integration
for A2163, respectively. These limits are stronger than those obtained with the
COBE-FIRAS spectral measurements of the CMB.
