  In this paper we consider signal-background interference effects in
Higgs-mediated diphoton production at the LHC. After reviewing earlier works
that show how to use these effects to constrain the Higgs boson total decay
width, we provide predictions beyond NLO accuracy for the interference and
related observables, and study the impact of QCD radiative corrections on the
Higgs width determination. In particular, we use the so-called soft-virtual
approximation to estimate interference effects at NNLO in QCD. The inclusion of
these effects reduce the NNLO prediction for the total Higgs cross-section in
the diphoton channel by about 1.7%. We study in detail the impact of QCD
corrections on the Higgs-boson line-shape and its implications for the Higgs
width extraction. Assuming an experimental resolution of about 150~MeV on
interference-induced modifications of the Higgs-boson line-shape, our NNLO
analysis shows that one could constrain the Higgs-boson total width to about
10-20 times its Standard Model value.
