  It is generally believed that any particle to be discovered will have a
TeV-order mass. Given its great mass, it must have a large decay width.
Therefore, the interference effect will be very common if they and the
Standard-Model (SM) particles contribute to the same final state.However, the
interference effect could make a new particle show up not like a resonance, and
it is difficult to search and measure its properties. In this work, a
model-independent method, F2F (Fit To Fourier coefficients), is proposed to
search for an unknown resonance and to determine its mass ($M$) and width
($\Gamma$) in the presence of interference. Basically we express the sum of
reosnant signal and the interference as a cosine Fourier series and relate the
Fourier coefficients with the mass and width. The relation is based on the
general propagator form, $1/(x^2-M^2+iM\Gamma)$. Thus it does not need any
signal model. Toy experiments show that the obtained mass and width agree well
with the inputs with a similar precision as using an explicit signal model. We
also show that we can apply this method to measure the Stardard-Model Higgs
width and to make statistic interpretation in searching for new resonance
allowing for interference.
