When in particle physics they plot the mass histogram we see a resonance on top of a decaying exponential background?
Why is the background modeled to be that way? what is the physical explanation?
Decaying exponentials are the result of there being a fixed probability of any specific member of a population being removed. Once removed, the population decreases and so the rate of decay also decreases. The rate of decay is therefore associated with a particular time scale related to the change in population. The same model works if the population is growing - it still leads to an exponential, just one that's growing.