If the particle is undisturbed in the two slit experiment, an interference pattern will show up. The weird quantum behavior here is that the particle can have "negative probabilities" associated with its position and momentum, thus they can cancel each other and form the pattern we see. When the particle interacts with the environment (decoheres), it is said that superposition disappears. Does this mean that the particle is still described by a probability distribution of outcomes, but those outcomes now look much more classical and can't interfere? So really when you say superposition disappears, you mean the negative amplitudes disappear, yes?