Well, each type of force is mediated by some type of boson. And the fact is that the new particle would be a new kind of boson - not fitting in existing categor

Well, each type of force is mediated by some type of boson. And the fact is that the new particle would be a new kind of boson - not fitting in existing categories. So it really is a new force that they would have discovered. As each type of force correspond to one type of boson - which mediates the interaction - it is indeed right, in this meaning, to call them also types of interactions. Also important is the fact that the discovered particle is a fundamental particule, so it is not a composite of other fermions and other bosons. That excludes the possibility of mistaking a mixture of existing forces for a new one.