There are two directions in which one can try to extend the presently known Standard Model, and these can essentially be characterized as follows:

(1) rare new particles and extremely weak new forces, and

(2) heavy new particles and new structures at very high energies.

Let us begin with the first. Particles could exist that are very difficult to produce and to detect, and, for that reason, they could have escaped our attention up till now. All particle types that exert strong forces on any of the known particles could never hide themselves from us. According to the forbidding laws of quantum particle theory, such particles would be frequently produced, either singly or in particle–antiparticle pairs – this would only not happen if their masses are too great, but that is covered by case (2). They would betray their presence by colliding against particles in a detector. The only light particles that can avoid our detection are ones that exert only very tiny forces upon most or all known species.

The first additional particle that springs to mind is a neutrino spinning to the right. Recall from Chapter 7 that neutrinos only rotate to the left (antineutrinos rotate to the right), if you take the axis of rotation parallel to the direction of movement. From a mathematical point of view, there is no objection to this, but it is somewhat unaesthetic.