This puzzle is a hex jigsaw puzzle that depicts interactions from the Standard Model of particle physics. No physics knowledge is required to put the puzzle together, but it helps. In particular, discovering our convention for the flow of time (for a given hex, particles going into the future leave through one of the top three sides, and particles coming from the past enter through one of the bottom three sides) simplifies things greatly because it constrains how many of the hexes may be oriented. For instance, a hex involving the interaction of an electron, a photon and a positron must be oriented such that the photon comes from the past and the electron-positron pair go to the future, or vice-versa, in order to conserve charge. If it were oriented so that, for example, the photon and electron collide to produce a positron, this violates charge conservation (-1 before the interaction, +1 after it), which is very nonphysical. The assembled grid is shown below.
Once the puzzle is assembled, there will be a set of particles entering from the bottom and a set leaving from the top; the two sets are identical. Reading clockwise, one finds
τ-, e-, e+, d, u, u, e-, e+
Replacing d, u, u with a p+ (quarks are never found outside of bound states; in this case a down quark and two up quarks form the bound state we call the proton) we find
τ-, e-, e+, p+, e-, e+
Making the obvious letter substitutions, we get the answer TEEPEE.
2006 MIT Mystery Hunt