Last year this time, a lot of things were different. What has not changed is the pressure to graduate with a physics degree so I can inhale more pressure in grad school. Even though not required, I will be taking a particle physics class this quarter and I hope I manage to squeeze time to write 5~10 posts along the way for both theory and ROOT related notes as this shall be what I do in the future (given humanity manage to build another collider in the next decade)


This is a reading note for The eighteen arbitrary parameters of the standard modelin your everyday life, by Robert N. Cahn

While the currently agreed model (the Standard Model) is fabulous and accurate through various experiments conducted in the last 5 decades, there are some fundamental questions the particle physics community is trying hard to answer.

Essentially, we are not satisfied with the 18(or 19) parameters we have in the SM. We have 6 masses for all the quarks, and 3 for the charges leptons and 9 for mediators (coupling constants). These parameters seem arbitrary and thus we have to measure them experimentally. This is a huge blow to the completeness of the theory and almost begs for a more fundamental model.

Consider the following "alternative universe", each deviates only a little bit from ours in terms of these constants. First, a universe where up quark is slightly heavier — just so that it's heavier than the down quark which is only weighs more by 2.5 MeV in our universe. This would essentially result a very different landscape, for that the protons are now unstable (instead of neutrons in our universe) and that after the big bang, the universe will be left with helium and neutrons. We would still have stars, but the evolution of them are totally different, as now they start from helium not hydrogen.

We can also play with the generation of quarks. Imagine the second generation is the first, and they are 'stable' somehow. In this case, because of the huge mass of c and s quarks, the only kind of nuclei we can get is the new hydrogen and Omega- (which is made up by 3 s quarks). This is because the strong force which binds the proton and neutrons are mediated by mesons, in this case the mesons are too heavy (~ 1GeV, made up by s+sbar) so the binding distance (check out asymptotic freedom) is too small for even two nucleus to bind. So the universe is fucked again.

The takeaway is, theorists find it hard to believe that the model has to be this delicate in order for us to exist (fine tuning), so they want something deeper so that we can calculate these 19 parameters from first principle. This gives birth to SUSY (supersymmetry), where it also has some popular feature such as grand unification of electroweak force and strong force. But so far we haven't seen anything at LHC and it's beyond the natural energy to the point where even if we discover some SUSY particles at this energy scale, it will be unnatural again :(.