Hello there readers (yes, all four of you), I'm back with the final installment of the Particle Physics Trilogy for Astronut. In my first and second posts, I gave a brief overview of the structure of the Standard Model of particle physics and went on to make a case for the existence of physics beyond the Standard Model. In this post I'm going to talk about one of these extensions of the SM, called Supersymmetry (you can call her SUSY)- which is also what will hopefully give me my PhD at the end of <till the funding runs out> years.
I'm afraid that this post is going to be a bit of a let-down, because I'm putting it together in a hurry but more so because we are now near the boundaries of my knowledge of particle physics- there is still a semester to go before I take the graduate level Beyond the Standard Model course! So please bear with the occasional vagueness.
Before I begin, I must point out that SUSY is a bit of a controversial topic in the particle physics community. She's the girl you think is The One, while your friends try to warn you that you are just in love with the idea of her. Err, I may or may not have binge-watched some chick-flicks today. A great man- my PhD supervisor, actually- once said: "Supersymmetry is too beautiful to not be true" (relevant xkcd, see alt-text). And indeed, it is the only extension of the Standard Model that to date provides a solution to the dark matter problem, the Hierarchy Problem of the Higgs mass, quantum gravity, origin of matter, the instability of vacuum and the cosmological inflation (source). Beautiful as it is, it still remains elusive at the TeV scales that we have probed at the LHC so far, even though theoretical predictions suggest that supersymmetric particles should have been observable at those energies.
But let's leave the SUSY debate to drunken after-conference dinner conversations, and try to make an acquaintance with the lady herself. Supersymmetry is a conjectured symmetry of space and time, and is the only possible non-trivial extension of the Poincaré symmetry. This basically means that the symmetry relates fermions to bosons. The basic premise of supersymmetric theories is that every fermion has a corresponding boson whose spin differs from the that of the fermion by half a unit and vice versa. The first thing you should notice that this immediately doubles the number of particles we have in the standard model. The supersymmetric algebra transforms a fermionic field to a bosonic field, and a bosonic field to a fermionic field. These fermion-boson pairs are grouped into supermultiplets, with equal number of fermionic and bosonic degrees of freedom. (Note: The fact that I am unable to do away with the jargon should tell you by now that I am still learning how-to-SUSY with the training wheels on). So if SUSY is a perfect fermion-boson symmetry, why haven't we observed these "superpartners" of the SM particles? Good question. The very fact that we haven't observed SUSY particles yet means that supersymmetry is only realised as a broken symmetry: the masses of the superpartners do not mirror those of the SM particles.
We're now getting to the fun and weird part of SUSY- the superpartners themselves. The supepartners of the fermions are bosons called sfermions. You read that right. The boson corresponding to the electron is called the selectron, that corresponding to the top quark is called the stop and so on. The fermionic superpartners of the SM bosons, on the other hand are named with suffix -ino. So we have the Higgsino corresponding to the Higgs, the wino corresponding to the W boson, the zino, gluino ... you get the idea. Moreover, SUSY hypothesizes not one, not two, not three- but FIVE physical Higgs states. There are several variants of supersymmetric theories: the simplest being the Minimal Supersymmetric Standard Model (MSSM) followed by the Next-to-Minimal Supersymmetric Standard Model (NMSSM). If you venture to Google these, you will notice the Wikipedia articles getting shorter and vaguer (like this blogpost), so these theories are clearly a work-in-progress.
However, there are two important things that one should know about SUSY: (1) It predicts the LSP- Lightest Supersymmetric Particle to be electrically neutral and weakly interacting with the Standard Model particles- exactly the properties of dark matter, and (2) It naturally suppresses the radiative corrections to the SM Higgs mass, thus solving the Hierarchy problem. How does it do that? In the last post, I wrote about how the quantum corrections account for the couplings of the Higgs to the SM particles. As it turns out, these corrections are cancelled out by those due to the coupling of the Higgs to the SM superpartners- thus providing a solution to the Hierarchy problem that arises out of symmetry, and not ad hoc fine-tuning.
So in all probability, SUSY seems weird and too far-fetched to you. But the reason why this theory appeals to physicists around the globe (and also why some really huge amount of money is being pumped into the LHC to search for SUSY) is that it's core principle is anchored in symmetry and by extension, in beauty and elegance. A brief acquaintance with Emmy Noether's remarkable theorem will tell you that symmetry dictates interaction- symmetries and invariance principles provide structure and coherence to the laws of nature. Whether or not SUSY continues to play hard-to-get in Run 2 of the LHC remains to be seen. No matter which way it goes, the next decade of particle physics is going to take us on a roller-coaster of a ride for sure.
As Hawking would say, we look to the future of physics standing on the shoulders of giants who came before us. Science in the 21st century, in my opinion, is the gateway for humanity to redeem itself. But in the end- and I am sure that anyone who has loved and delighted in physics, and conversed with it in the the language of equations would agree- the reason why we do science is not because we believe our science will save the world- or that we will transport the human civilization to a higher plane of understanding. We do it for deeply personal reasons, for that moment of clarity that comes with the Quad Erat Demonstrandum at the end of a proof and quite simply for the joy of fulfilling the demands of a curious mind.
It would be fitting to end this series of posts with the words of Henri Poincaré- physicist, mathematician, engineer, philosopher and master of words:
The Scientist does not study nature because it is useful to do so. He studies it because he takes pleasure in it; and he takes pleasure in it because it is beautiful. It is because simplicity and vastness are both beautiful that we seek by preference simple facts and vast facts; that we take delight, now in following the giant courses of the stars, now, in scrutinizing with a microscope that prodigious smallness which is also a vastness, and, now, in seeking in geological ages the traces of the past that attracts us because of its remoteness.
Over and out.