A Quantum Diaries Survivor

This morning at the CDF collaboration Meeting the theory talk was about Supersymmetry. Let me explain what this is about in a bit of detail…

Physicists have learned that nature likes symmetry the hard way.  

In the fifties, the mess of new particles created at the first particle accelerators were puzzling for their new properties. For a few years, new particles were discovered almost by the day. There was no understanding of the reason for the number and variety of properties of these new bodies, nor a clear scheme to fit them in.

It was only when a underlying symmetry among the produced particles was discovered, that order was put back in the variegated spectrum.

The symmetry is based on the composition in quarks of all observed hadrons. Let's say you have three kinds of quarks - "u", "d", and "s" - with which to build hadrons. (We later discovered there are actually six of them, but the higher mass "c", "b", and "t" quarks do not contribute to the particles observable in the low-energy accelerators of the fifties, so three quarks were sufficient back then).

Now, if you are allowed to combine them in pairs (called mesons) or in triplets (baryons) - for instance, "uud" and "uus" - you end up with sets of particles that exhibit some symmetry among them: the operation of exchanging one "s" quark with a "d" quark changes a particle into a different one belonging to the same multiplet, its "symmetrical image" with respect to the operation of "s <->d" quark exchange.

An example of a multiplet is the set uuu, uud, uus, udd, uds, uss, dds, dss, ddd, sss. Ten baryons you can obtain one by one from the first one by exchanging quarks.

It was a huge success of theory to explain the organization of hadrons in multiplets according to their quark composition, because more than 10 years afterwards quarks were identified as real particles, and not just mathematical entities. The identification of a symmetry in the organization of particles was the key to discovering a whole new level of subatomic particles - the constituents of what was then thought to be elementary, the quarks.

Now, let's come to SUSY. To discuss the idea of Supersymmetry, we first note that all elementary particles have a property called "spin", which is only partly understandable as the quantum analogue of classical rotation of a sphere around its own axis.

Elementary particles have either half-integer or integer spin, and according to this dicotomy they are labeled "fermions" and "bosons". It turns out that fermions are the constituents of matter, and bosons are the vectors of the interactions between the fermions. Although that is a rather simplified view, it explains that fermions and bosons are indeed quite different animals in the subatomic world.

Fifty years after the glorious first years of accelerator physics, theorists are speculating that for each subatomic particle we know there is a "superpartner": for each fermion there exists a "supersymmetric boson" with the same properties except spin, and for each boson there exists a supersymmetric fermion.

It might look like a simple assertion, but the idea is actually quite deep. And the existence of such a symmetry would go a long way to explain a few of the puzzles of contemporary physics and astrophysics.

However, there is a problem: these particles must have a large mass, otherwise we would have run into them with our accelerators. So Supersymmetric theories (yes, there are several) have to invoke a "soft breaking mechanism" that breaks the symmetry between ordinary and supersymmetric particles, endowing the latter with a large mass, above those explored at current accelerators.

Things get murky here. I will not describe the several appealing properties of supersymmetric theories in this post. There are indeed several good reasons to believe that supersymmetry exists in nature. However, to me it looks a bit too much of a stretch.

I am faithful to Occam's razor: "thou shalt not multiply entities". Meaning that one should not invoke the existence of things as yet unobserved, if not compelled by the need to explain observed phenomena.

I do not think we are quite at that point, at least not so desperate that we need to invoke a full doubling of all observed particles at once! So I remain skeptical with SUSY.  

However, there are many who believe SUSY is the right theory. If not, we would probably not have invested billions of dollars in the Large Hadron Collider at CERN: the LHC is our window of opportunity for the discovery of supersymmetry. If the LHC sees no supersymmetric particles, it will be much harder to believe that is the right theory of particle physics. The LHC can actually rule out SUSY, because to explain why no SUSY particles exist with a mass accessible to the LHC theorists would have to tweak too many things to make the whole thing still appealing.

So let's sit and wait…. That is, you sit. I have to work with these things. But I am not involved in searches for supersymmetry. Rather, I am working for the Higgs. I would be happy to find the Higgs boson, something I do believe exists!