Supersymmetry

I don’t know about you, but I think the Standard Model is quite elegant. If you’re not familiar with this term, it’s a theory which essentially describes all the matter and forces we have ever been able to observe. As I wrote in one of my previous articles, it is arguably the closest thing we have to a “Theory of Everything”. 

To me, what makes this theory so elegant is the fact that its idea that everything in the known universe is made out of only 17 building blocks. From black holes and badminton shuttles, to bacteria and the trajectory of a basketball. All of these things and forces are the result of a combination of just 17 fundamental particles. 

Apparently, for physicists, that’s not enough. 

This is why some have tried to find an even more basic theory of particles and physical forces. Supersymmetry offers a potential solution. 

This most notable issue which Supersymmetry tries to address which the standard model can’t is the so-called hierarchy problem. This relates how weak gravity is compared to the other 3 fundamental forces of nature: electromagnetism, the nuclear strong and the nuclear weak force. For instance, the weak force is a staggering 1024 times as strong as gravity. 

So what does Supersymmetry actually entail?  

The theory states that each of the 17 particles I mentioned earlier has a supersymmetric cousin, largely similar apart from its quantum spin. Quantum spin is a concept relating to angular momentum and doesn’t need to be explored in too much detail for now. 

The supersymmetric partner of each of the original 17 particles is also opposite in type; fermions have boson partners and bosons have fermion partners. Fermions are particles in the Standard Model which are components of matter while bosons are responsible for causing the fundamental forces. 

It’s also important to know that the predicted partners of Standard Model (SM) bosons are considerably more massive than the SM bosons themselves. In order to be able to solve the hierarchy problem we mentioned earlier, these superpartners need to have masses in the range of 100 to 1000GeV. This is the energy at which the weak nuclear force and the electromagnetic force merge into one. 

To clarify, GeV stands for Giga Electron Volts, which is a measure of mass on the ultra small scale. According to something known as mass-energy equivalence (relating to Einstein’s famous E = mc^2 equation), the electronvolt, which one would expect to be a unit of energy, can also be a unit of pass. 

Physicists were hoping that high energy collision in the Large Hadron Collider (LHC) would have by now produced evidence of one of these larger supersymmetric partners. Unfortunately, it hasn’t.  

This means that if these supersymmetric giants do exist, they are much more massive than scientists expected. As a result, their existence wouldn’t quite be able to solve the Hierarchy Problem as holistically as physicists had formulated. 

If you’re thinking that we need a bigger particle accelerator to yield evidence of these larger particles, you’re on the right track. Though, how much bigger than the LHC can conceivably be built? If it were laid out in a straight line (which it can’t be), the tunnel would nearly stretch from the London Eye to Ascot Racecourse, a whopping 27 kilometres. 

It turns out that the answer is in our backyard, our cosmic backyard at least. The universe itself is an abundant source of high energy particles, the source of which can be anything from gamma ray bursts and supernovae to black-hole magnetic fields. These particles, such as atomic nuclei and electrons, that are flung out into space are known as cosmic rays. 

These rays can have energies as much as 1000 greater than those at the LHC and hence should help with the search for any supersymmetric particles. However, cosmic rays at such intense energies are very rare so the best we can do is try to observe these cosmic rays in an indirect way. 

Notably, physicists rely on observations of what is produced when these high-energy rays interact with the light particles of the Cosmic Microwave Background (CMB). The CMB is essentially the heat left over from the creation of the universe. Partly why observation of such rays are so rare on earth is precisely because they lose energy to the CMB. 

Though when one of these rays interacts with a CMB photon, it can create a very high energy neutrino. These miniscule particles, whose mass is so small that it was long thought to be 0, can travel through the CMB largely unimpeded meaning that by observing these neutrinos, we can learn about the high-energy cosmic rays that produced them

However, very rarely, these high-energy neutrinos interact with nuclei of atoms here on Earth. When this happens, the neutrinos decay into a variety of particles, which can be observed. What does this observing is a nifty machine called ANITA, or the Antarctic Impulsive Transient Antenna. 

ANITA does a pretty incredible job, to say the least. It scans 50 million square kilometres of Antarctic ice sheet, when it is elevated 37km above the ground by a balloon. 

What ANITA does specifically is detect the resulting radiation from the decay of one of these high-energy neutrinos anywhere within a 700km radius away from it. This radiation comes in the form of electrons, tau or muon particles which the neutrino’s decay produced. The scientific name for this is Cherenkov radiation, and it can be picked up by ANITA even if the day happens in the ice sheet itself. 

Now, since other cosmic rays may produce similar radiation bursts as the neutrinos, ANITA is specifically designed to only detect neutrinos from below. In other words, these are the neutrinos that travel all the way along the diameter of the Earth and decay when they reach the Antarctic Ice sheet. 

However, scientists expect the most energetic neutrinos not to be the ones travelling all the way through the Earth since they lose energy to the magma, iron and other rock they travel through. Theoretically, the neutrinos with the highest energies observed should be coming in to the ice sheet at a shallow trajectory (see figure 1) since this means they travel through less Earth and hence slow down less. 

You can probably understand the confusion, then, when the ANITA scientists observed  two high-energy radio bursts which could only have been produced by a particle passing all the way through the earth. Without going into two much detail, the probability of the decays occurring which caused these bursts is about 1 in 3 trillion. It has been particularly difficult for physicists to account for these events with any particles in the existing standard model. 

This is where supersymmetry can offer a solution. Fox, Sigurdsson and team suggest that the supersymmetric partner of the tau particle (the stau) has exactly the right qualities to explain these high energy bursts. 

Here is what could have happened. A stau particle was made when a high energy neutrino arrived at the north pole. Given its particular qualities, the stau could then have travelled all the way through the Earth and decayed in the south pole ice sheet into a regular tau particle. When the tau came into existence and decayed, it would have produced bursts of Cherenkov radiation which could have been what was detected by ANITA. 

Of course, there are other explanations and the fact that the supersymmetry model fits doesn’t necessarily prove its validity. Therefore, while we haven’t quite yet developed a more elegant version of the Standard Model, we have come across a very tantalising hint that one might exist. 

I very much look forward to finding out whether physistics are able to formulate a more elegant and simplistically beautiful theory of everything. What a truly exciting time to be alive.  

This blog entry was primarily inspired by this superb youtube video on the channel PBS Space Time. For a highly in-depth explanation about supersymmetry, its significance and contemporary developments in the field, I would highly recommend any and all curious minds to watch the video mentioned above.