All particles can be divided into two distinct types known as bosons and fermions, as described in some of my earlier articles. For more information see: Bosons, Lasers and Superfluids and Fermions, Atoms and Neutron Stars. However, the ultimate aim of physicists is to reveal the unifying principles that bind the universe together – the simple laws from which all else follows. In the spirit of this adventure it is natural that they should seek a deep connection between these two apparently very different types of particle. This article is about the ongoing search for this connection.

**Good Vibrations!**

Many of today’s leading theorists are engaged in an ambitious programme called string theory that aims to explain the whole of physics within a single framework. In particular the aim is to encapsulate all the forces and particles of which the universe is composed. The new feature that string theory brings to the table is the idea that one physical entity – the string – can vibrate in many ways and each different vibration represents a different particle. For instance, one mode of vibration might be an electron, another mode of vibration might be a neutrino and a third mode of vibration might be a photon. This means that rather than positing the existence of a multitude of different particles, there is just one fundamental object.

This apparently simple idea has very deep consequences. Physicists and mathematicians have already spent several decades studying the implications of the quantum theory of strings. There are a great many weird and wonderful ideas that have grown out of this research, but the jury is still out about whether there is any connection to the real world. Experimental support is still lacking.

**Sexy Susy**

In the original incarnation of string theory all the string vibrations represented bosonic particles, such as photons, so the theory did not include any fermions which meant that it could never be a realistic model of physics. But in the early 1970s string theorists discovered ways in which a single string could vibrate as either a fermion or a boson. The theory now had a new type of symmetry built into its structure – a symmetry between fermions and bosons. This symmetry is called *supersymmetry*, a name that is sometimes abbreviated to Susy.

It didn’t take long for particle physicists to take the idea of supersymmetry and consider its implications for the more conventional physics that is being explored in particle accelerator experiments. New supersymmetric theories were constructed that were not necessarily connected to the esoteric world of string theory. The best theory of particle physics that we have is known as the standard model. The simplest supersymmetric extension of this theory is known as the minimal supersymmetric standard model, which gives us the neatly symmetrical acronym MSSM.

If the universe really is supersymmetric, then fermions and bosons must come in pairs. For instance, the electron (which is a fermion) must have a partner that is a boson, and the photon (which is a boson) must have a partner that is a fermion. Indeed, all the fundamental particles that we know of must have partners that are yet to be discovered. These pairs of particles would have the same mass if supersymmetry was a perfect unbroken symmetry, but we know that this cannot be the case, because if it were the partner particles would have been discovered long ago.

**Sleptons and Squarks**

The hypothetical new particles predicted by supersymmetry have already been named by theorists. The partner of the electron is known as the selectron. In general the name of the bosonic partner of a fermion is produced by adding the prefix ‘s’ for ‘supersymmetry’ to the name of the fermion, so that the partner of the neutrino is known as the sneutrino and the partners of the quarks are known as squarks.

**Winos and Zinos**

The supersymmetric partner of the photon is a fermion. Theorists call it the photino. In general, the supersymmetric partner of bosons all end in the suffix ‘ino’, which gives us Winos, Zinos, gluinos and Higgsinos.

The Large Hadron Collider is searching for signs of these new particles. If supersymmetry is a true symmetry of the universe, then there should be a wonderful harvest of new particles to be reaped. And their discovery could answer one of the biggest mysteries of the Cosmos.

**Dark Matter – A Possible Solution**

Almost all fundamental particles are extremely unstable, which means that they rapidly decay into other lighter particles. Matter is formed from the few types of particle that are stable. A very important consequence of supersymmetry is that the lightest supersymmetry partner particle is expected to be completely stable. This particle is usually referred to as the neutralino (because it carries no electric charge). If supersymmetry does play a role in the structure of the universe, then this particle would have been produced in great profusion in the earliest moments of the universe and because it is completely stable it will be as abundant now as it always was. This could explain why there seems to be much more matter in the universe than astronomers have been able to observe. See my article: Most of the Universe is Missing! In other words, dark matter could simply be made up of vast quantities of neutralinos.

**The Greatest Discoveries of Physics**

Supersymmetry has grown out of the belief that the laws of the universe must be beautiful. The prediction of a whole host of new particles simply to satisfy our demands for mathematical elegance is incredibly bold. The discovery of supersymmetry would rank as one of the greatest in the history of physics – even greater than the discovery of the Higgs boson.

The closest comparison in the history of physics was the prediction of antimatter by Paul Dirac. In 1928 Dirac formulated a new type of equation to describe the wave-like behaviour of electrons. His analysis of the equation led to his prediction of a particle with the same mass as an electron, but opposite charge. Dirac’s new particle was discovered by Carl Anderson in 1932 and is known as the positron. This was the first antiparticle to be discovered. It was followed by the antiproton, the antineutron and, indeed, we now know that all fermions have antiparticles and they are readily produced in particle accelerators such as the LHC.

**Not So Fast!**

However, so far the predictions of the standard model are standing up incredibly well and there is no sign of any new physics (such as supersymmetry) that goes beyond the standard model. This has led to recent reports that physicists are beginning to worry that their quest for supersymmetry might be in vain.

Particle physicists measure mass in electron Volts. One electron Volt is equivalent to the amount of energy that a charged particle such as an electron gains when moving through a one Volt electrical circuit. The mass of a proton is just under 1 GeV (billion electron Volts) and the mass of the Higgs boson is in the region of 125 GeV.

Most theorists expected that the mass of the lightest of the new particles predicted by supersymmetry would be comparable to the mass of the Higgs boson. While the LHC has not ruled out the existence of supersymmetry yet, it seems to have shown that the mass of the lightest supersymmetry partner particle must be greater than 1 TeV. (1 TeV is one trillion electron Volts.) So supersymmetry isn’t dead yet, but it would appear that the lightest of the predicted new particles must be much heavier than expected – at least ten times the mass of the Higgs boson.

The universe might still be supersymmetric, but on the other hand it might not be supersymmetric! We will have to wait and see.

**More Information**

For more information about Dirac and the discovery of antimatter, see my book *Higgs Force*:

http://quantumwavepublishing.co.uk/higgs-force/