Neat…orderly…beautiful even – the theory of ‘supersymmetry’, or SUSY for short, is a scientist’s dream. It tidies up many loose ends left by the so-called Standard Model of particle physics. It opens up the prospect of discovering a whole new array of exotic particles whose mysteries will need to be probed and unravelled. Above all, it potentially pushes our entire understanding of the Universe to a completely new level.
The trouble is, though the theory has secured widespread support since first framed over 40 years ago, so far not a single particle predicted by SUSY has definitely been observed. In 2012, Prof Chris Parkes (who is the spokesperson for the UK participation in the LHCb experiment) told BBC News: "Supersymmetry may not be dead but these latest results have certainly put it into hospital." If supersymmetry isn’t proven soon it could be a lot harder for particle physics to move further forward.
The Standard Model sets out how the subatomic particles we already know exist – quarks, electrons, photons and the rest – interact. But we need to resolve the discrepancies, inconsistencies and nagging questions it leaves us with – for example, why the famous Higgs boson is an astonishing one hundred thousand trillion times lighter than calculations say it should be. Put simply, there’s a lot we already know but we need to know an awful lot more.
That’s where SUSY comes in. The core idea behind supersymmetry is that every known type of particle has a partner differing from it in mass (super-particles are a lot heavier than their ‘regular’ counterparts) and a property known as ‘spin’. So the quark is partnered by the ‘squark’, the electron by the ‘selectron’, the Higgs boson by the ‘Higgsino’ and so on. (Coming up with names for new particles is surely one of particle physics’ most satisfying perks!)
Super-particles, if they exist could allow physicists to find a deeper theory which underlies the Standard Model that may explain (for example) why the Higgs boson is not much heavier than it is. Super-particles could even be responsible for the enigmatic ‘dark matter’ thought to hold galaxies together and to make up 80% of matter in the cosmos.
Like dark matter particles, though, no super-particles have ever been observed. Some experiments have produced tantalising results, but these ‘blips’ haven’t been conclusive. The race is on to generate hard evidence that supersymmetry is fact, not fantasy.
UK scientists are at the heart of key experiments such as ATLAS and CMS – located at the Large Hadron Collider (LHC) at CERN (the European Organization for Nuclear Research) in Switzerland – which are helping in the hunt. The hope is that, when the LHC generates much higher energies once it restarts in 2015, the proton collisions it creates will produce signals that provide incontrovertible proof of super-particles’ existence. Professor Alan Barr, whose research group at the University of Oxford has been hunting for evidence of supersymmetric particles at the LHC, says: “The higher energy will be a real game-changer – it’s going to be a very exciting time.” Dr Monica D'Onofrio of the University of Liverpool, former co-ordinator of the ATLAS experiment’s supersymmetry group, comments: "The LHC will deliver over 100 times more collisions than it has until now, so it will be a great opportunity to search for new particles."
There’s plenty of other activity too. For instance, the multinational MasterCode team led by Professor John Ellis of King's College London is forecasting the prospects for finding supersymmetry during the LHC’s next run; a team at Cambridge led by Professor Ben Allanach is hard at work interpreting ‘blips’ in current data; and a collaboration led by Professor Peter Richardson at Durham has invented a computer program that simulates particle production and decay and will help LHC experiments interpret their searches for super-particles.
Expert opinion remains divided on when – or even if – evidence of SUSY will be found. Discovery by June 2016 is reputed to be the subject of a small bet among scientists at CERN, with the balance of opinion evenly spread. If supersymmetry is discovered, it could trigger the big leap forward that particle physics has been waiting for. If it isn’t, SUSY will have to be extensively recalibrated… or radical new ideas will be needed to take our understanding of the Universe’s building blocks to the next level.
CERN people – unrequited love: supersymmetry
(Credit: Intelligent Channel)