Science & Environment

LHC scientists to search for 'fifth force of Nature'

Media captionPallab Ghosh on why a new theory of particle physics is needed

The next couple of years will be make or break for the next big theory in physics called supersymmetry - SUSY for short. It might make way for a rival idea which predicts the existence of a 'fifth force' of nature.

Next Spring, when the Large Hadron Collider (LHC) resumes its experiments, scientists will be looking for evidence of SUSY. It explains an awful lot that the current theory of particle physics does not. But there is a growing problem, provocatively expressed by Nobel Laureate George Smoot: "supersymmetry has got symmetry and it's super but there is no experimental data to suggest it is correct."

According to the simplest versions of the theory, supersymmetric particles should have been discovered at the LHC by now. One set of null results prompted Prof Chris Parkes, of the LHCb to quip: "Supersymmetry may not be dead but these latest results have certainly put it into hospital".

But other forms of the theory are still very much in play.

Next year will be an important year for SUSY. The LHC will be smashing atoms together at almost twice the energy it did in its first run. Even those who are still strong advocates of SUSY, such as Cern's revered professor of theoretical physics, John Ellis, agree that if LHC scientists do not find super particles in the LHC's second run, it might be time for the hospital patient to be moved to the mortuary.

"If it is not found in LHC run two then there will be relatively few corners it could hide," he told BBC News.

"I know that at that point the community may decide that the guys who predicted supersymmetry are dying off like flies and that young guys will be interested in different types of theories and supersymmetry may be forgotten. But I don't think we are at that point yet."

Image caption Engineers have spent more than a year upgrading the LHC's systems. The hope is that this will allow a new realm of physics to be opened up

One of those young guys is Thibaut Mueller, a 24-year-old PhD student at Cambridge University. He is already checking out alternatives to SUSY.

"A few years ago we thought it was a case of who will be first to find supersymmetry," he said.

"Now there is less and less focus on it and more people are starting to branch out into other models."

Mr Mueller's PhD looks at an alternative to supersymmetry called the composite Higgs model. This idea has been around for decades but is undergoing a resurgence as some researchers raise questions over supersymmetry. Physicists will be looking for evidence for it in the next run of the LHC in 2015.

Thibault's colleague Dr Ben Gripaios believes that the Composite Higgs theory is now a serious alternative to supersymmetry.

"SUSY was regarded by many people as the perfect theory. We have been looking really hard for it for a long time and we have not found it and so possibly there is a different explanation. For me the most compelling alternative is the Composite Higgs. It is just as plausible as supersymmetry," he told BBC News.

The current theory to explain the forces of nature was developed in the 1960s and is called the Standard Model. It elegantly explains how 13 particles, including the Higgs, interact to create three of the four forces of nature: electromagnetism, and the nuclear strong and weak forces.

But the Standard Model does not explain how gravity works, nor can it account for the matter and energy that makes up 95% of the Universe - referred to by physicists as the "Dark Universe".

Supersymmetry is an extension of the Standard Model and is an attempt to explain some of the things the current theory can't.

Image copyright SPL
Image caption The stage has been set for some years for the detection of super particles. But so far they have been a no show.

It predicts the existence of so-called superparticles which account for much of the missing mass and energy of the Universe.

Supersymmetry also neatly solves what physicists describe as the "fine tuning problem". In very crude terms, all subatomic particles can be thought to have two values for their mass: Their mass in isolation which is called their "bare" mass, and their experimental mass, which includes interactions with other sub-atomic particles.

For all particles the two masses are about the same, except for the Higgs, whose bare mass must be many times larger than its experimental mass.

Going from such a relatively big number to a small number is an unlikely occurrence, rather like a skydiver landing on the head of a pin each time they jump out of a plane. It can only happen if there is an overarching force guiding the skydiver on to the pin head - something that physicists call "fine tuning".

The existence of superparticles interacting with their normal counterparts fine tunes the Higgs's two masses perfectly. The drawback though is that there is no evidence of SUSY, at least not yet.

The composite Higgs theory also solves the fine tuning problem, albeit less elegantly and, just as with SUSY, there is no experimental evidence for it. It supposes that the Higgs is not a fundamental particle, but is instead made up of other fundamental particles bound together by a hitherto unseen fifth force of nature. This is similar to what is already known to happen with the strong nuclear force, which binds quarks together to produce nuclear particles like protons and neutrons.

Scientists at the LHC hope to detect evidence for one or other theory when they resume their experiments in April. In effect, the starting gun goes off in an invisible two-horse race where the winner emerges only at the finish line. Supersymmetry is still the favourite in the minds of most particle physicists, but Thibaut Mueller thinks that the likelihood of finding evidence for composite Higgs theory is not far behind.

Why then is this promising youngster gambling his still early career on the outsider?

High risk

"This is a high risk, high gain game," he explained. "If we find either (SUSY or the composite Higgs) this would be the biggest revolution in particle physics and possibly the whole of physics since quantum mechanics in the the 1940s.

"Even if we do not find evidence for SUSY or composite Higgs, we will still have learned important facts about the Standard Model, which will guide us to new theories".

Of course, the researchers may see neither, which raises the possibility that no fine tuning is needed to turn the big Higgs into the little Higgs.

That would mean that we live in a Universe where the dice are loaded to ensure that the Higgs experimental mass will always improbably land neatly on its bare mass each and every time.

Image caption As researchers smash particles at ever higher energies will they find a fifth force of nature that holds the Higgs together

In the absence of evidence for either theory, this anthropic principle might seem like a tempting option. But it's one that those on the front line of research vehemently resist.

According to Thibault Mueller that view is a "conversation stopper".

"It says that 'we are special because we as humans are here to observe it and so we exist'. If we accept that then we might as well give up science altogether.

"We (have established) that we as a species are not special, the Earth is not special, our Solar System is not special. Now we are saying: 'Ah! Our Universe is not that special either'."

Prof Rolf Dieter Heuer, the director-general of the European Centre for Nuclear Research (Cern) recently told researchers at the International Conference on High Energy Physics (ICHEP) in Valencia, that there was "a lot at stake" for the LHC's second run starting next year.

Indeed there is: careers, reputations and deeply cherished ideas.

But whatever the outcome, physicists are preparing themselves for the ride of their lives. As Prof Heuer told the physics community: "There's much more to be discovered in the Dark Universe".

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