Science & Environment

Higgs boson spills secrets as LHC prepared for return

Atlas Image copyright Cern
Image caption The LHC has been shut down since 2013 for a programme of upgrades and repairs

It's nearly time. After shutting down last year for vital repairs and upgrades, the Large Hadron Collider is being prepared for its comeback.

Engineers at Cern in Geneva have begun cooling the huge machine to its operating temperature of -271.3C, which is colder than deep space.

And the accelerator system that supplies the LHC with its proton particle beams - which are smashed together to recreate the conditions just after the Big Bang - is up and running for the first time since 2012.

Teams are working to get the LHC - located in a circular tunnel beneath the French-Swiss border - back online by January 2015 and this time it will operate at its full energy of 14 trillion electron volts.

After the $10bn machine was switched on for the first time in 2008, problems were found with many of the electrical splices between the 1,200 superconducting magnets that bend particle beams around the 27km-long underground ring.

To prevent serious damage, officials decided to run the collider at an energy of seven to eight trillion electron volts - about half what it was designed for.

"Much work has been carried out on the LHC over the last 18 months or so, and it's effectively a new machine, poised to set us on the path to new discoveries," said Cern's director-general Rolf Heuer at the EuroScience Open Forum in Copenhagen this month.

The low energy run from 2010-2012 was nevertheless sufficient to achieve a key scientific goal: Detecting the elusive Higgs boson particle.

The Higgs is the cornerstone of our current best theory of particle physics - the Standard Model. This is the "instruction booklet" that describes how elementary particles (the smallest building blocks of the Universe) and forces interact.

On 4 July 2012, two years ago this week, Cern announced that a five-decade-long search for the particle, first proposed by Edinburgh-based physicist Peter Higgs and others in the 1960s, had reached its conclusion.

Scientists working on Atlas and CMS, the two huge multi-purpose detectors placed at strategic points around the LHC tunnel, saw the Higgs at a 5-sigma level of significance - the statistical threshold for announcing a discovery.

While the LHC has been sitting idle of late, its scientists have not. They've continued to crunch the results from the first science run, and hundreds of them will gather to listen to the latest findings at the ICHEP physics meeting in Valencia, Spain, this week.

Particle physicists have learnt more about the Higgs boson's behaviour and how well it conforms to predictions. In a paper published last week in the journal Nature Physics, researchers outlined how they have watched the Higgs decay into the particles that make up matter (known as fermions), in addition to those that convey force (bosons), which had already been observed.

This is exactly as the Standard Model predicts. Physicists know that this framework, devised in the 1970s, must be a stepping stone to a deeper understanding of the cosmos. But so far, it's standing up exceptionally well. Searches at the LHC for deviations from this elegant scheme - such as evidence for new, exotic particles - have come to nothing.

Image copyright AFP
Image caption Physicists packed out the auditorium at Cern to hear the Higgs boson discovery announcement in 2012
Image copyright AFP
Image caption Francois Englert (L), Peter Higgs (R) and other originators of the Higgs boson theory were at Cern to hear the announcement. Englert and Higgs would later win a Nobel Prize for their work
Image copyright Paul Rincon
Image caption The announcement was a huge media event too

At ICHEP, other scientists are expected to outline details of a refined mass for the fundamental particle, which has been measured at approximately 125 gigaelectronvolts (GeV). For those outside the particle physics community, this might seem like a minor detail. But the mass of the Higgs is more than a mere number.

There's something very curious about its value that could have profound implications for the Universe. Mathematical models allow for the possibility that our cosmos is long-lived yet not entirely stable, and may - at some indeterminate point - be destroyed.

"The overall stability of the Universe depends on the Higgs mass - which is a bit funny," said Prof Jordan Nash, a particle physicist from Imperial College London, who works on the CMS experiment at Cern.

"There's a long theoretical argument which I won't go into, but that value is intriguing in that it sits on the edge between what we think is the long-term stability of the Universe and a Universe that has a finite lifetime."

To use an analogy, imagine the Higgs boson is an object resting at the bottom of a curved slope. If that resting place really is the lowest point on the slope, then the vacuum of space is completely stable - in other words, it is in the lowest energy state and can go no further.

Image caption The mass of the Higgs (inside rectangle) may hint at the stability of the Universe

However, if at some point further along this slope, there's another dip, the potential exists for the Universe to "topple" into this lower energy state, or minimum. If that happens, the vacuum of space collapses, dooming the cosmos.

"The Higgs mass is in that place where it gets interesting, where it's no longer guaranteed that there are no other minima," Prof Nash, who works on the CMS experiment at Cern, told the BBC. But there's no need to worry, the models suggest such a rare event would not occur for a very, very long time - many times further into the future, in fact, than the current age of the Universe.

This idea of a finite lifetime for the cosmos is dependent on the Standard Model being the ultimate scheme in physics. But there is much in the Universe - gravitation and dark matter, for example - that the Standard Model can't fully explain, so there are reasons to think that's not the case.

The existence of exotic particles, such as those predicted by the theory known as supersymmetry, would shore up the stability of the Universe in those mathematical models.

But as previously mentioned, searches for these particles, called superpartners, have so far drawn a blank, as have attempts to detect dark matter, extra dimensions, and other phenomena beyond the Standard Model. Hopes that the LHC would allow scientists to lift the veil on a whole new realm of physics have proved optimistic, at least during its initial run.

Image copyright Cern
Image caption Electrical connections between the superconducting magnets have been re-soldered
Image copyright Cern
Image caption Engineers have been working to prepare the machine for a planned re-start at the beginning of 2015

Some versions of supersymmetry have already been all but ruled out by the LHC. But the theory has many forms, depending on how you tweak the mathematical parameters.

"From the theory community's point of view, this is all very interesting because it fleshes out much better what the first run of the LHC has excluded," said Prof Dave Charlton, who leads the Atlas experiment at Cern.

"Therefore, it better establishes where we should be looking for new signals next year."

Assuming the theorists are indeed correct, supersymmetry will have to wait some time longer for its big reveal.

Other hypothesised particles, such as the W prime and Z prime bosons could possibly be detected soon after the LHC returns to particle smashing.

For now, all eyes are on the engineers at Cern. The LHC's initial switch on was marked by mishaps, including a magnet that buckled in the tunnel during a test in 2007. The following year, another magnet failure caused a tonne of helium to leak out, forcing controllers to shut the machine down just nine days after its big switch-on.

But after the re-start in 2009, the LHC performed flawlessly, and the rest, as they say, is history.

If all goes well, by the end of March 2015 scientists could begin colliding high-energy beams of particles at the LHC.

And that's when the real fun will begin.

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