Scientists restart Large Hadron Collider atom-smasher
Scientists have successfully restarted the Large Hadron Collider (LHC), the most powerful atom-smasher ever built, hoping to enter a new realm of physics and make history for the second time.
Two beams of particles travelling a whisker below the speed of light were sent flying in opposite directions through the LHC's 27 kilometres (16.7 miles) of circular underground tunnels straddling the Swiss-French border.
Amid scenes of jubilation in the LHC control room, Professor Rolf Heuer, director general of Cern, the European Organisation for Nuclear Research, said: " Congratulations. Thank you very much everyone... now the hard work starts."
Currently the £3.74 billion machine is running at a low "injection" energy of 450 giga-electron volts (GeV). In June, the energy level will be ramped up to a record-breaking 13 tera-electron volts (TeV) and experiments probing the fundamental building blocks of the universe can begin.
Two years ago the LHC team, which includes a number of British physicists, astounded the world with the discovery of the Higgs boson, an elementary particle that gives other particles mass.
Now the scientists have their sights set on an even more exotic trophy - dark matter, the invisible, undetectable material that makes up 84% of matter in the universe and binds galaxies together yet whose nature is unknown.
With a beam energy of 13 TeV - almost twice that which produced the Higgs boson - it is conceivable that the LHC will capture dark matter, marking a leap forward in our understanding of the universe.
Earlier Cern spokesman Arnaud Marsollier said: "The LHC will be running day and night. When we will get results we don't know. What is important is that we will have collisions at energies we've never had before.
"If something interesting appears in this new window we will see it. It might be two months from now or two years, we're not able to say. It took 50 years to find the Higgs boson and 20 years to build this machine, and it will be running at least until 2035, so we can be patient."
A technical hitch had delayed the restart of the LHC after a two year re-fit and upgrade.
An electrical short circuit prompted fears that operations could be put back weeks or even months. However, engineers quickly located the problem - a small piece of metal debris - and removed it.
As tension built during the final minutes before the restart, Frederick Bordry, Cern's director for accelerators and technology, handed out Easter eggs to staff in the LHC control room.
The particles of protons, the "hearts" of atoms, travel round the LHC at just three metres per second below the speed of light.
When experiments begin they will be smashed together in four giant detectors located around the beam ring, sparking the creation of new particles and hopefully opening up a new era in physics.
The search for dark matter involves stepping outside the Standard Model, the all-encompassing theory that describes the particles and forces of nature that has stood firm for the past 50 years.
A "new physics" model of the universe called supersymmetry predicts that every known particle has a more massive partner - and one of these elusive supersymmetry particles might be the source of dark matter.
At a Cern briefing in Geneva last month, British scientist Professor David Charlton, from the University of Birmingham, who heads the Atlas detector team, said: "We're heading for unexplored territory. It's going to be a new era for science."
Professor Jonathan Butterworth, from University College London, a leading member of the Atlas detector team, said: "It all seemed to go very well this morning. We'll all be watching very excitedly to see what develops over the next few weeks. The LHC's operating again for the first time in two years and that's a really important milestone in physics.
"You can think of the LHC as the world's greatest microscope looking into the heart of matter. At this higher energy level we don't know what we'll see - no-one has looked there before.
"The next milestone will be to ramp up the energy because what we're really interested in is the collisions. Where we are at the moment, we've found the Higgs boson but there are obvious things we haven't been able to describe and we'd like some clues.
"It would be absolutely lovely to find a candidate for dark matter. That's a fairly glaring hole in our theory. Supersymmetry particles are a good candidate, but they're not the only candidate.
"The fact that we don't know what most of the universe is made off is really what drives me on."
Professor Charlton said a couple of "splash events" - when the beam is directed at a test target - confirmed that the particles had arrived at the Atlas detector.
He said: "It was a happy moment in the Atlas experiment control room when we saw the first beam splash event.
"This is a first step towards the exciting physics we will do in the coming, second, multi-year run of the LHC, which should give us new insights into fundamental physics, when we have collected plenty of collisions at the new high energies to come. The Atlas experiment, along with the other experiments at the LHC, is ready and eagerly waiting."
The three other LHC detectors are called CMS, Alice and LHCb.
Professor David Colling, senior lecturer in high energy physics at Imperial College London, said: "The LHC is primarily designed as a discovery machine. In the first running period we discovered the Higgs boson, a particle for which we had been searching for 50 years. This was the last missing piece of the Standard Model.
"The increase in the machine energy during this second running period gives great scope for discovering physics beyond the Standard Model.
"Some discoveries could come very quickly (weeks after start up), others would require the collection of large amounts of data over several years. Which, if either, of these we will find depends on physics that we don't yet understand."
Professor Tara Shears, who heads the University of Liverpool LHCb detector group, said: " We have unfinished business with understanding the universe.
"We want to see what the new data shows us about antimatter, and why there's so little in the universe. We want to chase the hints we've seen in previous measurements, whose behaviour didn't quite match our expectations, in case these hints turn into discoveries.
"We've spent the shut-down readying and improving the LHCb detectors so that we can explore this new data with precision."
Professor Andy Parker, head of physics at Cambridge University, said: "This is a very exciting time for scientists as the Large Hadron Collider restarts after a major upgrade.
"The current Standard Model explains the known particles and forces, and the discovery of the Higgs completed that picture. But the Standard Model does not explain dark matter, which is believed to make up most of the universe, nor dark energy, a mysterious force driving the galaxies ever further apart.
"Interestingly, the answers to these problems in cosmology might lie in the realm of sub-atomic physics studied at Cern. The LHC might be able to produce dark matter particles for example, which would be glimpsed in the debris of collisions detected by the Atlas and CMS experiments.
"Even more exciting is the possibility that the universe could have more than three space dimensions, and that other spaces are hidden all around us."
Professor Roger Jones, head of the particle physics group at the University of Lancaster, said: "For less than the cost of a single Typhoon jetfighter, the upgraded LHC will push our understanding of physics to the brink. In a sense, we have entered a mode of more pure scientific discovery - and I for one cannot wait."
Dr Victoria Martin, an Atlas scientist from the University of Edinburgh, said: "The Atlas UK team are eagerly anticipating the extra data that LHC Run 2 will provide. Using data from LHC Run 1 we discovered the Higgs boson particle.
"However only a limited number of Higgs particles were produced and it has not yet been possible to test every prediction made by Peter Higgs and others.
"The higher energy and more frequent proton collisions in Run 2 will allow us to investigate the Higgs particle in much more detail. Higher energy may also allow the mysterious "dark matter" observed in galaxies to be made and studied in the lab for the first time."
Professor Craig Buttar, from the University of Glasgow's School of Physics and Astronomy, said: " The switch on of the LHC with increased energy and luminosity is incredibly exciting. It will allow us to extend our searches for evidence for new physics and to study the Higgs boson to see if it behaves as expected or is a window to new physics."
Professor Dave Newbold, who heads the particle physics group at the University of Bristol, said: "The discovery of the Higgs boson in 2012 was a huge breakthrough, but the real excitement starts now.
"The upgraded LHC will allow us to probe further than ever before, and to explore brand new scientific territory such as dark matter."