Collider has a bash at finding dark matter
A decade after its discovery of the ‘‘God particle’’, the world’s most powerful physics experiment is setting out on a new quest – the hunt for dark matter.
The Large Hadron Collider (LHC) is a 27km-long track built for smashing protons into each other at velocities close to the speed of light. Ten years ago yesterday its scientists announced that by analysing the wreckage of billions of these collisions, they had detected the Higgs boson, the sub-atomic particle that gives other particles mass.
This anniversary coincides with the LHC, which was built in a tunnel beneath the French-Swiss border, being switched back on after a three-year upgrade. Improvements to energy levels and sensitivity mean that researchers expect to collect as much data from it in the next three years as they have in the past 13.
During its first two runs, in 2009-13 and 2015-18, it tested accepted ideas. The results – including the discovery of the Higgs – reaffirmed the Standard Model, our best description of the forces and particles that form the universe.
Part of its mission will now involve searching for phenomena that the Standard Model does not explain – including the invisible ‘‘dark matter’’ that is thought to make up about a quarter of all matter. ‘‘This is the holy grail at the moment, no doubt about it,’’ said Dr Mike Lamont, the director for accelerators and technology at Cern, the European laboratory that runs the LHC.
Astrophysical evidence suggests that dark matter makes up about 27% of the universe, but it has never been observed.
Even so, Professor Barry Barish of the California Institute of Technology suggests that divining its nature is a relatively ‘‘accessible problem’’.
He believes it could be a stepping stone towards the biggest challenges faced by physicists, which include reconciling Einstein’s general relativistic theory – which explains the universe on a large scale – and its discordant cousin, quantum theory, which explains the behaviour of subatomic particles.
By colliding clouds of protons into each other, the upgraded LHC should create about one Higgs boson a second. There is a chance that these will interact with dark matter particles.
‘‘If there was anything with mass out there, the Higgs would be coupling to it,’’ Lamont said.
‘‘It is a useful ‘candle’, if you like, for looking for deviations from the Standard Model.’’
There is also a new experiment, Faser, that will detect dark matter particles that would previously have been missed.
Dr Mark Williams of the University of Edinburgh said: ‘‘We have really compelling astrophysical evidence that dark matter exists, so the chance of observing it as a new particle is quite tantalising. It would totally change our understanding of the universe at the smallest and largest levels.’’
He pushed back against the idea that physics has stalled since the discovery of the Higgs. ‘‘Ten years ago all we knew [about the Higgs] was that something with a mass of 125 GeV [giga-electron volts] was observed, which decayed into pairs of photons or into four muons.
‘‘Since then we’ve established the spin of the particle and confirmed that it does indeed couple to other particles with a strength proportional to their mass.
‘‘From an [initial] discovery to a high-precision [set] of measurements in less than a decade – it’s the equivalent of having a base on the Moon in 1979.
‘‘And the very fact that we haven’t seen a plethora of new particles at the LHC has told us a huge amount about the universe.
‘‘The LHC has killed a lot of theorist’s favourite models, or at least given them some nasty wounds.’’