The toil to discover ‘new physics’ after Higgs boson breakthrough
Six years after Cern scientists found the ‘god particle’, progress is slow
Clive Cookson
Nothing new has emerged, even after the Large Hadron Collider's operating power was almost doubled in 2015 © AFP
The summer of 2012 was the most glorious in the history of high-energy physics. To huge public acclaim, Cern, the European physics centre near Geneva, announced the discovery of the Higgs boson— frequently called the “god particle” in the media — which gives matter its mass.
Higgs particles emerged from the subatomic debris of trillions of collisions between hydrogen nuclei travelling at almost the speed of light around the world’s most powerful atom smasher, Cern’s $8bn Large Hadron Collider. Their detection after the LHC had been running for less than three years gave a finishing touch to the so-called Standard Model of physics, which provides an internally consistent description of some — but not all — of the fundamental forces and particles in our universe.
After the Higgs triumph, everyone associated with Cern hoped the LHC would move quickly on to make discoveries in what scientists often call “new physics”, helping to explain phenomena beyond the scope of the Standard Model.
One priority is to elucidate the dark matter and dark energy that appear to dominate the universe but remain a complete mystery. Another is to explore the supersymmetry theory, which holds that a panoply of “superparticles” overlays the subatomic particles described in the Standard Model. A third is to understand why the universe consists of matter rather than anti-matter.
Sadly, nothing new has emerged, even after the LHC’s operating power was almost doubled in 2015. Physicists have only been able to dot i’s and cross t’s within the Standard Model. Just this week, for instance, Cern announced the first detection of the predicted decay of the shortlived Higgs boson into a pair of “bottom quarks”. That is big news for Higgs researchers, but there have been no more discoveries to excite people beyond the world of particle physics.
Scientists who study the LHC’s vast data sets — accumulating at about a quadrillion bytes per day — have occasionally seen tantalising hints of new physics. But these have evaporated on further analysis, before reaching the level of statistical significance that would allow the researchers to claim a discovery. Although something exciting might leap out of the data, few physicists expect this to happen in the near future.
The biggest physics announcement since Higgs took place in 2016, far from Cern. A $1bn facility in the US for the first time detected gravitational waves, generated by a cataclysmic collision between black holes in the distant universe. Further discoveries in the past two years, including a neutron star collision, show that gravitational waves could open up a window into the most energetic events in the universe — perhaps including its birth in the Big Bang.
In retrospect, senior Cern scientists do seem to have been over-enthusiastic about the prospects for new physics as they celebrated the Higgs discovery. While some theories suggested that new particles would soon show up at the LHC, other versions would require collisions with more energy than even the upgraded collider can provide.
But it would be wrong to write off the LHC as a failure, even if it does nothing more than complete the Standard Model.
As Cern physicist Tim Gershon of Warwick university puts it, the Higgs can act as a “novel microscope” into the universe on the smallest scales through its interactions with other particles and forces. That is analogous to the idea that new gravitational wave telescopes will probe the largest cosmological scales.
With more than 20 years of life still left in the LHC — and several further upgrades planned to increase both the number and energy of collisions in its 27km ring — the prospects of moving firmly into new scientific territory remain excellent.
In addition to these hardware improvements, some physicists are beginning to change the way they handle LHC data. Until now everyone has concentrated on “targeted searches”, scouring the fallout from trillions of collisions for signs of specific particles predicted by theorists.
For instance, the hypothetical “neutralino” is a candidate to be a constituent of dark matter and to play a role in supersymmetry. New “general searches” would look for any anomaly that does not fit the Standard Model, increasing the chance of discovering something unexpected. Although the information processing challenge is immense, advocates of this approach are relying on artificial intelligence and machine learning to spot novel patterns in the data.
Looking further ahead, the world’s high energy physicists are already planning an even more powerful successor to the LHC, to come on stream in the late 2030s. Candidate designs include the Future Circular Collider with a 100km ring, and the straight International Linear Collider, which is about 40km long.
But it is hard to see governments committing billions of dollars to build a machine on that scale until the LHC comes up with some new physics for it to investigate. The field needs another achievement that can be greeted with as much acclaim as the Higgs discovery.
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