Has physics entered the nightmare scenario?
Since October last year, when a major upgrade to the Large Hadron Collider (LHC) particle accelerator was completed, it has run experiments at higher energy levels than ever. But as more and more data pours in, the one result that was completely unexpected appears more and more likely: there is nothing unexpected.
Physicists had already termed a phrase for it: the nightmare scenario. If it comes true, the LHC collider will reach its maximum energy without finding anything that points to new, unexplained physics, which could be the key to so many outstanding problems.
Particle accelerators seek to explore the workings of nature at the most fundamental level. Richard Feynman, the great 20th Century physicist and science communicator, described the logic behind them as so: “Atoms are complicated. Maybe like watches are—but atoms are so small that all we can do is smash them together and see all the funny pieces (gears, wheels, and springs) which fly out. Then we have to guess how the watch is put together.” Billions of particles smashed together at enormous energies, producing violent explosions in which new particles pop into and out of existence, create other particles, and interact with one another. Out of the chaos, physicists search for order: hints in the collision statistics for trails of missing energy left by new particles.
Such was how the Higgs Boson was discovered in July 2012 at the LHC, a giant, 27-km long circular tunnel in Switzerland. The detection was a capstone in the Standard Model of particle physics. The Higgs, which is believed to give other particles mass by their interactions with it, had been theoretically predicted in the 1960s but never experimentally observed.
But in the two years since, with the LHC running at higher and higher energies capable of finding even more massive particles, silence has reigned. No new particles have revealed themselves.
In December last year, the now infamous “diphoton bump” was believed to be precisely one such a particle. Two different, independent detectors at the LHC showed a spike in collisions at the same energy, suggesting a new particle at that mass. But the mass was far greater than anything predicted by current theory. The physics community was electrified by the potential discovery. New Scientist highlighted the implications with the title, “Bigger than the Higgs, bigger even than gravitational waves.”
The result, however, turned out to be a statistical anomaly. As more data poured in, the bump was drowned out—taking with it the most promising possibility of observing new physics at the LHC.
What does the nightmare scenario mean for particle physics? It leaves the Standard Model in a very awkward position. Speaking in a fascinating Quantum Magazine article on this question, physicist Raman Sundrum points to the deep principle of what physicists call “naturalness.” Plainly speaking, this says that there must be some reason why the Higgs, and other particles, have the mass that they do, and not some wildly different one. Why should nature preference one mass over another? The theory that had been developed in the 1980s to answer this question, dubbed “Supersymmetry,” requires the existence of new particles—particles that, in most cases, should have already been observed at the LHC.
In the history of physics, fundamental breakthroughs have so often been prepared by periods of growing tension and contradiction in current theory. Sabine Hossenfelder, a theoretical particle physicist, writes in Forbes that the nightmare scenario would mean “we’ve been doing something seriously wrong, that our experience from constructing the Standard Model is no longer a promising direction to continue… Sometimes things have to get really bad before they can get better.”