The LHC 'nightmare scenario' came and went and, here we are, still doing science.
It's been a month since physicists at the Large Hadron Collider dashed the hopes of a great many physicists and physics spotters that a new particle had appeared in collision data collected during the experiment's 2015 run. This was the 750 GeV diphoton resonance, an unexplained spike in the production of photon pairs with energies of around 750 gigaelectronvolts. There, in the particle detritus following the extreme high-energy collisions occurring between pairs of protons, was something unexplained. And, given the current state of particle physics research, something unexplained would have been fucking fantastic. The 750 GeV diphoton resonance would go on to spawn 547 theory papers attempting to explain it.
The excitement was justifiable. Physics has hit an uncomfortable impasse. This concerns the Standard Model of particle physics, a handy self-consistent description of the particles and forces that make up much of our reality, but not all of it. While the discovery of the Higgs boson filled in a crucial Standard Model gap—thus reinforcing its predictive value for experimental phenomena—the Model doesn't have much to say about the biggest lingering mysteries of physics today: dark energy, dark matter, gravity, neutrino oscillations, matter-antimatter asymmetry. It's incomplete.
This incompleteness is the essence of the hunt for New Physics, or physics that can't be explained by the Standard Model. Physics in 2016 is in dire need of mysterious events, such as the 750 GeV bump. This is why the mood since the August 5 announcement that said bump has not reappeared in 2016 LHC data has been so grim. Everyone went chasing after this (very convincing) mirage of a new unexplained particle only to find just more Standard Model sand.
At Backreaction, physicist and writer Sabine Hossenfelder sums it up:
Now that the diphoton bump is gone, we've entered what has become known as the "nightmare scenario" for the LHC: The Higgs and nothing else. Many particle physicists thought of this as the worst possible outcome. It has left them without guidance, lost in a thicket of rapidly multiplying models. Without some new physics, they have nothing to work with that they haven't already had for 50 years, no new input that can tell them in which direction to look for the ultimate goal of unification and/or quantum gravity.
As for the authors behind those 547 papers, a research physicist who blogs under the name ZapperZ wrote following the August announcement: "They had just created an explanation for the existence of the unicorn ... I guess that people would rather be FIRST to be correct rather than be cautious and not appear foolish. After all, how many of us would remember that such-and-such wrote a paper to explain something that never existed in the first place?" Fair enough.
The dark cloud is about more than just some unanswered questions. It's about the thing itself: how we're even supposed to be thinking about fundamental physics. In particular, the "failure" to find physics beyond the Higgs boson at the LHC is an affront to something more philosophical and, well, old. This is "naturalness."
Physics should be natural. Physics should be simple and beautiful. We should find coincidences in the fundamental laws of nature and those coincidences should point toward some satisfying aesthetic order. This is what we're told about the world, generally: That if we can ascribe to a natural, unmessy ordering of things, we will be more healthy and clear-headed. We will travel with the current of something deep and important instead of against it. In elegance, everything will just lock into place.
In physics, naturalness takes the form of numerical coincidence. The idea, which relates to quantum field theory, is technical, but the high-level sense is that the various numerical constants governing the universe shouldn't look too contrived. This is the appeal of the Standard Model, where everything just locks together, naturally.
Except it doesn't. The mass of the Higgs boson looks really contrived. It's small—despite endowing particles with masses potentially on the order of millions of billions of TeV, the Higgs boson as observed at the LHC weighs in at just .125 TeV. Like, somehow the Higgs boson started out with just enough mass to leave the universe as we see it, but no more. It's not the sort of thing you look at and can imagine "just happened." It's unnatural. There must be something else we're not seeing.
So, by extending the Standard Model, there's hope for resolving this unnaturalness, but physicists like Hossenfelder think that chasing after "natural" explanations is part of the problem. "That the LHC hasn't seen evidence for new physics is to me a clear signal that we've been doing something wrong, that our experience from constructing the standard model is no longer a promising direction to continue," she writes. "We've maneuvered ourselves into a dead end by relying on aesthetic guidance to decide which experiments are the most promising."
That is, attempting to resolve the Standard Model in accordance with some illusory nature will only make our problems worse.
We're not quite at a dead end, however, when it comes to the Standard Model. The LHC is producing statistics about a parameter relating to the Higgs boson known as tth production. This has to do with coupling between the Higgs and a particle known as the top quark. Current data shows this to be quite a bit out of whack with what's predicted by the Standard Model, though, as noted last week at the Résonaances blog, this, like the 750 GeV bump, is likely to be a statistical smudge as well. Still, it might soon become fodder for the next barrage of speculative physics papers.
Dark clouds portend rain, which washes things away, I guess. "Failures" as are they've always been in physics and science in general. By saying what something isn't, you're that much closer to saying what it is. The only real failure is not doing the experiment in the first place.