Sunday, May 15, 2016

Life at 750 GeV - Breakthrough Physics or Another Spurious Signal?

Back at base bugs in the software
flash the message "something's out there"
There is an unusual level of hype around the 2016 run of the Large Hadron Collider at CERN in Europe. You're seeing explainers in the mainstream press, deep dives in the technical press, and there have been no less than 350 academic papers written in the last year. Physicists are cautiously breathless, science reporters are forgetting their objectivity, and mainstream journalists are just struggling with the terminology and basic science. Meanwhile, all the cranks are out - from the 'Multiverse' to 'String Theory' to unconserved momentum propulsion systems, the "New Physics" is a new age set of magicks where physical laws are just suggestions and 'Quantum' means something less scientific and more spiritual, all while people are asking physics to offer some kind of meaning to the universe. It's really exciting to see the world getting interested in high energy physics and the coming breakthroughs, but at the same time people have always had a strong tendency to conflate the edge of science with their Woo, and the result is very often incoherent.

Whether this hint of something new and unpredicted turns out to be real or not, it has been invigorating to see the world recognize the amazing discoveries that have been made in the last 50 years of particle physics. We now understand so much about the universe, how it works and what it is, and even better, we know a tremendous amount of things that we still don't know.

So what's all the excitement about?

Last year the Large Hadron Collider restarted after a couple years of shutdown for large scale upgrades. When it restarted, it did so for the first time at it's rated energy - 2 beams of protons at very close to the speed of light - each beam with an energy of 6.5 TeV - colliding head on to produce collisions with a combined energy of 13+ TeV. Last year's run was carefully managed, so the amount of data that was collected - while massive on any real-world scale - was far below the full 'luminosity' speced into the collider.

When that data was analyzed, scientists saw something...odd. There was an excess in photon-photon pairs produced at mass of 750 GeV. (Wait - mass? I though electron volts were energy? Yes - remember E=mc² - energy and mass are the same thing, and can be expressed using either set of terms.) Now, the problem was that there were not a lot of data, and the statistical confidence that the so-called 'diphoton excess' was anything but noise was about 3σ (3 sigma). At that level, it would be ignored as background noise, but what makes this data interesting is the same diphoton excess was detected at the same mass by TWO DIFFERENT EXPERIMENTS. (In collider terms, that's like "The telephone calls are coming from INSIDE THE HOUSE!!) Both ATLAS and CMS experiments saw the same data (or noise). THAT'S exciting, even at 3σ.

High energy particle physics, for all it's 'peering into the universe' majesty, is ultimately just a very large exercise in statistics. Some collisions produce particles from the energy released - most don't. Make a LOT of collisions and count the results. There will be anomalous data, noisy data and just plain bad data. But just keep counting and measuring, and eventually you'll notice something that happens more often than it 'should'. If it happens enough - what statisticians call six sigma - then scientists will consider it a real phenomenon and start trying to figure out what's causing it.

Why the excitement?

The standard model was completely described in the 1970s. Since then, particle physics has been a process of confirming its accuracy. That is, detecting the various elementary particles it predicted. And with the confirmation of the Higgs boson, we are now at a point where - at least according to the Standard Model - we know what matter is, how it gets its properties and how it interacts. Of course, we also know there's other stuff out there - dark matter, dark energy - that probably requires an extension/addition to the Standard Model. And of course, we still don't have a complete theory of gravity - the Standard Model includes force carriers for all the other known forces, but if gravity is going to be considered a force like the strong force, and the electroweak interaction, it's going to need a boson to mediate it.

What could it be?

Heavier Higgs?
Physicists were surprised to discover the Higgs boson at the low, low mass of 125Gev. Everything they predicted about the only scalar boson in the standard model would indicate a much more massive particle. The interesting thing is that there is nothing in the model that precludes the existence of multiple Higgs - if this signal is real, the most likely scenario is that it is a more massive Higgs particle.

One fairly popular theory in the 'new physics' community is Super Symmetry or SuSy. The theory postulates that every particle in the standard model has a more massive version - a Super Particle if you will. At the energies the LHC is running at today, some of these SuSy particles may show up, indicating a much expanded standard model is necessary.

Dark Matter?
A quarter of the matter in the universe can not be detected by any means humans have developed. This dark matter doesn't interact with normal, baryonic matter, even though it provides a huge gravitational force distributed about the universe, and seems to be responsible for the large scale structures we observe, including galaxies and clusters of galaxies. If we find a particle that we never even suspected might exist, it would be hard not to consider the possibility that we are observing dark matter for the first time.

What scientists hope for the most is that it will be something utterly unexpected, new and shocking, a launchpad for the breakthrough in physics that will guide us through the next series of discoveries. Back in the 1950s, a similar observation - known as the Tau-Theta puzzle - led to the discoveries around symmetry breaking, electroweak unification and ultimately Quantum Chromodynamics (QCD).


In the end, however, the great likelihood is that by the end of summer we will have much more data, and the anomalous signal will have vanished into the background noise. Despite what everyone hopes it might be, the overwhelming odds are that it is routine experimental noise, and we will go back to working on the problems and questions we started out with.

But for now? For now we can dream, and think about where it might take us!


  1. This comment has been removed by the author.

  2. reporters are forgetting their objectivity, and mainstream journalists are just struggling with the terminology and basic science.

    Thanks for a clear and understandable explanation of what might be coming out of CERN. No matter how hard a person may try and keep up with real science news there seems to be ten other distorted stories in the media that might as well be pulled from an X-Files episode.

  3. I think if a proof for SUSY exists that we can perceive, it's going to have to be mathematical & not experimental - we're already getting close to the ceiling of energies that we can either afford to muster up or safely contain, with not one lousy neutralino or selectron in sight.

    I'm one of those kooks who thinks that a Theory Of Everything is a big fat Snark that we've been hunting for at least a century in vain. The cosmos neither requires one orderly & human-friendly set of principles to unify it nor does it present data that strongly suggests it. We've already graphed out the four fundamental forces, & although they do intersect, it's messy as hell & decidedly not symmetrical or unitary. Physics gets pretty wonky even at visible scales - just ask a water-strider or a gecko.