Sorry, Folks. The LHC Didn’t Find a New Particle After All
The last thirty years of particle physics have been a little disappointing. A scientist’s job is to prove themselves wrong, but despite their best efforts, despite recreating the conditions of the Big Bang, particle physicists just keep being correct. Aside from a few unexplained observations (meddling neutrinos!), the Standard Model, which describes interactions between all known particles, has exactly predicted the outcome of every experiment in the history of particle physics. Physicists try to prove it wrong, and they keep failing.
Last December exposed the field’s latent craving for novelty. That’s when CERN announced a collection of unexpected observations at the Large Hadron Collider. Scientists quickly submitted over 500 papers, each inventing a new way to explain the observations, which seemed to blast holes in the hull of the unsinkable Standard Model. But in a new paper uploaded last night, CERN makes it clear that the search will have to continue: The exciting measurements were nothing more than statistical blips. Scientists will discuss these results in more detail later today.
The LHC, CERN’s flagship project, searches for new physics by smashing together protons travelling vanishingly close to the speed of light. “The proton is a composite object; it’s something made of other particles,” explains Gian Giudice, head of the CERN Theoretical Physics Department. “So when you have a collision among protons, one has to understand which component of the proton is really responsible for the collision.” New particles form from the wreckage of the collision, usually decaying into common, easily detected particles like photons. “The goal is to reach the highest possible energy,” Giudice says. The more energetic the collisions are, the better chance there is of finding something new in the twisted wreckage.
The particle accelerator ramped up to its fastest, most energetic collisions yet in 2015, with initially surprising results. Two different experiments (ATLAS and CMS) examine the products of collisions for new and unexpected physics—each serving as a check on the other. And in December, both teams reported the exact same thing: more pairs of photons with a combined energy of 750 gigaelectronvolts than expected. (An electronvolt is the amount of energy an electron has when accelerated across a potential difference of one volt; a gigaelectronvolt is a billion electron volts.) No particle or process in the Standard Model could explain the extra photons; they seemed to be a hint of truly new physics.
Something similar happened four years ago. After improvements allowed the LHC to reach previously unachieved energies, the accelerator started up again, and ATLAS and CMS both saw extra photons summing to 125 gigaelectronvolts. Teams searched again a few months later to check their results—and continued to see photons at the exact same energy. They were definitely observing a brand-new particle. At the time, though, there remained a single unconfirmed piece of the Standard Model: ATLAS and CMS had found the Higgs boson, the final piece of the puzzle.
The photons at 750 gigaelectronvolts wouldn’t be completing any puzzle. They’d be extra pieces to the one already completed by the Higgs. “This was extremely intriguing,” Giudice says, “because it could not be explained by the Standard Model. This was absolutely clear.” The Standard Model does predict some photons summing to that energy, but nowhere near as many as ATLAS and CMS observed last year. Physicists flooded journals with papers, inventing potential new physics on the fly or adapting existing frameworks to account for the extra photons. Meanwhile, ATLAS and CMS continued to search for more data, just like they had for the Higgs.
Alas, there seems to be nothing new under the Sun. If you flip a coin enough times, you’re going to get a run of twenty “heads” in a row; if you crash enough photons, sometimes you’ll get a collection of 750 gigaelectronvolt photon pairs. The run of “heads” doesn’t necessarily mean that the coin is rigged, and the extra photons don’t necessarily mean the Standard Model is broken. There were just more of certain kinds of wreckage than are usually seen. The photons, in other words, were just a blip of particularly interesting noise. “This is pretty unfortunate,” theoretical physicist Michele Redi writes in an email, “as it would have been the greatest discovery in several decades in our field.”
It weathered this test, but the Standard Model won’t last forever. Neutrinos already fly through minute holes in its edifice, and its ignorance of gravity, dark matter, and dark energy pulls those holes even wider. Someday, an experiment will make it come crashing down—exposing the deeper and more wonderful underpinnings of reality. In this field, theoretical physicist Qaisar Shafi says, “theorists are desperate for new discoveries.” But until one is made, physicists are stuck with the most effective predictive tool in human history.
Really, it’s not such a bad fate.
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