Imagine that a friend you trust tells you a rumor. It’s an unlikely story and they aren’t completely sure of themselves. But a few minutes later another friend cautiously tells you the same thing. The combination of two similar stories from two reputable witnesses makes you want to explore further. This is what happened on December 15, 2015 when the ATLAS and CMS experiments at CERN shared an initial analysis of their highest-energy run, with the same hint of something interesting in the data. It’s too early to claim a new particle, but the situation in particle physics makes this the biggest news in our recent history.
The Large Hadron Collider’s (LHC) energy-breaking run was launched with a vengeance in 2010, following an incident that damaged the machine in 2008 and a cautious year in 2009. The 2011-2012 dataset delivered the discovery of the Higgs boson. As data streamed in, particle physicists around the world clustered in conversations around espresso machines and would land on this sobering scenario: What if we find the Higgs boson and nothing else? In other words, what if we neatly categorize the particles predicted by the Standard Model, incorporating the Higgs mechanism to provide mass, but have no progress toward understanding the nature of dark matter, dark energy, quantum gravity, or any clues to explain the 96 percent of the matter/energy content universe that is not incorporated in the theory?
If physics is a valid framework for understanding the universe, there is something else out there for us to discover. The prediction of the Higgs, the ultimately successful decades-long campaign to discover it, and the ongoing partnership between experiment and theory to characterize it, gives us confidence in the methods we are using. But the missing pieces could be beyond our imaginations, the current state of our technology, or both.
The terrifying possibility floating through these “Higgs and nothing else” conversations is that we might reach the end of exploration at the energy frontier. Without better clues of our undiscovered physics, we might not have sufficient motivation to build a higher energy machine. Even if we convince ourselves, could we convince the world and marshal the necessary resources to break the energy frontier again and continue to probe nature under the extreme conditions that teach us about nature’s building blocks?
In 2015, the LHC broke another energy barrier for hadron colliders with a jump from the 8 TeV center of mass collisions that delivered the Higgs discovery to 13 TeV center of mass. The ATLAS and CMS collaborations worked 24/7 to analyze the data in time for a presentation at an end of year event on December 15. Both experiments cautiously reported the hint of a new particle with the same signature in the same place. Two photons caught in high-energy collisions can be arranged together to form a mass, as if they originated from the same single particle. Using this technique, both experiments saw a slight clustering of masses—more than what was expected from the Standard Model alone—near 750 GeV/c2. The experiments are cautious for a good reason. These hints, in the same place for ATLAS and CMS, could disappear with the gathering of data in 2016. If the situation were less dire, this would not be big news. But for me it represents all of the promise of our current energy-frontier physics program.
Over the next few years we will gather ten times the data at 13 TeV than we currently have in hand, and we have theoretical motivation to expect something right around the corner. If the 750 GeV hint disappears with increased statistics, we will keep searching for the next hint that could break open our understanding of nature. We move forward with the next step potentially within our reach, determined to find it, if it’s there.