Scientists may have found a particle six times heavier than the Higgs bosonby Philip Ball / March 18, 2016 / Leave a comment
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From here on, it’s mostly guesswork and empiricism for particle physics. Informed guesswork, naturally—no field of science is better adapted to doing theory with minimal experimental data to guide it. But now that the Higgs boson has completed the Standard Model—the enumeration of all known subatomic particles and the interactions between them—what comes next is unknown. It will be largely a question to what experiments turn up. And none comes with higher expectations than those conducted at CERN’s Large Hadron Collider, which is currently undergoing test runs after an upgrade to take its particle collisions to even higher energies. The LHC will begin collecting data again in April.
Many particle physicists are now hoping to find evidence for the favourite candidate theory that takes physics beyond the Standard Model, called supersymmetry. This posits a fundamental relationship between the two currently distinct families of particle types, called bosons and fermions. If it holds true, then every current particle has a supersymmetric partner, all of them as yet unseen. The LHC will be looking for signs of these.
But already the collider has supplied a tantalizing hint of something that nobody predicted. Last December, CERN physicists reported that proton-smashing in both of the LHC’s principal detector systems, called ATLAS and CMS, had discovered an anomaly in the signals measured at energies higher than those needed to make the Higgs boson. The implication is that this is the signature of some extremely massive particle—a real monster as these things go, six times heavier than the ponderous Higgs boson—that decays into two gamma rays.
What could it be? It’s characteristic of this field that, rather than scratching their heads, researchers have already produced a flood of potential explanations: nearly 300 papers have been written and posted online so far. One might interpret that as an indication that the theories of high-energy physics are fluid and unconstrained enough to explain anything; a more positive spin would be that the field is open to, indeed hungry for, fertile speculation.
One idea is that this is a particle related to the Higgs itself, much as the humble electron has heavier cousins called the muon and tauon. Another, more exciting possibility is that this could be the particle generally assumed to be the “carrier” of the gravitational force, called the graviton. All the other fundamental forces have associated particles, and so the graviton is generally anticipated. But if the graviton has non-zero mass, the implications could be huge: an explanation for so-called dark energy, perhaps, or evidence for extra dimensions of space. In any event, if the new particle is real, it doesn’t look like there’s a place for it in the Standard Model and so we’ll finally get a glimpse of totally new physics.
Alternatively, of course, the blip in the LHC data could just be a glitch that will evaporate on closer inspection. The anomalies are outside the range of standard random error (especially for the ATLAS results), but only just. That they were seen in both detectors at the same energy is reason to take them seriously—but such things have been seen before and later proven to be artifacts. The very latest data from the two detectors, announced on 17th March, give a mixed message: confidence in the anomalous signal is a little higher for the CMS detector, but a little lower for ATLAS. CERN physicists are pretty confident that the data collected from the next set of runs will decide that question by the summer.
Right now, this is just what the doctor ordered: the unexpected. I was one of the curmudgeons slightly disappointed by the detection of the Higgs because it was essentially just what we anticipated and so pointed towards no new principles of physics. Surprise is the lifeblood of science, and we can only hope that this is just the first of many that the supercharged LHC will generate this year.
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