We were expecting it – but it’s only deepened the enigma. A slew of new results at the Large Hadron Collider at CERN near Geneva, Switzerland, have confirmed that the Higgs boson really does give all other fundamental particles mass.
The new measurements, by the ATLAS and CMS collaborations, have spotted particle processes in which the Higgs, discovered at CERN in 2012, is produced along with both a top quark and its antimatter equivalent, a top antiquark. It follows hot on the heels of the discovery by both collaborations of the Higgs boson decaying directly into tau leptons, heavier cousins of the electron.
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Interactions between the Higgs and the top quark are particularly significant, as the top is by far and away the most massive fundamental particle. Weighing in at 172 gigaelectronvolts, the top quark dwarfs even the Higgs’s own mass of 125 GeV. So whatever the Higgs does, it does it with the top more. “It drives why the universe is like it is,” says Fabio Cerutti of the ATLAS collaboration. “But it’s really puzzling, we don’t understand why the top mass is so heavy.”
The new results suggest there is exactly the same level of interaction between the Higgs and the top quark as predicted by the standard model, our best description of how particles and forces interact. The tau lepton results are similarly in line. Together they provide the first direct indication that interactions with the Higgs boson explain why fermions, the class of particles encompassing quarks and leptons that make up matter, have the masses they do.
Standard Model reigns
Understanding the Higgs mechanism better could help to explain the stability and structure of matter around us. Subtle differences in the masses of up and down quarks explain why neutrons within the atomic nucleus are heavier than protons, and so why stable nuclei and atoms can form.
“It means neutrons decay to protons but not the other way round,” says CERN theorist Gavin Salam. “So hydrogen and all the other nuclei exist.” Similarly the mass of the electron sets the size of atoms, and so determines how chemistry works.
The measured processes are incredibly rare, and both ATLAS and CMS had to analyse data sets built up over five years to achieve the necessary gold-standard statistical significance, equivalent to a chance of less than 1 in a million that the results came about by chance alone.
The top quark measurements particularly represent another bittersweet triumph for the standard model, says Jon Butterworth at University College London, a member of the ATLAS collaboration. “It is one of the benchmark processes we knew we had to see as part of the health check of the standard model,” he says. “And that health is disgustingly good.”
Disgusting, because we know that the standard model is not a complete theory of matter. It leaves open essential questions such as the make-up of dark matter, the stuff that cosmological measurements suggest makes up over two-thirds of all matter, and why matter apparently dominates antimatter in the material universe. “Every time we see something that agrees with the standard model, that means we don’t know the answers,” says Butterworth. “Any deviation would be a clue, and we want clues.”
No signs of supersymmetry
It also sheds no light on a crucial mystery within the standard model – why the Higgs mass itself is so low. Within the standard model, interactions with existing particles – above all the top quark – send the Higgs mass ballooning sky-high. In fact, the Higgs mass is about as low as it can be – so low that the universe it helps to construct seems to be only just on the borderline of stability.
One way to avoid this discrepancy is to propose the existence of a slew of new “supersymmetric” particles, one for each existing particle, whose interactions with the Higgs cancel out those of the known particles. Most of these particles are much more massive than their conventional partners, so evidence of them is difficult to come by. The exception is the top quark, whose supersymmetric partner would be expected to have around the same mass.
If this particle existed, you’d expect it to be having some ruffling effect on how top quarks are produced alongside Higgs bosons. Not one jot or tittle. It’s still too early to say on that basis that supersymmetry is on the way out, says Butterworth, “but every time we see something like this, it makes it less likely.”
There is a get-out clause: although these measurements have confirmed the existence of the effect beyond reasonable doubt, the rate of production of the Higgs together with two top quarks agrees with the standard model only to within around 20 per cent. That’s in line with what you’d expect given the as-yet small sample of events used to make the measurement.
The hope is that future, more precise measurements will see small deviations from standard model predictions indicating the existence of heavier particles or further unknown effects. “We are only seeing part of the puzzle,” says Cerutti. “And the part we see looks like the standard model.”
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Read more: The Higgs mass mystery: Why is everything so light?
