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Ping! In this event, two W bosons collide and then decay into particles called muons (red) while the quarks that emitted the W’s produce sprays of other particles (yellow).

Ping! In this event, two W bosons collide and then decay into particles called muons (red) while the quarks that emitted the W’s produce sprays of other particles (yellow).

ATLAS Collaboration

Had there been no Higgs boson, this observation would have been the bomb

Ever wonder what particle physicists would have done had the Higgs boson not existed? Even before they fired up the atom smasher that 2 years ago blasted out the Higgs—the $5.5 billion Large Hadron Collider (LHC) at the European particle physics lab, CERN, near Geneva, Switzerland—researchers said that if they didn't find that coveted quarry, it wouldn't be a total disaster. If there were no Higgs, they said, then a particular ordinary particle interaction should instead go haywire and hint at whatever nature was doing to get by without the Higgs. Now, physicists at the LHC have spotted the rare interaction in that "no-lose" theorem, which is known as WW scattering.

"I am thrilled," says Barbara Jäger, a theorist at the University of Tübingen in Germany who was not involved in the work. Of course, now that physicists know the Higgs exists, they don't expect WW scattering to go bonkers. But it could still play an important role in the hunt for new physics, as scientists look for deviations from the predictions of the field’s prevailing standard model. That approach would complement studies of the Higgs itself, Jäger says.

The Higgs boson is key to physicists' explanation of how all elementary particles—such as electrons and the quarks that make up protons and neutrons—get their masses. Theorists assume that otherwise massless particles interact with a quantum field a bit like an electric field that consists of Higgs bosons lurking "virtually" in the vacuum. Those interactions give each type of particle a certain amount of energy and, thanks to Einstein's famous equation E = mc2, mass.

That may seem like a lot of trouble to go through to make a particle massive. But it heads off a big problem with particles called the W boson and the Z boson, which convey the weak nuclear force that's responsible for a kind of radioactive decay. Those particles weigh 86 and 97 times as much a proton, respectively. In the mathematics of the standard model, however, theorists can't simply insert masses for the W and Z, as that would spoil a key mathematical symmetry that explains how the weak force arises in the first place. By starting with massless particles that interact with a field, the Higgs mechanism preserves that symmetry while enduing the W and the Z with their masses.

That connection also explains the importance WW scattering had as an alternative to finding the Higgs. Suppose two protons collide. Rarely, a quark in one proton and a quark in the other will each radiate a W boson. Those W bosons can bounce, or scatter, off each other, either by crashing directly into each other or by exchanging some other quantum particle. Thanks to quantum weirdness, the process in which the two W’s exchange a Higgs counteracts the ones in which they bounce off each other or exchange a Z, much as two waves rippling on a pond can cancel each other. That interference keeps the rate of WW scattering low.

If the Higgs did not exist, however, then the rate of WW scattering should skyrocket above a certain collision energy. Indeed, the probability of such collisions should exceed 100%. "If there were no Higgs, we knew that something had to happen there because we know that probability doesn't get bigger than 100%," says Marc-André Pleier, a physicist at Brookhaven National Laboratory in Upton, New York, and one of 3000 experimenters working with ATLAS, one of four huge detectors fed by the LHC. So physicists predicted that in addition to a rate increase in WW scattering, new effects should kick in: for example, novel and revealing correlations in the trajectories of the W’s.

Now, Pleier and colleagues have spotted evidence of WW scattering, as they describe in a paper in press at Physical Review Letters. Out of 1.5 quadrillion proton-proton collisions, ATLAS researchers spotted 34 instances of WW scattering. The signal isn't yet strong enough to claim a definite discovery, Pleier says, but it appears to be consistent with the standard model. Physicists with the rival CMS detector also have evidence for WW scattering, Pleier says.

WW scattering could be used to probe for new particles, such as those predicted by a scheme called supersymmetry. W’s might bounce off each other by exchanging such particles, changing scattering from the standard model predictions. But different ways of extending the standard model predict different changes in WW scattering, notes Jürgen Reuter, a theorist at the German Electron Synchrotron (DESY) laboratory in Hamburg. So physicists must first determine the most promising signals to search for, he says: "We don't know all the good observables at the moment."