Hints of a new particle seen by the ATLAS detector have disappeared.


Hints of exotic new particle fade at world’s largest atom smasher

CHICAGO, ILLINOIS—Particle physicists’ hopes have been dashed. Eight months ago, experimenters working with the world’s biggest atom smasher—the Large Hadron Collider (LHC) at the European particle physics laboratory, CERN, in Switzerland—reported hints of the first wholly unexpected new particle in decades, one that could have required a rethink of the prevailing theory of fundamental particles and forces. Now, however, physicists at the LHC have collected and analyzed roughly four times as many data as they had last December. And with those extra data, the signs of the new particle have faded away as mere statistical fluctuations, researchers reported here today at a conference.

“I’m not surprised,” says Beate Heinemann, an experimenter at the German Electron Synchrotron laboratory in Hamburg who works on the A Toroidal LHC Apparatus (ATLAS), one of four enormous particle detectors fed by the 27-kilometer-long LHC. “I always thought that the chances were less than 5% that it would hold up.”

A new particle would thrill physicists. For nearly 40 years the prevailing theory of the fundamental forces and building blocks of nature, the standard model, has been incredibly successful, but ultimately lacking. It doesn’t account for gravity, for example. Yet, the theory explains everything seen at atom smashers—including the famed Higgs boson, the last particle in the standard model roster, which was discovered at the LHC in 2012. Stymied by their own success, most particle physicists hope to see some new and unpredicted particle that might point them to a deeper theory.

The hints last December pointed to just such an unpredicted particle. It appeared to decay into a pair of high-energy photons, which is one way the Higgs boson decays. However, the putative new particle appeared to be much heavier, weighing 800 times as much as a proton, whereas the Higgs weighs about 133 times as much, ATLAS experimenters said. Most tantalizing, researchers working with the Compact Muon Solenoid (CMS), another of the detectors fed by the LHC, seemed to see corresponding, albeit weaker, evidence of the particle. The results caused a stir because when the Higgs was discovered, it began to emerge in much the same way, showing up first as a small excess of particles decaying to two photons seen by both ATLAS and the CMS.

There were reasons to doubt that there was a new particle there, however. For example, physicists saw no evidence of the particle decaying into the other combinations of standard model particles that the Higgs decays into, so it could not simply be a heavier version of the Higgs; instead, it would require a much more complicated explanation. Moreover, the LHC ran from 2010 until early 2013 before shutting down for 2 years for repairs to design flaws that had limited its energy, and physicists had seen no evidence of the particle during that first run. Still, the possibility of something new tantalized theorists, who whipped up nearly 400 possible explanations of the new particle on the online preprint server.

Unfortunately for them, the particle just isn’t there in the new data, CMS and ATLAS researchers reported today at the biennial International Conference on High Energy Physics. That suggests that the twinned photon signals were just statistical flukes—produced by pairs of random, unrelated photons in their detectors. Natural statistical variation demands that from time to time, pure chance will lead to this. “It was a small effect, and you’re going to get those from time to time,” says Kenneth Bloom, an experimenter at the University of Nebraska, Lincoln, who works on the CMS. Bloom says the fact that the particle was never seen decaying into other combinations of particles already suggested that “there was never anything there.”

The null results also set up one of the more humorous situations at the meeting. Immediately following the talks in which experimenters said the purported particle didn’t exist, five different theorists took turns explaining what the particle might be.

Still, Bloom says, the whole episode will be useful. Researchers are now collecting in a week as many data as they collected all of last year, and they are hopeful that there will be something new in that torrent. “The next time we see something unexpected,” Bloom says, “if it holds up we’ll be even better at explaining what it is.”

*Correction, 5 August, 2:27 p.m: This story has been updated to reflect the correct rate at which the LHC is now collecting data.