A particle collision from the ALEPH experiment that started in 1989
Jean-Luc Caron /CERN
Could old colliders point to new discoveries? An energy excess spotted in data from a 20-year-old experiment suggests so. The bump hints at the existence of a previously undetected particle, which could change our understanding of physics. Then again, it could also be a statistical fluke.
The ALEPH particle detector at CERN near Geneva, Switzerland, switched on in July 1989. For the next 15 years, it analysed the decays of millions of Z particles created in the Large Electron-Positron (LEP) collider – located in the same tunnel that now houses the Large Hadron Collider.
Although scientists scrutinised those decays throughout the experiment’s lifetime, they tended to look only for signals that theorists had predicted.
In the years since, theorists have made many more predictions. So Arno Heister at CERN decided to take a second look at the old data. Without looking for anything in particular, he found a slight excess of decay products at an energy of about 30 gigaelectronvolts (GeV).
Just as the famous Higgs boson first showed itself with a tiny bump that swelled as more data was gathered, this could signify the existence of a new particle. But this particle, unlike the Higgs boson, was not predicted by the standard model of particle physics, and so could point toward a more complex theory of nature.
That would be a dream come true. Physicists are hungry for a model that can probe questions the standard model cannot, like why there is more matter than antimatter or what constitutes dark matter.
Proceed with caution
But Heister doesn’t jump to that conclusion. “I cannot tell you if it’s something really new, if it’s a statistical fluctuation, or if it’s something we didn’t understand in the standard model,” he says. “I have no clue what it could be.”
The issue is that the statistical significance of the bump is at most 3 sigma, meaning the odds that it’s due to chance are about 1 in 740. At that level, the bump could disappear fast. Earlier this year, for example, a similar signal had physicists all abuzz before it disappeared after several months of additional data. In order to claim a discovery, particle physicists require a 5-sigma result, meaning the chance that the signal is a fluke is about 1 in 3.5 million.
More evidence will either boost or destroy the signal. “The good news is that there are three other LEP experiments and three other LHC experiments that can all weigh in,” says Matthew Strassler from Harvard University. “So this should be settled very quickly.”
And even if the excess turns out to be a fluke, the paper highlights an important issue, Strassler says: major colliders like the LHC may have more secrets to reveal, even within experiments that are 20 years old. Strassler hopes that the findings will urge other scientists to further examine old data. Because without a particular theory in mind, you never know what might turn up.
Journal reference: ArXiv, DOI: 1610.06536v3