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Decades-Old Collision Data Offers Tantalizing Hint of New Particle

The Large Electron-Positron collider returns to haunt the LHC.

by Michael Byrne
Oct 22 2016, 2:00pm

Image: CERN

The Large Hadron Collider, now in its second operational phase, may be the most powerful particle collider ever built and it may smash together billions of protons per second like it's not even a thing, but the latest hint of a new particle comes courtesy of the predecessor to the LHC's predecessor. This is according to a paper posted Friday to the arXiv preprint server by a physicist named Arno Heister who had worked on the ALEPH experiment at the Large Electron-Positron collider (LEP), which formerly occupied the LHC's 27-kilometer tunnel.

The LEP collider was constructed in 1989 and later upgraded (in 1995) to the LEP II. It has several claims to fame, including further refinements to the masses of the W and Z bosons and a would-be hint of the Higgs boson. Surely the project's resulting data has been analyzed and analyzed again by now, but Heister offers an intriguing reconsideration—one that yields a small anomalous signal in the 30 GeV mass range. Is this data bump real, or is it just a statistical artifact?

Bumps like this are generally how new particles are found in collision experiments. Two particles are smashed together at extreme energies and the result is an energetic shower of particle byproducts; you might imagine pairs of wine glasses impacting dead-on at bullet speeds. Statistics describing these post-collision particle showers are collected over time and then analyzed for unknown quantities.

LEP was built to analyze the decays of one particular sort of particle: the Z boson. Z bosons mediate the weak force—the fundamental force that governs radioactive decay. As they're produced in collisions between electrons and positrons, the Z bosons themselves decay into other particles. So, what the LEP is ultimately observing is a decomposition of the weak force itself—an observation that it achieved at much higher energies than were ever before possible.

The bump in question:

Data from LEP and its four detectors (including ALEPH) was indeed analyzed and reanalyzed at the time, but Heister's new analysis is coming at things from a new perspective. Crucially, this is a perspective that allows for "hidden valley" models, which only began appearing in the years following LEP's shutdown in 2000.

The hidden valley idea came courtesy of two physicists, Matt Strassler and Kathryn Zure. In a blog post published Friday discussing Heister's results, Strassler explains: "Hidden Valleys are theories with a set of new forces and low-mass particles, which, because they aren't affected by the known forces excepting gravity, interact very weakly with the known particles. They are also often called 'dark sectors' if they have something to do with dark matter."

It's only when hidden valleys are allowed that Heister's bump appears, which is why it shows up only now. It should be cautioned that the statistical significance for the bump is small. Many a would-be discovery has been nullified by additional data and this is no exception. Still, given the current sense of bleakness among many physicists in the absence of New Physics at the LHC, it's a welcome hint.

"Though we should be skeptical that today's paper on ALEPH data is the first step toward a major discovery," Strassler concludes, "at minimum it is important for what it indirectly confirms: that searches at the LHC are far from complete, and that discoveries might lie hidden, for example in rare Z decays (and in rare decays of other particles, such as the top quark.) Neither ATLAS, CMS nor LHCb have ever done a search for rare but spectacular Z particle decays, but they certainly could, as they recently did for the Higgs particle; and if Heister's excess turns out to be a real signal, they will be seen to have missed a huge opportunity."