Physicists struggle to squeeze new particles from the LHC

Large Hadron Collider

It's a fairly silent night as the Large Hadron Collider shuts down for the holidays. Particle physicists at CERN today presented the first results since the LHC was switched back on for its second run, but had no discoveries to report.

Instead, early hints of new physics from the end of the LHC's first run seemed to drop away. But there is one glimmer of hope: a glimpse of a possible new particle.

With rumours of new particles flying around earlier in the week, physicists were crammed into the main lecture hall at CERN - the location of the announcement of the Higgs boson discovery in 2012. But this time there were no early Christmas presents.

Researchers from the CMS and ATLAS experiments presented their initial findings based on data taken since the LHC was switched back on in April this year.

Promising hints

First up was Jim Olsen of Princeton University, who is part of the CMS team. He started with a list of results confirming the standard model of particle physics - a good way to confirm the new run is working as expected. "The standard model at this early stage is still going strong," he said. They don't yet have enough data to "rediscover" the Higgs, but should do next year.

Next he addressed two promising hints of new discoveries seen in data taken before the LHC shut down for upgrades in February 2013. Both CMS and ATLAS had weak evidence for a boson with a mass of 2 teraelectronvolts (TeV), much larger than the 125 gigaelectronvolt (GeV) Higgs. But in the new run, CMS has seen no such signal. There isn't enough data to fully rule it out, said Olsen, but it looks possible that the previous signal was just a blip.

The same was true of another signal of a potential supersymmetric particle, nicknamed "the edge". Supersymmetry, also known as SUSY, is a theory that extends the standard model with a host of heavier partner particles. Finding any evidence for the theory is one of the LHC's main goals in this second run, but it looks like we don't have anything yet.

But there is a glimmer of hope for new physics. Olsen presented results showing an excess of events producing two photons at 760 GeV, which could point to a new particle (shown above). The results had a statistical significance of 2.6 sigma, close to the particle physicists' criteria of 3 sigma for "interesting" results, but still a long way from the 5 sigma necessary to claim a discovery.

Further statistical analysis taking into account the "look-elsewhere effect" - the chance that you are seeing a spurious result due to the large number of possible events you are searching for - reduced this to just 1.2 sigma, so it may be nothing.

Unlucky fluke?

Marumi Kado of the Linear Accelerator Laboratory in Orsay, France, spoke for the ATLAS team. He confirmed the robustness of the standard model and said they had started to see some signs of the Higgs, but with low statistical significance.

Like CMS, ATLAS has also been looking for SUSY particles, but there's just not enough data to draw any firm conclusions yet. "We made a host of searches for supersymmetry," said Kado. "No excess was found."

There was a bit more of a signal from "the edge" compared to what CMS saw, however, with a 3 sigma result. But this is still less than either detector had seen in the first run. "This is certainly something to watch in the near future," he said.

On the two photon front, ATLAS also saw an excess at around 750 GeV, tantalisingly close to the similar signal at CMS. Their significance was 3.6 sigma, dropping to 1.9 sigma with further "look-elsewhere" analysis.

The fact that both experiments are seeing a similar signal suggests there might be a new particle on the horizon in 2016, but it could still be an unlucky fluke. "We are eagerly waiting for much larger datasets in 2016," said Kado.

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