Warning for allergy sufferers, this post contains multiple sigmas 🙂
When ATLAS and CMS first published their results based on 2011 data in December, an unofficial combination of the results gave an excess with significance 3.74 sigma, a long way short of the 5 sigma needed to claim a discovery. Adding the Tevatron results available at that time only made it worse with a drop to 3.69 sigma. In February CMS added some extra diphoton events that pushed the LHC combination up to 4.3 sigma, then at the Moriond conference on March both CMS and ATLAS updated their combinations with the result that the significance dropped to 3.64 sigma. At the same meeting CDF and Dzero presented an update using the full dataset from the Tevatron. This time the combination with the Tevatron data improved the result pushing the significance back up to 4.25 sigma.
With all the data in use it looked like new data from the 2012 LHC run would be needed to reach discovery significance. Three days before the ICHEP conference the Tevatron collaborations presented updated combinations using some updated analysis from Dzero. This pushed the significance of the global combination up to 4.39 sigma. Then of course ATLAS and CMS added their 2012 data to reach 5.0 sigma individually with the combination reaching an impressive 7.45 sigma.
Later when the data was published as analysis notes more detail was given including data for the diphoton and 4 lepton data at 7 TeV. These had been updated yet again with ATLAS improving their analysis technique and CMS finding an extra 0.33/fb of 2011 data. Using these a new unofficial combination for the 2011 data can be generated and the result is dramatic. The LHC combination jumped from 3.67 sigma to 4.64 sigma while the global combination with all the 2011 data jumped from 4.4 sigma to 5.27 sigma. Even taking into account the error margins for the unofficial combination this means that the global combination has risen to discovery level significance level based on 2011 data alone, an impressive result.
So where did this increase of nearly 1 sigma from the 2011 data come from? Looking at the individual contributions, the CMS combination increased by 0.35 sigma and the ATLAS combination increased by 0.85 sigma. Of course all these results are approximate, unofficial and not endorsed by the experiments.
You can generate all the combinations here using the unofficial Higgs combination tool.