When the Lepton-Photon conference started 10 days ago there was a report in the Guardian that the signal for the Higgs boson reported at the earlier Europhysics conference had “faded”. They even put figures on it saying that the excess observed by ATLAS had decreased from 2.8 to less than two and in CMS from 2.8 to 2.3. The message was echoed in other papers who picked up the story and was also reported at the conference by the collaborations themselves with CMS saying that “Excess in the low mass range seems to persist but with reduced significance.”
The cause of this change was said to be the addition of some new data into the analysis, but I think this has to be looked at in more detail, so I have been doing some more combinations of the LHC data and am now working from the individual decay channel plots. The first clue that the story is not quite as straightforward as it seems comes when you look at what was said by CMS about the WW decay channels at the conferences. here first is the slide from Europhysics.
You may need to click to see full-sized. This slide shows that there are two distinct analysis methods available, “Cut based” and “MVA based”. The MVA gives a much better result as shown by the lower expected CLs line. In fact it is about as good as twice as much data. You will also notice that the excess from the MVA analysis was bigger which is what you would expect if the signal is real. Indeed the MVA analysis was the one used in the final CMS combination for Europhysics.
Now look at what they said at Lepton-Photon.
This shows just the Cut-based analysis with a note that the MVA-based result is coming soon! They have used 1.5/fb compared with 1.09/fb at EPS but remember that the MVA method is as good as twice as much data, so in fact the data used for WW at EPS was better and they took a backward step. The WW channel dominates the plot over the crucial range where the biggest excess was observed. You can even see directly that the expected CLs line went up higher in the LP plot compared to EPS, so really they took a step backward. A fading excess is therefore exactly what we should expect.
To see how much the excess actually changed we can reconstruct the EPS plot using the cut-based data and do the same thing for LP data. This is what we get
What we find is that there has been a small decrease in the excess in places, but not by much.
The natural thing to do next is to construct the plot using the MVA based data from EPS for the WW channel with the other channel data from LP. Since the EPS data was better for WW we should get the best possible results this way.
This brings back the broad excess previously seen with almost 3 sigma significance at 140 GeV. So what we can now say is that the observed decrease in the excess for CMS was mostly due to a change in the analysis rather than a statistical fluctuation as implied.
What about the ATLAS data? They were reported to have an even bigger decrease in the excess from 2.8 to less than 2 sigma. Here are my plots reconstructed from individual channel data.
There is some decrease in the excess but not as much as advertised. In fact the signal does not appear to have been as strong as originally claimed in the first place. Of course my combinations may not be as accurate as the official ones, but at least I can be sure the analysis has not changed, just the data.
Conclusion: The CMS excess did not fade at all, the difference was due to a change in the analysis from Cut-based to MVA-based for the dominant WW channel. The ATLAS combinations when reconstructed consistently only show a small decrease in the excesses. Not the large decrease advertised. Higgs boson hints are still alive.