ATLAS and CMS have updated their Higgs publications based on last years data. These results were released rapidly at the CERN council meeting in December but since then they have had time to polish the reports and as a bonus CMS have added some new data into the diphoton channels. This has already been covered nicely on the other blogs including QDS, NEW, TRF, OPS and the best report from Resonaances which gives a nice account of where the extra events come from.
“CMS added a new category of events which, apart from 2 photons, contain 2 energetic jets in the forward (closer to the beam pipe) region of the detector. Such events could arise in the so-called vector boson fusion (VBF) process, where each of the 2 colliding quarks emits a W or Z boson which coalesce to create a Higgs boson.” Jester
The outcome of this is a stronger signal at 124 GeV making it look very similar in strength to the one in ATLAS ar 126 GeV. Here is a plot showing the new CLs for diphoton from CMS with the December version in red for comparison
As you can see, the peak at 124 GeV is stronger while the other bumps have gone down. I have plotted this on a log CLs scale rather than the linear scale that CMS use because you need a log scale to read the significance properly. You can see that it has now gone up to 2.5 sigma for this channel alone. For comparison here is the same plot but with the results from ATLAS for comparison (which has not changed since December).
If ATLAS looks better don’t forget that its expected value (the dotted blue line) is also higher so in fact the statistical significance is now about the same for both.
One mystery is why the peaks are about 2 GeV apart. This could simply be a statistical deviation or it may be a sign that they still have some work to do on calibrating the calorimeters which measure the energies of the photons. The two experiments have different systems for detecting these photons. ATLAS use a tank of liquid argon, a clear liquid that looks like this.
CMS on the other had uses some Lead Tungstate crystals which look like glass but are as heavy as lead. See this old article from Symmetry Breaking for the remarkable story behind them.
When the CMS detector is finally laid to rest in about 40 years time they will be able to take the crystals out and make a great chandelier out of them as a memorial.
Another mystery about the diphoton results is that the strength of the signal is about twice as strong as expected. This can be seen clearly in this signal plot.
The excess over the standard model Higgs signal is about 1.5 Sigma for each experiment. It could be a sign that the particle is not as standard as expected, but more probably this is just a fluctuation that will go away with more data. It does mean that we should be cautious about how much we should expect the signal to improve if we double the data. It wont be as much as you might think and in fact the signal could get worse. This is why you should not expect conclusive results until all this years data is in.
The unofficial Higgs combinations now need updating and here is the new ATLAS+CMS combo for the diphoton channel with the December version in red for reference.
The significance is not as good as it would be if the two excesses coincided better in mass but it is now almost 3 sigma.
The CMS all channel combination has also been updated of course. Here is a replot to show how it has changed vs the December levels in red. there is no change at higher masses.
The significance on this plot has now just passed 3-sigma at the peak of the excess.
Which means that the unofficial Higgs combination needs to be updated so here it is
The peak significance here has now reached about 3.5 sigma.
I should remind you that this is an unofficial approximate combination that ignores correlations and un-normal probability distributions. The official version from the Higgs combination group should be due out soon. It will be similar but the differences are important.