A year ago I started to get fired up about the prospects for the Higgs boson discovery as it become clear that the Large Hadron Collider was performing so well that they would either find it, or prove that it does not exist, at least not in the form most expected. We had three major progress updates from the LHC last year with the amount of data being analysed doubling each time bringing better and better signs that a signal was emerging from the noise. At first the heavier ranges for its mass were ruled out. Then, in December the last major announcement left many theorists such as myself cautiously optimistic that the Higgs boson has finally been glimpsed in its last refuge at a mass of about 125 GeV. Officially the physicists who speak for the experiments have remained cautious but now they have enough data to settle the matter conclusively. This years initial runs of the proton accelerator have already delivered as much collision data as it produced last year, and CERN has announced another meeting to update the figures once again.
Rumours have spread that the new data contains the same signal seen before by both the large detectors CMS and ATLAS that have been searching for the Higgs boson at the collision points of the Large Hadron Collider. If this is true then it is just possible that either or both of the teams that run the detectors will be able to tell us that they have seen a signal with the 5 sigma significance required to claim a discovery. If they don’t reach that goal individually, the combination of the two almost certainly will.
As I write the auditorium at CERN is letting in the physicists who have been queueing all night for their place. Several will be live blogging from there but I will be reporting from home using the live webcast.
Is 5-sigma necessary for a discovery?
We have been assuming that a discovery announcement would require a level of significance of 5 sigma equivalent 30 a one in 3 million chance of the signal happening as just a background fluctuation is there is really nothing there. This morning some of the live bloggers are playing down this requirement which suggests that they might not reach 5 sigma but that the overall levels of significance could be considered sufficient. We will see what they actually say shortly.
08:55 Higgs applauded as he takes his seat
09:00 DG opens the meeting
Incandela, CMS spokesman starts with pile-up slide. Pile-up could be an excuse for any anomalies.
8:24 Far too much detail for time allowed
8:30 Amazing signal from combining 7 TeV + 8 TeV in diphoton channel for CMS
They have used 5.5/fb from 2012 data.
Here is the exclusion plot
4lepton also looks good. Combined significance is 5 sigma! = Discovery
WW looks OK too, only 8 TeV not combined with 7 TeV
Mass of Higgs is 125.3 += 0.6 GeV, combined significance 4.9 sigma
All channels consistent with SM but diphoton a little enhanced
8:53: Now starting the ATLAS presentation
Diphoton channel for ATLAS also showing a distinct signal. They get 4.5 sigma combining 2011 with 2012, used 5.9 sigma
Signal is nearly twice the standard model
Even in the 4-lepton channel the signal looks clear on the evnt plots
3.6 sigma in this channel
In combined channels ATLAS reach 5 sigma at 126.5 GeV = discovery!
Interesting that the mass value is still a little inconsistent with CMS.
Both experiments are showing exvess anove standard model in diphoton channel. This is even nore exciting than the discovery
DG says “I think we have it, do you agree?”
“We have a discovery, a particle consistent with the Higgs boson”
Now I have to combine those diphoton channels to see how significant the excess really is, BRB
11:47 This is what DG warned you against…
The combined diphoton plot gives a 6 sigma signal. It is 2.4 sigma stronger than the standard model.
This is what the signal plot lokks like. Rememner the grenn line is the standard model level, red line is background level
I will refine these when I have clearer plots to work from
The slides are now online.
13:44 I have been occupied with other things but will add some more combos later. There are lots of plots to digitise,
14:10 For those patiently waiting here is the unofficial combination for ZZ to four leptons. Significance is an impressive 4.6 sigma
The signal plot shows that in this channel it matches perfectly the standard model Higgs
For completeness here is the combination of the two low resolution channels across ATLAS+CMS. This one gives 7.4 sigma
Notice that we have now eliminated any possibility of a second boson nearby, unless they are too close to separate.