Just because the Higgs has been discovered does not mean there is any rest the the physcists at the LHC. They are now a few days out of the technical stop and have already returned to running with maximum bunch numbers (at slightly reduced intensity for now) and have added 0.33/fb, and they have done some special runs for TOTEM.
The CERN directorate would not feel that they are doing their job if they did not change the plan at least three times a year, so at the 4th July press conference the DG announced that they had decided to run the LHC for an extra three months and then shut down the collider for a longer period of two years. It will not restart at higher energy until 2015. There will be about 2 months extra proton physics this year which could add another 5/fb to 10/fb to the total delivered. The hope is that it will give enough data to study the properties of the Higgs and perhaps find something else so that the physicists have plenty of analysis to work on during the two year break while the LHC and the detectors are being upgraded. For more details see the ICHEP talk here
The Unofficial Higgs Combination Tool has now been updated with all the new Higgs plots released in the last few days, including the Tevatron updates and the new 8 TeV data from the LHC. There will probably be more to add on 7th and 9th July from ICHEP. Feel free to play around with it.
At the CERN press conference yesterday the Director General Dr Rolf Heuer warned journalists about unofficial combinations. What he said exactly was at follows (It is 26:50 in if you are looking on the recording):
“The fact that they [CMS and ATLAS] have not yet combined their results today is that they did not have enough time. We should have shifted the Melbourne conference by 2 weeks or 3 weeks or 4 weeks but that was not possible. You have to stay tuned until at some time they combine their results. Whatever combination you get beforehand is unauthorised and is certainly not valid because you have to take into account the different correlations, one has to be very careful.”
I agree with what he says. The unofficial combinations you find on this blog are approximate and unofficial and should be used with caution. I have always made that clear. It is not just the correlations that are neglected. The quick combination method assumes that the statistical errors have a flat normal distribution and that is not quite correct. The detector collaborations don’t provide detailed likelihood data to outsiders so this is the best I can do. Luckily all statistical errors tend towards the normal Gaussian as the quantity of data increases (central limit theorem) and in most cases there is enough data for the results to be good, with a few exceptions.
Whether the combinations are “valid” or not depends on what you are using them for. I don’t consider them valid for writing up published results of any kind, but they are good enough as a rough guide to theorists looking for possible signals in the data and there is nothing wrong with showing them at conferences as some eminent theorists have already done, provided they come with appropriate caveats.
I have previously shown some comparisons between official combinations and my unofficial ones to show how accurate they can be (or not). I think it is worth doing a few more now using some of the recent results where the amount of data has increased. In all the plots below the red line is the official result and the black is the unofficial. First up is the latest version of the Tevatron combination compared with an unofficial combination of the updated Dzero and the latest CDF plot that was updated in March. You can click on the plots to get a larger version.
The combinations across all channels have always worked quite well because they use lots of data. The last time that the LHC provided an official combination for ATLAS + CMS was in November when there was only 2.3/fb. here is how it looked next to the unofficial combination that I had done 10 weeks earlier.
Notice here how the accuracy gets worse at higher energies where there is less data available. Heuer seemed to be implying that there should be another combination due out soon. If so it will be interesting to see if the comparison improves as I would expect.
The combinations for single channels have been less successful in the past, but now they are improving. Here is a reconstruction of the ATLAS combination for 7 TeV + 8 TeV data in the diphoton channel
But the results don’t always come out so well even now. The 4 lepton channel uses very few events in both the signal and the background. Here is the result of a similar combination (Update: There was an error in the digitisation that I now fixed and it is not so bad now)
The combination across ATLAS and CMS should be better because it involves twice as much data. They should also have twice as much again by the end of the year so by then combination should work OK even in this channel.
If you want to try more the Higgs combination tool is easy to use and free.
Update: I said that I dont think these combination methods should be used in published papers but other theorists are apparently not as reticent. arXiv:1207.1347 is one example of paper showing a combined signal plot as well as combined channel values and other fits. There conclusion is that everything fits the standard model except that the diphoton rate is 2.5 sigma too high, in agreement with my figure.
CERN have happily announced the arrival of a new Boson but so far are being a bit cagey about what to call it. Is it the Higgs? Their caution as experimenters is perfectly laudable. They should show that they are keeping an open mind, but theorists are independent of the process of discovery and do not need to be so reticent.
The facts are that the boson discovered with a mass of about 125 GeV or 126 GeV interacts with a wide range of particles in exactly the way the Higgs boson should. Its decay modes to Z, W, b and tau have just the right ratios and its production has also been tested in different ways confirming indirectly that its coupling to the top quark is also about right. Its spin could be 0 or 2 but 0 is much more likely. All these features point to the standard model Higgs boson.
The only fly in the ointment is its decay rate to two photons. This is nearly twice as large as expected. The significance of the discrepancy with the standard model is about 2.5 sigma. It could be a fluke. We have learnt to show some healthy skepticism when it comes to observations of physics beyond the standard model. However it is also consistent with an enhancement due to the presence of another charged boson. If that boson exists it must have a mass at least a bit larger than the W otherwise the Higgs would decay to this particle in pairs and we would see the effect on the other decay rates. It can’t be too massive otherwise it would not enhance the diphoton rate enough. But it is likely to be possible to find a range of masses and properties that is consistent with all the observations.
So it is not necessary to invoke any properties for the observed boson that are any different from the standard model. Separate new physics will suffice. So the observed boson passes several tests required by the Higgs and I think that it is reasonable to assume that is indeed the Higgs boson until some observation suggests otherwise. It will always be possible to think of other models that could fit the facts, but they are not likely to be quite as economical as the standard model. It would be a disservice to the theorists who provided the theory 50 years ago if we continue to refuse to acknowledge the clear nature of this discovery when there is no evidence to the contrary. They predicted it would be just like this and It is the Higgs boson. Congratulations to all the experimenters and theorists who made this dream come to life.
Update: Here is the global Higgs combination. Is that conclusive enough now?
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.
Now that the results from the Higgs searches really have been seen by the teams of physicists from CMS and ATLAS we can expect a few rumours that might be based on fact. They start with this leaked video with CMS spokesman Joe Incandela talking about what they have seen. There is a strong signal from the diphotons and something weaker in four-leptons. As predicted here he is being careful with his wording to say that they have discovered a new particle consistent with the Higgs boson but further observations are required to know more details. There is no talk of discovery here but he seems to believe that the results might indicate that new particles are within reach of the LHC.
Video was pulled just after I viewed it.
Tomorrow CERN will announce an important update on their search for the Higgs Boson at the LHC (in case you have just got back from another planet) Expectations are high with some news reports saying they will reach the critical 5 sigma discovery level. Peter Higgs himself has flown over to CERN where he has joined other physicists responsible for the theoretical breakthrough back in 1964 that predicted the Higgs Boson. Sources from within CERN point out that final results will not be ready until today so all prior reports of a discovery can be no more than speculation, we will se tomorrow.
What we do know is that the two experiments that have been searching for the Higgs, CMS and ATLAS will each provide updates including data collected this year at 8 TeV. This could be combined with the results from last years 7 TeV run that were published at the last big update in December. We know that they will not attempt to combine the CMS and ATLAS data together because they have stated that they are now aiming for independent discoveries from the two detectors. Whether they can reach that important 5-sigma level will depend on how much data they can prepare in time. The amount of data available is 6/fb to be compared with the 5/fb from last year. The higher energy gives another 15% advantage in the crucial diphoton channel where the Higgs is seen most clearly.
If they can get that data together it adds up to a signal about 55% stronger than last year when they each had about 3 sigma in the diphoton channel, so this year we might expect at least 4.5 sigma, but it is not that simple. Last years signal was stringer than expected. If that was a statistical fluke for both experiments then it should be weaker this year bringing expectations down to more like 4 sigma. On the other hand if the enhancement was due to real physics it will still be there and they may even be lucky with the random quantum fluctuations and get nearer 5 sigma.
There is one last thing they can do to improve the signal. They can combine information from another channel such as the decay of the Higgs to 4 leptons. Last year this did not provide much help and only added about 0.2 sigma, but with higher energies it may just be a little better, perhaps enough to take them over the finish line. In the end it will be the luck of the fluctuations that counts. They have two experiments so two tosses of te coins. One may make it while the other falls short.
Of course the next set of data due out in September will certainly finish the game for both of them but nobody wants to wait for that after all the build-up. If neither experiment makes it individually they will be close enough to say that the combination of the two certainly adds up to an unofficial discovery, even if they do not do that combination immediately, Which ever way you look at it they will be able to spin the conclusion to provide the media with the result they are waiting for.
And if they don’t? Here is a ray of hope from an AP report
“Scientists with access to the new CERN data say it shows with a high degree of certainty that the Higgs boson may already have been glimpsed, and that by unofficially combining the separate results from ATLAS and CMS it can be argued that a discovery is near. Ellis says at least one physicist-blogger has done just that in a credible way.”
That physicist-blogger is of course yours truly. As usual I will be carrying out the full cube of combinations using my unofficial methods as soon as the plots are available. The viXra combination applet has already been updated with yesterday’s new data from the Tevatron. The results will be a little approximate and certainly not endorsed by CERN, but unofficial discovery is at least guaranteed or your money back.