Post-Higgs LHC Update

July 7, 2012

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

Are unofficial Higgs Combinations “Valid” ?

July 5, 2012

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.

What’s the deal with H → WW ?

June 29, 2012

In a few days we will get the next big update from CERN on the Higgs boson and it is likely that the main question they are investigating will switch from “Is there a Higgs Boson?” to “Is it the standard model Higgs Boson?” Already the 2011 data shown during the winter carried signs that the cross-sections for some decay signatures are quite different from the standard model predictions. In particular the digamma rate is high and the WW is very low. Significance levels were not strong but if this is reinforced by the 2012 data people are going to suspect that new beyond-standard-model physics is at play. Many theory papers will be written as I predicted a year ago, but how well can the numbers be relied on? The ATLAS and CMS discuss many of the details behind closed doors and do not publish every detail. If theorists want to be sure that the results are good they will have to ask some probing questions at the talks. They need to go along to the conference prepared.

So let’s look at the data so far. Using the unofficial combinations for CDF, D0, CMS and ATLAS the μ signal at 126 GeV for the accessible decay channels looks like this

In this diagram the green line represents the prediction for a standard model Higgs while the red line is the background level with no Higgs. The first observation is that the Higgs is clearly favoured across the channels. After that, much has been said about the digamma excess because it is a high-resolution channel and an enhancement of this branching ratio could indicate new physics such as a new heavy charged boson. But what about the WW channel? It is now quite a bit below the standard model and is more consistent with no branching to WW. Even taking into account that these combinations are unofficial and approximate there is clearly something odd here.

From a theoretical point of view this is very perplexing because the digamma decay is (in theory) dominated by loops involving the W boson. If the direct decay to WW is lower than predicted then the diphoton decay should be too. This really cannot be made sense of and if it is correct then the Higgs nature of the observed resonance would have to be questioned, but let’s not be too hasty.

There are several sources of error that can affect these results so let’s take a step back and think about those first. They can be broken down along these lines

  1. Statistical errors from the limited amount of experimental data
  2. Theoretical errors in the approximate calculations of the standard model production rates and branching ratios
  3. Errors from the measured standard model parameters such as the masses of the W,Z, top etc.
  4. Statistical and other errors from the monte carlo simulations used to predict the background and signal
  5. Measurement errors from the detectors

All these things should have been taken into account and included in the error bars but before we draw too many conclusions and new theories we should ask questions, especially since the results do not make good theoretical sense. I think it is instructive to look at how the WW channel plot has evolved in ATLAS and CMS from the early days when they had 1/fb to the full 5/fb from last year. I’m not going to copy all the plots here but you can look at them on the Higgs combination plot. When there was only 1/fb of data we got excited because of an excess in the WW channel. It was most significant at about 144 GeV with over 3-sigmas but it was a broad excess which at the time suggested a Higgs in the range 135 ± 10 GeV, so with hindsight it was consistent with the present signal at 125 GeV. Sadly this signal faded as more data came in even though it was present in both CMS and ATLAS and is now nearly completely gone. What happened? Let’s walk through some of te possibilities.

1. Statistical fluctuations – On the face of it this seems like the most likely explanation. The original excess after just 1/fb faded slowly in both CMS and ATLAS. This then was a remarkable fluke but given enoigh things to look at we will always find remarkable flukes somewhere, so perhaps this is it. The present low signal for the Higgs in WW at 125 GeV where digamma is strong could equally well be part of that fluke. The 2012 data will tell us whether it is or not but the WW analysis is harder and we may not get full results until after ICHEP

2. and 3. Theoretical errors – The calculation of production rates is thought to be good to about 15%, but some theorists sat less and some more. The branching ratios are known to about 5%. Background estimates are another source of theoretical errors. Putting it altogether we may expect errors as high as 25% and it is not clear that this much error has been included in the analysis. This could eat into some of the significance of the observed deviations from the standard model.

4. Errors from the monte carlo – We have to assume that the monte carlo simulations have been run long enough so that statistical errors are sufficiently small to be negligible, but what about other errors. As far as I understand it, ATLAS and CMS have detailed simulators of their detectors that include everything from pile-up to the efficiency of the parts in the detector. One thing that could be very relevant is the effect of the pile-up. WW at low Higgs masses decays to leptons and neutrinos so there is missing energy to be accounted for. Pile-up has been said to make this difficult because particles from one event contaminate another. The simulations must include not just the pile-up but also the triggers and the algorithms used to reconstruct the individual events. How well has this been done? The first inverse femtobarn of data had low pile-up numbers so if they have not understood the effects of pile-up correctly it could account for the fact that the signal faded as high pile-up data was added. I dont know if this is a plausible explanation but it is something the collaborations should be talking about and if they don’t say anything about it theorists should be asking them questions.

5. measurement errors –  From the 2011 data it was noticed that the CMS signal peak was at a lower mass than the ATLAS peak in the digamma channel. The difference was only about 1 or 2 GeV, well within the expected errors from the detectors, but this can still be significant. The WW channel has much lower mass resolution so how good is the estimate for the reconstructed Higgs mass? The reason that this is so important id that the WW branching ratio increases rapidly at around 125 GeV. If there are systematic errors that result in a mass offset they could be comparing experimental measurements with theoretical branching ratios and backgrounds at a slightly offset mass and this could result in big errors. For the digamma channel the problem is less acute because the branching ratio is a maximum at 125 GeV so it varies slowly in this region. the background also varies quite slowly.

Another part of the measurement process that could affect the result is the resolution of the detectors. How well is this resolution understood for different parts of the detector? This effects how much the signal is spread out over different energy bins. If the resolution is better than expected there would be more events in the central bin than expected and the signal would be bigger than expected. The opposite happens if the resolution is worse than expected. How well have they taken this into account?

The moral of this story is that if CMS and ATLAS do report significant deviations from the standard model next week, we as theorists should keep an element of skepticism in our interpretations. It ios easy to get excited by results that appear to agree with what we want to see, i.e. new physics rather than plain old standard model Higgs Boson. It will be impossible to resist speculating about what new physics can explain it and it will be a healthy excercise to do so, but don’t be surprised if more careful analysis sees some of the results fade away.

We have become addicted to the beguiling green and yellow brazil-band plots that have been produced in hundreds to show  where the Higgs does and does not show up, but as we move into the next stage of exploration at the electro-weak scale these need to be put to one side. What will count next is estimates for the mass of the Higgs and the actual cross sections for the different decay channels with error bars. The cross-sections need to be independent of the mass estimate so that we don’t get messed around by the ways these errors combine when branching ratios are varying rapidly, It may take a little longer before we can really be sure whether or not we are seeing the SM Higgs or a BSM Higgs. Reults from the LHC may improve as we head into the long-shutdown next year and we may need a linear collider to get really good measurements of the Higgs Boson properties. But meanwhile theorists imaginations may run wild.

Lots of Higgs webcasts coming up

June 28, 2012

We are used to looking at the slides from conferences and seminars around the world but we miss what the speaker actually says and the all important questions. So it is good to know that many of the talks to be given in the next few days about the Higgs boson will be webcast live from three continents. For your convenience (and mine) here is a short list with the appropriate links:

  • 2nd July : Not to be outdone by CERN, Fermilab will present its Higgs results 2 days earlier, but probably there is nothing new because they already gave us the full details at Moriond. If you are still interested the webcast is here
  • 4th July : CERN give their important update and possible discovery announcement followed by a press conference. This may just cover the most critical channels (e.g. diphoton and 4-lepton) Webcast from here.
  • 7th July: The Higgs results for each decay channel in fine detail will be presented at the ICHEP conference in the parallel sessions. Most of these will be updates with 2012 data. I am informed that one stream of talks will be webcast live from ICHEP for 5th-7th July and this will include the Higgs searches. Link to come later.
  • 9th July: The combined Higgs results from CERN for ATLAS and CMS (separately) will be presented in the plenary sessions at ICHEP. This will again be webcast.

While we wait there is an interesting audio interview with Peter Higgs at PhysicsWorld

Yet another LHC Update

June 19, 2012

The Large Hadron Collider has now entered a Machine-Development/Technical-Stop phase that will last eleven day. That means they have collected all they can for the ICHEP conference in July. The amount delivered is 6.6/fb, just a shade short of the projected figure of 6.8/fb and well over the 5/fb target,  so they have done very well. As usual it was not easy with many problems holding down run efficiency but in the last week they amassed an impressive figure of 1.3/fb to make up for some time lost at the beginning of this run. They have two more runs of similar length to the last so they are well on the way to reaching the 13.3/fb figure that was given as the amount needed to ensure that both experiments can independently discover or rule out the Higgs Boson. They should also have time to complete other priority tasks before the long shutdown next year, such as testing runs with 25ns that will be needed when they restart at 13 TeV.

Now it is over to the experiments to see what they can achieve in time for ICHEP which starts in just 15 days time. ATLAS has recorded 6.23/fb and CMS has 6.15/fb and they have already been beavering away with the first chunk of this data. There is some chance that they can get to the critical 5-sigma level for independent Higgs discoveries (For the (well-meaning) nit-pickers I mean discovery of a neutral boson resonance (probably) consistent with the standard model Higgs, sigh)  Contrary to what some blogs are saying it is no more difficult to combine the 7 TeV and 8 TeV data than it is to combine all the different channels, and whether or not they do this could be largely a political choice depending on how and where they want the discovery announcement to be made. However, time constraints could also be critical. It will be tempting for them to try to use as much of the 6/fb of data as possible for the Higgs search but that will require maximum computing resources. They may have to face a difficult choice between getting discoveries in the Higgs search in time for ICHEP or looking for other searches for more exotic physics. If it comes to that face-off my bet would be on them prioritising the Higgs search, but whatever the outcome it will be an enthralling conference.

I should also mention that LHCb have 0.65/fb added at 8 TeV so good progress there as well.

By the way, viXra Log has made it into the final of this years science blog contests at 3 Quarks Daily. It is up to Sean Carroll to pick the three winners. Now I wish I hadn’t said all those bad things about him, perhaps he didn’t notice, LOL 🙂 .

ICHEP Preparations

June 15, 2012

In just three weeks time ICHEP 2012 will be underway with the biggest expected news to come from Higgs Searches presented on 9th July in Melbourne. Meanwhile closer to home a much more low-key meeting at CERN has given us an update on the running of the LHC. As far as I can tell from the images of the auditorium the only people who attend these meetings in person are the speakers, but there are high quality webcasts so nobody else has to.

The usual LHC Machine Status Report by Steve Myers is worth watching if you are interested in where they stand with beam operations. Tonight they will have a celebration for collecting 5/fb although they have already passed the 6/fb mark. This merged slide shows progress as we approach the next technical stop compared to his prediction from Chamonix. It is good news that they are almost on target and will decide not to use the two months of extra running that had been set aside just in case. They will need the extra time to do the growing pile of tasks scheduled for the long shutdown which could easily extend to nearly two years long if they are not careful. If you are observant you will also notice that the main goal is illustrated with a picture I took for this blog last year but Myres will have to nick a lot more of my stuff if he is to get even for all the ones I took from him 🙂

Myers also reported a scary story about a situation that nearly led to LHC armagedon last week when they discovered in testing that the beam dump system relied on a power supply that formed a single point of failure. A simple fault could have led to a situation where the beams could not be dumped even when a failure signal would normally abort the run. The beams would have kept circulating like an unstoppable train with the only possible outcome being the loss of 120 MJ of beam energy around the accelerator ring with the potential to destroy almost anything and everything in the collider.

The progress talk from ATLAS is also interesting in that they revealed three new versions of Higgs plots using 2011 data with improved analysis methods. From this I deduce that at ICHEP they will update only the critical digamma and Z to 4l channels with 2012 data and will use these new versions of the 2011 data for some of the other channels. These will be combined to form the new ATLAS plot. Although the cut-off for collecting data for ICHEP has passed I think there is a good chance they will continue collecting up to the technical stop because the collection rate is now very high. This may give them close to 6/fb of data in the two high-resolution channels.

In the ICHEP abstracts CMS indicate that they will update all their channels with 2012 data. This difference between the two experiments is similar to what was presented at the December council meeting.  As far as I know there is no technical reason why the two experiments should not combine all the 2011 data and 2012 data in the high-resolution channels in time for ICHEP, but this would risk arriving at the unfair situation where one of the two experiments gets the discovery level significance first. I think there is a good chance that they will compare preliminary results in the next few days and if it is clear that they can both reach the critical 5 sigma level they may go for full combinations and announce the joint discovery. Whether they can do this depends on how much data they use, how well they deal with pile-up, how well they control the background, how lucky they are with the fluctuations and even how big the cross-section is if the Higgs is seeing BSM enhancements. My prediction is that they wont quite get there this time round, however an unofficial discovery using ATLAS+CMS is much more likely.

Apart from these Higgs results there will be a lot of other new results presented at ICHEP including a few exotic searches (such as heavy gauge bosons) using 2012 data from both ATLAS and CMS. I expect ATLAS to hold their 2012 supersymmetry searches back for SUSY 2012 in August but the abstracts from CMS indicate that a few new SUSY at 8 TeV will be shown at ICHEP. LHCb, Tevatron and many others from the accelerator labs, neutrino experiments and astronomical observatories have the potential to produce new discoveries in particle physics and ICHEP 2012 is the place to grab the headlines so we should expect the unexpected.

If you have not yet voted in the 3 Quarks Daily science blog awards viXra log would appreciate your support. Update: voting is complete and we appear to have made it through to the next round in about 9th place with 68 votes. Thanks.

LHC Update May

May 17, 2012

The Large Hadron Collider crawled out of a scheduled technical stop two weeks ago and passed through a rocky patch. There was a series of cryogenic failures that slowed the build up back to normal luminosity. They are currently running with the worst hit sectors around point 8 at a temperature of 2.0 Kelvin rather than the normal 1.9 K. This appears to have fixed the problem but as an uninformed outsider I can’t help wondering what extra risks this entails. Another issue was emittance blow-up from the SPS that was limiting peak luminosity to around 4.3/nb/s. This was fixed in the last couple of days and now luminosities have returned to the record levels set before the technical stop of around 5.7/nb/s with bunch intensities up to 138 billion protons per bunch. previous discrepancies between luminosity recorded by CMS and ATLAS have been resolved by data from the Van de Meer scans run just before the technical stop. The two experiments are now in perfect agreement and previous record numbers from CMS have been rescaled downwards. The present luminosity should be close to the maximum they can achieve this year unless they have kept back some tricks for later.

On the plus side, minimum turnaround times are well under two hours which is about half last years waiting time. Recovery from loss of cryogenics also looks much faster than before. This means that if they can avoid problems with cryogenics and RF they should be able to accumulate data at a high rate. As I write they are passing the 2/fb mark for this year’s total with a little under 5 weeks before the next technical stop. It should be a breeze to reach the stated 5/fb target in time for the summer conferences.

There are a few conferences coming up over the next three weeks that could be opportunities for the experiments to present some early results using the 2012 data at 8 TeV. In particular Recontres de Blois opens on 27th May and Physics at LHC begins in Vancouver on the 4th June. However we may need to wait for the big ICHEP conference in Melbourne where they should be able to add about 5/fb from this years data at 8 TeV to last years similar total at 7 TeV. This looks likely to be a watershed moment for the Higgs search with a likelihood of at least an unofficial discovery moment if the combined significance exceeds 5-sigma (it is currently around 4.2 sigma) There is even the possibility that one of the two experiments could pass the discovery threshold with the diphoton decay mode. It depends on how lucky they are with the stats. There is also the possibility that this years data will tell a different story from last year and we will be left waiting for the full year’s dataset to complete the story. Whichever way it goes the ICHEP conference is billed as a historic moment for the Higgs boson, and it is just seven weeks away.