viXra log

The latest Higgs updates are now being presented at Moriond. CMS have kicked off this morning with a presentation of bosonic decays including WW and ZZ but still not including the important diphoton channel. The full LHC run 1 dataset is now being used including 19.6/fb at 8 TeV

In ZZ they get a very clear signal on the event plot

Higgs Mass from ZZ is 125.8 +- 0.5(stat) +- 0.2(Syst)

The cross section relative to standard model is 0.91 +- 0.27

ATLAS also updated ZZ with 20.7/fb at 8 TeV to produce a similarly impressive plot

Higgs mass for ZZ from ATLAS is 124.3 +- 0.6(stat) +- 0.4(syst)

cross-section 1.7 +- 0.5

Unlike CMS, ATLAS have presented their diphoton results giving a mass estimate of 126.8 +- 0.2(stat) +- 0.7(stat)

diphoton cross-section is 1.65 +- 0.24(stat) +- 0.21(syst)

Rumour puts the CMS diphoton excess at 1.0 +- 0.2, to be shown at Moriond QCD next week perhaps (via Jester on twitter)

The excess over the standard model remains high but its significance has not increased because the value has gone down as more data has been added. When we first saw this excess a year ago we were excited that it may be real physics and we hoped that by this time we would have a truely significant effect. This has not happened. We still need to wait for CMS to show their diphoton results before we can draw any conclusion but rumours are that their overexcess has fallen even more dramatically. This means that expectations of significant BSM effects from run 1 are now lower.

CMS also gave us a plot of excesses in the WW channel over the standard model with Higgs at 125 GeV. In other words this plot should only show any excesses attributed to any other Higgs like particles. They said they are now doing this analysis for all the high mass searches which is a good move.

The WW cross-section from CMS is 0.76 +- 0.21

This shows that there are not yet any signs of higher mass Higgs particles as would be expected in Higgs multiplet models. If they exist then they must be quite well decoupled from the observed Higgs boson. The usual combined ZZ channel plot tells a similar story with no significant excesses beyond the known Higgs.

By the way, we are still waiting for the AMS-02 results due out soon. They had hoped to reveal them yesterday at Moriond but approval was not ready in time. Next oportunity could be the Moriond Cosmology conference next week

The CMS collaboration have kindly posted a pleasant video that reveals the moments when they “unblinded” their Higgs diphoton results within the collaboration in the run-up to the public discovery announcement in July.

I find it interesting to look back and see how these events relate to what was going on publicly on the blogs at the time. From the video we learn that the CMS collaboration were shown the first results on 15th June. The Higgs analysis group within CMS must have seen it at least a day or two before in order to prepare the plots for the talk. We can assume that ATLAS were seeing their results at about the same time. For two days the collaborations were able to walk around knowing that they knew stuff that the outside didn’t until Peter Woit blew the lid with his leak about the new results. This was a little upsetting for them at first as shown by the response from CMS blogger Michael Schmitt who later calmed down a bit. One argument they gave was that they did not want the information to pass between the ATLAS and CMS collaborations because it would spoil their independence but if the rumour can reach the outside world so quickly it is clear that it would not be kept secret at CERN and other institutions where 3000 excited physicists from each collaboration share the same social spaces.

The rumours were saying that the diphoton excess was at around 4 sigma and the video shows that it was indeed around 4.3 for CMS. In my own analysis the next day I estimated that this was based on about 3/fb of the data which turns out to be exactly right for CMS as seen in the video when the camera zooms in at 2:20. I also pointed out that when they add the full dataset the signal could easily go down and in fact it did descend to 4.1 sigma as seen in the next part of the video. I am not always right but I was this time. Subsequently the New York Times reported an email from the spokesperson for ATLAS saying that they should not believe the blogs. Now we know that this was a euphemism for “please don’t report what they are saying because it is perfectly accurate and we were hoping to keep it as a surprise for the next conference”.

Another point worth making here is that the collaborations like to make big statements about how they do their analysis blind. This is supposed to mean that they don’t look at the results until they have fixed the parameters of the analysis so that they cannot introduce any bias. From this video we can see what this really means in practice. They unblind the data as an early check then they “re-blind” it while they adjust the analysis. Then they unblind it again two weeks later with just 30% more data added. Come-on guys own up, this is not quite in the spirit of how blind analysis is meant to work. Luckily the signal is so clear that it is indisputable in any case.

Getting more up-to-date, remember that CMS have not yet published the diphoton update with 13/fb at 8 TeV. Rumours revealed that this was because the excess had diminished. At the Edinburgh Higgs symposium some more details about the situation were given. The talks are not online but Matt Strassler who was there has told us that the results have now been deemed correct. It may be understandable that when the results are not quite what they hope for they will scrutinize them more carefully, but I find it wrong that they do not then publish the results once checked. It was clear that they intended to publish these plots at HCP2012 in November and would have done so if they showed a bigger excess. By not releasing them now they are introducing a bias in what is publicly known and theorists are left to draw conclusions based on the ATLAS results only which still show an over-excess in the diphoton channel. It will all be history once the final results with about 20/fb are released soon but it would be helpful if they could keep this sort of biasing factor to a minimum.

The March meeting at Moriond is slated as the occasion for the final update but only of they are happy with the results. Their analysis has been used and refined many times in the last two years and by now they should be confident enough to say that they will publish regardless of the result they get. The data for the last proton run was available before Christmas so by now the collaborations should have completed their final analysis. The fact that we don’t have any rumours suggests that this time they have decided to confine knowledge of the results to the smaller Higgs groups within the collaborations and they may actually succeed in keeping them secret until the conference.

Update: See Tommaso Dorigo’s response here

This week marks the end of proton physics runs at the LHC. The last days are dedicated to machine development and in particular test runs at 25 ns. This shot shows the scrubbing runs during which they filled the collider to its full capacity for the first time. Record intensities of 270 trillion protons per beam were reached with 2748 bunches injected in 288 bunch trains with 25ns spacing. This doubles the intensity numbers used in the proton physics runs this year but it comes at a cost. In the pictures you can see how fast the beam intensity drops due to losses from the e-cloud effect. The purpose of the scrubbing runs this weekend was to clean out the e-cloud and improve beam lifetime. After nine runs the effect was significantly reduced but not fully removed. During the last few remaining days we may see some runs bringing 25ns beams into collision, but perhaps not at these intensities.

The point of these tests is to work out if and how the next runs can work at 25ns spacing rather than 50ns. That will happen when the LHC restarts at 13 TeV in 2015 after the long shutdown. We still have some heavy ion runs before the shutdown but otherwise it is going to be a long wait for new data.. During the LHCC meeting last week Steve Myres gave an overview of the main considerations for running at 25ns vs 50ns. You can watch the video from here. Myres revealed that other tests had shown that they can increase the brightness of the beams from the injectors by 50% using new optics. In addition to this the beta* in the next runs will come down to 0.5m or perhaps even 0.4m, so with all other things being equal luminosities could be three times as high. The problem is that pile-up with 50ns spacing is already near the limit of what the experiments can take. Switching to 25ns will half the pile-up making the situation much more tolerable. The other alternative would be to use luminosity levelling to artificially keep the luminosity down during the first part of any run.

This means the pressure to run at 25ns is high, it will make a big difference to the physics reach, but the technical issues get very troublesome. As well as the e-cloud problems which could mean losing maximum luminosity far too fast, they also have to worry about excess heating which has already been a problem with this years run forcing them to wait for things to cool down before refills. Another big worry is that UFO events become much more frequent at 25ns so even if they can maintain the luminosity they may keep losing the beams through unplanned dumps. Switching between 25ns and 50ns can lose a week of runs so they must decide which setting to use from the start of 2015 and try to stick to it. This makes the present 25ns tests very important. they had been planned for a few weeks ago to allow plenty of time but some injector problems set them back as explained by Myres in his talk. hopefully they will get all the data they need this week.

Meanwhile this week is also the occasion of the annual Cern Council Meetings. Remember that last year this was the event where they announced the first signs of an excess at 125 GeV in the Higgs searches. There are rumours coming in via twitter of new updates from CMS on Wednesday and ATLAS on Thursday (see calendar comments). There is nothing yet scheduled in indico that I can find apart from a status update on 13th (not physics) and the CCM open session on Thursday. We are still waiting for reports of the analysis using 12/fb at 8 TeV that were missing this year at the HCP meeting in Tokyo, especially the diphoton channel. In anticipation here is the latest CMS combo plot that has been around for a few weeks but which has not been much discussed.

The peak at 125 GeV is clear but what about the excesses that continue up to 200 GeV? No doubt these are due to systematic errors and fluctuations that will go away, bur any new updates will be keenly awaited, just in case.

The LHC has now delivered 23/fb to CMS and ATLAS at 8 TeV of which about 20/fb will be usable data. The complete analysis could be ready in time for Moriond in March with the diphoton over-excess being the most likely centre of attention.

Update: Indications are that the CMS and ATLAS updates were cancelled.

Update: Peter Woit thinks that ATLAS will give new diphoton and ZZ results at the LHC status meeting tomorrow. Meetings with this title usually indicate technical updates on the running of the collider and its experiments, not new physics results. It looks a lot like they are trying to spring a surprise by stealth A presentation later at KITP confirms that they are planning to talk. It still seems that CMS are not ready to give their diphoton update but they do have a status update.