CMS diphoton result approved

March 13, 2013

Todays LHCC meeting is currently being webcast and slides are going online here. Colin Bernet presenting the CMS update has confirmed that the crucial diphoton results have now been approved. He said they will be presented at Moriond today but the schedule suggests that it will be tomorrow. This part of the Moriond meeting is not being webcast so we will have to wait for slides to come online for the results.


Looking Forward to Moriond

February 15, 2013

The LHC has ended its three-year long physics run this week and is now finishing off with some quench tests. Tomorrow (Saturday) morning the beams will be dumped for the last time for nearly two years while the collider is upgraded so that it can work at a higher energy of 12.5 – 13 TeV.


But the fun is not quite all over yet. In just over two weeks time we should start to see the first of the final results from proton physics run that ended in December 2012. The event to watch is the Electro-Weak section of the Moriond Meeting that opens on the 2nd March. The schedule has not yet been published but when it is you should see it at this link. The following week they will hold the Moriond QCD meeting whose schedule is now available at this link. There will be a quick summary of the Higgs searches which I presume will have already been revealed the week before.

What we expect to see is an update for all the Higgs decay channels from both ATLAS and CMS. Remember that we have seen all the results for 5/fb @ 7TeV + 13/fb @ 8TeV, except that CMS choose not to publish the diphoton result because it was smaller than expected. This means that the public values for the diphoton cross-section are currently subject to a selection bias that needs to be put right. The hope is that we will get full results at something like 5/fb @ 7TeV + 20/fb @ 8TeV, In other words we will have 40% more data for most channels and about 75% more for the diphoton channel. We know that all channels other than diphoton are perfectly in line with the standard model Higgs while the diphoton channel cross-section is a bit too large. However, we need to remove the CMS selection bias before we can get excited about it.

In addition to the cross-sections we can hope for an update to the tests of spin parity on the Higgs boson. This is the final step required before CERN will be happy to declare that the Higgs-Very-Like-Boson is indeed the Higgs-Boson so that Nobel prizes can be handed out. It is unlikely that the individual results from CMS and ATLAS will be quite sufficient. These tests do not need a 5-sigma significance because they are property measurements rather than discoveries. I think they will settle for 3 or 4 sigmas but this will require the combination of CMS and ATLAS data.

The ATLAS and CMS collaborations have had plenty of time to analyse their results and have kept them under wraps with no rumours leaking out yet. This may mean that they are keeping them “blind” until the last-minute. If that is the case it will probably mean that there is not time to do an ATLAS+CMS official combination for Moriond. Unofficial combinations of the channel cross-sections can be done quickly by hand but the spin-parity is more subtle so there will be one final Higgs cliff-hanger until the summer.

Normally the biggest HEP conference of the year is either the European EPS-HEP conference or the ICHEP conference. These normally alternate in a two-year cycle but this year an extra ICHEP conference in Switzerland has been laid on. (UPDATE: It turns out that this is not an official ICHEP conference. Same committee appears to be organising over 1000 conferences this year. Do not register, ) The EPS-HEP conference looks legit and will be in Stockholm. Please always check before paying conference fees.

Update: 16-Feb-2013


See Mike Lamont’s final “run 1” report in the CERN bulletin

String Theory returns to symmetry

July 31, 2012

The strings 2012 conference has finished and it is great to see that all the talks are online as slides and videos. Despite what you hear from some quarters, string theory is alive and progressing with many of the brightest young people in physics still wanting to do strings. Incredibly the next three strings conferences in Korea, US and India are already being organised. How many conference series have that many groups keen to organise them?

It has become a tradition for David Gross to give some kind of outlook talk at these conferences and this time he said there were three questions he would like to see answered in his lifetime

  • How do the forces of nature unify?
  • How did the universe begin and how will it end?
  • What is string theory?

The last of these questions is one he has been asking for quite a few years now. We know string theory only as a small set of perturbative formulations linked together by non-perturbative dualities. There has to be an underlying theory based on some unifying principle and it is important to find it if we are to understand how string theory works at the all-important Planck scale. This time Gross told us that he has heard of something that may answer the question. Firstly he now thinks the correct question to ask is “What are the underlying symmetries of string theory?” and he thinks that work on higher spin symmetries could lead to the answer. What is this about?

For about 16 years it has been known that an important element of quantum gravity is the holographic principle. This says that in order to avoid information loss is black holes, the amount of information in any volume of space must be bounded by the area of a surface that surrounds it in Planck units. This might mean that the theory in the bulk of spacetime is equivalent to a different theory on the boundary. How can that happen? How can it be that all the field variables in the volume of spacetime only carry an amount of information that can be contained on the surface. We can reason that measurement below the Planck length is not possible, but even then there should be at least a few valid field parameters for each plank volume of space. If the holographic principle is right there must be a huge amount of redundancy in this volumetric description of field theory. Redundancy can be taken to imply symmetry. Each degree of symmetry or dimension of the group Lie algebra tells us that one field variable is redundant and can be taken out by gauge fixing it. In gauge theories we get one set of redundant parameters for each point in spacetime but if the holographic principle is correct there must be a redundancy for almost every field variable in the bulk of spacetime and we will need it to be supersymmetry to deal with the fermions. I call this complete symmetry and I’ve no idea if anyone else appreciates its significance. It means that the fields of the theory are given by a single adjoint representation of the symmetry. This does not happen in normal gauge theories or in general relativity or even supergravity, but it does happen in Chern-Simons theory in 3D which can be reduced to a 2D WZW model on the boundary, so perhaps something is possible. Some people think that the redundancy aspect of symmetry means that it is uninportant. They think that the field theory can be reformulated in a different way without the symmetry at all. This is incorrect. The redundant nature of the local symmetry hides the fact that it has global characteristics that are not redundant. In holographic theories you can remove all the local degrees of freedom over a volume of space but you are left with a meaningful theory om the boundary.

If there is symmetry for every degree of freedom in the bulk then the generators of the symmetries must match the spin characteristics of the fields. Supergravity only has symmetries corresponding to spin half and spin one fields but it has fields from spin zero scalars up to spin two. String theory goes even further with higher excitations of the string providing an infinite sequence of possible states with unlimited spin. This may be why the idea of higher spin symmetries is now seen as a possible solution to the problem.

Surprisingly the idea of higher spin symmetry as a theory of quantum gravity is far from new. It goes back to the 1980s when it was founded by Vasiliev and Fradkin. It is a difficult and messy idea but recent progress means that it is now becoming popular both in its own right and as a possible new understanding of string theory.

There is one other line of development that could lead to a new understanding of the subject, namely the work on supersymmetry scattering amplitudes. Motl has been following this line of research which he calls the twistor mini-revolution for some time and has a nice summary of the conference talk on the subject by Nima Arkani-Hamed. It evolved partly out of the need to calculate scattering amplitudes for the LHC where people noticed that the long pages of solutions could be simplified to some very short expressions. After much thought these expressions seem to be about permutations and Grassmanians with things like infinite dimensional Yangian symmetry playing a big role. Arkani-Hamed believes that this is also applicable to string theory and could explain the holographic principle. The Grassmanians also link nicely to algebraic geometry and possibly work on hyperdeterminants and qubits.

I have to confess that as an undergraduate at Cambridge University in the late 1970s I was completely brainwashed into the idea that symmetry is the route to the underlying principles of nature. At the time the peak of this idea was supergravity and Stephen Hawking – who had just been inaugurated into the Lucasian chair at Trinity college – was one of its greatest advocate. When string theory took over shortly after, people looked for symmetry principles there too but without convincing success. It is true that there are plenty of symmetries in string theory including supersymmetry of course, but different sectors of string theory have different symmetry, so symmetry seems more emergent than an underlying principle. I think the generations of undergraduates after mine were given a much more prosaic view of the role of symmetry and they stopped looking out for it as a source of deep principles.

Due to my brainwashing I have never been able to get over the idea that symmetry will play a huge role in the final theory. I think that all the visible symmetries in string theory are remnants of a much larger hidden symmetry so that only different residual parts of it are seen in different sectors.  In the 1990s I developed my own idea of how infinite dimensional symmetries from necklace algebras could describe string theory in a pregeometric phase. The permutation group played a central role in those ideas and was extended to larger string inspired groups with the algebra of string creation operators generating also the Lie algebra of the symmetry. Now that I know about the importance of complete symmetry and higher spin symmetry I recognise that these aspects of the theory could also be significant. Perhaps it is just a matter of time now before string theorists finally catch up with what I did nearly twenty years ago 🙂

In any case it is good to see that there is now some real hope that the very hard problem of understanding string theory from the bottom up may finally have some hope of a solution. It will be very interesting to see how these ideas mature over the next few strings conferences.

LHC Prepares for ICHEP

June 10, 2012

According to an earlier commenter on this blog, today is the cut-off date for the LHC to collect data for the ICHEP conference.  Despite technical issues,  overall the LHC has been “enjoying remarkable availability” Right on cue the luminosity delivered has passed the 5/fb mark promised by the operations group. The total LHC delivered luminosity is now about 11.2/fb for CMS and will soon pass the 11.87/fb total for the Tevatron run II.

Now the computer grid will light up as ATLAS and CMS push through the analysis for the Higgs and SUSY plots in time to get them approved for the massive ICHEP conference in one months time.

The schedule of talks  shows that each experiment will be giving detailed talks for each individual Higgs channel (plus possibly a new H -> Z+γ analysis) using 2012 data at 8 TeV leading up to the final combinations for each experiment. They will leave it up to bloggers to complete the full combination. It is likely that they will fall just short of discovery significance in diphoton channels and combined plots for each experiment. The full combined significance for the LHC will probably pass the 5 sigma finish line but no official combination will show that. This is not certain because the statistical fluctuations are not predictable.

In the lead up to ICHEP there have been many lesser HEP conferences already but none have had data beyond 2011 available from the LHC. Even at this late stage ATLAS has added a new conference note with an update to the WW channel using multivariate analysis which improves the sensitivity from 2011 data. There have been a few interesting talks about the significance of the Higgs excess at 125 GeV and some comments about the fact that the diphoton channels are showing a stronger signal than expected while the WW channel is noticeably deficient. These irregularities are likely to be statistical but they are a sign of the interesting speculations that will follow further results.

Some questions we will be looking to answer:

  • How much will the excesses seen in 2011 data be strengthened?
  • Will the CMS and ATLAS peaks still be at slightly different masses?
  • Will the channel branching ratios remain significantly different from standard model predictions?
  • Will the 115GeV to 120 GeV window be excluded?
  • Will the DG continue to call the observations just “interesting fluctuations” and get away with it?

On a slightly different topic, the 3quarksdaily blog has launched its science blog competition for 2012 which will be judged by fellow blogger Sean Carroll. About a hundred shameless bloggers have nominated themselves for the title and voting will begin for the final cut in the next couple of days. I have nominated my coverage of the December Higgs announcement so please support viXra by voting in our favour. Update: link for voting is here

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.

Mini Higgs Update

March 2, 2012

Today at La Thuile physicists from the Tevatron and LHC have been giving out a few teasers in preperation for the next Higgs updates expected at Moriond.

Dzero have released a diphoton plot as their first Higgs channel result using the full Tevatron dataset of 9.7/fb (Satish Desai Desai)

This is a companion to the equivalent plot from CDF published a month ago. At the LHC this is the most exciting channel but at the Tevatron it does not reach the sensitivity required to tell us anything about a standard model Higgs.

Slightly more interesting is this WW channel plot from CDF which improves on previous limits by about 10% (Richard StDenis). This is close to the sensitivity where some excess could have emerged but nothing is apparent.

The real interest for the Tevatron is the Higgs decay to two bottom quarks (bb channel) . For that and the combinations we are told to wait until next week which probably means Moriond.

ATLAS and CMS have not provided any new plots yet but ATLAS have reminded us that they still have to update WW, bb and ττ at 5/fb and we are also told to expect news at Moriond from them.

The ATLAS+CMS combinations previously expected for Moriond have apparently been abandoned. With the peak excesses from the two experiments in slightly different places the benefit of doing the combination may not justify the resources needed to produce it. Instead they look set to aim for independent discoveries from both ATLAS and CMS by the end of the year. This will not be an easy task as this plot at the bottom of the ATLAS talk shows (Junichi Tanaka) The 8 TeV energy improves the cross section by 30% and 3 sigma sensitivity is within easy reach with 2012 data, perhaps even in time for ICHEP, but 5 sigma discovery quality results require the full years run and some good luck. A run extension and a combination with 2011 data may be needed to polish it off. The same goes for CMS of course, and there is always the possibility that they will end the year with one team having better luck than the other.

In any case the arrow on this plot shows that they already know where the Higgs is 🙂

CMS also presented today (Josh Bendavid) but they have already given us everything they have for Higgs in 2011 data.

Higgs 2012

February 23, 2012

2011 lived up to all expectations and hopes for news about the Higgs Boson, but 2012 promises to be it’s crunch year and the excitement is about to begin

LHC startup

The Large Hadron Collider is getting ready to restart operations after the winter shutdown. The first part of the schedule looks like this

As you can see they should be starting full powering tests today which means the complete circuit of magnets should be cooled down to its working temperature of 1.9 Kelvin. However, as you can see in the picture below one of the RF cavities is still at room temperature. I hope it is something that can be sorted soon and they will be on their way.

A summary of how the LHC will run during 2012 can be found in a presentation by Rogelio Garcia . This slide in particlular says it all.

They expect 15/fb to 19/fb integrated luminosity for the whole year. Last year they expected 1/fb and delivered more than 5/fb, but this year we should not expect such a large overshoot. The machine has been brought close to its present operating capabilities and the peak luminosity cannot be pushed much beyond the numbers they are aiming for. The main uncertainty is in how efficiently it will run. last year there were times when it ran smoothly for two weeks and other times when technical issues held up progress for almost as long. The estimates for 2012 are based on the assumption that an average of the two extremes will be seen, but reality may differ. The decision to stick with 50ns at least means that the running will not be two different, although the higher energy and tighter squeeze than last year will be challenging enough.

ICHEP 2012

This year the International Conference for High Energy Physics will be taking place in July in Melbourne. This is the largest meeting on the HEP calendar and it is only held every two years. The experiments will be keen to have something new to say about the Higgs for the occasion so they have asked for 5/fb by June. With the 5/fb already analysed at 7 TeV and another 5/fb at 8 TeV there is a good chance that very convincing evidence for the Higgs will be found. However, I understand that they will not be combining the results at different energies immediately. I will of course perform my usual party trick of combining the plots unofficially to provide combinations over the different energies, channels, and experiments. I expect to be very busy. However, the approximate combinations do not give a definitive answer to how many sigmas of statistical significant have been observed. That will have to wait for official combinations to provide the pvalue plots.

Moriond 2012

Much sooner than ICHEP we will have the Moriond Meetings. This is split into several parts including the Electro-Weak conference and the QCD conference (there is also Theory and Cosmology).  The Higgs reports should be in the electroweak section but from the preliminary programs you can see that there is more about the Higgs at the QCD meeting with Sunday 11th of March being the crunch day. One talk that is so far noticeable by its absence is the ATLAS+CMS Higgs combination. I am led to believe that this will not now be ready in time due to the recent update by CMS. Producing the combinations is a long process and as the amount of data to analyse increases it can only get longer. It is also possible that the difference in position of the peak excess from the two experiments is giving some cause for delay while they improve their calibration methods to see if they can be brought closer together. I would not be surprised if they abandon the full combination and aim to just get decisive results from both individual experiments instead.

Since the LHC has nothing new to show about the Higgs, the interest will be in what the Tevatron can produce from its complete 10/fb of data. In the last month they have published their final results for the diphoton channel (already reported at AP and TRF) here is the plot

This plot tells us almost nothing because the limits are at ten times the expected Higgs cross-section. Any bumps at this level of sensitivity would be almost certainly statistical fluctuations. The Tevatron is not very sensitive in the diphoton channel because Higgs production is lower at the lower energy and because the mass resolution is not very good compared to the LHC. However, the Tevatron does much better in the bb decay channel and their complete combination should be quite good.

The overall expected sensitivity of the Tevatron to a 125 GeV Higgs is 3-sigma we are told. Previous published results reached 2-sigma sensitivity but only a 1-sigma excess was seen, they were probably unlucky. Due to the inferior energy resolution of the Tevatron any excess at the low mass region should also be expected to be quite wide compared to what we have seen recently at the LHC. Here is my simulation of what we might see as the final result.

Hopefully we will see the real deal on 11th March if they are ready in time. If the excess is any bigger than this fake version we should be happy, any less will be a bit disappointing, but it is all down to chance.