Seminar Watch 2012

January 28, 2012

The New Year is well under way and already ATLAS has released its first conference note for 2012 updating the search for supersymmetry at 2/fb. They see a small excess of events, i.e. four where 1.7 ± 0.9 were expected. This is nothing that trial error cannot account for but it shows that the update to 5/fb still has a potential to produce ground-breaking results. CMS will be the first to provide that update with a seminar scheduled for Tuesday covering various searches including SUSY and exotica. We can rule out the possibility of anything really startling being announced, by the lack of rumours :) but there can always be first tantalizing hints.

After that the next big events for HEP results are the Aspen conference from 11th February and Moriond from 3rd March. The latter is the most likely venue for further results at 5/fb to be presented. The official Higgs combination for the LHC should appear and final Higgs results from the Tevatron using their full dataset are also expected.

Meanwhile the LHC itself has already entered its hardware recommissioning phase at the end of the Winter shutdown with half of the eight cryogenics plants already cooling down. The running schedule for 2012 is available showing a start date of 11st March for beams. This is 17 days later than last year. By then they will have decided on the running parameters for next year. The main ones to consider are the energy, bunch spacing and beta*. This year hopes are running high that the centre of mass energy will go up from 7 TeV to 8TeV but this depends on the results of hardware tests. They will not want to take any risk. The bunch spacing could go down from 50ns to 25ns which would please the experiments because it reduces pile-up, but realistically I think this would require too much scrubbing and preparation. It may result in a much lower luminosity for the year. They would be better to stick to 50ns and be sure of a good clean run. A better squeeze with a lower beta* is a promising option for pushing the luminosity to higher levels without risking higher intensities. These matters will be discussed at Chamonix from 6th February. If that is too technical a summary session will be webcast on 15th February.

It is too soon to list all the conferences where new results may appear this year but the biannual ICHEP meeting deserves a special mention. This is the largest conference for HEP and this year it will take place in Melbourne from 4th July. By then they may have doubled the LHC dataset if everything has run very smoothly. Discovery potential rises another notch.


Seminar Watch, Higgs Special

December 1, 2011

On the 12th December the CERN council will meet and announce the latest news about the search for the Higgs boson to its member states. This will be done in closed meetings but the next day the spokespersons for CMS and ATLAS will deliver 30 minute talks each in public. There will then be a discussion period of one hour. Hopefully this indicates that some meaningful result has been obtained and they will be able to tell us what the Higgs mass is or that it does not exist in the Standard Model form.

Unless they do their own approximate combinations I will be doing them myself here. This means I will have to digitise all the points in the CMS and ATLAS plots and run them through my spreadsheet. However, the real interest may come from the diphoton and ZZ channels so I will have to digitise another four plots and combine those too. I am going to be very busy but I will aim to have it all done before the end of the discussion period, unless they find some way of foiling my evil plot such as by not posting the plots online until later.

See also NEW and TRF.


New Higgs Combinations Released

November 18, 2011

The LHC Higgs combination group is presenting their ATLAS+CMS Higgs combination plot at the Hadron Collider Physics conference in Paris today at noon and the slides of the talk (Gigi Rolandi) are already online. It includes some nice individual channel combinations as well as the full one we have been expecting. Before I look at those here is my approximate version of the full combination that I showed here two months ago. This version of it is taken from a slide shown by “Bill and Vivek” for the Higgs Combination Group themselves at a kickoff meeting in September for the plots finally shown today.

So how did I do? Here is a version of the new combination that conveniently shows some of the variations you can get just by using different methodologies.

The viXra version of the plot was produced using the minimal data available in the individual ATLAS and CMS Higgs Combination plots shown at Lepton Photon 2011 and approximates the probability distribution function by flat normal error curves. the calculation takes a few milliseconds. The full combination from the HCG goes back to the original data using the real log likelihood numbers and takes into account all known correlations between the data and background calculations. The calculation takes hundreds of thousands of hours of CPU time, yet the difference between the viXra plot and the official HCG one is no bigger than the differences of using alternative methodologies such as Bayesian. This is a nice demonstration of the power of the central limit theorem which says that and error distribution becomes normal given enough data and a finite variance. It also confirms that the effect of correlations on the plot cannot be very big.

To be clear, I think it is important that the full official combinations are worked out carefully because if you want to claim a discovery you have to make sure you have covered all the sources of error correctly. The Higgs Combination Group have done a good job. But if you just want to see the signal in the data we now know that an approximate combination is good enough.

If you want to compare more closely here is the official version with the viXra combination overlaid in red. The areas where it deviates are regions at high mass where there is low background and few events have been recorded. The approximation is not so good there because the normal distribution approximation is less accurate.

Here is the zoom onto the lower mass region

I Like that the combination group have also produced combinations for all the individual channels. My own verisons of these are a little less reliable because there is less data in each case so the normal distribution is not such a good approximation. Even so my plots were not far out which means that with the next batch of data using two to three times the statistics I can expect to get good results.

Here is the crucial combination for the golden channel. This is one of the best hopes for a signal  because its high resolution and good branching ratio at low mass. If you want to compare with my earlier combination it is here.

The other channel that has the potential to find a low mass Higgs is the direct diphoton decay and there is a new combination for that too

I think it is striking that both these plots have healthy excesses at around 140 GeV and perhaps again at lower mass. To see this better we need to combine them both together.

But this data is by now very old and it is no longer worth speculating on the basis of what the plots might show. The story has already been superceded by rumours over at Résonaances that the 5/fb plots show no more than a 2-sigma excess at 120 GeV. If all goes well we may get first results via the CERN Council Meeting during the week starting 12th December.


Higgs excluded from 130 GeV to 480 GeV (Illustrative)

August 29, 2011

There are a few interesting workshops and conferences on today that are presenting results from LHC and Tevatron. In particular the “Implications of LHC results for TeV-scale physics” meeting at CERN all this week is the most likely place to look for new results, and indeed the following plot has just been shown by Eilam Gross.

This is an “illustrative” combination of the ATLAS and CMS Higgs searches which appears to be based on the data presented at lepton-Photon-2011. If you look carefully at where the black line crosses the 95% confidence level limit you will see that it excludes the standard model Higgs between 130 GeV and 480 GeV.

A Higgs below 130 GeV disfavours the standard model on its own because of vacuum instability. It might be OK if the vacuum remains metastable with a sufficiently long lifetime but if the mass is a bit smaller then such a universe becomes a very dangerous place to live. The safer explanation would be that the light Higgs is stabilised by extra particles which would have to look very much like a Higgsino or a stop. I.e. SUSY.

The excesses above 130 GeV are still there. It is difficult to read their size from this plot but they are obviously not due to a standard model Higgs. They could be from another boson with a smaller cross-section, or they may just be the effects of uncertainty in measuring the missing energy of the neutrinos in the WW channel.

No doubt more data will added soon and some possibilties are:

  • Excesses at 120 GeV could grow to a robust signal of a light Higgs, suggesting supersymmetry
  • The curve may continue to descend until the whole mass range is excluded according to the standard model
  • The 140 GeV excess could bounce back up from the grave to provide a standalone Higgs boson solution
  • The Higgs could appear at higher mass than 480 GeV, posing other problems for the standard model.
  • Some completely unexpected signal of electro-weak symmetry breaking could emerge.

Of course the plot is marked as “Illustrative” and I have no idea what other caveats the speaker has added (but see remarks in comment section from the speaker).

Update: For the record it turns out that the above plot used a combination formula which is not too good for observed CLs. See comments from its constructor below. It would be wrong to use it to draw any conclusions. You should think of it as an illustration of how missleading a combination can be if not done correctly. :) It was removed from the uploaded slides.

My own combinations use a different formula which I believe is much better. They do not yet show an exclusion at 140 GeV.


Higgs Hunting 2011

July 31, 2011

After the hectic EPS conference last week there are a number of followup workshops organised for people to discuss the new results concerning the Higgs boson and possible new physics. The first is the three day meeting “Higgs Hunting 2011″ in Orsay which ended yesterday. For such a workshop the words of the presenters and discussions after are what count, but these are not webcast so all we have to go on as outsiders are the slides (Update 5-Aug-2011: video recordings of the talks have now also been made available at the same link). Nevertheless there are some interesting points in the slides and it is worth picking out some highlights. The workshop started with a talk by Massimiliano Grazzini with this slide showing the main new Higgs results and how it makes the theorists feel These exclusion plots only tell part of the story and it is easy to be misled by excesses that look convincing because they have lots of substructure that makes them appear to show complex signals. In truth the excess comes from a small number of events often seen in just one channel, with the detailed noise coming from the background. A slide from James Olsen for CMS shows the event data from the diphoton channel. On the lefthand plot you can see some excesses at 120 GeV and 140 GeV that make bumps in the exclusion plots but on their own they don’t count for very much. If you look at enough plots you are bound to see excesses of this size somewhere. A slide shown by Elisabetta Pianori shows some signals at around 120 GeV in the same diphoton channels. These are still weak and they are not seen elsewhere. It’s easy to get carried away if you are selective about what you show Here is an more extreme example from Aurelio Juste (see also Paul Thompson). This slide shows events recorded by ATLAS in the H-> ZZ->4l channels. As you can see there are not a lot of events there. This leads to the exclusion limits on the right. As you can see there are bumps giving nearly two sigma excesses, but they correspond to single events. These are not signals on their own. When we combine all the channels and all the experiments we do get some slightly better signals, but still the signal is quite weak. Ben Kilminster has conveniently lined up the plots to show us where they agree, Draw your own conclusions. Here is the update from Matthias Schott on behalf of the gfitter group As you can see they wont include the ATLAS and CMS data anymore claiming that it is “not trivial anymore”. This did not stop John Ellis using the “bloggers combination” to draw some tentative conclusions about the standard model Higgs

The discussion was not just about Higgs but I just have the energy to show one slide summarising the mass limits on various possible new particles according to Paris Sphicas on behalf of ATLAS


Big Day for Higgs Boson

July 22, 2011

Today at the EPS conference in Grenoble the worlds largest hadron colliders will be revealing the results of their latest searches for the Higgs boson, using data collected up until the last few weeks. We will be posting the plots here as they appear.

The individual experiments Dzero, CDF, ATLAS and CMS will each show their all channel combined plots. There will also be separated plots for individual channels and some separate searches for a charged Higgs as predicted in some models such as MSSM.

Our expectation is that the Tevatron plots (Dzero and CDF) will show some good exclusion limits but we will have to wait for the plenary talks next week to see the full Tevatron combined plot. From a press release last night we already know that they will claim to limit the Higgs to a region of 114 geV to 137 GeV, but that is not the end of the story. Above 185 GeV they only use indirect measurements to exclude the Higgs and these assume that no new particles beyond the standard model exist. That could be a weak assumption.

Later today the CMS and ATLAS plots will tell us about those heavier mass regions with direct searches. They should be able to exclude a heavy Higgs or provide a plausible signal above 190 GeV, so what will it be?

We wont get a full combined LHC plot at this confernece but the individual  plots for ATLAS and CMS will already have strong results.

Click on big titles below to bring up the full slide presentation.

Dzero

First up is Dzero with this combined plot that we first saw a couple of days ago

It shows a good exclusion from 162GeV to 170 GeV, not a new result but good to see that the limits imposed by the individual experiments at Fermilab are already strong.

CDF

The CDF combined plot is not very different.

CMS ττ

In the Higgs to tau lepton pair decay channel CMS produce this plot. Remember that the observed limit has to drop below the horizontal line at 1.0 to provide a 95% confidence level exclusion. There is not enough data to do that here, but this data will go into the combined plot later too. It is good to see that 1.1/fb is being used. The same presentation also provides good SUSY exclusion results.

CMS γγ

The Higgs decay into two photons is a crucial channel for finding the Higgs. The LHC do not yet have enough data in this region but with this new plot we see just how close they are getting. A full combination of Tevatron and LHC data at this time might almost have something to say about low mass Higgs if this is anything to go by.

ATLAS γγ

Much the same from ATLAS

ATLAS bb

ATLAS have looked at the decays of a Higgs to decay into a bottom quark pair in conjunction with a W or Z boson. They see no excess at twenty times the standard model Higgs signal.

CMS ZZ

Tow Z bosons from a Higgs can decay into pairs of leptons, quarks or neutrinos giving different channels to search in. First the 2 leptons plus two quarks plot. This is an exciting result that comes close to exclusion at some points, but why has it been cut-off below 220 GeV?

The decay into two leptons and two neutrinos gets even closer

Finally the golden channel of four leptons crosses the line with a tiny exclusion around 185 GeV

ATLAS ZZ

The story from ATLAS is pretty much the same

The 2lepton+2 neutrino channel even has a good exclusion on its own

The golden channel is close to expectation levels for no Higgs

ATLAS WW

This splits into two main channels, first each W decays to a lepton and a neutrino. Here we get an impressive exclusion.

The lepton neutrino 2 quark channel is not so strong

CMS WW

For CMS just one combined plot for the WW channel is all we need

ATLAS

Combined result. WOW!


CMS

Finally the CMS combined plot. The exclusions are from CMS are 149-206 and 300-340 GeV with some large exclusions in the space between.


Conclusion

We still have to see the combined CMS plot and the combined Tevatron plot but already we have some strong results. Much of the Higgs mass range has now been excluded leaving just a window from about 114 GeV to 137 GeV and 205 GeV to 295 GeV. the higher range is excluded by precision tests for a standard Higgs, but a combination of massive particles is not ruled out.

There are excess in the 140 GeV to 150 GeV and a curious deficit at 350 GeV, seen consistently across the data. These results are compatible with a number of options including a light Higgs and a multiplet of Higges. More data will be required to finish and we should get enough this year but already we see that a standard model Higgs on its own only just fits the data if around 130 GeV. This is an outstanding result.

See also TRF for more discussion especially about the deficit.


Highlights of EPS first morning

July 21, 2011

Undoubtedly the most talked about presentation so far will be the CDF search for ZZ resonances (Robson) with this striking peek at 327 GeV. This is the only talk so far that has not seen everything consistent with standard model. The talks themselves have not been broadcast so we only have the slides to go by. It would be nice to know what questions were asked at this one.

Another controversial topic that is not so new is the Wjj bump observed by CDF. This morning we had new discussions about this from CDF (Cavalier) themselves  and D0 (Sekaric) who have refuted the bump with their data. CDF points to some differences between the analysis, especially some morphing that D0 use to remove systematics. Did this remove the bump too? A task force will compare the two calculations step by step to see where the discrepancy comes from. (see also below for the ATLAS contribution on Wjj)

The first talk with all new LHC data came from Kai Yi who presented searches for new physics in all hadronic final states at CMS. This included dijet resonances in 1.01/fb with nothing showing up to very high masses. A number of exotics including black holes are excluded up to one or two TeV

This was followed by Gibson who provided the matching dijet resonance data from ATLAS with 0.81/fb. The negative conclusion was the same.

CMS returned with another null search from leptons plus gamma presented by Leonidopoulos.

ATLAS also covered the same channel and found nothing in their talk about exotics by Berger-Hryn’ova. This presentataion covered a number of interesting areas but one worthy of particular note is a repeat of the Wjj search with 1.02/fb. They found nothing so they are tending to side with D0 in refuting the bump seen by CDF

One last search by CMS was for two electrons or muons presented by Tucker. As usual they produced some colourful plots but no resonances.

Although these negative search results are disappointing there is still plenty of space to find exotics with more data this year, or with more energy in a few years time. Meanwhile it is the Higgs and SUSY searches that are the most promising and those are still to come


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