viXra log

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.

Yesterday the Sunday Times and the Daily Mail reported that five leading physicists have been invited to CERN for the Higgs search update on Wednesday. Peter Higgs is one of those called to the press conference so the five physicists could be the five living Higgs founders who were honoured with the Sakurai prize in 2010 i.e. Englert, Higgs, Kibble, Guralnik and Hagen. Kibble confirmed that he was called but is unable to make it and told the Sunday Times ‘My guess is that is must be a pretty positive result for them to be asking us out there.’

Frank Close on twitter has indicated that it was not five physicists and that they were not so much invited as “welcomed to attend”. He should know because he just tweeted this picture of himself with Higgs waiting at Palermo airport for a plane to Geneva.

Close is well-known for his own contributions to particle physics as well as classic text books and his popular book The Infinity Puzzle about the history of the standard model and the LHC.

By now you should know that physicists working on the CMS and ATLAS experiments at the Large Hadron Collider are about to announce important new results in the search for the Higgs boson. The announcement will be made on the morning of the 4th July at CERN in advance of the ICHEP conference in Melbourne where more details may emerge. The expectation is that this update will actually be a discovery announcement for the Higgs Boson. This is based on vague rumours, plus the fact that CERN PR are not saying that it is not a discovery, plus the fact that it would make no sense to have such an update at CERN before a big conference unless it were a discovery, plus the fact that they would not have been so sure so soon that there was something big to say unless the signal had come through very clear and strong.

The details will have to wait for the day and of course I will be here to add my independent analysis and unofficial Higgs combinations as the story unfolds. Others will be live blogging including Tommaso Dorigo of CMS who says he will be in the auditorium. I hope he has a seat reserved for him so that he does not have to camp outside the door overnight to get in. I will be watching the live webcast from home instead.

How do they know it is the Higgs Boson?

This is now the most frequently asked question, how do they know it is the Higgs boson and not some other particle they are seeing? In the scientific papers we can expect that the physicists of the collaborations will be careful about how they word the discovery. They will say something like: “We have found a new resonance (i.e. particle) in the search for the Higgs boson which is consistent (or maybe not) with the standard model Higgs Boson. Further measurements will be needed to confirm that its properties are as predicted.” And of course they will quantify what they mean by this with a slew of numbers and plots. In the press you will simply hear that they have discovered the Higgs boson. Dont by upset by this, you can’t expect a report in the New York times to read like a paper in Physical Review D, but it is fair to ask to what extent its known properties so far indicate that it really is the Higgs boson.

What is the Spin?

The most distinctive characteristic of the Higgs Boson is that it is a scalar, i.e. it has no spin. Other elementary particles in the standard model are either fermions with spin one-half or gauge bosons with spin one. Particles with spin that is any multiple of one half are possible and it is a quantity that needs to be checked experimentally. The channel where they are seeing the signal for the Higgs boson most strongly is through its decay into two high energy photons. The photons have spin one but spin is conserved because the two photons take away spin in opposite directions that cancel. It is not possible for fermions that have a odd-integer spin to decay without producing at least one new fermion so we know already that the particle observed is a boson. By a theoretical result known as the Landau-Yang theorem it is not possible for a spin-one particle to decay into two photons either, but it is possible for a spin-two particle to decay into two photons with spins in the same direction.

So we know already that the new particle has spin zero or spin two and we could tell which one if we could detect the polarisations of the photons produced. Unfortunately this is difficult and neither ATLAS nor CMS are able to measure polarisations. The only direct and sure way to confirm that the particle is indeed a scalar is to plot the angular distribution of the photons in the rest frame of the centre of mass. A spin zero particle like the Higgs carries no directional information away from the original collision so the distribution will be even in all directions. This test will be possible when a much larger number of events have been observed. In the mean time we can settle for less certain indirect indicators.

In March the Tevatron presented their final observations in their search for the Higgs boson. Their detectors are more sensitive to the decay of the Higgs to two bottom quarks. A weakly significant signal was seen at the same mass of 125 GeV where the LHC is seeing its resonance. This too will be confirmed with more certainty by the LHC later.  This shows (or will show) that the particle can decay into two spin half fermions. This is certainly possible for a spin zero particle and also for a spin one particle but is it possible for a spin two particle? If not we would know that the spin must be zero by a simple process of elimination. In fact it is possible for a spin-two particle to decay into two spin halfs provided the extra spin one is carried away either as orbital angular momentum (p-wave) or as a soft photon that is not seen, but neither of these possibilities is very likely. We can therefore be reasonably sure already that the observed particle is indeed spin zero, but for absolute certainty we will have to wait for more detailed studies.

What about other quantum numbers?

As well as spin, any elementary particle is partially classified by other quantum numbers including electric charge, colour charge, baryon number,  CP, etc. The charges are strictly conserved due to gauge invariance and are zero in the decay products so we know for sure that the particle is neutral. We also know that the baryon number is zero otherwise the particle would provide a mechanism for baryon number violation that would probably destabilise the proton. The quantity CP can be either even or odd but it is hard to know for sure which it is because CP is known to be unconserved at an observable level. Given that the decay modes are predominantly into a particle and its anti-particle or into two particles that are the same, it is unlikely that the CP is odd, but we will have to wait for more carefull tests to be reasonably sure. In any case there are versions of the Higgs boson in theories outside the standard model that have odd CP so this question does not really affect whether or not they are seeing the Higgs.

What about other Higgs properties?

The mass of the Higgs boson is the last parameter of the standard model to be determined. With the imminent discovery we now believe it to be about 125 GeV. With this quantity known every other property of the standard model can in principle be calculated, but it is not always easy due to non-perturbative effects that are difficult to model. Uncertainty in other measurements also adds more uncertainty to any calculation. The decay time ( or width ) of the Higgs boson can be calculated but because 125 GeV is less than twice the W or Z masses, the boson is relatively stable and the width is a few MeV. This is far too narrow to be measured at the LHC where the mass resolution is in the order of a GeV.

However, the most distinctive characteristic of the Higgs boson is its coupling to massive particles. By the nature of the Higgs mechanism that gives mass to the fundamental particles in the standard model, the coupling is always proportional to the mass. according to the theory the fermions and gauge bosons do not have any mass in the unbroken electroweak phase due to gauge symmetry and chiral symmetry (however the fact that neutrinos have a small mass already takes us beyond the standard model)   This affects all the production rates and branching ratios for the decays so if these are measured and found to be in agreement with the standard model we will have a useful test that what we have found really is the Higgs boson. Only by producing the unbroken state can we get a clearer sign that it is the real Higgs mechanism that breaks electro-weak symmetry but that is not accessible to present day technology.

The decay rates for the Higgs to ZZ, WW and bb all go by direct couplings to the Higgs boson so these provide particularly good tests. We can’t measure them directly because the rates at which we see these processes also depend on the production rate for the Higgs boson. The predominant mechanism for Higgs boson production is gluon fusion. This can be calculated in the standard model to an accuracy of about 15%, but it can be suppressed or enhanced by physics beyond the standard model. This is because the process involves a quark loop that is dominated by the top quark in the standard model. In some SUSY theories it is enhanced due to the bottom quark getting a stronger role, or it can be suppressed if there is a stop quark with a mass near that of the top quark. Even if the production rate is unreliable the ratios of the decay rates to ZZ, WW and bb should be fairly robust and will make a good test of the Higgs mechanism.

What would enhance the diphoton channel?

In the 2011 data we saw an enhancement of the diphoton channel amounting to 80% above the standard model in the unofficial ATLAS + CMS combination. The local significance is about 1.6 sigma, so nothing special, but the fact that they have opted for a special update so soon after looking at the new 2012 data with perhaps only 3/fb revealed suggests that this enhancement could have persisted. Even the collaborations wont know for sure until the final results which will probably not be ready yet. However it is certainly something worthwhile for a blogger to speculate about. So what could cause such an enhancement and does it mean this particle may not be the Higgs boson?

The decay mode to photons is more interesting because it also involves a loop that is dominated by the W boson but which also has (negative) contributions from the top quark. This can also easily be suppressed or enhanced by new physics such as any new massive charged particle with mass near the electro-weak scale. A boson will tend to enhance it while a fermion has a negative sign in any loop so will tend to suppress it. Prime candidates for enhancement would be a scalar top (stop) or a scalar tau (stau) The stop also suppresses the Higgs production rate because it has colour so it works both ways, but the stau is pure enhancement.

The diphoton channel can also be enhanced indirectly along with the ZZ and WW if the dominant bb channel is suppressed, e.g. if the Higgs is partially fermiophobic. We can distinguish this from the direct enhancements by observing the ZZ and WW channels, especially through the ZZ to 4 leptons decays which is a very clean and predictable measurement.

Together, observations of these channels should add up to an excellent test for the presence of beyond standard model physics and will provide narrow clues as to what type of physics it is. However the Higgs boson will still be a Higgs boson even if it is not quite the standard model Higgs boson.

Can they say they discovered the Higgs boson then?

Once we have the data from the first 2012 run in our hands in ten days time we will already have enough data to say that the new particle looks like a Higgs boson. We may even be able to make some preliminary statements about any deviations from the standard model. These will improve in time.

There will always be those who say that we dont really know for sure that this is the Higgs boson rather than some other scalar neutral particle that happened to be around, but the fact is that this particle turned up just about where the Higgs boson was most expected and with the right properties. We already know from the discovery of the W and Z bosons and many other tests that the standard model is a good one and it is a model based on electroweak symmetry breaking. Something is required to break that symmetry and now we have found a particle that fits nicely the characteristics of such a particle. Only the most obstinate skeptic would complain if CERN claim to have discovered the Higgs boson given the evidence we expect to see very soon.

If it swims on a pond and quacks like a duck it is not unreasonable to say it is a duck, especially when you were expecting to find a duck. Further observations will just tell us more about what kind of duck it is.

CERN have issued a press release to announce that an update on the search for the Higgs boson will now be given at CERN on 4th July. This will come before the talks at the ICHEP conference in Melbourne begin. Presentations detailing the individual decay channel searches are scheduled for 7th July at the conference with the combinations detailed in plenary sessions on the 9th, but now it looks like any important news will be delivered before then.

The update is likely to be similar in format to the talks given in December when the spokespersons for ATLAS and CMS delivered there results in detail. On that occasion the CERN director warned that no big discovery was in the offing as press and blog speculation ran rife. This time round no such statements have been made which can only lead to even more intense speculation. Rumours have already been indicating that this years results that will be added to the data from 2011 have been confirming the apparent Higgs signal seen at 125 GeV in the diphoton decay channel. This could increase the level of certainty to the five sigma significance from each experiment as required for an official discovery. Further rumours have even been suggesting that the strength of the signal is more than predicted by the standard model. An announcement of the discovery of a new particle consistent with the standard model Higgs would be the biggest in particle physics for some time, but if it is significantly different from predictions it will have enormous implications for the future direction of the whole field.

A possible reason for conducting the update at CERN is that the director general Rolf Heuer has said that he wants the news of a discovery to take place at the centre in front of the dedicated staff who have worked so hard for this result. The need is made stronger because Australia is not a member state of CERN so to announce the discovery there would be inconvenient politics. Originally an announcement was scheduled a few days ago for the 6th July (You may have seen it added on our blogs calendar) but this has bow been brought forward. Presumably this is to avoid the conflict with talks that would be going on in Australia at that time.

The update will be webcast but from previous experience of these events the video stream is likely to be overloaded and very difficult to watch.

The first batch of 2012 data from the LHC has started to be analysed ready for the ICHEP conference in three weeks time so it is no surprise that the first rumours about the Higgs data has surfaced. At NEW Peter Woit revealed his latest information that both ATLAS and CMS are seeing excesses in the diphoton channels at the same places where they saw them last year. The significance is said to be 4 sigma in each experiments for the combination of 2011 and 2012 data. Could this mean the Higgs Boson discovery has arrived?

Remember that from last year’s data the local significance with diphotons was 3.1 sigma for CMS and 2.9 sigma for ATLAS. According to the mew rumour the total significance has risen to about 4 sigmas in both cases. This is not yet sufficient to claim a discovery for each experiment independently but the two results together could easily take the combined significance over the 5 sigma line that is required for discovery. Note that this would be the discovery of something consistent with a standard model Higgs boson. More work would be required to show that it has all the characteristics of the particle they were searching for. The total combined local significance across all channels might be expected to be even better but don’t forget that ATLAS had only 2.5-sigma for its combination, and CMS got 3.1 sigma, so even if the diphoton channel provides a discovery the combination may or may not reach 5 sigma depending on what happens in other channels.

Here are a few points just to underline how much uncertainty remains

1. The rumours are a little vague and may not be completely reliable.
2. If this years peaks are not exactly in the same place as last years then the combined significance could be considerably less.
3. They may insist on using a Look Elsewhere Effect to reduce the significance to a global value. HEP protocol says they can claim a discovery on local significance, but if they want to draw out the game even longer they could still do this.
4. We don’t know how much data had been analysed. They should have about 4.5/fb but due to the processing lag I estimate that both CMS and ATLAS might be looking at something like 3/fb at this time. This might make the 4 sigma seem even better but it also means that it could go down as more data is added. The LHC has been running very well over the last few days and there are a couple more days left before the technical stop with over 6/fb recorded for each experiment and no reason why it should not all be used at ICHEP. If this is the case we will most likely get even better results and possibly even Independent discoveries from each experiment.
5. Credible information tells us that ATLAS and CMS don’t plan to show the combination of the 2012 results at 8 TeV with the 2011 results at 7 TeV. However, I think that if both experiments each have a clear 5 sigma discovery with the combined results they will not be able to resist making the combos, if they have time.

Here is a bonus video about Peter Higgs

See also Dorigo at QDS who is not giving away what he knows as a member of the CMS collaboration but he points out that these levels of significance are about what is expected from the amount of luminosity provided.

See also an article on the collider blog questioning the practice of spreading these rumours. For what its worth, I agree that it would be better if no rumours emerged before the conference and I have resisted the temptation to reveal unapproved information from the experiments I have come across in the past whenever I knew it to be private, but once one blog has revealed something like this everyone will see it and follow-up postings like this one do not spread it any further.