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.