True, I wrote about Higgs week ago, but in less than 24 hours Higgs week starts and there so much to keep an eye on. Yes, less than 24 hours. But, first things first. A Higgs boson is an excitation - a fleeting, grainy representation - of the Higgs field, which extends throughout space and gives all other particles their mass. At the instant of the Big Bang, everything was the same as everything else, a state of symmetry that lasted no time and was immediately broken. Particles of matter called fermions emerged from the sea of energy (mass and energy being interchangeable), including quarks and electrons that would much later form atoms. Along with them came force-carrying particles called bosons to rule how they all were related. All had different masses -sometimes wildly different masses. Using the concepts of a Higgs field and Higgs boson, the Standard Model explains why quarks, protons, electrons, photons, and a wide-ranging zoo of other particles have the specific masses they do. Oddly, however, the Standard Model can't predict the mass of the Higgs itself. That will only be learned from experiment.
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. But, this is not from where first hints will come from. Tevatron is a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory (also known as Fermilab). The Tevatron ceased operations on 30 September, 2011, due to budget cuts (shame on you!); it is not as powerful as the LHC, which began operations in early 2010. The main ring of the Tevatron will probably be reused in future experiments, and its components may be transferred to other particle accelerators. Nevertheless, Fermilab folks decided to get into spotlight before CERN ones and they have results from data they previously collect now analyzed and ready to be shared with public. When? Tomorrow! I'm not sure I get reasons behind as probably there is nothing new because they already gave us the full details at Moriond conference earlier. Still, if you wish to follow their webcast, click here. Their show starts from 09:00am to 10:30am CDT (2:00pm to 3:30pm UTC).
Then, as noted above and earlier, CERN comes on menu two days later. It is ambigous to expect all channels analyzed so most likely you should except most important ones. It's loads of data and I can't see how all of that could have been covered in so short time. Expect to hear indications which should be either stronger than last year or perhaps they are gone (woopsie). Webcast can be found here and show starts at 9am Zurich time (7:00 am UTC).
On Saturday, 7th of July, Higgs results for each decay channel in fine detail will be presented at the ICHEP conference in the parallel sessions. Most of these will be updates with 2012 data. No webcast information yet (I will update an article once it becomes available).
Week away, on 9th of July, the combined Higgs results from CERN for ATLAS and CMS (separately) will be presented in the plenary sessions at ICHEP. No webcast information yet (I will update an article once it becomes available).
What to expect? Well, everyone wants to know that so you will just have to wait. But one of following may happen:
- a particle has been found which seems to have expected properties of Higgs boson
- a partcile has been found which seems not to have expected properties of Higgs boson
- no particle has been found
In first two cases, expect to hear that more work will be needed to establish precise and exact properties and until that is not done, we still say in realm of guess work. That will take couple of years - it may be Higgs, but which one? Mainstream media will probably talk rubbish as always - everyone can be journalist these days so expect many screaming titles about God particle soon.
How will they know this is Higgs? One property which is associated with Higgs boson is spin 0. 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. This is not trivial. 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. Scientists have some other methods, indirect ones, to use and try to address this issue. Scientists will also examine quantum numbers and compare it against theoretical assumptions among different models for Higgs out there (that includes those beyond standard model). The way decays in different channels are observed will be most interesting result and those will eliminate some of theories out there for sure. According to the Standard Model, the Higgs can decay by half a dozen different patterns of tracks, or channels. The probability of each path varies. For example, there's a low probability that a Higgs with mass equivalent to 100 billion electron volts (100 GeV) of energy would decay into a pair of W bosons, carriers of the weak interaction. Yet if its mass were 170 GeV, the probability of its decaying by that channel would be very high. But earlier measurements, including those made last year at the LHC and at Fermilab's Tevatron, have already excluded many possible masses for a Standard Model Higgs. Of the narrowing possibilities, the hints that ATLAS and CMS saw in 2011 were in the neighborhood of 125 or 126 GeV. The two channels involved, called the two-photon channel and the four-lepton channel for short, are certainly not the most likely decay routes. The probability that a 125-GeV Higgs would decay into two gamma rays is about two tenths of one percent, and the likelihood that it would decay into four muons or electrons is even smaller. Background noise is the key. Even though the two-photon and four-lepton channels have a low probability, they are relatively free of noise from particle debris that obscures evidence of other channels. More probable routes for the decay of a Higgs with mass near 125 GeV would be to a bottom quark and antibottom quark, or a pair of W bosons (see link for interesting discussion), or a pair of tau particles, but all these are much harder to detect.
Above sounds as there hard to expect claim that Higgs boson has been discovered next week since there are so many parameters to be checked (that includes me). Philip Gibbs on his blog covered this aspect. There will always be those who say that we don't really know for sure that this is the Higgs boson rather than some other scalar neutral particle (spin 0 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. Phillip goes on by saying 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. I agree with this - but I'd like to add that those with high expectation should be aware that nature is as it is - not as we expect it to be. I have no doubts to whatever result it will be that this is next step - an indirect step in shaping reality that surrounds us.
In order to declare a new discovery we need to have a 5 sigma excess and projecting our sensitivities using results from the 2011 data, to the data we have accumulated so far suggests that either experiment might see something close to 5 sigma at ICHEP. Of course, there is an option to combine Higgs searches in order to increase the sensitivity of the datasets even further. Aidan Randle-Conde argued on his blog this is wrong. In his words, the reason we have two experiments at the LHC looking for the Higgs boson is because if one experiment makes a discovery then the other experiment can confirm or refute the discovery. If we combine measurements from two different experiments we end up losing the vital crosscheck. The best way to proceed is two wait a few more weeks until both experiments can produce a 5 sigma discovery and see if these results agree. Lubos Motl, on the other hand, has opposite opinion - click here to read it.
Whatever happens, this isn’t the end of a journey culminating in the Standard Model, but the beginning of an expedition for exciting new physics at the terascale. If Higgs get announced around 125GeV I expect to see further work on SM (as preferred value is 100GeV) and inclusion of supersymmetry or extra dimensions and explained here. Pretty much everyone would like to see some new physics. Excess in two-photon channel and lack of it in WW in 2011 data might already suggest something new under the hood.
Credits: Philip Gibbs, Aidan Randle-Conde, Lubos Motl