Today in La Thuile, in Italy, both the ATLAS and CMS experiments from the Large Hadron Collider (LHC) at CERN, and CDF and D0 from the Tevatron presented their updated results in their search for the Higgs boson, a hypothetical particle that physicists have been trying to find for nearly five decades.
The biggest surprise came from the competing experiments from the Tevatron, another particle accelerator located near Chicago. CDF and D0 announced that they were both seeing a small excess of events for a Higgs boson decaying into a pair of b quarks for a Higgs mass between 115-135 GeV, the same area where CMS and ATLAS are seeing an excess. The observed combined effect corresponds to a 2.2 sigma, i.e. the probability this comes from something else than the Higgs is 1.4%. Since the Tevatron stopped operation last September, these results are nearly final, although both experiments still expect to improve their analyses.
The morning session of the Moriond conference had opened with a lively lecture from Prof. François Englert, one of the key contributors to what is now known as the « Higgs mechanism », a theoretical description of how all elementary particles could acquire their mass. The current theoretical model, the Standard Model, fails to give mass to elementary particles unless this mechanism comes to the rescue. If this theory is right, we must find a new particle called the Higgs boson.
Both CMS and ATLAS are large experiments operating at the Large Hadron Collider (LHC), at CERN. Already, preliminary results were presented last December but today, both experiments showed the results obtained using all data collected in 2011 and involving nearly all possible “decay channels”.
The Higgs boson is expected to be unstable and will “break apart” in many different ways called decay channels. These are just like the different ways a machine can give change. The sum given should always be the same, no matter which small coins come out. Studying different decay channels is like checking they all correspond to the same particle and all give the same mass value for this particle. i.e. the value of the initial coin.
Now, there is only a very narrow range of about 7.5 GeV where the Higgs might be hiding around 124 GeV. At this time last year, it spanned a range of about 470 GeV.
In December, ATLAS only showed the results of the main two decay channels. Having now included 12 channels, ATLAS consolidates its previous findings and excludes a Higgs boson with a mass below 122.7 GeV (except marginally at 118 GeV), and from 128.6 GeV up to 539 GeV. Similarly, CMS rules out all masses between 127.5 all the way to 600 GeV.
The low mass region is therefore where we can expect some action in the coming months. Given the amount of data collected in 2011, both experiments expected to exclude the whole region between roughly 115 and 540 GeV. They exclude less at low values because they see more events in this area than expected if there were no Higgs boson.
This is observed in many decay channels and for both experiments, which have worked independently. This is now also the case for CDF and D0. Therefore, this is one more sign going toward reinforcing the possibility of a Higgs boson around 125 GeV.
This means these could be the first faint signs of a Higgs boson, most likely between 125-126 GeV. This is a bit like seeing the first pimple when a child develops chicken pox. Until she is covered with them, despite some early tell-tale signs, it is too early for a definitive diagnostic.
The most probable value observed by CMS is for a Higgs mass around 125 GeV where the excess has increased slightly since December after adding one new decay channel involving two photons and two jets. The excess is now a 2.8 sigma deviation, meaning the probability there is nothing there but “background” – other types of events – is 0.26 %.
Here is a quantity equivalent to the amount of events expected as a function of possible Higgs masses. The dashed line shows what ATLAS expected to see given the amount of data analyzed and in the absence of a Higgs boson. The solid black line gives the observed values. The green and yellow bands represent the one and two sigma statistical fluctuations. The fact that the black line exceeds the yellow region means that more events are observed than if there was only background. The most pronounced excess is 2.5 sigma for a Higgs boson mass of 125 GeV.
For ATLAS, the effect is now slightly reduced after adding the Higgs to two W bosons channel. The most pronounced excess is still around 126 GeV but now corresponds to a 2.5 sigma deviation or 0.6 % probability it comes from background only. It was 3.6 sigma in December. But if one adds the “look-else-where” effect, namely not only looking at one local deviation but taking into account the probability that statistical fluctuations can occur anywhere, then the probability that this could be due to a fluctuation from the background reaches to 10%-30%, depending how far elsewhere one looks. Similar effects were shown by CMS.
An excess of events is also shown by the CMS experiment in the low mass range. The largest excess of events is observed at 125 GeV at the 2.8 sigma level.
On the other hand, if there is a Higgs boson at a mass of 126 GeV, ATLAS should see locally an excess of 2.8 sigma when 2.6 sigma are observed. This means the data are also compatible with the hypothesis that this excess really comes from Higgs boson decays.
The LHC accelerator is due to restart on March 14 and will operate at a higher energy this year – 8 TeV instead of 7 TeV like in 2011, slightly increasing the chances of producing a Higgs boson. If all goes well, both experiments expect to have the final answer some time this year. Knock on wood!
The bottom line is: although the picture has not changed much for CMS and ATLAS since last December, more channels and more data are now analysed, yielding a more robust picture. With the Tevatron now bringing the news of a similar excess in their data, starts to build a coherent and convincing picture. Many physicists will have a hard time deciding in advance when to book their summer vacation…
Pauline Gagnon
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