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CERN | Geneva | Switzerland

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Latest news on the Higgs boson

After six hours of presentations dedicated to the search for the Higgs boson at the Moriond conference, here is a summary of the many new results shown today. Both the CMS and ATLAS experiments presented their latest updates, and no matter the angle studied, the new boson is still perfectly compatible with being a Higgs boson. More will be presented next week, once further checks are completed.

The experiments are now trying to establish not only how the new boson decays but also how it is produced. This will eventually help determine if the new boson is really a Higgs boson, either the one prescribed by the Brout-Englert-Higgs mechanism or one associated with supersymmetry, or even not a Higgs boson at all. To answer this question, both teams measured several of the new boson properties, quantities like the signal strength in various production modes, the different decay channels as well as its mass, spin and parity.

Only two decay channels, namely when the boson decays into two photons or four leptons, are used to measure its mass but for all channels, one can measure the signal strength (how many events are found compared to what the Standard Model predicts) and the spin.

An unambiguous signal obtained by the CMS collaboration in the search for a Higgs boson decaying into two Z bosons, each one decaying in turns into two leptons. This is the so-called four-lepton channel. We can see the data (black dots) matching the simulation of a Higgs boson shown by the red line.

The experiments had already checked that the new boson can decay into a pair of other bosons, namely W, Z ou photons, but it had not been established for fermions, the particles of matter like quarks and leptons. This is now a done thing since CMS observes decays into two tau leptons after analyzing the whole data sample. This remains to be proven for b quarks, which might have to wait until more data become available given the high background in this channel. Across the Atlantic though, the Tevatron experiment reported today seeing it at the 3 sigma level, i.e. three times stronger than possible statistical fluctuations.

Other novelty: ATLAS presented the first limit on possible decays of the new boson into invisible particles such as dark matter. This is not expected to happen in the framework of the Standard Model and indeed, with a limit placed at 68% of the time, it is compatible with the model.

The latest signal strength and mass measurements are shown in bold types in the table below along with the older results from last December.

CMS observes a number of events slightly inferior to the expected value in the four-lepton and WW channels while ATLAS reported small excesses for the number of events observed when the new boson decayed either in two photons or four leptons. This is still statistically too weak to draw any conclusion except to notice that all values are still compatible with the Standard Model predictions, all deviations being at most 2.3sigma for ATLAS.










The signal strength for different decay channels as seen by CMS (left plot) and ATLAS (right plot). A value of 1.0 is expected if everything behaves as prescribed by the Standard Model for a Higgs boson.

It will be particularly interesting to see what CMS obtains in the two-photon channel in their next update. If any deviation gets confirmed, it will draw a lot of attention from theorists due the possible huge consequences. A significant deviation with respect to the theoretical predictions would reveal a flaw in the model and help zoom on the right solution.

It is a well-known fact that the current theory, the Standard Model, has its limits. Everyone agrees that there should be a more encompassing theory to describe phenomena like the existence of dark matter, something the Standard Model fails to explain. But what is this new theory is the big question. All attempts so far have failed to find a crack in the Standard Model. Hard to improve on an impressive  theory that can make predictions accurate up to the tenth decimal.

New mass measurements were also presented today.  No anomalies here either. Last December, with only a third out of the 2012 data sample (21fb-1) analysed, ATLAS had obtained two different mass values for the new boson when measuring it using two different decay channels. Although an impressive series of crosschecks were performed, no experimental mistake was uncovered. The difference was ascribed to a statistical fluctuation. Today, after analyzing the whole data set, the difference is getting smaller, but so are the uncertainty margins. Nevertheless, this is probably a non-issue.

Finally, a few new spin and parity measurements were shown today, such that both experiments observe that the new boson is more compatible with a spin-parity of 0+ as expected for a Higgs boson than with any other spin-parity hypotheses. This is reinforcing the hypothesis that we are indeed dealing with a type of Higgs boson.

CMS checks to see if the new particle is more likely to have a spin-parity of 0+ (in yellow) as expected for a Higgs boson than other hypotheses (all shown in blue). The red arrow shows the value obtained for the new boson. The compatibility with each hypothesis is measured by the amount of the curve lying to the right of the arrow. There is always more yellow remaining than blue, meaning in all cases, the new boson is more likely to have spin 0+ than any other values.

While we are waiting for new results, some of which will be announced next week, you can entertain yourself by watching an animation (or another)recreating how the new boson signal appeared in ATLAS data over time. Meanwhile, as the information is trickling in, the identity of the new boson is slowly being revealed.

Pauline Gagnon

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