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Wrapping it up on the Higgs boson

As the Large Hadron Collider (LHC) is preparing to shut down for the end of the year holidays, the LHC experiments presented on Thursday morning a summary of the last three years of operation. For CMS and ATLAS, the highlight was of course the discovery of what looks more and more like the Higgs boson.

The certainty for the presence of a new boson has been reinforced. As Sara Bolognesi, speaking on behalf of the CMS collaboration, put it: “The signal is so strong, the probability of having it wrong is as low as the chance of flipping a coin 40 times and getting 40 heads in a row”. Marumi Kado, representing ATLAS, showed that even when using a single decay channel, the signal is strong enough to claim a discovery. Hence, the focus is now on finding the exact properties of this new boson to reveal its identity.

ATLAS showed their first results on the spin and parity of the new boson. The parity seems positive, as expected for the Standard Model Higgs boson, reaching the same observation as CMS. But the jury is still out on the value of its spin although the results are more compatible with 0, the value expected by the Standard Model, but a value of 2 is still possible. A clearer answer might come once the 23 inverse femtobarns of data delivered this year by the LHC will have been processed and combined for the two experiments.

What’s new on the more-and-more-Higgs-like new boson? CMS showed the first results on a Higgs boson decaying into a Z boson and a photon. This decay channel should be very small unless there are contributions from processes predicted by theories going beyond the Standard Model, and these could be huge. Nothing is seen so far but this is a promising avenue.

A few facts are nevertheless puzzling. For example, ATLAS measures two different masses when the Higgs decays to two photons as opposed to four leptons, the two decay channels giving the best precision on the mass measurement.

Each one of these decay channels represents one way the Higgs boson can break apart. It is very much like making change for one dollar. No matter if you give the change with coins of ten, twenty or fifty cents, the total sum should always add up to one dollar. As it stands, it is as if ATLAS obtains $1.05 and $0.95 when adding up all the coins, despite having checked each channel with extreme scrutiny for a possible mistake.

This is most likely due to a statistical fluctuation since the data gives only one mass value in the global combination but it might take more data than is at hand to resolve this apparent discrepancy. CMS obtains similar masses in both channels but the results need to be updated with more data for the two-photon channel.

Another slightly intriguing fact: both experiments measure more Higgs boson decays into two photons than what is predicted by the Standard Model. I summarized the situation in the table below.

The error margins are still fairly large which means more data will be needed to sort it all out. The LHC will undergo a major upgrade starting in March 2013, to restart at higher luminosity and higher energy beginning of 2015.  It takes a lot of patience to do high energy physics!

Pauline Gagnon

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