Singing happy birthday slightly off-key but in good spirit. This is how several hundred physicists gathered for the 37th International Conference on High Energy Physics in Valencia, Spain closed the day on July 4th. Two years before, the CMS and ATLAS experiments had announced the discovery of the Higgs boson on the eve of the same conference that was then held in Melbourne, Australia. Lots of people reminisced about the day of the announcement, where they were when they heard the news since many were traveling.
Two years later, the two experiments have now gathered an impressive amount of knowledge on the Higgs boson. Both groups have measured with high precision the Higgs boson mass, how it is produced and how it decays. ATLAS presented its published Higgs boson mass combination, namely 125.36 ± 0.41 GeV also in perfect agreement with the CMS measurement, presented for the first time at this conference, of 125.03 ± 0.30 GeV.
By presenting its final Higgs boson decay to two photons results, the CMS Collaboration has now completed its analysis of all the data taken so far. The obtained value for the combined signal strength, which is how many Higgs bosons are observed compared to the number predicted by the theory, is 1.00 ± 0.13. ATLAS measured 1.3 ± 0.18. Both results indicate that, within errors, this boson is compatible with what the Standard Model predicts.
Its spin and parity, two properties of fundamental particles, are also known. These are like fingerprints. Knowing them reveals the identity of a particle and that is how we know the boson discovered two years ago is really a Higgs boson.
The question is still open though to see if this is the unique Higgs boson that was predicted by Robert Brout, François Englert and Peter Higgs in 1964 in the framework of the current theory, the Standard Model. But it could also be the lightest of the five Higgs bosons predicted by a more encompassing theory like Supersymmetry that would fix some problems of the Standard Model and open the door to the so-called “new physics”.
Several measurements from ATLAS on the signal strength, i.e. how often Higgs bosons are produced in different ways, and decay into different types of particles, compared to the theoretical predictions. The result should therefore be equal to 1.0 if the theoretical predictions are right. The black “+”symbol indicates the predicted value while the various circles give the zone where the experiment expects the real value to be with 68% or 95% confidence level.
Nearly all the data collected up to the end of 2012 – before the Large Hadron Collider (LHC) was shutdown to undergo a massive consolidation and maintenance program – were used for the many analyses presented at the conference. Everything measured so far agrees within experimental uncertainties with the predictions of the Standard Model. Not only did the experiments improve the precision on most measurements, but they are also looking at new aspects all the time. For example, CMS and ATLAS also showed the distribution of the momentum of the Higgs boson and its decay products afterwards. All these measurements test the Standard Model with increasing precision . Experimentalists are looking for any deviation from the theoretical predictions in the hope of finding the key to reveal what is the more encompassing theory lying beyond the Standard Model.
A series of results by the CMS Collaboration on the signal strength. With the current level of precision, all these measurements agree with a value of 1.0, as predicted by the Standard Model. A deviation would suggest the manifestation of something beyond the Standard Model.
But none of the numerous direct attempts to find particles related to this new physics has proved successful yet. Despite having looked at hundreds of different possibilities, each one corresponding to a particular scenario involving one of the hypothetical particles of Supersymmetry, no sign of their presence has been discovered yet.
However, this is quite similar to doing archaeology: one needs to shovel a lot of dirt before extracting something meaningful. Each analysis is like one bucket of dirt removed. And each small piece of information found helps get the bigger picture. Today, with the wealth of new results, theorists are in a much better position to draw general conclusions, eliminate wrong models and zoom in on the right solution.
The whole community is eagerly awaiting the restart of the LHC in early 2015 to collect more data at higher energy to open up a new world of opportunities. All hopes to discover this new physics will then be renewed.
Pauline Gagnon
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