ATLAS and CMS experiments observe new particle consistent with long-sought Higgs boson

Today the ATLAS and CMS experiments have reported the observation of a strong excess of proton-proton collision events compatible with the Higgs boson.

The observed excess is obtained by combining 5 channels in the case of CMS to reach a level of 4.9 sigma of statistical significance. ATLAS has presented so far the result from two channels and observes an excess of 5 sigma. The number of events and the type of decays observed are both compatible with the standard model Higgs boson with a mass of about 125 GeV, and given the statistical significance of both ATLAS and CMS observations this can no longer be a statistical fluctuation. So today we have the discovery of a new particle.

So that’s it we got a Higgs boson! Yes! The atmosphere at CERN right now is pretty amazing and there is a palpable feeling we are living a historical moment, one which will be mentioned in text books. While a few other fundamental particles have been discovered in the 1990’s such as the top quark and the tau neutrino, we probably have to go back to the discovery of the J/Psi in 1974 which validated the quark model, to find a discovery of today’s significance.

Mass distribution for the two-photon channel. The strongest evidence for this new particle comes from analysis of events containing two photons. The smooth dotted line traces the measured background from known processes. The solid line traces a statistical fit to the signal plus background. The new particle appears as the excess around 126.5 GeV. The full analysis concludes that the probability of such a peak is three chances in a million.

So this has taken almost 40 years. It was deeply moving to see and hear the comments from Peter Higgs and Francois Englert right after the presentation, see the people queueing to the CERN conference room at 2 in the morning….

The observations by ATLAS and CMS are just enough to state a discovery of a new particle compatible with the Higgs boson, but it is not yet enough to precisely measure the properties of this new particle (well we know its mass already pretty well). Is it just a standard model Higgs boson? In supersymmetry there are five Higgs bosons, could it be one of them?

To determine this we need to now measure the Higgs production in all its possible decay channels, into two photons, two Z bosons, two WW, into two tau leptons, into two b quarks and so on. Today not even all these channels have been observed or presented yet, let alone measuring precisely enough the branching ratios into the various channels. So that is the next important step. In 2012 the LHC should provide 3 times more data than we have analysed so far so we should get a bit on the way towards checking all these channels. In some sense we are lucky that the Higgs boson has a mass of just 125 GeV since around that mass in particular it decays into so many channels. This will give us some extra help to analyse its properties in details. We also have to see whether there could be additional Higgs bosons, as predicted by Supersymmetry and other theories of physics beyond the Standard Model.

It also remains to understand why the Higgs boson mass is 125 GeV, as theoretical calculations diverge when attempting to predict it. It is only with a new theory of physics beyond the Standard Model that we could explain why the Higgs mass is 125 GeV and fix the hierarch problem (read this)

We do not know what is the right theory to fix this problem, but all alternatives are very exciting, could it be Supersymmetry which could also explain dark matter? could it be large extra-dimensions which would also explain why gravity is so much weaker than the other elementary interactions (electroweak and strong interactions)? Could it be that the Higgs boson is not elementary but that there is yet an even smaller level of organisation of matter?

All that remains to be seen and this is now the central focus of the work at the LHC experiments.

Experimental limits from ATLAS on Standard Model Higgs production in the mass range 110-600 GeV. The solid curve reflects the observed experimental limits for the production of a Higgs of each possible mass value (horizontal axis). The region for which the solid curve dips below the horizontal line at the value of 1 is excluded with a 95% confidence level (CL). The dashed curve shows the expected limit in the absence of the Higgs boson, based on simulations. The green and yellow bands correspond (respectively) to 68%, and 95% confidence level regions from the expected limits. Higgs masses in the narrow range 123-130 GeV are the only masses not excluded at 95% CL.

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