14 December 2011
This week, the latest results from the ATLAS and CMS experiments at the Large Hadron Collider (LHC) were presented at CERN to an expectant audience of physicists around the world.
The results are based on the analysis of all of the data provided by the collider in 2011, with a factor more than 100 increase in data compared to 2010.
These data were sufficient to make significant progress in the search for the Higgs boson, but not enough to make any conclusive statement on the existence or non-existence of the elusive Higgs. The main conclusion is that the Standard Model Higgs boson, if it exists, is most likely to have a mass constrained to the range 116-130 GeV by the ATLAS experiment, and 115-127 GeV by CMS. Tantalising hints have been seen by both experiments in this 15 GeV mass window, but these are not yet strong enough to claim a discovery.
Higgs bosons, if they exist, are very short lived and can decay in many different ways. Discovery relies on observing the particles they decay into rather than the Higgs itself. Both ATLAS and CMS have analysed several decay channels, and the experiments see small excesses in the low mass 15 GeV window that has not yet been excluded.
Taken individually, none of these excesses is any more statistically significant than rolling a die and coming up with two sixes in a row. What is interesting is that there are multiple independent measurements pointing to the region of 124 to 126 GeV. It’s far too early to say whether ATLAS and CMS have discovered the Higgs boson, but these updated results are generating a lot of interest in the particle physics community.
Huge progress has been made in the last two months, following the end of 2011 LHC data taking. We can view this like the Apollo missions to the moon. 2011 data corresponds to Apollo 10 - the technologies required to find the one in ten billion events, corresponding, to the Higgs boson, have been fully deployed and tested. We know the path and the Higgs channels we need to follow next year. The LHC and experiments will need to operate even better next year, but given this year’s excellent progress we will finally discover what nature has in store.
Over the coming months, both experiments will be further refining their analyses in time for the winter particle physics conferences in March. The next step is to combine the information from both experiments in a statistically rigorous procedure. However, a definitive statement on the existence or non-existence of the Higgs will require more data, and is not likely until later in 2012.
A Standard Model Higgs boson would confirm a theory first put forward in 1964 by Peter Higgs, but there are other possible forms the Higgs boson could take, linked to theories that go beyond the Standard Model.
A Standard Model Higgs could still point the way to new physics, through subtleties in its behaviour that would only emerge after studying a large number of Higgs particle decays. A non-Standard Model Higgs, currently beyond the reach of the LHC experiments with data so far recorded, would immediately open the door to new physics, whereas the absence of a Standard Model Higgs would point strongly to new physics at the LHC’s full design energy, set to be achieved after 2014. Whether ATLAS and CMS show over the coming months that the Standard Model Higgs boson exists or not, the LHC programme is opening the way to new physics.