Higgs spotting at CERN

Archana Sharma, staff physicist at Europe's high-energy physics laboratory CERN near Geneva, says hunting down the elusive Higgs boson make for some thrilling times in the history of physics. However, we still need more data to say 'Eureka', she reports from CERN.

doi:10.1038/nindia.2011.182 Published online 13 December 2011

Archana Sharma

Physicists at CERN have been busier than bees for the last few weeks, with expectations about the famous Higgs boson sighting, or not. The two large experiments at the Large Hadron Collider (LHC) — ATLAS and CMS — have been combining their data and poring deeper into the regions where some evidence was expected.

CERN director general Rolf-Dieter Heuer said in an invitation to the much-awaited meeting on December 13: "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 Higgs." This built up excitement significantly.

Physicists on these two experiments have now presented signals that point to the possible appearance of the Higgs boson, the particles that give all other particles their mass. However, researchers at both experiments have clarified that more data is needed to scale this 'observation' up to the level of a 'discovery'. That announcement — this way or that — is expected next year.

The elusive Higgs

The Higgs boson has been evading discovery for a long time now. It is a tough call since we do know theoretically from the Standard model how many Higgs bosons should be produced at a specific mass and how they would decay. Unfortunately the mass is not known. Hence CMS and ATLAS are searching for it in all possible decay modes, albeit a bit blindly.

Physicists making the big announcement at CERN.

© Archana Sharma

Given previous studies, the region at low mass, between 114 and 141 GeV, is where we expect to see three different decay channels contributing, i.e. decays into two photons, two Z bosons going into four leptons, or two W bosons decaying into two leptons and two neutrinos.

Due to the techniques employed, both experiments have their forte in respective discovery channels, and an observation from one or the other channel would be a great indication for each. But the 'evidence' would be when both experiments see hints in a couple of decay channels.

A discovery would at least require a high degree of certainty — 5 sigma — by both experiments. Informed sources at CERN have said it is at about 2.5 sigma for CMS and 3.5 for ATLAS — enough to qualify the sightings as "observation", but excluding a discovery.

The boson was postulated in 1964 by British physicist Peter Higgs as the particle that gave mass to matter less than a fraction of seconds after the Big Bang 13.7 billion years ago, thereby resulting in the formation of the universe as we know today.

The missing link

Archana Sharma at CERN's LHC.

The Standard Model formulated thereafter during the 1970s, has been experimentally verified throughout the 80s and 90s. The Higgs particle is the only missing link in this model. Its discovery, if confirmed, especially if it is at the low mass levels where ATLAS and CMS have found it, would open the way to what experts at CERN call the "new physics" of super-symmetry (SUSY) and dark matter.

Let's take the famous example of searching for a needle in not one but several barns of hay. The probability of finding the needle in a barn is very low, and the barns of ATLAS and CMS are different. Let's say both CMS and ATLAS found a couple of clumps which look suspicious, and that they may contain the needle, or it may just be a clump due to other conditions.

On the other hand, if you were to find identical clumps and many of them, you would start getting suspicious. The sensitivity of the experiments allows them to separate as much hay as possible but not all to be able to clearly see the needle. Therefore finding more than a certain minimum number of identical clumps is necessary to be able to claim 'evidence', while cleaning up the clumps — getting more data — would enable us to say "Eureka, we found it".

A clear cut conclusion is on the horizon some time next year. These are thrilling times in the history of physics and it is an incredible privilege to be a tiny part of it.