Physicists like to describe nature in the simplest and most elegant theoretical framework. A significant development towards such a description of the sub-atomic world during the 1970s was the unification of disparate forces of nature (excluding gravity) in a single theoretical framework. But this unification came at a price. The elegant mathematical symmetry that made it possible required all elementary particles to be massless, which is not the real world we know. So the underlying universal symmetry had to be ‘broken' to some degree for particles to have a range of masses and forces to have different strengths, and yet described by a single theory. This was achieved through the introduction of a hypothetical particle called Higgs — after Peter Higgs who proposed it — and an associated force. One can imagine Higgs as an all-pervasive ether-like force-field, which endows particles with mass (or inertia) because of the drag that the field exerts as particles move through space. This model of the universe, called the Standard Model (SM), seems to be along the right lines; this is so particularly after the discovery during the 1980s of particles W and Z — the carriers of the weak nuclear force just as the massless photon carries the electromagnetic force — with masses exactly as predicted by SM (about 100 times the mass of the proton). Since then the model has held up superbly in experimental tests. But Higgs itself has remained elusive and is the only missing piece in this otherwise enormously successful theory. The model itself does not predict a mass for the Higgs. So, for the last three decades physicists have been combing the entire energy ranges available to particle accelerators for signatures of Higgs in the decays of particles produced in these high-energy particle collisions. With the advent in 2009 of the highest energy accelerator, the Large Hadron Collider (LHC) at CERN, Geneva, which opened up a new energy domain, hopes of discovering Higgs have been high.
A discovery in particle physics is a long, painstaking process. It requires sifting through data related to trillions of collisions and picking out a statistically significant signal that stands out as an excess over the background of events from other processes that mimic a decaying Higgs. Two entirely independent experiments at CERN, ATLAS and CMS, have seen an excess of events that are attributable to Higgs. By summer, these experiments had excluded vast regions of mass where Higgs could exist, leaving just a narrow window. The latest results, announced on December 13, have squeezed the window further to around 125 times the mass of a proton. Since two independent experiments have arrived at the same conclusions, these are tantalising signals — but not good enough to be called a discovery. At present there is just about one per cent chance of the excess being due to fluctuations in the background. The golden rule for discovery in particle physics is that such a chance should be less than one in a million. A definitive statement on the existence or non-existence of Higgs requires more LHC data running through 2012. If Higgs does not show up even then, there will be an upheaval in the current understanding of the sub-atomic world, with the crucial question on the origin of mass remaining unanswered. But that, as we have seen before in the history of physics, is only likely to throw up even more revolutionary ideas.
