4th July 2012 was a day to go down in the history of science. Before the eyes of the world, Rolf-Dieter Heuer, Director-General of the European Organization for Nuclear Science (CERN), announced the discovery of the Higgs boson. It was the long awaited confirmation that, after a search lasting nearly five decades – an inspiring and relentless quest that stretched imaginations and tested convictions across the global particle physics community – the previously unseen particle first predicted in the 1960s had finally come to light.
But discovery is never the end of a journey. In this case, it was just the prelude to intensive efforts to study the precocious new kid on the subatomic block. We may have a basic understanding of the Higgs – the crucial particle that helps explain how infinitely small particles can still have finite mass. Now, though, the focus is on finding out much more about the only subatomic particle that can justifiably claim global ‘celebrity’ status.
The Higgs boson takes its name from Professor Peter Higgs of Edinburgh University, one of six scientists who predicted its existence half a century ago. But the star in the drama of its eventual discovery was CERN’s Large Hadron Collider (LHC), with UK researchers prominently involved. Deep down in its 27 km tunnel spanning the Franco-Swiss border, high-energy beams of protons crashed together and two experiments – ATLAS and CMS – produced data used to prove the Higgs boson’s existence as it disintegrated into other bosons. Since then, ATLAS and CMS have gathered evidence that the Higgs can decay into fermions too (fermions and bosons being the two key classes of particles in quantum mechanics).
In fact, by the end of its first three-year running period, the LHC had produced an impressive 14,000 Higgs bosons for study. But the second run due to start in 2015, with its higher-energy proton beams and higher rate of collisions, is expected to produce up to ten times more. Throw in a range of new equipment, more computing power and smarter algorithms to make sense of the data generated – and the result will be a substantial improvement in our ability to probe the particle’s properties.
The basics of the Higgs boson
(Credit: Dave Barney and Steve Goldfarb)
So what exactly is our current state of knowledge about a particle that provides vital evidence for the existence of a ‘Brout-Englert-Higgs field’ giving matter mass and enabling it to form into atoms? We know how much the particle weighs – 125.36 (plus or minus 0.41) gigaelectronvolts, around 130 times more than a proton. We have a good idea of how long it ‘lives’ before it decays away – around a tenth of a billion-trillionth (10-22) of a second – and we’ve started to explore those decay patterns. But more precise measurements of its lifespan are needed, as well as answers to a raft of questions. Exactly how many collisions at the LHC does it take to produce a Higgs? What are all the possible final states that it decays into? How exactly does it couple to other particles? Are there more types of Higgs bosons to be discovered? Above all, what are the full implications of the Higgs and its properties for the Standard Model of particle physics and the theoretical explanation this model puts forward regarding the interactions between subatomic particles?
Unravelling the mysteries of the Higgs and penetrating its opaque world won’t just help us understand why the Universe is as it is now. It’s like taking a time machine back to when the cosmos was in its infancy – just after the Big Bang created everything in an explosion of unimaginable violence, an event whose fall-out is still evident in a background ‘hiss’ of microwave radiation. This can give us a glimpse of when the Higgs field first appeared, and helped to shape the Universe we know today.
Seen in these terms, the painstaking process of more detailed observation and more precise measurement – and the relentless