Higgs-like particles exist: the head scratching begins

Physicists working at the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN), Geneva, have announced the observation of a new particle, widely thought to be the elusive Higgs boson.

Announced via a live two-way video link to the International Conference on High Energy Physics (ICHEP) in Melbourne, the result is the most significant finding in particle physics for decades, and is also potentially capable of solving a long-standing mystery concerning the origin of mass.

All matter in the universe is ultimately composed of subatomic particles, and the hugely successful Standard Model of particle physics mathematically describes what these particles are and how they interact.

The physicists of the 1960s knew most of the details, but found that any attempt to give particles a non-zero mass broke the theory.

As any personal trainer will tell you, mass is a fact of life. It was not long before a handful of physicists developed an elegant solution: if the universe is filled with a particular quantum field, particles can interact with it to gain mass.

One way to envision this is to consider a small ant — it has little or no intrinsic mass and may thus be called mass-less; however, should this ant walk through some treacle, it feels a great inertia from its interaction with it.

Our particles have the same experience, except that some of them feel the treacle more strongly than others and, hence, are heavier. This "background of particle treacle" was christened "the Higgs field" after Peter Higgs, a key proponent of the idea. Experimental physicists commenced a 50-year journey to obtain conclusive evidence of the field’s existence.

By smashing particles together at high energy in giant colliders, scientists aimed to produce a particle associated with the field known as the Higgs boson, recognisable through the particular pattern of signals that it would induce in giant detectors, buried deep underground.

The quest for the Higgs boson has involved decades of hard labour from thousands of physicists, engineers and computer scientists, and is the last piece of the puzzle that is required to complete the Standard Model.

That quest may now be over. Two experiments at the LHC, at CERN in Geneva, Switzerland, have seen strong enough evidence for a Higgs-like particle to declare a formal discovery of something new, and both are seeing similar signals.

Produced in the collision of high energy protons at the foot of the Jura mountains, the new particle has a mass of roughly 125 "giga-electronvolts" and has been seen to decay into a variety of other particles, with the pattern of these decay products being broadly consistent with the precisely calculated signatures of the Higgs.

Physicists worldwide have greeted the news with a mixture of excitement, delight, astonishment and, typically for the most sceptical profession on Earth, a good deal of head-scratching. Is the new particle really the boson predicted by the Standard Model, or is it a yet more exotic version?

Theoretical physicists have worked for years on theories that may one day supersede the Standard Model, and many of these theories predict more than one Higgs boson or a Higgs boson that would decay differently. The LHC has not yet produced enough Higgs bosons to definitely answer these questions, but the precision of the measurements will increase over the year, as the LHC collects more data.

For now, many are speculating that the particle may, indeed, be an exotic Higgs, rather than the common garden variety — a result that would be as popular as it would be earth-shattering.

As the champagne glasses are cleared from the tables of the Melbourne Exhibition Centre, those not lucky enough to attend ICHEP are watching Melbourne, where the details of the new results will be presented and discussed this weekend.

This international focus on Melbourne occurs at a time when Australian particle physics is enjoying a significant increase in funding, thanks to the creation of a new Australian Research Council Centre of Excellence in Particle Physics, home to many physicists — including this author.

While the experimental researchers in this centre continue to make leading contributions to Higgs experiments at the LHC, Australian theoreticians are sharpening their minds and pencils to spear the correct explanation, should the Higgs prove a trickier beast than first thought.

Dr Martin White works closely with members of the Centre of Excellence for Particle Physics, referred to in the article.