Australasian Science: Australia's authority on science since 1938

The Hunt for the Higgs Boson

By Stephen Luntz

Elisabetta Barberio spent the past two decades designing and carrying out experiments that helped to find the Higgs boson.

When the 2013 Nobel Prize for Physics was awarded to Prof Peter Higgs and Prof Francois Englert it was widely seen to be also recognising the work of the 3000 scientists who collaborated to find the Higgs boson.

While the Nobel Prize for Peace can be given to an organisation, the science prizes cannot be shared between more than three people. The rule is something of an anachronism in the modern era, where many of the greatest discoveries involve large teams, but never more so than in the case of the Higgs boson. Along with the Moon landing it represents one of the great triumphs of collective science, and Prof Elisabetta Barberio was one of those who played a part.

Barberio says she was always interested in science as a child. “In Italy, physics is a compulsory part of the high school curriculum. I decided I liked it and would study it at university,” she says. After an undergraduate degree at the world’s oldest university, Bologna, she did her PhD at Siegen, Germany, on the electroweak interaction.

The discovery that electromagnetism and the weak nuclear force are two aspects of the same phenomenon, known as electroweak, was the first step towards proving that the four fundamental interactions of nature are united. Under the conditions we experience today, the two interactions behave as very different phenomena. However, the two merge at energies above 100 GeV. Such energies existed shortly after the Big Bang, when the universe was more than 1015°K. Work showing that the two interactions can be linked was awarded the 1999 Nobel Physics Prize for Professors Gerardus ‘t Hooft and Martinus Veltman.

“The Higgs is very important for the electroweak research,” Barberio says. “I did a lot of things to test the standard model, and the Higgs was the last piece missing.” She describes most of her research as “flavour physics,” referring to the classification system for particles in the Standard Model.

Barberio did not find that the scale of the project to find the Higgs boson restricted her capacity to choose the direction for her own research. “I made the decision 20 years ago, when the Large Hadron Collider (LHC) was proposed, to design and build this experiment for it,” she says. While there were “the usual problems with getting funding,” she believes the need to coordinate with other aspects of the vast program did not uncomfortably limit her research.

The LHC was set up to send two beams of protons towards each other with a combined energy of 7 TeV. From the collisions it was hoped Higgs boson particles would appear before quickly decaying to lower energy particles.

One feature of the Higgs boson is that it decays very rapidly. Barberio’s work involved analysis of this decay to jets of hadrons, including tracking that allowed more detailed study of what was occurring in the high-energy particles being created and destroyed in the LHC.

“The most challenging part was to keep up with the rate the data was arriving. We were working 24/7 with very little sleep,” Barberio says. The 24/7 description does not apply to each individual, as the data took several years to collect and analyse.

In order to verify the results obtained, the scientists seeking the Higgs boson were split into two teams, Atlas and CMS, using parallel particle detectors to observe the same collisions. Barberio was part of the ATLAS team. If they were unable to keep up with the data coming in they risked being scooped by CMS and left out of the credit for the results. An agreement bound each team to the principle that, when discoveries were made, the other team would be alerted and, if they could find the same results within 2 weeks, publication would occur together. Otherwise, one team would publish on its own.

The two teams gradually narrowed the energies at which the Higgs boson might exist, producing overlapping ranges where it might lie until the value of 125 GeV became the focus of attention, with more and more data suggesting that something important was happening at that point. In 2012 both teams produced evidence that they had found the Higgs boson, although with slightly differing energies.

Although the peak excitement has passed, Barberio still has plenty to do. Now based at the University of Melbourne, she received the Australian Institute of Physics Women In Physics lectureship and is working to encourage girls to consider a career in the traditionally male-dominated field. “ATLAS was led for a long time by a woman, and women are doing some amazing research in this area,” she says.

Through a series of public lectures, some for girls only, she is conveying to the public the excitement that comes with being part of one of the great steps forward in human knowledge.