Autumn 2012 Edition
UCD Search
Published: 12 September 2012
The hunt for the Higgs
In July, CERN in Geneva announced a landmark scientific discovery: evidence for a particle consistent with the long-sought Higgs boson. Claire O'Connell (BSc, PhD) finds out more from Dr Ronan McNulty about UCD's role in the hunt for the Higgs.
On the morning of July 4th this year, the eyes of the world were on Geneva. There was going to be a big announcement from the research organisation CERN and the anticipation was high. Over previous months, CERN had been narrowing the search for a type of fundamental particle in physics called the Higgs particle. The quest to find this particle - a missing piece in the model of physics that describes matter in the universe - had been going on for decades. Had they discovered something? And if so, what would it mean?
The Higgs boson, which was proposed by British scientist Peter Higgs and others around 50 years ago, is one of 17 fundamental particles that everything in our universe can be decomposed into, explains Dr Ronan McNulty, a senior lecturer at UCD School of Physics and a visiting professor at the University of Liverpool.
Event recorded with the CMS detector in 2012 at a proton-proton centre of mass energy of 8 TeV. The event shows characteristics expected from the decay of the SM Higgs boson to a pair of Z bosons, one of which subsequently decays to a pair of electrons (green lines and green towers) and the other Z decays to a pair of muons (red lines). The event could also be due to known standard model background processes. © Image, courtesy of CERN
"At that time, theoretical physics was at an impasse," he says. "It could successfully describe all of electricity, magnetism and radioactivity, but only if the particles involved were massless. Higgs and others came up with an ingenious mechanism by which mass could be introduced into the theory."
Developing the theory was one major advance, but what about finding evidence for it? A Higgs boson would be a relatively large sub-atomic particle, and the massive scale of energy needed to produce it existed about one billionth of a second after the Big Bang, more than 13.7 billion years ago. So to make a Higgs, you would effectively have to reproduce the Big Bang in an environment where you can watch what happens. Enter the Large Hadron Collider, an approximately 27km-long ring-shaped underground track where beams of protons can be smashed together at speed.
"The LHC fires two protons together at high energies and this energy turns into mass," explains Dr McNulty. "Sometimes a Higgs boson is produced and this decays instantaneously into other particles which we can detect. By reconstructing the decay fragments, we can infer the presence of the Higgs."
So what did CERN have to tell the world in July? People queued through the night to stake a place in the small auditorium where the announcement was scheduled for the next morning. Then, as the time came for the news, viewers from around the world were watched over the Internet.
What greeted them didn't disappoint: results from two experiments at the Large Hadron Collider - ATLAS and CMS - had each pinpointed a particle consistent with the predicted Higgs. Present at the conference was Peter Higgs himself, and he was moved to tears.
"This is probably the most important physics discovery for 80 years; the last time we had an event of such magnitude was the discovery of anti-matter," says Dr McNulty. "It is the culmination of 50 years of experimental searching which has involved thousands of scientists throughout the world, including Ireland."
Dr Ronan McNulty
Dr McNulty's group made an important contribution to the search for the Higgs through their work on the 'LHCb' experiment at CERN, one of the four experiments that o perate on the LHC collider. The UCD group made a particular breakthrough by analysing W and Z bosons, which are heavy, like the Higgs.
"A careful investigation of the properties of the W and Z, such as we made in 2008, is a window onto the Higgs," he explains. "Of even greater importance is that fact that when the Higgs boson decays, it often produces W and Z bosons. Therefore, being able to reconstruct W and Z bosons is a necessary first step to being able to reconstruct the Higgs."
Higgs bosons are around 10,000 times rarer than W and Z bosons, but as the UCD researchers work through the information from the LHCb experiment, Dr McNulty is hopeful they will also see a hint of the presence of the Higgs there. "Seeing the Higgs in LHCb too is very important, because it gives us more information about the properties of the Higgs and how it behaves."
He describes how the next few years are going to be "incredibly exciting" as researchers figure out the nature of the Higgs: "Many people expect that the Higgs will reveal much deeper truths about the nature of the universe - it may be the product of supersymmetry for example. To answer these questions we must gather as many Higgs bosons as possible and see how they behave."
UCD will play its part through ongoing work on the LHCb experiment.
And more generally, Dr McNulty notes how the 'hunt for the Higgs' has captured the imagination of the public.
"I believe it sends an inspirational message that science is fascinating, fun, and makes the world a better place," he says. "And beyond the noble aim of understanding the very nature of our universe, we reap the rewards of this curiosity in technology which gives us new medical detectors like PET scans and hadron [anti-cancer] therapy, more efficient electricity generation or a new social order [CERN played a role in the development of the worldwide web]." Indeed, in a case of spectacular timing, during the Euroscience Open Forum (ESOF2012) this summer the Royal Irish Academy hosted a 'Higgs master class' a mere 10 days after the CERN announcement. Dr McNulty was a driving force behind the event, which brought some of CERN's top scientists and engineers to Dawson Street to talk to a packed and diverse audience.
Dr McNulty has long been an advocate of Ireland joining CERN, and he hopes the recent announcement about the Higgs and new changes in membership subscriptions could help to make that a reality.
"CERN has recently introduced a partial membership scheme where you can pay a proportion of the full subscription and receive a portion of the benefits," he says. "This greatly lowers the bar and means Ireland could be involved for about €1m a year."
Ronan McNulty was in conversation with freelance journalist Claire O'Connell (BSc 1992, PhD 1998))
Produced by UCD University Relations
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