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Captivating collisions

The discovery of the Higgs in 2012 was made possible due to the involvement of across the world. Dr Andreas Warburton and his group played their part by developing a novel technique that was used to search for the infamous

the objective of establishing evidence for the predicted but elusive particle. Unlike the Large at CERN, which collides against matter, we were

DR ANDREAS WARBURTON able to exploit the unique strengths of the matter- collisions in the to search for Higgs produced along with a companion particle called a W boson. Once a Higgs boson was created in the collider, it only survived for a fraction of a second before disintegrating to lighter . Our team was specifically searching for Higgs boson candidates that disintegrated into particles called bottom (or beauty) .

As a scientist involved in the discovery of the Higgs, what was the feeling amongst the community at the ?

Your research interests lie in high- It was truly beautiful to see that the most matter-antimatter, matter-matter particle probable Higgs values measured in the and multipurpose detector Fermilab evidence were consistent with those technologies. How did you become involved in the CERN experiments’ . I was in this area of study and what continues to at the International Conference on High Energy fascinate you? Physics in Melbourne, Australia when the CERN discovery results were announced. Never have I have always wanted to understand how I seen so many physicists expressing such things work, from the bottom up. High- elation at the same time and place. My first energy particle colliders, and the detectors thoughts were centred on the realisation that that we build around the collision interaction we had definitely discovered a new boson, but regions, have helped us to progress in this was it truly the predicted Higgs particle? understanding of . I’m fascinated by the links to the bigger picture, the relationships How are you communicating the meaning between the subatomic particles and the story of the Higgs to the scientific community at of our . When we collide particles in large and the public? our accelerators, we synthesise and observe not only pieces of ordinary matter but also new As a scientist involved in the Higgs discovery, short-lived variants that only existed naturally I consider it essential that I devote a portion during the first few moments after the Big of my efforts to interpreting for the public the Bang. Since I began working in this , importance and value of this new knowledge observations in cosmology and astrophysics that we are now uncovering. Unlike advances have informed us that the explained, or in, for example, medicine or astronomy, which observed, fractions of the energy and matter have obvious connections to the human composition of our Universe have in fact condition and imagination, explaining to decreased. As a , I find this both the public the profundity of our work with explains a few per cent of the matter-energy humbling and inspiring. subatomic particles can be a challenge. Social content of the Universe means that there is media has played a useful role in my efforts, as much necessary exploration ahead. Quarks, The Canadian CDF II group that you has have some blog articles. which so far appear to be fundamental, may recently been involved in the search for the in fact have substructure and be constituted Higgs boson. Could you outline the group’s For you personally, where do you see your from tinier components. , which seems role within the Higgs project, citing the core research progressing in the future? What are so important in our everyday human lives, aims and objectives of its work? your aspirations? is very poorly understood at the subatomic level. I am working with colleagues on the The Canadian CDF II group worked with Notwithstanding the technological spinoffs ATLAS experiment at CERN to search for new colleagues at other collaborating institutions to and the wealth of understanding about the phenomena not currently explained by the develop innovative techniques to sift through subatomic world that our field has cultivated of , particles nearly a decade’s worth of collision data with over the past century, the fact that this only like excited quarks and black holes.

48 INTERNATIONAL INNOVATION DR ANDREAS WARBURTON The hunt for the Higgs particle

As key contributors to the search for the Higgs boson particle, scientists working with the Collider Detector at Fermilab II experiment based at the Fermi National Accelerator Laboratory in Illinois have been influential in seeking, discovering and interpreting Higgs bosons

AN IMPORTANT ASPECT of humanity’s companion W boson’s discriminating traces left development of is the notion that much in the CDF-II detector. “The rarity of Higgs boson can be learnt about the world around us by production also motivated us to combine several studying and understanding its fundamental data samples, collected over many years of constituents – the basic building blocks. To Tevatron collider operation,” Warburton explains. examine matter at the very smallest length “The criteria used to trigger the detector to record scales, scientists need to work at extremely potential Higgs events varied significantly during high energy densities, hence the requirement this extended period. Our Canadian group led the for powerful accelerators such as the Large invention and development of a novel technique Hadron Collider at the CERN laboratory in to combine these differently triggered datasets, , which powered the collisions behind thereby significantly enhancing our reach to find the discovery of the Higgs boson particle. evidence for Higgs particles.”

SEEKING THE HIGGS PARTICLE DISCOVERING THE HIGGS PARTICLE The search for the Higgs boson was supported Several complementary analyses, which by an international effort, and one of the included those carried out by Warburton’s collaborative projects which developed novel team, were combined with analyses from the techniques for investigating Higgs particles wider CDF-II collaboration to enhance the was the Collider Detector at Fermilab (CDF) statistical power of the findings. The CDF results II experiment in the US. One of two large were then, in turn, statistically combined with detectors situated on the - results from the CDF’s sister Tevatron collider Tevatron collider at the Fermi National experiment, the DZero collaboration. It was Accelerator Laboratory (Fermilab) in Illinois, in this Tevatron-wide combination that the the CDF-II experiment observed proton- first published evidence for Higgs production, antiproton collisions for nearly a decade, and subsequent decay to bottom quarks, was until late 2011. Since then, the team has been established. All of the current Canadian CDF-II analysing the dataset collected to complete the collaborators are also members of the 3,000 CDF-II physics programme. The collaboration physicist-strong ATLAS detector experiment currently comprises about 450 physicists from on the at CERN, which 57 institutions in 14 countries in North America, announced the unequivocal discovery of a Asia and Europe. new boson particle on 4 July 2012, but using collision events where the Higgs candidates Since 2007, Dr Andreas Warburton has led the disintegrated into daughter particles different Canadian part of the CDF collaboration, and from the Tevatron’s bottom- results. his team contributed directly to finding initial evidence for Higgs bosons prior to the definitive Although Professor predicted the discovery made in Europe. Once a collider is existence of the particle in 1964, for a long capable of creating collisions with enough energy period scientists did not know the potential mass to produce Higgs particles there are two main ranges within which the Higgs particle could lie, obstacles that remain. First, the Higgs boson only thus a good deal of searching was necessary. becomes synthesised in an extremely small “In this kind of energy-frontier particle physics, fraction of collisions, and secondly, in those whenever you build a higher-energy accelerator, rare cases where a Higgs particle is created, the time investment is significant before you the detector’s recording of the collision can use those higher to search for new is only subtly different from that phenomena,” Warburton reflects. “There are of the majority of the recorded events. not only the technical and financial challenges Due to these two factors combined, in building higher-energy facilities, but also Warburton’s team was confronted the extreme rarity of creating Higgs particles with a minuscule signal-to- compared with the abundance of collisions background ratio, which producing phenomena that are already well they improved markedly understood.” It is only in recent years that by taking advantage of accelerator and computing technologies have certain characteristic advanced enough to enable the energies and features of the high rates of collisions necessary, yet still ATLAS EXPERIMENT © 2013 CERN WWW.RESEARCHMEDIA.EU 49 INTELLIGENCE have the capacity to perform the computing- Higgs discovery lecture, Purkal Youth Development intensive data analyses on all the recorded Society, Dehradun, Uttarakhand, India. THE COLLIDER DETECTOR AT FERMILAB collision events. (CDF) II EXPERIMENT OBJECTIVES The CDF II collaboration is now completing its final analyses of the full data sample. Deeper The CDF II experiment is enabling the study of studies into the properties of the newly the highest energy proton-antiproton collisions discovered boson have only just begun to be at a centre-of-mass energy near 2 TeV. The CDF collaboration consists of scientists from conducted within the CERN experiments. Many 14 countries, including a group of Canadian years of further data collection, as well as a new physicists affiliated with the Institute of Particle matter-antimatter collider, will be required in Physics (IPP). order to establish an adequate understanding of the particle’s behaviours, including the different KEY COLLABORATORS ways it can disintegrate. “There are also strong Dr Adrian Buzatu, University of Glasgow, UK theoretical arguments suggesting that there banks of the Ganges River to represent may in fact be more than one kind of Higgs and heavy quarks. Dr Nils Krumnack, Iowa State University, USA particle, so this is by no means the end of our Dr Wei-Ming Yao, Lawrence Berkeley National Higgs search,” Warburton adds. Warburton explains that the Higgs boson is Laboratory, USA more than just another to add to the plethora of known fundamental INTERPRETING THE HIGGS PARTICLE PARTNERS forms of matter: “If the theory is shown to be Institute of Particle Physics (IPP), Canada As a scientist involved in the Higgs boson correct, then Higgs bosons are the quantum • University of Alberta • Simon Fraser discovery, a key aspect for Warburton has been carriers of a Higgs field that pervades all space University • University of Toronto • TRIUMF, outreach to and education of the public. One in the Universe,” he elucidates. “Theoretically, Canada’s National Laboratory for Particle and way he has communicated with the public the idea for this field was conceived because, has been using to disseminate when you include it in the equations describing the results, for example by live-tweeting the the fundamental particles and their FUNDING definitive discovery announcements from interactions, it provides a natural and elegant Natural and Engineering Research Melbourne, Australia. He also recently visited way of explaining why the weak nuclear Council of Canada (NSERC) an NGO school for disadvantaged children in force is so different from the electromagnetic rural India where he described the discovery force.” A useful side effect of this is that all Universities Research Association, Inc. (URA), using props such as a pomegranate fruit to fundamental subatomic particles with non-zero USA represent the Higgs boson and stones from the can, in principle, be endowed with their CONTACT characteristic masses based on their degree of interaction with the Higgs field. Without such Dr Andreas Warburton a mechanism in effect, the Standard Model Associate Professor of particle physics predicts that all particles Department of Physics would be massless, which in turn means that McGill University they would be moving at the . “If Rutherford Physics Building all the fundamental particles were massless, 3600 rue University the Universe as we know it would not have Montréal, Québec H3A 2T8, Canada evolved to its current state. It turns out that we T +1 514 398 6519 owe our very existence to the Higgs field. Our E [email protected] scientific curiosity has compelled us to search for the marker of this field, the Higgs boson,” www.physics.mcgill.ca/~awarburt Warburton enthuses.

ANDREAS WARBURTON is an Associate Professor of Experimental High-Energy Subatomic Particle Physics at McGill University with a BSc from the University of Victoria, a PhD from the University of Toronto, and postdoctoral work at Cornell University. He has collaborated on the ATLAS, CDF, and CLEO experiments and is co-author of the and Higgs boson discoveries at Fermilab and CERN.

There are strong theoretical arguments suggesting that there may in fact be more than one kind of Higgs particle, so this is by no Working in the ATLAS cavern in January 2011 ATLAS Experiment © 2013 CERN. means the end of the Higgs search 50 INTERNATIONAL INNOVATION