The discovery of the Higgs boson in 2012 was made possible due to the involvement of physicists 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 particle

Your research interests lie in high-energy matter-antimatter, matter-matter particle colliders and multipurpose detector technologies. How did you become involved in this area of study and what continues to fascinate you?

I have always wanted to understand how things work, from the bottom up. High-energy particle colliders, and the detectors that we build around the collision interaction regions, have helped us to progress in this understanding of Nature. I’m fascinated by the links to the bigger picture, the relationships between the subatomic particles and the story of our Universe. When we collide particles in our accelerators, we synthesise and observe not only pieces of ordinary matter but also new short-lived variants that only existed naturally during the first few moments after the Big Bang. Since I began working in this field, observations in cosmology and astrophysics have informed us that the explained, or observed, fractions of the energy and matter composition of our Universe have in fact decreased. As a physicist, I find this both humbling and inspiring.

The Canadian CDF II group that you lead has recently been involved in the search for the Higgs boson. Could you outline the group’s role within the Higgs project, citing the core aims and objectives of its work?

The Canadian CDF II group worked with colleagues at other collaborating institutions to develop innovative techniques to sift through nearly a decade’s worth of collision data with

the objective of establishing evidence for the predicted but elusive Higgs boson particle. Unlike the Large Hadron Collider at CERN, which collides matter against matter, we were able to exploit the unique strengths of the matter-antimatter collisions in the Fermilab Tevatron to search for Higgs bosons 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 particles. Our team was specifically searching for Higgs boson candidates that disintegrated into particles called bottom (or beauty) quarks.

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

It was truly beautiful to see that the most probable Higgs mass values measured in the Fermilab evidence were consistent with those in the CERN experiments’ observations. I was at the International Conference on High Energy Physics in Melbourne, Australia when the CERN discovery results were announced. Never have I seen so many physicists expressing such elation at the same time and place. My first thoughts were centred on the realisation that we had definitely discovered a new boson, but was it truly the predicted Higgs particle?

How are you communicating the meaning of the Higgs to the scientific community at large and the public?

As a scientist involved in the Higgs discovery, I consider it essential that I devote a portion of my efforts to interpreting for the public the importance and value of this new knowledge that we are now uncovering. Unlike advances in, for example, medicine or astronomy, which have obvious connections to the human condition and imagination, explaining to the public the profundity of our work with subatomic particles can be a challenge. Social media has played a useful role in my efforts, as have some blog articles.

For you personally, where do you see your research progressing in the future? What are your aspirations?

Notwithstanding the technological spinoffs and the wealth of understanding about the subatomic world that our field has cultivated over the past century, the fact that this only

explains a few per cent of the matter-energy content of the Universe means that there is much necessary exploration ahead. Quarks, which so far appear to be fundamental, may in fact have substructure and be constituted from tinier components. Gravity, which seems so important in our everyday human lives, is very poorly understood at the subatomic level. I am working with colleagues on the ATLAS experiment at CERN to search for new phenomena not currently explained by the Standard Model of particle physics, particles like excited quarks and quantum black holes.

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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 development of science is the notion that much can be learnt about the world around us by studying and understanding its fundamental constituents – the basic building blocks. To examine matter at the very smallest length scales, scientists need to work at extremely high energy densities, hence the requirement for powerful accelerators such as the Large Hadron Collider at the CERN laboratory in Geneva, which powered the collisions behind the discovery of the Higgs boson particle.

SEEKING THE HIGGS PARTICLE

The search for the Higgs boson was supported by an international effort, and one of the collaborative projects which developed novel techniques for investigating Higgs particles was the Collider Detector at Fermilab (CDF) II experiment in the US. One of two large detectors situated on the proton-antiproton Tevatron collider at the Fermi National Accelerator Laboratory (Fermilab) in Illinois, the CDF-II experiment observed proton-antiproton collisions for nearly a decade, until late 2011. Since then, the team has been analysing the dataset collected to complete the CDF-II physics programme. The collaboration currently comprises about 450 physicists from 57 institutions in 14 countries in North America, Asia and Europe.

Since 2007, Dr Andreas Warburton has led the Canadian part of the CDF collaboration, and his team contributed directly to finding initial evidence for Higgs bosons prior to the definitive discovery made in Europe. Once a collider is capable of creating collisions with enough energy to produce Higgs particles there are two main obstacles that remain. First, the Higgs boson only

becomes synthesised in an extremely small fraction of collisions, and secondly, in those

rare cases where a Higgs particle is created, the detector’s recording of the collision

event is only subtly different from that of the majority of the recorded events.

Due to these two factors combined, Warburton’s team was confronted

with a minuscule signal-to-background ratio, which

they improved markedly by taking advantage of

certain characteristic features of the

companion W boson’s discriminating traces left in the CDF-II detector. “The rarity of Higgs boson production also motivated us to combine several data samples, collected over many years of Tevatron collider operation,” Warburton explains. “The criteria used to trigger the detector to record potential Higgs events varied significantly during this extended period. Our Canadian group led the invention and development of a novel technique to combine these differently triggered datasets, thereby significantly enhancing our reach to find evidence for Higgs particles.”

DISCOVERING THE HIGGS PARTICLE

Several complementary analyses, which included those carried out by Warburton’s team, were combined with analyses from the wider CDF-II collaboration to enhance the statistical power of the findings. The CDF results were then, in turn, statistically combined with results from the CDF’s sister Tevatron collider experiment, the DZero collaboration. It was in this Tevatron-wide combination that the first published evidence for Higgs production, and subsequent decay to bottom quarks, was established. All of the current Canadian CDF-II collaborators are also members of the 3,000 physicist-strong ATLAS detector experiment on the Large Hadron Collider at CERN, which announced the unequivocal discovery of a new boson particle on 4 July 2012, but using collision events where the Higgs candidates disintegrated into daughter particles different from the Tevatron’s bottom-quark results.

Although Professor Peter Higgs predicted the existence of the particle in 1964, for a long period scientists did not know the potential mass ranges within which the Higgs particle could lie, thus a good deal of searching was necessary. “In this kind of energy-frontier particle physics, whenever you build a higher-energy accelerator, the time investment is significant before you can use those higher energies to search for new phenomena,” Warburton reflects. “There are not only the technical and financial challenges in building higher-energy facilities, but also the extreme rarity of creating Higgs particles compared with the abundance of collisions producing phenomena that are already well understood.” It is only in recent years that accelerator and computing technologies have advanced enough to enable the energies and high rates of collisions necessary, yet still

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DR ANDREAS WARBURTON

There are strong theoretical

arguments suggesting that there

may in fact be more than one kind

of Higgs particle, so this is by no

means the end of the Higgs searchWorking in the ATLAS cavern in January 2011 ATLAS Experiment © 2013 CERN.

THE COLLIDER DETECTOR AT FERMILAB (CDF) II EXPERIMENT

OBJECTIVES

The CDF II experiment is enabling the study of the highest energy proton-antiproton collisions at a centre-of-mass energy near 2 TeV. The CDF collaboration consists of scientists from 14 countries, including a group of Canadian physicists affiliated with the Institute of Particle Physics (IPP).

KEY COLLABORATORS

Dr Adrian Buzatu, University of Glasgow, UK

Dr Nils Krumnack, Iowa State University, USA

Dr Wei-Ming Yao, Lawrence Berkeley National Laboratory, USA

PARTNERS

Institute of Particle Physics (IPP), Canada • University of Alberta • Simon Fraser University • University of Toronto • TRIUMF, Canada’s National Laboratory for Particle and Nuclear Physics

FUNDING

Natural Sciences and Engineering Research Council of Canada (NSERC)

Universities Research Association, Inc. (URA), USA

CONTACT

Dr Andreas Warburton Associate Professor

Department of Physics McGill University Rutherford Physics Building 3600 rue University Montréal, Québec H3A 2T8, Canada

T +1 514 398 6519 E awarburt@physics.mcgill.ca

www.physics.mcgill.ca/~awarburt

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 top quark and Higgs boson discoveries at Fermilab and CERN.

have the capacity to perform the computing-intensive data analyses on all the recorded collision events.

The CDF II collaboration is now completing its final analyses of the full data sample. Deeper studies into the properties of the newly discovered boson have only just begun to be conducted within the CERN experiments. Many years of further data collection, as well as a new matter-antimatter collider, will be required in order to establish an adequate understanding of the particle’s behaviours, including the different ways it can disintegrate. “There are also strong theoretical arguments suggesting that there may in fact be more than one kind of Higgs particle, so this is by no means the end of our Higgs search,” Warburton adds.

INTERPRETING THE HIGGS PARTICLE

As a scientist involved in the Higgs boson discovery, a key aspect for Warburton has been outreach to and education of the public. One way he has communicated with the public has been using social media to disseminate the results, for example by live-tweeting the definitive discovery announcements from Melbourne, Australia. He also recently visited an NGO school for disadvantaged children in rural India where he described the discovery using props such as a pomegranate fruit to represent the Higgs boson and stones from the

banks of the Ganges River to represent protons and heavy quarks.

Warburton explains that the Higgs boson is more than just another subatomic particle to add to the plethora of known fundamental forms of matter: “If the theory is shown to be correct, then Higgs bosons are the quantum carriers of a Higgs field that pervades all space in the Universe,” he elucidates. “Theoretically, the idea for this field was conceived because, when you include it in the equations describing the fundamental particles and their force interactions, it provides a natural and elegant way of explaining why the weak nuclear force is so different from the electromagnetic force.” A useful side effect of this is that all fundamental subatomic particles with non-zero masses can, in principle, be endowed with their characteristic masses based on their degree of interaction with the Higgs field. Without such a mechanism in effect, the Standard Model of particle physics predicts that all particles would be massless, which in turn means that they would be moving at the speed of light. “If all the fundamental particles were massless, the Universe as we know it would not have evolved to its current state. It turns out that we owe our very existence to the Higgs field. Our scientific curiosity has compelled us to search for the marker of this field, the Higgs boson,” Warburton enthuses.

Higgs discovery lecture, Purkal Youth Development Society, Dehradun, Uttarakhand, India.

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INTELLIGENCE