CERN Accelerating science

Interview with Jon Butterworth
by Panos Charitos (CERN)

Prof Jon Butterworth (University College London) is Head of the UCL Physics Department and member of the ATLAS Collaboration (Image:

Panos Charitos of Accelerating News sat down with John Butterworth, Head of the Physics Department at University College London (UCL) and author of the book “Smashing Physics: The Inside Story of the Hunt for the Higgs” to discuss his work. We covered his involvement in one of the most important physics discoveries, the present landscape in high-energy physics and the plans for future colliders and ongoing R&D efforts that inspire technological innovation and could lead to ground-breaking science in the course of this century.

PC: What is your view on the latest results from the LHC and other experiments presented earlier this summer in ICHEP16 ?

JB: From the point of view of the experimentalist, the LHC has done an incredible work offering a significant leap in the energy scale. The fact that the 750 GeV bump was not confirmed caused some disappointment but this doesn’t mean that our search for new physics came to an end as we have just started scratching the surface.

Perhaps one could compare the situation with the first flight over a newly discovered island, where new physics may lie. We first fly at 30,000 ft., which is what we did in 2015, and then at 10,000 ft., where we may see signs of a new civilization. However, discovering nothing unexpected does not mean that there is no new physics on the ground. We just have to land carefully and explore the territory in detail.

On the one hand, it would be great to have a breakthrough discovery announced at ICHEP 2016, but on the other hand, the fact that the accelerator and detectors are doing so well means that we experimentalists have a lot of work to do.

It seems strange that nothing has appeared yet, but the next discovery may be just around the corner and there might be something to discover in higher energies. I would like to see the theoretical net cast a little wider. In any case, however, I am looking forward to the next three years for more data with higher precision.

PC: Do we need a new way of interpreting experimental results given the success of the Standard Model?

JB: Presently we experience a strange situation, because the Standard Model of particle physics — so complete and consistent that every calculation fits new data with remarkable accuracy not to mention the fantastic success of the Higgs discovery— leaves a number of questions open. It does not explain dark matter nor what caused the observed matter–antimatter asymmetry; both are fundamental problems that challenge our present understanding of nature.

In other words, the more we look closely at the Standard Model, the more surprised we are at its success. Looking at the latest results, I think that a large part of the motivation for theories postulating new physics tied to electroweak symmetry breaking is becoming slightly less attractive.

So to answer your question, I think that there might be more to it than we thought and maybe approaching it from a different angle will reveal answers to some of the open questions Maybe the Standard Model is even more wonderful than it appears. 

PC: To which extent should the concept of naturalness continue inform our research?

JB: We know that at the LHC energies special things happen in physics. The force carriers of the weak interaction – W and Z bosons – have masses in this energy range and we have discovered a Higgs boson with mass lying in this energy range.

However, from our theory, the Higgs mass gets lots of big quantum corrections, positive and negative, which cancel each other out in an apparently miraculous way for the Higgs mass to be where we see it. The exact cancelation of terms seems a bit strange to be merely a coincidence of the model. . In this context, naturalness is the assumption that the parameters in a theory should be about unity, and should not have to be fantastically fine-tuned in order to make the theory work.

Supersymmetry tries to answer this question by avoiding the concept of fine-tuning. It does so by introducing a new particle for every existing one, with the opposite sign thus accounting of all these cancellations that we observe. However, though it is conceptually a beautiful theory there is yet a lack of experimental evidence to confirm it.

The concept of naturalness boils down to the so-called Hierarchy problem and is related to the fact that we have different hierarchy scales: the QCD scale, the electroweak scale and the Planck scale at very high energies. The electroweak scale is closely linked to the mass of the Higgs boson but we still don’t know why the Higgs boson has a mass at this energy scale and how to deal with the quantum corrections predicted by the theory. Theories like supersymmetry are introduced to cancel those corrections and thus make it more natural to have this mass. Usually a lower than expected energy scale for the mass of a particle, as in the case of pion mass, is due to an approximate symmetry. In the case of the electroweak scale the approximate symmetry would be supersymmetry that fine-tunes the Higgs mass to where we see it.

To conclude, naturalness presents an interesting problem in modern physics which becomes very pressing in light of recent LHC data. The motivation for and significance of naturalness in quantum field theory is a hotly contested topic that we need to rethink. A concept which I think may evolve – rather than guide- as we get more data from the LHC and other experiments. On a personal note, I think that we have other reasons to believe that the Standard Model is not the whole story, with dark matter being one of the main motivations for future research.

PC: How important is our understanding of gravity for answering some of the open questions?

JB: Presently the best theory we have for the description of gravitiy is the General relativity which explains the geometry and development of the universe on macroscopic scales. Quantum field theory, in the Standard Model of particle physics, describes the other three fundamental forces and describes the universe of the very small.

However, at very high energies their spheres of applicability - the very large and the very small - overlap, and the theories conflict. Both cannot be valid and it seems that we still lack a more profound understanding.

 We face a great anomaly which is the absence of any treatment of gravity on the same footing as the other forces. There is a hierarchy problem of gravity being so ridiculously weak compared to the other forces while the same applies to the masses of particles like neutrinos that are extremely small compared to other particles. These two apparent unrelated observations may be linked and could mark a radical shift in our understanding of nature as well as to rethinking or rephrasing some of the so-called open questions.

PC: How could we decide about the next step in particle physics research?

JB: We need to understand the scale at which new physics may exist. Before committing my scientific career, I would like to know that there is an energy scale after which physics is not the same. In the case of the LHC — although there are still many ongoing searches — we knew that it could answer whether the Standard Model Higgs boson exists. We need a similarly well-posed question about the new leap in energy.

In the meantime I think is important to work on R&D to make future high-energy accelerators cost-effective, as well as diversify our experiments until we find a clue of new physics and think how we could probe it. I hope that this would be within the reach of a 100 TeV machine and I would love to work towards this direction to explore the physics options present by such a machine. However,  I think we still have to learn more from the LHC, as well as from some precision experiments and from astrophysics as well.

PC: Do you think that maybe we should also reconsider the speculative character of science?

JB: I never believed that there is a hard divide between exploratory and theoretically driven science. I think any good large-scale project would be based on a mix of the two. We had a huge theoretical motivation with the Higgs at the LHC, but we also pursued, and still pursue, an exploratory aspect. One of my favourite plots is the charge current and neutral current cross section in Deep Inelastic Scatter from HERA. You could see the weak and electromagnetic forces coming together around 100 GeV — that is a real change in high energy physics that we knew that the LHC could probe. This is motivated partly by theory and partly by experiment.

The bigger and longer-term a project is, the stronger its motivation has to be. For a small project you can take a long-shot and come up with a high-reward, high-risk plan. There is, however, a trade-off between doing a large number of these experiments and constructing a large accelerator, since resources, including physicists who can work on such projects, are not infinite. This balance of large and small experiments should be examined case by case given also the long lead times for these projects.

Finally, one should bear in mind that we live in a kind of ecosystem in which is important to advance our R&D efforts for new technologies. New developments have a strong impact, even if not directly applied to fundamental physics, including the development of new accelerators, high-field magnets and fast computing needed to process data from future detectors.

PC: Do you think that nowadays there is a strong complementarity between research in HEP and in astrophysics?

JB: I am chair of a department that is home to a very strong astrophysics and cosmology group. I found their combination of theoretical motivation and exploratory driven science very interesting. Much of astronomy is pure exploration — going to Pluto is not about fundamental physics but about investigating the solar system. Of course, studying cosmology and trying to understand dark matter or dark energy and how the Universe evolved is closely linked to the fundamental questions that particle physics tries to answer. Some of our undergraduate students found an exoplanet, and another group found a supernova. I slightly envy them. It might not be a fundamental breakthrough in the theory of supernovae but they discovered something new, that lies out there.

PC: Finally, I would like to discuss your motivation to communicate science and what is the personal reward.

JB: I have always enjoyed writing something other than a scientific paper. As a field, being able to explain our work to a non-scientific audience is just as important as publishing in peer-reviewed journals, in my opinion – though not everyone has to do both! We live in a complex society and people often cannot understand and differentiate between fiction and fact. As our lives are heavily based on science and technology, we need scientists to engage with society and discuss their work with the people. Not to mention that it can be very fun as well.