An interview with Stephen King, physicist

By Colin Adcock on December 1, 2015

Professor Stephen F King is a world renowned expert in neutrino physics, beyond the standard model physics and the early universe.

At the cutting edge of this Nobel Prize– and Breakthrough Prize– winning topic, his new open access review ‘Models of neutrino mass, mixing and CP violation‘ paints a positive outlook for the field. Let’s find out a bit more about the author and his research.

What research projects are you and your group currently working on?

We are working on various models of neutrino mass and mixing, ranging from low energy theories at the TeV scale, with experimental signatures at both high energy hadron colliders and and high luminosity lepton experiments, to high scale seesaw models which may provide the origin of matter-antimatter asymmetry in the Universe via so called “leptogenesis”. Our low scale approach postulates a doubly charged Higgs-like particle with a mass in the 100 GeV range, while our seesaw approach proposes a very simple but highly predictive scheme called the “Littlest Seesaw” leading to distinctive predictions for the next generation of high precision neutrino experiments.

The most ambitious seesaw models can lead to an understanding of the wider problem of the pattern of all quark and lepton masses, within some unified framework. We are exploring various unified gauge groups which can encompass the Standard Model gauge group, while at the same time introducing new discrete flavour symmetry groups such as A4 (the tetrahedral symmetry of the diamond lattice) which can help pin down the number of quark and lepton families and their mass spectrum. Such unified flavour models also incorporate the “littlest seesaw”.

We are also working on various Supersymmetric Models which can be tested at the Large Hadron Collider (LHC) at CERN. Some of these models are motivated by unified gauge theories with discrete flavour symmetry groups, leading to a characteristic spectrum of Supersymmetric particles which could be produced directly in Run2 at the LHC. It is also possible to observe the indirect effects of such particles by studying rare flavour changing processes at the LHC, or elsewhere, which we are also analysing.

We are particularly interested in Higgs extensions of the Standard Model which predict new phenomena associated with either deviations from the observed Higgs boson’s properties, or indeed additional Higgs bosons which have not been observed. Such extensions may or may not involve the idea of Supersymmetry and we are pursuing both possibilities, searching for extensions of the Standard Model which might show on in Run-2 of the LHC at CERN.

In addition we are also pursuing more theoretical studies of multi-Higgs boson theories, such as their implications for dark matter, and also the nature of the multi-Higgs potential when controlled by a discrete flavour symmetry.

Last but not least, we are also working on the ultimate unification of all forces on nature, including gravity, via F-theory and M-theory string and brane extensions to the Standard Model, investigating the features of such theories as the ultimate origin of Supersymmetric Grand Unified Theories including Flavour Symmetry

What motivated you to pursue this field of research?

The Standard Model, while highly successful, leaves many unanswered questions in its wake. For example the origin of neutrino mass is unknown. The answer to this may be related to the origin of flavour, namely why are there three families of quarks and leptons with the observed pattern of masses and mixings?

The mystery of flavour has been with us from the discovery of the muon in 1936, when (Nobel Laureate) Isidor Isaac Rabi famously asked of the muon “who ordered that?” to the discovery of neutrino mass and mixing in 1998, leading to the award of the 2015 Nobel Prize for “the discovery of neutrino oscillations which shows that neutrinos have mass”.

The Standard Model, extended to include neutrino mass, is described by at least 26 parameters, of which no less than 20 are flavour parameters: 10 from the quark sector and at least 10 from the lepton sector. At least two of these parameters are related to CP violation in the quark and lepton sectors, although the latter has not yet been definitively observed. This is surely too many parameters for a fundamental theory of nature!

The above considerations, together with the intrinsic desire for beauty and unification, going back to Maxwell’s work on unifying electricity and magnetism, suggests that there must some unified theory, beyond the Standard Model, which also provides a theory of flavour. Such a theory ideally should provide the cosmological asymmetry between matter and antimatter, as well as also providing a candidate for the dark matter particle which makes up a large fraction of the mass of the Universe.

The search for such an elegant and all-encompassing theory beyond the Standard Model is the driving motivation for all our research.

Where do you think the field is heading?

All of our research depends on experimental information provided by the current and planned generation of high energy and high luminosity experiments in particle physics, in particular the LHC and the neutrino experiments. While we believe we are making progress towards our goal of discovering the nature of whatever theory lies beyond the Standard Model, the only way we can check these ideas is by performing more and more accurate experiments capable of finding discrepancies with the Standard Model. We pin great hopes on the current Run2 of the LHC, but only time will tell if any disagreements with the Standard Model will emerge, either in the form of new particles discovered, such as Higgs or Supersymmetric particles, or by revealing cracks in the Standard Model edifice as observed in rare or flavour changing processes. Dark matter is currently being actively pursued by direct or indirect methods, but again we must wait to see if there are any signals. The neutrino experiments will be certain to measure the mass and mixing parameters ever more precisely enabling our theories to be confronted with the new precise data in the future. In addition the open questions in neutrino physics such as the neutrino mass ordering and hierarchy will be answered.

A common intriguing theme of many of our areas of research is that of charge conjugation C and parity P, which combined together as CP, are very accurately respected in particle physics. Indeed in the Standard Model CP is so accurately respected that it is impossible to understand the matter-antimatter asymmetry of the Universe. However if neutrino mass results from the seesaw mechanism, then there may be enough CP violation to generate the observed matter-antimatter asymmetry via “leptogenesis”. So far, CP violation has not been directly observed in neutrino oscillation experiments, but over the next decade, ever more precise neutrino experiments will enable CP violation to be revealed. Many theories of neutrino mass and mixing such as the Littlest Seesaw model give fairly precise predictions for such leptonic CP violation and those predictions will be put to the test.

So, in a nutshell, we live in exciting times where theory and experiment are working hand in hand in particle physics towards revealing the answers to the deep and pressing questions left unanswered by the Standard Model.

What do you consider to be the hot topics in physics at the moment?

CP violation, Dark Matter, LHC Collider and Higgs phenomenology and Neutrino Physics.

Inside the Super-Kamiokande detector.

What has been the most exciting development in physics during the course of your career?

The discovery of neutrino oscillations in 1998 which implies neutrino have mass and mixing. This was recently acknowledged by the 2015 Nobel Prize for Physics to Takaaki Kajita (Super-Kamiokande Collaboration, University of Tokyo, Japan) and to Arthur B. McDonald (Sudbury Neutrino Observatory Collaboration, Canada).

In 1998 Takaaki Kajita presented to the world the discovery that neutrinos produced in the atmosphere switch between two identities on their way to Earth. Arthur McDonald subsequently led the Canadian collaboration which demonstrated that neutrinos from the sun do not disappear on their way to Earth, but change identity by the time of arrival to the SNO detector.

Takaaki Kajita is collaborator and the scientist in charge of the University of Tokyo node of our European ITN network “Invisibles: Neutrinos, Dark Matter and Dark Energy”, which has neutrino oscillations as one of its major research lines. This network is coordinated by the team of the Universidad Autónoma de Madrid and it includes 29 European and extra-EU nodes, which includes the BSM sub-group of SHEP in Physics and Astronomy at the University of Southampton. Recently two new EU Networks have been announced namely the ELUSIVES ITN and InvisiblesPlus RISE network which will continue this line of research from where the Invisibles network leaves off in April 2016.

What do you find to be the most rewarding aspect of your job?

I enjoy both the possibility to perform independent research and also teaching the next generation of students.

And finally, tell us an interesting fact about yourself.

I am married with two sons who are both presently studying for PhDs in physics.

Read more works from Stephen King:

Review: “Models of neutrino mass, mixing and CP violation”

Paper: “Testing constrained sequential dominance models of neutrinos”

Paper: “Discrete Symmetries and Models of Flavour Mixing”

Review: “Neutrino Mass and Mixing: from Theory to Experiment” [not open access]

Review: “Neutrino Mass and Mixing with Discrete Symmetry”

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