A roller-coaster ride
6 June 2022
I’m a researcher in the Institute of Particle and Nuclear Studies at KEK, Japan. I was born and brought up in the UK, but if you go back a few generations many of my roots spread across central Europe. I remember always being interested in science – I come from a pretty scientific family, and was regularly shown things through microscopes – and got interested in physics during high school, thanks to a combination of several inspiring teachers and my inability – or probably rather unwillingness – to memorise too many facts, names, or reactions. At university my undergraduate advisor was a particle physicist, and I suppose his enthusiasm for particles somehow infused into me.
When I started as a doctoral student, around the turn of the century, I chose to do my research as part of the DELPHI collaboration, which operated a detector at the LEP collider at CERN. I was fortunate to spend two years stationed at CERN, where I enjoyed the international “CERN spirit” and the excitement of data flowing from the detector. The thrill of being deep underground, climbing around the detector to check on “my” muon detectors while particles were colliding just a few metres away stays with me to this day. Precision measurements being made at LEP were telling us that the Higgs boson was just around the corner, with a mass close to the limit of what was in reach at LEP. The accelerator physicists were pushing the energy as high as possible, to give us the best chance to glimpse this Higgs boson. My research was on W bosons, which are closely intertwined with the Higgs, so I was watching closely. As new candidate Higgs events arrived in the various detectors, rumours would spread like wildfire through the coffee rooms and cafeteria across the CERN site!
In hindsight it turned out that LEP did not have sufficient energy to produce Higgs bosons, and that these candidate events were actually due to other processes… So although LEP had over many years made fantastic precision measurements of many processes, it felt pretty sad and anti-climactic when LEP was turned off for the last time without having discovered the Higgs, in order to make space for installation of the Large Hadron Collider.
As a newly minted doctor in physics, it was time to look for a new experiment, maybe also a new environment? I settled on Italy’s INFN section in Rome (Sapienza), and the CDF experiment at Fermilab. I’d never visited Italy before, but it looked nice and I got on well with the many Italians in DELPHI! I remember the shock of summer heat as I stepped onto the tarmac. The Tevatron collider was in full swing, producing copious top quarks which were being used to make ever more precise measurements of this heaviest SM particle. I got involved in developing tools for the identification of b-jets, which are produced in almost all top quark decays, and were also expected to dominate the decays of the Higgs boson. It turns out that during my 5 years at CDF, thousands of Higgs bosons were produced in the experiment, however they could not be cleanly identified because of the “messy” environment of hadron collisions.
After the Higgs boson discovery at LHC, and long after I had left the collaboration, CDF published “evidence” for Higgs production; the analysis made some use of the b-tagging tools I had earlier developed, so I made some contribution to confirmation of the Higgs discovery, although it’s rather small and tenuous!
Having been present close to the end of large collider experiments DELPHI and CDF, I thought it was time to see the other end of such big endeavours; and thanks to my then-future spouse, I wanted to see Japan. At this time  there was lots of excitement about the International Linear Collider, a proposed high energy electron-positron collider. Several competing groups working on different accelerating technologies around the world had just agreed to pool their resources into a single project, the ILC. Japan was one of the main players, so it was a great match to what I was looking for. I showed my partner a list of institutes working on ILC R&D, and she chose Kobe as the best place to live – so we ended up there. It turned out to be a great choice in many ways. I was working on the development of calorimeter technology for future experiments at the ILC with the CALICE collaboration, as well as trying to understand how well ILC would be able to measure the Higgs boson’s properties. Constructing a prototype detector, taking it to a test beam and trying to get it to work was a new experience for me, and very satisfying when it eventually came together. (It should be said that any such successes were entirely due to my colleagues, and usually in spite of my best efforts!)
After my fellowship in Japan was over, I continued working on calorimeters and Higgs physics in my next position, at the Laboratoire Leprince Ringuet near Paris. The ILC group has a long history in e+ e- experiments and had made seminal contributions to “particle flow” calorimetry; it also benefited enormously from the lab’s strong professional in-house engineering groups. This was around the time that the LHC was starting up, and the next group along the LLR corridor was working on LHC’s CMS experiment. Excitement was building, and some of it rubbed off on us during the communal coffee breaks. We heard many rumours, especially about Higgs, but were never sure how much to believe: colleagues were always rather ambiguous when asked directly!
The official announcement of the Higgs boson discovery arrived in 2012, just as I was preparing to move back to Japan, to the University of Tokyo. Even though it wasn’t a complete surprise (!), it was super exciting, with Particle Physics splashed across the news media. Now that we knew that the Higgs boson really existed, we wanted to be able to measure it as precisely as possible. The ILC, whose design was already rather advanced, would provide a supreme experiment in which these precise investigations of the Higgs boson could be made. High energy physicists in Japan soon expressed their wish to realise the ILC in Japan, and in 2013 went on to select the most suitable site, in the Kitakami mountains. Proposals were made to the Japanese government to host the ILC.
Mounting a multi-billion-$ international science project is an enormous task which requires a delicate mix of conditions among scientists, industries, local and national government, the general public, both domestically and internationally, to come together into a real project. One of my (French) colleagues memorably likened it to hand-making mayonnaise. We are now somewhere in this phase, trying to get it to emulsify. My senses of optimism and frustration wax and wane with the arrival of various ILC review reports, politician’s speeches, prioritisations by particle physics communities, government Ministers’ declarations, and local government actions. It often feels like a roller-coaster ride! In addition, several rival Higgs factory projects are making great progress, providing an extra frisson of competition.
One thing remains constant during all this: the ability of an electron-positron collider such as the ILC to make wonderful measurements of the Higgs boson and its interactions, and the fundamental importance that these will have to better understand the workings of our universe. This goal helps keep me focused on my day-to-day activities developing seemingly small corners of the ILC ecosystem, working with my colleagues young and old to push our dream towards reality.