In conversation with John Rutherfoord, a leading designer of the ATLAS Calorimeter
22nd April 2021 | By
John P. Rutherfoord is a professor at the University of Arizona and a long-standing member of the ATLAS Collaboration. His extensive career has taken him from searching for the Upsilon particle at Fermilab to CERN to leading the design and development of the ATLAS Forward Calorimeter.
At a very early age, I pictured myself working in a college atmosphere. There was no question in my mind that I was going to be a physics major and that I was going to go to grad school. I’m not sure where my conviction came from – I hadn’t yet taken a physics course and I didn’t really know what grad school was – but I set my mind to getting there.
It wasn’t simple, though. In my junior year of high school, I realised that I needed to understand calculus to address the physics questions that I was playing with. But my school didn’t offer the subject and I found that almost none of the math teachers knew calculus. Luckily there was one teacher, Mr. Barlow, who did and he was willing to teach it to me after school. Each afternoon I would have a class with him and he gave me homework problems to work on before our next meeting. He made a huge difference in my life, not just by teaching me calculus but also by encouraging me to pursue math and science. I learned an awful lot from him.
The work paid off and I did my BSc in Physics at Union College (US), a Ph.D. in Experimental Particle Physics at Cornell University (US) and did some of my first post-grad work at the Cornell Synchrotron Lab.
Small experiments, big results
Back in those days, experiments were much smaller and, as a consequence, there were many more of them. My thesis experiment at Cornell consisted of just three scintillation counters, a beam line I constructed, and a few home-made fast electronics modules constructed by an earlier grad student, Walter LeCroy.
That was typical of the time. Experiments were specialised in doing one thing and doing it well. I was able to see experiments through their entire lifetime: from concept and funding, to design and construction, and finally data-taking and data analysis.
As a postdoc at the University of Washington I was stationed at Fermilab. My first job was as deputy spokesperson for an experiment that observed the Upsilon particle shortly after Leon Letterman’s team. Our detector’s mass resolution wasn’t as good as Leon’s – so our signal wasn’t as clear – but we could take higher rates and so we saw more of the Drell-Yan continuum above the Upsilons. These were exciting times.
I was able to see experiments through their entire lifetime: from concept and funding, to design and construction, and finally data-taking and data analysis.
Shortly after, I was invited to join Leon’s team for a follow-up experiment similar in concept to the Upsilon discovery experiment, but at a much bigger scale. Having learned the importance of good mass resolution, I realised that a different approach, which would require only minor modifications to the experiment, would improve our resolution by a factor of ten. My concept was immediately accepted by the team, and we put it together in just a few months.
It was a fantastic detector with mass resolution comparable to what's obtained at electron-positron colliders. In 1984, while we were taking data, a team at DESY announced that they’d observed a new particle thought to be a Higgs boson. I realised that if they had found the Higgs boson, our detector would have also seen it. We scoured our data and were able to rule out that possibility. Somewhat later, the DESY team found they had made a mistake in their analysis and withdrew their claim.
Arizona Bound! From the SSC to ATLAS
In 1988, I was asked by the University of Arizona to consider forming a new experimental-particle-physics group. I faced a difficult decision, as I loved my position at University of Washington, but this was an opportunity to build a team from the ground up. I accepted the offer. One of the first things I did was to recruit my friend and colleague Mike Shupe, who is well known to many in the ATLAS Collaboration. We spent the next several years building up our team, developing expertise and really putting the university on the particle physics map.
Even before moving to Arizona, I had started to get involved in the Superconducting Super Collider (SSC), and so it was one of our team’s first projects. We led the Forward Calorimeter effort for the GEM experiment, which became one of the two approved experiments for the collider. It was an interesting time for the particle-physics community, as physicists on the SSC were somewhat in competition with those at CERN. Despite that, there were still opportunities for collaboration.
Similar to ATLAS, the Forward Calorimeter design for GEM used liquid argon. And so when Daniel Fournier – who first proposed the accordion calorimeter design – visited the SSC headquarters in Dallas in the early ‘90s, it was a rare opportunity to share with him what we were doing. Shortly thereafter, we were invited (very indirectly, mind you) to test our forward calorimeter prototype at a test beam in CERN’s North Area.
That’s where we were in the autumn of 1993, when the US Congress decided to kill the SSC. Our little calorimeter prototype was working marvellously and we were overjoyed. People would come by – standing at what we now call a social distance – just to watch our progress. But by the end of our trip, our eyes were only on our emails, as we watched the cancellation decision come through.
It was rather natural that, very soon after the SSC’s demise, our team approached ATLAS about the possibility of joining. We knew we could make a big contribution and we had resources that would really benefit the project. We became unofficial members that same year and started attending regular meetings in early 1994 – arguing for the design of the ATLAS Forward Calorimeter system.
Several good proposals for the Forward Calorimeter were being considered, and no decision had been reached in early 1994. At the time, it was not appreciated that the ATLAS cavern would light up with background radiation. My colleague Mike Shupe made a huge contribution very early on, by making a detailed calculation of the expected background radiation in the ATLAS cavern. He was able to show, very clearly, that several of the proposals still on the table would have led to unacceptable backgrounds, particularly in the muon system.
Despite the physical distance between Arizona and Geneva, we never felt like we were out here by ourselves. Our work was done in a close collaboration with the whole liquid-argon team, and we all became lifetime friends as a result.
An ATLAS calorimeter review committee was appointed to choose one of the Forward Calorimeter proposals. In the summer of 1994, our proposal was selected. One of the first things we did was to get more teams on board, in particular the University of Toronto and Carleton University in Canada, and ITEP in Moscow. Our partnership with them worked out perfectly as, when we moved into the construction phase, they were interested in doing hadronic modules and we focused on the electromagnetic modules.
Despite the physical distance between Arizona and Geneva, we never felt like we were out here by ourselves. Our work was done in a close collaboration with the whole liquid-argon team, and we all became lifetime friends as a result. Most of these interactions took place at meetings at CERN, so we were traveling back and forth all the time. I remember one year I was at CERN more than 25% of my time, all while teaching classes here in Arizona. In retrospect I'm not sure how that worked out; I grew quite accustomed to the long journey.
Once our Forward Calorimeter concept was accepted, I became the leader of the Forward Calorimeter Construction Project within ATLAS. Arizona led the construction and the electromagnetic modules were actually built in our physics department’s basement, where we involved a lot of students. The finished modules were shipped to CERN in 2003 and a lot of our technical people moved out with them. They spent several years integrating the modules into the two forward calorimeter units, and then testing and installing them into the experiment’s cryostats.
Want to construct a detector? Seek out new opinions
Having a broad perspective, some gained during the SSC days, and input from several different experts was critical throughout the construction process. I remember sitting in a committee meeting in the early 1990s, discussing how we would maintain certain aspects of the detector. One rather prominent physicist suggested that we could just put up a stepladder to work on a piece of the calorimeter. He had no concept of how far off the floor the detector was, or even how big it was. It caught me by surprise.
This is where engineers are invaluable and I’ve worked with some very impressive engineers over the course of my career. They would often sit in on our meetings, listening to our debates over particular types of technology. In one meeting, I remember one of these top-notch engineers making a point. He said, “what we can do is make sure that your detector stands up. Not that it works. Just that it stands up.” That’s why we need different types of expertise. We were worried about whether we were developing the best technology for the application, while the engineers worried about whether it would stand up.
A top-notch engineer told us, “What we can do is make sure that your detector stands up. Not that it works. Just that it stands up.”
When the construction was largely completed, we all were asked to write what I call an as-built document describing a part of the ATLAS detector. I wrote the section on the Forward Calorimeter, along with Gerald Oakham, a Canadian colleague. Different groups took different approaches to this document, but I chose to focus on why the Forward Calorimeter was designed and built the way it was. This is something I think is missing from a lot of experimental papers which just focus on listing facts. That ignores a fundamental part of the process: the why is the art of designing a successful detector.
When we don’t share how we arrived at our decisions, we communicate a strange message to the physicists who come after us. A lot of my students don’t think to ask why ATLAS is the way it is. They just accept it. But that assumes the detector was the best it could be, which it wasn’t. It came about from past experience, technological breakthroughs, aggressive thinking, political necessity, the best decisions we could make at the time, and a lot of compromises.
Speaking to those younger physicists I would say you don’t need to wait until you’re a senior researcher to make change – so long as you’re willing to push for an idea and to gather people around it. The whole process of launching your own project is so much fun, and I would certainly encourage young people to try their hand at it. Choose a compelling physics topic that would be key to the collaboration, and then figure out how to get it done. Some of the most exciting new projects under development right now came about exactly this way.
You don’t need to wait until you’re a senior researcher to make change – so long as you’re willing to push for an idea and to gather people around it.
My focus these past few years has been on the upgrades to the detector. In particular, I’m interested in understanding how the electrical pulses from some parts of the calorimeters will change as the luminosity of the LHC increases well beyond the original design value. These changes will require modifications to our treatment of the data and it’s not yet known what these modifications will be.
I see the focus of the ATLAS Collaboration shifting even further towards perfecting the performance of our detector to get high-quality data, first by following through on our upgrade plans which will enhance the detector’s capabilities and second, through continuous improvement of data analysis strategies. We’re looking at a future with an exceptional quantity and quality of physics data – and with it, hopefully, big new discoveries.
ATLAS Portraits is a series of interviews presenting collaborators whose contributions have helped shape the ATLAS experiment. Discover more ATLAS Portraits here.