Hadron colliders in perspective

15 December 2021
50 years of hadron colliders at CERN featured
Carlo Rubbia gave the opening talk at the October event. Image credits: CERN-PHOTO-202110-155-52

From visionary engineer Rolf Widerøe’s 1943 patent for colliding beams, to the high-luminosity LHC and its possible successor, the 14 October symposium “50 Years of Hadron Colliders at CERN” offered a feast of physics and history to mark the 50th anniversary of the Intersecting Storage Rings (ISR). Negotiating the ISR’s steep learning curve in the 1970s, the ingenious conversion of the Super Proton Synchrotron (SPS) into a proton–antiproton collider (SppS) in the 1980s, and the dramatic approval and switch-on of the LHC in the 1990s and 2000s chart a scientific and technological adventure story, told by its central characters in CERN’s main auditorium.

Former CERN Director-General (DG) Chris Llewellyn Smith swiftly did away with notions that the ISR was built without a physics goal. Viki Weisskopf (DG at the time) was well aware of the quark model, he said, and urged that the ISR be built to discover quarks. “The basic structure of high-energy collisions was discovered at the ISR, but you don’t get credit for it because it is so obvious now,” said Llewellyn Smith. Summarising the ISR physics programme, Ugo Amaldi, former DELPHI spokesperson and a pioneer of accelerators for hadron therapy, listed the observation of charmed-hadron production in hadronic interactions, studies of the Drell–Yan process, and measurements of the proton structure function as ISR highlights. He also recalled the frustration at CERN in late 1974 when the J/ψ meson was discovered at Brookhaven and SLAC, remarking that history would have changed dramatically had the ISR detectors also enabled coverage at high transverse momentum.

A beautiful machine

Amaldi sketched the ISR’s story in three chapters: a brilliant start followed by a somewhat difficult time, then a very active and interesting programme. Former CERN director for accelerators and technology Steve Myers offered a first-hand account, packed with original hand-drawn plots, of the battles faced and the huge amount learned in getting the first hadron collider up and running. “The ISR was a beautiful machine for accelerator physics, but sadly is forgotten in particle physics,” he said. “One of the reasons is that we didn’t have beam diagnostics, on account of the beam being a coasting beam rather than a bunched beam, which made it really hard to control things during physics operation.” Stochastic cooling, a “huge surprise”, was the ISR’s most important legacy, he said, paving the way for the SppS and beyond.

Former LHC project director Lyn Evans took the baton, describing how the confluence of electroweak theory, the SPS as collider and stochastic cooling led to rapid progress. It started with the Initial Cooling Experiment in 1977–1978, then the Antiproton Accumulator. It would take about 20 hours to produce a bunch dense enough for injection into the SppS , recalled Evans, and several other tricks to battle past the “26 GeV transition, where lots of horrible things” happened. At 04:15 on 10 July 1981, with just him and Carlo Rubbia in the control room, first collisions at 270 GeV at the SppS were declared.

Poignantly, Evans ended his presentation “The SPS and LHC machines” there. “The LHC speaks for itself really,” he said. “It is a fantastic machine. The road to it has been a long and very bumpy one. It took 18 years before the approval of the LHC and the discovery of the Higgs. But we got there in the end.”

Discovery machines

The parallel world of hadron-collider experiments was brought to life by Felicitas Pauss, former CERN head of international relations, who recounted her time as a member of the UA1 collaboration at the SppS during the thrilling period of the W and Z discoveries. Jumping to the present day, early-career researchers from the ALICE, ATLAS, CMS and LHCb collaborations brought participants up to date with the progress at the LHC in testing the Standard Model and the rich physics prospects at Run 3 and the HL-LHC.

Few presentations at the symposium did not mention Carlo Rubbia, who instigated the conversion of the SPS into a hadron collider and was the prime mover of the LHC, particularly, noted Evans, during the period when the US Superconducting Super Collider was under construction. His opening talk presented a commanding overview of colliders, their many associated Nobel prizes and their applications in wider society.

During a brief Q&A at the end of his talk, Rubbia reiterated his support for a muon collider operating as a Higgs factory in the LHC tunnel: “The amount of construction is small, the resources are reasonable, and in my view it is the next thing we should do, as quickly as possible, in order to make sure that the Higgs is really what we think it is.”

It seems in hindsight that the LHC was inevitable, but it was anything but

Christopher Llewellyn Smith

In a lively and candid presentation about how the LHC got approved, Llewellyn Smith also addressed the question of the next collider, noting it will require the unanimous support of the global particle-physics community, a “reasonable” budget envelope and public support. “It seems in hindsight that the LHC was inevitable, but it was anything but,” he said. “I think going to the highest energy is the right way forward for CERN, but no government is going to fund a mega project to reduce error bars – we need to define the physics case.”

Following a whirlwind “view from the US”, in which Young-Kee Kim of the University of Chicago described the Tevatron and RHIC programmes and collated congratulatory messages from the US Department of Energy and others, CERN DG Fabiola Gianotti rounded off proceedings with a look at the future of the LHC and beyond. She updated participants on the significant upgrade work taking place for the HL-LHC and on the status of the Future Circular Collider feasibility study, a high-priority recommendation of the 2020 update of the European strategy for particle physics which is due to be completed in 2025. “The extraordinary success of the LHC is the result of the vision, creativity and perseverance of the worldwide high-energy physics community and more than 30 years of hard work,” the DG stated. “Such a success demonstrates the strength of the community and it’s a necessary milestone for future, even more ambitious, projects.”

Videos from the one-off symposium, capturing the rich interactions between the people who made hadron colliders a reality, are available here.

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