High energy physics in the postwar era
Having led the world in the 1920s and 30s, nuclear research at the Cavendish went into decline following the death of Rutherford in 1937 and the departure of many of the lab’s leading physicists. After the Second World War, Cavendish high energy physics increasingly turned towards large international facilities, as Val Gibson explains.
Image above: The High Tension Hall at the Cavendish
Otto Robert Cavendish Professor Nevill Frisch was the father of high energy physics in Cambridge. Having been central to work on the atomic bomb and the Manhattan Project, he succeeded John Cockcroft as Jacksonian Professor in 1947 and became Head of the Cavendish in 1954. By the late 1950s, the Cavendish housed a cyclotron, two ‘MeV’ Cockcroft- Walton machines, and a Van de Graaff electrostatic generator, capable of accelerating particles up to 3 MeV. Housed in the newly built High Tension Hall, these machines were beloved of the media as being the shape of science to come, with tall columns of polished metal electrodes that could generate crashing sparks. All three machines were used to study energy levels of light nuclei until decommissioned when they became uncompetitive. This marked the demise of nuclear physics in Cambridge and the dawn of the era of high energy particle physics.
The 1950s also saw the development of billion-volt ‘GeV’ machines. In Cambridge, attempts were made to build a linear electron accelerator, which was terminated by Mott, who concluded that it was “too little too late”. Likewise, Cockcroft and Frisch’s proposed electron synchrotron was not approved. Cambridge also failed in its bid to host a new European laboratory with a large circular accelerator to be built in Norfolk, which eventually went to Geneva with the establishment of CERN in 1954. How UK high energy particle physics could have been so different!
Image: SWEEPNIK
Frisch met the inventor of the bubble chamber, Donald Glazer, who was concerned that, although his invention could photograph tracks of particles from high-energy collisions in large numbers, there would be a serious bottleneck unless equipment was developed to cope with the flow of film. In 1964, Frisch’s responded by inventing SWEEPNIK, a semi- automatic measuring machine, which benefitted from new commercial lasers with intense small spots of light and cheap computing. SWEEPNIK was an amazing success and Frisch, alongside Australian PhD student John Rushbrooke went on to found Laser Scan Limited in 1969, the first company to take premises in the Cambridge Science Park.
By the 1970s, the High Energy Physics (HEP) group numbered eight people in the Austin Wing of the Old Cavendish, carrying out research on accelerators run by the (inter)national laboratories RAL and CERN. Experiments were small scale by today’s standards, based on one or two groups. Frisch had retired but was still around. Led by academics Rushbrooke and Bill Neale were post-docs Richard Ansorge and Janet Carter, PhD students David Ward and David Munday; and Sven Katvars and Patrick Elcombe, who provided technical and computing support. An electronics engineer, Maurice Goodrick, joined in 1973 to replace the SWEEPNIK engineer who went to Laser Scan. Bubble chamber pictures were scanned in the New Exams Hall, which had space for the scanning machines and up to 20 staff, while data analysis was performed on the University IBM mainframe, conveniently situated on the New Museums Site. By the time of the move to West Cambridge in 1974, the group’s research focus was hybrid bubble chamber and electronic detector experiments at SLAC and Fermilab.
Image: SWEEPNIK
Frisch met the inventor of the bubble chamber, Donald Glazer, who was concerned that, although his invention could photograph tracks of particles from high-energy collisions in large numbers, there would be a serious bottleneck unless equipment was developed to cope with the flow of film. In 1964, Frisch’s responded by inventing SWEEPNIK, a semi- automatic measuring machine, which benefitted from new commercial lasers with intense small spots of light and cheap computing. SWEEPNIK was an amazing success and Frisch, alongside Australian PhD student John Rushbrooke went on to found Laser Scan Limited in 1969, the first company to take premises in the Cambridge Science Park.
By the 1970s, the High Energy Physics (HEP) group numbered eight people in the Austin Wing of the Old Cavendish, carrying out research on accelerators
run by the (inter)national laboratories RAL and CERN. Experiments were small scale by today’s standards, based on one or two groups. Frisch had retired but was still around. Led by academics Rushbrooke and Bill Neale were post-docs Richard Ansorge and Janet Carter, PhD students David Ward and David Munday; and Sven Katvars and Patrick Elcombe, who provided technical and computing support. An electronics engineer, Maurice Goodrick, joined in 1973 to replace the SWEEPNIK engineer who went to Laser Scan. Bubble chamber pictures were scanned in the New Exams Hall, which had space for the scanning machines and up to 20 staff, while data analysis was performed on the University IBM mainframe, conveniently situated on the New Museums Site. By the time of the move to West Cambridge in 1974, the group’s research focus was hybrid bubble chamber and electronic detector experiments at SLAC and Fermilab.
The Cavendish is renowned for exchange of ideas between experimentalists and theorists, including Paul Dirac, a father of quantum physics. One of Dirac’s PhD students, Richard Eden, created an HEP theory group in the Cavendish when many chose to move to the new Department of Applied Mathematics and Theoretical Physics in 1959. One of Eden’s students, Michael Green became a co-founder of String Theory, succeeded Stephen Hawking as Lucasian Professor, and received the 2013 Breakthrough Prize in Physics. In 1971, Eden also persuaded Bryan Webber, a postdoc at Lawrence Berkeley Laboratory, to join the theory group.
The 1970s was a game-changing period. The standard model of particle physics was being formed and neutral currents, the charm quark, and the bottom quark were all discovered. At CERN, the first hadron collider, the Intersecting Storage Rings (ISR), had been commissioned in 1971, the Super Proton Synchrotron (SPS) in 1976, and the proton-antiproton collider (Spp̅ S) in 1981. Following the move to West, CERN was the natural place for the Cambridge group’s research.
The 1970s was a game-changing period. The standard model of particle physics was being formed and neutral currents, the charm quark, and the bottom quark were all discovered.
In 1976, Rushbrooke, now Head of HEP and was joined by a new academic staff member, the effervescent Tom White. Whilst on sabbatical leave at CERN in 1977, Rushbrooke proposed the UA5 experiment at the Spp̅ S to provide an inclusive overview of hadronic interactions at the centre-of-mass energy of 540 GeV. A selling point were the uexplained ‘Centauro events’ claimed by cosmic ray emulsion experiments, which if validated, would have been a sensational discovery. The idea was to mount a pair of streamer chambers above and below the collision point, with light from the streamers be photographed using image intensifiers and the images analysed in Cambridge. A collaboration was formed, Rushbrooke became Spokesperson, and Cambridge led the physics analysis until about 1986.
Around the same time, Carter, now a senior member of HEP, joined the WA42 hyperon experiment at the SPS, followed by the Axial Field Spectrometer (AFS) at the ISR in 1983. The AFS was the first hadron collider experiment designed to study pp and pp ̅ collisions, making the first measurement of jet production cross-sections at a centre-of-mass energy of 45 GeV.
In 1983, Webber started to develop the world-acclaimed HERWIG Monte Carlo generator, which simulates quantum chromodynamic (QCD) processes in particle collisions. Over 40 years later, HERWIG remains at the forefront of event generator development and is used by all the major Large Hadron Collider (LHC) experiments. Webber received the 2021 European Physical Society High Energy Particle Physics Prize for this outstanding achievement.
Image: The UA2 experiment. © CERN
The discoveries of the W and Z bosons in 1983 by the UA1 and UA2 experiments at the Spp̅ S had a major impact on particle physics. In the late 1970s, CERN had started thinking about its next major project – the Large Electron Positron Collider (LEP), designed to make detailed measurements of W and Z bosons, to be constructed between 1983 and 1988. Cambridge joined the Omni Purpose Apparatus at LEP (OPAL), one of four large experiments. Carter led the group into OPAL (with Pat and David Ward, Goodrick and John Hill) to work on the central vertex tracking chamber, the track trigger, and the endcap lead glass calorimeters. Ward meanwhile became OPAL software coordinator in 1986 when the experiments were moving from large mainframe computing to workstations and PCs allied to data farms.
Meanwhile White and Munday made a major contribution to the 1985 upgrade of the UA2 experiment, allowing more precise measurements of the W and Z bosons. The team built a novel scintillating fibre detector that improved the tracking and electron identification significantly.
By 1989, Rushbrooke had resigned to become Head of Physics at Australia’s Bond University, Neale had also retired, and Carter became Head of HEP. The departures led to new academic appointments: Richard Batley to work on OPAL and Andy Parker to work on the UA2 upgrade which would search for the top quark. They were joined by senior post-docs Phil Allport and I to also work on OPAL. These arrivals provided the means to expand into the novel area of silicon detectors. Parker and Munday led the development of the UA2 silicon pad detector, installed in 1989, which further improved electron identification. This was the first silicon tracker at a collider experiment.
Together with Carter, Allport and Batley, we played a lead role in the OPAL silicon micro-vertex detector, placed around the beam-pipe. The detector provided very accurate measurements of particle trajectories close to the interaction point allowing for the recognition of short-lived particles, especially tau leptons and hadrons containing bottom quarks.
Image: The OPAL silicon micro-vertex detector. © CERN
The first phase of LEP and OPAL ran between 1989 and 1995, collecting millions of Z events, as the group led precision electroweak measurements, studied the underlying events to understand QCD, and measured bottom meson (B) production and decay processes. During the second phase of LEP, with the collision energy increased to produce WW pairs, the group made precision electroweak measurements and searched for new particles, until the collider completed data-taking in 2000. The UA5, UA2 and OPAL experiments saw a plethora of PhD students passing through, including current researchers Chris Jones, Dave Robinson and Steve Wotton.
I also brought expertise in experiments that measure charge-parity (CP) symmetry violation to address the question ‘why is our universe made of matter and not antimatter’?
This provided impetus for Batley, Munday, White and Wotton to join the NA48 kaon experiment at CERN, for which they were responsible for large planes of muon detectors and associated high-speed electronics, and understanding kaon beam backgrounds. The muon detector was so large that White took great pleasure in constructing it in the Rutherford building stairwell. NA48 started taking data in 1997, discovering ‘direct’ CP violation in the process.
Image: ATLAS semiconductor tracker © CERN
During the 1980s, CERN was already considering its next major project – the LHC – to search for the Higgs boson and physics beyond the standard model. Parker became a key player in the development of the ATLAS experiment, which Cambridge joined in 1992 to work on the semiconductor tracker with the expertise of Dave Robinson and later Bart Hommels. Construction on the LHC and ATLAS began in 1998 with first collisions in 2009.
While many ATLAS institutes searched for the Higgs boson, the Cambridge team made a strategic decision to seek out exotic new physics, including mini black holes, extra dimensions, supersymmetry (SUSY) and dark matter, developing strategies in collaboration with the Cambridge Phenomenology (then SUSY) Working Group. The ATLAS team expanded with the arrival of new academics: Mark Thomson who proposed an upgrade to the calorimeter trigger; and Christopher Lester who significantly enhanced the combined ATLAS/ Phenomenology Working Group.
I was appointed to an academic position in 1994 and led Cambridge alongside Jones and Wotton into the LHCb experiment, designed to study charm and bottom quarks, make precision measurements of CP violation and search for new physics. The Cambridge team became major players in particle identification, providing readout electronics for the ring-imaging Cherenkov detectors, and the global pattern recognition.
The LHC scored its first major triumph with the discovery of the Higgs boson in 2012, resulting in a Nobel Prize for Peter Higgs and Francois Englert. Today, the ATLAS team continues its search for new physics and while conducting benchmark measurements of the standard model and the Higgs. Meanwhile, the LHCb team makes precision measurements of CP violation in B decays, has discovered very rare B decays and rattled the standard model with enticing anomalies.
Alongside the success of the LHC, the HEP group continued to expand. James Stirling arrived as Jacksonian Professor in 2008, becoming Head of the Cavendish in 2011. Parker succeeded Carter as Head of HEP in 2009 and Stirling as Head of the Cavendish in 2013. Stirling appointed me as Head of HEP in 2013. The HEP theory group also had new appointments: Ben Gripaios (2011) and Alex Mitov (2013), international leaders in theories of new physics and top quarks, respectively. With these appointments the theory group was in excellent hands when Webber retired in 2010 and Stirling became Provost of Imperial College London in 2013.
Throughout our history, HEP has continued Frisch’s interest in developing particle detectors that get the best physics from experiments, nurturing a culture for collaboration between the theory and experiment, and producing world-leading physics results.
Thomson also expanded the HEP group’s research into neutrino physics, developing an event reconstruction tool, PANDORA, that identifies energy deposits from individual particles in fine granularity detectors, with applications in several neutrino experiments. Thomson took leave from Cambridge in 2018 to become Executive Chair of STFC and is currently the UK’s delegate for the next Director General of CERN.
The HEP group today stands at about 50 academics, post- docs, PhD students, engineers and technicians. Parker and I retired at the end of 2023, and Tina Potter took the reins as Head of HEP. As we move to the Ray Dolby Centre, the group remains very much at the heart of the LHC and neutrino experimental programmes. We also have significant roles in AI applications, searching for long-lived dark matter, and emerging quantum technology detectors.
We also lead preparations for the Future Circular Collider at CERN and the Deep Underground Neutrino Experiment at Fermilab. Throughout our history, HEP has continued Frisch’s interest in developing particle detectors that get the best physics from experiments, nurturing a culture for collaboration between the theory and experiment, and producing world-leading physics results.
Val Gibson is an Emeritus Professor of High Energy Physics and former Head of the HEP Research Group. She was awarded an OBE in 2021 "For services to Science, Women in Science and to Public Engagement".
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