On 3 December, the LHC’s second run came to an end after three fantastic years. Over the course of Run 2, our flagship machine truly came of age. The LHC accelerator, detectors and computing all performed with metronomic reliability, while demonstrating great versatility through a number of special runs. As well as running with protons and lead ions, the LHC also collided xenon ions to provide an extra data point in the quest to understand the mysteries of Quark Gluon Plasma. At Point 8, it became a fixed target machine with a neon gas jet target in the beam pipe, allowing LHCb to collect proton-proton collider data at the same time as proton-neon fixed-target data. The proton-neon data allow nuclear effects in particle production processes to be studied, and enable LHCb physicists to look into the physics of cosmic ray proton collisions with gas atoms in the upper atmosphere.
There were also runs with protons on protons, protons on lead, and lead on lead, some with seemingly curious centre of mass nucleon-nucleon collision energies tuned to, for example, 5.02 TeV. These were designed to make a bridge between the Run 1 and Run 2 heavy-ion data sets, as well to allow comparisons between data from the three types of collisions. In Run 2, it’s also worth noting that all four experiments took data with heavy ions: adding new analyses to their portfolios is a sure sign of maturity, and strengthens the overall reach of the LHC physics programme.
The landmark Higgs boson discovery in Run 1 presented us with a wonderfully rich and diverse physics programme. In Run 2 we learned a lot more about the Higgs boson, notably how it couples to the heaviest, third generation of quarks and leptons, thus establishing the Yukawa coupling as a separate term in the Lagrangian of the Standard Model – more familiar to many as the formula proudly displayed on T-shirts sold at the CERN shop! The coupling to top quarks was a particular bonus: measuring it was not expected to be within the reach of the LHC experiments until much more data had been recorded. The fact that the Higgs to top quark coupling has been measured already is testimony to the great progress the experiments have made in refining their analysis techniques.
Thanks to Run 2, we now know the masses of the Higgs boson, top quark and W boson to considerably greater precision. Such measurements are important for constraining the Standard Model as a stable theory. Our understanding of CP-violation emerges from Run 2 with much improved measurements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. The quantity of data collected means that teams will be busy throughout the long shutdown analysing it. There could be exciting results in store if early hints turn out to be more than a statistical fluke. Flavour physics, for example, looks at rare transitions between generations of particles, and there’s enough data that subtle effects might be seen. With direct searches for new physics still revealing nothing new, the road to physics beyond the Standard Model may emerge through measurements such as these.
It was not just the big LHC experiments that produced exciting results in Run 2, the forward experiments also had important contributions to make. They took us back to an earlier era, when CERN was in its infancy and high in the lexicon of particle physics students were words like pomeron, coined in the early 1960s, and odderon, in the late 1970s. These hypothetical particles, later considered to be composed of an even or odd number of temporarily associating gluons, were put forward to describe elastic scattering. While the precision LHC measurement is not hard proof, it’s strong evidence that the odderon model bears some truth.
The many facets of LHC physics, including all that was revealed in Run 2, explain the beauty of the LHC, but without accelerator physics at an advanced level, none of it would be possible. With the LHC running so well, it is easy to forget what a complex beast it is, and what a triumph of human ingenuity. As well as running smoothly at 13 TeV, delivering a greater harvest in terms of luminosity than the ambitious target we had set ourselves, the LHC juggled with custom particle combinations, and fine-tuned energies. All in all, it has firmly established itself as a remarkably versatile instrument.
There was a very poignant moment as Run 2 came to an end and the venerable Linac 2 delivered its final protons destined for the LHC. Linac 2 has been faithfully providing beams for all proton experiments at CERN since 1978. It has been the lynchpin of the proton injector chain. Without its remarkable performance, along with the equally remarkable performance of the whole of the LHC injector chain, the LHC would not have achieved all it did in Run 2.
Run 2 has advanced our knowledge hugely, and left us at the beginning of the long shutdown with one inescapable conclusion. The physics harvest to date underscores more than ever the need for the High-Luminosity LHC, and for the full design energy of 14 TeV. The upcoming long shutdown, LS2, is the shutdown for the LHC Injectors Upgrade, LIU, project. All the careful preparations to replace Linac 2 with Linac 4, along with upgrades throughout the whole chain from the particle sources, for both protons and ions, through the Booster, the PS and the SPS, will come to fruition over the next two years. It’s going to be a busy shutdown as we prepare for the future, but we are already looking forward to the next instalment as Run 3 gets underway in 2021.