At 4.47 p.m. on Tuesday 5 July, applause broke out in the CERN Control Centre as LHC operators declared Stable Beams. After more than three years of upgrade and maintenance work across the machine and experiments, ALICE, ATLAS, CMS and LHCb started recording their first proton–proton collisions at an unprecedented energy of 13.6 TeV.
LHC Run 3 is set to last until December 2025. In addition to a slightly higher centre-of-mass energy than Run 2, the machine will operate at an increased average luminosity thanks to larger proton intensities and smaller transverse beam sizes. New or upgraded detectors and improved data readout and selection promise the experiments their greatest physics harvests yet. ATLAS and CMS each expect to record more collisions during Run 3 than in the two previous runs combined, while LHCb and ALICE hope for three and 50 times more data, respectively. Two new forward experiments, FASER and SND@LHC (CERN Courier July/August 2021 p7), also join the LHC-experiment family.
While pilot beams circulated in the LHC for a brief period in October 2021, the countdown to LHC Run 3 began in earnest on 22 April, when two beams of protons circulated in opposite directions at their injection energy of 450 GeV. Since then, operators have worked around the clock to ensure the smooth beginning of the LHC’s third run, which was livestreamed to the media on the afternoon of 5 July. True to form, the machine added drama to proceedings: a training quench that morning generated enough heat to warm up several magnets well above their operating temperature. The cryogenics team sprang into action, managing to recuperate operational conditions just in time for the live event, watched by more than 1.5 million people.
Since then, the intensity of the beams has been increased in carefully monitored steps. As the Courier went to press, 900 bunches each containing around 120 billion protons were circulating, with 2748 bunches expected by September. “Run 3 is going to be a game-changer for us,” says operations group leader Rende Steerenberg. “In Run 2, we exploited the LHC in its ‘normal’ hardware configuration as constructed. Now, after the injectors have been adapted, we can push the brightness and the intensity of the beams much more. Run 3 is also an important stepping-stone to the High-Luminosity LHC upgrade.”
In March, the CERN management announced a change to the LHC schedule. Long Shutdown 3 will now start in December 2025, one year later than in the previous baseline, and last for three instead of 2.5 years. Production schedules across the LHC’s lifetime will remain unaffected, while the change will allow work for the HL-LHC to be completed with appropriate schedule margins. The extended year-end technical stop (EYETS) is now scheduled to take place in 2024/2025 and to last for 17 weeks, while the two preceding EYETSs will be of the standard length of 13 weeks beam-to-beam.
The preferred scenarios and duration of ion runs during Run 3 remain to be confirmed, but are likely to take place in four week-long periods towards the end of each year. While the majority of the LHC’s heavy-ion runs employ lead ions, a novel addition to the Run 3 programme will be a short period of collisions between oxygen ions in 2024. As with the first xenon runs in 2017, colliding ions with masses that are intermediate between protons and lead allows the experiments to scan important physics regimes relevant to the study of high-energy QCD.
“Every time you make a step in energy, even if it’s not that large, and a step in the amount of data, you open up new physics opportunities,” said CERN Director-General Fabiola Gianotti. “And every time we start a new run, it’s always a new adventure. You have to recalibrate the detectors and the accelerator, so it’s always uncharted territory and always a big emotion.”
- For full coverage of the physics targets at LHC Run 3, please see the May/June 2022 issue of CERN Courier.