On Oct. 27, 2021, stable proton beams circulated and collided in the Large Hadron Collider at CERN for the first time since 2018. The CMS experiment has spent the last three years preparing for the upcoming data-taking period, Run 3, which is scheduled to start in the spring of 2022. The month of October was dedicated as a special period of commissioning with the first LHC beams—essential for ensuring that CMS is ready to collect data in Run 3. This data will be used to search for new physics and make precision Standard Model measurements.
“We achieved all our objectives for the three-year-long shutdown, despite the challenges of the pandemic, a testament to our fantastic teams in CMS,” said Lothar Bauerdick, US CMS program manager. “We are looking forward to taking data again, with a refurbished and upgraded detector and at higher beam energies. This should yield great physics in the coming years.”
The CMS magnet was ramped to its full strength, 3.8 T, and data from cosmic rays passing through the detector was collected. Cosmic data had been recorded throughout the shutdown, but this was the first time with the full magnetic field.
The first major milestone in the LHC pilot beam program was to observe beam splash events in the CMS detector, beginning on Oct. 19. When a proton beam hits a collimator upstream of the detector, it creates a beam splash, or a big spray of particles, designed to hit a large area of the detector. CMS can use this very distinct splash event, coming unmistakably from the LHC beam, to test its data-taking chain and prepare for stable beam operation. Figure 1 shows a dramatic beam splash event.
Figure 1: A beam splash recorded by the CMS detector on Oct. 22, 2021. The innermost red region represents data in the electromagnetic calorimeter, the blue in the hadronic calorimeter, and the outermost region shows data in the muon chambers. Photo: CMS Experiment
Finally, on Oct. 27, the LHC began delivering stable beams. Each beam was at the LHC injection energy of 450 GeV, resulting in a center-of-mass proton-proton collision energy of 900 GeV. Figure 2 shows one of the first events from stable beam collisions with the pilot beams. Stable beams are particularly important because the high voltage in all subdetectors can be safely turned on and all systems can be tested. While the tracker high voltage cannot be turned on during beam splashes, it was during stable beam collisions.
CMS took advantage of this pilot beam period to perform important calibrations and tests. For example, timing calibrations were performed in the tracker and calorimeters, and the inclusion of GPU nodes in the High-Level Trigger was tested. Particularly notable is the inclusion of the gas electron multiplier detectors in the central CMS data-taking chain for the first time, marking a partial upgrade to the Muon system.
Figure 2: A collision event from stable beams with pilot beams recorded by the CMS detector on Oct. 27, 2021. The image shows data collected by the tracker (yellow), electromagnetic calorimeter (red) and hadronic calorimeter (blue). The muon system is not shown. Photo: CMS Experiment
The pilot beams concluded on Nov. 1, after a successful period of detector operations for CMS and machine operation for the LHC. During the 40 hours of stable beams, the LHC delivered 2.5 nb^-1 of data, of which CMS recorded 2.0 nb^-1, as measured by CMS luminosity monitoring. This exciting period showed that the work performed over the last three years has been highly successful.
The CMS collaboration looks forward to recording high-quality physics data from the LHC Run 3 at 13.6 TeV, expected to begin early next year.
With the CMS magnet running at full strength, the team observed beam splashes and pilot collisions in the LHC for the first time since 2018. Photo: CMS Experiment
Kaori Maeshima is a Fermilab scientist and the physics performance and dataset coordinator for CMS, and Lesya Horyn is a research associate in the CMS department at Fermilab.
The CMS department communications are coordinated by Fermilab scientist Pushpa Bhat.