CERN Accelerator Complex Enters Full LS3 Shutdown for Upgrades, Paving the Way for the High-Luminosity LHC Era

On the morning of August 31 local time, the injector complex and the Antimatter Factory at the European Organization for Nuclear Research (CERN) ceased operation after completing their final beam deliveries, officially entering the third Long Shutdown (LS3) phase. Earlier, the Large Hadron Collider (LHC) had been closed in June, marking the flagship accelerator as the first to kick off LS3 upgrade work at the center.

Bettina Mikulec (left), head of the Operations Group at CERN, handed a symbolic baton—a miniature superconducting magnet from the LHC—to Jean-Philippe Tock (right), head of the LS3 Coordination Group, with CERN Director-General Mark Thomson (second from left) and Oliver Brüning (third from left), head of Accelerators and Technology, in attendance. (Image: CERN)

After the LHC shutdown, the injector system that supplies its beams continued operating throughout the summer, delivering proton and ion beams to related experimental facilities. CERN stated that during the final weeks of operation, teams used the remaining beam time to complete physics programs, collect measurement data, and further optimize accelerator performance. These data will support preparations for Run 4 and provide a reference for the extensive maintenance and upgrades during LS3.

During LS3, all relevant CERN machines will undergo maintenance, consolidation, upgrades, and preparations for new experiments. These modifications aim to enhance the reliability of the accelerator complex and lay the groundwork for the High-Luminosity LHC (HiLumi LHC) operational phase. According to the plan, the CERN accelerator complex will resume operations in phases starting in 2028, with injectors and experimental facilities restarting progressively; the HiLumi LHC will be the last accelerator to restart, expected to resume operation around mid-2030.

Meanwhile, CERN recently also published research progress on oxygen nucleus collision experiments conducted using the LHC. By analyzing oxygen–oxygen collisions with the ALICE detector, researchers observed parton energy loss, commonly referred to as jet quenching. According to the data, this is one of the smallest nuclear systems in which such features have been observed in studies to date.

The research focuses on questions related to the state of matter in the very early universe after the Big Bang. Scientists are investigating the properties of matter existing as a quark–gluon plasma in the microseconds following the Big Bang, under conditions of extremely high temperatures where protons and neutrons had yet to form. Researchers have previously produced quark–gluon plasmas primarily through collisions of heavy nuclei such as lead, studying the energy loss of high-energy quarks or gluons propagating through them.

In this study, the ALICE detector measured the production of neutral pions in oxygen–oxygen collisions and found that their production is suppressed, a phenomenon similar to observations in lead–lead collisions. By comparing with oxygen–proton collisions as a control, researchers concluded that there are indications of jet quenching or parton energy loss in oxygen–oxygen collisions.

The study also notes that such measurements indicate that, under the relevant conditions, subatomic particles still exhibit strong collective motion characteristics, behaving more like a nearly perfect liquid rather than a dilute gas. The research findings have been published on the arXiv preprint platform.