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Accelerator Report: Reviving antimatter physics and fine-tuning luminosity measurements in the LHC


On Friday, 30 June, an exciting milestone was reached: the much-awaited antiproton physics season finally commenced. Originally scheduled for 11 May, the start had to postponed due to an unforeseen water leak that occurred on 14 March in a special quadrupole magnet located in the Antiproton Decelerator (AD) machine. As a result, the magnet had to be removed for repair in the workshop before being reinstalled to finalise hardware and beam commissioning. Consequently, the start of antimatter physics was rescheduled for 30 June. The AD-ELENA operations team, together with many experts, have been working hard to meet – with success – this important deadline.

The delay caused by the leaking magnet resulted in a loss of 50 physics days for the experiments behind the ELENA machine. To partially compensate for this significant loss of precious physics time, the 2023 run for the antimatter factory has been extended by 12 days. The extended run will now conclude at 6 a.m. on 13 November. This adjustment is intended to maximise scientific output and make best use of the available time for the experiments, without compromising on the many activities scheduled for the 2023–2024 year-end technical stop (YETS).

On the LHC side, the technical stop mentioned in the last Accelerator Report has been successfully completed. Following the stop, special physics runs were conducted along with a short intensity ramp-up to revalidate the LHC machine for luminosity production. Despite some delays caused by technical issues, including a power cut affecting part of CERN, the machine has now resumed normal operation with the aim of maximising luminosity production.

One of the special physics runs was the so-called “van der Meer” run, which plays a crucial role in precisely calibrating the experiments’ luminosity measurements. This calibration involves establishing a precise relationship between the beam separation and the observed rate of particle interactions. During the van der Meer scan, the separation between the colliding beams is intentionally varied, leading to changes in the number of particle interactions. Through meticulous control of the beam separation and thorough analysis of the resulting data, the experts in the experiments can accurately determine the relationship between beam separation and observed interactions. This relationship is referred to as the “luminosity calibration curve”, which serves as vital input towards an accurate – in the order of one per cent – determination of the number of collisions recorded by the LHC experiments.

This is just one part of the LHC luminosity scan application, where the beam separation steps for the CMS van der Meer scan are clearly visible (second row of plots): on the left-hand side we see the horizontal separation, and on the right-hand side the vertical separation. The resulting luminosity is given in the bottom left-hand plot. (Image: CERN)

The LHC will continue its proton collisions and luminosity production until a brief technical stop in mid-September. Subsequently, the focus will shift to lead-ion collisions until 30 October, when the YETS is set to commence.