Nerve-wracking: revamped LHCb detector closes around LHC beam for first time

19 October 2022

In a nerve-wracking operation of days, the heart of the revamped LHCb detector at CERN closed precisely around the proton beam in the LHC accelerator for the first time today.

The two movable halves of the Vertex Locator (VELO) were very carefully pushed toward the collision point until a passage remains one meter long and the diameter of a pencil. Still with millimeter sized spacers to have some space left to manoeuvre.

The two halves of the VELO inside the LHCb detector just leave room for the proton beam from the LHC accelerator at CERN. PHOTO LHCB

The detector encloses proton beams that are as thin as a hair but have an energy of 400 MegaJoules, comparable to the energy of a TGV train at full speed. At no time is this scorching beam allowed to touch the VELO. Only when the beam is completely stable, according to CERN operators, will the halves of the VELO be pushed all the way together.

When closing, the VELO continuously measures itself exactly where the beam is. This can vary somewhat in successive LHC beams.


The so-called VELO closure is the literal capstone of the upgrades that have been performed at the detector at CERN over the past 3.5 years. During the second long-shuwdown of the LHC, a much more precise version of the earlier VELO was built in. Engineers from Nikhef collaborated on the installation of the device (video Marco Kraan).

Nikhef was previously closely involved in the construction of the new VELO, both of the pixel sensors inside the device and the ultra-thin metal box (RF box) that encloses it. Nikhef collaborated intensively on the placement of the VELO detector before the summer. On July 5, the LHC delivered proton beams again for the first time, now completely enclosed.


The VELO detector is the first layer of sensors in LHCb around the collision point of the proton beams in the LHC. The device contains millions of microscopic pixels measuring 55 by 55 micrometers, which can capture the trajectories of particles at a rate of 40 million times per second. This makes it possible to determine the exact location of a proton-proton collision, known as the vertex.

The LHCb experiment was built to study particles with so-called bottom quarks (the “b” in the detector’s name). Such relatively heavy quarks are made in proton-proton collisions and then fly away about a centimeter before decaying into lighter particles.

The LHCb experiment was built primarily to detect any differences between matter and antimatter in the particle world. These could possibly explain why the universe consists mostly of matter and not anti-matter.

UPDATE Oct. 21: After removing the spacers, VELO is now really closed as close as possible around the bundle.