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LHCb’s new VELO springs into action

The subdetector aligned more closely with the LHC beam than ever before, marking an important milestone in LHCb data taking


Installation of Vertex Locator (VELO) sub-detector in LHCb
The LHCb VELO installation earlier this year (Image: CERN)

On Friday, 21 October, at 10.15 p.m. CEST, the Large Hadron Collider beauty (LHCb) experiment passed an important milestone for Run 3. Its state-of-the-art vertex locator, or VELO upgrade, aligned more closely with the LHC beam than ever before. This process, known in LHCb jargon as “VELO closing”, allows the experiment to reconstruct the trajectories of the particle collisions at LHCb with extreme precision.

LHCb analyses particles thrown forward from the collision point of the two LHC beams. In particular, the LHCb collaboration searches for a type of particle called a B meson, which is characterised by containing a “beauty” quark. B mesons are important for particle physics research because their interactions may hold the clues to the limitations of the Standard Model, which currently dictates all of particle physics. Earlier this year, using data from the previous version of the detector, LHCb announced the discovery of new types of matter–antimatter asymmetry and new exotic particles. Both of these topics, and many more, will be probed further with the new detector.

To be able to understand these particle interactions fully, physicists need more data. The VELO’s job is to completely reconstruct the trajectories of particle collisions, pick out the important interactions involving B mesons, and analyse them.

“It has to be incredibly close to the beams in order to get the maximum accuracy,” says Paula Collins, LHCb experimental physicist. The VELO achieves this by gradually moving pairs of plates closer to the beam, so close that they even enter the vacuum of the LHC beam pipe. The subdetector is composed of millions of pixels, which act like a camera, taking pictures of the interaction at a rate of 40 million times per second.

The plates start at a width of about 3 cm apart, and carefully move to centre around the beam. “When we’re closed, the aperture where the LHC beams pass is just 3.5 mm,” continues Collins. “This is something like the diameter of a pencil, and the 400 mega-joule LHC beams have to pass through this very narrow space.”

This impressive feat could not have been achieved without a huge team effort to design, install and operate the VELO. The LHCb detector underwent a complete overhaul in preparation for Run 3 of the LHC, which began on 5 July 2022. The VELO is only one of a number of brand-new subdetectors that have increased LHCb’s precision and data-taking capacity. Other new subdetectors include the new upstream tracker (UT) and the scintillating fibre tracker (SciFi), which analyse the beam either side of the central LHCb magnet.

Since the LHC began, physicists have discovered the existence of 68 new hadrons, 60 of which were discovered by the LHCb experiment. The unprecedented accuracy achieved by the new VELO marks an exciting new era for the experiment, with hope for plenty more discoveries to come.

You can find out more about this milestone in the video below:

Alignment of the LHCb experiment's VELO subdetector (Video: CERN)