Today the two halves of new Vertex Locator (VELO) approached the LHC proton-proton collision point for the first time. This process is called “VELO closing” in the LHCb jargon. The images below were taken during installation of the VELO detector inside LHCb cavern and its connection to the LHC vacuum system. The mounting of the VELO half is shown in the image to the left.
On July 5th, protons collided again at LHC after a 3.5 year break known as Long Shutdown 2 (LS2). During this period the original LHCb experiment has been largely dismantled and an almost completely new detector has been constructed. The old Vertex Locator (VELO), located around the proton-proton collision point, was replaced by a new, more precise, VELO detector. It contains millions of microscopic pixels, each measuring 55 x 55 micrometers. It can recreate particles’ trajectories at the full proton-proton collision rate of 40 million times per second. It is divided into two halves which separate during beam setup, and were brought together for the first time during the VELO closure. When closed, the two halves are shaped to form at their centre a tube more than one metre long but with a diameter no bigger than that of a typical pencil. The 400 MegaJoule LHC beams must thread themselves precisely through this tube and collide at the centre. This is why the precision of this step is so important and why it is a huge milestone for the experiment. Learn more about VELO construction and installation at the video below.
The “b” in LHCb is related to the name of beauty quark. The beauty particles, containing b quarks, are produced during proton-proton collisions and are decaying at a distance of about one centimetre from the proton-proton collision point. The VELO can reconstruct perfectly trajectories of particles produced at the collision point as well as particles created at the point of the beauty particle decay, as seen in the image.
Proton beams have the size of human hair. There are so many protons in the beams that the energy of beams at the LHC is equivalent to the energy of a fast train such as the TGV that runs between Geneva and Paris. Therefore the two halves of the VELO detector are located some centimetres from the expected proton-proton collision point during the injection of protons into the LHC and during the different steps of beam acceleration and adjustment by the LHC operators, before the state of perfect and safe conditions of proton-proton collisions named “Stable Beams” is achieved. At this moment LHCb physicists move the two VELO halves close to the collision point. The two possible locations of the LHCb VELO detector are indicated on the main LHC control screen as “VELO OUT” and “VELO IN”. The image below, taken at the LHCb exhibition, shows the two modules, face to face, in the “VELO IN” position, accompanied by the happy VELO team.
The proton-proton collision point varies from fill to fill. Therefore LHCb physicists measure the location of these collisions using the VELO detector itself. This location is measured approximatively when the detector halves are located relatively far from the beam. The decision is then made to move both halves a little bit closer, measure again, decide how far to move again, etc., until the two halves reach the final position. This process was made today for the first time in Run 3 by the VELO team crew located in front of VELO control screens in the LHCb control room. This procedure was filmed and is shown in the video.
The VELO team crew will continue to close the VELO manually for the next period. Later on the process will be automatised and monitored on a special control screen by the LHCb main shift crew. This video shows the control screen during the automatised closing process of the previous Run 1 and 2 VELO system.