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A short VELO presentation

The Vertex Locator (Fig.1) will provide precise measurements on straight tracks (no magnetic field) in an area close to the LHCb interaction region.

1. The Vertex Locator

To this end, it is constituted of two series of 21 stations (Fig.2), each station consisting into two plane silicon sensors (shown in blue on fig. 2): one giving the R position, the other the Phi position of the track. Each sensor is divided into a certain number of silicon strips. A particle passing trough a sensor will let charge into the strips, then these charges will be collected to form clusters, and clusters into different sensors will be used to fit the tracks.

2. VeLo stations

As we said before, the distance between the sensors and the beam is very small (only 8 mm). In fact it is smaller than LHC aperture during injection, so that the stations should be able to retract during this phase.

Furthermore, in order to minimize the material seen by the particles during their journey trough the VELO, the stations are into two aluminium boxes (one for each side) at a very low pressure (less than 10-4 mbar). These boxes (shown in part on fig. 3) are also shielding the stations against beam RF pickups.

3. Open RF box

Precise track measurements are needed as soon as the LEVEL 1 of LHCb trigger. Indeed this LEVEL is mainly based on secondary vertex reconstruction. That means that we need to know precisely the position of each sensors at each moment during the run. This is the role of the alignment. One of the most critical point concerning LHCb alignment is that the detector is moved between each run. So the alignment should be performed at the start of each run, within few minutes.

It defines all the problematic: find a quick but nevertheless robust method to align the VELO online.

To learn more about the VELO, have a look at the VELO website, the LHCb light TDR and the LHCb Detector paper. To learn even more, look at VELO notes on CERN document server.