LHC report: Level best

The number of the events the detector can handle per bunch crossing is limited. Even so, LHCb wants to integrate as much luminosity as possible

The LHCb experiment is special: there is a limit to the number of the events the detector can handle per bunch crossing. Consequently the maximum luminosity provided in 2012 has been around 4 x1032 cm-2s-1 (compared to the maximum of 7.7 x1033 cm-2s-1 seen by ATLAS and CMS). Nonetheless LHCb still wants to integrate as much luminosity as possible. 

To meet LHCb's requirements a luminosity leveling technique is used. A machine setup is chosen that would give a peak luminosity well above the required maximum if the beams are collided head-on at LHCb's interaction point. This peak luminosity is then reduced to the required maximum by moving the two beams transversely apart at the interaction point. As the beam current goes down during a fill, the beams can be moved together in small increments to keep the collision rate constant throughout the fill.

In practice, when the LHC goes into collisions in LHCb, the initial luminosity is safely below LHCb's demanded level. There is then a semi-automatic system that allows LHCb to pass instructions to the LHC control system to nudge the beams towards each other. Any changes in the corrector settings used to move the beams have to be confirmed by the LHC operations team. To notify the team of a new LHC request, the LHC announcer gives an audible message and the phrase "Luminosity leveling requested by LHCb" is a now a familiar part of life in the control room.

The LHCb data taking in 2012 has been characterized by a stable trigger configuration and very stable data taking at the optimized instantaneous luminosity of 4 x1032 cm-2s-1 and with an operational efficiency of 95%. 2 fb-1 has been delivered to them so far this year. (Elsewhere in the ring, ATLAS and CMS have now passed the 21 fb-1 mark.)

As of Friday 16 November, the rest of the 2012 runs foresees another 18 days of proton physics, 3 days of machine development and 8 days of 25 ns operation. The 25 ns slot will include a scrubbing run, machine development and, finally, a short 24 hours physics run with relatively low intensity 25 ns beam.

Did you know?

The beam size at LHCb's interaction point is around 160 microns (4 Gaussian sigma) and the beam centres are separated by 100 microns at the start of a fill, and of the order of 40 microns towards the end of a long fill. The beams can also be collided head-on at the end of a very long fill, when there are fewer particles in the beam.