The LHC has restarted this year, with a couple of differences, the most obvious being the energy change from 3.5 TeV per beam to 4 TeV per beam. However there are lots of more subtle changes between this year’s and last year’s running. One of these is the collision setup at Point 8, where LHCb is located.
I should probably pause here to warn you that this post is going to be a little technical. But hopefully you’ll get something out of it, even if it’s only that the LHC is a very complicated machine.
In most of the LHC, there are two separate beam pipes, one for the clockwise beam one and one for the anticlockwise beam two. These two beam pipes can be seen in the above image. However, since we want to collide the two beams, there are sections of the LHC where there is only one beam pipe. As seen below, these naturally are at each of the points where the experiments are located. In these regions, the beams are kept physically separated using magnets (horizontally at CMS and LHCb and vertically at ATLAS and ALICE) and brought into collision at a finite crossing angle to avoid unwanted collisions.
This is where it starts getting more complicated… As I’ve mentioned earlier, each experiment has some sort of magnet system, which could affect the trajectory of the circulating beams. The LHCb dipole magnet produces a deflection of around 180 μrad at the top energy of 7 TeV per beam. The field direction is in the vertical plane and the deflection therefore in the horizontal plane. This deflection must be compensated for to ensure beam stability. The situation can be seen in the image below.
For our physics analyses, we would like to take data with our dipole magnet in both directions (the N pole at the top and S pole at the bottom, and vice versa). We would also ideally like the crossing angle between the two beams to remain the same in both situations to reduce the errors on our results.
The problem here is that both the LHCb dipole compensation and the beam separation and crossing are in the horizontal plane. For the LHC, the beam exchange should always occur in the same direction, that is, the clockwise traveling beam one (the blue one) should always cross from the outside beam pipe to the inside at LHCb. This means that when we change our dipole polarity, we can’t ask the LHC to switch the beams around so we get the same crossing angle in both directions.
So for the 2010 and 2011 LHC runs, we’ve been running with different crossing angles when we switch magnet polarities. This year however, a new scheme has been proposed, where instead of horizontally separating the beams at the LHCb collision point, the beams will be vertically separated. With the vertical separation, and the horizontal compensation, this actually creates a tilted crossing plane, which you can see in the image below.
Now when we change the direction of our dipole magnet, we just change the direction of the tilted crossing plane (from NE-SW to NW-SE), but the crossing angle remains the same. So this year’s results will be even better than last year’s!