Why does LHC have 8 interaction points?

[kelly0303]

Hello! I attached a picture of the LHC main rings. I see 8 interactions points labeled, even if there are only 4 experiments (and the other 4 points don't even intersect). What are these 4 other points? Thank you!

 

[websterling]

From here-
The Large Hadron Collider (LHC) at CERN

The LHC has 8 arcs and 8 straight sections between the arcs. The straight sections are around 528 m long. Each straight section has associated with it surface and underground installations, lifts, and a wide variety of technical infrastructure. These locations are referred to as the LHC points.

• The four main experiments are situated at point 1 (ATLAS), point 2 (ALICE), point 5 (CMS) and point 8 (LHCB).
• Injection of clockwise beam (beam 1) takes place at point 2. Injection of anti-clockwise beam (beam 2) takes place at point 8.
• The main collimator installations are at points 3 and 7.
• The radio frequency (RF) system is situated at point 4.
• The beam dump system is situated at point 6.

Functionally the LHC is divided into 8 sectors (namely sectors 12, 23, 34, 45, 56, 67, 78, 81). A sector spans the underground installation between 2 LHC points - thus sector 12 lies between point 1 to point 2 etc. Importantly a sector can be cooled and powered independently.

 

[mfb]

LHC was built in the same tunnel as LEP, an electron-positron collider. At LEP the particles lost a significant amount of energy to synchrotron radiation in each turn, so LEP needed long straight sections for RF cavities to accelerate the particles again. That lead to a design with eight 45 degree turns with straight sections in between. Four of them were occupied by experiments (ALEPH, DELPHI, OPAL, L3).
For LHC the energy is limited by the curvature radius and it doesn't need so many straight sections - a more circular tunnel with shorter straight sections would be better. Changing the tunnel layout would have been very expensive so it was decided to use the existing tunnel.

One experiment (ATLAS) is directly at the main CERN site, two more are reasonably close, only CMS is quite far away. There are shuttle buses but it is still annoying if you have to move between experiments and CERN site every day (e.g. for shifts).