The magnificent seventh

MoEDAL is the LHC’s newly approved seventh experiment. Unlike its general-purpose partners installed along the LHC ring, MoEDAL will search for very specific exotic objects such as the highly ionizing magnetic monopoles and massive, conventionally charged, supersymmetric particles. The experiment is relatively small, cheap and quick to install but its physics potential is huge and a true complement to the already wide exploration range of the LHC experiments.


MoEDAL collaborators install the first layers of plastic in the LHCb VELO cavern at LHC Point 8.

In its final configuration, MoEDAL will consist of ten layers of plastic attached to the walls and ceiling of the cavern that houses the VELO detector of LHCb at Point 8 of the LHC ring. “When a heavily ionizing stable particle, such as a magnetic monopole or a massive stable super-symmetric particle, crosses the MoEDAL detectors, it produces damage in the plastic at the level of polymeric bounds in a small cylindrical region around its trajectory”, explains James Pinfold, spokesperson of the newly approved experiment. “The subsequent etching of the solid nuclear detectors leads to the formation of etch-pit cones. These conical pits are usually of micrometre dimensions and can be observed with an optical microscope. Their size, shape and alignment yield accurate information about the charge and the direction of motion of the incident highly ionizing particle”.

The particles for which MoEDAL is designed will have a highly characteristic signature in the detector to the point that, unlike other experimental set-ups, the background is not really an issue. “If it exists, a magnetic monopole will leave a very characteristic set of collinear etch-pits”, explains James. “There is no other conventional particle that could produce such a distinctive track – one event will be very significant”.

Simulation of collisions in MoEDAL.

The main LHC experiments are designed to detect conventionally charged particles traveling fast enough to cross the various layers within the trigger window of 25 ns. Also, very highly ionizing particles will usually be absorbed well before fully traversing the general-purpose detector. Further problems could arise with saturation effects in the detectors and the electronics. Thus, the detection of massive slowly moving and/or highly ionizing exotic particles is a significant challenge for the main LHC experiments. However, MoEDAL is a passive device, unaffected by trigger considerations, consisting of detectors that are capable of accurately measuring ionization rates that are thousands of times that of a minimum ionizing particle. This is why James Pinfold maintains that the MoEDAL detector complements the main LHC experiments in the search for new physics. As he says, “ If MoEDAL sees only a few candidates it will be a very clear indication that something new and very interesting has been produced. At that stage I would expect the other LHC experiments to look very closely for corresponding signals in their detectors”.

MoEDAL is an international collaboration of about 25 members from nine institutes worldwide. They have already installed the first square metre of plastic in the VELO cavern – as shown in the figure. The maximum possible surface area available for detectors is slightly less than 25 m2 per layer. During the long run of the LHC, MoEDAL scientists will study the general radiation levels in the cavern. “If the LHC has a technical stop in December, we hope to install the next five square metres of plastic. During the long shut-down we will install the full detector”. Even with just the initial deployment, MoEDAL will be able to give a first indication of the possible existence of new heavy and highly ionizing particles. There is no doubt that, like its other partners, the seventh LHC experiment is up to meeting the expectations of the very demanding physics community.

Further information about MoEDAL can be found here.

by CERN Bulletin