Résumé
MoEDAL devient la septième expérience du LHC

Le 2 décembre 2009, la Commission de la recherche du CERN a approuvé la septième expérience du LHC : le détecteur MoEDAL (Monopole and Exotics Detector At the LHC). Le premier objectif de cette expérience est de rechercher la production directe du monopôle magnétique auprès du LHC. Elle s’efforcera également de découvrir des particules massives stables (ou pseudo-stables) fortement ionisantes à charge électrique conventionnelle. De dimensions modestes, par rapport aux autres détecteurs du LHC, le détecteur MoEDAL est constitué d’un ensemble d’environ 400 détecteurs de traces nucléaires (NTD). Ceux-ci sont déployés à la région d’intersection au point 8 de l’anneau du LHC, à proximité du détecteur VELO de l’expérience LHCb.

On 2 December 2009 the CERN Research Board approved the LHC's seventh experiment: the Monopole and Exotics Detector At the LHC (MoEDAL). The prime motivation of this experiment is to search for the direct production of the magnetic monopole at the LHC. Another physics aim is the search for exotic, highly ionizing, stable (or pseudo-stable) massive particles (SMPs) with conventional electric charge. Although MoEDAL is a small experiment by LHC standards it has a huge physics potential that complements the already wide vista of the existing LHC experiments.

The scientific quest for the magnetic monopole – a single magnetic charge, or pole – began during the siege of Lucera in 1269 with the Picard Magister, Petrus Peregrinus. He was a Franciscan monk, a soldier, a scien-tist and a former tutor to Roger Bacon, who considered him the fore-most experimentalist of his day. It was during this siege that Peregrinus put the finishing touches to a long letter entitled the Epistole de Mag-nete, which is his only surviving work. In this document, Peregrinus scientifically established that magnets have two poles, which he called the north and south poles.

In 1864 the Scottish physicist James Clerk Maxwell published the 19th-century equivalent of a grand unified theory, which encompassed the separate electric and magnetic forces into a single electromagnetic force (Maxwell 1864). Maxwell banished isolated magnetic charges from his four equations because no isolated magnetic pole had ever been observed. This brilliant simplification, however, led to asymmetric equations, which called for the aesthetically more attractive symmetric theory that would result if a magnetic charge did exist. Thirty years later, Pierre Curie looked into the possibility of free magnetic charges and found no grounds why they should not exist, although he added that it would be bold to deduce that such objects therefore existed (Curie 1894).

Paul Dirac, in a paper published 1931, proved that the existence of the magnetic monopole was consistent with quantum theory (Dirac 1931 and 1948). In this paper, he showed that the existence of the magnetic monopole not only symmetrized Maxwell's equations, but also explained the quantization of electric charge. To Dirac the beauty of mathematical reasoning and physical argument were instruments for discovery that, if used fearlessly, would lead to unexpected but valid conclusions. Perhaps the single contribution that best illustrates Dirac's courage is his work on the magnetic monopole. Today, magnetic-monopole solutions are found in many modern theories such as grand unified theories, string theory and M-theory. The big mystery is, where are they?

In the 1980s, two experiments found signals induced in single superconducting loops that could have indicated the passage of monopoles, but firmer evidence with coincidences in two loops was never found. Cosmic-ray experiments have also searched for monopoles but so far to no avail. For example, the Monopole, Astrophysics and Cosmic Ray Observatory (MACRO) detector in the Gran Sasso National Laboratory has set stringent upper limits (CERN Courier May 2003 p21). High-energy collisions at particle accelerators offer another obvious hunting ground for monopoles. Searches for their direct production have usually figured at any machine entering a new high-energy regime – and the LHC will be no exception.

New limits

At CERN, the search for magnetic monopoles – using dedicated detectors – began in 1961 with a counter experiment to sift through the secondary particles produced in proton–nucleus collisions at the PS (Fidecaro 1961). Over the following years, searches took place at the Interacting Storage Rings and at the SPS. At the Large Electron–Positron (LEP) collider, the hunt for monopoles in e+e collisions was carried out in two experiments: MODAL (the Monopole Detector at LEP), deployed at intersection point I6 on the LEP ring (Kinoshita et al. 1992); and the OPAL monopole detector, positioned around the beam pipe at the OPAL intersection point (Pinfold et al. 1993). These established new limits on the direct production of monopoles.

The international MoEDAL collaboration, made up of physicists from Canada, CERN, the Czech Republic, Germany, Italy, Romania and the US, is preparing to deploy the MoEDAL detector during the next long shutdown of the LHC, which will start late in 2011. The full detector comprises an array of approximately 400 nuclear track detectors (NTDs). Each NTD consists of a 10-layer stack of plastic (CR-39 and MAKROFOL) and altogether they have a total surface area of 250 m2. The detectors are deployed at the intersection region at Point-8 on the LHC ring around the VErtex LOcato (VELO) of the LHCb detector, as figure 1 indicates. The MoEDAL collaboration positioned 1 m2 of test detectors before the LHC was closed for operation in November 2009. Figure 2 shows the detectors being installed. If feasible, they will be removed for analysis during the planned short shutdown at the end of 2010 and a substantial subset of the full detector system will be deployed for the run in 2011.

The MoEDAL detector is like a giant camera for photographing new physics in the form of highly ionizing particles, and the plastic NTDs are its "photographic film". When a relativistic magnetic monopole – which has approximately 4700 times more ionizing power than a conventional charged minimum-ionizing particle – crosses the NTD stack it damages polymeric bonds in the plastic in a small cylindrical region around its trajectory. The subsequent etching of the NTDs leads to the formation of etch-pit cones around these trails of microscopic damage. These conical pits are typically of micrometre dimensions and can be observed with an optical microscope. Their size, shape and alignment yield accurate information about the effective Z/β ratio, where Z is the charge and β the speed, as well as the directional motion of the highly ionizing particle.

The main LHC experiments are designed to detect conventionally charged particles produced with a velocity high enough for them to travel through the detector within the LHC's trigger window of 25 ns – the time between bunch crossings. Any exotic, highly ionizing SMPs produced at the LHC might not travel through the detector within this trigger window and so will have a low efficiency for detection. Also, the sampling time and reconstruction software of each sub-detector is optimized assuming that particles are travelling at close to the velocity of light. Hence, the quality of the read-out signal, reconstructed track or cluster may be degraded for an SMP, especially for subsystems at some distance from the interaction point.

Another challenge is that very highly ionizing particles can be absorbed before they penetrate the detector fully. Additionally, the read-out electronics of conventional LHC detector systems are usually not designed to have a wide enough dynamic range to measure the very large dE/dx of highly ionizing particles properly. In the case of the magnetic monopole there is also the problem of understanding the response of conventional LHC detector systems to particles with magnetic charge.

The MoEDAL experiment bypasses these experimental challenges by using a passive plastic NTD technique that does not require a trigger. Also, track-etch detectors provide a tried-and-tested method to detect and measure accurately the track of a very highly ionizing particle and its effective Z/β. Importantly, heavy-ion beams provide a demonstrated calibration technique because they leave energy depositions very similar to those of the hypothetical particles sought. If it exists, a magnetic monopole will leave a characteristic set of 20 collinear etch-pits. There is no other conventional particle that could produce such a distinctive signature – thus, even one event would herald a discovery.

One of the world's leading string theorists, Joseph Polchinski, has reversed Dirac's connection between magnetic monopoles and charge quantization. He has posited that in any theoretical framework that requires charge to be quantized, there will exist magnetic monopoles. He also maintains that in any fully unified theory, for every gauge field there will exist electric and magnetic sources. Speaking at the Dirac Centennial Symposium at Tallahassee in 2002, he commented that "the existence of magnetic monopoles seems like one of the safest bets that one can make about physics not yet seen" (Polchinski 2003). The MoEDAL collaboration is working to prove him right.