The MoEDAL experiment: a new light on the high-energy frontier

Abstract

MoEDAL is a pioneering LHC experiment designed to search for anomalously ionizing messengers of new physics, such as the magnetic monopole. After a test run at 8 TeV centre-of-mass energy (E_cm), it started official data taking at the LHC at an E_cm of 13 TeV, in 2015. Its groundbreaking physics program defines a number of scenarios that yield potentially revolutionary insights into such foundational questions as: are there extra dimensions or new symmetries; what is the mechanism for the generation of mass; does magnetic charge exist; do topological particles exist; and what is the nature of dark matter? After a brief introduction, MoEDAL's progress to date will be reported, including its past, current and expected future physics output. Additionally, an upgrade to the MoEDAL detector consisting of two new subdetectors: MAPP (MoEDAL Apparatus for Penetrating Particles) now being prototyped at IP8; and MALL (MoEDAL Apparatus for very long-lived particles), will be presented. Finally, a possible astroparticle extension to MoEDAL, called Cosmic-MoEDAL, will be briefly described. This high altitude detector will allow the search for magnetic monopoles to be continued from the TeV scale to the GUT scale.

This article is part of a discussion meeting issue ‘Topological avatars of new physics’.

1. Introduction

MoEDAL is the LHC's first experiment dedicated entirely to the search for new physics [1]. After an initial test run, it started official data taking in 2015. The MoEDAL detector, which is radically different from other collider detectors, is comprised of passive tracking, using plastic nuclear track detectors (NTDs) and trapping subdetectors that are capable of retaining a permanent direct record of discovery and even capturing new particles for further study in the laboratory. It also has a small TimePix pixel device array for monitoring beam-related backgrounds.

MoEDAL is a pioneering experiment designed to directly search for highly ionizing avatars of new physics that include not only magnetic monopoles but also electrically charged massive (pseudo-) stable particles [2]. A full GEANT4 model of MoEDAL is now available. A visualization of this model is shown in figure 1. The primary aim of the LHC's general-purpose detectors (GPDs), ATLAS and CMS, is to discover the Higgs boson and study its properties as precisely as possible. As we all know the discovery of a new particle by ATLAS and CMS, which is now largely identified with the standard model (SM) Higgs boson, was announced on 4th July 2012 [3]. In a similar way, the main aim of the MoEDAL experiment is to search for the magnetic monopole. The modern conception of the magnetic monopole is that it is a topological excitation of the Higgs field, of the underlying theory. In this way, the main physics aims of the GPDs and MoEDAL are complementary. Their experimental sensitivity is also complementary in that GPDs have quite limited sensitivity to highly ionizing particles (HIPs), which MoEDAL is designed to detect.

Figure 1.

A visualization of the GEANT-4 simulation of the MoEDAL experiment, represented using Geant-4's PANORAMIX. (Online version in colour.)

But MoEDAL, like the LHC's GPDs, can do much more. Its groundbreaking physics program defines over 34 scenarios [2] that yield potentially revolutionary insights into such foundational questions as: are there extra dimensions or new symmetries; what is the mechanism for the generation of mass; does magnetic charge exist; do topological particles exist, what is the nature of dark matter; and, how did the big-bang develop from the earliest times?

Another important development is the planning and prototyping of a new MoEDAL subdetector, MAPP (MoEDAL apparatus for penetrating particles). MAPP is planned for deployment during LHC's Run-3. A prototype MAPP detector is already deployed. MAPP will extend MoEDAL's physics reach by allowing the search for milli-charged particles and new weakly interacting very long-lived neutral particles. A further new subdetector, in the longer term planning phase, is MALL (MoEDAL apparatus for extremely long-lived particles). This detector is aimed at the search for new massive charged particles with lifetimes that can reach to of the order of 10 years.

The high-risk nature of MoEDAL's extensive program is justified not only by the prospect of a revolutionary breakthrough with impact beyond the realm of particle physics but also by its now proven ability to provide unique and wide-reaching constraints on new physics. MoEDAL is now preparing a proposal to continue data taking during LHC's Run-3.

2. The MoEDAL detector

The innovative MoEDAL detector employs unconventional methodologies tuned to the prospect of discovery physics. The largely passive MoEDAL detector is deployed at Point 8 on the LHC ring and shares an experimental cavern with the LHCb experiment as shown in figure 1. The innovative MoEDAL detector has a dual nature. First, it acts like a giant camera, comprised of NTDs—analysed offline by ultra fast optical scanning microscopes. Second, it is uniquely able to trap the particle messengers of new physics for further study. MoEDAL's radiation environment is monitored by a state-of-the-art real-time TimePix pixel chip array.

(a). The nuclear track detector array

The MoEDAL detector consists of three detector subsystems. A depiction of part of the MoEDAL detector, showing examples of all MoEDAL subdetector systems is shown in figure 2. The main NTD subsystem, referred to as the low threshold (LT), NTD array, is the largest array of (320) plastic NTD stacks (25 × 25 cm²) ever deployed at an accelerator, comprised of CR39 (3) and MAKROFOL (3) plastic NTD sheets. A depiction of a MoEDAL LT-NTD stack is shown in figure 3a. The track of the HIP damages the long chain molecular bonds of the plastic as shown in figure 3b. Etching in a hot sodium hydroxide etchant as shown in figure 3c reveals this damage zone. The passage of the HIP though the stack, illustrated in figure 3d, allows it to be tracked and its characteristic ionization signature to be recognized.

Figure 2.

(a) A view of part of the MoEDAL deployment showing the LHCb's VELO detector as well as various elements of the MoEDAL detector. (b) The 4sqm VHCC detector plane that is deployed in the downstream acceptance of the LHCb detector. (c) Part of MoEDAL's ‘C-side’ deployment, including part of MoEDAL's NTD array, one of three MMT detector stacks and one of 5 TMPX devices. (d) ‘A-side’ deployment. (Online version in colour.)

Figure 3.

(a) A MoEDAL LT-NTD stack. (b) A depiction of a HIP damaging NTD plastic as it traverses the NTD film. (c) The etching of the NTD plastic to reveal the passage of the particle. (d) A passage of the HIP through the NTD stack defines a track of etch-pits. (Online version in colour.)

The CR39 NTDs have an excellent charge resolution (approx. 0.1e, where e is a single electric charge) and low threshold, allowing the detection of (HIPs) particles with an ionizing power equal to more than approximately five times that of a minimum ionizing particle (MIP). The LT-NTD array has been enhanced by the high charge catcher (HCC) subdetector—with threshold around 50 MIPS—comprised of thin low mass stacks with 3 MAKROFOL sheets in an Al-foil envelope applied directly inside LHCB's forward tracking system—increasing the overall geometrical acceptance for monopoles of the NTD system to 65%. The NTD detectors are calibrated at two centres. The first is the NA61 experiment at CERN where we have access to very high-energy heavy-ion beams. This year it will be lead-ions. The second is the NASA Space Radiation Laboratory (NSRL) facility at the Brookhaven National Laboratory (BNL). We have already gone through a few calibration cycles at both these centres.

After exposure for 1 year, the NTD detectors are removed for etching under controlled conditions. The etching removes plastic preferentially along the damage trail caused by the passage of a HIP through the NTD. This process reveals etch pits on one or both sides of the plastic sheet, depending on whether the particle penetrates the sheet. The size and depth of the pit are proportional to the charge of the particle from which the pit was derived. The etching conditions are of two kinds: strong etching and soft etching. Soft etching is used when low-threshold signals are being sought otherwise strong etching is used. The etching conditions are determined from the calibration process where NTDs stacks are exposed to heavy-ion beams of known charge and energy. Etching takes place at INFN Bologna using facilities that were originally developed for the MACRO [4] and SLIM [5] experiments.

After the NTDs sheets are etched they must be scanned using optical microscopes, since the feature (etchpit) sizes lie in the range $20\to 50$ μm. The etching and scanning of MoEDAL's NTDs take place at INFN Bologna using a system of manual microscopes and computer-assisted optical scanning microscopes. For MoEDAL data taking during Run-3, we will also deploy a computer controlled optical scanning microscope system developed at the University of Helsinki. The system will be controlled by special artificial intelligence (AI) software developed by MoEDAL's machine learning group, designed to recognize signal etch-pits in the presence of beam-induced backgrounds. A photograph of the Helsinki scanning systems is shown in figure 4.

Figure 4.

The Helsinki computer controlled optical scanning microscopes. (Online version in colour.)

(b). The trapping detector system

The magnetic monopole trapper (MMT) is another vital MoEDAL subdetector system. Its sensitive volume consists of 794 kg of aluminium trapping volumes deployed around the MoEDAL cavern at IP8. A photograph of the C-side MMT stack is given in figure 5a comprised of aluminium bar sections shown in figure 5b. Similar sized MMT stacks are also deployed on the A-side of the cavern and in front of LHCb's VELO detector. Aluminium is used to fabricate the trapping volumes due to its anomalously large nuclear magnetic moment.

Figure 5.

(a) The C-side MMT detector stack (with two LT-NTD detectors planes attached to its front) is one of three similar arrangements. (b) The aluminium bars that comprise the MMT stacks. (c) The response of the SQUID to MMT volumes prior to exposure and presumably with no trapped magnetic charge. (d) Is MoEDAL SQUID facility based at ETH Zurich. (Online version in colour.)

The exposed bars are monitored for the presence of monopoles using a SQUID magnetometer at the Geomagnetische Messstation Addisberg Facility, shown in figure 5d. A fraction of the monopoles traversing the MMT detector stack will slow down, stop and be trapped by the magnetic effect of the nucleus. After exposure the MMT trapping volumes are monitored at the ETH-Zurich SQUID facility for the presence of captured monopoles. The response of the MoEDAL SQUID magnetometer, shown in figure 5c, has a resolution of around 0.1g_d, where g_d is a Dirac charge. The SQUID is calibrated with very long thin solenoids that are effective monopoles.

3. Physics results

In August 2016, MoEDAL published its first physics analysis paper [6] on the search for magnetic monopoles based on data taken at a centre-of-mass energy (E_cm) of 8 TeV. In this paper, MoEDAL placed the best limits in the world in the search for magnetic monopoles with Dirac magnetic charge greater than or equal to 2g_d. In February 2017, we published the results of our first paper on LHC Run-2 data taken at an E_cm of 13 TeV [7]. In this paper, we were still the only LHC experiment placing limits on g_d > 1. Additionally, we pushed the direct search for multiple magnetically charged particles to 5g_d, with masses up to 6 TeV, for the first time ever. Using a Drell-Yan (DY) model for monopole-pair production with spin-1/2 monopoles, this translates into monopole mass limits exceeding 1 TeV, the strongest to date at a collider experiment [8] for charges ranging from $2\to 4$ times g_d.

The Run-2 results published placed limits on spin-0, spin-1/2 and, for the first time, spin-1 monopole production. In addition, the Run-2 analyses placed limits for both β-dependent and β-independent monopole couplings, another first for direct searches for monopoles. Additionally, we placed monopole-pair direct production cross-section upper-limits in the range $40\to 105$ fb for magnetic monopole charges up to 5g_D and monopole masses up to 6 TeV.

We recently completed an extension of our monopole search criteria to include, for the first time at the LHC, monopole-pair production via photon-fusion [9]. This paper follows on from a detailed theoretical study of these processes carried out by MoEDAL authors [10]. In this paper, we have employed duality arguments to justify an effective monopole-velocity-dependent magnetic charge in monopole-matter scattering processes. Based on this, we conjecture that such β-dependent magnetic charges might also characterize monopole production. In addition, we introduced a magnetic-moment term proportional to a new phenomenological parameter κ describing the interactions of these monopoles with photons for spins 1/2 and 1. The lack of unitarity and/or renormalizability is restored when the monopole effective theory adopts an SM form.

The motivation behind our introduction of the concept of the magnetic-moment of the monopole introduced above is to enrich the monopole phenomenology with the (undefined) correction terms to the monopole magnetic moment to be treated as free parameters, potentially departing from those prescribed for the electron or W^± bosons in the SM. As we lack a fundamental microscopic theory of magnetic poles, such an addition appears reasonable. This creates a dependence of the scattering amplitudes of processes on this parameter, which is passed on to the total cross sections and, in some cases, to kinematic distributions. Therefore, the parameter is proposed as a new tool for monopole searches that can be used to explore different models. An important conclusion from this paper [10] is that photon fusion production is an important mechanism for direct monopole searches at the Large Hadron Collider (LHC). A comparison of the above MoEDAL results with the published results of the ATLAS [11,12] and CDF [13] experiments is given in figure 6.

Figure 6.

A summary plot of the mass limits placed on monopole-pair production by MoEDAL, ATLAS and CDF. (Online version in colour.)

Our analysis of photon-fusion and DY production of monopole pairs used the full MoEDAL trapping detector and [9] 4.0 fb⁻¹ of 13 TeV proton–proton collisions at the LHCb interaction point. Magnetic charges equal to or above a single Dirac charge were excluded in SQUID scans of all samples. Monopole spins 0, 1/2 and 1 were considered and both velocity-independent and -dependent couplings are assumed. This search provides the best current laboratory constraints for monopoles with magnetic charges ranging from two to five times the Dirac charge.

No candidates remained after our scanning procedure was completed and cross-section upper limits as low as 11 fb were set, improving previous limits of 40 fb also set by MoEDAL [8]. We considered the combined photon-fusion and DY monopole-pair direct production mechanisms; the former process for the first time at the LHC. Mass limits in the range 1500–3750 GeV c⁻² were set for magnetic charges up to 5g_D for monopoles of spins 0, 1/2 and 1—the strongest to date at a collider experiment [14] for charges ranging from two to five times the Dirac charge. The results are summarized in figure 7. The most recent limit on photon-fusion, carried our nearly ten years ago, was made using CDF data with the result that the monopole mass must be greater than 370 GeV [15]. Previously, roughly 13 years ago, the H1 Collaboration at HERA carried out a search, based on monopole-pair production via photon fusion [16].

Figure 7.

95% CL mass limits in models of spin-0, spin-1/2 and spin-1 monopole pair direct production in LHC pp collisions. The present results are interpreted for Drell-Yan and combined DY and photon fusion production with both β-independent and β-dependent couplings.

(a). Examples of ongoing analyses

MoEDAL has a number of ongoing analyses and planned analyses. We shall briefly describe three here. The most advanced ongoing analysis is a search for highly electrically charged objects (HECOs) with spin-1/2, spin-0 and spin-1. In this search, we used data from MoEDAL's NTD system for the first time. No such particles were observed in 1–2 fb⁻¹ of data. To date, the only LHC limit we can find comes from the ATLAS Collaboration [12] where they placed limits on electrically charged particles with charge (Z) in the range: 10e < Z < 60e. Even though this first MoEDAL limit on HECO production only uses Run-1 data, we envisage that the limits on HECO productions will be the best in the world.

As we ramp up analyses that use our NTD system, we will be able to extend our searches to include less ionizing particles that nevertheless exceed the detection threshold of our most sensitive (CR39) NTD detectors of Z/β≥5, for example, massive (meta)-stable slowly moving singly charged particles. We have completed a preliminary study of such objects arising from SUSY scenarios [17]. In this study, using Monte Carlo simulations, we compared the sensitivities of MoEDAL versus ATLAS/CMS, for various long-lived particles (LLPs) in supersymmetric models. An example of a competitive result is shown in figure 8 where the MoEDAL sensitivity is compared with a CMS analysis [18]. The scenario, in this case, is gluino ($\tilde{g}$) pair production where the $\tilde{g}$ decays to a long-lived neutralino (${\tilde{\chi }}_1^0$) decaying to a metastable stau (${\tilde{\tau }}_1$) and a τ is considered. The mass splitting between the ${\tilde{\chi }}_1^0$ and the $\tilde{g}$ (${\tilde{\tau }}_1$) is 30 GeV (300 GeV). A ${\tilde{\tau }}_1$ detection threshold Z/β≥5 is assumed. It seems that for longer-lived particles moving with Z/β≥5 in more complex topologies MoEDAL does appear to be able to make a useful contribution to the search for such phenomena.

Figure 8.

MoEDAL discovery reach requiring two signal events versus CMS 95% CL exclusion plot in the $\tilde{g}$ mass versus ${\tilde{\chi }}_1^0$ lifetime plane for Run 2 (13 TeV) and for Run 3 (14 TeV) integrated luminosities. (Online version in colour.)

In March of 2019, the beryllium section of the CMS beam-pipe, originally deployed for Run-1 running and removed in the first long shutdown of the LHC (LS1), was donated to the MoEDAL Collaboration. A photograph of the removal of this beam-pipe section is shown in figure 9. During the spring and summer of 2019 the MoEDAL Collaboration examined the central section of this beryllium beam-pipe section for the presence of trapped monopoles. This required the beam-pipe to be cut into small enough pieces and passed through the ETH Zurich SQUID Magnetometer to test for the process of trapped magnetic charge. Such a search is sensitive to high magnetic charges of around 6g_d and above, which would be too highly ionizing to penetrate the beam-pipe walls and enter the detector. Consequently, a search for monopoles trapped in the walls of this beam-pipe would be complementary to MoEDAL searches which are sensitive to magnetic charges up to 6–7g_d. However, it should be noted that the issue of whether or not monopoles would be trapped in Be is not completely clear [19,20].

Figure 9.

A photograph of the old CMS-beam pipe being removed. (Online version in colour.)

4. The program for Run-3

The luminosity delivered to IP8 during LHC's Run-2 is shown in figure 10. The average luminosity delivered per year over the four years of Run-2 to MoEDAL is 1.7 fb⁻¹. The luminosity available over the at IP8 will increase substantially to 25 fb⁻¹ to 30 fb⁻¹ over the three years of Run-3, an increase in the average luminosity taken per year at IP8 by roughly a factor of five. In order to push the search for new physics to higher E_cm (14 TeV) and also to take advantage of the expected significantly higher luminosity available at Point 8 during Run-3, the MoEDAL Collaboration is requesting to take data as part of the LHC's Run-3 program. This future physics program will take the search for HIPs such as magnetic monopoles, and new massive long-lived/pseudostable electrically charged particles, to higher luminosity (30 fb⁻¹) and slightly higher energy, from 13 TeV to 14 TeV. Importantly, we are presently preparing two new subdetectors for MoEDAL that will expand its discovery horizon for LHC's Run-3. These two new subdetectors are MAPP (MoEDAL Apparatus for Penetrating Particles) and MALL (MoEDAL Apparatus for the detection of extremely long-lived particles).

Figure 10.

A plot of the luminosity delivered to MoEDAL and LHCb at IP8. (Online version in colour.)

(a). The MAPP subdetector

The MoEDAL Apparatus for Penetrating Particles (MAPP) will be installed in the UGC1 gallery adjacent to the existing MoEDAL/LHCb detectors in order to take data during LHC's Run-3. The purpose of this deployment is to expand the physics reach of the existing MoEDAL experiment to include the search for fractionally charged particles with charge as low as 0.001e (where e is the charged of the electron) and for long-lived weakly interacting neutral messengers of new physics. A sketch of the MAPP detector is given in figure 11. MAPP is protected from SM particles from interactions at IP8 by roughly 30 m of rock and from cosmic rays by an overburden of approximately 100 m of limestone. The MAPP detector can be deployed in a number of positions ranging from approximately 5° at distance of approximately 55 m from IP* to 30° at a distance of approximately 25 m from IP8. Additionally, the MAPP detector has a decay zone of 7 m to 10 m in front of the detector to the midpoint of the detector that makes it possible to measure the decay of new neutral particles in flight at large distances from the IP.

Figure 11.

A sketch of the MAPP subdetector. (Online version in colour.)

The MAPP Detector is composed of two subdetectors each with a different purpose. The compact central section of MAPP is made up of two collinear sections, with cross-sectional area of 1.0 m², each comprised of 100 (10 cm × 10 cm) plastic scintillator bars each 1.5 m long. Each bar is readout by two low noise PMTs. Thus, each through-going particle from the IP will encounter 3.0 m of scintillator and be registered by a coincidence of 4 PMTs, The detectors are protected from cosmic rays and from particle interactions in the surrounding rock by charged particle veto detectors.

A physics scenario [21] in which milli-charged particles (mQp), i.e. particles with an electric charge much smaller than the electric charge e, have been discussed in connection with the mechanism of electric charge quantization and possible non-conservation of electric charge [22]. The existence of mQps has also been proposed in various extensions of the SM. In particular, in the hidden (dark) sector models with a new U(1) gauge group [23,24]. We use as an example a scenario in which a new U(1) in dark sector with a massless dark-photon (A′) and a massive dark-fermion (ψ), is predicted. The dark-fermion is charged under the new U(1) field A′ with charge e′. The mQp (ψ) couples to the photon and Z⁰ boson with a charge κe′cosθ_W and −κe′sinθ_W, respectively. The fractional charge in units of the electric charge is therefore, κe′cosθ_W/e.

A study of the sensitivity of the MAPP detector to such a possibility [21] is given in figure 12 shows that with the luminosity expected at IP8 during Run-3 we can expect to be sensitive to mQps with charge in the range 0.01e to 0.001e. An estimate of the sensitivity of the MAPP detectors to this scenario is given in figure 12. The MAPP detector is competitive with the MilliQan experiment [21] that is also preparing to take data, adjacent to the CMS experiment at Run-3.

Figure 12.

The maximum reach of the MAPP detector using 30 fb⁻¹ of data in LHC's Run-3. The solid line represents that points at which three events would be observed assuming 100% efficiency and no background. The dotted line represents the case where three events are observed with an overall signal detection efficiency of 10%. (Online version in colour.)

The second purpose of the MAPP detector is to pursue the quest for new weakly interacting long-lived neutral particles, i.e. those with macroscopic decay length. Observed particles have lifetimes spanning many orders of magnitude, ranging from that of the Z boson at 2 × 10⁻²⁵ s through to the free proton which, if it does decay, is constrained to be at least 1.67 × 10³⁴ years. The free neutron is the longest-lived unstable particle with a lifetime of around 15 min. Of course, many elementary particles are considered to be stable such as the electron and neutrinos. But, all known longer lived unstable neutral particles are strongly interacting. Consequently, at the LHC a clear signature for physics beyond the SM would be a weakly interacting neutral penetrating particle with a long enough lifetime to be able to clear the smog of SM particles closer to the LHC's interaction points.

LLPs arise in a number of well-motivated models of physics that supercede the SM. These scenarios include the much considered minimal supersymmetric standard model (MSSM) [25] as well as more recently proposed theoretical frameworks such as hidden sector dark matter [26–30] and neutral naturalness [31–33]. LLPs can naturally arise due to the presence and breaking of symmetries, which can be motivated by cosmological considerations; solutions to the hierarchy problem; and the issue of neutrino masses. Importantly, LLPs are a generic prediction of new hidden sectors at or below the weak scale [34–42]. A comprehensive review of theoretical motivations for LLPs can be found elsewhere [43].

The MAPP-LLP detector has three pairs of hodoscopes, each detector plane is 3 m × 4 m, and is designed to track particles consistent with originating from the decay zone defined by a charged particle veto on the tunnel wall and the 1st/2nd pair of hodoscopes. At the 5° position of the MAPP detector placed at a distance of roughly 50 m from the IP the fiducial volume of MAPP for LLPs is defined by the size of the tracking detectors the forward charged particles veto and the total depth of approximately 10 m in which decays in-flight could be detected. A sketch of the MAPP-LLP detector is shown in figure 11. The LLP design is currently under review and is likely to be upgraded in the near future.

The sensitivity of MAPP to LLPs is investigated by using various benchmark processes. One such derives from a dark sector model where a Higgs mixing portal admits exotic inclusive $B\to X_s\varphi$ decays, in which ϕ is a light CP-even scalar that mixes with the Higgs, with mixing angle θ≪1 [44]. We use the exclusive decay $B\to K\varphi$ to estimate the fiducial efficiency for the process $B\to X_s\varphi$. The fiducial efficiency was estimated to be at maximum several times 10⁻⁴. This is a competitive number that will allow us to substantially expand the discovery reach for the above process at Run-3.

A number of backgrounds need to be considered even though the MAPP detector is separated from the IP by 60 interaction lengths The backgrounds from cosmic muons are highly suppressed for a number of reasons, these are: the shielding provided by the approximately 100 m rock overburden; the fact that cosmics are out of time with respect to LHC collisions; and, the ‘verticality’ of the cosmic muons as opposed to the dominant ‘horizontal’ component of particles from the IP. Once these factors are taken into account it is clear that muons from the IP are an important background. In this case, muons directly incident on the detector would be ruled out by the forward charged particle veto. Another possibility is that muons that just by-pass the forward veto scatter on air in the tunnel to produce particles that move back into the fiducial volume of MAPP.

Neutral particles, such as neutrinos, neutrons and K⁰_Ls, provide another important source of backgrounds, to which neutrons make the main contribution. Neutrino-air events in the volume of the MAPP detector arise from two sources: atmospheric neutrinos; and neutrinos from the IP. The neutrino background from the IP requires a detailed simulation of the material budget around MoEDAL extending out to MAPP, which is currently in preparation. But, initial estimates show that when an initial energy cut of E_ν > 0.5 GeV is applied the contribution to the neutrino induced backgrounds in MAPP from hadron decays in the rock shielding is negligible.

There are a number of ways that backgrounds from the IP and from cosmics will be severely reduced in MAPP. The main background reduction factor comes from by the approximately 60 nuclear interaction lengths (nils) of rock between the IP and the MAPP detector. Additionally, powerful background rejection factors arise from the tracking and timing. Additionally, we shall include in front of each pair of scintillator tracking detectors an iron radiator layer (six radiation lengths, approx. 10 cm, wide) this will enable us to identify photons and electrons providing another means of separating signal from background. Although studies of the various backgrounds are continuing, we have found no show stoppers so far.

(b). The MALL subdetector

The existence of hyper long-lived particles (HLLCPs), with lifetimes well in excess of 1 s, has been suggested in superWIMP dark matter scenarios [45]. Because superWIMP dark matter interacts only gravitationally, searches for its effects in standard dark matter experiments are challenging. At the same time, this superweak interaction implies that WIMPs decaying to it do so after Big Bang nucleosynthesis. However, the search for HLLCPs at the LHC is challenging but possible. One possible way to achieve this goal is to look for the subsequent decays of HLLCPs that slow down and stop in the mass of the detector. An ATLAS search sought HLLCP decays when they occurred in empty bunch crossings and out of time with p-p interactions [46]. Another suggestion was to leave the large detectors in data-taking mode for a period after collisions cease for the year and monitor the subsequent decays of HLLCPs in the absence of SM backgrounds form p-p interactions [47].

The MALL subdetector, which is in its planning phase, is intended to push the search for the decays of new HLLCPs, with lifetimes ranging up to 10 years. This is achieved by monitoring the MoEDAL Trapping volumes, contained within the central portion of an hermetic scintillator array, for the decays of trapped particles. A sketch of the MALL sub-detector is shown in figure 13. Remarkably, this detector will not be positioned at IP8 on the LHC but rather in a remote deep underground laboratory, such as SNOLAB. In this way, cosmic ray backgrounds can be reduced to a minimum.

Figure 13.

A sketch of the MALL subdetector proposed for MoEDAL. (Online version in colour.)

(c). Cosmic MoEDAL

Cosmic monopoles that are detectable on Earth can either be light, for example, produced locally in cosmic-ray interactions with the atmosphere with mass less significantly less that the GUT scale, or else so heavy that they can only have been produced soon after the birth of the Universe in the Big Bang. Most of these searches have been based on the premise that the monopoles are produced in a symmetry-breaking phase transition in the early Universe as topological defects via the Kibble mechanism [48]—typically with the GUT mass approx. 10¹⁶ GeV c⁻². Some GUT models and some supersymmetric models predict intermediate mass monopoles (IMMs) with masses 10⁵ < m_MM < 10¹² GeV and with magnetic charges of multiples of g_D; these IMMs may have been produced in later phase transitions in the early Universe and could be present in the cosmic radiation [49–52].

Detectors underground, underwater and under ice would mainly have sensitivity for IMMs coming from above [53,54]. Detectors at the Earth surface could detect IMMs coming from above if they have masses larger than 10⁵–10⁷ GeV [55] with velocities (β = v/c) of 10⁻³ to 10⁻¹. Lower mass IMMs may be detected with detectors located at high mountain altitudes or in balloons and satellites. SLIM was the first experiment to extend the search for cosmic monopoles with masses well below the GUT scale, with a high sensitivity [55]. However, SLIM's modest size (approx. 400 m²) precluded it searching for a flux of cosmic monopoles below the Parker Bound [56]. I have proposed [57] a very large array (approx. 10 000 m²) of CR-39 detectors—‘Cosmic-MoEDAL’—to be deployed at very high altitude, for example at Mt Chacaltaya laboratory. in Bolivia with an elevation of 5400 m. Such an array would be able to take the search for cosmic monopoles with velocities β∼0.1, from the LHC's TEV scale all the way to the GUT scale, for monopole fluxes well below the Parker Bound. An artist's impression of such an array is shown in figure 14. Monopole lux upper limits achievable with Cosmic-MoEDAL are shown in figure 15.

Figure 14.

An artist's impression of the Cosmic-MoEDAL experiment deployed on Mt Chacaltaya. (Online version in colour.)

Figure 15.

Flux upper limits for cosmic MMs of charge = g_d and β > 0.05 versus monopole mass. The figure shows the 90%CL limits obtained by the SLIM [5], MACRO [58.61] and OHYA [62] experiments. MMs with masses smaller than 5 × 10¹³ GeV are detected only if coming from above; monopoles with masses larger than 5 × 10¹³ GeV can traverse the Earth, so an isotropic flux is expected. (Online version in colour.)

5. Concluding remarks

The MoEDAL experiment has already placed the world's most stringent limits on multiply charged magnetic monopole production. It is now poised to push the search for HIPs to lower charge thresholds, where it is sensitive to new physics scenarios involving, for example, supersymmetry and extra dimensions. Importantly, MoEDAL will be seeking to continue data taking during LHC's Run-3. This will enable MoEDAL to continue its search for highly ionizing avatars of new physics to smaller cross-sections and higher energy. Also, the deployment of the new MAPP subdetectors will enable MoEDAL to expand its physics horizons to include the search for fractionally charged particles and new very long-lived neutral particles. Additionally, the new MALL subdetector will allow MoEDAL to search for massive charged HLLCPs. It is envisaged that MoEDAL will also take data at the High Luminosity LHC. Plans for Cosmic-MoEDAL, the astroparticle extension of MoEDAL, are also underway, allowing MoEDAL to continue the search for the magnetic monopole up to the GUT scale.

Data accessibility

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Competing interests

I declare I have no competing interests.

Funding

Computing support was provided by the GridPP Collaboration, in particular, from the Queen Mary University of London and Liverpool grid sites. This work was supported by grant no. PP00P2150583 of the Swiss National Science Foundation; by the UK Science and Technology Facilities Council (STFC), via the research grant nos ST/L000326/1, ST/L00044X/1, ST/N00101X/1 and ST/P000258/1; by the Generalitat Valenciana via a special grant for MoEDALand via the Project No. PROMETEO-II/2017/033; by the Spanish Ministry of Science, Innovation and Universities (MICIU), via the grant nos FPA2015-65652-C4-1-R, FPA2016-77177-C2-1-P, FPA2017-85985-P and FPA2017-84543-P; by the Severo Ochoa Excellence Centre Project No. SEV-2014-0398; by a 2017 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation; by the Physics Department of King's College London; by a Natural Science and Engineering Research Council of Canada via a project grant; by the V-P Research of the University of Alberta; by the Provost of the University of Alberta; by UEFISCDI (Romania); by the INFN (Italy); and by the Estonian Research Council via a Mobilitas Plus grant MOBTT5.