Magnetic monopole search with the MoEDAL test trapping detector
EPJ Web of Conferences
Magnetic monopole search with the MoEDAL test trapping de- tector
0 Particle physics department, University of Geneva , 1211 Geneva 4 , Switzerland
MoEDAL is designed to search for monopoles produced in high-energy Large Hadron Collider (LHC) collisions, based on two complementary techniques: nucleartrack detectors for high-ionisation signatures and other highly ionising avatars of new physics, and trapping volumes for direct magnetic charge measurements with a superconducting magnetometer. The MoEDAL test trapping detector array deployed in 2012, consisting of over 600 aluminium samples, was analysed and found to be consistent with zero trapped magnetic charge. Stopping acceptances are obtained from a simulation of monopole propagation in matter for a range of charges and masses, allowing to set modelindependent and model-dependent limits on monopole production cross sections. Multiples of the fundamental Dirac magnetic charge are probed for the first time at the LHC.
Magnetic monopoles were first postulated by Dirac in 1931, who showed that with their existence,
electric charge quantisation could be explained as a natural consequence of angular momentum
]. Their introduction would also add symmetry to Maxwell’s equations of electromagnetism.
’t Hooft and Polyakov, in 1974, independently demonstrated that a Grand Unified Theory (GUT)
scheme possesses a monopole solution when a U(1) subgroup of electromagnetism that is embedded
into a larger gauge group is spontaneously broken by the Higgs mechanism [
solutions have been proposed to arise within the electroweak theory itself , which relies on spontaneous
gauge symmetry breaking. The Cho-Maison electroweak monopole would have a mass of the order
of several TeV [
] and is possibly within the range of the Large Hadron Collider (LHC). With the
abundance of monopole theories and no definite estimate on its mass, the search for free magnetic
charges in nature is compelling.
Dirac proposed that the monopoles should carry a magnetic charge g equal to the multiple of
a fundamental unit of magnetic charge referred to as the Dirac charge gD: g = n.gD, where gD is
equivalent to 68.5 times the charge of an electron. The minimum value of the quantisation number
n varies between different theories. According to Dirac, n = 1, 2, 3.., while according to Schwinger,
as well as, Cho and Maison, n = 2, 4, 6.. [
], and n = 3, 6, 9.. or n = 6, 12, 18.. if one considers
the elementary charge to be carried by the down quark. As a result of the high value of the Dirac
magnetic charge, a high velocity monopole is expected to suffer energy losses in matter over ≈4700
times higher than a muon [
]. Thus, in general monopoles in nature will manifest themselves as
highly ionising particles. A free magnetic charge will also induce a persistent current when passed
through a superconducting loop. Searches for monopoles using these two signatures (high ionisation
and magnetic induction) were performed in cosmic rays and in matter, and at accelerators each time a
new energy regime was reached .
At colliders, three kinds of techniques are commonly used to search for magnetic monopoles:
(1) General-purpose detectors with high ionisation energy loss detection capabilities (e.g. OPAL at
] and CDF at the Tevatron [
]); (2) dedicated deployment of nuclear-track detectors [
around the interaction points (e.g. at LEP [
] and at the Tevatron ); and (3) the induction
technique applied to accelerator and detector material in which monopoles have stopped and remained
trapped (e.g. at HERA [
] and at the Tevatron [
]). These searches have excluded the presence
of monopoles with charges equal to or above the Dirac charge and masses up to 400 GeV. Masses
higher by one order of magnitude can be probed at the LHC. For complementary searches, the LHC
programme should include all three of these techniques . The general-purpose ATLAS detector
has performed searches for monopoles at 7 and 8 TeV centre of mass energies [
]. The MoEDAL
experiment near the LHCb interaction point uses a combination of in-flight detection with
nucleartrack detectors and re-usable aluminium trapping detectors .
2 Experimental setup
The Monopole & Exotics Detector at the LHC (MoEDAL) experiment is a dedicated experiment
designed to enhance, in a complementary way, the physics reach of the LHC. The primary objectives
of MoEDAL is to directly search for the Magnetic Monopole or Dyon and other highly ionising Stable
(or meta- stable) Massive Particles (SMPs) at the LHC [
Deployed around the LHCb experiment’s Vertex Locator (VELO) cavern, it is a unique and largely
passive LHC detector that comprises four sub-detector systems. The largest sub-detector systems are
arrays of two nuclear-track detectors which are optimised to probe a range of particle charges. The
TimePix pixel devices form a real-time radiation monitoring system devoted to monitoring of
highlyionising backgrounds in the MoEDAL cavern. The Magnetic Monopole Trapper (MMT) is the other
passive sub-detector system that is capable of capturing long-lived charged particles and measure
directly magnetic charge properties of particles.
2.1 MMT sub-detector system
The MoEDAL MMT test detector system is an aluminium volume placed in the vicinity of the LHCb
interaction point. Aluminium is a good choice for the trapping volume material for three important
reasons. First, the anomalously large magnetic moment of aluminium nucleus means that it will
strongly bind a trapped monopole. Second, aluminium does not present a problem with respect to
activation. Lastly, aluminium allows a cost effective approach to the construction of the MMT detector.
In September 2012, an initial MMT test array comprising 11 boxes each containing 18 rods of 60 cm
length and 2.5 cm diameter was installed and exposed to 0.75 fb−1 of 8 TeV proton-proton collisions.
The boxes were placed downstream of the LHCb detector, 1.5 m away from the interaction point,
immediately in front of the LHCb VELO vacuum vessel, covering 1.3% of the total solid angle (Fig.
1). After the completion of the run, the rods are cut into samples of 20 cm length (except for samples
from box 11, which were cut into a mix of 10, 15, 20 and 30 cm samples), which total to 606 samples
that are later passed through the magnetometer for measurements.
A DC-SQUID rock magnetometer based at the Laboratory of Natural Magnetism (ETH-Zurich)
was used. The magnetometer is capable of detecting monopole charges much less and much greater
than the Dirac charge as was demonstrated in previous studies [
]. The calibration was performed
with a convolution method applied to a dipole sample, and cross-checked using thin solenoids which
mimic a monopole of well known magnetic charge [
]. The response of the magnetometer was found
to be linear and charge symmetric which allowed to express the measured currents in units of Dirac
magnetic charge. Fig. 2 shows the magnetometer current response as one sample traverses through the
magnetometer. The dashed lines show the expected response from the sample if a magnetic monopole
of charge ±gD.
Each of the samples was passed through the magnetometer at least once over a 7-day measurement
campaign in September 2013 1. On average, every tenth measurement was performed with an empty
1The samples from Box 11 were measured earlier in May 2013
9 Sep 10 Sep
Box 5 part of Box 6
part of Box 6 pBaorxt 4of Box 3
11 Sep 17 Sep
part of Box 3 Box 1
sample holder for an offset subtraction. The relevant value is the persistent current, defined as the
difference between the currents measured after and before passage of the sample through the sensing
coil, and then subtracting the difference obtained with a nearby empty holder measurement (Fig. 3).
Whenever the persistent current differs from zero by more than one fourth of the Dirac charge,
the sample is considered as a candidate and measured again several times. A sample that contained
a trapped monopole would consistently yield the same value for repeated measurements, while
deviations in the first measurement correspond to instrumental effects (Fig. 4). The factors that affect
the magnetometer response could be due to flux jumps occurring when the slew rate is large, noise
currents in the SQUID feedback loop, small variations in the length of the sample holder from one
run to the next and accumulation of condensed water and ice in the magnetometer tube near the cold
For model-dependent and model-independent interpretations of the results, magnetic monopoles were
simulated using Drell-Yan (DY) and Single-monopole production [
se 120 ×103
MoEDAL simulation preliminary
DY spin-1/2 monopole
m = 500 GeV
m = 1000 GeV
m = 1500 GeV
m = 2000 GeV
The leading-order DY process produces a pair of heavy monopoles from initial pp state by
quarkantiquark annihilation into a virtual photon using MADGRAPH5 Monte-Carlo event generator. The
monopole coupling to the Z boson is set to zero. PYTHIA is used for the initial-state radiation and
the hadronisation and the underlying event. The kinematic distributions for the spin-1/2 monopoles
are shown in Fig. 5.
00 100 200 300 400 500 600 700 800 900 1000
The Single-monopole samples are generated with a flat kinetic energy distribution ranging from
0 to 10000 GeV and a flat θ and φ distribution which encompasses the angular distribution of the
trapping detector, i.e., 2.4 rad < θ < 3.0 rad and -2.7 rad< φ < -0.5 rad.
The acceptance of the trapping detector is defined on an event-by-event basis as the probability
that the monopole stops inside one of the aluminium rods2. It is determined by propagating monopoles
into the geometry model. The acceptance is defined in a unique way such that it only depends on the
geometry of the detector and not on the production model. Since the collisions are symmetric with
respect to the azimuthal angle φ, only two kinematic variables are needed to define the acceptance in
a model-independent manner. The two variables chosen here are the longitudinal kinetic energy Ekin
and the polar angle θ, after restricting the denominator of the acceptance definition to the range -2.7
rad < φ < -0.5 rad (encompassing the extent of the trapping detector). Thus for Single-monopole
Monte-Carlo samples, the acceptance is mapped for all mass and charge combinations as a function
of Ezkin and θ (with -2.7 < θ < -0.5 rad), as show in Fig. 6 for monopoles with m = 1000 GeV. These
two-dimensional histograms contain all the information needed to obtain the acceptance in any given
pair-production model to a good approximation.
The DY acceptances can be computed in two ways. The first method requires full simulated
pair-produced monopoles to be tracked through the geometry of the MoEDAL detector in order to
compute the acceptances. The more computationally efficient way is to use the Single-monopole
efficiency maps (described above) folded with the DY pair-production kinematics for both spin-1/2
and spin-0 monopoles distributions.
The uncertainty in the amount of material in the geometry description of MoEDAL is the dominant
source of systematic error. The VELO region within the acceptance of the LHCb detector is designed
2In the case of pair-produced DY monopoles, at least one of the two monopoles is required to stop inside the aluminium
to minimise the amount of material obstructing the passage of measured particles. This region has
been well modelled by GEANT4. However, the MoEDAL test trapping detector considered here
lies just upstream of the VELO vacuum vessel in a more complex region containing: cables,
manifolds, and piping, where detailed drawings are not always available. In order to account for possible
imperfect understanding of the material in this arena two geometry models were constructed to
conservatively describe the minimum and maximum amount of material. These models are based on
a detailed physical examination of the material in this region. The uncertainty on the material
description has a direct consequence on the acceptance maps, that affects both model-independent and
model-dependent results. At the time of writing these proceedings, the determination of these
systematic uncertainties is ongoing and will be documented in future publications [
5 Results and conclusions
MoEDAL is designed for passive detection of magnetic monopoles, both in-flight (with the track-etch
technique) and trapped (with the induction technique). This is the first time at a collider experiment
with a dedicated scalable and recyclable monopole trapping array has been deployed. Under the
assumption of monopole capture by aluminium nuclei, there were no magnetic monopoles candidates
found in 606 samples of the MMT test array exposed to 0.75 fb−1 of 8 TeV proton-proton collisions.
Despite its small acceptance and modest received luminosity MoEDAL’s test MMT detector is
capable of probing ranges of charge, mass and energy that were not accessed by other LHC
experiments. Additionally, this technique can yield results very quickly and would allow for and
unambiguous background-free assessment of a signal. A new, larger trapping detector array with a sensitive
mass of around 1 tonne was deployed in 2014 in the LHCb cavern, allowing to perform a search in 13
TeV collisions in the near future.
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