arXiv:0801.4913v2 [hep-ex] 28 May 2008
Magnetic Monopole Search at high altitude with the SLIM
S. Balestra1,2, S. Cecchini1,3, M. Cozzi1,2, M. Errico1,2, F. Fabbri2, G. Giacomelli1,2, R. Giacomelli2, M.
Giorgini1,2, A. Kumar1,4, S. Manzoor1,5, J. McDonald6, G. Mandrioli2, S. Marcellini2, A. Margiotta1,2,
E. Medinaceli1,7, L. Patrizii2, J. Pinfold6, V. Popa2,8, I.E. Qureshi5, O. Saavedra9,10, Z. Sahnoun2,11,
G. Sirri2, M. Spurio1,2, V. Togo2, A. Velarde7and A. Zanini10
(1) Dip. Fisica dell’Universit´ a di Bologna, 40127 Bologna, Italy
(2) INFN Sez. Bologna, 40127 Bologna, Italy
(3) INAF/IASF Sez. Bologna, 40129 Bologna, Italy
(4) Physics Dept., Sant Longowal Institute of Eng. & Tech., Longowal, 148 106, India
(5) PD, PINSTECH, P.O. Nilore, and COMSATS-CIIT, No. 30, H-8/1, Islamabad, Pakistan
(6) Centre for Subatomic Research, Univ. of Alberta, Edmonton, Alberta T6G 2N4, Canada
(7) Laboratorio de F´ ısica C´ osmica de Chacaltaya, UMSA, La Paz, Bolivia
(8) Institute for Space Sciences, 077125 Bucharest-M˘ agurele, Romania
(9) Dip. Fisica Sperimentale e Generale, Universit´ a di Torino, 10125 Torino, Italy
(10) INFN Sez. Torino, 10125 Torino, Italy
(11) Astrophysics Dept., CRAAG, BP 63 Bouzareah, 16340 Algiers, Algeria
The SLIM experiment was a large array of nuclear track detectors located at the Chacaltaya high
altitude Laboratory (5230 m a.s.l.). The detector was in particular sensitive to Intermediate Mass Mag-
netic Monopoles, with masses 105< MM< 1012GeV. From the analysis of the full detector exposed for
more than 4 years a flux upper limit of 1.3 · 10−15cm−2s−1sr−1for downgoing fast Intermediate Mass
Monopoles was established at the 90% C.L.
In 1931 Dirac introduced Magnetic Monopoles (MMs) in order to explain the quantization of the electric
charge, obtaining the formula eg = n?c/2, from which g = ngD= n?c/2e = n 68.5e = n 3.29 · 10−8in
the c.g.s. symmetric system of units ; n is an integer, n = 1,2,3,... MMs possessing an electric charge
and bound systems of a magnetic monopole with an atomic nucleus are called dyons. An extensive
bibliography on MMs is given in ref. . Relatively low mass classical Dirac monopoles have been
searched for at high energy accelerators [3, 4].
Magnetic Monopoles are present in a variety of unified gauge models with a wide range of masses.
Grand Unified Theories (GUT) of the strong and electroweak interactions at the mass scale MG∼
1014÷1015GeV predict the existence of magnetic monopoles, produced in the early Universe at the end
of the GUT epoch, with very large masses, MM≥ 1016GeV. Such monopoles cannot be produced with
existing accelerators, nor with any foreseen for the future. In the past, GUT poles were searched for in
the cosmic radiation. These poles are characterized by low velocities and relatively large energy losses
. The MACRO experiment set the best limits on GUT MMs with g = gD, 2gD, 3gDand dyons at the
level of ∼ 1.4 · 10−16cm−2s−1sr−1for 4 · 10−5< β = v/c < 0.7 .
Some GUT models and some supersymmetric models predict Intermediate Mass Monopoles (IMMs)
with masses 105< MM < 1012GeV and with magnetic charges of multiples of gD; these MMs may
have been produced in later phase transitions in the early Universe and could be present in the cosmic
radiation [7, 8].
IMMs may be relativistic since they could be accelerated to high velocities in one coherent domain of
the galactic magnetic field. In this case one would have to look for downgoing, fast (β > 0.03), heavily
The main purpose of the SLIM (Search for LIght Monopoles) experiment at the Chacaltaya laboratory
in Bolivia at 5230 m a.s.l., was the search for IMMs . An exposure at a high altitude laboratory allows
to search for MMs of lower masses, higher magnetic charges and lower velocities, see Fig. 1.
The searches for IMMs by Earth based detectors are essentially limited to downgoing particles .
Water Cherenkov detectors are limited to fast downgoing IMMs (with β > 0.5), and a search can be done
if the detectors are able to discriminate against the large background of cosmic ray muons .
Figure 1: Accessible regions (above lines) in the plane (mass, β) for monopoles with magnetic charge
g = gD coming from above for an experiment at altitudes of 20000 m, 5230 m, and for an underground
detector at the Gran Sasso Lab. (average rock overburden of 3700 m.w.e.)
The SLIM detector was also sensitive to Strange Quark Matter nuggets [12, 13] and Q-balls . The
results on these Dark Matter candidates are discussed in ref. .
In the following, we present a short description of the SLIM apparatus, the calibrations of the Nuclear
Track Detectors (NTDs), the etching and analysis procedures, and the limits obtained by the experiment
on IMMs and GUT Magnetic Monopoles.
1The interest in MMs was also connected with the possibility that they could yield the highest energy cosmic rays .
Figure 2: Left: the SLIM modules installed at Chacaltaya. Right: composition of one of the 7410
modules; each module was enclosed in an aluminized mylar bag filled with dry air at a pressure of 1
2 Experimental procedure
The SLIM experiment was an array of NTDs2with a total surface area slightly greater than 400 m2.
The array was organized into 7410 modules, each of area 24 × 24 cm2. All modules were made up of:
three layers of CR39R
Lexan each 0.25 mm thick and one layer of aluminum absorber 1 mm thick (see Fig. 2 right). The CR39
used in about 90% of the modules (377 m2) was of the same type used in the MACRO experiment .
The remaining modules, 50 m2, utilized CR39 containing 0.1% of DOP additive, CR39(DOP).
Each module (stack) was sealed in an aluminized plastic bag (125 µm thick) filled with dry air at a
pressure of 1 bar. The modules were transported to La Paz, Bolivia, from Italy in wooden boxes and
their position with respect to the other modules in the shipping crate was recorded. The stacks were
deployed under the roof of the Chacaltaya Laboratory, roughly 4 m above ground (see Fig. 2 left). The
installation of the SLIM detectors started in February 2000 and ended in February 2002. The return of
the material to Italy was organized in batches, after the completion of the 4 years exposure.
The atmospheric pressure at Chacaltaya is about 0.5 bar; before shipping to Chacaltaya, in Bologna
we checked the air tightness of the envelopes sealed with air at a pressure of 1 bar by placing a sample of
them in an airtight tank at a pressure of 0.3 atm for a few months; no significant leakage was detected.
From the experience gained with the MACRO Nuclear Track Subdetector , we know that the used
CR39 does not suffer from “aging” or “fading” effects for exposure times as long as 10 years . Further
calibrations with 1 AGeV Fe26+ions in 1999 and 2005 and with 158 AGeV In49+in 2003 confirmed the
quality and the stability of the CR39 used in the SLIM experiment .
? 3, each 1.4 mm thick; 3 layers of Makrofol DER
? 4, each 0.48 mm thick; 2 layers of
2Another 100 m2of NTDs were installed at Koksil (Pakistan, 4275 m a.s.l.) since 2002 and were not used in the present
3The SLIM CR39 was produced by the Intercast Europe Co, Parma, Italy according to our specifications.
4Manufactured by Bayer AG, Leverkusen, Germany.
During the first phases of the detector deployment we evaluated possible effects of climatic conditions on
the detector response and possible backgrounds. Previous tests had shown that the CR39 response does
not depend on the time elapsed from its production and the passage of the particle if the ambient tem-
perature ranges between -20◦C and +30◦C. The minimum and maximum values of the air temperature
in each detector hall in Chacaltaya was recorded 3 times a day over the lifetime of the experiment. The
temperature values usually ranged from 0◦C to 30◦C with an average value of 12◦C for the whole year
and from one year to the other; however in the summer months in very few cases temperatures down
to -5◦C were measured in the early morning. Therefore, no significant variations were expected in the
detector response over the exposure period.
We performed measurements of the radon concentration in different locations of the experimental
rooms where the SLIM detectors were placed. We used for this purpose E-PERMR
The measured radon activity was about 40 ÷ 50 Bq/m3of air. According to our previous experience
with the MACRO NTDs, we concluded that this level of radon induced radioactivity did not present a
problem for the experiment, even in case of radon diffusion into the module bags.
Two different types of neutron detectors (BTI bubble counters and a BF3 counter detectors) were used
to measure the neutron flux at Chacaltaya, during the first installation shift of 2001 over the energy range
of a few hundred keV to about 20 MeV . Neutrons of these energies interacting inside the detectors
could induce background tracks, and their density could affect the scanning speed and efficiency. Both
types of neutron detectors measured the accumulated dose. Consistent results were obtained by both
types of detectors. The accumulated dose measured in open air and near the detectors was very similar.
The absolute neutron flux was computed using the BTI bubble counters for which the efficiency is known.
A value of (1.7 ± 0.8) · 10−2cm−2s−1was obtained, which is in agreement with other reported neutron
flux data at the altitude of Chacaltaya and with more recent measurements at the same location .
The necessity to reduce the neutron induced background in CR39 required us to study special etching
procedures, mainly based on the addition of ethyl alcohol to the etching solutions. As discussed in the
next section, the addition of alcohol reduces the background tracks on the detector sheets and improves
the surface quality (i.e. greater transparency), at the expense of a higher threshold .
2.2 Etching procedures
The passage of a magnetic monopole in NTDs, such as CR39, is expected to cause structural line damage
in the polymer (forming the so called “latent track”). Since IMMs have a constant energy loss through
the stacks, the subsequent chemical etching should result in collinear etch-pit cones of equal size on both
faces of each detector sheet. In order to increase the detector “signal to noise” ratio different etching
conditions [16, 17] were defined. The so-called “strong etching” technique allows better surface quality
and larger post-etched cones to be obtained. This makes etch pits easier to detect under visual scanning.
Strong etching was used to analyze the top-most CR39 sheet in each module. “Soft etching” was applied
to the other CR39 layers in a module if a candidate track was found after the first scan. This process
allows to proceed in several etching steps and study the formation of the post-etched cones.
For CR39 and CR39(DOP) the strong etching conditions were: 8N KOH + 1.5% ethyl alcohol at
75◦C for 30 hours. The bulk etching velocities were vB= 7.2 ± 0.4 µm/h and vB= 5.9±0.3 µm/h for
CR39 and CR39(DOP), respectively.
The soft etching conditions were 6N NaOH + 1% ethyl alcohol at 70◦C for 40 hours for CR39 and
CR39(DOP). The bulk etching rates were vB= 1.25 ± 0.02 µm/h and vB= 0.98 ± 0.02 µm/h for CR39
and CR39(DOP), respectively.
Makrofol NTDs were etched in 6N KOH + 20% ethyl alcohol at 50◦C for 10 hours; the bulk etch
velocity was vB= 3.4 µm/h.
0 4000800016000 12000
0 5000 100002000015000
Figure 3: Calibrations of CR39 nuclear track detectors with 158 A GeV In49+ions and their nuclear
fragments with decreasing charge. The base areas (1 pixel2= 0.3 µm2) of the etched cones were
averages over 2 faces. The CR39 was etched in (a) soft and (b) strong etching conditions.
The CR39 and Makrofol nuclear track detectors were calibrated with 158 A GeV In49+and Pb82+beams
at the CERN SPS and 1 A GeV Fe26+at the Brookhaven National Laboratory (BNL) Alternating
Gradient Synchrotron (AGS). The calibration layout was a standard one with a fragmentation target
and CR39 (plus Makrofol) NTDs in front of and behind the target . The detector sheets behind the
target detected both primary ions and nuclear fragments of decreasing charge.
We recall that the formation of etch-pit cones (“tracks”) in NTDs is regulated by the bulk etching rate,
vB, and the track etching rate, vT, i.e. the velocities at which the undamaged and damaged materials
(along the particle trajectory), are etched out. Etch-pit cones are formed if vT > vB. The response of
the CR39 detector is measured by the etching rate ratio p = vT/vB.
After etching the standard calibration procedure was the following:
(i) measure the base area of each track in NTDs with an automatic image analyzer system . The
projectile fragments carry the same β and approximately the same direction of the incident ion; the Z of
each resolved peak is identified via the base area spectrum. The average base area distributions of the
In49+ions and of their fragments in CR39, etched in soft or strong conditions, are shown in Figs. 3a,b
REL [MeV cm
0500 10001500 20002500
p - 1
REL [MeV cm
0 5001000 15002000 2500
p - 1
Figure 4: Reduced track etch rate (p − 1) vs REL for the CR39 (left) and CR39(DOP) (right)
detectors, exposed to the 158 A GeV indium ion beam, etched in soft and strong etching conditions.
(1 pixel2= 0.3 µm2).
(ii) For each calibration peak the Z/β is obtained and the reduced etch rate (p−1) is computed. The
Restricted Energy Loss (REL) due to ionization and nuclear scattering is evaluated, thus arriving to the
calibration data of (p − 1) vs REL shown in Fig. 4 for both strong and soft etching conditions for CR39
and CR39(DOP). For soft etching the threshold in CR39 is at Z/β ∼ 7 corresponding to REL ∼ 50 MeV
cm2g−1. For strong etching the threshold is at Z/β ∼ 14, corresponding to REL ∼ 200 MeV cm2g−1.
The extrapolation of the calibration curves to p = 1 gives REL<
at Z/β ∼ 13 corresponding to REL ∼ 170 MeV cm2g−1; the threshold in strong etching conditions is
at Z/β ∼ 21 corresponding to REL ∼ 460 MeV cm2g−1. The extrapolation of the calibration curves to
p = 1 gives REL<
For magnetic monopoles with g = gD, 2gD, 3gDwe computed the REL as a function of β taking into
account electronic and nuclear energy losses, see Fig. 5 .
With the used etching conditions, the CR39 allows the detection of (i) MMs with g = gDfor β ∼ 10−4
and for β > 10−2; (ii) MMs with g = 2gDfor β around 10−4and for β > 4·10−3; (iii) the whole β-range
of 4 · 10−5< β < 1 is accessible for MMs with g > 2gDand for dyons.
For the Makrofol polycarbonate the detection threshold is at Z/β ∼ 50 and REL ∼ 2.5 GeV cm2g−1
; for this reason the use of Makrofol is restricted to the search for fast MMs.
∼40 MeV cm2g−1for soft etching and
∼160 MeV cm2g−1for strong etching. For CR39(DOP) the threshold in soft etching conditions is
∼240 MeV cm2g−1for strong etching.
After exposure at Chacaltaya the modules were brought back by air flights to Italy in order to be etched
and analyzed in the Bologna laboratory. Three “reference” holes of 2 mm diameter were drilled in each
module with a precision machine (the hole locations were defined to within 100 µm). This allowed us to
follow the passage of a “candidate” through the stack. The bags (envelopes) were opened, the detectors
were labeled and their thicknesses were measured, using a micrometer, in 9 uniformly distributed points
on the foil surface.
The analysis of a SLIM module started by etching the uppermost CR39 sheet using strong conditions
in order to reduce the CR39 thickness from 1.4 mm to ∼ 0.9 mm. After the strong etching, the CR39
sheet was scanned twice, with a stereo microscope, by different operators, with a 3× magnification optical
10 1010 10
REL (MeV cm??g )
Figure 5: REL vs beta for magnetic monopoles with g = gD, 2gD, 3gD. The dashed lines represent
the CR39 thresholds in soft and strong etching conditions and the Makrofol threshold (see Sect. 2.2).
lens, looking for any possible correspondence of etch pits on the two opposite surfaces. The measured
single scan efficiency was about 99%; thus the double scan guarantees an efficiency of ∼ 100% for finding
a possible signal.
Further observation of a “suspicious correspondence” was made with an optical 20 ÷ 40× stereo
microscope and classified either as a defect or a candidate track. This latter was then examined by an
optical microscope with 6.3ob×25ocmagnification and the axes of the base-cone ellipses in the front and
back sides were measured.
A track was defined as a “candidate” if the computed p and incident angle θ on the front and back
sides were equal to within 20%. For each candidate the azimuth angle ϕ and its position P referred to
the fiducial marks were also determined. The uncertainties ∆θ, ∆ϕ and ∆P defined a “coincidence” area
(< 0.5 cm2) around the candidate expected position in the other layers, as shown in Fig. 6.
In this case the lowermost CR39 layer was etched in soft etching conditions, and an accurate scan
under an optical microscope with high magnification (500× or 1000×) was performed in a square region
around the candidate expected position, which included the “coincidence” area. If a two-fold coincidence
was detected, the CR39 middle layer was also analyzed.
The bottom CR39 sheet was etched in about 50 cases; the third CR39 sheet was etched only in few
cases, when there was still a possible uncertainty, and for checks (∼ 16 times). Some Makrofol foils were
etched for reasons similar to the previous point and for other checks concerning the Makrofol itself (∼ 12
Figure 6: Illustration of the procedure used to define the “confidence” area where the possible
continuation of a candidate track inside two (or more) sheets of the same module was searched for
(see text for details).
From the detector calibration we computed the SLIM acceptance for downgoing IMMs with g = gD, 2gD, 3gD
and for dyons. For the ithmodule of area Sithe acceptance was computed as
The total acceptance is the sum of all the individual contributions.
Since no candidates were found, the 90% C.L. upper limit for a downgoing flux of IMMs and for dyons
was computed as
(SΩ) · ∆t · ǫ
where ∆t is the mean exposure time (4.22 y), SΩ is the total acceptance, ǫ is the scanning efficiency
estimated to be ∼ 1.
The global 90% C.L. upper limits for the flux of downgoing IMMs and dyons with velocities β > 4·10−5
were computed, as shown in Fig. 7. The flux limit for β > 0.03 is ∼ 1.3 · 10−15cm−2s−1sr−1.
Two “strange events” were observed and were finally classified as manufacturing defects in a small
subset of CR39 NTDs. These “strange events” are discussed in detail elsewhere .
We etched and analyzed 427 m2of CR39, with an average exposure time of 4.22 years. No candidate
passed the search criteria. The 90% C.L. upper limits for a downgoing flux of fast (β > 0.03) IMM’s
coming from above are at the level of 1.3 · 10−15cm−2sr−1s−1. The complete β-dependence for MMs
with g = gD, 2gD, 3gDand for dyons is shown in Fig. 7.
Figure 7: 90% C.L. upper limits for a downgoing flux of IMMs with g = gD, 2gD, 3gD and for dyons
(M+p , g = gD) plotted vs β (for strong etching). The poor limits at β ∼ 10−3arise because the
REL is below the threshold (for gD and 2gD) or slightly above the threshold (for 3gD and dyons),
see Sect. 2.3.
Superheavy GUT magnetic monopoles in the cosmic radiation can traverse the Earth. Therefore the
SLIM limit on their flux is one half of the IMM flux: φGUT < 6.5 · 10−16cm−2s−1sr−1for β > 0.03 for
g = gD.
Fig. 8 shows the flux upper limits for MMs of charge g = gD and β > 0.05 vs monopole mass.
Note that the SLIM limit is 1.3 · 10−15cm−2sr−1s−1for MM masses smaller than ∼ 5 · 1013GeV and
0.65 · 10−15cm−2sr−1s−1for masses larger than ∼ 5 · 1013GeV. In Fig. 8 are also shown the limits
obtained by the MACRO  and OHYA  experiments for g = gDmagnetic monopoles with β > 0.05.
SLIM is the first experiment to extend the cosmic radiation search for Magnetic Monopoles to masses
lower than the GUT scale with a high sensitivity.
The addition of SLIM data to the MACRO data would improve the MACRO limits by only 18%.
Large scale underwater and under ice neutrino telescopes (Amanda, IceCube, ANTARES, NEMO)
have the possibility to search for fast IMMs with β > 0.5 to a level lower than the Parker bound [11, 25].
We thank the engineering staff in BNL and in CERN for their help for the heavy ion calibration
exposures. We acknowledge the collaboration of our technical staff, in particular L. Degli Esposti, G.
Grandi and C. Valieri of INFN Bologna, and the technical staff of the Chacaltaya Laboratory. We thank
A. Casoni for typing and correcting the manuscript. We thank INFN and ICTP for providing grants to
Figure 8: Flux upper limits for cosmic MMs of charge g = gD and β > 0.05 vs monopole mass. The
figure shows the 90% C.L. limits obtained by the SLIM, MACRO  and OHYA  experiments.
MMs with masses smaller than ∼ 5 · 1013GeV are detected only if coming from above; MMs with
masses larger than ∼ 5·1013GeV can traverse the Earth, so an isotropic flux is expected. The Parker
bound , obtained from the survival of the galactic magnetic field, and the limit obtained from the
mass density for a uniform density of monopoles in the Universe  are also plotted.
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