The new large area (400 m2) experiment—SLIM—to search for magnetic monopoles and other exotic massive particles is presented. It uses of nuclear track detectors and is being deployed at the Chacaltaya cosmic ray laboratory for at least 4 years. The detection capability of the experiment is discussed

Cecchini, S.

Giacomelli, G.

Giorgini, M.

Mandrioli, G.

Manzoor, S.

Margiotta, A.

McDonald, J.

Nogales, J.

Patrizii, L.

Pinfold, J.

Popa, V.

Qureshi, I.E.

Rana, M.A.

Saavedra, O.

Serra, P.

Shahzad, M.I.

Spurio, M.

Ticona, R.

Togo, V.

Velarde, A.

Zanini, A.

English

Scientific Open-access Literature Archive and Repository

IL NUOVO CIMENTO Vol. 24 C, N. 4-5 Luglio-Ottobre 2001Search for magnetic monopoles at the Chacaltaya cosmic raylaboratory(∗)The SLIM CollaborationS. Cecchini(1)(7), G. Giacomelli(1), M. Giorgini(1), J. McDonald(3)G. Mandrioli(1), S. Manzoor(1)(4), A. Margiotta(1), J. Nogales(6)L. Patrizii(1), J. Pinfold(3), V. Popa(1)(5), I. E. Qureshi(4), M. A. Rana(4)O. Saavedra(2), P. Serra(1), M. I. Shahzad(4), M. Spurio(1), R. Ticona(6)V. Togo(1), A. Velarde(6) and A. Zanini(2)(1) Dipartimento di Fisica dell’Universita` di Bologna and INFN - 40127 Bologna, Italy(2) Dipartimenti di Fisica Sperimentale e Generale dell’Universita` di Torino and INFN10125 Torino, Italy(3) Centre for Subatomic Research, University of AlbertaEdmonton, Alberta T6G 2N4, Canada(4) RPD - Pinstech, P.O. Nilore, Islamabad, Pakistan(5) Institute for Space Sciences - 76900 Bucharest, Romania(6) Laboratorio de Fisica Cosmica de Chacaltaya - UMSA, La Paz, Bolivia(7) Istituto TeSRE/CNR - 40129 Bologna, Italy(ricevuto il 13 Ottobre 2000; approvato il 12 Febbraio 2001)Summary.— The new large area (400 m2) experiment—SLIM—to search for mag-netic monopoles and other exotic massive particles is presented. It uses of nucleartrack detectors and is being deployed at the Chacaltaya cosmic ray laboratory forat least 4 years. The detection capability of the experiment is discussed.PACS 14.80.Hv – Magnetic monopoles.PACS 29.90 – Other topics in elementary-particle and nuclear physics experimentalmethods and instrumentation.PACS 01.30.Cc – Conference proceedings.1. – IntroductionGrand Unified Theories (GUT) of electroweak and strong interactions predict the ex-istence of superheavy magnetic monopoles (MM) with masses larger than 1016 GeV [1].They would have been produced at the end of the GUT epoch, at the mass scale(∗) Paper presented at the Chacaltaya Meeting on Cosmic Ray Physics, La Paz, Bolivia,July 23-27, 2000.c© Societa` Italiana di Fisica 639640 THE SLIM COLLABORATIONa) b)Fig. 1. – Left: Accessible regions in the plane (mass, β) for monopoles with magnetic chargeg = gD coming from above for an experiment at altitudes of 20 km, 5230 m, and for an un-derground detector at the Gran Sasso Lab. (like the MACRO detector). It is assumed thatmonopoles interact only via the electromagnetic interaction, and no radiative effects are con-sidered [9]. If a light monopole attaches a nucleon or if it has some strong interaction, it couldinteract in the higher atmosphere, like primary protons and nuclei of the cosmic radiation.Right: Accessible region in the plane (mass, β) for nuclearites. Only the energy losses via elec-tromagnetic interaction are considered (bremsstrahlung is not considered). If nuclearites arepart of the dark matter they would have typical velocities of β ∼ 10−3. It is highly probablethat low mass nuclearites interact strongly; thus they may not reach the lower atmosphere.∼ 1014 GeV and cosmic time of ∼ 10−34 s. Such monopoles cannot be produced withexisting accelerators, nor with any foreseen for the future, but can be searched for in thecosmic radiation.Several experiments, using different techniques, have tried to detect superheavy mon-opoles with velocities greater than 10−5c. The MACRO experiment at LNGS is the oneproviding the best experimental flux upper limit for GUT MMs over the widest velocityrange [2].The existence of MMs with masses around 106–1010 GeV that require a phase tran-sition in the early universe in which a semisimple gauge group yields a U(1) factor ata lower energy scale, has also been proposed (see, e.g., [3]). One of the recent inter-ests in relatively low mass MMs is connected also with the possibility that relativisticMMs could be the sources of the highest energy cosmic rays, with energies larger than1020 eV [4, 5]. For monopoles possible acceleration mechanisms are known: since thebasic magnetic charge should be very large, relatively light monopoles can be acceleratedto relativistic velocities and to energies of the order of 1020 GeV in one coherent domainof the galactic magnetic field, or in the intergalactic field, or in many astrophysical sites,like in the magnetic field of Active Galactic Nuclei (AGN) and even of neutron stars.Experimental signatures for these monopoles can be established considering how theyinteract in the upper atmosphere and yield electromagnetic showers [6]. Other detectiontechniques can be devised in the case a monopole forms a bound state with a proton (adyonic system) which may interact with a cross-section typical of a relativistic hadron(σ ≥ 10−26 cm2) [7]. Monopole masses of 106–1010 GeV could be consistent with a fluxat the Parker limit [8], i.e. ∼ 10−15 cm−2 s−1 sr−1.We present here the capabilities of the SLIM (Search for LIght Magnetic monopoles)experiment, an array of 400 m2 of passive nuclear track detectors, that is currently underSEARCH FOR MAGNETIC MONOPOLES AT THE CHACALTAYA COSMIC RAY LABORATORY 641Fig. 2. – Left: Restricted Energy Losses of MMs vs. β in the CR39 nuclear track detector [17].The detector threshold of the SLIM CR39 is also shown. Right: Flux upper limits for MMs vs.monopole mass. The expected results (90 % CL) in the absence of candidates, from the SLIMexperiment (dashed line) and the combined MACRO [18] and SLIM experiments (dash-dottedline) are shown. The Parker bound, due to the survival of galactic magnetic field and massdensity limits for a uniform density of monopoles in the Universe are also plotted.contruction at the high-altitude Chacaltaya lab (5230 m a.s.l.) for the search for relativelylight MMs. In > 4 years of operation it should be able to reach a sensitivity at the levelof the Parker bound.Figure 1 (left) shows the accessible region in the plane (mass, β) for MMs, withcharge g = gD for an experiment, like MACRO, located in the Gran Sasso undergroundlaboratory (at an average depth of 3700 hg/cm2), at the Chacaltaya altitude (540 g cm−2of atmosphere) and at 20 km height. An exposure at a high-altitude laboratory will allowto search for MMs of lower masses, higher magnetic charges and lower velocities [9].It must also be remembered the possibility that MMs could be multiply charged,g = 2gD, as in some SUSY theories, and g = 3gD, as in some superstring models (gD =h¯c/2e = 68.5 e is the basic Dirac monopole charge) and that the basic electric chargecould be 1/3 e [10].Byproducts of this MM search are the searches for relatively light nuclearites [11] andQ-balls [12]. We recall that nuclearites (strangelets, strange quark matter) are nuggets ofstrange quark matter (aggregates of u, d, and s quarks in approximately equally propor-tions); they could be the ground state of QCD and could be part of the cold dark matterwith typical galactic velocities β ∼ 10−3. Q-balls are supersymmetric coherent states ofsquarks, sleptons and Higgs fields, predicted by minimal supersymmetric generalizationsof the Standard Model; they could be produced in the early universe. Relic Q-balls arealso candidates for the cold dark matter.Since both nuclearites and charged Q-balls lose a large amount of energy for β > 4 ×10−5 [13, 14] they would be easily detectable with the SLIM apparatus. For low massnuclearites one would reach a level of sensitivity more than one order of magnitude lowerthan any of the existing limits [13, 15]. Figure 1 (right) shows the accessible regionin the plane (mass, β) for nuclearites, at MACRO depth, at the Chacaltaya altitude642 THE SLIM COLLABORATIONFig. 3. – p = vT/vB vs. REL for CR39 exposed to Pb82+ ions of 158 A GeV and Fe 26+ions of1 A GeV at different times after production. This was done to estimate possible aging effects.The dashed line indicate the systematic uncertainty arising mainly from fluctuations of the bulketching rate vB.(540 g cm−2 of atmosphere) and at 20 km height assuming that the nuclearites havestandard energy losses [11]. Lower-mass nuclearites should be much more abundant thanhigher mass ones [16].The high-altitude exposure will allow detection of the above-mentioned particles evenif they had strong-interaction cross-sections which could prevent them from reaching theearth surface. From this point of view, it is important that the site be at the highestpossible altitude.2. – Experimental methodThe SLIM apparatus will consist of 400 m2 of CR39 and Makrofol nuclear trackdetectors. The CR39 allows to search for magnetic monopoles with one unit Diraccharge (g = gD), for β = v/c around 10−4 and for β > 10−3, the whole β-range of4× 10−5 < β < 1 for MMs with g ≥ 2gD, for dyons, for nuclearites and for Q-balls.Figure 2 (left) shows the Restricted Energy Loss (REL) vs. β for MMs of magneticcharges g = gD, 2 gD and 3 gD, in the CR39; the detector threshold is also shown [17].The polycarbonate has a higher threshold, and it is useful for monopoles, nuclearitesand Q-balls with β > 10−4.The track-etch detector is organised in modules of 24 cm × 24 cm, each made of 3layers of CR39, 3 layers of polycarbonate and of an aluminium absorber 1 mm thick; thismodule is sealed in an aluminized plastic bag filled with dry air. Since the atmosphericpressure at Chacaltaya is 0.5 atm, we made a test in which some envelopes filled with1 atm of air were sealed and placed in a chamber at a pressure of about 0.3 atm forSEARCH FOR MAGNETIC MONOPOLES AT THE CHACALTAYA COSMIC RAY LABORATORY 643Table I. – Background activity measured in different locations of the experimental room.Location no. Activity (Bq/m3)3 42 ±12%5 35 ±10%6 40 ±12%7 45 ±11%8 69 ±9%three weeks; no significant leakage was detected in any of them. From our experiencewith MACRO, where the same CR39 have been used we know that such material doesnot suffer from “aging effects”, at least for exposure times shorter than 8 years, that is,there is no appreciable dependence of the detector response on the time elapsed betweenthe date of production and the passage of the particle. Tests were made for searchingfor such effects. Two sets of samples, 0.8 y and 2.5 y old, respectively, were exposed inNovember 1994 to 158A GeV Pb82+ ions. For each detected nuclear fragment the reducedetch rate p = vT/vB (vT and vB are the track and bulk etching rates, respectively) wascomputed and plotted vs. REL (fig. 3). The lines represent the limits of the systematicuncertainties coming mainly from 1 standard deviation uncertainty on vB. A more recenttest was made by exposing CR39 samples 10 y old to 1A GeV Fe26+ ions. The resultsindicate that within experimental uncertainties aging effects in the MACRO CR39 arenegligible and confidently the same will happen for the SLIM CR39.3. – Environmental backgroundWe have installed 100 m2 in March 2000, and an additional 90 m2 at the end of July2000. We expect to complete the apparatus by July 2001. We are presently making testsby exposing nuclear track detectors in Bologna and at the Chacaltaya mountain station,in order to study the effects of possible backgrounds and of possible climatic conditions.Preliminary results of radon concentration in the experimental room have been ob-tained by using E-PERM radon dosimeters. In table I we present the measured activityin six different positions. From our experience at LNGS with the MACRO experiment,we can conclude that the measured levels of radon activity are not a problem for theexperiment.4. – ConclusionsThe new CR39 array currently being built at the Chacaltaya high-altitude laboratoryhas good capability to detect a magnetic monopole flux at the Parker bound level in areasonable time interval over a wide range of masses, charges and velocity (see fig. 2,right). This experiment will also be able to search for low mass nuclearites and forsupersymmetric dark-matter Q-balls at a high-altitude laboratory; in this respect it is atthis moment unique.∗ ∗ ∗We would like to acknowledge fruitful discussions with A. De Ru´jula, S. Glashow,J. Steinberger. We would like to thank E. Bottazzi, L. Degli Esposti, D. Di644 THE SLIM COLLABORATIONFerdinando and C. Valieri of INFN, Sezione di Bologna for their help in preparingthe experiment and G. Landini of ITeSRE/CNR, Bologna for the vacuum test.REFERENCES[1] Giacomelli G., Riv. Nuovo Cimento, 7 (1984) 1; in Theory and Detection of MagneticMonopoles in Gauge Theories, edited by N. Craigie (World Scientific, Singapore) 1986.[2] MACRO Collaboration (Ambrosio M. et al.), Proceedings of the 26th ICRC (Salt LakeCity), vol. 2 (1999) p. 337; hep-ex/9905023 (1999).[3] De Ru´jula A., Nucl. Phys. B, 435 (1995) 257.[4] Weiler T. J. and Kephart T. W., Nucl. Phys. B. Proc. Suppl., 51 (1996) 218;Kephart T. W. and Weiler T. J., Astropart. Phys., 4 (1996) 217.[5] Escobar C. O. and Vazquez R. A., Astropart. Phys., 10 (1999) 197.[6] Wick S. D. et al., astro-ph/0001233 (2000).[7] Huguet E. and Peter P., hep-ph/9901370 (1999).[8] Turner M. S., Parker E. M. and Bogdan T. T., Phys. Rev. D, 26 (1982) 1296.[9] Derkaoui J. et al., Astropart. Phys., 9 (1998) 173.[10] Sutcliffe P. M., Int. J. Mod. Phys. A, 12 (1997) 4663; Lazarides G. et al., Phys. Rev.Lett., 58 (1987) 1707.[11] Witten E., Phys. Rev. D, 30 (1984) 272 De Ru´jula A. and Glashow S. L., Nature,312 (1984) 734.[12] Coleman S., preprint HUTP-85/A050 (1985); Kusenko et al., preprint CERN-TH/97-346 (1997); hep-ph/9712212 (1997).[13] MACRO Collaboration (Ambrosio M. et al.), Eur. Phys. J. C, 13 (2000) 453.[14] Bakari D. et al., hep-ex/0003003 (2000).[15] Nakamura S. et al., Phys. Lett. B, 263 (1991) 529; astro-ph/0005103 (2000).[16] Wilk G. and Wlodarczyk Z., hep-ph/9606401 (1996).[17] Derkaoui J. et al., Astropart. Phys., 10 (1999) 339.[18] Giacomelli G., MACRO Status Report in LNGS 1999 Annual Report (2000).

Search for magnetic monopoles at the Chacaltaya cosmic ray laboratory

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Search for magnetic monopoles at the Chacaltaya cosmic ray laboratory

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