We discuss the possible imprints of strangelets (i.e., lumps of Strange Quark
Matter) in Chacaltaya experimental data using model of propagation of such
objects through the atmosphere developed by us recently.Comment: gz-compressed .tar file containing LaTeX file and 4 PS files with
  figures, 5 pages altogether (Nuovo Cimento cimento.cls style file attached)
  Presented at Chacaltaya Meeting On Cosmic Ray Physics, La Paz - Bolivia,
  23-27 July 2000. To be published in Nuovo Cimento (Proc. Suppl.

Rybczynski, M.

Wilk, G.

Wlodarczyk, Z.

English

arXiv

We discuss the possible imprints of strangelets (i.e. lumps of strange quark matter) in Chacaltaya experimental data using model of propagation of such objects through the atmosphere developed by us recently

Rybczyński, M.

Scientific Open-access Literature Archive and Repository

IL NUOVO CIMENTO Vol. 24 C, N. 4-5 Luglio-Ottobre 2001Strangelets at Chacaltaya(∗)M. Rybczyn´ski(1)(∗∗), Z. Wlodarczyk(1)(∗∗∗) and G. Wilk(2)( ∗∗∗)(1) Institute of Physics, Pedagogical University - Kielce, Poland(2) The Andrzej Soltan Institute for Nuclear Studies - Warsaw, Poland(ricevuto il 14 Settembre 2000; approvato il 12 Febbraio 2001)Summary. — We discuss the possible imprints of strangelets (i.e. lumps of strangequark matter) in Chacaltaya experimental data using model of propagation of suchobjects through the atmosphere developed by us recently.PACS 96.40 – Cosmic rays.PACS 01.30.Cc – Conference proceedings.1. – Introduction: strangelets and their propagationChacaltaya Laboratory offers a unique possibility to observe possible imprints of stran-gelets arriving from the outer space. They are lumps of Strange Quark Matter (SQM), anew possible stable form of matter (cf. [1-6] for details). Following [7] it is fully sensibleto search for strangelets in cosmic ray experiments, especially at Chacaltaya level (540g/cm2 of atmosphere) because [7] the specific features of strangelets [4] allow them topenetrate deeply into the atmosphere [8]. The point is that there exists some critical sizeof strangelet given by the critical value of its mass number A = Acrit ∼ 300–400 suchthat for A > Acrit strangelets are absolutely stable against neutron emission. (However,small strangelets might probably also gain stability due to the shell effect [5].) Below thislimit strangelets decay rapidly evaporating neutrons. The geometrical radii of strangeletsturn out to be comparable to the radii of ordinary nuclei [7], i.e. their geometrical cross-sections are similar to the normal nuclear ones. To account for their strong penetrabilityone has to accept that strangelets reaching deeply into the atmosphere are formed inmany successive interactions with air nuclei by the initially very heavy lumps of SQMentering the atmosphere and decreasing due to the collisions with air nuclei (until their(∗) Paper presented at the Chacaltaya Meeting on Cosmic Ray Physics, La Paz, Bolivia,July 23-27, 2000.(∗∗) E-mail: mryb@pu.kielce.pl(∗∗∗) E-mail: wlod@pu.kielce.pl( ∗∗∗) E-mail: wilk@fuw.edu.plc© Societa` Italiana di Fisica 645646 M. RYBCZYN´SKI, Z. WLODARCZYK and G. WILKFig. 1. – (a) An example of the predicted decrease of the actual size of strangelet A with depthh of the atmosphere traversed for two different initial sizes: A0 = 1000 and 2000 (dotted linescorrespond to A < Acrit). (b) Number of nucleons released in 1 g/cm2 at depth h of theatmosphere from the strangelet with mass number ratios A0/Acrit = 1, 2, . . . , 8, respectively.A reaches the critical value Acrit [7]). The opposite scenario advocated recently in [9]faces some difficulties and will not be discussed here.Such scenario is fully consistent with all present experiments [7]. In this scenariointeraction of strangelet with target nucleus involves all quarks of the target located inthe geometrical intersection of the colliding air nucleus and strangelet. It is assumed thateach quark from the target interacts with only one quark from the strangelet; i.e. duringinteraction the mass number of the strangelet is diminished to the value equal to A−At atmost. This procedure continues unless either strangelet reaches Earth or (most probably)disintegrates at some depth h of the atmosphere reaching A(h) = Acrit. This results, in afirst approximation (in which At  Acrit < A0), in total penetration depth of the orderof Λ  43λNAt(A0/At)1/3. The characteristic features of strangelets propagation areillustrated in fig. 1. All numerical calculations presented here were done using suitablemodifications of the SHOWERSIM [10] modular software. Strangelets propagation andnuclear-electromagnetic cascades through the atmosphere were simulated with primaries(p, Fe and strangelets) initiating showers sampled from the power spectrum F (E) ∼E−2.7 with energies above 1000 TeV per particle. EAS detected at Chacaltaya withNe = 106–107 were then analysed.2. – Cosmic nuclearities and exotic eventsThere are several reports suggesting existence of direct candidates for SQM [11] (char-acterized mainly by their very small ratios of Z/A). All of them have mass numbers Anear or slightly exceeding Acrit (including Centauro event which contains probably ∼ 200baryons [12]). Analysis of these candidates for SQM shows [7] that the abundance ofstrangelets in the primary cosmic ray flux is FS(A0 = Acrit)/Ftot  2.4 · 10−5 at thesame energy per particle. Efficiency for registration of strangelets at Chacaltaya level isshown in fig. 2a. To detect strangelets with A > Acrit at Chacaltaya the mass of theinitial strangelet should be A0  7Acrit that leads to ∼ 10−11 as the relative abundanceof such strangelets. For normal flux of primary cosmic rays [13] the expected flux ofSTRANGELETS AT CHACALTAYA 647Fig. 2. – (a) Registration efficiency for strangelet with mass number A > Acrit at the Chacaltayalevel as a function of initial mass number A0. Consecutive full circles indicate (for A0 > 1600)points where A0/Acrit = 5, 6, 7, 8, 9, 10, respectively. (b) Mass distribution of strangelets atChacaltaya level (solid histogram) resulting from primary mass spectrum A−7.5 (solid line). Thecorresponding initial mass distribution for detected strangelets is shown by dotted histogram.strangelets is then equal to FS = 7 · 10−6 m−2h−1sr−1 for the energy above 10 GeV perinitial strangelet. The high-altitude exposure of passive nuclear track detector arrays [14]and their operation for one year allow therefore detection of such objects. In fact, theexposed CR39 detector should detect at least 6 strangelets with masses above Acrit andwith energies per registered strangelet above 1 GeV. The mass distribution of strangelets,that can be observed at Chacaltaya, is shown in fig. 2b.Experimental results obtained at Chacaltaya show a wide spectrum of exotic events(Centauros, superfamilies with “halo”, strongly penetrating component, etc.) which areclearly incompatible with the standard ideas of hadronic interactions known from theaccelerator experiments. Some new mechanism or new primaries are therefore needed.Assuming that strangelets represent such new primaries one is able to explain [15] (atleast to some extent) a strong penetrating nature of some “abnormal” cascades associatedwith their very slow attenuation and with the appearance of many maxima with smalldistances between them (about 2−3 times smaller than in the “normal” hadron cascades).Already mentioned Centauro (and mini-Centauro) events, characterized by the ex-treme imbalance between hadronic and gamma-ray components among produced secon-daries, are probably the best known examples of such exotic events. They require deeplypenetrating component in cosmic rays. We claim that they can be products of strangeletspenetrating deeply into the atmosphere and evaporating neutrons [8]. Both the flux ratioof Centauros registered at different depths and the energy distribution within them canbe successfully described by such a concept.Another example of exotic event is the phenomenon of alignment of structural objectsof gamma-hadron families near a straight line in the plane at the target diagram [16].The excess of aligned families observed is incompatible with any conventional concept ofinteraction. One can speculate therefore that it is caused by the arrival of strangelets withhigh spin (J ∼ A2 ) gradually dispersing their masses A(h) when propagating throughthe atmosphere.Anomalous events have been reconfirmed by measuring extensive air showers648 M. RYBCZYN´SKI, Z. WLODARCZYK and G. WILKFig. 3. – Multiplicity distribution of (a) hadrons and (b) muons in EAS with size Ne = 106–107detected at Chacaltaya and initiated by primary protons (dashed), iron nuclei (dotted) andstrangelets with A0 = 400 (solid histogram).(EAS) [17]. Among them was the striking observation [18] of extremely long-delayedneutrons in conjunction with the large EAS which cannot be explained by the knownmechanism of hadronic cascades development. Also muon bundles of extremely highmultiplicity observed recently by ALEPH detector (in the dedicated cosmic ray run) canoriginate from strangelets collisions with the atmosphere [19]. As an illustration of sensi-tivity of EAS characteristics on primary strangelets we show in fig. 3 our predictions forthe corresponding distributions of hadrons and muons in EAS detected at Chacaltaya.Fig. 4. – (a) The expected flux (our results) of strangelets compared with the upper experimentallimit, compiled by Price [20], and predicted astrophysical limits: Big Bang estimation comesfrom nucleosynthesis with quark nuggets formation; Dark Matter one comes from local fluxassuming that galactic halo density is given solely by quark nuggets. (b) Comparison of theestimated mass spectrum N(A0) for strangelets with the known abundance of elements in theUniverse [21].STRANGELETS AT CHACALTAYA 6493. – Final remarksThe experimental data mentioned before lead to the flux of strangelets which isconsistent (cf. fig. 4a) with the astrophysical limits and with the upper limits givenexperimentally [20]. There follows the A−7.50 behaviour, which coincides with the be-haviour of abundance of normal nuclei in the Universe (fig. 4b) [21]. The fascinatingsubject of searches for strangelets as a new form of matter can be successfully realisedat experiments located in the mountain region of Chacaltaya. Interpretation of indi-rect observations (anomalous events observed in emulsion chambers and results from themeasurements of EAS) can provide signals of strangelets. Moreover, direct identification(by implementing passive nuclear track detector arrays) of SQM is quite realistic in thenear future. All this justifies interest in further experimental search for the SQM and forits cosmological and elementary particle physics aspects.∗ ∗ ∗Authors ZW and GW are grateful to the organizers of the Chacaltaya Meeting onCosmic Ray Physics for their support and hospitality. The partial support of PolishCommittee for Scientific Research (grants 2P03B 011 18 and 621/E-78/SPUB/CERN/P-03/DZ4/99) is acknowledged.REFERENCES[1] Cf., for example, Klingenberg R., J. Phys. G, 25 (1999) R273 and references therein.[2] Witten E., Phys. Rev. D, 30 (1984) 272; Madsen J. and Haensel P. (Editors), Nucl.Phys. B (Proc. Suppl.), 24 (1991) ; Alcock C. and Olinto A., Ann. Rev. Nucl. Part.Phys., 38 (1988) 161.[3] Panagiotou A. D. et al., Phys. Rev. D, 45 (1992) 3134; Asprouli M. N. et al., Astropart.Phys., 2 (1994) 167. Cf. also Price P. B., Phys. Rev. D, 47 (1993) 5194.[4] Alcock C. A. and Farhi E., Phys. Rev. D, 32 (1985) 1273; Farhi E., Comm. Nucl.Part. Phys., 16 (1986) 289; Farhi E. and Jaffe R. L., Phys. 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