Recently the CERN ALICE experiment, in its dedicated cosmic ray run, observed
muon bundles of very high multiplicities, thereby confirming similar findings
from the LEP era at CERN (in the CosmoLEP project). Significant evidence for
anisotropy of arrival directions of the observed high multiplicity muonic
bundles is found. Estimated directionality suggests their possible
extragalactic provenance. We argue that muonic bundles of highest multiplicity
are produced by strangelets, hypothetical stable lumps of strange quark matter
infiltrating our Universe.Comment: 4 pages, Proceedings for 17th International Conference on Strangeness
  in Quark Matter, Utrecht, the Netherland

Kankiewicz, P.

Rybczynski, M.

Wilk, G.

Wlodarczyk, Z.

EPJ Web of Conferences

English

arXiv

Recently the CERN ALICE experiment, in its dedicated cosmic ray run, observed muon bundles of very high multiplicities, thereby confirming similar findings from the LEP era at CERN (in the CosmoLEP project). We found significant evidence for anisotropy of arrival directions of the observed high multiplicity muonic bundles. The distribution on celestial sphere and the estimated directionality suggests their possible extragalactic source. We argue that muonic bundles of highest multiplicity are produced by strangelets, hypothetical stable lumps of strange quark matter infiltrating our Universe

P. Kankiewicz

M. Rybczyński

Z. Włdarczyk

G. Wilk

EDP Sciences OAI-PMH repository (1.2.0)

On muon bundles from the Universe

Recently the CERN ALICE experiment, in its dedicated cosmic ray run, observed muon bundles of very high multiplicities, thereby confirming similar findings from the LEP era at CERN (in the CosmoLEP project). Significant evidence for anisotropy of arrival directions of the observed high multiplicity muonic bundles is found. Estimated directionality suggests their possible extragalactic provenance. We argue that muonic bundles of highest multiplicity are produced by strangelets, hypothetical stable lumps of strange quark matter infiltrating our Universe.</jats:p

Z. Włodarczyk

Crossref

Muon bundles from the UniverseP. Kankiewicz1,, M. Rybczyn´ski1,, Z. Włodarczyk1,, and G. Wilk2,1Institute of Physics, Jan Kochanowski University, 25-406 Kielce, Poland2National Centre for Nuclear Research, Department of Fundamental Research, 00-681 Warsaw, PolandAbstract. Recently the CERN ALICE experiment, in its dedicated cosmic ray run, ob-served muon bundles of very high multiplicities, thereby confirming similar findings fromthe LEP era at CERN (in the CosmoLEP project). Significant evidence for anisotropy ofarrival directions of the observed high multiplicity muonic bundles is found. Estimateddirectionality suggests their possible extragalactic provenance. We argue that muonicbundles of highest multiplicity are produced by strangelets, hypothetical stable lumps ofstrange quark matter infiltrating our Universe.1 IntroductionThis talk is based on [1], where the relevant details can be found. Cosmic ray physics is our uniquesource of information on events in the energy range which will never be accessible in Earth-boundexperiments [2, 3]. This is why one of the most important aspects of their investigation is the under-standing of the primary cosmic ray (CR) flux and its composition. In this respect the recent measure-ment performed by the ALICE experiment at CERN LHC in its dedicated cosmic ray run [4, 5] is ofgreat importance. A number of events with muon bundles of high multiplicity was registered in the socalled Extensive Air Showers (EAS) produced by cosmic ray interactions in the upper atmosphere [4].A special emphasis has been given to the study of high multiplicity events containing more than 100reconstructed muons. Similar events have already been studied in the previous LEP experiments atCERN (in the so called CosmoLEP program) such as ALEPH, DELPHI and L3.In this talk we shall concentrate mainly on the first, high multiplicity events. To describe them wepropose seriously to consider, for a moment, the possibility of the existence in the flux of incomingCR a component with very high atomic number, of the order of A ∼ 103. In fact we propose to returnto our old idea that muon bundles of extremely high multiplicities could be produced by strangelets,hypothetical stable lumps of strange quark matter (SQM) infiltrating our Universe [6]. Strangeletswith such masses, much larger than the masses of ordinary nuclei, could easily produce extremelylarge groups of muons in collisions with the atmosphere.2 Searches for Strange Quark Matter in cosmic raysThe idea of SQM originated some time ago in Ref. [7], but it remains alive, cf. [8]. In short, the basic,commonly accepted view is that that SQM (understood as a combination of roughly an equal numbere-mail: maciej.rybczynski@ujk.edu.pl© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).EPJ Web of Conferences 171, 20003 (2018) https://doi.org/10.1051/epjconf/201817120003SQM 2017F(>Nµ)NµALICEpFeSQMFigure 1. Integral multiplicity distribution of muons for the ALICE data (circles) published in [4]. Monte Carlosimulations for primary protons (dotted line); iron nuclei (dashed dot line) and primary strangelets with mass Ataken from the A−7.5 distribution (full line) with abundance of the order of 2 · 10−5 of the total primary flux.of up, down and strange quarks) might be the true ground state of quantum chromodynamics (QCD).It is therefore reasonable to expect that it exists in some form in the Universe and can be detected. Thissupposition resulted in a number of searches for strange stars and quark stars, in which such a form ofmatter would be dominant and which could therefore be a possible source of strangelets penetratingouter space [9]. The experimental data mentioned above lead to a flux of strangelets which followsthe A−7.5 behaviour, which in turn coincides with the behaviour of the abundance of normal nucleiin the Universe. Strange quark matter fills a vast gap in the distribution of all known forms of stablematter, cf. chart of nuclides presented in Ref. [10]. It places itself exactly between the heaviest atomicelements and neutron stars. The estimated flux of strangelets is also consistent with the astrophysicallimits and with the upper limits given experimentally [11].3 High multiplicity muon bundles from SQMWe start with a short reminder of our proposed possible scenario of propagation of strangelets throughthe atmosphere [12]. The apparent contradiction between the large initial size of the incomingstrangelets and their required strong penetrability in the atmosphere can be resolved by assumingthat strangelets reaching deeply into the atmosphere are formed from the original large strangeletwhich loses mass in many successive interactions with air nuclei when penetrating the atmosphere.To provide numerical estimate we limit ourselves to the two most extreme pictures of the collisionof a strangelet of mass number A with an air nucleus target of mass number At: (i) All quarks of Atwhich are located in the geometrical intersection of the two colliding projectiles are involved and oneassumes that each quark from the target interacts with only one quark from the strangelet; (ii) Allquarks from both nuclei which are in their geometrical intersection region participate in the collision.In the first case, during the interaction up to (3At) quarks from the strangelet could be used up and itsmass could drop to a value of A − At, at most. The second case is an analogue of the so called tubemodel used occasionally in nuclear collisions. After the collision the atomic mass of the strangeletdiminishes to a value equal to A− A1/3 · A2/3t . While this is a rather extreme variant, it is still useful inproviding an estimate of the maximum possible destruction of the quarks in the strangelet.2EPJ Web of Conferences 171, 20003 (2018) https://doi.org/10.1051/epjconf/201817120003SQM 2017F(>Nµ)NµALICEpFeSQMFigure 1. Integral multiplicity distribution of muons for the ALICE data (circles) published in [4]. Monte Carlosimulations for primary protons (dotted line); iron nuclei (dashed dot line) and primary strangelets with mass Ataken from the A−7.5 distribution (full line) with abundance of the order of 2 · 10−5 of the total primary flux.of up, down and strange quarks) might be the true ground state of quantum chromodynamics (QCD).It is therefore reasonable to expect that it exists in some form in the Universe and can be detected. Thissupposition resulted in a number of searches for strange stars and quark stars, in which such a form ofmatter would be dominant and which could therefore be a possible source of strangelets penetratingouter space [9]. The experimental data mentioned above lead to a flux of strangelets which followsthe A−7.5 behaviour, which in turn coincides with the behaviour of the abundance of normal nucleiin the Universe. Strange quark matter fills a vast gap in the distribution of all known forms of stablematter, cf. chart of nuclides presented in Ref. [10]. It places itself exactly between the heaviest atomicelements and neutron stars. The estimated flux of strangelets is also consistent with the astrophysicallimits and with the upper limits given experimentally [11].3 High multiplicity muon bundles from SQMWe start with a short reminder of our proposed possible scenario of propagation of strangelets throughthe atmosphere [12]. The apparent contradiction between the large initial size of the incomingstrangelets and their required strong penetrability in the atmosphere can be resolved by assumingthat strangelets reaching deeply into the atmosphere are formed from the original large strangeletwhich loses mass in many successive interactions with air nuclei when penetrating the atmosphere.To provide numerical estimate we limit ourselves to the two most extreme pictures of the collisionof a strangelet of mass number A with an air nucleus target of mass number At: (i) All quarks of Atwhich are located in the geometrical intersection of the two colliding projectiles are involved and oneassumes that each quark from the target interacts with only one quark from the strangelet; (ii) Allquarks from both nuclei which are in their geometrical intersection region participate in the collision.In the first case, during the interaction up to (3At) quarks from the strangelet could be used up and itsmass could drop to a value of A − At, at most. The second case is an analogue of the so called tubemodel used occasionally in nuclear collisions. After the collision the atomic mass of the strangeletdiminishes to a value equal to A− A1/3 · A2/3t . While this is a rather extreme variant, it is still useful inproviding an estimate of the maximum possible destruction of the quarks in the strangelet.In this study we have used suitably modified SHOWERSIM [13] modular software. We performedMonte Carlo simulations for primary nuclei composed of protons and iron nuclei and for primarystrangelets with mass A taken from the A−7.5 distribution. The results of our simulations are shownin Fig. 1. Note that whereas the lower and medium multiplicities can be reproduced by the ordinarynuclei, the extremely large groups of muons can be described only by allowing (a relatively minute,of the order of 10−5 of the total primary flux) admixture of SQM of the same total energy.4 Anisotropy of arrival directions of strangeletsThe ALICE data also turns out to be very valuable for another reason. Namely, they show the angulardistribution of the muon events in the spherical reference frame with zenith angle (θ) and azimuthangle (Φ).Taking into account the orientation of the axes, these coordinates can be transformed into a hori-zontal reference frame: (Φ, θ) → (Az, h), where Az is the Azimuth used in navigation/astronomy (inthis particular case: Az = Φ + α0), and h is the elevation above the horizon (where h = 90◦ − θ).Finally, we can transform the coordinates from the horizontal to the equatorial reference frame:(Az, h) → (α, δ). Such a system of spherical coordinates is often used in astronomy and is expressedby right ascension (α) and declination (δ).Using the usual spherical triangle formulas or rotation matrices one can convert between the hori-zontal and equatorial coordinates. For the five high-multiplicity muon events we obtained the celestialequatorial coordinates (α2000, δ2000) and the estimated limits of their positional errors are ∼ 10◦ [1].The centroid (average location) of the five considered events is marked by a star in Fig. 2. Thecoordinates of this centroid are: αcentr = 11 h 15 m , δcentr = 39◦ 29′.We can conclude that the distribution of the directions of these five events suggests their possibleextragalactic provenance. Most of them are located in the vicinity of the galactic north pole, far fromthe galactic plane. If we temporarily assume that all these events have the same source, the mostprobable source would be the blazar Mrk 421 which is located close to the centroid of the five events.It is one of the brightest blazars known, with major outbursts, active at all wavebands [14]. However,there are also many other known extragalactic high energy sources located close to the coordinates ofour events [14], [15] and, at the moment, they cannot be excluded as potential sources.Figure 2. Aitoff projection of the UHECR map in equatorial coordinates taken from Telescope Array Collabora-tion data [16]. Location of the centroid of five ALICE events is marked with a star.3EPJ Web of Conferences 171, 20003 (2018) https://doi.org/10.1051/epjconf/201817120003SQM 20175 ConclusionsThere is an ongoing discussion concerning the possibility of the existence in the Universe of somestable forms of SQM and the feasibility of its detection. Whereas until now we do not have fully con-vincing observations of SQM, nevertheless there are numerous indications of the possible existenceof strange stars, or even of the existence of quark stars. Strangelets could originate, for example, fromviolent collisions between such objects. The recent results of ALICE reporting the existence of bun-dles of high-multiplicity muons could serve, notwithstanding the weakness of this signal, as a direct(almost) signal of stable lumps of SQM called strangelets arriving at the Earth from outer space. Asa result, we have found that whereas the low multiplicities of muon groups measured by the CERNALICE experiment favour light nuclei as primaries and the medium multiplicities show behaviourspecific for heavier primaries, the muon groups (bundles) of really high multiplicities (of the orderof ∼ 100) apparently cannot be described by the common interaction models. We have shown herethat the situation can be rescued by allowing for a relatively small (of the order of 10−5 of the totalprimary flux) admixture of strangelets of the same total energy. Our estimation of their flux does notcontradict the results obtained recently by the SLIM Collaboration [11]. The ALICE data allowed usto estimate the directionality of the five events of the highest muon multiplicities, and to identify theirmost probable extragalactic source(s).AcknowledgmentsThis research was supported by the Polish National Science Centre, grant 2016/23/B/ST2/00692 (MR)and contract 2016/22/M/ST2/00176 (GW).References[1] P. Kankiewicz, M. Rybczynski, Z. Włodarczyk and G. Wilk, Astrophys. J. 839, no. 1, 31 (2017).[2] A. Dar and A. De R ujula, Phys. Rep. 466 179 (2008).[3] A. Letessier-Selvon and T. Stanev, Rev. Mod. Phys. 83 907 (2011).[4] J. Adam et al. [ALICE Collaboration], J. Cosm. Astrop. Phys. 01 032 (2016).[5] J. Adam et al. [ALICE Collaboration], ALICE-PUBLIC-2016-003.[6] M. Rybczyn´ski, Z. Włodarczyk, G. Wilk, Nucl. Phys. B (Proc. Suppl.) 97, 85 (2001).[7] E. Witten, Phys. Rev. D 30 272 (1984).[8] M. G. Alford, Nucl. Phys. A 830 385c (2009).[9] K. S. Cheng, Z. G. Dai, D. M. Wei and T. Lu, Science 280 407 (1998).[10] H. J. Crawford and C. H. Greiner, Sci. Am. 270 58 (1994).[11] Z. Sahnoun et al., Radiat. Meas. 44 894(2009).[12] G. Wilk and Z. Wlodarczyk, J. Phys. G 23 2057 (1997).[13] A. Wrotniak, SHOWERSIM/84, University of Maryland Preprint, 85 (1984).[14] D. Horan and T. C. Weekes, New Astr. Rev. 48 527 (2004).[15] C. F. Turley et al. [Astrophysical Multimessenger Observatory Network) Collaboration], Astro-phys. J. 833, no. 1, 117 (2016).[16] R. U. Abbasi et al. [Telescope Array Collaboration], Astrophys. J. 790, L21 (2014).4EPJ Web of Conferences 171, 20003 (2018) https://doi.org/10.1051/epjconf/201817120003SQM 2017

Muon bundles from the Universe

Recently the CERN ALICE experiment, in its dedicated cosmic ray run, observed muon bundles of very high multiplicities, thereby confirming similar findings from the LEP era at CERN (in the CosmoLEP project). We found significant evidence for anisotropy of arrival directions of the observed high multiplicity muonic bundles. The distribution on celestial sphere and the estimated directionality suggests their possible extragalactic source. We argue that muonic bundles of highest multiplicity are produced by strangelets, hypothetical stable lumps of strange quark matter infiltrating our Universe.</jats:p

Recently the CERN ALICE experiment, in its dedicated cosmic ray run, observed muon bundles of very high multiplicities, thereby confirming similar findings from the LEP era at CERN (in the CosmoLEP project). Significant evidence for anisotropy of arrival directions of the observed high multiplicity muonic bundles is found. Estimated directionality suggests their possible extragalactic provenance. We argue that muonic bundles of highest multiplicity are produced by strangelets, hypothetical stable lumps of strange quark matter infiltrating our Universe

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Muon bundles from the Universe

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