A search for long-distance correlations between Extensive Air Showers (EAS) detected by pairs of MRPC telescopes of the Extreme Energy Events (EEE) network was carried out. A dataset obtained by all possible pairings between ten EEE cluster sites (hosting at least two telescopes) with an overall exposure of 3968 days was analyzed. A few candidate events with unusually small time difference and angular distance were observed

Abbrescia, M.

Baldin Ferroli, R.

Baldini, L.

Batignani, G.

Riggi, F.

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DOI 10.1393/ncc/i2017-17196-0Colloquia: Physics in Collision 2017IL NUOVO CIMENTO 40 C (2017) 196Time and orientation long-distance correlations between extensiveair showers detected by the MRPC telescopes of the EEE ProjectF. Riggi(1)(18)(∗), M. Abbrescia(1)(2), L. Baldini(1)(3), R. Baldin Ferroli(1)(4),G. Batignani(1)(3), M. Battaglieri(1)(17), S. Boi(1)(8), E. Bossini(1)(5),F. Carnesecchi(1)(6), A. Chiavassa(1)(7), C. Cicalo(1)(8), L. Cifarelli(1)(6),F. Coccetti(1), E. Coccia(1)(9), D. De Gruttola(1)(11), S. De Pasquale(1)(11),F. L. Fabbri(1)(4), V. Frolov(16), P. Galeotti(1)(7), M. Garbini(1)(6),G. Gemme(1)(17), I. Gnesi(1)(7), S. Grazzi(1), C. Gustavino(1)(12),D. Hatzifotiadou(1)(6)(15), P. La Rocca(1)(18), G. Mandaglio(1)(19),O. Maragoto Rodriguez(14), G. Maron(13), M. N. Mazziotta(1)(20),S. Miozzi(1)(4), R. Nania(1)(6), F. Noferini(1)(6), F. Nozzoli(1)(21),F. Palmonari(1)(6), M. Panareo(1)(10), M. P. Panetta(1)(10), R. Paoletti(1)(5),W. Park(14), L. Perasso(1)(17), F. Pilo(1)(3), G. Piragino(1)(7),S. Pisano(1)(4), G. C. Righini(1), C. Ripoli(1)(11), G. Sartorelli(1)(6),E. Scapparone(1)(6), M. Schioppa(1)(22), A. Scribano(1)(3), M. Selvi(1)(6),S. Serci(1)(8), S. Squarcia(1)(17), M. Taiuti(1)(17), G. Terreni(1)(3),A. Trifirò(1)(23), M. Trimarchi(1)(23), M. C. Vistoli(13), L. Votano(1)(12),M. C. S. Williams(1)(6)(15), L. Zheng(1)(14)(15), A. Zichichi(1)(6)(15)and R. Zuyeuski(1)(15)(1) Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi - Roma, Italy(2) INFN and Dipartimento Interateneo di Fisica, Università di Bari - Bari, Italy(3) INFN and Dipartimento di Fisica, Università di Pisa - Pisa, Italy(4) INFN, Laboratori Nazionali di Frascati - Frascati (RM), Italy(5) INFN Gruppo Collegato di Siena and Dipartimento di Fisica, Università di Siena - Siena,Italy(6) INFN and Dipartimento di Fisica e Astronomia, Università di Bologna - Bologna, Italy(7) INFN and Dipartimento di Fisica, Università di Torino - Torino, Italy(8) INFN and Dipartimento di Fisica, Università di Cagliari - Cagliari, Italy(9) INFN and Dipartimento di Fisica, Università di Roma Tor Vergata - Roma, Italy(10) INFN and Dipartimento di Matematica e Fisica, Università del Salento - Lecce, Italy(11) INFN and Dipartimento di Fisica, Università di Salerno - Salerno, Italy(12) INFN, Laboratori Nazionali del Gran Sasso - Assergi (AQ), Italy(13) INFN CNAF - Bologna, Italy(14) ICSC World Laboratory - Geneva, Switzerland(15) CERN - Geneva, Switzerland(16) JINR Joint Institute for Nuclear Research - Dubna, Russia(17) INFN and Dipartimento di Fisica, Università di Genova - Genova, Italy(18) INFN and Dipartimento di Fisica e Astronomia, Università di Catania - Catania, Italy(19) INFN Sezione di Catania and Dipartimento di Scienze Chimiche, Biologiche, Farmaceu-tiche e Ambientali, Università di Messina - Messina, Italy(20) INFN Sezione di Bari - Bari, Italy(∗) E-mail: Francesco.Riggi@ct.infn.it (Corresponding author)Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 F. RIGGI et al.(21) INFN and ASI Science Data Center - Roma, Italy(22) INFN and Dipartimento di Fisica, Università della Calabria - Cosenza, Italy(23) INFN Sezione di Catania and Dipartimento di Scienze Matematiche e Informatiche,Scienze Fisiche e Scienze della Terra, Università di Messina - Messina, Italyreceived 6 April 2018Summary. — A search for long-distance correlations between Extensive Air Show-ers (EAS) detected by pairs of MRPC telescopes of the Extreme Energy Events(EEE) network was carried out. A dataset obtained by all possible pairings betweenten EEE cluster sites (hosting at least two telescopes) with an overall exposure of3968 days was analyzed. A few candidate events with unusually small time differenceand angular distance were observed.1. – IntroductionThe possibility to observe time correlations between cosmic ray detectors separatedby distances much larger than the extent of the highest energy Extensive Air Showers(EAS), i.e. a few km, has been discussed for many years. A possible physical mechanismwhich could justify the existence of such events was originally proposed in [1] and laterdiscussed by many authors.In this picture, a heavy primary nucleus A could be photodisintegrated (for instanceinto a proton and a nucleus A − 1) by interaction with the solar photons, since the rel-ative energy between a high energy proton and the photons from the solar field couldcorrespond to an enhancement in the photodisintegration cross section. The two parti-cles could then propagate in the interplanetary magnetic field and arrive on the Earthatmosphere producing two individual yet correlated showers.A few experimental searches for such events have been reported without any definiteconclusion [2-6]. Current rate expectations are very low and range from 10−3 to 1 eventper km2 per year. The Extreme Energy Events (EEE) network [7], based on MultigapResistive Plate Chamber (MRPC) telescopes, located in Italian high schools over a largeterritory, reconstructs cosmic muons with high efficiency and good angular resolution.The wide coverage, number of sites, and large time exposure due to continuous datataking are unique characteristics of the EEE network which allow for the current investi-gation. We report preliminary results of a search for long-distance time and orientationcorrelations between independent extensive air showers detected by telescope pairs.2. – The EEE ProjectThe EEE Project is a joint educational and scientific initiative by Centro Fermi (En-rico Fermi Historical Museum of Physics and Research and Study Centre) [7], in Collab-oration with INFN (Italian National Institute for Nuclear Physics), CERN and MIURTIME AND ORIENTATION LONG-DISTANCE CORRELATIONS ETC. 3(the Italian Ministry of Education, University and Research). The Project has built andinstalled an array of cosmic ray detectors.The detection technique employed by the EEE telescopes is based on three MRPCs,each 0.82×1.58m2, which reconstruct the incoming muon tracks with high efficiency andgood angular resolution. The absolute time of each event is recorded with a GPS unit,to a precision of a few tens of nanoseconds, for later combination with data from othertelescopes. Presently, 53 telescopes have been taking data for several years, collectingmore than 5 × 1010 muon tracks. A complete description of the experimental setup andrecent physics results from the EEE Project are reported in [8-11].The research goals of the Project are focused on the study of the properties of thelocal muon flux and its dependence on the Earth and solar environment, the detection ofhigh-energy extensive air showers created in the Earth’s atmosphere and the search forpossible long-distance correlations between telescopes. Other physics items of interestinclude the study of the upward muon flux and the search for small anisotropies in theequatorial coordinate sky map. A powerful impact on education is also envisaged bythe EEE Project, which has already introduced a large number of school teachers andstudents to particle and astroparticle physics.3. – Experimental results and discussionTo enhance the probability of observation of rare events and minimize the amountof spurious coincidences, different strategies were considered. The correlation betweenindependent telescopes —due to their individual single rates— is not selective enough;even with a cut imposed on the relative orientation between the detected muons, thenumber of random coincidences per day is still of the order of 103–104 in a time windowof 1 ms. Since in the EEE network there are a few (cluster) sites which are equippedwith at least two telescopes in the same town, we may consider however the correlationbetween telescope pairs, each detecting a local shower. With this strategy, the accidentalcoincidence rate in a time window of 1 ms goes down to the level of about 1 event peryear. A dataset originating from 10 EEE cluster sites, each equipped with two MRPCdetectors was considered in the present analysis. The distances between these sites rangefrom 86 km to 1200 km, and the overall time exposure (summing all 45 pairings) wasabout 11 years.All candidate events which stay within the time window dictated by the distance be-tween the sites were considered, and for each of them we evaluated the p-value (accordingto a Poisson distribution) to obtain such an event by chance, considering the number ofexpected events in a time window corresponding to the measured Δt and with a relativeangle between the two showers not exceeding the actual observed value of ϑrel.The uncertainty in the relative angle between the two showers depends on severalfactors. The angular resolution in the reconstruction of individual muon tracks in eachtelescope is of the order of 1◦, resulting from the spatial and time resolution of theMRPC detectors, which was experimentally measured and also compared to simulations.The geographical orientation of the telescopes also plays some role in the overall angularuncertainty. However, a larger contribution (a few degrees) originates from the intrinsicuncertainty associated with the shower axis, when estimated by the average direction ofthe two detected muons, which is only roughly correlated to the direction of the primary.The angular spread of an extensive air shower may indeed amount to a few degrees,depending on the energies of the primary particle and of the secondary muons, and hasbeen evaluated by EAS shower simulation codes in a previous paper [8].4 F. RIGGI et al.Table I. – List of candidate events observed within a time window compatible with the distancebetween the sites. The columns report the site pair observing the event, the relative distance be-tween the sites, the measured time difference, the relative angle between the showers, the expectednumber of accidental coincidences in that time window, and the corresponding p-value. The la-bels identifying the various sites refer to the geographical locations of Bologna (BOLO), Cagliari(CAGL), Catania (CATA), CERN, Grosseto (GROS), L’Aquila (LAQU), Torino (TORI).Event EEE pairs Distance (km) Δt (μs) ϑrel (deg) Expected events p-value(A) BOLO-CAGL 614 86 27.1 0.0069±0.0002 0.007(B) BOLO-LAQU 290 740 9.1 0.014±0.001 0.014(C) CATA-TORI 1040 88 9.2 0.0265±0.0005 0.026(D) GROS-TORI 377 297 14.4 0.032±0.001 0.031(E) CERN-CATA 1200 248 9.3 0.049±0.001 0.048From this analysis we selected five candidate events for which the p-value is relativelylow (smaller than 0.05), given the time difference between the two sites, the relativeangle, and the number of expected accidental events in that time window. Such eventsare reported in table I, with their main properties. For some events (A, C), the timedifference is smaller than 100 μs, and three events have a relative angular distance smallerthan 10◦.For the events reported in table I, the trend of the number of accidental events asa function of the coincidence time window, with a relative angle not greater than thatactually observed, may be analyzed. Figure 1 shows an example of the analysis performedfor a pair of telescopes (Event A in table I). The event of interest has the smallest timedifference, which may be compared in the plot with the observed trend of the accidentalrate as a function of the time window.Table I only reports the most 5 significant events observed (i.e. those events whichhave a p-value smaller than 0.05). However, the total number of events observed withinthe correct time window, dictated by the mutual distance between the telescope pairs andby the maximum detectable angle for the two showers, is 96, arising from all 45 pairings.Fig. 1. – Number of coincident events as a function of the time window, with an angular cutcorresponding to the value observed for the event A reported in table I.TIME AND ORIENTATION LONG-DISTANCE CORRELATIONS ETC. 5To estimate an overall significance, we considered all the observed events, comparingthe result with the corresponding number of background events, which turns out to beequal to 77.8, roughly corresponding to a 2σ effect. A more detailed discussion of thisprocedure is reported in a recently published paper [11].4. – Conclusion and future prospectsA first analysis of a representative dataset of cosmic ray events detected by EEEtelescope clusters located at large distances has been carried out. A few candidate eventswhich are characterized by small values of the time difference —and in some cases alsoby a small angular difference— were extracted from the analysis. The observation of thiskind of anomalous events is one of the goals of our experiment. To investigate in moredetail a possible evidence for long-distance correlations will require higher statistics. Anincrease in the available statistics in the EEE Project could come within a few years fromlonger data taking periods, larger duty cycles, greater telescope detection efficiency, andthe inclusion of additional clusters in the EEE network.∗ ∗ ∗We are grateful to the many teachers and students of the Italian high schools involvedin the EEE Project, who have made this experiment possible thanks to their enthusiasmand skilful application. We also acknowledge the continuous support of CERN and INFN.REFERENCES[1] Gerasimova N. M. and Zatsepin G. T., Sov. Phys. JETP, 11 (1960) 899.[2] McBreen B. et al., in Proceedings of the 17th ICRC, Paris, Vol. 6 (Commissariat àl’Énergie Atomique, International Union of Pure and Applied Physics, CEN Saclay, France,1981) p. 300.[3] Ochi N. et al., J. Phys. G: Nucl. Part. Phys., 29 (2003) 1169.[4] Fujiwara Y. et al., Nucl. Phys. B Proc. Suppl., 151 (2006) 481.[5] Blaschke F. et al., Astrophys. Space Sci. Trans., 7 (2011) 69.[6] Iyono A. et al., Astrophys. Space Sci. Trans., 7 (2011) 327.[7] Centro Fermi Web site: http://eee.centrofermi.it.[8] The EEE Collaboration (Abbrescia M. et al.), Eur. Phys. J. Plus, 130 (2015) 9.[9] The EEE Collaboration (Abbrescia M. et al.), Nucl. Instrum. Methods A, 816 (2016)142.[10] The EEE Collaboration (La Rocca P. et al.), Nucl. Instrum. Methods A, 845 (2017)383.[11] The EEE Collaboration (Abbrescia M. et al.), Eur. Phys. J. Plus, 133 (2018) 34.

Time and orientation long-distance correlations between extensive air showers detected by the MRPC telescopes of the EEE Project

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Time and orientation long-distance correlations between extensive air showers detected by the MRPC telescopes of the EEE Project

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