EAS selection in the EMMA underground array

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. EAS selection in the EMMA underground array Sarkamo, J.; Bezrukov, L.; Enqvist, Timo; Fynbo, H.; Inzhechik, L.; Joutsenvaara, J.; Kalliokoski, Tuomo; Kuusiniemi, Pasi; Loo, Kai; Lubsandorzhiev, B.; Monto, Tiia; Petkov, V.; Räihä, T.; Slupecki, M.; Trzaska, Wladyslaw; Virkajärvi, A. Sarkamo, J., Bezrukov, L., Enqvist, T., Fynbo, H., Inzhechik, L., Joutsenvaara, J., Kalliokoski, T., Kuusiniemi, P., Loo, K., Lubsandorzhiev, B., Monto, T., Petkov, V., Räihä, T., Slupecki, M., Trzaska, W., & Virkajärvi, A. (2013). EAS selection in the EMMA underground array. In 23rd European Cosmic Ray Symposium (and 32nd Russian Cosmic Ray Conference) 3–7 July 2012, Moscow, Russia. Institute of Physics. Journal of Physics: Conference Series, 409. https://doi.org/10.1088/17426596/409/1/012086 2013 This content has been downloaded from IOPscience. Please scroll down to see the full text. Download details: IP Address: 130.234.75.141 This content was downloaded on 15/01/2016 at 10:55 Please note that terms and conditions apply. EAS selection in the EMMA underground array View the table of contents for this issue, or go to the journal homepage for more 2013 J. Phys.: Conf. Ser. 409 012086 (http://iopscience.iop.org/1742-6596/409/1/012086) Home Search Collections Journals About Contact us My IOPscience EAS selection in the EMMA underground array J Sarkamo1, L Bezrukov2, T Enqvist1, H Fynbo3, L Inzhechik2,4, J Joutsenvaara1, T Kalliokoski5, P Kuusiniemi1, K Loo5, B Lubsandorzhiev2, T Monto5, V Petkov2, T R¨aih¨a1, M Slupecki1, W H Trzaska5, A Virkaj¨arvi1 1Oulu Southern Institute and Department of Physics, University of Oulu, Finland 2Institute of Nuclear Research, Russian Academy of Sciences, Moscow, Russia 3Department of Physics and Astronomy, University of Aarhus, Denmark 4Moscow Institute of Physics and Technology, Russia 5Department of Physics, University of Jyv¨askyl¨a, Finland E-mail: [email protected] Abstract. The first measurements of the Experiment with MultiMuon Array (EMMA) have been analyzed for the selection of the Extensive Air Showers (EAS). Test data were recorded with an underground muon tracking station and a satellite station separated laterally by 10 metres. Events with tracks distributed over all of the tracking detector area and even extending over to the satellite station are identified as EAS. The recorded multiplicity spectrum of the events is in general agreement with CORSIKA EAS simulation and demonstrates the array’s capability of EAS detection. 1. Introduction Primary cosmic-rays above the energy of 1015 eV are studied by measuring the properties of the Extensive Air Showers (EAS). Several experiments (see, for example, ref. [1]) using various measurement methods have reported an increase in the average mass of the primary particles in the energy range of 1015 −1016 eV. A novel approach to the study of the primary spectrum and mass composition is the measurement of the high-energy muon lateral density distribution of EAS. The Experiment with MultiMuon Array (EMMA) [2] is an underground EAS array designed for the measurement of the high-energy muon component (Eµ>50 GeV). It consists of several muon tracking detectors at the depth of 75 metres (210 m.w.e) in the Pyh¨asalmi Mine, Finland (63◦39.6 N, 26◦02.5 E). This contribution focuses on the results of 2011 test data taking. The data, taken with two stations of the array, have been analyzed for the selection of EAS events from all the recorded events. The presented methods are an essential progress towards the analysis of the primary cosmic-ray spectrum by the experiment. 2. Measurement configuration and data taking Two underground detector stations were used in the data taking. Station C is a muon tracking detector with the active dimensions of 4220 ×3650 ×2250 mm3and a geometric acceptance of 18 sr m2for three-layer tracking. It consists of 105 drift chambers of the former DELPHI [3] 23rd European Cosmic Ray Symposium (and 32nd Russian Cosmic Ray Conference) IOP Publishing Journal of Physics: Conference Series 409 (2013) 012086 doi:10.1088/1742-6596/409/1/012086 Published under licence by IOP Publishing Ltd 1 experiment. Station B is a prototype one-layer station. The horizontal distance between the station centres is 10 metres. The configuration is depicted in figure 1. Figure 1. A schematic view of the measurement configuration in the data taking. The chamber configuration of Station C is depicted on the left side. The right side shows Station B. Individual chambers are indicated by shaded rectangles. The sublayers within a layer are referred to as X and Y. The chamber heights (20 mm) are exaggerated for clarity. The lengths of the chambers are 3650 mm. Other dimensions are indicated in the figure. DAQ and trigger electronics are situated inside the stations. Signals are exchanged between the stations via cable canals running in the cavern. HV is supplied separately within the stations. Ar:CO2-gas mixture is delivered from a surface supply. The anode and delay line signals from the chambers are read out by CAEN V767b TDCs and the charged particle coordinates can be extracted from the recorded signals. The trigger is generated from the chamber anode signals. In the data taking, a trigger input generated from Station C was a triple-coincidence between any chambers from the three separate layers. The trigger input from Station B was a coincidence of any chambers from the separate Xand Ysublayers. The final trigger was OR of the Station C and Station B inputs. This trigger logic was used to guarantee efficient recording for the muon events. Given a relatively high singles rate from the chambers (∼100 Hz per chamber) and the coincidence gate widths (5µs), not all triggers correspond to muon events. Test data for the analysis were taken from 13th April to 4th June 2011 for an effective period of 44.0 days. In total, 184 million triggers were recorded, out of which the EAS can be extracted. 3. EAS extraction The track reconstruction relies on an algorithm of hit extraction and track fitting through the hits. A hit is defined as an extracted avalanche coordinate within the detector. The position accuracy of the detector is ∼1 cm2. The tracks are reconstructed using the extracted hit positions and a parallelity (δθ < 3◦) criterion. Some hits are left with no corresponding track. The shower arrival direction is determined as an average of the track arrival directions. A study of multiple-track events recorded by Station C reveals that two classes of events exist. Some events consist of tightly clustered hits and tracks localized well within the detector area. Other events consist of hits and tracks which are more uniformly distributed over all of the detector area. Two variables are used to classify the multiple-track events. The bundle size is defined as hRi= ΣNtrack i=1 ri/Ntrack, where Ntrack is the number of tracks and riare the distances between the individual tracks and the mass centre of all the tracks in the event. The distance is calculated on the detector plane. The value of the bundle size is affected by the detector dimensions and the expectation value is ∼150 cm for uniformly distributed tracks. The ’purity’ of the event reconstruction is described by P=Nhit/Ntrack, where Nhit is the 23rd European Cosmic Ray Symposium (and 32nd Russian Cosmic Ray Conference) IOP Publishing Journal of Physics: Conference Series 409 (2013) 012086 doi:10.1088/1742-6596/409/1/012086 2 number of reconstructed hit coordinates. The value of Pis expected to be larger for events with accompanying electromagnetic subshowers in comparison to pure muon events. One parametre is used in the case of Station B. NBis defined as the number of hit pairs recorded by the station. A hit pair is counted given that |zX−zY|<10 cm, where ziare the hit positions recorded in the Xand Ylayers of the station. Figure 2. The event distribution in the (hRi, P)-plane for one array station (left panel) and two array station (right panel) analyses. The cuts used are: Ntrack >4 (one station), Ntrack >4 and NB>0 (two stations). The binning of x-axis is in units of 10 cm and in y-axis in units of 1. The number of events in indicated by colours. The horizontal bars express the CORSIKA expectation for hRifor the multiplicity of 5. Figure 2 depicts the event distributions of Station C in the (hRi, P)-plane. Events with reconstructed zenith angles <35◦are selected for the analysis. Further cuts are based on the track multiplicity in Station C (Ntrack >4) and the number of hit pairs in Station B (NB>0). It is evident that two different classes of events exist. The background contribution is dominant in one station data and is characterized by small bundle sizes (<100 cm). The peak maximum of background events is diminished by a factor of ∼100 if a coincidence hit in Station B is required. The EAS events are characterized by hRi-values corresponding to the expectation from CORSIKA-QGSJET01 [4] simulation and many also extend to Station B, thus passing the coincidence criterion NB>0. The track multiplicity distributions in Station C are shown in figure 3 for two cases. For one-station analysis, the EAS can be selected by P < 1.375 ·(hRi − 48 cm). For two-station analysis, a cut hRi>70 cm is used. The number of events decreases if the coincidence of two stations is required. This is due to the fact that not all EAS, which yield multiple muon tracks in Station C, yield coincidence hits also in Station B. The highest-multiplicity events pass both of the cuts, as is expected due to the correlation of local muon densities in the shower. The data are compared with a CORSIKA simulation for the muon multiplicities expected in Station C. The spectrum is pure proton with spectral index γ=−2.7 below E= 4 ·1015 eV and γ=−3.1 above. The simulation is in general agreement with the data further proving the EAS extraction as valid. 23rd European Cosmic Ray Symposium (and 32nd Russian Cosmic Ray Conference) IOP Publishing Journal of Physics: Conference Series 409 (2013) 012086 doi:10.1088/1742-6596/409/1/012086 3 Figure 3. Preliminary muon multiplicity distribution in Station C for 44 days of data taking. Extraction with one-station method (open circles) and extraction with two-station method (open squares). The symbols overlap for the highest multiplicities. A CORSIKA simulation expectation normalized appropriately is drawn as a solid red line. Different primary energies contribute up to different multiplicities. The energy labels denote the approximative energies with the largest yields to the multiplicity. 4. Discussion The background events are interpreted as single-muon-induced particle showers. Such events may exhibit a multitude of hits around a straight trajectory, which may cause false interpretations of multiple parallel tracks. The average number of hits contributing to the fitted track is lower in the background events than in the EAS. The multiplicity distribution of background events is close to exponential indicating a stochastic origin. These events can be discarded with the methods presented, but are also to be separately studied in more detail. The comparison shown in figure 3 demonstrates the EAS selection method. More systematical effects related to the optimal tracking parametres and multitracking efficiencies, both on the simulation and reconstruction, must be investigated before proceeding to composition analysis. Therefore figure 3 is not to be interpreted as a result of such an analysis. 5. Conclusion The EAS selection in the EMMA underground array can be approached by two different means. Events recorded by one array station can be classified according to the bundle size and reconstruction purity, which disentangles the Extended Air Showers and the single-muoninduced background. A requirement of a coincidence between two detector stations diminishes the background. In upcoming data taking with multiple stations, the use of these methods will result in background-free EAS selection. Acknowledgments The support from the Magnus Ehrnrooth Foundation, the Vilho, Yrj¨o and Kalle V¨ais¨al¨a Foundation, the Finnish Cultural Foundation, and the Jenny and Antti Wihuri Foundation is gratefully acknowledged. The work is funded by the European Union Regional Development Fund and it has also been supported by the Academy of Finland (projects 108991, 7108875 and 7106570). References [1] Bl¨umer J, Engel R and H¨orandel J 2009 Progress in Particle and Nuclear Physics 63 293-338 [2] Kuusiniemi P et al. 2012 Underground cosmic-ray experiment EMMA this proceedings [3] Aarnio P et al. and the DELPHI Collaboration 1991 Nucl. Inst. Meth. in Phys. Res. A303 233 [4] Heck D, Knapp J, Capdevielle J N, Schatz G and Thouw T 1998 CORSIKA: A Monte Carlo Code to Simulate Extensive Air Showers Report FZKA 6019 23rd European Cosmic Ray Symposium (and 32nd Russian Cosmic Ray Conference) IOP Publishing Journal of Physics: Conference Series 409 (2013) 012086 doi:10.1088/1742-6596/409/1/012086 4