New technique and results of cosmic ray investigations in the energy interval ev

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1 EPJ Web of Conferences 53, (2013) DOI: /ejconf/ C Owned by the authors, ublished by EDP Sciences, 2013 New technique and results of cosmic ray investigations in the energy interval ev A.A. Petrukhin 1, A.G. Bogdanov 1, D.V. Chernov 1, L.I. Dushkin 1,R.P. Kokoulin 1, G. Mannocchi 2, O. Saavedra 3, G. Trinchero 2, V.V. Shutenko 1 and I.I. Yashin 1 1 National Research Nuclear University MEPhI, Moscow, Russia 2 Istituto di Fisica dello Sazio Interlanetario INAF, Torino, Italy 3 Diartimento di Fisica dell Universita di Torino, Torino, Italy Abstract. New technique of EAS investigations based on the measurements of local muon density sectra (LMDS) is develoed. Alication of this method to investigations of inclined EAS allows exloration of CR energy interval from to ev by means of a relatively small detector with area 100 m 2 due to very strong deendence of EAS muon density on zenith angle. During , long-term NEVOD- DECOR exeriment (about 20,000 h live time) was conducted, and more than two million muon bundles in zenith angle interval degrees were registered. Comarison of exerimental data with results of CORSIKA-based simulations showed that the new method is sensitive to all main eculiarities of CR energy sectrum: the knee, increase of the energy sectrum sloe with energy, the second knee. But the observed rogressive excess of muon bundles with the increase of rimary CR energy in comarison with simulations (even for ure iron comosition) can indicate the aearance of new rocesses of muon generation. In this case, for correct investigations of EAS, the exerimental arrays must be sulemented by detectors which can measure or evaluate the energy of muons. 1. INTRODUCTION This year coincides with 100 anniversary of cosmic ray discovery and more than 50 years from the first observation of the knee in EAS energy sectrum (more correctly, in the sectrum of EAS size in charged articles). During this eriod the following results above ev were obtained: the knee, steeening energy sectrum, the second knee, the ankle, GZK cut-off. All these results were obtained in frame of existing models of hadron interaction in which accelerator data are extraolated u to uer limit of cosmic ray energies. At the same time, information about mass comosition of cosmic rays is not so definite and often even contradictory. The reasons of these disagreements can be connected with different techniques of EAS detection and narrow energy intervals, which are accessible for each EAS array. From this oint of view the develoment of new methods and aroaches to EAS investigations is an imortant task. At resent, the following arameters of EAS are measured: number of electrons N e (in fact, mixture of charged articles); number of muons N μ (in fact, ionization effect in scintillation detectors, roduced by muons and ossible secondary articles); energy deosit of EAS core, E h (in calorimeter measurements); fluorescence radiation flux, F f ; Cherenkov radiation flux, F ch ; radio emission flux, F r (first results aeared recently); acoustic radiation flux, F a (ideas of the use of this method are ermanently discussed, but no real results were obtained). Two new methods are connected with EAS muon comonent investigations: local muon density, D μ (which was realized in the NEVOD-DECOR This is an Oen Access article distributed under the terms of the Creative Commons Attribution License 2.0, which ermits unrestricted use, distribution, and reroduction in any medium, rovided the original work is roerly cited. Article available at htt:// or htt://dx.doi.org/ /ejconf/

2 EPJ Web of Conferences df / d (log r), arb. units ( a ) ), arb. units dn µ / d ( log E µ ( b ) Distance r from shower axis, m lg (E µ, GeV) Figure 1. (a) Contribution of muons detected at various distances from the shower axis to the sectrum of events selected in article density (solid curve) and to the total number of EAS muons (dashed curve); (b) energy sectra of all EAS muons (dashed curve) and muons detected in bundles (solid curve). Zenith angle θ = 60. Effective event collection area S eff, m SIBYLL, rotons QGSJET01, rotons SIBYLL, iron SIBYLL, rotons (without EMF) E 0 = ev With EMF Without EMF o 40 o 50 o 60 o 70 o 80 o 90 o Zenith angle Figure 2. Zenith-angular deendence of effective EAS collecting area in LMDS method. exeriment); muon bundle energy deosit, E μb (the use of this method in NEVOD-DECOR exeriment is lanned). 2. METHOD OF LOCAL MUON DENSITY SPECTRA (LMDS) IN INCLINED EAS INVESTIGATIONS The detailed descrition of LMDS method is given in aer [1]. Here the main advantages of this method in comarison with N μ measurements are discussed. The first one is the range of distances from EAS axes which give basic contribution to total number of EAS muons estimated from N μ measurements and to sectrum of events selected by muon density. Figure 1a illustrates this difference. It is seen that the main contribution to the local muon density sectrum give about ten times smaller distances from the EAS axes than to the total number of muons. Corresondingly, muon energies which give main contribution to LMDS and to the total muon number will be different, too. In Fig. 1b, energy sectra for these two cases are shown. Of course, LMDS method can be used for any EAS arrival directions, but it is esecially effective for investigations of inclined EAS. Due to a strong deendence of characteristics of the Earth s atmoshere on zenith angle, the lateral size of EAS muon comonent raidly increases, and the shower may be detected at large distances from the axis, so that the region from which such events are collected is determined in this case by the cross-sectional area of the shower rather than by the detector size (Fig. 2). As can be seen, at large zenith angles very imortant role in the increase of the collecting area lays the

3 UHECR 2012 Figure 3. Layout of the NEVOD DECOR exerimental array (for descrition, see the main body of the text). geomagnetic field (EMF). Therefore at detecting nearly horizontal muon bundles this area (more than 1km 2 ) is sufficient for obtaining statistically significant results u to rimary article energies of ev and even higher. In this situation, the muon bundle detector with sizes of several tens square meters can be considered as a oint-like device. 3. NEVOD-DECOR EXPERIMENT The layout of exerimental comlex NEVOD-DECOR is resented in Fig. 3. It consists of two main detectors. The DECOR coordinate detector [2], which is a multilayer system of lastic streamer tubes featuring resistive coating of the cathode, is deloyed around the NEVOD Cherenkov water calorimeter [3] of internal volume m 3. The DECOR side art, laced in the gallery of the building on the three sides of the NEVOD tank, includes eight suermodules of working area 8.4 m 2 each, which consist of eight vertical lanes of streamer tubes hung at a distance of 6 cm from one another. The uer art of the coordinate detector consists of four suermodules 11.5 m 2 in area each, which are mounted on horizontal latforms. The DECOR lanes involve an external two-coordinate readout system from aluminum stris, which ensures determination of ositions of charged-article tracks to a recision about 1 cm in each of the coordinates (X, Y). The angular recision in reconstructing muon tracks intersecting suermodules is about 0.7 and 0.8 for rojected zenith and azimuth angles, resectively. The detecting system of the NEVOD calorimeter is a satial lattice of quasi-sherical measuring modules (crosses in Fig. 3), which make it ossible to detect Cherenkov radiation from relativistic charged articles in water from any direction, to estimate the energy deosition in the detector volume, and to reconstruct the direction of motion of these articles. The total live time of oeration of the array with fully equied DECOR side art (eight suermodules, the set of triggering event-selection conditions being invariable) was 19,922 h. An examle of an event with a muon bundle in the coordinate detector is resented in Fig. 4. Results of selection of muon bundles with various multilicities and zenith angles are given in Table 1. Since the detected number of muon bundles with small multilicity and low zenith angles is very large, for the analysis only a art of total statistics was taken. In Fig. 5, the regions of rimary energies and zenith angles which corresond to the obtained exerimental data (Table 1) are shown. For these events, the sectra of local muon density were obtained, which are comared with results of calculations by means of the CORSIKA code [4] under various assumtions on the sectrum and comosition of rimary cosmic rays and on hadron interaction models (Fig. 6). From the figure, the

4 EPJ Web of Conferences Figure 4. Results of geometric reconstruction of the event with 50 quasi-arallel tracks, the zenith angle being 78 (rojection onto the horizontal lane of the detector). < log ( E, GeV ) > o 80 o 70 o 60 o 50 o 35 o 6 5 D, muons / m 2 Figure 5. Average logarithms of the energy of rimary articles resonsible for events with different local muon densities D for various zenith angles. The olygons corresond to categories of events indicated in Table 1. Table 1. Statistics of selected muon bundles. Category of Muon multilicity Zenith angle Live time, Number of events range (hour) events following conclusions can be done. At energies between and ev the first knee is observed and mass comosition is nearly normal (as measured at lower energies). In the energy interval the sectrum of local muon density moves to the rediction for ure iron comosition and at higher energies corresonds to this comosition. At that, the second knee at energy about ev is observed. At energies about ev, the exerimental LMDS is higher than redicted ones. To comare the obtained results with other exerimental data at these energies, the LMDS were converted into rimary sectra by using traditional assumtions about comosition and interaction of UHECR. The results are given in Fig. 7. One can see that estimations of rimary sectrum on the basis of DECOR data are incomatible with HiRes, Auger and TA data (even under assumtion of iron comosition). If we assume that the energy

5 UHECR 2012 D 3 df/dd, 1 / ( s sr m 4 ) ev ev ev 35 o ( a ) ev ev ev ( b ) 50 o ev ev ev ev ev ev D 3 df/dd, 1 / ( s sr m 4 ) 10-5 ( c ) 65 o ( d ) 78 o 10-6 D, m - 2 D, m - 2 Figure 6. Exerimental and calculated differential sectra of the local muon density at zenith angles of (a) 35,(b) 50,(c)65,and(d)78. Points reresent exerimental data; the solid and dashed curves corresond to results of calculations erformed by using the QGSJET01 and SIBYLL2.1 models, resectively. In each anel, the lower air of curves corresonds to rimary rotons, while the uer air corresonds to iron nuclei. E 3 dn/de / 10 24, ev 2 m -2 s -1 sr Only rotons Only iron θ > 75 o S Q2 Q1 E1.9 E1.6 θ > 80 o S Q2 E1.9 Q1 E1.6 AGASA HiRes-1 HiRes-2 Auger-2011 (combined sectrum) TA-2011 (surface detectors) DECOR log (E, ev) Figure 7. Differential sectrum of rimary cosmic rays at ultra high energies. The closed symbols stand for the intensity reconstructed on the basis of DECOR data for two limiting assumtions on the comosition of rimary cosmic rays (rotons and iron nuclei) by using the SIBYLL2.1, QGSJET01, QGSJET-II, EPOS1.61 and EPOS1.99 hadron-interaction models (S, Q1, Q2, E1.6 and E1.9 labels at the oints). The oen symbols reresent well-known data (boxes) HiRes-1 [5], (circles) HiRes-2 [5], (inverted triangles) AGASA [6], (triangles) PAO [7] and (diamonds) TA [8] results. calibration of the fluorescent method used by HiRes and PAO collaborations is close to a true one, then results resented in Fig. 7 indicate inalicability of aforementioned interaction models in calculating the roerties of the muon comonent of extensive air showers in the energy region around ev. In order to exlain the observed intensity of muon bundles at large zenith angles, one needs considerably higher (by a few tens of ercent) muon density in the shower central region

6 EPJ Web of Conferences 4. DISCUSSION Alication of the new aroach to studying extensive air showers that is based on measuring sectra of the local muon density at various zenith angles makes it ossible to obtain information about the roerties of the flux and interaction of cosmic rays over a broad range of rimary article energies (which covers more than three orders of magnitude) within a single technique by using the same exerimental facility of relatively small size. The sectra of the local muon density are sensitive to the shae of the rimary sectrum, the mass comosition of rimary cosmic rays, and the roerties of the interaction of ultrahigh-energy hadrons in the forward kinematical region, where the uncertainties in the existing theoretical models with increasing of energy are maximal. An analysis of the DECOR exerimental data on muon bundles which is based on the comarison of the measured local muon density sectra with the results of simulation erformed by using the CORSIKA code indicates that the muon density increases gradually in relation to the value exected for a fixed comosition of rimary cosmic rays. A robable interretation of this is that the comosition becomes heavier. But the measured intensity of muon bundles roduced in extensive air showers by rimary articles of energy about ev at large zenith angles is significantly higher than that which could be exected on the basis of data obtained for rimary cosmic rays by the fluorescence method even for ure iron comosition and any extensively used models of hadron interactions. Of course, it is ossible to develo a model of hadron interaction which will give higher multilicity of secondary articles in the forward region to exlain obtained exerimental data. But there are some evidences that excess of muons exists at very high energies [9, 10]. To exlain these data, serious changes of interaction model are required, including a new hysical rocess aearance. 5. CONCLUSION Since the main task of the Symosium is discussion of further develoment of UHECR investigations, it is ossible to do the following conclusion: new exeriments must give a ossibility to evaluate muon energy in detected EAS. References [1] A.G. Bogdanov et al. Phys. Atom. Nucl. 73 (2010) 1852 [2] N.S. Barbashina et al. Instrum. Ex. Tech. 43 (2000) 743 [3] V.M. Aynutdinov et al., Inzh. Fiz., no. 4 (2000) 71 [4] D. Heck and T. Pierog, Extensive Air Shower Simulations with CORSIKA: A User s Guide (Karlsruhe, 2009); htt://www-ik.fzk.de/corsika/usersguide/corsika_tech.html [5] R.U. Abbasi et al. (HiRes Collab.) Phys. Rev. Lett. 100 (2008) [6] M. Takeda et al. Astroart. Phys. 19 (2003) 447 [7] J. Abraham et al. (PAO Collab.) Phys. Rev. Lett. 101 (2008) [8] D. Ikeda et al. (TA Collab.) Proc. 32 nd ICRC, Beijing, 2011, vol. 2,. 238 [9] A.G. Bogdanov et al. Astroart. Phys., in ress (2012) [htt://dx.doi.org/ / j.astroarthys ] [10] P. Berghaus and Ch.Xu. Presentation at 32nd ICRC (Beijing),