Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition

Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition S. P. Knurenko 1 and A. Sabourov 2 1 s.p.knurenko@ikfia.ysn.ru, 2 tema@ikfia.ysn.ru Yu. G. Shafer Institute of cosmophysical research and aeronomy Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 1

1. Introduction The Yakutsk EAS array is designed for detection of cosmic rays (CR) with energy 10 15 10 19 ev. It provides measurements of main components of extensive air showers (EAS): electrons, muons and Cherenkov light emission. Recently, experiments on radio-emission detection have been re-initiated. The array itself consists of several instruments, combined in a single system (see next two slides): the main array, small Cherenkov array, Cherenkov tracking detector based on camera-obscura, large muon detector, weather-station and a λ = 532 nm lidar to measure atmosphere parameters. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 2

1.1 The schematics of Yakutsk EAS array network Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 3

1.2 The Yakutsk EAS array layout Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 4

2. Energy spectrum The readings of light-integrating Cherenkov detectors can be analyzed separately from other detectors. The energy of CR particle initiating air shower is determined in almost model independent way involving Cherenkov light measurements. The depth shower maximum x max is derived from observations of Cherenkov light lateral distribution. Knowing the x max for protons and iron nuclei from simulations (QGSJET01), the mean logarithmic mass can be derived from measured x max : lna = x max x p max x Fe max x p max lna Fe The experimental data set ( 75000 events at E 0 > 10 17 ev) is considered within two, dip and ankle scenarios. In both cases low energy part of CR spectrum J(E) is produced in supernova remnants (SNRs). The galactic part of spectrum J g (E) and composition are calculated within kinetic nonlinear theory (Berezhko and Völk (2007)). Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 5

2.1 The dip-scenario: The second CR component J eg (E) is produced by extragalactic sources assuming that they produce CR spectrum J eg s /E 2.7 at E > 10 18 ev and taking into account the modification of this spectrum due to the propagation effects in the intergalactic space (Aloisio et al (2007)). On the next slide: The dip-scenario compared to with several experiments. The dashed line represents the Galactic component. The dash-dotted line represents the assumed extragalactic component. It is seen, that experimental CR in a satisfactory way is consistent with theoretically expected spectrum within the dip scenario. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 6

2.1 The dip-scenario: Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 7

2.2 The ankle scenario: In the ankle scenario the extragalactic source spectrum is assumed to be much harder Js eg /E 2 : J eg (E) becomes dominant above 10 19 ev and therefore the observed CR spectrum needs the third component reacceleration mechanism (spiral shocks in the galactic wind, pulsar vicinity (Völk & Zirakishvili (2004), Bell (1992), Berezhko (1994))). Instead of J g Z (E) for every element with nuclear charge number Z we use J g Z (E) which coincides with J g Z (E) at E < E max1 and at E > E max2 : ( ) J g Z (E) = J g Z (E (E) max1 Z E E Z max1 ) γ exp( E E Z max2 ) where Emax1 Z minimal energy of particles involved into reacceleration process and Emax2 Z maximal particle energy achieved in reacceleration. On the next slide: The ankle scenario compared to experimental data. Dashed line galactic component (including SNRs and reaccelerated CRs). Dash-dotted line extragalactic component (corresponding to Js eg /E 2 ). Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 8

2.2 The ankle scenario: Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 9

2.3 Mean CR atomic number Due to dependence of maximal (cutoff) energy of CR produced in SNRs E max 3 Z 10 15 ev on the atomic charge number Z CR composition becomes progressively heavier as the energy increases from E 10 15 ev to E 10 16 ev, where iron nuclei become dominant. At higher energies within the dip scenario the contribution of extragalactic CR becomes essential, therefore ln A goes down with the increase of the energy towards the value lna 1.5. Completely different CR composition is expected at E > 10 16 ev within the ankle scenario is expected at E > 10 16 within the ankle scenario. Due to reacceleration heavy CR with dominant CR iron nuclei extend from 10 15 ev to about 10 19 ev (see next slide). In this case the transition towards lighter extragalactic component occurs at E 10 19 ev. Next slide: Mean logarithm of the CR nucleus atomic number as a function of energy calculated within the dip and ankle scenario are represented by solid and dashed lines respectively. Experimental data obtained at ATIC-2, JACEE, KASCADE, HiRes, PAO and Yakutsk EAS experiments. It follows that the Yakutsk EAS array data better agree with the ankle scenario. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 10

2.3 Mean CR atomic number Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 11

3. Longitudinal distribution It is known fact that the depth of shower maximum (x max ) and fluctuations in EAS development are sensitive to atomic number of primary particle and for this reason they are used to estimate the CR mass composition (Efimov et al (1987)), Dyakonov et al (1989), Knurenko et al (2005)). Here we present the data on longitudinal EAS development reconstructed from Cherenkov emission data. These data were obtained after modernization of the Yakutsk array when the accuracy of main EAS characteristics increased as compared to previous series of observations. It is important to consider not only mean EAS parameters, e.g x max, muon content ρ µ /ρ ch but also their fluctuations in given energy intervals. In order to minimize the latter, it is also a good idea to analyze them at fixed energies. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 12

3.1 Technical aspects Determination of x max in individual shower is based on methods developed at the Yakutsk array and utilize the measurements of EAS Cherenkov light emission at different core distances. the x max is determined by parameter p = lgq 200 /Q 550 (a relation of Cherenkov light fluxes at 200 and 550 m from the core); involving the reconstruction of EAS development cascade curve, using Cherenkov light lateral distribution function and a reverse solving (Knurenko et al (2001); based on half-width and half-height of Cherenkov light pulse recorded at 200 m from the core; fourth method includes recording of Cherenkov track with several detectors based on camera-obscura located at 300 500 m from the array center (Petrov et al (2008)). Following slides: examples demonstrating these techniques for x max estimation. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 13

3.1.1 Estimation by parameterp = lgq 200 /Q 550 10 10 Q (R ), photon/m 2 10 9 10 8 10 7 10 6 E 0 = 1.3 10 19 ev; θ = 25 ; x max = 738 g/cm 2 100 1000 core distance, m Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 14

3.1.2 Estimation by the shape of Cherenkov pulse Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 15

3.1.3 Estimation with Cherenkov tracking detector Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 16

3.1 Technical aspects The accuracy of x max determination in individual showers was estimated in simulation of EAS characteristics measurements at the array involving Monte-Carlo methods and amounted to 30 45 g/cm 2, 35 55 g/cm 2, 15 25 g/cm 2, 35 55 g/cm 2 accordingly for first, second, third and fourth methods. Total error of x max estimation included errors associated with core location, atmospheric transparency during observational period, hardware fluctuations and mathematical methods used to calculate main parameters. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 17

3.2 Mean depth of maximal shower development 1000 900 x max, g/cm 2 800 700 600 500 400 300 10 6 10 7 10 8 10 9 Q (R ), photon/m 2 A cloud of points in x max distribution for showers with energy above 10 17 ev. These data were obtained using all four methods and reflect an alteration of x max towards lower atmosphere depths with growth of energy. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 18

3.2 Mean depth of maximal shower development Next figure shows x max values averaged over energy intervals together with the data from other experiments. On the same picture results of different hadron models calculations are shown. All experimental data coincide within experimental errors and demonstrate irregular shift with energy. Up to 3 10 18 ev E.R. has value 60 80 g/cm 2 and within the interval of 3 10 18 5 10 19 ev it equals to 40 60 g/cm 2. This might be interpreted as a possible alteration in mass composition at very high energies. A comparison with calculations revels the tendency of light nuclei abundance starting from 5 10 17 ev to 2 10 19 ev and some abundance above 2 10 19 ev. Next slide: Filled circles represent Yakutsk data open circles CASA-MIA, squares AUGER data, blue triangles preliminary results of the Telescope Array experiment. Solid lines results obtained with QGSJet II, dashed EPOS 1.6, point line SIBYLL 1.62. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 19

3.2 Mean depth of maximal shower development 850 800 x max, g/cm 2 750 700 650 600 550 10 17 10 18 10 19 10 20 E 0, ev Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 20

3.3 Fluctuations of x max Fluctuations of x max play a huge role in EAS longitudinal development as they are associated with the point of first interaction (and, hence, with cross-section of inelastic interaction, σ A-air ), energy transfer to secondary hadron particle (inelastic coefficient K inel ) and, to a great extend, depend on the kind of primary particle initiating a shower. So, the amount of fluctuations measured in different energy intervals could characterize CR mass composition at given energy and on the whole determine the dynamics of its change with the energy of primary particle. Next figure demonstrates energy dependence of σ(x max ) obtained at the Yakutsk array. To compare with, the same figure shows HiRes data (Abbasi et al (2009)). The data from HiRes experiment virtually reproduce the data from Yakutsk but have a slight tendency of σ(x max ) change: a small increase in the region of 10 17 10 18 ev and decrease at 2 10 18 5 10 19 ev. The curves representing simulation results, obtained with QGSJet01, QGSJet II and SIBYLL models, are also shown on this figure. Next figure: filled squares Yakutsk data, open squares HiRes data, open triangles data from Pierre Auger Observatory. Straight line results obtained with QGSJet01, dashed line QGSJet II, dotted line SIBYLL 1.62 for various primary nuclei. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 21

3.3 Fluctuations of x max 80 70 RMS(x max ), g/cm 2 60 50 40 30 20 10 10 17 10 18 10 19 10 20 E 0, ev Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 22

3.3 Fluctuations of x max It should be pointed out that according to previous slide, the portion of heavy nuclei in CR flux energy above 2 10 18 ev is small and helium and CNO-group nuclei might play a significant role. We came to the same conclusion (Knurenko et al (2005)) where the shape of x max distribution was analyzed within the framework of QGSJet01 model at fixed energies 10 18 ev and 10 19 ev (see next slide). Next slide: x max distribution at fixed energy 10 18 ev. Solid line represents Yakutsk data (8 10 17 < E < 2 10 18 ev, E = 1.0 10 18 ev, 857 events); dotted line QGSJet01 for mixed composition (70 % p, 30 % Fe); dashed line QGSJet01 for primary protons, solid grey line QGSJet01 for CNO group nuclei, dash-dotted line QGSJet01 for iron nuclei (Knurenko et al (2005)). Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 23

3.3 Fluctuations of x max 200 number of events 150 100 50 0 400 500 600 700 800 900 1000 1100 x max, g/cm 2 Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 24

3.4 Cosmic ray mass composition Next figure displays mean natural logarithm of the CR atomic number ln A concluded from the x max data from four experiments Yakutsk, HiRes, Auger and Telescope Array (Tameda et al (2010)). For lna derivation x max values were utilized, obtained in simulations within the framework of the QGSJet II models for proton and iron nuclei. At first glance, all data reveal a tendency to alter lna with energy. For instance, in energy interval2 10 17 3 10 18 ev, the value of lna drops from 3 to 1.3 and above 10 18 ev a slight growth is noted. Such a behaviour is close to the dip-scenario (Berezhko (2008)), where two peaks are presented in the energy dependence of ln A. First one, at 10 17 ev, corresponds to the ending of galactic component, second at 10 19 to the start of CR intensity change due to GZK-cutoff. Next slide: Mean mass number of primary particle as a function of energy. Circles represent Yakutsk data, triangles HiRes data, squares results obtained at Auger observatory, blue empty triangles preliminary data from the Telescope Array experiment, dotted line computational results (Berezhko (2008)). Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 25

3.4 Cosmic ray mass composition 3.5 3.0 2.5 ln A 2.0 1.5 1.0 0.5 0.0 10 17 10 18 10 19 10 20 E 0, ev Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 26

3.4 Cosmic ray mass composition However, there is still a significant data dispersion in this energy region due to poor event statistics. Thus, the reliability of our statement is quite limited. For more precise conclusion on ultra-high cosmic rays origin, a few conditions must be fulfilled: improved statistics, improvement of x max estimation precision, adaptation of a single hadron interaction model that well describes experimental data and involving several alternative methods for x max evaluation. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 27

4. Muons with ε thr 1 GeV The next figure shows an example of mean lateral distributions for muons at different energies. Muon lateral distribution function (LDF) is significantly lower then that of charged component and can be effectively measured in individual events at E 10 17 ev within the core distance range 100 800 m. Thus, as a classification parameter in this energy region, a parameter ρ µ (600) could be used the density of muon flux at 600 m from shower core. Next slide: Solid and dotted lines on the figure denote computational results obtained with QGSJet(UrQMD) models for proton (solid) and iron (dotted), red line EPOS+UrQMD for proton, green carbon, blue iron. It s seen that muon LDF from proton is more steep than one from iron nucleus and this difference is especially pronounced at large core distances. Qualitative comparison of computational results with the experiment reveals a better agreement with a heavier component of primary CR at E 10 18 ev and with lighter at E 10 19 ev. This feature could be stressed out if one puts parameter r 2 ρ(r) on the y-axis of a plot instead of simple ρ(r). Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 28

4.1 Muon LDF 10 7 r 2 ρ µ (r) 10 6 10 5 10 19 ev 10 18 ev 10 4 10 17 ev 10 3 10 100 1000 10000 r, m Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 29

4.2 Muon content We considered the dependence of ρ µ /ρ ch on the length of shower development after the maximum λ = x 0 /cosθ x max. In highly inclined showers muon content increases proportionally to x 0 /cosθ value, where x 0 = 1020 g/cm 2 for Yakutsk. It is known fact that the depth of maximum EAS development differs significantly, depending on the kind of primary particle and, hence, this feature could be used in the analysis of the CR mass composition. For instance, by fixating the λ parameter and studying the fluctuations of ρ µ /ρ ch value. This technique is rather similar to one proposed earlier (Atrashkevich et al (1981)). Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 30

4.2 Muon content Shower parameters calculated with CORSIKA code were modified by applying distortions according to experimental errors. Parameters measured in experiment (e.g. cosθ,x max,ρ ch (r),ρ µ (r)) for every shower were rolled with the normal distribution with σ parameter according to the experiment: σ(θ) = 3 secθ; σ(x max ) = 40g/cm 2 ; σ(ρ r ) = ρ 2 r ( 0.025 + 1.2 s det ρ r cosθ ) where s det is area of the detector. Next slides: A dependence of muon portion with ε thr 1 GeV on the length of track in the atmosphere for individual showers with θ = 0 50 and energy 10 18 ev. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 31

4.2 Muon content: QGSJet II+FLUKA ρ µ / ρ ch 0.8 0.7 0.6 0.5 0.4 0.3 0.2 p C Fe Yakutsk 0.1 QGSJet II, FLUKA 0 200 300 400 500 600 700 800 900 x 0 / cos θ - x max Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 32

4.2 Muon content: EPOS+UrQMD ρ µ / ρ ch 0.8 0.7 0.6 0.5 0.4 0.3 0.2 p C Fe Yakutsk 0.1 EPOS, UrQMD 0 200 300 400 500 600 700 800 900 x 0 / cos θ - x max Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 33

4.2 Muon content It is seen from figures that a strong correlation is observed between the muon content and the length of track in the atmosphere after the shower maximum. It is also seen that experimental data are in a good agreement with simulation results. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 34

4.3 Fluctuations of ρ µ /ρ ch relation Showers initiated by different nuclei have differing altitudes of the maximum which in turn means that different numbers of muons are generated in these showers. It also means that they cover different paths in the atmosphere. By analyzing the tracks of muons that they pass in the atmosphere after the maximum of shower development we can try to estimate the composition of cosmic rays. With this aim in view, by choosing the mean zenith angle 36 (which corresponds to the track length after the maximum λ = 500 g/cm 2 ), let s normalize the values of muon content to this level and consider their fluctuations. Results are presented on the next slides. There are also shown the computational results obtained with QGSJet II and EPOS models for various nuclei. Obtained results have shown that within this method fluctuations of ρ µ (600)/ρ ch (600) parameters don t allow to estimate the CR mass composition. However, mean values from different nuclei differ. Besides, QGSJet II hints a heavier composition than that of EPOS: according to first one, the composition of selected showers shifts towards heavier nuclei; according to second one, showers correspond to nuclei of intermediate group. On the whole, both models argue for mixed composition. Next slides: A distribution of the ρ µ /ρ ch relation, normalized to the track length 500 g/cm 2. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 35

4.3 Fluctuations ofρ µ /ρ ch relation: QGSJet II+FLUKA w 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Yakutsk p C Fe 0 0.1 0.2 0.3 0.4 0.5 0.6 ρ µ / ρ ch Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 36

4.3 Fluctuations ofρ µ /ρ ch relation: EPOS+UrQMD w 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Yakutsk p C Fe 0 0.1 0.2 0.3 0.4 0.5 0.6 ρ µ / ρ ch Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 37

4.2 Fluctuations of ρ µ /ρ ch relation However, if one takes into account gamma-photons generated in ground covering muon detectors, the mean value of ρ µ (600)/ρ ch (600) relation decreases and composition shifts towards lighter nuclei (protons-helium-carbon) (Dedenko et al at Moscow CR conference (2010)). Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 38

4.3 Fluctuations ofρ µ /ρ ch relation, normalized to the length of track500 g/cm 2 QGSJet II, FLUKA EPOS, UrQMD Yakutsk Yakutsk a p C Fe p C Fe ρ µ /ρ ch 0.3145 0.2768 0.2687 0.3025 0.3193 0.2893 0.3170 0.3381 σ 0.0747 0.0657 0.0517 0.0541 0.0536 0.0563 0.0511 0.0539 A comparison of the muon portion distribution with computational results points towards mixed cosmic ray composition near E 10 18 ev. Large fluctuations of the muon portion prevent revealing of a single determined group of nuclei. A more detailed analysis is required, involving possible systematics of muon density measurement in the Yakutsk experiment. a With respect to contribution from gammas generated in the shielding of detector (Dedenko, 2010) Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition p. 39