In this contribution I briefly review the most recent results obtained by experiments studying the primary cosmic rays spectrum at energies around the knee, i.e. the change of the slope observed at E0 ∼ 3 × 1015 eV. The contribution

is completed discussing the arguments that still remain controversial, the measurements that will allow to clarify them and the experiments actually operating

Chiavassa, A.

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DOI 10.1393/ncc/i2009-10468-6Colloquia: IFAE 2009IL NUOVO CIMENTO Vol. 32 C, N. 3-4 Maggio-Agosto 2009The primary cosmic rays spectrum at knee energies.Experimental results and future developmentsA. ChiavassaUniversita` di Torino - Torino, Italy(ricevuto il 19 Settembre 2009; pubblicato online il 23 Novembre 2009)Summary. — In this contribution I briefly review the most recent results obtainedby experiments studying the primary cosmic rays spectrum at energies around theknee, i.e. the change of the slope observed at E0 ∼ 3 × 1015 eV. The contributionis completed discussing the arguments that still remain controversial, the measure-ments that will allow to clarify them and the experiments actually operating.PACS 96.50.sb – Composition, energy spectra and interactions.PACS 96.50.sd – Extensive air showers.1. – IntroductionThe primary cosmic rays energy spectrum has been measured in a very large energyrange, from ∼ 1012 to ∼ 1020 eV. Its shape is very regular and can be described by apower law with a nearly constant slope (γ). Two main features have been detected: atE0 ∼ 3 × 1015 eV, the so-called knee with γ steepening from ∼ 2.7 to ∼ 3.0, and atE0 ∼ 5 × 1018 eV, where γ returns to ∼ 2.8 (the so-called ankle). A recent review ofexperimental results, taken from [1], is shown in fig. 1 (for the single experiments seereferences therein).The knee was first detected, with an array measuring the electromagnetic componentof the extensive air showers (EAS, i.e. the cascade generated by the interaction of theprimaries at the top of the atmosphere) almost fifty years ago [2], but the mechanismoriginating it is not yet completely understood. In the last decade experiments sampling,for all events, several EAS components (mainly, but not only, CASA-MIA [3], EAS-TOP [4] and KASCADE [5]) obtained high-precision results constraining the hypothesisproposed to explain the knee.In the following section I review the main results obtained in the recent past, dis-cussing their consequences on the different hypotheses proposed to explain the knee.The contribution is then concluded discussing the measurements that could give impor-tant information about the knee, and the experiments that are performing them.c© Societa` Italiana di Fisica 303304 A. CHIAVASSA10 610 710410510610710810910101011Energy E0 [GeV]Flux dΦ/dE0 ⋅ E0 3 [m-2 sr-1 s-1 GeV1.5 ]++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++++++++++++++++++✜✜✜✜✜✜✜✜✜✜✜✜✜✜✜✜✧✧✧✧✧✧✧✧✧✧✧✧✧✧✧✧✧✧✧✢ ✢✢ ✢✢ ✢✢ ✢ ✢ ✢ ✢✕✕✕✕✕✕✕✕✕✕✕✕✕✕✕✕✕✕✕✕✕✕✕✕ ✡✡✡✡✡✡✡✡✡✡✡✡✡✡✡✡✡✡✡✡✡✡✡✡⊕⊕⊕⊕⊕⊕⊕ ⊕ ⊕ ⊕⊕⊕⊕⊕⊕⊕⊕⊕⊕⊕⊕⊕⊕⊕⊕⊕ ⊕ ⊕⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕⊕ ⊕∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅∅ ✲✲⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗ ⊗⊗∇∇∇∇∇∇∇∇∇∇∇∇∇∇∇∇∇∇∇∇∇∇✣ ✣✣✣ ✣ ✣✣✣ ✣✣ ✣✣ ✣ ✣❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄❄¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤✲✲✲✲✲✲✲✲✲✲✲✲✲✲✲+ AGASA+ Akeno 20 km2+ Akeno 1 km2✜ AUGERBLANCA✧ CASA-MIA◊ DICE✢ BASJE-MAS✕ EAS-Top✡ Fly's Eye✣ GRAPES QGS✣ GRAPES SIBHaverah ParkHaverah Park FeHaverah Park pHEGRA⊕ HiRes-I⊕ HiRes-II⊕ HiRes/MIA¤ KASCADE (e/m QGS)¤ KASCADE (e/m SIB)KASCADE (h/m)❄ KASCADE (nn)✲ KASCADE-Grande∅ MSUMt. Norikura✲ SUGAR⊗ Tibet ASγ⊗ Tibet ASγ-III∇ Tunka-25direct: JACEERUNJOBSOKOLGrigorovknee ankle2nd knee1 23-9 10-2425-2728-92galacticextragal.Fig. 1. – Review of measurements of the primary cosmic rays spectrum.2. – Experimental results about the kneeDue to the low fluxes the energy range around the knee has to be studied with largearea experiments operating at earth, thus measuring the secondary components of theEAS. The characteristics of the primaries must then be indirectly inferred using completesimulations of the EAS development in the atmosphere.The change of slope has been observed in the spectra of all EAS components (elec-tromagnetic [6-8], muonic [9,10] and hadronic [11]) and at different stages of the showerdevelopment, both with experiments operating at different altitudes above see level andwith each array measuring in different intervals of zenith angle. In fig. 2 a compilationof spectra of the electromagnetic component, taken from [12], is shown. It can be seenas all of them show the change of slope and that the shower size at the knee (Ne, i.e.the number of electrons at observation level) decreases with atmospheric depth. Theattenuation of the shower size at the knee is compatible with what we expect for EASdevelopment.Moreover the characteristics of the observed change of slopes of the spectra of differ-ent components, namely the integral fluxes above the knee and the number of electronsand muons at the knee, agree with the expectations of shower development in the atmo-sphere [9].All these measurements lead to the conclusion that the knee is a characteristic of theprimary cosmic ray spectrum and it is not due to a change in the interaction mechanism(affecting the EAS development).The EAS-TOP [9] and CASA-MIA [13] experiments have measured, correlating themean values of electrons and muons in EAS, that the primary chemical compositionbecomes heavier at energies crossing the knee.Using a more refined analysis the KASCADE experiment shows that: the knee isdue to the light component of cosmic rays [10] and that the spectra of single elementsshow knees at energies increasing with the primary atomic number [14]. A similar resulthas been obtained by the EAS-TOP Collaboration correlating the shower size both withGeV [9] and TeV [15] muons. The resolution reached (by both experiments) does notallow to discriminate between a Z or A dependence.THE PRIMARY COSMIC SPECTRUM AT KNEE ENERGIES 305Fig. 2. – Shower size (Ne) spectra measured by different experiments at different atmosphericdepths. The dashed line is simply a reference to guide the eye over the Ne values where thechange of slope is observed.All these analyses concerning the primary chemical composition heavily depend (atleast for the quantitative aspects) on EAS simulation, thus being affected by the lackof knowledge of primary interactions at the energies under investigation. Moreover atcolliders the forward region of the interactions (the one relevant for the EAS development)is not studied.We can conclude that the favored scenario to explain the knee deals with astrophys-ical mechanisms: either the acceleration or the propagation of galactic cosmic rays. Inboth scenarios a change of the slope of the spectra of single elements is expected atan energy scaling with the charge Z, thus we expect the iron knee at an energy equalto ZFeEknee ∼ 1017 eV. Its detection would be a conclusive argument in favor of anastrophysical mechanism. It is thus necessary to extend the measurements in the 1016to 1018 eV energy range with high-resolution experiments allowing a separation betweendifferent elements or at least between light and heavy components.This lead to the start of new experimental projects. Most of them are in the con-struction phase: ICE-TOP [16], TUNKA-133 [17], the low-energy extensions of the PierreAuger Observatory [18] and of the Telescope Array [19]. The only experiment that isnowadays taking data in the full configuration is KASCADE-Grande [20].3. – The KASCADE-Grande experimentThe KASCADE-Grande experiment [20] is a multi-detector set-up consisting of theKASCADE [5] experiment, the trigger array Piccolo and the scintillator detector arrayGrande. Additionally, KASCADE-Grande includes an array of digitally read out dipoleantennas (LOPES) to study the radio emission in air showers at E > 1016 eV [21]. TheKASCADE experiment is itself a multiple detector set-up and its major parts are anarray of 252 scintillator detector stations, a streamer tube Muon Tracking Detector (Eμ >800MeV) [22], and a multiwire proportional chamber muon detector (Eμ > 2.4GeV).The Grande array is formed by 37 stations of plastic scintillator detectors, 10m2 each(divided into 16 individual scintillators) spread on a 0.5 km2 surface, with an averagegrid size of 137m.306 A. CHIAVASSAKAchlg N5.8 6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6KAch / NKAch-NGrchN-0.3-0.2-0.100.10.20.30.40.5MeanRMSFig. 3. – Mean value and RMS of the relative difference between the shower size values obtainedby the KASCADE and the Grande arrays.The arrival direction of the events is determined fitting the arrival time of the particlesin the Grande detectors to a curved shower front [23]. The core position, the shower ageand the shower size are obtained fitting the particles densities measured by the Grandestations with a NKG like function [23]. The total number of muons is then calculatedusing the core position determined by the Grande array and the muon densities measuredby the KASCADE muon detectors [24].The precisions obtained in the reconstruction of the shower parameters are evaluatedexploiting the unique feature of the KASCADE-Grande experiment of having two inde-pendent samplings of the same event by the KASCADE and the Grande arrays. Forevents at energies greater than 1016 eV (threshold for 100% Grande detection efficiency)the measured resolutions are: better than 1◦ for the arrival direction; lower than 10mfor the core position and lower than 20% for Ne (see fig. 3 including also the systematicshift between the two measurements). Thus showing that the array performances arethose required to perform the previously mentioned studies.4. – ConclusionsThe results recently obtained, by experiments studying the cosmic rays spectrum atknee energies, favor the astrophysical scenario, even if some details remain unclear. Tosolve them high-precision measurements in the energy range from 1016 to 1018 eV arerequired. As an example the performances of the KASCADE-Grande experiment havebeen described.REFERENCES[1] Ho¨randel J., Adv. Space Res., 41 (2008) 442.[2] Kulikov G. and Khristiansen G. B., JEPT, 35 (1958) 635.[3] Borione A. et al., Nucl. Instrum. Methods A, 346 (1994) 329.[4] Aglietta M. et al., Nucl. Instrum. Methods A, 336 (1993) 310.[5] Antoni T. et al., Nucl. Instrum. Methods A, 513 (2003) 490.THE PRIMARY COSMIC SPECTRUM AT KNEE ENERGIES 307[6] Aglietta M. et al., Astropart. Phys., 10 (1999) 1.[7] Antoni T. et al., Astropart. Phys., 19 (2003) 703.[8] Glasmacher M. A. K. et al., Astropart. Phys., 10 (1999) 291.[9] Aglietta M. et al., Astropart. Phys., 21 (2004) 583.[10] Antoni T. et al., Astropart. Phys., 16 (2002) 373.[11] Ho¨randel J. et al., Proc. of the 26th ICRC, 1 (1999) 337.[12] Haungs A. et al., Rep. Prog. Phys., 66 (2003) 1145.[13] Glasmacher M. A. K. et al., Astropart. Phys., 12 (1999) 1.[14] Antoni T. et al., Astropart. Phys., 24 (2005) 1.[15] Aglietta M. et al., Astropart. Phys., 20 (2004) 641.[16] Gaisser T. et al., arXiv:0711.0353v1 (2007).[17] Budnev N. M. et al., arXiv:0801:3037 (2008).[18] Klages H. O. et al., Proc. of the 30th ICRC, 5 (2007) 849.[19] http://www.telescopearray.org.[20] Navarra G. et al., Nucl. Instrum. Methods A, 518 (2004) 207.[21] Falcke H. et al., Nature, 435 (2005) 313.[22] Doll P. et al., Nucl. Instrum. Methods A, 488 (2002) 517.[23] Di Pierro F. et al., arXiv:0906.4007 (2009).[24] Fuhrmann D. et al., arXiv:0906.4007 (2009).

The primary cosmic rays spectrum at knee energies.

Experimental results and future developments

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The primary cosmic rays spectrum at knee energies. Experimental results and future developments

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