The Fluorescence Detector of the Pierre Auger Observatory measures the
atmospheric depth, $X_{max}$, where the longitudinal profile of the high energy
air showers reaches its maximum. This is sensitive to the nuclear mass
composition of the cosmic rays. Due to its hybrid design, the Pierre Auger
Observatory also provides independent experimental observables obtained from
the Surface Detector for the study of the nuclear mass composition. We present
$X_{max}$-distributions and an update of the average and RMS values in
different energy bins and compare them to the predictions for different nuclear
masses of the primary particles and hadronic interaction models. We also
present the results of the composition-sensitive parameters derived from the
ground level component.Comment: Proceedings of the 12th International Conference on Topics in
  Astroparticle and Underground Physics, TAUP 2011, Munich, German

Allen J

D Garcia-Gamez for the Pierre Auger Collaboration

D Garcia-Pinto for the Pierre Auger Collaboration

Facal P for the Pierre Auger Collaboration

Klages H

L Cazon

Ulrich R

Wahlberg H

Journal of Physics : Conference Series

English

arXiv

The Fluorescence Detector of the Pierre Auger Observatory measures the atmospheric depth, $X_{max}$, where the longitudinal profile of the high energy air showers reaches its maximum. This is sensitive to the nuclear mass composition of the cosmic rays. Due to its hybrid design, the Pierre Auger Observatory also provides independent experimental observables obtained from the Surface Detector for the study of the nuclear mass composition. We present $X_{max}$-distributions and an update of the average and RMS values in different energy bins and compare them to the predictions for different nuclear masses of the primary particles and hadronic interaction models. We also present the results of the composition-sensitive parameters derived from the ground level component.Peer Reviewe

Cazon, L.

Repositório Comum

arXiv:1201.6265v1  [astro-ph.HE]  30 Jan 2012Studying the nuclear mass composition of Ultra-HighEnergy Cosmic Rays with the Pierre AugerObservatoryL. Cazon1 for The Pierre Auger Collaboration2,31 LIP, Av Elias Garcia 14-1,1000-149 Lisboa, Portugal2 Av. San Mart´ın Norte 304 (5613) Malargu¨e, Argentina3 A full author list and affiliations can be found athttp://www.auger.org/archive/authors 2011 09.htmlE-mail: cazon@lip.ptAbstract.The Fluorescence Detector of the Pierre Auger Observatory measures the atmospheric depth,Xmax, where the longitudinal profile of the high energy air showers reaches its maximum. Thisis sensitive to the nuclear mass composition of the cosmic rays. Due to its hybrid design, thePierre Auger Observatory also provides independent experimental observables obtained from theSurface Detector for the study of the nuclear mass composition. We present Xmax-distributionsand an update of the average and RMS values in different energy bins and compare them tothe predictions for different nuclear masses of the primary particles and hadronic interactionmodels. We also present the results of the composition-sensitive parameters derived from theground level component.1. IntroductionThe determination of the nuclear mass composition of UHECR (Ultra High Energy Cosmic Rays)is fundamental to unveil the origin of the most energetic particles known in nature. UHECRare detected by means of the extensive air showers created in the Earth’s atmosphere, which arecomposed by a cascade of hadrons and electromagnetic (EM) particles. The depth at which theEM shower reaches its maximum, Xmax, strongly correlates with the depth where the primaryfirstly interacted. The Xmax-distribution carries information about the primary particle and thephysical processes in the cascade. The hadronic cascade is composed mostly by pions, of whichthe charged pions might decay in flight into a muon and a neutrino. Due to their long lifetimeand low cross section, muons can leave the core of the hadronic cascade traveling kilometersaway and be detected. Electrons spread out in time and space because of Coulomb scattering,whereas muons practically travel following straight lines. The time structure of the shower discencodes the history of the shower, allowing us to recover information about the longitudinalevolution by analyzing the time distribution of the particles arriving at ground level.The Pierre Auger Observatory, located on the high plateau of the Pampa Amarilla, is thelargest cosmic ray observatory ever built. Its hybrid design allows to collect the shower particlesby a surface detector (SD) and to observe the longitudinal development of the EM profileby collecting the UV light with a fluorescence detector (FD). The SD spans 1600 Cherenkovdetectors in a 1.5 km triangular grid over 3000 km2, whereas the FD is composed by 24 telescopesdistributed over 4 sites overlooking the array. The baseline design is being complemented withenhancements both on the SD and FD, and new detection techniques [1].In this paper we present the latest 〈Xmax〉 and RMS-Xmax results as a function of energy, andtheXmax-distributions for the highest and lowest energy bins. We also present the evolution withenergy of SD observables extracted from the time distributions of the signal which are sensitiveto the longitudinal evolution of the shower. We compare our results with the predictions ofhadronic interaction models [2].2. Electromagnetic Shower MaximumWe have used data taken between December 2004 and September 2010, following the analysisreported in [3] and updated in [4]. We have considered those showers reconstructed by the FDthat have at least a signal in one of the SD stations. The longitudinal profile was fitted witha Gaisser-Hillas function, and a series of cuts depending on the atmospheric optical conditionsand on the quality of the reconstructed profile were applied [3, 4]. In order to avoid that thedata sample is biased regarding to the nuclear mass, a number of cuts in the geometry wereimposed.The resolution in Xmax is around 20 g cm−2. Uncertainties in the atmospheric conditions,calibration, event selection and reconstruction result in a systematic uncertainty of ≤ 13 g cm−2.The average values as a function of the energy are displayed in the left panel of Fig. 1 (thirdplot from top), alongside with the predictions for different hadronic interaction models.The elongation rate D10 = d〈Xmax〉/d log10E is better described using two slopes, withχ2/ndf=7.4/9, compared to 54/11 if we used a single power law. The change of the elongationrate could be interpreted as a change on the nuclear mass composition, provided that there isno change on fundamental properties of the hadronic physics. The comparison of 〈Xmax〉 withthe different predictions of the hadronic interaction models could be interpreted as a changeof composition towards heavier elements at the highest energies. The left panel of Fig. 1 alsodisplays the RMS-Xmax, which has been corrected by the detector resolution. It rapidly decreaseswith energy around the same energy of the change on the elongation rate.The distributions of Xmax for all energy bins have been published in [4]. Fig. 2 plots the(Xmax − 〈Xmax〉)-distributions for the highest and lowest energy bins. The predictions of thedifferent hadronic models show a nearly universal shape. At low energies, the shape of the datadistribution is compatible with a very light or mixed composition, whereas at high energies aheavier composition is favored. The high Xmax tail, characteristic of very light components, issuppressed with respect to the lowest energies.3. Asymmetry of the signal risetimeIn each SD event, the Cherenkov detectors record the signal as a function of time. The risetime,t1/2, defined as the time elapsed between the 10% and 50% of the integrated signal, dependson the distance to the shower maximum [5], the zenith angle θ and the distance to the corer. In inclined showers, the average risetime depends on an angle defined on the perpendicularplane ζ as 〈t1/2/r〉 = a + b cos ζ, due to the different effective distances of the tanks to theshower maximum. The evolution of b/a with zenith angle reaches a maximum at Θmax which isdifferent for different primary masses [6]. In the right central panel of Fig. 1 an example of b/aas a function of ln(sec θ) is shown for the energy bin log10(E/eV ) = 18.85 − 19.00.Events with energy above 3.16× 1018 eV and θ ≤ 60◦ were selected [8]. Results of Θmax as afunction of energy are displayed in the left panel of Fig. 1. The systematic uncertainty amountsto <∼ 10% of the proton-iron separation predicted by models. The number of muons predicted bythe hadronic interaction models differs from data [9]. A preliminary study including this effectindicates a possible change of about ≤ 5% of the proton-iron difference.Figure 1. Left Panel: Results on shower evolution sensitive observables compared with modelspredictions. The error bars correspond to the statistical uncertainty, whereas the systematicuncertainty is represented by the shaded bands. Right Panel, top: Typical longitudinaldevelopment of the muon production. Middle: Average asymmetry in the risetime. Bottom:Typical longitudinal development on the energy deposit.4. The muon production depthAn approximated relation between the arrival time delay of a muon with respect to the showerfront plane and the distance of the muon production point to ground can be used to transformthe arrival time distributions of muons into production depth distributions [10]. In [11], thistechnique was applied to Auger data in an angular window between 55◦ and 65◦ for stationswith r ≥ 1800 m, as a result of a trade off between EM contamination rejection and the intrinsicresolution of the method. In Fig. 1, right top panel, an example for a real event at E = 94±3 EeVis shown. Similarly to the EM component, the depth at which the muon production rate reachesa maximum, Xµmaxis a good indicator of the primary mass composition. After a series of cuts[11], the measured values of 〈Xµmax〉 are presented in the upper panel of Fig. 1. The systematicuncertainty due to reconstruction bias, core position, rejection of the EM component and qualityFigure 2. Centered distributions Xmax − 〈Xmax〉, for the lower and highest energy bins. Themixed composition corresponds to 50% p and Fe.cuts amounts to 11 g cm−2, corresponding to 14% of the proton-iron separation. Note that thediscrepancy on the total number of muons observed between models and data does not affectthis result, but only the normalization of the muon production depth distribution.5. ConclusionsProvided that the present models give a fair description of the physical processes and theirsystematics, the average primary composition of UHECR could be inferred from the left panelof Fig. 1, showing a change towards heavier mass composition at high energies. The evolutionof 〈Xmax〉, Θmax , and 〈Xµmax〉 is similar in the overlapping energy regions, despite the fact ofhaving completely independent systematics. RMS-Xmax evolution can be accommodated witha variety of compositions since it is influenced by the shower-to-shower fluctuations and therelative differences on 〈Xmax〉. The analysis of the shape of the Xmax-distributions also pointsto the direction of heavier composition as we increase the energy. On the other hand, it isnot guaranteed that current hadronic interaction models describe well particle physics at theseenergies [9], and a different interpretation would therefore apply.References[1] H. Klages for the Pierre Auger Collaboration, these proceedings[2] N.N. Kalmykov, S.S. Ostapchenko, and A.I. Pavlov, Nucl. Phys. Proc. Suppl. 52B (1997) 17-28; E.J. Ahn etal., Phys. Rev. D80 (2009) 094003; T. Pierog and K. Werner, Phys. Rev. Lett. 101 (2008) 171101[3] The Pierre Auger Collaboration, Phys. Rev. Lett. 104 (2010) 091101.[4] P. Facal for the Pierre Auger Collaboration, Proceedings of the 32nd ICRC at Beijing, China (2011).arXiv:1107.4804[5] H. Wahlberg, for the Pierre Auger Collaboration, (2009) Proc. 31st ICRC, Lodz, Poland (2009).arXiv:0906.2319v1[6] M. T. Dova, M. E. Mancenido, A. G. Mariazzi, H. Wahlberg, F. Arqueros, D. Garcia-Pinto, Astropart. Phys.31 (2009) 312-319.[7] The Pierre Auger Collaboration, Nucl. Instrum. Meth. A613 (2010) 29-39.[8] D. Garcia-Pinto for the Pierre Auger Collaboration, Proc. 32nd ICRC, Beijing, China (2011). arXiv:1107.4804[9] J. Allen for the Pierre Auger Collaboration, Proc. 32nd ICRC, Beijing, China (2011). arXiv:1107.4804 ;R. Ulrich for the Pierre Auger Collaboration, these proceedings[10] L. Cazon, R. A. Vazquez, E. Zas, Astropart. Phys. 23 (2005) 393-409.[11] D. Garcia-Gamez for the Pierre Auger Collaboration, Proc. 32nd ICRC, Beijing, China (2011).arXiv:1107.4804

Studying the nuclear mass composition of Ultra-High Energy Cosmic Rays with the Pierre Auger Observatory

null null

Crossref

Journal of Physics Conference Series

Studying the nuclear mass composition of Ultra-High Energy Cosmic Rays
  with the Pierre Auger Observatory

Studying the nuclear mass composition of Ultra-High Energy Cosmic Rays with the Pierre Auger Observatory

Abstract

Similar works

Full text

Available Versions

Repositório Comum

Crossref