Observation of UHE Cosmic Rays and upper limit on the diffuse neutrino flux bythe Pierre Auger Observatory

D. Gora for the Pierre Auger Collaboration

Karlsruhe Institute of Technology (KIT), D-76021 Karlsruhe, GermanyInstitute of Nuclear Physics PAN, ul. Radzikowskiego 152,31-342 Cracow, Poland

Abstract

The Pierre Auger Observatory has already collected more ultra high energy cosmic ray data than all previousexperiments. The hybrid detection technique provides results on the energy spectrum and arrival directions of thehighest energy cosmic rays, and characterize the extensive air showers in order to probe characteristics of the primaryparticle and its interactions. The Pierre Auger Observatory is also sensitive to ultra high energy neutrinos and photons.In this paper recent results from the Pierre Auger Observatory including a limit on the diffuse flux of neutrinos above1017 eV and limit on the fraction of photons to the total cosmic ray flux are shown.

Keywords: Pierre Auger Observatory, cosmic rays

1. Introduction

The detection of ultra-high energy cosmic rays(UHECRs), above 1017 eV is important as it may allowus to answer some of the most important questions inastrophysics: what are the sources of high energy cos-mic rays, where are they produced, what are the corre-sponding acceleration mechanisms, and what is their el-emental composition. In fact, although the existence ofUHECRs is experimentally proven for at least 50 years,these questions are still open, mainly due to the smallflux of UHECRs which is one of the reasons for theslow progress in their understanding. The detection ofcosmic rays is also important because UHECRs may al-low us to probe particle physics at an energy scale be-yond TeV energies. When cosmic rays arrive at Earth,they collide with nuclei in the atmosphere at center-of-mass energies which are orders of magnitude above theones available in man-made particle accelerators. Cos-mic rays also propagate through the interstellar and in-tergalactic media and are thus subject to magnetic fieldsand to interactions with cosmic matter and radiation,having the possibility to give information about phe-nomena which arise only at large distances and highest

energies.

Cosmic rays interact with nuclei creating a largenumber of lower energy particles in a showering pro-cess. These nuclei will produce charged pions whichcan interact or decay, producing both the atmosphericneutrino flux and high energy muons. Neutral pionswill decay immediately into gammas feeding the elec-tromagnetic component of the shower which will carrymost of the initial energy. Shower induced by CRsabove about 100 TeV can reach the ground level andcan be efficiently sampled by an array of surface detec-tors. Such a shower is commonly called Extensive AirShower (EAS). The electromagnetic component of anEAS produces isotropic fluorescence light by excitingnitrogen molecules in the air. For very high energies ofthe primary particle, enough fluorescence light is pro-duced by the large number of secondaries in the cas-cading process so that the shower can be recorded froma distance of many kilometers by an appropriate opti-cal detector system (fluorescence telescopes). As theamount of fluorescence light is correlated with energydissipated by shower particles, it provides a calorimet-ric measure of the primary energy.

Available online at www.sciencedirect.com

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doi:10.1016/j.nuclphysbps.2012.09.040

Figure 1: Left: Map of the southern site of the Pierre Auger Observatory. The 4 telescope buildings are marked in blue with lines indicating thefield of view of the telescopes. The surface detectors are represented by black dots. Right: Example of an event seen simultaneously by the SDsand FDs (hybrid event). The first plot inset shows the measured energy deposit profile of an EAS from FD, the second plot shows the measuredsignal from stations of the SD with fitted lateral distribution function (LDF). The LDF fit allows determining the particle density at a distance 1000m from the shower axis.

The Pierre Auger Observatory [1] measures EASwith both surface detectors and fluorescence telescopes.This hybrid detection technique enables combining thecalorimetric measurement of the shower energy throughfluorescence light with the high-statistics data of the sur-face array.

In this paper after a brief description of the Auger Ob-servatory we report recent results from the Pierre AugerObservatory i.e. the measurement of UHECR spectrum,the anisotropy signal of arrival directions, and the com-position. Finally limits on the UHE neutrino flux andlimits on the flux of UHE photons are presented.

2. The Pierre Auger Observatory

The Pierre Auger Observatory operating since 2004is the largest project to measure UHECRs. It is locatednear Malargue in the province of Mendoza, Argentina.As a hybrid detector the Auger Observatory consists ofthe surface detector array and the fluorescence detector.

The surface detector (SD) array consists of about1600 water Cherenkov detectors (tanks) arranged in atriangular grid with 1.5 km spacing (see Figure 1 left).Each Cherenkov detector consists of three photomulti-pliers (PMTs) on the top, which sample the shower sig-nal. A Tyvek liner of the internal wall of each stationguarantees a complete diffusion of the Cherenkov lightproduced by particles crossing the water which fills a

station. At each station of the surface detector array,the 40 MHz flash analog-to-digital converter (FADC)samples the current generated at the PMTs and returnsa measure of the Cherenkov light. The signal detectedat each station refers to a common calibration unit, theso-called vertical equivalent muon or VEM [2].

The fluorescence detector (FD) consists of 4 tele-scope buildings (eyes) overlooking the detector array asit is shown in Figure 1 (left). Each building houses 6telescopes with a 30◦ × 28.6◦ viewing angle. The fluo-rescence light is focused in each telescope onto a cam-era consisting of 440 PMTs through its Schmidt-opticsand a spherical mirror of � 11 m2. The fluorescence de-tector probes the longitudinal development of EAS bymeasuring the photon emission from atmospheric nitro-gen, which is excited by shower charged particles. De-tails of the design and status of the Observatory can befound in [2, 3].

3. Results

High Energy Cosmic Ray Spectra

The Hybrid nature of the Auger Observatory enablesus to determine the energy spectrum of primary cosmicrays without strong dependence on our limited knowl-edge of the mass composition and hadronic interactionmodels. The Auger approach is to use a selected sample

D. Góra / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 251–257252

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Figure 2: Left: Correlation between lg S 38◦ and lg EFD for the 661 hybrid events used in the fit. The full line is the best fit to data. Right: TheAuger energy spectrum with two empirical fit functions (see Ref. [5]) in comparision with data from HiRes [7]. The systematics uncertainty of theflux scaled by E3 due to uncertainty of the energy scale of 22% is indicated by arrows.

of hybrid events in which the energy EFD can be esti-mated accurately using the FD. The calibration curve,which is used to find energies of SD events is shown inFigure 2 (left). The parameter chosen to charaterise thesize of SD event is the signal at 1000 m from the showeraxis, normalized to mean zenith angle of the events of38◦ (S 38) according to the method described in [4].

In Figure 2 (right) the Auger combined energy spec-trum derived from more than 35 000 events is shown.The energy spectrum derived from hybrid data is com-bined with the one obtained from surface detector datausing a maximum likelihood method [5]. Using hybridevents allows us to extend the flux in a region wherethe SD is not fully efficient. From the plot we cansee some characteristic features of the Auger combinedspectrum: the so called “ankle” and the flux suppres-sion at highest energies. The position of the ankle atlog 10(Eankle/eV) = 18.61 ± 0.01 has been determinedby fitting the flux with a broken power law E−γ. An in-dex of γ = 3.26 ± 0.04 is found below the ankle. Abovethe ankle the spectrum follows a power law with indexγ = 2.55 ± 0.04. In comparison to the power law ex-trapolation, the spectrum is suppressed by a factor twoat log 10(Ebreak/eV) = 19.43 ± 0.03. The significanceof the flux suppression is larger than 20σ. The flux sup-pression is similar to what is expected from the Greisen-Zatsepin-Kuzmin (GZK) effect [6], but could also berelated to a change of the shape of the mean injectionspectrum of the sources.

In addition in Figure 2 (right) the spectrum obtainedwith stereo measurements of the HiRes instrument [7]

is shown. We can see basically the same features like inthe Auger spectrum, but the ankle feature seems to besomewhat more sharply defined in the Auger data. Thisis possibly due to a systematic energy offset betweenthe experiments as it is also seen in the plot. However,for a complete comparison, care must also be taken toaccount for energy resolution and possible changes inthe aperture with energy.

Arrival directions

An anisotropy in the arrival direction distribution ofcosmic rays in the energy range of the GZK suppres-sion is expected because of the highly anisotropic mat-ter distribution on distance scales of 100 Mpc. Also themagnetic fields are not strong enough to deflect signifi-cantly high energy cosmic rays during their propagationto the Earth, so the observed source distribution in prin-ciple should follow the matter distribution in the nearbyuniverse.

In November 2007 the Auger Collaboration pub-lished a report in Science [8] providing evidence for acorrelation between the arrival directions of the high-est energy cosmic rays and the celestial positions ofnearby active galactic nuclei (AGNs) from the 12th edi-tion of the catalog of quasars and active galactic nucleiby Veron-Cetty and Veron [9] (VCV catalog). The cor-relation gave 99% confidence that the cosmic rays above57 EeV are not arriving isotropically. The 99% confi-dence was based on a chance probability less than 1%that a specific single-trial test would have been passed

D. Góra / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 251–257 253

Figure 3: The most likely value of the degree of correlation p = k/N(k correlating out of N) is plotted with black dots as a function of thetotal number of time-ordered events (excluding the explanatory scandata set). The 1σ and 2σ confidence level intervals around the mostlikely value are shaded. The horizontal dashed line shows the isotropicvalue piso = 0.21. The current estimate of the signal is (39 ± 6)%.

if the cosmic ray flux were isotropic. The single-trialtest was motivated by a strong correlation of arrival di-rections with AGN positions prior to 27 May 2006 (Pe-riod I). Specifically, a large fraction of the cosmic raysabove 55 EeV arrived within 3.1 degrees of the positionof AGNs closer than 75Mpc (z = 0.018), whereas forisotropic cosmic rays the fraction of correlating direc-tions should be only 0.21.

Figure 3 summarizes the present status [10] of thatcorrelation with AGN positions in the VCV catalog.The exploratory data set (Period I) is not plotted, since itcould be plausibly assumed that the hypothesis resultedin part from a positive fluctuation in the data that wasfound through searching and parameter tuning. PeriodII, an independent data set collected between 27 May 06and 31 August 2007, is the confirming data set after thestart of the prescribed trial up to the last data used in thepublished Science result. Period III a new data set from1 September 2006 to 31 March 2009. The plot showsa remarkable run of non-correlating arrival directionsabove 55 EeV starting in the latter part of the publisheddata. The best estimate of the true correlation is now39%, based on all the data taken after the exploratoryset which led to the hypothesis of an AGN correlation.While this is lower than the estimate at the time of pub-lication, it is nevertheless significantly above the 21%isotropic expectation, and the confidence in anisotropyremains stronger than 99% based on this correlation.

The Auger publications emphasized that the VCVcorrelation does not prove that AGNs are the sources ofthe trans-GZK cosmic rays. They themselves are tracersof the large-scale matter distribution in the nearby uni-verse and they are therefore correlated with other candi-date astrophysical sources [11].

Composition

The atmospheric depth Xmax (see Figure 4 (left))where an air shower reaches its maximum size dependson the primary nuclear mass. For a fixed energy, show-ers from heavy nuclei reach their maximum size on av-erage at smaller Xmax than those from protons (Figure 4(right)) and shower-to-shower fluctuations in Xmax aremuch less for heavy nuclei than for protons. Both ofthese properties are robust expectations that follow froma heavy nucleus being composed of many nucleons ofmuch lower energy.

In Figure 5 we show the measured Xmax and theshower-to-shower fluctuations, RMS(Xmax) from theAuger Observatory [12]. While at low energies the dataare reasonably compatible with the expectation for pro-tons, at higher energies above about 18.24 EeV, the datadeviate from the expectation of protons approaching theprediction for heavy nuclei, indicating mixed composi-tion with average mass between proton and iron. Thelinear fit, Xmax = D10 · lg(E/eV) + c, yields an elon-gation rate (variation of 〈Xmax〉 per decade of energy)of D10 = (106+35

−21) g/cm2/decade below the knee andD10 = (24 ± 3) g/cm2/decade above the knee. Thisconfirms the break in the distribution around energy18.24 EeV and suggests a smooth composition transi-tion. Also the RMS(Xmax) shows a similar behavior,Figure 5 (right). As it can be seen from the plot theshower-to-shower fluctuations decrease from 55 g/cm2

to about 25 g/cm2 as the energy increases. This decreas-ing fluctuations are an independent signature of an in-creasing average mass of the primary particles.

It is not clear yet, in the context of results presentedin the previous section if this reflects a transition fromproton to iron primaries. Arrival direction results in theenergy range of the GZK suppression suggest rather alighter composition at highest energies.

The interpretation of the Xmax and RMS(Xmax) resultshown in Figure 5 is however uncertain. This is becausethe present interpretation relies on air shower simula-tions that use hadronic interaction models to extrapolateparticle interaction properties two orders of magnitudein center-of-mass energy beyond the regime where theyhave been tested experimentally. The possible differ-ent interpretation can be that the proton-air interaction

D. Góra / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 251–257254

Figure 4: Left: Example of the longitudinal profile of energy deposited by EAS recorded by the Auger Observatory; Right: Illustration of thechanges of the Xmax for different primaries.

cross-section or multiplicity, or both, are increased lead-ing to a faster shower development, as it would happenin a heavy nuclei collision [13]. In other words the in-terpretation of the Auger Xmax and RMS(Xmax) result isstill an open question.

The SD also measures shower parameters that aresensitive to the speed of air shower development and canbe used to study the mass composition of UHECR. Forexample for each event the water Cherenkov detectorrecords the signal as a function of time. Muons travel instraight lines through the atmosphere with hardly anyinteraction, whereas the electromagnetic particles un-dergo multiple scattering on their way to ground. Thefirst portion on the signal of the stations is dominatedby the muon component which tends to arrive earlierand over a period of time shorter than the electromag-netic particles, which are spread out in time. Air show-ers with more muon content (like thouse produced byheavy primary cosmic rays) have a narrower distribu-tion in arrival times than showers with large fractions ofelectromagnetic particles (like those produced by lightprimaries). Then the useful indirect observable is therisetime of the signals in detectors. The risetime is de-fined as the time to go from 10% to 50% of the totalintegrated signal in each station. It was shown that therisetime is sensitive to primary mass composition, andit is highly correlated with the shower development andthe depth of the shower maximum. Other indirect SDobservables like the risetime asymmetry and the devia-tion from the average risetime confirm the energy trendtowards iron primaries or very high cross-section seen

by the Pierre Auger Observatory [14].

Photons and Neutrinos

Another important issue concerning compositionstudies is the search for photons and neutrinos in pri-mary cosmic rays. Photons and neutrinos may traceback the origin of the UHECR. Essentially all mod-els of UHECRs production predict neutrinos as a resultof the decay of charged pions produced in interactionsof cosmic rays within the sources themselves or whilepropagating through background radiation fields. Forexample, UHECR protons interacting with the cosmicmicrowave background (CMB) give rise to the so called“cosmogenic” or GZK neutrinos [15]. The cosmogenicneutrino flux is somewhat uncertain since it depends onthe primary UHECR composition and on the nature andcosmological evolution of the sources as well as on theirspatial distribution [16]. In general, about 1% of cosmo-genic neutrinos from the ultra-high energy cosmic rayflux is expected.

Due to their low interaction probability, neutrinosneed to interact with a large amount of matter in or-der to be detected. One of the detection techniques isbased on the detection of EAS in the atmosphere bylooking for very inclined young showers. The neutrinoevents would have a significant electromagnetic compo-nent leading to a broad time structure of detected signalin contrast to nucleonic-induced showers.

Propagating through the Earth only the so-calledEarth skimming (up-going) tau neutrinos may initiate

D. Góra / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 251–257 255

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Figure 5: The Xmax (left) and RMS(Xmax) (right) obtained by the FD as a function of the energy. The predictions of different hadronic interactionsmodels, for pure proton and iron compostion, are also shown.

detectable air showers above the ground [17]. In thiscase tau neutrinos may interact within the Earth and pro-duce charged leptons which decay and produce show-ers with lower energies. Since the interaction length forthe produced tau lepton is a few kilometers at the en-ergy of about 1 EeV, the leptons produced close to theEarth’s surface may emerge from the Earth, decay abovethe ground and produce EAS potentially detectable by asurface detector of the Auger Observatory [18, 19, 20].The surface detector of the Pierre Auger Observatory isalso sensitive to down-going neutrinos in the EeV en-ergy range [21]. Down-going neutrinos of any flavourmay interact through both charged (CC) and neutral cur-rent (NC) interactions producing hadronic and/or elec-tromagnetic showers. Details about the search tech-nique was presented at this meeting [22].

Up to now, no candidate events were found that ful-filled the selection criteria. The absence of neutrino can-didate events yields the best upper limit on the diffuseflux of neutrinos in the EeV energy range [23] as shownin Figure 6 (left). The figure includes a new down-goinglimit for all flavours as well as the recent limit on tauneutrinos derived using the absence of up-going neu-trino shower candidates.

The Auger Observatory has also set new photon lim-its with both the hybrid and SD detection methods [24].The new limits are compared to previous results and totheoretical predictions in Figure 6 (right) for the pho-ton fraction. In terms of the photon fraction, the currentbound at 10 EeV approaches the percent level while pre-vious bounds were at the 10 percent level. A discoveryof a substantial photon flux could have been interpreted

as a signature of top-down (TD) models. In turn, theexperimental limits now put strong constraints on thesenodels.

4. Conclusions

The measurements at the Pierre Auger Observatory,containing data equivalent to 2 years of operations ofthe full-size Auger Observatory, indicate a change in thenature of comic rays at around 3 EeV and show a changein the shape of the energy spectrum and the elongationrate. These measurements add support to the hypothesisthat an extragalactic component of mixed compositionstarts to dominate in this energy range. The near futureparticle accelerator results will constrain the hadronicinteraction models and the interpretation of the evolu-tion of the shower maximum with energy will be moreconclusive. The photon limits exclude most of the top-down scenarios above 2 EeV. In the next 20 years of op-eration the photon fraction measurement will be sensi-tive to a level of less than 0.1%. and the neutrino limits,if no neutrino is observed, will improve by more thanan order of magnitude. These determinations, togetherwith the arrival directions and mass composition anal-ysis will help solving the origin of the highest energycosmic rays.

5. Acknowledgements

The author gratefully acknowledges the financial sup-port by the HHNG-128 grant of the Helmholtz associ-ation and the Ministry of Science and Higher Educa-

D. Góra / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 251–257256

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Figure 6: Left: Differential and integrated upper limits (90% C.L.) from the Pierre Auger Observatory for a diffuse flux of down-going neutrinos inthe period 1 Nov 07 – 28 Feb 09 and up-going tau neutrinos (1 Jan 04 – 28 Feb 09). Limits from other experiments are also plotted. A theoreticalflux for GZK neutrinos is shown. Right: Upper limits on the photon fraction in the integral cosmic ray flux from different experiments. The linesindicate predictions from top-down models and shaded region shows the expected GZK photon fraction.

tion under Grant 2008 No. NN202 127235 and N N202207238.

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