The Auger Observatory: Status and Potential for Neutrino Detection

Enrique Zasa∗ (For the Auger Collaboration)

aDepartamento de Fısica de Partıculas, Universidad, E-15706 Santiago de Compostela, Spain, andKavli Institute for Cosmological Physics, 5640 South Ellis Av., Chicago, IL 60637.

The southern Auger observatory which is being constructed in Malargue, Mendoza, Argentina, is the largestcosmic ray detector in operation. Its concept and design, progress in its construction and its excellent preformanceare summarized paying special attention to inclined showers. Data are being collected at an ever increasing rateand some representative examples of reconstructed events are shown. The connection between Ultra High EnergyCosmic Rays and neutrinos is discussed. Progress in understanding the differences between inclined showersproduced by protons and nuclei and those expected from neutrinos and calculations of the neutrino acceptanceare reviewed.

1. The Ultra High Energy Challenge

The study of Ultra High Energy Cosmic Rays(UHECR) has become one of the priorities of as-troparticle physics in the past decade. There is atwofold motivation for it, one coming from par-ticle physics because they give access to interac-tions at energies much higher than accelerators,and another from astrophysics because we do notknow what particles they are nor where and howthey acquire these energies. This has led to muchspeculation on their possible origin which rangesfrom conventional acceleration mechanisms basedon Fermi acceleration, in which charged particlesare gradually accelerated to the highest energiesby electromagnetic processes, to “top down sce-narios” in which particles of these energies arecreated otherwise, and the cosmic rays followfrom their decay chain [1].

If Ultra High Energy Cosmic Rays (UHECR)are protons or nuclei they should interact withthe Cosmic Microwave Background (CMB) to de-grade their energy in relative short cosmologi-cal distances. This is a threshold effect for pro-tons which requires an energy exceeding about4 1019 eV with an interaction length of a few Mpc.Iron nuclei get degraded through photodissocia-

∗This work was partially supporte by Xunta de Galicia(PGIDIT02 PXIC 20611PN) by Ministerio de Educacion yCiencia (FPA 2001-3237, FPA 2002-01161 and FPA 2004-01198). We thank the CESGA for computer resources.

tion at similar energies and photons interact evenmore rapidly in this thermal background. Parti-cles that travel distances of 100 Mpc can hardlyexceed about 1020 eV when they reach us. Thishas been known since the 1960’s, and its effect isthat the cosmic rays must display a suppressionat these energies, the so called Greisen ZatsepinKuzmin (GZK) cutoff[2], but there is no unam-biguous evidence for it in the data.

The highest energy particles cannot deviatemuch in the magnetic field of the galaxy andthey should arrive from preferred directions ifthey were produced in the galaxy. But thisis not observed, since the arrival directions areconsistent with isotropy, suggesting extragalacticsources. Still, using nominal guesses of 1 nG forthe strength and 1 Mpc for the coherence scale,the deviation of a proton is expected to be of or-der 2.5 degrees after travelling 50 Mpc [3]. Thiscould open a new window for astronomy or con-tribute to the establishment of basic properties ofthe magnetic fields [4].

While Fermi acceleration is the favoured mech-anism, there is a limit to the maximum achievableenergy which involves the size and the magneticfield of the objects involved. Few astrophysicalobjects are capable of reaching these energies [5],particularly if they are close to Earth as the nonobservation of the GZK cutoff would imply. Topdown scenarios predict relatively high fluxes of

Nuclear Physics B (Proc. Suppl.) 143 (2005) 373–380

0920-5632/$ – see front matter © 2005 Published by Elsevier B.V.

www.elsevierphysics.com

doi:10.1016/j.nuclphysbps.2005.01.133

neutrinos and photons and it is difficult to ar-range them to naturally account for the observa-tions of cosmic rays at the GZK cutoff and below,its apparent isotropy, the data on lower energyphoton fluxes as well as experimental bounds onneutrinos.

Two types of experiments based on very dif-ferent techniques (fluorescence and arrays of par-ticle detectors) have undoubtedly detected par-ticles well exceeding the GZK cutoff [6–9]. Un-fortunately the data are scarce. The largest ex-periments disagree (at about the 2 σ level) onthe flux measured and on arrival direction corre-lations. HiRes using the fluorescence techniqueclaims that they detect a suppression of the fluxat the GZK energies, with no evidence for clus-tering in the arrival directions[6]. Arrays detectno supression [8,7]. and a possible clustering ofthe highest energy events [7]. What has becomeclear is that more and better data is badly neededfor the field to advance.

2. The Auger Observatory

The increasing interest in the observation ofthe EeV cosmic rays with more statistics has re-sulted in an international effort to construct thelargest detector of this type combining the twotechniques that have lead to the discovery of thehighest energy events. Its objectives are to haveuniform exposure to the south and north parts ofthe sky, to intercalibrate the two techniques, toestablish the spectrum of the cosmic rays in theregion of the GZK cutoff and above, and to es-tablish the mass composition of the arriving parti-cles. The Auger project is conceived as two 3,000km2 twin observatories in the northern and south-ern hemispheres, situated at mid latitudes. Eachobservatory is a hybrid experiment combining anarray of particle detectors and a fluorescence de-tector (see Fig. 1). The southern observatory isnow in full construction in la Pampa Amarilla,near Malargue, in the province of Mendoza, Ar-gentina. The Northern observatory is planned tobe sited in the U.S., either in Utah or in Colorado.

Surface Detector: The ground array willconsist in its final form of 1600 cylindrical wa-

A n g l e

C a s c a d e p l a n e

"Fly's Eye" with someact ive photodectors

I m p a c t p o i n t

Cherenkov Tanks

Figure 1. Schematic representation of the PierreAuger Hybryd concept. Showers can be detectedin the water tanks at ground level as particles gothrough them, and alternatively they can be de-tected through the fluorescence light emitted bythe atmospheric nitrogen. Each optical modulein the fluorescence detector sees a small fraction(pixel) of the 2π sr of the upper hemisphere.

ter Cerenkov detectors of 10 m2 surface areaand 1.2 m of height, each instrumented withthree photodetectors. They are arranged in ahexagonal grid, each tank separated 1.5 km fromanother, and extending over a surface area of3,000 km2. The integrated signal in each pho-totube is digitized in 25 ns intervals, with FlashAnalog to Digital Converters (FADC). Each tankis powered with solar cells and batteries, and thedata are transmitted from the detectors to a cen-tral station by conventional Local Area Network(LAN) radio links (Fig 2). All the stations are

E. Zas / Nuclear Physics B (Proc. Suppl.) 143 (2005) 373–380374

controlled remotely. The detectors are monitoredcontinuously at regular intervals by setting themto measure the muon spectrum. The single muonspectrum is used for calibration and the unit usedfor the signal is the that produced by a verticalmuon, a Vertical Equivalent Muon (VEM). Tanksare set to trigger at a 20 Hz rate for a signal to besent to the central station, where the trigger forcosmic ray showers is made requiring three tanksto have signals above 3.2 VEM within a shorttime window.

Figure 2. Photograph of a characteristic watertank of the Engineering Array of the Pierre Augerobservatory in Pampa Amarilla, Mendoza, Ar-gentina, diplaying the main components.

When a very high energy cosmic ray interactsin the upper layers of the atmosphere it producesa cascade of chain reactions of secondary parti-cles with air nuclei that end up in an extensiveair shower. The shower contains a very largenumber of particles at maximum (close to 1012

particles for a 1020 eV shower), mainly photons,electrons, positrons and muons, and this maxi-mum is reached not too far from the surface ofthe Earth. The particle density distribution de-creases rapidly as we move away from the showeraxis. Close to the axis electrons and photonsdominate, but as we get to distances of order 1 kmthe density of muons dominates, because it has amilder slope. As a result high energy showers canbe detected over surfaces that exceed 30 km2 onthe ground.

When electrons and photons reach the tankthey are typically absorbed and they give aCerenkov light signal which is proportional tothe total energy carried by them. Muons travelthrough the whole tank and give a light signalthat is proportional to their track-length. Fora proton initiated shower, the relative contribu-tions of each particle species, electrons, photonsand muons, to the tank signal are comparable be-cause the average energies carried by each partlycompensate for the diferences in particle densi-ties. Close to shower core the time spread of thesignal is rather small but at distances of order1.5 km (the separation between tanks), the sig-nals in the tanks can spread over intervals of order2 µs.

The arrival directions of the incident cosmicrays is determined from the relative arrival timesof the shower front detected at the triggeredtanks, with an accuracy that will possibly be bet-ter than one degree. On the other hand the ima-pact point of the core and the lateral distribu-tion of the signal are fitted to well studied showermodels. The signal measured at about 1 km fromshower axis has been established as a good es-timator of the shower energy, because it is leastsensitive to fluctuations and the composition ofthe primary particle.

Fluorescence Detector: The fluorescencetechnique requires mirrors with imaging capabil-ities and covering a sufficient field of view to cap-ture the depth development of the shower. Thefluorescence detector in the southern observatorywill consist on four “eyes”, at the perimeter of thesurface array, at four locations which are slightlyelevated with respect to the rest of the detector,chosen to cover all the atmosphere on top of theground array. Each eye is based on Schmidt op-tics and consists of mirror modules each with a2.2 m diaphragm, including a corrector ring of25 cm, preceeding a 3.8 m diameter mirror andlimiting the field of view of each mirror to ap-proximately a 30◦× 30◦ fraction of the sky. Eacheye views 30◦ upwards from the horizon and com-bines six of these mirror modules in the azimuthaldirections. The focal plane of each mirror is in-strumented with a camera, an array of 20×22 op-tical modules (or pixels), each of viewing aprox-

E. Zas / Nuclear Physics B (Proc. Suppl.) 143 (2005) 373–380 375

imately 1.5◦ × 1.5◦, and covering between themthe 30◦ × 30◦ solid angle viewed by the mirror.

As the shower flies past the fluorescence de-tector, the eye captures the light from the at-mospheric ionized nitrogen, which corresponds toonly about 4 photons per meter of ionizing par-ticle track emitted isotropically. The arrival di-rection of the cosmic ray particle is determinedby the pattern of pixels hit in the camera, whichdetermines a plane containing the shower. Thefinal shower orientation within that plane can beobtained from the timing sequence of the pixels.Once the geometry of the shower is establishedthe amount of light received at each pixel, whencorrected for propagation distance and attenua-tion, is converted to emitted light at different po-sitions in space.

The shower development profile can be recon-structed because the emitted light is proportionalto the total tracklength of the ionizing particlesof the shower. The shower energy can be deducedfrom it in a similar way to a calorimetric detec-tor used in accelerator physics. The uncertaintyin the energy determination is therefore reducedwith respect to a particle array which is measur-ing the particle content at a particular plane inshower development and thus is subject to fluc-tuations between different showers. The abilityto follow the depth development of the showeris also an important advantage because showermaximum can be determined directly. Such mea-surements of depth of maximum are most impor-tant for the establishment of primary composi-tion.

The detection of 1020 eV showers with the fluo-rescence technique necessarily requires the collec-tion of light which is produced over 20 km awayand thus subject to significant dispersion and at-tenuation in the atmosphere. As a result theatmospheric conditions must be monitored reg-ularly in order to have real time information onthe relavant attenuation and dispersion parame-ters. There is a complete program of atmosphericmonitoring, including, presure and temperature,infrared cameras for cloud coverage, several lidarsystems, a central laser facility, star monitoringand ballon flights [10]. The conversion of lightto shower signal is also affected by uncertainties

in the geometrical reconstruction of the showerdirection. Much of the geometrical uncertaintyin the reconstruction is eliminated if the showersare detected from two or more eyes, that is fromat least two different locations (stereo viewing).This constitutes an important advantage of usingseveral locations. In the Auger observatory manyshowers seen will be viewed in stereo and even bythree and four eyes.

The Auger observatory is the first hybrid de-tector combining the fluorescence technique witha ground array for the detection of EeV cos-mic rays. The angular resolution of the fluores-cence technique improves when used in combina-tion with the ground array, because much of thegeometrical uncertainty in the reconstruction ofthe shower profile is eliminated when the groundimpact time of the shower is determined by thesurface array. The power for composition of usingmethods that establish both the depth of maxi-mum and the muon content will help to elimi-nate part of the ambiguity associated to the inter-dependence between composition and interactionmodels.

3. Status of the Array and Performance

The southern Auger observatory has now over500 tanks deployed and fully operational andtwo complete flourescence detector locations eachwith six telescopes. The Auger detector is thelargest and most complete cosmic ray detector inthe world. Triggers are continuously being col-lected at an always incresing rate and the dataare being sent worldwide where it is analyzed bymany groups in the collaboration. The sample ofcollected showers is quite large.

Large events have been detected both with thesurface and the fluorescence detectors. The ar-rival direction in the surface detector is deter-mined by performing a plane fit to the arrivaltimes. The energy of the surface detector eventis reconstructed performing a fit to the lateraldistribution (LDF) of the signal and the impactpoint of the shower. The LDF is then used toestablish S(1000) from which the energy is esti-mated (See Fig. 3. The recorded time structureof the received signals is very rich and allows in

E. Zas / Nuclear Physics B (Proc. Suppl.) 143 (2005) 373–380376

Figure 3. Vertical shower observed by the SurfaceDetector, θ � 29◦, with the lateral distribution fit(Event 573928).

some cases to identify narrow spikes from individ-ual muons, particularly well away from the showercore (see Fig. 4).

Figure 4. Comparison of FADC traces in twosample tanks for a close to vertical (left) and aninclined event (right).

For the fluorescence detector events, once theshower detector plane is determined by the ac-tive pixels, the shower direction within this planeis obtained making a fit of the arrival times of thesignals at each pixel. In the data sample there aremany hybrid events as well as events detected instereo, that is from two fluorescence locations andalso with the surface detector. Fig. 5 displays asample of a hybrid stereo event. The synchro-nization of the fluorescence and surface detectors

allows for a hybrid reconstruction procedure inwhich the time signal of the tank can be used inconjunction with the times of the pixels to con-strain the time fit in the reconstruction of theangle in the shower detection plane. This greatlyimproves the angular resolution and the recon-strucion procedure on the whole. When the ar-ray is fully completed and operational most of thefluorescence events whill be hybrid (which repre-sents about 10% of the total data).

Figure 5. Stereo and hybrid shower, the arrayconfiguration is shown as well as the images inthe cameras of the two eys. Signal pulses in twoof the tanks are also displayed (Event 673411).

Inclined events are also detected up to anglesof 86◦ at a high rate because the tanks are sensi-tive to these showers and because the sampling ofthe shower front is more effective at high zeniths.A representative example of an inclined shower ispresented in Fig. 6. Inclined showers are charac-terized by very sharp peaks and very little cur-vature in the shower front as can be expected

E. Zas / Nuclear Physics B (Proc. Suppl.) 143 (2005) 373–380 377

for early proton or nuclei showers that are ab-sorbed in the top layers of the atmosphere. In-clined showers of zenith angle exceeding ∼ 60◦

have characteristic time stamps in the surfacedetectors corresponding to fast muons travelingalmost parallel to shower axis as illustrated inFig. 4. The behavior of these showers accordingto expectations from protons, nuclei or photons,is sufficiently distinctive to allow the identifica-tion of showers produced by deeply penetratingneutrinos at high zenith angles.

Figure 6. Shower of θ � 80◦ in the Surface De-tector, as reconstructed in the transverse plane(Event 577421).

The analysis of inclined showers enahances theaperture of the array that will eventually become9000 km2sr when the detector is finished. Thiswill contribute to increase the statistics in thesearch for answers to some of the most presssingissues concerning UHECR, the arrival directionsand the GZK cutoff.

4. The Neutrino Cosmic Ray Connection

The relation beween UHECR and neutrinosincludes production, transport and detection.Practically all mechanisms proposed as the ori-gin of the UHECR also produce Ultra High En-ergy neutrinos. The neutrinos are produced ei-ther through the cosmic ray interactions withmatter or radiation, (in acceleration models), orthrough fragmentation, (“top-down” scenarios).In both cases the neutrinos arise mainly fromcharged pion decay.

Accelerated protons will interact with sur-rounding matter or radiation to produce pions

and neutrons that will decay into neutrinos. As-suming that each proton produces one chargedpion, we can expect a ratio of neutrinos to pro-ton flux of order 3:1, but this is highly variablebecause it depends much on model details. Inthe models in which the neutrinos are producedthrough pion fragmentation and its subsequentdecay, the ratio of charged pions to protons is ex-trapolated from electron positron colliders, whichis known to be of order 20:1. As a result one ex-pects a fairly fixed ratio of neutrinos to protons60:1 which is about an order of magnitude higherthan for acceleration models. The ratio of theproton to the neutrino flux at high energies car-ries important information concerning the originof the UHECR.

In either case neutrinos can be expected fromthe GZK interactions themselves because theseinteractions produce pions. These neutrinos arefairly unavoidable, provided that the cosmic raysare protons or nuclei but unfortunately the fluxis very uncertain [11]. By the argument givenabove, it is clear that the neutrinos produced byGZK interactions could be dominant in acceler-ation mechanisms, but if the origin of UHECRwere “top-down” models, the GZK neutrinoswould be shadowed by those coming from thefragmentation process itself.

On the other hand neutrinos can be detectedat high energies by the very same detectors thatmeasure the cosmic ray spectrum because neu-trino interactions in the atmosphere also produceextensive air showers. The dificulty lies preciselyin separating the neutrino induced air showersfrom those produced by cosmic rays. This canbe done by looking for very inclined showers. Inthe horizontal direction the atmosphere becomes36,000 g cm−2 deep at sea level. High zenith angleair showers produced by cosmic rays reach a max-imum in the top of the atmosphere, where mostof the electrons and photons are absorbed, andonly the long range muon component survives toground level. As a result the showers induced byprotons, nuclei or phtons, detected at high zenithangles are very different at ground level from ver-tical showers.

The neutrino, having a small cross section, canhowever interact at any point in the atmosphere

E. Zas / Nuclear Physics B (Proc. Suppl.) 143 (2005) 373–380378

inducing deep showers. If their shower maxi-mum develops close to observation level they willhave electrons photons and muons in proportionsclose to vertical showers induced by protons. Thesearch for neutrino events has thus a clear signa-ture, namely inclined showers that have a largeelectron and photon component, and are thussimilar to a vertical shower induced by a protonor a nucleon. The Auger observatory has a largepotential for detecting neutrino showers. Thesecan come from neutrino interactions deep in theatmosphere, but also for earth skimming tau neu-trinos that produce tau leptons exiting the surfaceof the Earth.

5. Inclined Showers

The water Cerenkov detectors used in theAuger Observatory are very effective in detect-ing large zenith angle showers. The array can beused for neutrino detection by looking for deeplypenetrating showers at very high zenith angles.The acceptance has been shown to be compara-ble at EeV showers to contained events in conven-tional underground neutrino detectors [13]. Fig 7displays the estimated acceptance of the Augerdetector for deeply penetrating inclined showers.

This acceptance was obtained in the absence ofoscillations, requiring the showers to have zenithangles above 75◦ and demanding that the show-ers are detected with shower maximum intercept-ing the surface detector array. This implies thata large electromagnetic component is present toensure that they are indeed produced deep intothe atmosphere, in contrast to the inclined show-ers produced by cosmic rays. These requirementsare possibly very conservative. It is possible tomeasure the radius of curvature of the showerfront indicating the production depth. A methodis being devised to deduce the production depthfor the muons from the time structure of the sig-nals at large distances to shower core [14]. Ob-servations of inclined showers produced by cosmicrays, which are routinely done at the observatoryindicate that showers at 60◦ display the charac-teristic sharp signals in the detectors. All theseconsiderations will enhance the aperture to neu-

Figure 7. Expected acceptance of the PierreAuger observatory as a function of shower en-ergy to near horizontal neutrinos (θzenith ≥ 75 de-grees) for electromagnetic showers (dashed) andhadronic (solid). Lower set of curves correspondsto only showers with a core inside the array. Up-per set considers all showers. Volume units arekm3 of water equivalent.

trino showers.Calculations of the Auger acceptance to the

showers produced by Earth skimming tau neu-trinos have also been made [15]. They producetau leptons in the Earth surface that fly out ofit to decay in the atmosphere. The most effi-cient grammage of Earth is such that the solidangle of the outgoing lepton is small and con-centrated around the horizonal direction givinga practically horizontal shower developing at alow altitude above the ground. The results arevery encouraging since it turns out that the mostfavourable energy for the competing processes oftau production and tau exit is in the 1018 eVrange, just right for the Auger observatory. Theacceptance calculated with simulations exceedsthat of Fig. 7 by close to an order of magnitudein the most favourable energy region. Providedthat inclined showers produced by neutrinos canbe identified, the Auger observatory will measurethe neutrino flux at high energies or produce com-petitive bounds.

It is for this reason that a large effort has been

E. Zas / Nuclear Physics B (Proc. Suppl.) 143 (2005) 373–380 379

made to study the background, inclined showersproduced by protons, nuclei and photons. Thefirst analysis of inclined showers was performedfor Haverah Park data after the distorted geo-metrical patterns of the muons at ground levelwere understood in terms of a relatively simplemodel [16]. The relative rate of inclined showersto vertical showers detected at Haverah Park hasbeen succesfully used to establish the first limiton Ultra High Energy photons and to comple-ment compositon studies [17]. Similar methodsare being developed to analyse inclined events athe Auger observatory [18]. The much improvedperformance of the Auger observatory will extendthese results and help to tie down the most diffi-cult task, namely determining the mass compos-tion of the primaries. The inclusion of the in-clined events analysis will also enhance both theaperture and the sky coverage of the Auger ob-servatory.

6. Conclusions

The construction of the southern Auger obser-vatory in Malargue is reaching its mid point and ifthe present rate of deployment can be mantainedit will be finished early in 2006. The detectoris performing very well and events are routinelydetected in both the surface and fluorescence de-tectors. Many of the fluorescence events are hy-brid and there are also stereo events and detectedby both flourescence detectors and by the sur-face detector. The detection of inclined showersis also done routinely and these showers have acompletely different time structure in the surfacedetector signals to vertical as expected for pro-tons or nuclei.

The Auger observatory has a large potential todiscover (or establish competitive bounds for) in-clined showers produced by Ultra High Energyneutrinos deep in the atmosphere or from Earthskimming tau neutrino events. The identificationof the deep inclined showers produced by neutri-nos is expected to be easy because they have aclear signal. In any case the reconstruction of in-clined showers on an individual basis will providenew limits on composition and increase the aper-ture of the surface array when the observatory is

completed. The analysis of the large data samplethat is accumulating is under way and the firstscience results are expected in the near future.

REFERENCES

1. For reviews see for instance, A.V. Olinto,Phys. Rept. 333-334 (2000) 329. P. Bhat-tacharjee and G. Sigl, Phys. Rept. 327, 109(2000)

2. K. Greisen; Phys. Rev. Lett., 16 (1966) 748.G.T. Zatsepin and V.A. Kuz’min, JETPLett., 4 (1966) 78.

3. J.W. Cronin, Nuc. Phys. B28 Proc. Suppl,213 (1992).

4. G. A. Medina-Tanco, et al., Astrop. Phys. 6,337 (1997)

5. A.M. Hillas et al., Proc. of XI ICRC, Bu-dapest (1969), Acta Physica Academiae Sci-etiarum Hungaricae 29, Suppl. 3, p. 533, 1970.

6. R. U. Abbasi et al. Phys. Rev. Lett. 92,151101 (2004); Astrop. Phys. 22, 139 (2004).

7. M. Takeda et al. Astropart. Phys. 19, 135(2003).

8. M A Lawrence J. Phys. G 17, 733 (1991).9. M. Nagano and A. A. Watson, Rev. Mod.

Phys. 72, 689 (2000).10. P. Veberic et al. (Auger Colab.), Proc. of XVI

ICRC 2003, Tsukuba, Japan, vol 1, pp. 461;M.A. Mostafa, (Auger Colab.), Ibid. pp. 465.

11. R. Engel et al. Phys. Rev. D 64, 093010(2001), M. Ave, et al. astro-ph/0409316.

12. J. Abraham et al. Nucl. Instrum. Meth. A523, 50 (2004).

13. K. S. Capelle, et al. Astrop. Phys. 8, 321(1998).

14. L. Cazon, et al. Astropart. Phys. 21, 71(2004).

15. X. Bertou, et al., Astrop. Phys. 17, 183(2002).

16. M. Ave, et al., Astrop. Phys. 14, 109 (2000),and 14, 91 (2000).

17. M. Ave, et al., Phys. Rev. Lett. 85, 2244(2000).

18. M. Ave (Auger Colab.) 28th ICRC , Tsukuba,Japan, vol 1, pp. 365; M. Ave, et al., Phys.Rev. D67, 043005 (2003).

E. Zas / Nuclear Physics B (Proc. Suppl.) 143 (2005) 373–380380