On the pampas of western Argentina the Pierre Auger Observatory is studying extensive air showers caused by the universe’s highest energy particles. These cosmic ray air showers are generated by collisions of particle and atmospheric nuclei with center-of-mass energies well over 400 TeV. Ever since the discovery that cosmic rays have an extraterrestrial origin, these energetic particles have challenged our understanding of where they come from and what they are.
The cosmic ray spectrum falls very steeply, so that the flux of particles with energy above 10 EeV (1 EeV = 1018 eV) falling on the Earth’s atmosphere is about 1 per km2 per year. Therefore, detectors are deployed over vast areas to accumulate useful numbers of events. Remarkably, this approach is practical because the cosmic rays generate giant cascades of particles with more than than ten billion particles for
a 10 EeV primary cosmic ray and spread over about 20 km2 when they reach the ground.
The Pierre Auger Observatory uses two techniques to measure the extensive air shower properties by observing both their longitudinal development in the atmosphere and their lateral spread at ground level. Charged particles and photons that reach the ground are sampled with the Surface Detector (SD) array which consists of 1660 water Cherenkov detectors (WCDs) spread on a triangular grid of 1.5 km spacing over 3000 km2, Each detector is filled with 12 tons of ultra-purified water each and equipped with three photomultiplier tubes (PMTs) to detect the Cherenkov light emitted in the water. The fluorescence light generated in the atmosphere by the charged particles of the air shower through excitation of nitrogen molecules is detected by the Fluorescence Detector (FD) which consists of 27 telescopes in five different buildings that overlook the SD array. Light is focused with a spherical mirror of 13m2 on a camera of 440 PMTs. The duty cycle of the FD is about 13-15%. The combination of both technologies is a great asset allowing for cross calibration and independent confirmation of results. The Observatory was completed in 2008, and has been taking data since 2004. We are currently collecting approximately 40,000 events per month.
After completion of the baseline construction, enhancements were added to further the Observatory as a multi-detector facility. The AMIGA (Auger Muons and Infill for the Ground Array) project is a joint system covering an area of 23.5 km2 that consists of an infill of 61 WCDs that are spaced 750m apart and accompanying buried scintillator stations each with an area of 30m2. The High Elevation Auger Telescope (HEAT) comprises of three additional telescopes that can be tilted upwards to have an elevated field of view. The infill and HEAT combination allows a hybrid trigger down to below 0.1 EeV. The HEAT started taking data in 2009 and the infill deployment was completed in 2011.
The Auger Observatory has several ongoing R&D projects that are pursuing new detection techniques. These activities rely on using the data of the Auger Observatory to correlate their measurements with the shower parameters derived from the standard data analysis.
I) The Auger Engineering Radio Array (AERA) is used to study emission of radio waves from extensive air showers. The 124 radio stations cover an area of 6km2 and operate in the frequency range from 30 to 80 MHz. The duty cycle is close to 100% and only excludes times of close-by thunderstorms since strong atmospheric electric fields significantly affect the radio emission of the air showers. Some of the open questions AERA attempts to address are as follows: 1) Can radio measurements compete in precision for energy and composition with the fluorescence technique? 2) Is it possible to operate radio arrays as stand-alone? 3) Can the radio technique be extended to very large scales, i.e. can it be used for future cosmic ray experiments studying the end of the energy spectrum with high statistics?
II) The discovery that microwave radiation is emitted by the passage of charged particles through a dielectric (Askaryan effect) has opened a new window for mapping extensive air showers. Three different R&D projects (AMBER, MIDAS, EASIER) are being used to test air shower detection in the GHz. The EASIER prototype installed 61 tanks with GHz receivers tightly integrated in the hosting SD station. In June 2011, the first clear air shower signal in the GHz wavelength was detected by EASIER in coincidence with the main SD array. AMBER and MIDAS are imaging telescopes similar in concept to a fluorescence detector, instrumenting an array of feed horn antennas at the focal plane of a parabolic dish reflector.
The Observatory has provided answers and new questions about the universe’s highest energy particles. Below, some of the major scientific achievements are listed.
We now know that the energy spectrum is suppressed at energies above 40 EeV, with fewer particles found above this energy than a simple extrapolation from lower energies would suggest. Presently it is not possible to determine whether this suppression is due to energy losses in transit (such as the GZK cutoff where protons undergo photo-pion production as it interacts with the CMB photons or photodisintegration of heavier nuclei) or if it reveals the maximum energy of the natural accelerators at the sources.
Photon and neutrino limits:
The limits on the fluxes of photons and neutrinos have eliminated the so-called “top-down models” where high energy cosmic rays are produced by the decay of other objects such as superheavy dark matter and topological defects, and through Z-bursts (particle production via hypothetical flux of high energy neutrinos annihilating against a sea of relic neutrinos from the Big Bang). The limit on the neutrino flux also eliminates some of the astrophysical models.
It had been assumed that these particles were mostly protons. However it appears that other nuclei may also be present. The current analyses indicate that the composition becomes heavy with increasing energy. However, the door is not closed to the possibility that assumptions we have made about particle physics at these high energies may be incorrect and need to change.
The depth of shower maximum is directly related to the depth of the first interaction of cosmic rays in the atmosphere. Based on this correlation the proton-air cross section has been measured at a center-of-mass energy of 57 TeV using the hybrid data. This cross section can be converted to an equivalent proton-proton cross section by applying the Glauber approximation. The cross section is found to be consistent with model extrapolations that describe the LHC data. An unexpected, rapid increase of the cross section directly above the LHC energy is disfavored.
The muonic component of air showers is sensitive to hadronic particle interactions at all stages in the air shower cascade and to many interaction properties such as multiplicity, elasticity, fraction of neutral secondary pions, and the baryon-to-pion ratio. Currently the number of muons in the air shower can be estimated indirectly. Utilizing events that are reconstructed by both the surface and fluorescence detectors, an excess of muons has been observed.
The arrival direction distribution is one of the key observables to search for the transition from Galactic to extragalactic cosmic rays and for sources or source regions of high energy cosmic rays. The amplitude and phase of the dipole anisotropy in the equatorial plane has been measured, which indicates a less than 1% change of isotropy and suggests that cosmic rays above 1 EeV to be of extragalactic nature. Cosmic rays with energy above 55 EeV indicates a correlation with the distribution of nearby AGNs.
Measurements from Auger, while shedding light to the nature and origins of the highest energy cosmic rays, have also led to a number of puzzling observations that indicate a much more complex astrophysical scenario. Moreover, the interpretation of the Auger data relies on the accuracy of modeling air showers and hadronic multi-particle production. Therefore the astrophysical scenarios have to be considered in the context of our current understanding of hadronic interactions. It is possible that the overall features of hadronic interactions are significantly different at energies and in phase space regions which are not accessible to current colliders.
The collaboration aims at an upgrade of the Auger Observatory to ensure that the data collected after 2016 will provide additional information to allow us to address the following questions:
- Elucidate the origin of the flux suppression and mass composition at the highest energies;
- Search for a flux contribution of protons up to the highest energy;
- Study hadronic interactions and extensive air showers to above center of mass energy 70 TeV;
Understanding the origin of the flux suppression will not only provide fundamental constraints on the astrophysical sources, but it will subsequently allow much more reliable estimates of neutrino and gamma ray fluxes. This will be a decisive ingredient for estimating the physics potential of existing and future cosmic ray, neutrino and gamma ray detectors.
To accomplish these science objectives, it will be of central importance to improve the composition sensitivity and to extend it into the energy region of the flux suppression. The most promising way is to discriminate between electromagnetic and muonic components of the shower with the SD array. The data readout of the enhanced SD stations will be facilitated by replacing the current readout electronics by modern state-of-the-art electronics providing three times faster sampling, a significantly enhanced dynamic range, and enabling enhanced trigger and monitoring capabilities.
The upgrade will be proposed to the funding agencies by the international collaboration.
- The official Pierre Auger Observatory site
- Link to the Education and Outreach section
- Time lapse video of the Observatory by Steven Saffi
- Voices of the Universe video
- 60 seconds with Alan Watson
- Peter Kasper
- Paul Mantsch
- Paul Lebrun