1 A. Zech, Instrumentation in High Energy Astrophysics Chapter 6.3: Ultra-high energy cosmic rays & indirect detection

2 A. Zech, Instrumentation in High Energy Astrophysics The ultra-high energy range ● The "ultra-high" energy range begins at around eV. There is no strict definition, though. ● An energy of about 2x10 16 eV corresponds to the upper limit that will be reached at the LHC (Large Hadron Collider) in the center of mass frame for Pb nuclei. ● Thoretical mechanisms to explain cosmic ray acceleration (e.g. in shockwaves, AGN jets, etc.) above ~10 19 eV are still very uncertain. ● At ~6 x eV a flux suppression is expected to be observed in the extragalactic cosmic ray spectrum due to the GZK effect (photo pion-production with the CMBR). Limite LHC

3 A. Zech, Instrumentation in High Energy Astrophysics The "GZK Controversy" Until recently, two experiments had measured the cosmic ray spectrum at ultra-high energies: HiRes (two air fluorescence telescopes) and AGASA (a 100 km 2 array of scintillators). AGASA observed a continuation of the cosmic ray spectrum up to the highest energies without any sign of the GZK feature. HiRes did observe the GZK cut-off in the spectrum. The Auger experiment combines the two techniques. It has confirmed the flux suppression discovered by HiRes.

4 A. Zech, Instrumentation in High Energy Astrophysics A bit of history...some UHECR experiments ● late 1940s: ground array technique developed by scientists from MIT (USA). ● 1959: Volcano Ranch (New Mexico, USA), 10 km 2 array of plastic scintillators, energies estimated to be up to eV. ● 1962: Haverah Park (England), 12 km 2 array of water Cherenkov detectors, spectrum measured from 0.3 to 10 EeV ● 1964: Fluorescence technique tested by scientists from Cornell (USA). ● 1968: First successful air fluorescence detection near Tokyo. ● 1976: Air fluorescence observed in coincidence with Volcano Ranch detectors. ● Yakutsk (Siberia), 18 km 2 of muon counters, scintillators and PMTs for Cherenkov light detection, highest E event: 1.3 x eV ● SUGAR (Australia), 60 km 2 of muon counters ● Akeno (Japan), 20 km 2 of muon counters and scintillators, later extended to AGASA ● 1982: Fly's Eye (Utah, USA), two air fluorescence detectors, spectrum measured above 0.4 EeV, the highest ever energy event was measured at 3 x eV.

5 A. Zech, Instrumentation in High Energy Astrophysics A bit of history...some UHECR experiments ● 1990: AGASA, extension of Akeno to 100 km 2, energy spectrum does not show any sign of the GZK feature, 11 events above eV. ● : HiRes/MIA is the first experiment to employ a hybrid (fluorescence + ground array) technique, it consists of a HiRes prototype telescope and the MIA muon counter array ● : HiRes (High Resolution Fly's Eye) consists of two air fluorescence telescopes located in Utah (USA), measurement of the energy spectrum above ~0.1 EeV, observation of the GZK cutoff in the spectrum ● 2004: Auger South is taking the first data ● at present: development of radio detection methods are very promising

6 A. Zech, Instrumentation in High Energy Astrophysics Ground-based experiments: The Pierre Auger Observatory ● Hybrid technique: combination of surface detectors and fluorescence telescopes ● very large area (~3000 km 2 ) for good statistics at the highest energies (around GZK threshold) ● study of the energy spectrum, arrival directions and composition of ultra-high energy cosmic rays.

7 A. Zech, Instrumentation in High Energy Astrophysics The Auger South array Auger South is located in the province of Mendoza (Argentina), near Malargüe. Layout: 1600 surface detectors and 4 fluorescence stations (6 telescopes each). Advantages of the site: Far from light-pollution, clear atmosphere, large flat area, good infrastructure.

8 A. Zech, Instrumentation in High Energy Astrophysics Instrumentation - Auger ● More than 1600 surface detectors are arranged in a grid with a 1.5 km spacing between two stations. They cover ~3000 km 2. Almost all of the stations are deployed by now. ● 4 stations with 6 fluorescence telescopes each view the atmosphere above the site. ● Several stations (laser, lidar, weather station...) are included in the array to collect information on the local weather conditions. ● The Pierre Auger Observatory South site has started taking data in January 2004 with a small proportion of the full array. ● A similar, but larger array is planned for the northern hemisphere (Colorado): Auger North

9 A. Zech, Instrumentation in High Energy Astrophysics

10 A. Zech, Instrumentation in High Energy Astrophysics Surface Detectors (SD) ● The surface detectors are water Cherenkov tanks. ● They are each filled with 12 tons of ultra-clean water. Charged particles from the air shower will leave a trace of Cherenkov light when traversing the tank. e+,e-: provide a signal, but are quickly absorbed (E~10MeV) muons: provide a strong signal as they traverse the tank (E~GeV) photons: pair produce e+,e- ● 3 PMTs observe the Cherenkov photons and record traces. ● solar panel for power supply ● antenna for GPS and communication.

11 A. Zech, Instrumentation in High Energy Astrophysics Antenne electronics reflecting bag batteries ultra-clean water photomultiplier solar panel Temps Signal A water Cherenkov detector Cherenkov cone

12 A. Zech, Instrumentation in High Energy Astrophysics FADC traces seen in the SD detectors Station selection : shape of the signal muon + electromagneticmuon ns

13 A. Zech, Instrumentation in High Energy Astrophysics Reconstruction of the Shower Geometry (SD) When an air shower triggers several SD stations, their pulse sizes and trigger times are used in the reconstruction of the shower front and of the complete shower geometry. Below: a footprint of a highly energetic shower. The angular resolution of the shower axis and thus of the cosmic ray direction is < 2.2 o.

14 A. Zech, Instrumentation in High Energy Astrophysics distance to axis (m) recorded signal S1000 Lateral distribution function – Energy reconstruction ● LDF = distribution of signal size as a function of distance from shower axis ● The signal size at 1000 m is called "S1000". It is used as an estimator of the primary energy E. S1000 ~ const * E. ● The distance of 1000 m is determined from Monte Carlo simulations for the given array configuration. For the tank distance of 1.5 km, fluctuations in the LDFs are at a minimum at 1000 m. ● The conversion factor between S1000 and E has to be determined from simulations or from additional shower measurements (FD, radio...).

15 A. Zech, Instrumentation in High Energy Astrophysics Deeper showers have smaller radii of curvature. This helps to distinguish photon showers from hadronic showers. One can also try to distinguish between proton and iron showers, which is more difficult. X max Proton Radius of Curvature (SD) – Composition Measurement X max photonproton

16 A. Zech, Instrumentation in High Energy Astrophysics Photon showers develop deeper in the atmosphere and over a longer range => longer risetime of the FADC pulse. There is an additional effect from the difference in muon fraction. The risetime of the pulse can be used to distinguish photons and hadrons. One can also use it to try and distinguish between protons and iron nuclei, which is more difficult. Risetime (SD) – Composition Measurement Photon Proton

17 A. Zech, Instrumentation in High Energy Astrophysics Air Fluorescence Detectors (FD) ● 4 fluorescence stations view the atmosphere above the ground array. Each station consists of 6 telescopes. ● Operation only during clear, moonless nights. ● Antenna for communication with surface detectors and with central trigger. ● Each time an FD is triggered, SD stations are read out as well to yield hybrid information.

18 A. Zech, Instrumentation in High Energy Astrophysics Mirrors ● UV fluorescence light from the air shower passes through the aperture (with filter and a corrector lens) and hits the mirror. ● The mirror reflects the light onto a camera in its focal point.

19 A. Zech, Instrumentation in High Energy Astrophysics Camera ● Each camera consists of 440 (20 x 22) PMTs arranged in a hexagonal pattern. ● Each PMT sees ~1.5 o. One telescope covers 30 o in azimuth and 30 o in elevation. The six telescopes of each station view 180 o in azimuth. ● Reflectors similar to Winston cones are put in front of the PMTs to enhance their angular range. ● When an air shower traverses the sky, its image traverses the camera. A track of triggered pixels can be recorded.

20 A. Zech, Instrumentation in High Energy Astrophysics Reconstruction of the Shower Geometry (FD) 1 above: shower track seen in two telescopes; below: reconstruction of a stereoscopic event (seen by two stations) The first step in the geometry reconstruction is the reconstruction of the "shower-detector- plane". Next, the position of the axis inside the plane has to be determined with stereo information or timing information of the PMTs on the track

21 A. Zech, Instrumentation in High Energy Astrophysics Reconstruction of the Shower Geometry (FD, Hybrid) (2) The geometry of the axis is determined (in the absence of stereo information) from a time vs. angle plot. The additional information from the tanks improves this fit considerably. (Hybrid reconstruction) PMT traces of a stereo event. The pulse sizes give information on the longitudinal profile of the air shower.

22 A. Zech, Instrumentation in High Energy Astrophysics Reconstruction of the cosmic ray energy (FD, Hybrid) ● The recorded signals are transformed into a photon flux at the detector. ● This flux is traced back through the atmosphere to the shower axis. Atmospheric attenuation is taken into account. ● The photon flux is converted into a distribution of charged particles per slant depth (= the longitudinal shower profile). ● Cherenkov light from the shower is subtracted in an iterative procedure. ● The integral over the shower times the mean ionisation loss rate ~2.9 MeV / (g cm -2 ) = energy deposit of the shower. ● ~10% is added for "missing energy" to get the total energy of the cosmic ray.

23 A. Zech, Instrumentation in High Energy Astrophysics Using hybrid information for SD reconstruction ● The information from hybrid events can be used to determine the conversion factor between S1000 and the primary energy. ● One determines this factor for a subset of hybrid events with well reconstructed S1000 (from SD) and energy (from FD). ● One reconstructs the S1000 of all SD events and applies the factor to determine their energy. ● This way, the huge statistics from the SD and the very good energy reconstruction of the FD are combined. Log (S(1000)/VEM) Log (E/EeV) 10EeV 1 EeV

24 A. Zech, Instrumentation in High Energy Astrophysics Measurement of the Cosmic Ray Composition (FD) X max top of atmosphere X max FD Xmax is used as an estimator for the composition of the cosmic ray. Heavy nuclei lead to showers that develop higher up in the atmosphere than light nuclei. The measured Xmax are compared to Monte Carlo simulations. A big problem lies in the fact that fluctuations of the Xmax between different showers of the same composition and energy are large. Longitudinal profile of a eV proton shower.

25 A. Zech, Instrumentation in High Energy Astrophysics Calibration (FD) ● photon flux -> p.e. : each PMT is calibrated against NIST calibrated photodiode ● p.e. -> ADC counts : – absolute calibration every few months with an LED light source. The light source illuminates a drum, which is plugged onto the aperture of each telescope. The mirror is illuminated uniformly. – verification with laser shots of known energy – nightly relative calibration with light sources at the mirrors, camera, aperture.

26 A. Zech, Instrumentation in High Energy Astrophysics Calibration (SD) ● The SD stations are "self-calibrating" ● Atmospheric muons (from air showers) are used to calibrate all the stations. There are counts per minute from this background. ● The number of ADC counts at the peak is the signature of a vertically traversing muon. ● The peak ADC value defines a "VEM" (vertically equivalent muon). All signals are then expressed in multiples of 1 VEM.

27 A. Zech, Instrumentation in High Energy Astrophysics Atmospheric Monitoring ● There are two main processes for atmospheric attenuation in the UV band of interest (~ nm): – molecular (Rayleigh) scattering – aerosol (Mie) scattering ● The molecular density of the atmosphere is measured with ~5 radiosonde launches per month. The air pressure is measured at different altitudes. ● The aerosol density and distribution has to be measured during the nights when data are taken, since aerosols can change rather rapidly. Several detectors are used (lasers, lidars, horizontal attenuation detectors) ● Clouds are observed with infrared cameras. ● A weather station records temperature, wind speed, precipitation.

28 A. Zech, Instrumentation in High Energy Astrophysics Some Scientific Results from Auger - Spectrum The energy spectrum measured by Auger using the SD data (and calibrated against the FD measurements) confirms the GZK flux suppression detected by HiRes.

29 A. Zech, Instrumentation in High Energy Astrophysics Some Scientific Results from Auger - Anisotropies (1) Auger does not see any anisotropy towards the Galactic Center (as claimed by SUGAR, AGASA) at EeV energies

30 A. Zech, Instrumentation in High Energy Astrophysics Some Scientific Results from Auger - Anisotropies (2) Auger sees for the first time some evidence for a correlation between the arrival direction of UHECRs of the highest energies (circles) and a catalogue of nearby AGN (red stars).

31 A. Zech, Instrumentation in High Energy Astrophysics Some Scientific Results from Auger – Photon Flux Limit Auger (latest results = black arrows) puts a stringent upper limit on the photon fraction at ultra-high energies. Previous measurements from Haverah Park, Yakutsk, Agasa and previous Auger results (A and FD) are also shown. These upper limits rule out many "Top-Down" models (Super Heavy Dark Matter, Topological Defects).

32 A. Zech, Instrumentation in High Energy Astrophysics The Future of Cosmic Ray Physics ● The Telescope Array (TA) is a hybrid array in Utah (USA). It combines fluorescence telescopes with an array of 1000 km 2 of plastic scintillators. Its unique advantage is a large coverage of energies from ~30 PeV to the highest energies. ● Auger North is planned as a 5000 km 2 array in Colorado (USA), similar to Auger South. It would also use the hybrid technique. It would complete the sky coverage of Auger and increase its overall aperture. ● JEM/EUSO is a Japanese/European proposal for a detector to be deployed on the space station or on a satellite. It would observe extensive air showers in the atmosphere from above and significantly increase the experimental aperture at the highest energies.

33 A. Zech, Instrumentation in High Energy Astrophysics More Information ● HiRes: ● AGASA: ● AUGER: ● TA: