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Introduction
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The Extreme-Universe Space Observatory ( EUSO) is a European Space Agency (ESA) mission to investigate the nature and origin of extreme-energy cosmic rays (EECRs) -the window on the extreme-energy universe. EUSO will pioneer measurements of EECR-induced extensive air showers (EASs) from space. It will make accurate measurements of the primary energy, arrival direction, and composition of EECRs using a target volume far greater than is possible from the ground. These data will be used to extend the EECR spectrum to the highest energies, determine their nature, and make an all-sky survey of their arrival directions. EUSO will investigate radiations produced under the most extreme physical conditions in the universe. These are conditions beyond our present understanding and possibly involve the early history of the Big Bang and the grand unification of the fundamental forces of nature. EUSO’s objectives support the Structure and Evolution of the Universe theme and are closely related to the first two objectives of NASA’s Office of Space Science (OSS) Strategic Plan1 for 2003 and beyond. These objectives are-
The role of the United States (U.S.) in the EUSO mission is to provide the wide-angle optics, expert advice on the focal plane and electronics design, participate in simulations for instrument definition, development of production data analysis software and analysis and publication of results. ESA is depending on the U.S. for the optics system. We propose to bring the U.S. expertise to bear on key scientific and technical issues for EUSO. Our EUSO team includes the Space Optics Manufacturing and Technology Center SOMTC), who will provide large, wide-angle space optics; effective physics data analysis and simulation groups that will maximize the science return; and experts to provide advice to our international collaborators on photon detectors and front-end electronics. This team possesses a strong theory component that will assist with testing of the many EECR models and with the interpretation of the results. The OSS cost for U.S. participation in this pioneering experiment is modest, as ESA will bear the major mission expense. The EUSO instrument has been selected for study as an International Space Station (ISS) payload by ESA, following completion of an ISS accommodation study in 2000. The 1-year European phase-A study, beginning in December 2001, will develop the concepts for the EUSO instrument and its accommodation on the Columbus module. ESA will depend on a U.S. phase-A study to develop the concept for the optical system. Raw data from EUSO will be processed at the Science Data Center (SDC) in Europe. The raw and processed data will be archived and distributed to the entire EUSO collaboration. All EUSO data will be made publicly available within 1 year after the completion of the mission.
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Science Objectives
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EUSO data analysis will address the following crucial experimental questions:
Past Investigations: Six ground-based experiments have reported a total of 22 events >1020 eV (super-Greisen-Zatsepin-Kuzmin (GZK) events) during the last 40 years 1 event from Volcano Ranch, 4 from Haverah Park, 1 from Yakutsk, 1 from Fly’s Eye, 8 from the Akeno Giant Air Shower Array (AGASA), and 7 from the preliminary High-Resolution Fly’s Eye Experiment (HiRes-1)). This corresponds to a flux of about one event per km2 per century. At the August 2001 International Cosmic-Ray Conference, HiRes reported two super-GZK events. Haverah Park revised their results downward to zero, and AGASA, with its new extended aperture, increased their statistics to 17 super-GZK events. Since their exposures are comparable, the AGASA and HiRes results are in conflict. Nevertheless, both experiments indicate the existence of these super-GZK events. The conflict could arise from a shift of unknown origin in the experimental energy scale of the two experiments. This murky experimental situation can be resolved by a high statistics and high-energy resolution experiment with different systematic errors. Such an experiment is proposed here. Cosmic rays (CRs) with lower energies of ~1018 eV show a small but significant anisotropy toward the galactic center. If a similar fraction of higher energy events have a galactic origin, they should show a stronger anisotropy due to their increasing magnetic rigidity. However, AGASA and a world data summary of CRs above EGZK (~5x1019eV) show an isotropic distribution in the sky on large scales (fig. 1.1, foldout 1), clearly suggesting an extragalactic origin. Among 58 events observed by AGASA, small-scale clustering (events within 2.5°) has been found. AGASA counts six pairs and one triplet of spatially correlated events with arrival times differing by <2 years. The world data shows nine pairs and two triplets. The chance coincidence probability for these clusters arising from an isotropic distribution is <0.04 percent. The inference to be drawn is that particles within each cluster may have the same source and travel with minimal magnetic deflection. At present, the two highest energy CRs measured have the energy of ~3.2x1020 eV (Fly’s Eye) and ~3.4x1020 eV(AGASA). The origin of these events is mysterious since there are no visible source candidates within the GZK horizon except possibly M87, a radio-loud AGN about 20 Mpc away from us, and Cen-A (NGC5128), a radio galaxy at 3.4 Mpc. Neither of these is in the direction of any observed events. Large-scale isotropy of observed events suggests that many sources, rather than one or a few sources, are probably required. Figure 1.2 shows a compilation of the most recent data published from AGASA, Fly’s Eye, and HiRes. The overall E-3 dependence has been removed from this figure to reveal the detail. This figure clearly shows the spectrum extending beyond EGZK. The dashed line shows the effect of the GZK cutoff, assuming that a homogeneous source population fills the universe. Because the universe is transparent only to neutrinos at these energies, much speculation has arisen about the distances of the sources and the nature of the primary particles. Present and Planned Ground-Based Observations: The largest planned ground-based experiment facility is the Pierre Auger Observatory presently under construction in Argentina. It will consist of an array of 1,600 particle detectors, and 4 fluorescence light detectors similar to the ones used in the HiRes experiment. This hybrid detector will allow cross calibration and a check of the systematic uncertainties inherent in each of the techniques. Construction of the Pierre Auger Observatory is expected to be completed in 2004. It will have an aperture of 7,000 km2 sr. Extrapolating the spectrum as a power law above 1020 eV, we expect about 70 events/yr if the data follow the AGASA spectrum (fig 1.2) or 30 events/yr if the data follow the HiRes spectrum. In about half a year, it will produce a number of events comparable to all previously observed events above 1020 eV. A second Auger observatory is proposed for the northern sky. If the HiRes spectrum is correct, Auger may still be too limited to follow the CR spectrum much higher in energy, or to obtain the detailed form of the spectrum with small statistical errors. It is possible that only a few events above 1x1021 eV (ZeV) will be recorded in 10 years of operation with both Auger observatories. Only 20 events are expected, even if the AGASA flux is right. At least an order of magnitude more statistics is desirable. Future Space-Based Observations: John Linsley first suggested that the Earth’s atmosphere at night, viewed from space, constitutes a huge calorimeter for remotely observing EECRs. The collecting power of the night sky on the whole Earth, 4x108km2 sr, is the ultimate limit for space observatories. By comparison, ground-based observatories are reaching a practical limit at ~104 km2 sr. The next generation of EECR detectors must be space based. The EUSO instrument will be the first of this new generation. A geometrical factor of 5x105km2 sr, combined with an efficiency of 0.1, gives EUSO an effective geometry factor of 5x105 km2 sr in its nadir viewing orientation. This will be increased to 5x105 km2 sr during the last year by tilting EUSO toward the horizon. Table 1.1 shows how EUSO compares with present and future ground experiments. A single mission’s discoveries rarely answer all the outstanding scientific questions and they usually raise even more compelling ones. The EUSO mission will be followed by the U.S. Orbiting Wide-Angle Light Collector (OWL) mission, which is in the OSS midterm strategic plan. OWL is a much more ambitious project. It uses two satellites, with larger entrance pupils, in higher orbits that are tilted to allow stereo imaging of the same volume in the atmosphere. This strategy will provide redundant measurements of each EAS. EUSO is an outgrowth of OWL planning and will become the pathfinder mission for EAS measurements from space. It will teach us about interference from background light and scattering and attenuation in the atmosphere. The last year of operation will be devoted to tilted observations. The EUSO data and on-orbit experience will help determine the focus of the OWL investigation as well as its design. EUSO will provide the heritage and experience needed for NASA to confidently proceed with the OWL mission. The Need for This Investigation Present data suggest intriguing, new scientific opportunities. These data confront us with the existence of super-GZK events for which we have no understanding and intrigue us with apparent coincidences, where multiple events arrive from nearly the same direction. Events have been observed with energies so high that no known sources, either near or far, seem capable of producing them. Origin, propagation, and the fundamental science considerations indicate that systematic high-quality measurements of the energy, direction, and composition of EECRs with high statistics are likely to produce important new discoveries. Because of the very low event rate, an extremely large detector is required to make these discoveries. EUSO will provide this detector. The following paragraphs explain the compelling need for EUSO to resolve these mysteries. The Origin Question: The origin of EECRs is completely unknown. The EUSO instrument will provide the high statistics and high-quality data that will enable investigation of the nature of these particles (their elemental composition, if they are nuclei) and their arrival direction, which may hold the clues to their origin. Using EAS data, the nature of the particles can only be identified on a statistical basis by examining the distribution of the depths of shower maxima. Similarly, statistical analyses of large samples of events are required to find clues hidden in the arrival direction distributions. The GZK Cutoff: Soon after Penzias and Wilson discovered the cosmic microwave background (CMB)radiation, Greisen and, independently, Zatsepin and Kuzmin pointed out that this radiation would make the universe opaque to CRs of sufficiently high energy. For protons, this occurs when the pion production threshold is reached (~5x1019 eV). The reaction N+g-->D-->N+p,where N represents a nucleon, gives the energy attenuation factor e-x/27Mpc at 1020 eV, leading to an effective range of ~50 Mpc. This is nearby on the scale of the universe. Super-GZK protons from distant sources will lose energy and pileup at sub-GZK energies. A complicated shape for the observed EECR energy spectrum is predicted, with details reflecting the evolution of the universe as shown in figure 1.3 (foldout 1). The measured spectrum (fig. 1.2) flattens above 1019 eV. This may be due to an event pileup or to an extragalactic flux overtaking the galactic flux. No conclusion can be drawn with the present limited statistics. Additional data from present and planned experiments may permit us to discern a GZK pileup. Not all models predict a GZK cutoff. Examples include nearby source models, the Z-burst model, and models with broken Lorentz invariance (LI). The high statistics of EUSO may be required to make a sensitive search before a conclusion can be drawn about the existence of the cutoff. Clear evidence for the GZK cutoff would show that the LI is valid up to a Lorentz factor (g)~1011. Outstanding Problems and Opportunities Super-GZK Particles: Existing data show an excess of super-GZK particles. The present number of super-GZK events is too low to allow a quantitative investigation of their origin. Several explanations have been proposed. Some employ extreme values of the key parameters of known astrophysical objects. Other explanations invoke new physics and/or particular particle types that avoid the GZK cutoff. Inferences to be derived from the highest energy events in nature may span the fields of traditional astrophysics, particle and neutrino astrophysics, and possibly cosmology and fundamental physics. The following paragraphs present several proposed explanations for super-GZK events. No known astrophysical object is able to easily accelerate particles to 1020 eV. This is illustrated in figure 1.4; objects below the diagonal line cannot accelerate particles to 1020 eV by shock acceleration. The green solid line indicates the limit for protons, the dashed line is the limit for iron nuclei, both with extreme shock speeds of b=1. The upper blue line is for protons at more realistic shock speeds of b=1/300. Even if 1020 eV could be achieved, it is unclear how the accelerated particles can emerge from the dense radiation fields near the acceleration region without significant energy loss. In addition, most of the radio galaxies and AGN are at distances >100 Mpc from Earth. Acceleration, even to ZeV in these distant sources, is insufficient to explain the observed events. Super-GZK particles may come from numerous nearby sources of an unknown kind. Analysis of the angular distribution in EUSO’s high statistics data may reveal point sources that can be identified with the objects responsible for EECRs or anisotropy pointing to the source regions. Pulsars or magnetars, dead quasars, long-lived shocks due to galaxy collisions or the formation of galaxy clusters, and even rare and recent nearby GRBs have been suggested, but these too have theoretical difficulties. It has also been suggested that EECRs are iron nuclei that survive the GZK cutoff and photodisintegration to somewhat higher energies. It is possible that all the EECRs originate from a single nearby source. The radio-loud quasar M87, at 20 Mpc, and Cen-A, at 3.4 Mpc, have been sug-gested, but only if the intervening magnetic field is unexpectedly large, and the small-angle clustering data are discounted. A more speculative class of explanations follows: The first is the so-called top down model, where EECRs are provided by the decay of supermassive particles (SMPs) or topological defects (TDs) that were produced in an early-universe phase transition or in the inflationary stage of the universe. The decay of these massive objects to fermion-antifermion jets is expected to produce copious numbers of photons, neutrinos, leptons, and a smaller fraction of nucleons; thereby providing a useful signature. Angular distributions from EUSO measurements should reveal SMP clustering. If the SMPs cluster in our galactic halo, the propagation problem is solved. If there are no sources within 50 Mpc, the EECRs must come from distant sources. Among the known particles, only neutrinos can propagate unimpeded to Earth from distant acceleration sites at super-GZK energies. Exploiting this fact is the Z-burst model. In the Z-burst hypothesis, neutrinos from distant sources annihilate with the cosmic neutrino background (CNB) nearby to create a flux of nucleons and photons above EGZK. The standard model (SM) of particle physics and the local enhancement of the CNB yield a probability of 1 percent for a neutrino, with resonant energy to annihilate to a Z-burst as it crosses the local cluster of galaxies. The Z-burst energy is EvR=Mz2/ 2mv=(4 eV/mv)x1021 eV for a neutrino mass, mv, in the range of 0.1-2.0 eV. Each burst produces, on average, 2 baryons and 20 photons with energies exceeding EGZK. This hypothesis is consistent with the observed large-scale isotropy and small-scale clustering of events on the celestial sky. It requires a large neutrino flux that would be directly observable with the EUSO instrument, whose measurements of composition and angular distribution could also provide evidence for this model. If Z-bursts are found to be the explanation for the EECRs, they would allow us to actually make the first measurements of the relic-neutrino density liberated only a second after the Big Bang. The arrival of super-GZK protons from distant sources in the universe can also be explained by the breakdown of LI. A breakdown at extreme Lorentz factors (gs) has been proposed in several theoretical works. It would cause an absence of photo-pion production above EGZK and possibly a suppression of neutron decay. With a sample of proton events at ZeV energies, the sensitivity of the EUSO instrument for LI verification extends to gp~1012. The EUSO instrument will measure a few thousand super-GZK events with an angular resolution <2° and the ability to discern the nature of these events. With these data we should be able to discriminate among the postulated models. This places the potential for new discoveries within the reach of EUSO. Neutrino Astronomy: Astronomy, at the highest energies, must ultimately be performed by neutrinos because the universe is transparent to no other known radiation. Detection of astrophysical neutrinos demands an extraordinarily large volume. EUSO will significantly increase the target volume compared with ground-based detectors, enabling exploration of the neutrino universe. The effects of the CMB on potential astronomical channels are summarized in table 1.2. According to the SM of electroweak and quantum chromodynamic (QCD) interactions, the neutrino-nucleon cross section increases indefinitely with energy. Quantum chromodynamic theory predicts svN  ELAB0.4+0.1 above 10 TeV, yielding an ~0.1 microbarn cross sectionat 1020 eV. This extrapolated cross section is uncertain by a factor of a few. However, we expect it to be large enough for cosmic neutrinos to produce observable numbers of atmospheric showers in the 1013 tons of atmosphere observed by EUSO. If the predicted flux of GZK neutrinos is present, the EUSO mission is expected to observe a few events from interactions in the Earth’s atmosphere. If other predictions are correct, even more neutrino events will be observed. The observation of an EECR neutrino, in coincidence with a GRB or AGN flare, would imply that these sources are accelerating hadrons. EUSO has the possibility to detect tau neutrinos (nt) at even much lower energies by measuring the Cherenkov light from the upward showers they produce. Tau neutrinos interact near the Earth’s surface after penetrating the whole Earth (fig. 1.5, foldout 1) and produce ts that exit the Earth and decay in the atmosphere. While electron neutrinos (ue) and mu neutrinos (um) are fully absorbed by the Earth at energies >1014 eV, a t will regenerate through the Earth. Regeneration is an inevitable consequence of the repeated charged current processes nt+N-->t+X and t-->nt+X. The end result is to produce an emerging upward shower of energy, 1015-1018 eV. Above the energy threshold of ~1x1015 eV, the EUSO instrument can detect collimated beams of Cherenkov light emitted in a narrow cone by these upward showers. Figure 1.6 (foldout 1) compares two predicted ut spectra with EUSO’s energy-dependent threshold for detecting 10 uts per year. The highly collimated light beam from upward-going showers is well adapted to the search for point sources of neutrinos (e.g., from the galactic center or AGNs). Observing uts coming from an AGN would be evidence that the AGN accelerates protons to energies above 1015 eV and that cosmological um<-->ut oscillations occur (where u m is a mu neutrino). Like the horizontal EECR neutrinos, upward u ts, if they can be observed, offer a window on the high-energy neutrino universe. Earth-skimming neutrinos are another class of neutrino-initiated showers that have been recently discussed. These neutrinos graze the Earth and travel through a small column density of Earth in which they interact. The resulting shower emerges into the atmosphere. The rate of such Earth-skimming events actually grows with a decreasing cross section, as 1/s uN. If EUSO, viewing the horizon, can detect Earth-skimming neutrinos then EUSO can measure the neutrino-nucleon cross section from the angular dependence of the Earth-skimming rate. Such a measurement would establish the value of this cross section at high energies. High-Energy Gamma Rays and Quantum Gravity: It is expected that gamma rays in the range 1013-1020eV are attenuated by pair production on infrared, microwave, and radio backgrounds, therefore, no gammas from distant sources are expected. It has been conjectured that quantum gravity effects may make the universe transparent to such gamma rays. EUSO observations of EECR gamma rays originating from distant sources would be evidence for this effect. Some quantum gravity theories suggest that the speed of light (c) is reduced for very high-energy gamma rays. EUSO could observe this effect by a time delay between a gamma-ray burst and the arrival of the high-energy gamma rays.
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Mission Overview
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The EUSO instrument is a large, high-resolution digital camera (~2.5x105 pixels) with a 60° field of view (FOV) (fig. 1.7, foldout 1) that records nitrogen (N2) fluorescence from EAS events (fig. 1.8, foldout 1). EUSO will provide full-sky coverage, allowing a sensitive search for point sources and anisotropy over the whole sky with a single instrument. Taking into account acceptance of the fiducial volume viewed, the aperture is 5.3x105 km2 sr. The instrument will be deployed on the Columbus External Payload Facility (CEPF) (figure 1.9, foldout 1). EUSO will look down from ~400 km on a 1.7x105 km2 FOV that will be divided into 0.8x0.8 km pixels. The ISS is stabilized to ±2° with knowledge to ±0.1°. EAS images will be recognized by the trigger electronics, and the luminosity along the EAS track will be recorded. The EUSO instrument will observe EAS events only in the night sky, over the dark Earth, and under low-moonlight conditions (fig. 1.8 and fig. 1.10, foldout 1). We believe the likely duty cycle will be 0.1-0.15 and we have used 0.1 to estimate event rates in this proposal, which is more than adequate to meet our scientific objectives. The useful observing duty cycle is estimated by taking into account the ISS orbit, other natural and manmade background light (fig. 1.11, foldout 1), the lunar cycle, clouds, other atmospheric conditions, and interfering ISS activities. The estimated fraction of time that acceptable atmospheric conditions will prevail will be dependent on the effectiveness of our atmospheric monitoring data to correct for atmospheric conditions. EUSO data will be recorded on the ISS, transmitted to the ground, and provided to the EUSO SDC on a telescience resources kit (TReK) workstation via the Internet. The SDC will process the raw data. Details of the products from the production data analysis will be worked out during the phase-A study in Europe. Both raw and processed data will be archived at the SDC and made available to the entire collaboration for analysis and interpretation via the Internet. For the last year of its mission, EUSO will be repositioned on CEPF and tilted at ~40° to the nadir. The oblique view in this mode covers the area-- S = p(H tanq0) x {R x [p/2-arcsin[R/R+H]] - H tan(q-q0)} = 1.55x106km2 where q is the angle to the horizon (<70°) and q0 is the angle to the instrument FOV (60°). R denotes the Earth’s radius of 6,400 km and H denotes the orbital altitude of ~400 km. The viewed area in the tilted mode is as much as 9.2 times larger for a total aperture of 4.9x106 km2 sr. The energy threshold will be much higher with the instrument tilted, and the spatial resolution will degrade the far end of the FOV. In phase A, we plan to simulate the tilted mode in order to understand these effects. Experience with tilted-mode operation will benefit the future OWL mission. Experimental Approach Primary CRs and neutrinos, interacting with atmospheric nuclei, produce a propagating cascade of secondary particles (fig. 1.10, foldout 1), an EAS. These relativistic particles excite atmospheric N2 and produce a fluorescence signal that develops in the lowest 25 km of the atmosphere and ends in a Cherenkov flash (fig. 1.10 and fig. 1.12, foldout 1). Nitrogen fluorescence has a broad spectrum, but the signal-to-noise ratio is most favorable in the 330-400-nm band (fig. 1.13, foldout 1). The fluorescence yield for this band is 4.2 photons per electron per meter in air. It is practically independent of altitude below 20 km. Above 20 km it gradually increases with altitude (measured in g/cm2). Electrons are the most numerous particles in an EAS. Their fluorescence at the maximum of the shower’s development and the total fluorescence of the EAS are proportional to the energy of the primary. Practically all the particles in the EAS are relativistic and their fluorescence is isotropic. Because the N2 molecules fluoresce quickly after excitation, the EAS is observed as a thin luminous disk ~100 m in diameter, streaking through the atmosphere at c. In addition, forward collimated Cherenkov light scatters from the Earth, giving a flash that indicates the precise EAS landing point and timing (fig. 1.10 and fig. 1.12, foldout 1). The fundamental formulae for the observable signal strength are well known as a function of energy, calorimetric material, depth, and radius. They are summarized in figures 1.14 and 1.15 (foldout 2). The fine pixelization in the EUSO focal plane signi-ficantly reduces the background per pixel. Taking the fluorescence yield from figure 1.14 (foldout 2) and the background from figure 1.11 (foldout 1), we see that the EUSO instrument is signal limited at a detection threshold of E~3x1019 eV. Uncertainty in the background does not alter the efficiency of the EUSO instrument trigger. The EUSO instrument will also detect upward directed showers by uts at E>1015 eV because the t generated in the last neutrino interaction emerges from the Earth and decays in the atmosphere. The resulting Cherenkov light is collimated within 1.3° and the intensity in the wavelength range of 330-400 nm is very high, ~30 photoelectrons/pixel (at E~1015 eV). This low threshold is extremely useful for the observation of uts. Because the Cherenkov light is relatively uniform across the upward cone, the light falling inside the entrance pupil of the EUSO instrument allows us to estimate the energy. Figure 1.6 (foldout 1) shows the energy dependence of EUSO’s sensitivity for uts.
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Additional Information |
For any more information about the EUSO mission, EUSO USA's main point of contact is Dr. Carl Pennypacker. Carl leads our education and public outreach effort in cooperation with his successful educational program Hands-On Universe. An e-mail list of collaborators is also on the Team Directory on our website.
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