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Its theoretical studies include dark energy, dark matter, inflation, cosmic neutrinos, classical and quantum gravity, black hole physics, etc. Experimentalists with a background in CMB data analysis or laser-plasmascience and technology are especially encouraged to apply. These positions are available on September 1, Search for:. News The Leung Center for Cosmology and Particle Astrophysics LeCosPA of National Taiwan University is pleased to announce the availability of several Post-Doctoral Fellow or Assistant Fellow positions in theoretical and experimental cosmology and particle astrophysics, depending on the seniority and qualification of the candidate.

It is natural to want to extend observations of. Thus, at the upper end of the electromagnetic spectrum, space-borne instruments have been used to observe gamma rays up to about 10 9 eV. Higher energy observations are difficult because of rapidly falling fluxes. The Gamma Ray Observatory, GRO, which will soon be launched, will extend space-borne observations to their practical limit, a few 10 10 eV. At about 10 11 eV, gamma rays begin to produce showers in the earth's atmosphere which can be detected by ground-based detectors.

In addition, it is possible to build detectors on earth that have large collection areas and good sensitivity for very low fluxes. Attempts to observe astrophysical objects by detection of air showers have been made for more than twenty years. For the most part, these efforts have been an offshoot of cosmic ray studies.

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In the energy range 10 11 - 10 13 eV, ground-based optical detectors can observe the Cerenkov light produced by the shower in the upper atmosphere. In the U. However, both techniques, the atmospheric Cerenkov technique, and the air shower technique, have a problem with severe backgrounds from ordinary cosmic rays, which arrive isotropically. The ordinary extensive air showers are initiated in the upper atmosphere by protons and heavier nuclei, but the air showers they produce are not easily distinguishable from showers produced by the gamma rays being sought.

The technique which can separate the signal from the background has primarily been the identification of an excess of showers from a given source direction. If the source is identified to have a well-established periodic modulation in another wavelength region, one can search for the same periodicity in the gamma ray signal. Other methods to suppress cosmic ray background depend on the technique. For the atmospheric Cerenkov technique, the Cerenkov light is imaged on the focal plane of a mirror which follows the source.

The Cerenkov image is quite different for a gamma ray shower from that of a hadronic shower.

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For the air shower technique, one exploits the fact that a gamma ray shower is expected to have at least twenty times fewer muons accompanying it than a proton-induced shower of similar size. The separation techniques are statistical in nature and encounter all the dangers of statistical fluctuations when the signals are weak. Only recently has a steady flow of gamma rays in the energy range 0. Figure 4 shows a view of the telescope.

The imaging algorithms used improved the signal to background ratio by a factor of Both results showed an excess of showers from the source as well as a periodicity that was characteristic of the source. In the case of Hercules X-l, the period is slightly different from that of X-rays. The most curious aspect of both observations was the fact that the muons accompanying the shower were not reduced as would be expected for a gamma-induced shower. Imaging of the Cerenkov light from lower energy showers coming apparently from Hercules X-1 and showing the same slightly displaced period, leads to the same puzzling conclusion that showers seem hadronic.

Such observations are inconsistent with the expectations of particle physics for T rays and, if confirmed, would require the existence of new, light, and strongly interacting particles or of dramatic new interaction thresholds. Furthermore, these results imply new compact acceleration mechanisms at the source.

Particle Astrophysics and Cosmology - Theory

While radiation at 10 12 eV from the Crab nebula can be understood with conventional astrophysical mechanisms, radiation at 10 14 eV from compact objects, if it is gamma radiation, must come from neutral pions produced at the source. Given the exciting consequences which follow from the air shower observations, there are many groups throughout the world which are engaged in the search for point sources emitting radiation at energies greater than 10 14 eV.

So far there have been no compelling confirmations of the Cygnus X-3 or Hercules X-1 results, a consequence, perhaps, of source variability. By the end of an array built by the Universities of Utah, Michigan, and Chicago will be operating with detectors covering 2. These two installations are located at similar longitudes so that even short bursts of an object will observable by both detectors. Whether a genuine astronomy will develop at energies greater than 10 14 eV depends on confirmation of the results discussed above. Detectors of sufficient sensitivity exist and it is a matter of time and patience before it will be known whether new instruments are required.

Figure 5 The Cygnus extensive air shower detectors. Several counters of the expanded array can be seen in the immediate foreground. On the other hand, astronomy at 10 12 eV is truly beginning. At present, a second 10m telescope GRANITE is being constructed at the Whipple Observatory, which will greatly improve the sensitivity so that sources other than the Crab are likely to be seen. There are new technologies using large coverage or tracking detectors whereby the threshold of the air shower technique may be lowered to nearly overlap the atmospheric Cerenkov technique, especially if the detector is located at high altitude.

One of these techniques involves the instrumentation of more than 10 4 m 2 of a clear lake with photomultipliers so that the observation of an entire air shower by Cerenkov light in the water is possible. The advantage of such a method is that its operation is not restricted to dark moonless nights. It should be remarked that the American teams are currently mostly involved in arrays located in the northern hemisphere.

Since most of the galaxy is visible only at southern latitude, it is likely that the need for southern instruments will arise. Because of its altitude and the fact that the same patch of the sky is always visible, the South pole may be particularly interesting. High energy neutrinos may provide another window onto acceleration mechanisms in the astrophysical environment, and if observed from point sources, would give a clear proof that hadronic processes play an important role. Such neutrinos are best detected by their production of upward-going muons.

The neutrinos that pass through the earth may produce muons in the ground just below a large detector. One possibility is a water Cerenkov detector installed at the bottom of the ocean. Such experiments will have negligible background but the signal may be very small. The Dumand experiment recently approved by DOE represents a first attempt at exploring this virgin territory at a sensitivity sufficient to observe a few neutrinos from Cygnus X-3 if the continuous gamma fluxes are at the level reported by Kiel.

Alternative techniques using the lake Cerenkov concept or the polar ice as a Cerenkov light or microwave radiating medium have been proposed. If the existence of point sources radiating 10 14 eV gamma rays is confirmed, then this effort will become even more interesting. If a compact source produces neutral pions, it must produce charged pions as well. Decays of these pions will produce a powerful source of neutrinos. Depending on the details of the source, the flux of these neutrinos can greatly exceed the gamma ray flux since the neutrino absorption cross section is so small.

Observation of both neutrinos and gamma rays from a source would provide important information about the mechanism of the compact accelerator. Cosmic rays which can be observed near Earth cover a large range of energies, from the MeV region to 10 20 eV, and comprise the nuclei of all known elements, as well as electrons, positrons, and antiprotons. The study of this tenuous plasma of relativistic particles addresses scientific questions closely related to the themes discussed above: high energy particle acceleration, stellar and galactic astrophysics, and cosmology and particle physics.

However, contrary to the observational techniques reviewed thus far, cosmic ray measurements up to energies around 10 14 eV should be conducted in space or on high altitude balloons. It is for that reason that cosmic ray research in space is summarized separately in the following. We conclude the discussion with an account of the present status of air shower measurements from the ground which cover the range 10 14 20 eV. Particle acceleration is perhaps the most immediate scientific theme of cosmic ray studies. It is ubiquitous in nature, occurring in solar and stellar flares, interplanetary and interstellar shocks, pulsars, supernova explosions, and perhaps in shocks on the scale of entire galaxies.

A major goal is the understanding of these cosmic accelerators and the determination of their energy sources and the physical mechanisms by which they are able to efficiently extract a small number of highly energetic particles from a nearly thermal distribution. The main observational tools available are studies of the energy spectra of the individual cosmic ray components over a wide energy range.

The persistence of the power-law spectra of cosmic ray primaries over a large energy range lends support to the assumption that first-order Fermi acceleration in supernova-driven shocks acts as the prevailing acceleration mechanism. The spectral measurements also provided indications that during their propagation through the galaxy, the cosmic rays become enriched in heavy elements relative to protons at energies above approximatively 10 TeV.

Significant progress in understanding the mechanism by which particles are selected for acceleration resulted from the discovery that cosmic rays undergo fractionation which is strongly correlated with the particle first ionization potentials. The great. The origin and evolution of matter in the Galaxy is the major theme addressed by studies of the elemental and isotopic composition of cosmic rays.

Measurements on balloons, followed by more precise data from spacecraft, have led to the first high resolution observations of the isotopic composition of heavy primary cosmic ray nuclei, and to the discovery of excesses of the neutron rich isotopes of Ne, Mg, and possibly Si relative to solar system composition , indicating differences in the nucleosynthesis history of these two samples of matter. The first reliable measurements of abundances of ultraheavy nuclei above the iron group demonstrated that both slow and rapid neutron capture nucleosynthesis contribute to these elements, and that the heaviest nuclei are not produced solely by recent explosive nucleosynthesis as might occur if cosmic rays were both synthesized and accelerated in a supemova.

Figure 6 Heavy element abundances derived from energetic particle measurements are plotted relative to abundances in the solar photosphere with the Si ratio defined to be 1 , as a function of the element's first ionization potential FIP. Panel a shows the composition of the galactic cosmic ray source, while panel b shows the composition of the solar corona, as derived from measurements of solar energetic particles SEP.

Observational tests of cosmology constitute another active area for cosmic ray investigations. For instance, the determination of the contribution of cosmic ray-induced spallation reactions to the production of light isotopes, particularly 7 Li, is essential for deriving the cosmogenic yields of these species and establishing the baryonic density in the universe.

In the search for antiparticles in the cosmic rays, recent balloon measurements have placed stringent limits on possible contributions of annihilations of supersymmetric dark matter particles to cosmic ray antiprotons at low energies. However, in the GeV region, there are indications of fluxes of p's and positrons that are larger than those expected to arise from interstellar nuclear interactions of cosmic rays.

It remains. Particle acceleration can be studied in the heliosphere in much more detail than in more remote regions of the galaxy, and will provide invaluable tests for theoretical models that are applicable at larger scales and higher energies. In order to establish the role of low energy cosmic rays in the heating and ionization of the interstellar medium, in situ measurement of these particles will be required in local interstellar space, as pointed out in the Astronomy and Astrophysics Survey Report Field Report. An "Interstellar Probe" mission to carry out such measurements remains a high priority objective in this field, but will not be discussed in detail here since it falls outside the scope of the present study.

This brief discussion illustrates the importance of cosmic ray observations in space for the understanding of a variety of astrophysical phenomena relevant to fields ranging from cosmology to radio and gamma ray astronomy. In the coming decade, major advances are expected in a number of areas:.

Antimatter studies. High precision observations of antiprotons and positrons over a wide range of energies with magnetic spectrometers on balloons and in Space "Astromag" should make it possible to determine conclusively whether there is a significant contribution of these particles from a source other than interstellar interactions of cosmic ray protons. In addition, the sensitivity of searches for heavy antinuclei should be improved by 2 to 3 orders of magnitude.

Isotopic composition studies. The abundances of essentially all stable and long lived isotopes of elements with should be measured, and exploratory isotope observations should be extended up to.

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These data should clarify the pattern of isotopic anomalies, making it possible to determine the dominant nucleosynthesis process contributing to the production of cosmic ray source material. The observations made in particular with the Advanced Composition Explorer should establish the time between nucleosynthesis and acceleration, using the abundances of primary electron capture nuclides such as 57 Co. With the Astromag magnetic spectrometer, isotopic separation will be extended to GeV energies, making it possible, for instance, to use radioactive "clock" isotopes in a regime where their half lives are increased by relativistic time dilation.

Ultraheavy element studies. These will make possible studies of neutron capture nucleosynthesis and determination of the relative importance of steady state and explosive processes in the production of cosmic ray source material. Studies of high energy spectra and composition.

Direct measurements of the spectra of major elements at high energies can be achieved through exposure of large instruments in space. Such measurements will not only test models for particle acceleration at high energies, but will also provide an essential overlap and calibration for the indirect measurements by ground based air shower arrays that provide information up to the highest particle energies known in nature.

This could be accomplished by a one-year flight of the transition radiation detector system that was successfully tested on the shuttle Spacelab-2 in , or by new instrumentation to be developed for attachment to the Space Station. Studies of interaction with the solar system.

A number of space missions selected for flight in the 's will measure the nuclear composition and atomic charge states of solar energetic particles and anomalous cosmic rays. These should significantly improve our knowledge of the composition of the solar surface, and should conclusively establish the origin of the anomalous component. Information on the three dimensional structure of the heliosphere and its role in the modulation of galactic cosmic rays will be greatly improved through particle observations at high heliographic latitude by the Ulysses mission, and by ongoing studies using the Pioneer and Voyager space probes in the outer heliosphere.

Investigation of differences in the modulation of particles of opposite charge sign, particularly electrons and positrons, will also contribute to understanding the physical processes responsible for the solar modulation of cosmic rays. The highest energy region is particularly interesting. The mere existence of cosmic rays up to 10 20 eV is rather surprising and there is at present no understanding of a process which could accelerate or produce directly particles of such energy.

Moreover, at energies greater than 10 19 eV there are sufficiently distinctive phenomena which should permit the identification of the source of the radiation galactic or extragalactic and the identity of the radiation, protons, heavy nuclei, or gamma rays.

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The radius of curvature of a proton of energy greater than 10 19 eV in the galactic magnetic field is larger than 10 Kpc, which is comparable to the size of galactic disk. Heavier nuclei will have correspondingly smaller radii of curvature. In addition, if the cosmic rays at this energy consist of extragalactic protons, their spectrum should have a sharp cutoff at 10 20 eV because of photon-pion production on the 2. Below this cutoff, the spectrum is expected to flatten because of the pile-up of particles produced in these interactions.

The Fly's Eye experiment of the University of Utah has collected the largest number of events with energy greater than 10 19 eV. Their technique of observing the cosmic rays by their fluorescence in the atmosphere allows many details of the induced shower to be observed. The direction, energy, and longitudinal development of the shower can be measured.

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  • The longitudinal development permits incident gamma rays, protons, and heavier nuclei to be distinguished on a statistical basis. The energy spectrum of the upper end of the spectrum is shown in Fig. It suggests that there is a flattening of the spectrum above 10 19 eV and a possible cutoff based on the absence of events at energy higher than 10 20 eV. These cosmic rays arrive isotropically and the longitudinal development suggests that they are protons.

    Thus, there is some evidence that these cosmic rays come from outside our galaxy. Other groups using conventional extensive air shower techniques dispute these findings but their sample is also small and their energy resolution worse. This instrument would have much finer resolution for the measurement of the longitudinal development of the shower. Measurement of the depth of maximum of the shower and its longitudinal extent would permit a clear separation between incident protons, heavy nuclei such as iron, and gamma rays.

    Thus, by a combination of the measurement of the spectrum, composition and anisotropy it would be possible to establish the galactic or extra-galactic nature of these cosmic rays. The ability to collect over several years events with energy greater than 10 19 eV would make this determination possible.

    Cosmic rays in the 10 15 19 eV region also present mysteries. They may be galactic in origin but there is no established galactic acceleration mechanisms that can produce protons with energies greater than about 10 The existing air shower arrays referred to in the gamma ray section, or the combination of surface arrays with underground detectors should be able to make crude composition measurements which will help answer these questions. As mentioned above, cross calibration with space instruments will be essential.

    One would also like to know whether the galactic cosmic rays are produced in localized compact sources, or are accelerated slowly over much larger regions in the galaxy. The observation of discrete gamma ray sources at energy greater than 10 14 eV may resolve this question, since a discrete source implies the production of neutral pions by considerably higher energy primary particles. From the discussion above, we can extract the most likely themes in the next decade:. The nature of dark matter and the possibility that it is made of nonbaryonic particles produced in the early universe.

    More generally, the imprints left by physics at ultrahigh energy on the universe are fundamental to the understanding of both cosmology and particle physics. The solar neutrino problem and the use of neutrinos to understand supernovae. In the process, stellar models may be refined and the intrinsic properties of neutrinos will also be better delineated.

    The nature of particle acceleration mechanisms, especially if gamma rays are indeed observed as high as 10 14 eV. Confirmation of a muon anomaly would require major revisions of particle physics.

    In any case, detection of gamma rays in the 10 12 eV region will complement observations at longer wave length. The origin of cosmic rays, their composition, the nature of their acceleration mechanisms and the possibility that they may partially be extragalactic. We can be assured that other concepts and fascinating questions will be generated along the way, but these problems are useful tools to define the highest scientific priorities. The highest priority is to implement and strengthen the current program.

    As explained above, very important experiments are coming on line or have just been approved which will shed critical light on the scientific problems tackled, while small developments explore the feasibility of new techniques. It is to the credit of the funding agencies and to the community that this represents a good first approximation for a balanced program and it is critical that these experiments and developments be vigorously supported at the fastest rate feasible.


    Concurrently a strong theoretical effort should be continued. In order to expand on these points, we reiterate the fields considered above:. Although they may not yet have the required sensitivity to probe the entire available parameter space, first generation experiments attempting to detect dark matter particles axions and weakly interacting massive particles are essential to explore an already large class of models, develop the technology and master the experimental problems. For axions, the current technological development should be aggressively pursued and a very interesting next step would be to extend the sensitivity of the search to the low mass region.

    It is also important to continue the weakly interacting massive particle searches with improved ionization detectors possibly with isotopically enriched materials to improve the sensitivity to Majorana particles and eventually with the transformation of these set ups into pilot experiments using cryogenic detectors of a few tens of grams. Indirect searches for dark matter particles with existing neutrino detectors [mostly DOE] and the antiproton, positron and gamma ray observations [NASA] should also be actively pursued.

    MACRO [supported by DOE] is expected to reach an important milestone in the search for monopoles with a sensitivity below the Parker bound and to improve significantly our searches for high energy neutrinos potentially produced by annihilation of dark matter particles in the sun or the earth. More generally, its large volume and area allows this instrument to detect supernova neutrinos and high energy muons from cosmic rays.

    In the past ten years, theory has led the way in establishing the bridge between particle physics and cosmology. It is critical for this effort to be expanded at the time when experiments begin to be implemented.

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    This experiment will produce a high counting rate study of 8 B solar neutrinos, including the energy spectrum of the electron type neutrinos and a measurement of the total flux of neutrinos independent of their type. Participation of the US teams in the gallium experiments [DOE] is important, since their results will determine a portion of the overall picture of the solar neutrino problem. It is important to keep the 37 Cl experiment running over the long term, until a larger 37 Cl detector planned by the Soviet Union becomes operational.

    It is critical to organize a well coordinated supernova watch. Support of the theoretical efforts in understanding the sun and the supernova explosions are obviously an important complement of this experimental program. This region is particularly interesting, with the only unambiguous observation of a steady flux from a high energy source, the Crab nebula, and the expectation that many other sources may be observable in this energy region where the flux is expected to be larger than at higher energies. Moreover such studies will complement nicely the observations in space made by the Gamma Ray Observatory, which will be sensitive up to 10 10 eV.

    They will be critical in clarifying the observational situation in the 10 14 eV region: existence of pulsed sources and muon content anomaly. Since they are located at approximately the same longitude, the two experiments will be able to cross check each other. DUMAND II is a useful first exploration of the virgin field of high energy neutrino astrophysics and an important pilot implementation of a technology which, if successful, could be extended to a larger size, if this is required for the study of neutrino sources.

    High priority should be given to carry out expeditiously the space missions recently selected for cosmic ray investigations:. The Advanced Composition Explorer ACE will measure isotopic, elemental and charge state composition over six decades of energy 0. The Astromag facility selected for the Space Station Freedom, will be a unique facility with a large magnetic spectrometer which should allow major advances in the study of cosmic antimatter components and isotopic composition over a wide range of energies.

    The Heavy Nucleus Collector HNC , another Space Station payload, will make the first high resolution determinations of the abundances of individual ultra-heavy elements with. The POEMs experiment on the Earth Observation System, will measure spectra of electrons and positrons below 1 GeV, and determine the relative contributions of primary and secondary production. These space observations have to be complemented at the highest energy by observations from the ground:. In addition, important new information is being collected at very high energy by the Fly's Eye detector [NSF] which has been a recognized experimental success and should continue to be supported.

    The program described above is well balanced with a number of new experiments. However, the scientifically essential supernova watch requires a better coordination, and the proper exploration of the highest energy cosmic rays and the elucidation of the old puzzle of their origin requires a new instrument. In addition it is clear that the experiments currently implemented and the present technological developments will lead to major new initiatives, the character of which will greatly depend on the results obtained in the intervening time.

    In charting the future of particle astrophysics it is then essential to take into account decision points that we see occurring naturally in a few years when the following information is obtained: feasibility of cryogenic detection of dark matter particles, nature of the solar neutrino problem, existence of sources at 10 14 eV. This is summarized in table I. The two projects that we recommend for immediate action are the systematic organization of a supernova watch and the construction of the High Resolution Fly's Eye , currently proposed to the NSF, provided the technical review of the instrumentation and the institutional arrangements is favorable.

    The first project involves mainly the continuation of the present support to existing instruments at least until new ones of similar supernova detecting capabilities come on line. The cost involved in this transfer period will be minimal. In the long run,. We will come back to the institutional problems involved. The second project is technically ready to go, has a capability of collecting ten times more events than the present instrument, and a multi-university collaboration has been established. Prototype mirrors and detectors have been built. It has the capability with several years of operation to observe a Greisen cut off, to determine the extragalactic origin of the particles, and to establish whether they are protons or heavier nuclei.

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