Report to the 2005 General Assembly for 2002-2005
Cape Town, South Africa
26 – 28 October 2005
C4 is the standing international advisory committee for the International
Cosmic Ray Conference (ICRC) series. The 28th ICRC was held at Tsukuba,
Japan, 31 July - 7 August 2003. There were 760 attendees, including
160 students. The O'Ceallaigh Award was presented to Frank McDonald
(USA) for his major contributions to cosmic-ray physics over many years.
The Yodh Prize for important and pioneering contributions to the field
of cosmic-ray physics went to B.V. Sreekantan (India). Pasquale Blasi
(Italy) won the Shakti P. Duggal Award for outstanding young scientists
in the field of cosmic-ray physics.
The 29th ICRC was held in Pune, India, 3-10 August 2005. It was attended
by 580 scientists. Tom Gaisser (USA) received the O'Ceallaigh Medal.
The Yodh Prize went to Michael Hillas (UK). Jim Hinton (UK) was presented
with the Duggal Award.
The 30th ICRC is scheduled from 3 - 11 July 2007 in Mérida,
the capital of the Mexican state of Yucatán.
Web site: http://www.icrc2007.unam.mx
A unique aspect of the ICRC series is the opportunity for interdisciplinary
interactions between particle physics and cosmic-ray physics on the
one hand and between space physics and high-energy astrophysics on
the other. There have been major new results in both areas that are
driving major new endeavours (see below).
C4 met during both conferences. The Commission recommended IUPAP support
for the following topical conferences: 8 th International Workshop
on Topics on Astroparticle and Underground Physics, TAUP2003 (Seattle,
USA, 5 - 9 September 2003); 9 th International Conference on Topics
on Astroparticle and Underground Physics, TAUP2005 (Zaragoza,
Spain,11 -14 September 2005) ; the 20 th European Cosmic Ray Symposium
(Lisbon, Portugal, 5 – 8 September 2006).
C4 is also the IUPAP liaison (along with C11, C12 and C19) to IUPAP
Working Group 4, the Particle and Nuclear Astrophysics and Gravitation
International Committee (PaNAGIC), Working Group 4 of IUPAP. The PaNAGIC
report will be presented separately.
C4 maintains its homepage and newsletter COSNEWS on the IUPAP web
site. C4 also provided a chapter to ‘Physics Now’, a compilation
by C14 of reviews by scientists from various IUPAP Commissions (IUPAP-39,
The proceedings of the ICRC series can be found via the Astronomical
Data System (ADS).
Web site: http://ads.harvard.edu/
The Voyager 1 spacecraft, after more than 28 years in space and at
a distance of 94 AU from the Sun, crossed the solar-wind termination
shock and is now exploring the final layer of the solar plasma environment,
the heliosheath. The strongest evidence that Voyager 1 passed through
the termination shock was the enhanced magnetic field, as expected
in the subsonic flow in the heliosheath. Contrary to predictions, the
low-energy anomalous cosmic rays (ACR) were not observed, indicating
that their source region is remote from the location of Voyager 1.
The highest energy cosmic rays may reveal their secrets over the next
few years. These particles, at energies in excess of 10 20eV, are expected
to be extra-galactic in origin. These particles should show a “cutoff” due
to interactions with the microwave background radiation, as originally
proposed by Greisen, Zatsepin and Kuzmin (GZK). The progress of the
Piere Auger Observatory in Argentina has made a high statistics observation
of this feature possible within the next few years. The establishment
of an extra-galactic contribution to the observed cosmic rays through
observation of the GZK effect will be a major scientific result. The
absence of this effect will be puzzling indeed, since a source within
our galaxy producing these enormous energies has yet to be identified.
In the last two years the new generation of TeV (10 12 eV) gamma-ray
telescopes have begun to operate. The production of new results is
presently led by the H.E.S.S. group which has had a telescope array
operating in Namibia since 2003. In the near future the CANGAROO, MAGIC
and VERITAS groups are expected to become similarly productive as these
telescopes become fully operational. These new results have been spectacular;
for the first time images of high-energy gamma-ray sources have been
produced, including shells of supernovae explosions. If it can be demonstrated
that these gamma-rays are produced through hadronic processes, the
sources of most of the cosmic rays in our galaxy will finally be identified
through these images. The increased sensitivity of the new measurements
revealed many new TeV sources both galactic and extragalactic in nature.
Some of the sources are unknown in other wavelength regions. This field
is expected to provide many new scientific results over the next few
STATE OF THE FIELD:
One of the most important contributions of cosmic-ray physics to particle
physics in the last decades has been the discovery, using natural fluxes
of neutrinos, of the phenomenon of neutrino oscillations (flavour transitions),
i.e. the transformation of a type (or flavour) of neutrinos into another.
The neutrino flavour transitions have been observed for solar neutrinos,
produced in nuclear reactions in the centre of the Sun, and for atmospheric
neutrinos, produced in cosmic ray showers in the Earth’s atmosphere.
The results obtained from both observations are compatible with each
other and give important constraints on the neutrino masses and the
family structure of elementary particles.
The experimental study of solar neutrinos began in the late
1960’s with the Homestake detector that measured the highest
energy part of the solar neutrino flux, obtaining a flux approximately
one third of the expectations. After this result several other experiments,
most notably the water Cherenkov detector Super-Kamiokande in Japan,
the heavy water Sudbury Neutrino Observatory (SNO) experiment in Canada,
GALLEX-GNO in Italy and SAGE in Russia, also measured the flux of solar
neutrinos with greater accuracy. These experiments established that
while all solar neutrinos are created in the core of the Sun with the
electron flavour, only approximately one third of them reach the Earth
in this original state, while the remaining fraction arrives transformed
into muon and/or tau neutrinos. The long standing solar neutrino problem
no longer exists.
The experimental study of atmospheric neutrinos started in
the 1980’s with large mass underground detectors, originally
built to search for the existence of proton decay. These studies demonstrated
that approximately one half of the muon neutrinos that travel on trajectories
that cross the Earth transform into tau neutrinos.
Both results on the flavour transitions of solar and atmospheric neutrinos
have been confirmed in studies using man-made neutrinos: the solar
neutrino results by studying the propagation of anti-electron neutrinos
generated in nuclear reactors over a mean distance of 180 km with the
Japanese KamLAND experiment, and the atmospheric neutrino results by
studying the propagation of accelerator neutrinos over a distance of
250 km with the Japanese K2K experiment. Several other “long-baseline” neutrino
experiments are under construction for future more detailed studies.
It is rewarding for this field that two pioneers of neutrino detection,
Raymond Davis and Masatoshi Koshiba, were amongst the 2002 Physics
Sudbury Neutrino Observatory (SNO): http://www.sno.phy.queensu.ca/
The solar wind, a turbulent, magnetized plasma emanating from the
Sun with a velocity of 400-800 km/s, dominates the region around our
Sun out to approximately 100 AU. A global picture of the solar wind
and other activity in the heliosphere has emerged from data collected
from a variety of spacecraft, including Ulysses that pioneered the
exploration of the regions over the solar poles, and the two Voyager
spacecraft in the outer heliosphere. Various kinds of shocks driven
by solar activity, including huge coronal mass ejections, accelerate
particles to velocities orders of magnitude greater than that of the
ambient plasma. These events have been studied in situ by
spacecraft that can associate specific transient populations of energetic
particles with specific shocks. The heliosphere thus serves as a laboratory
for cosmic-ray acceleration on larger scales by distant galactic and
extra-galactic sources not accessible to direct observation.
On 16 December 2004 Voyager 1, at a distance of 94 AU from the Sun,
crossed the solar-wind termination shock and entered the solar system's
final frontier, the heliosheath. The strongest evidence that Voyager
1 has moved into the slower, denser wind beyond the weak shock is the
measured increase (a factor of two and a half) in the strength of the
magnetic field carried by the solar wind and the inferred decrease
in its speed. The energetic particle measurements revealed new surprises:
the spectrum of the anomalous cosmic rays (ACR) did not unroll at the
shock, indicating that the ACR source is a remote region of the shock
not connected to Voyager 1 along the magnetic field. Episodes of enhanced
intensities of termination shock particles (TSP) of energies below
the ACR have been observed upstream of the shock and in the heliosheath.
The future evolution of these particle populations should reveal new
aspects of the acceleration processes and their sources. With the termination
shock moving inward as the solar wind pressure declines, Voyager 2,
now at 75 AU, may also encounter the shock within a few years.
The turbulent solar wind modulates the intensity of galactic cosmic
rays (GCR) with an 11-year periodicity as they diffuse upstream against
the outward flowing wind to reach the inner heliosphere. The 11-year
cycle of solar activity is characterized by a reversal of the solar
magnetic field associated with each solar maximum. Thus there is a
22-year cycle, with alternating decade-long intervals of relatively
quiet positive and negative solar magnetic fields.
2007 will see an international program of scientific collaboration:
the International Heliophysical Year (IHY). Its goal is to advance
the understanding of the universal heliophysical processes that govern
the Sun, Earth and heliosphere as a system.
Ulysses: http://ulysses.jpl.nasa.gov/, http://ulysses.esa.int
International Heliophysical Year (IHY): http://ihy.gsfc.nasa.gov
GCR spectra, composition and antimatter :
A new generation of measurements with magnetic spectrometers has greatly
improved our knowledge of the fundamental observables in cosmic-ray
physics: the energy spectra of proton, helium and light nuclei, the
isotope abundances of light elements, including the radioactive 10Be
isotope, and the abundances of antiprotons and positrons. While the
discovery of heavy antimatter would have a major impact on our understanding
of the origin of matter in our universe, the accurate studies of cosmic
ray abundances and their variations with energy are the key science
motivations for these observations.
These measurements were to a large part performed with balloon-borne
experiments such as BESS, CAPRICE and others. A test flight of the
Alpha Magnetic Spectrometer (AMS 01) on the Space Shuttle in 1998 also
contributed to this effort. The cosmic ray spectra of protons and helium
up to some hundreds of GeVs are now known to a precision of about 10%.
The observed flux of antiprotons as measured in the energy range from
a few hundred of MeV up to 40 GeV is consistent with a secondary origin
from collisions between high energy cosmic-ray protons and nuclei
and the interstellar medium. There is yet no sign in the antiproton
data for the existence of exotic particles, such as hypothetical weakly
interacting massive particles that are believed to be dark matter candidates.
There is also no sign for primodial antideuterons and antihelium.
A series of balloon flights of the BESS experiment through 1993 to
2002 (covering nearly a solar activity cycle) provided a unique set
of observations which unambiguously
indicate a charge sign effect which can help to clarify the interplay of magnetic
drifts and diffusion in the process of solar modulation. Recently a BESS-Polar
long-duration balloon flight of 9 days was successfully performed to extend
the antiproton spectrum down to 100 MeV with better statistics.
The wealth of information contained in cosmic-ray isotope abundances
makes it possible to study aspects of acceleration, propagation and
lifetime in the interstellar medium. Based on isotope data from the
Cosmic Ray Isotope Spectrometer (CRIS) on ACE and the ISOMAX experiment
the time between nucleosynthesis and acceleration of 10 5 yrs and a
mean confinement time in the galaxy of about 15 Myrs have been determined.
Significant new data on the cosmic ray composition have also been
recently acquired in the poorly known energy interval around and above
the TeV (10 15 eV) region. The spectra and composition of cosmic rays
up to tens of TeV were measured by ATIC (Advanced Thin Interaction
Calorimeter) as well as by CREAM (Cosmic Ray Energetics And Mass) on
long-duration balloon flights around the south pole, the latter with
a recent record breaking 42-day flight. The TIGER (Trans-Iron Galactic
Element Recorder) experiment had two successful flights around the
south pole making a measurement of trans-iron elements in the GeV/amu
energy range. The TRACER (Transition Radiation Array for Cosmic Energy
Rays) had a successful balloon flight measuring spectra and composition
from oxygen to iron in the 0.5 - 10 TeV/amu range. Another major experimental
challenge is to study directly the composition of cosmic-ray nuclei
at energies above 100 TeV, where the conventional acceleration
models (based on shock acceleration in supernova remnants) fail.
Future long-duration exposure in space of the PAMELA instrument and
of the large AMS 02 on the International Space Station (ISS) will extend
the measurements of antiprotons and positrons into the region of some
hundreds of GeVs with high precision and will increase the sensitivity
to detect antimatter. PAMELA, to be flown aboard the Russian Resurs
DK-1 satellite with tentative launch in December 2005, combines a permanent
magnet with other detectors to measure primarily antiprotons and positrons
in the range 10 8-2×10 11eV and a search for antihelium with
sensitivity for antiHe/He < 10 -7. (PAMELA will also be able to
measure cosmic ray spectra of light elements up to some hundreds of
AMS 02, with a planned launch in 2008, uses a large superconducting magnet
spectrometer. Its emphasis is on the search of antimatter aiming for a sensitivity
of antiHe/He to about 10 -9. It also will measure antiprotons and positrons
with very high statistics in an energy regime comparable to PAMELA and further
improve the measurements of cosmic ray spectra of elements up to iron in the
energy range from some GeVs up to 1 TeV. AMS 02 is also capable of measuring
individual light isotopes. A good measurement of 10Be in the energy regime
from 100 MeV up to 10 GeVs as planned in AMS 02 can help to distinguish between
different propagation models.
The Japanese CALET (CALometric Electron Telescope) mission being developed
for the ISS is a large electromagnetic calorimeter primarily aimed
at detecting electrons and gamma rays into the 10 12eV energy range.
The spectrum of high-energy cosmic-ray electrons is expected to “cutoff” in
this energy range because of radiative losses in the interstellar medium.
NASA balloon program: http://www.wff.nasa.gov/~code820/index.shtml
Advanced Composition Explorer (ACE): http://www.srl.caltech.edu/ACE/
Alpha Magnetic Spectrometer (AMS): http://ams.cern.ch/AMS/ams_homepage.html
Payload for AntiMatter Exploration and Light-nuclei Astrophysics (PAMELA): http://wizard.roma2.infn.it/pamela/pam_home.htm
Above 100 TeV the flux is too low to be accessible to current direct measurements
above the atmosphere. This has long been the province of large arrays
of detectors on the ground that measure the extensive air
shower (EAS) cascades of secondary particles from the initial interaction
in the atmosphere of a high energy primary cosmic-ray nucleus. In this
situation it is a challenge to measure the total energy and even more
difficult to determine the mass of the incident nucleus. Systematic
uncertainties in the models used to interpret the data may soon become
the limiting factor. The well-instrumented KASCADE air shower array
at Karlsruhe, Germany recently succeeded in separating on a statistical
basis the spectra of several groups of nuclei in the region of the "knee" of
the spectrum above 10 15 eV. The measurements show for the first time
the pattern of spectral steepening ordered by the masses of the primary
Ultra-high energy cosmic rays:
Ultra-high energy (UHE) cosmic rays are the highest energy messengers
of the present universe. Their origin is one of the most profound mysteries
in high-energy astrophysics. Between 10 18 and 10 19 eV the composition
appears to shift to lighter elements. At the highest end of the cosmic-ray
spectrum, several different experiments report events around 10 20
eV and above. In particular the AGASA (Akeno Giant Air Shower Array)
reported about a dozen of events with energies above 10(20) eV. This
is remarkable because it had been expected that the energy spectrum
would become steeper above 5 x 10 19 eV as a consequence of energy
loss by inverse photo-pion production as UHE protons propagate through
the microwave background radiation from sources at cosmological distances.
Due to the GZK effect, there should be fewer such events. These few
UHE events have generated great excitement because their explanation
would require novel physics. Measurements from the HiRES (High Resolution
Fly’s Eye cosmic ray observatory) atmospheric fluorescence detectors
(in monocular mode) indicate smaller fluxes above 10 20 eV which appear
to be consistent with the expected steepening of the spectrum. Events
at the high energy end of the spectrum are extremely rare, and more
data are needed to resolve the problem. Measurements with Stereo HiRes
are underway. The Pierre Auger Observatory in Argentina, consisting
of giant air shower arrays, water Cherenkov detectors and fluorescence
telescopes with larger acceptance, is completed to 50% (area of 1500
km 2). A first estimate of the UHE spectrum has been presented. The
Extreme Universe Space Observatory (EUSO) is a fluorescence detector
that is envisaged to look down onto the Earth’s atmosphere with
a wide angle telescope from the ISS. Its implementation is uncertain.
High-energy gamma-rays probe the workings of active galactic nuclei,
supernova remnants, pulsar wind nebulae, gamma-ray bursts and other
energetic astrophysical objects. Gamma-ray astronomy has long been
associated with cosmic-ray physics because the gamma-rays imply the
existence of energetic particles (such as electrons or protons) from
which they are radiated. Gamma-rays are therefore also probes of potential
sources of cosmic rays.
Major discoveries about such gamma-ray sources began about twenty-five
years ago. Space-borne instruments, most notably EGRET on the Compton
Gamma Ray Observatory (CGRO), detected sources up to a GeV. Ground-based
optical telescopes to register the Cherenkov light produced by the
cascades generated when high-energy gamma-rays interact in the atmosphere,
extended the energy range into the TeV region. The imaging technique,
introduced by the Whipple Observatory group, allowed the separation
of hadronic showers due to charged particles from those due to gamma-rays.
As a result the Whipple telescope could detect the Crab Nebula with
high significance. These discoveries motivated construction of a new
generation of detectors.
The last decade has seen a remarkable technological breakthrough
with the introduction of sophisticated imaging systems on Cherenkov
telescopes, which have so improved the background rejection that
real observational astronomy is now possible in the TeV region.
As usual, when a new observational window is opened, something unexpected
was found. In this case the Whipple, and subsequently other groups,
discovered strong and rapidly varying emission from several nearby Blazars.
These are thought to be galaxies with active nuclei (powered by
accretion onto a central, super-massive black hole) where we happen
to be looking almost straight down the axis of the relativistic jet
of material emitted by the nucleus. During outbursts the power registered
in the TeV region from these objects exceeds that at all other wavelengths!
By now these observations are placing important limits on the intergalactic
optical/infrared background radiation field and on possible quantum
gravity effects on the propagation of high-energy photons.
In addition to extragalactic sources, several galactic sources have
been detected apart from the Crab Nebula. Most importantly, TeV emission
has been detected from at least eight shell-type supernova remnants
(including SN1006, RX J1713-3946, and Cassiopeia A). Both the Inverse
Compton radiation from electrons of energies up to about 100 TeV and
the neutral pion decay emission due to collisions of high energy protons
with gas atoms inside supernova remnants could be responsible for this
TeV emission. Their successful separation is expected from synchrotron
measurements at radio and X-ray wavelengths, with the hope to find
out whether supernova remnants are indeed the sources of the bulk of
the cosmic rays.
TeV astronomy has now reached a sound level of maturity. A most exiting
aspect of the recent results is the diversity of objects that are proving
to be sources of TeV gamma-ray sources; many of them were not detected
by EGRET. Most new results have come from the H.E.S.S. (High Energy
Stereoscopic System) group using an array of four imaging atmospheric
Cherenkov telescopes situated in Namibia since 2003. Together with
other telescopes (CANGAROO-III, MAGIC, VERITAS) that will soon be fully
operational, these ground-based systems will dominate the TeV observational
arena for the next decade. With the improved sensitivity of the GLAST
( Gamma Large Area Space Telescope ) the 100 MeV components of these
sources are expected to be detected.
Compton Gamma Ray Observatory: http://cossc.gsfc.nasa.g ov /cgro/
Atmospheric Cherenkov Telescopes: http://icrhp9.icrr.u-tokyo.ac.jp/c-experiments.html
Fred Lawrence Whipple Observatory: http://www.cfa.harvard.edu/flwo/
Very Energetic Radiation Imaging Telescope Array System (VERITAS): http://veritas.sao.arizona.edu/
A qualitatively different probe of potential cosmic-ray sources will
be provided by neutrino telescopes capable of detecting high-energy
neutrinos from deep inside cosmic accelerators. Whereas photons are
radiated prolifically by electrons, as well as from decays of neutral
pions, observation of neutrinos would require the presence of higher
energy protons to produce the charged pions from which neutrinos originate.
There are currently two working neutrino telescopes, the Baikal detector
in Lake Baikal and the AMANDA detector in Antarctica. Both have detected
upward-moving muons produced by atmospheric neutrinos that have penetrated
the Earth. What is measured is the Cherenkov light generated as the
muon passes through the detector. These measurements provide a proof
of principle that both clear water and clear ice are feasible as the
detection medium for neutrino astronomy. Estimates of signals that
might be expected from sources such as gamma-ray bursts, flares of
active galaxies and other cosmic accelerators, show that larger, kilometer-scale
detectors are needed to have a good expectation of seeing a signal.
Two observatories (km 3 size) will be necessary to see the two hemispheres
and to fully exploit the complementarity of water and ice as detecting
media. Major efforts are underway to reach the kilometer scale, both
in Antarctic ice (IceCube) and in the Northern hemisphere. Joining
of the European efforts (NESTOR in Greece, Antares in France and NEMO
in Italy) aims towards the construction of a single kilometer-cube
neutrino detector (KM3NeT) in the Mediterranean Sea. More details are
described in the PANAGIC report. New techniques (radio, acoustic, and
EAS measurements) are also being developed to detect the highest energy
neutrinos (E above 10 17 eV). The ANITA balloon experiment is planned
for long-duration flights in Antarctica to detect the radio emission
of UHE neutrinos interacting in ice.
For links to neutrino telescope projects see: http://neutrinooscillation.org/neutrino_telescopes.html