Physics issues with Auger

If any opinion is expressed in the following, it only shows some personal prejudice within a somewhat controversial field. Some links forward you to the bibliographic lists (many downloadable preprints or references to easy-to-find articles). The references given as "GAP-yy-nnn" are GAP (Giant Array Project) notes which you can access through the Fermilab Page (Technical note Index).

To be read as a very profitable introduction to the field: Origin and Propagation of Extremely High Energy Cosmic Rays by P.Bhattacharjee and G.Sigl, Physics Reports 327 (2000) 109-247. An electronic preprint version is astro-ph/9811011.

Location of the sources of the ultra-high energy cosmic rays (UHECR)

The highest energy cosmic rays detected by past and present experiments are known to come from nearby sources (less than 100 Mpc or so) and to be weakly deflected by the galactic and intergalactic magnetic fields (unless they are heavy nuclei, an unlikely hypothesis because of photodisintegration processes as they propagate through the 3K microwave background). Therefore their reconstructed direction should point, within a few degrees at most, toward the sources where they were produced. Such remarkable astrophysical sources are very few in our neighbourhood. The existing data does not show any strong indication on the existence of such sources detected by other means (optical, radio- or gamma astronomy etc...). This is one of the major puzzles concerning the origin of the UHECR. Some indication was found (see Stanev et al (1995) in the Bibliography) correlating the direction of the cosmic rays above the GZK cutoff and the supergalactic plane (a region of the sky with a high concentration of radio-galaxies) but the evidence is far from being convincing and even controversial: see Hayashida et al (1996) and Uchihori et al (2000) in the Bibliography.

The Auger Observatory with its full sky coverage (hence the need for sites in both hemispheres), its very good angular resolution (down to a fraction of a degree for the hybrid events) and its high statistics should be able to locate point sources (if they exist), decide if there is isotropy in the incoming direction or find correlations with any dense mass distribution in the neighbourhood of our galaxy.

The idea that one can locate the position of the astrophysical sources by proton astronomy is challenged by some authors who argue that the inter-galactic magnetic fields have much larger intensities than what is usually admitted (i.e. in the microgauss rather than nanogauss range). Such challenges are also challenged.

Understanding the production mechanisms of the UHECR

There are many acceleration mechanisms in the universe but they can be roughly grouped in two categories. The first is the bottom-up processes where a particle is accelerated from lower to higher energies either progressively (e.g. the Fermi mechanism) or in a one-shot process (e.g. by a local, high-gradient, electric field). The second is the top-down processes where the particles are produced with an initial ultra-high energy (e.g. through the decay of a super-heavy GUT particle). The latter process is reviewed in the "Topological Defect" paragraph.

When a cosmic ray is detected in the 10²⁰ eV range, and given the energy losses either at the source or during propagation, it has to be accelerated at much higher energies (say, in the ZeV - Z for zetta or 10²¹ - range for the highest energies observed). What makes the Auger physics particularly exciting is that the conventional astrophysical theories are unable, within our present knowledge, to propose a fully convincing model where such gigantic (macroscopic) energies can be reached through any mechanism. Therefore there is no doubt that whatever the outcome, the interpretation of the observations, when enough statistics will be accumulated, will open windows on new physics. Either we'll discover new, not yet understood, astrophysical processes or we'll confirm the TD theories and enter a new era of particle physics with the first direct observation of GUT physics.

The signature of a given accelerating process is mainly the shape of the energy spectrum (hence the need for the highest possible statistics) and the nature of the detected cosmic rays (which gives an indication of the medium where the accelerating mechanism operates). This is why Auger aims for high statistics and deploys unprecented efforts for particle identification (in particular by the hybrid detection of a sub-sample of events).

It is very difficult for a neutral observer to decide if there is any favoured mechanisms among the tens proposed every year, since this seems very strongly dependent on personal prejudice. One can however indicate a few recently explored directions. Hot spots of powerful radio-galaxy lobes is one of the well explored mechanisms. This is the region where the extremity of a jet emitted by the central engine of an AGN interacts with the intergalactic medium. The basic process is the first order Fermi mechanism. Does not explain why no such object is visible in the direction of the observed events (see above for an attempt to circumvent this objection).

Another recent suggestion is the acceleration by the magnetic winds generated by rapidly rotating young neutron stars. The primary cosmic rays would then be heavy nuclei (iron) and the sources located inside our galaxy. Both hypotheses should be easily tested by Auger.

For reviews on conventional accelerating mechanisms, see: Norman et al (1995); Cocconi, Protheroe, Halzen (1996); Medina-Tanco, Anchordoqui et al, Sigl et al (1997) and Blanford (1999) in the Bibliography.

Neutrino astronomy

The detection of neutrinos is not the main aim of the Auger experiment, but it can be considered as a very rich by-product for many reasons.

First, neither the GZK cutoff nor magnetic fields operate on neutrinos. Therefore the sources can be at cosmological distances and the reconstructed directions should point directly to the source, with the intrinsic angular precision of the detector. Second, the existence of detectable fluxes of neutrinos with energies in excess of 10¹⁸ eV is in principle one of the golden signatures of Topological Defect theories. Third, and if the detection efficiency enables us to reach the lower energies (say, 10¹⁷ eV or so), we'll be close to the highest energies accessible to the large future "Neutrino Telescope" projects and give some indications, several years before such detectors become operational with their cubic-kilometer full-size, on the validity of the models used in the design of such projects (which aim mainly to detect neutrinos produced in AGNs).

The detection of the neutrinos with Auger is based on the observation of large zenith angle showers (HAS or Horizontal Air Showers). The vertical thickness of the atmosphere is about 1000 g/cm², hence a good calorimeter. When one goes to the atmospheric thickness seen by a cosmic ray incident tangentially to the earth, this thickness increases by a factor of 36, and the atmosphere becomes the equivalent of a good beam-dump. A normal (hadronic or electromagnetic) shower with incident angles larger than 70^o or so is mostly absorbed and only its penetrating muon component reaches the ground. Whereas a neutrino interacts uniformly throughout the atmosphere. Therefore if we detect a HAS with the Auger ground array and this shower has a characteristic electromagnetic component, this is a quite strong signature of an event due to an incident neutrino. Other differences between showers due to hadrons or photons (interacting high in the atmosphere, or even outside the atmosphere for the photons of the highest energies) and to neutrinos (uniformly distributed over the atmospheric thickness) should make possible the extraction of even a weak neutrino signal from the background. The shape of the shower front is one of the strongest discriminating parameters we could use.

An interesting hypothesis is that of the Z-bursts which solves the problem of propagation without imposing that the detected UUHECRs be neutrinos. The idea is that an UHE neutrino (whatever its production mechanism) interacts with a relic (and massive) local neutrino to produce a Z⁰ whose subsequent decay into leptons and quarks is the source of the incident cosmic rays.

Ongoing studies show that, provided the fluxes are appropriate, Auger should easily detect neutrino showers of energies as low as 1 EeV, and even lower. In the relevant energy range, Auger is equivalent to tens of km³ of water. With a few events per year, we should be able to extract the signal from the hadronic background. If no signal is observed, this will put severe constraints on models where high-energy neutrino production is envisaged.

See the full UHE neutrino astronomy page in the Bibliography, in particular Capelle et al and also the GAP notes: GAP-97-49 and GAP-97-58 (Billoir), GAP-97-052 and GAP-97-046 (Bordes et al). In issue of great interest is the detection of tau-neutrinos interacting with the earth in the vicinity of the Observatory. An in-depth study on the subject can be found in articles by A.Letessier-Sevon and collaborators: astro-ph/0009444; X.Bertou et al, Astropart. Phys., 17 (2002) 183.

Proton astronomy - Magnetic Field studies

Large scale structures of magnetic fields in the universe are of the utmost cosmological importance. They are characterized by the intensity and the coherence sizes of the fields. For the astrophysicists, there is hardly any method available to have a good knowledge of those but the Faraday Rotation measures.

The Auger experiment provides a new and independent way of studying the magnetic fields in the universe, namely "charged particle astronomy". The method is new because it can be used only with the highest energy cosmic rays and those cannot be detected in enough large numbers except by a detector with the size of Auger. And it is of course independent because the Faraday rotation deals only with the detection of polarized light from distant sources, which has no relation to cosmic rays.

The only constraint for Auger to be able to bring new information on intergalactic magnetic fields (IGMF) is that the sources are point-like and the UHECR have low charges (e.g. protons, which is the most likely hypothesis on their nature). In this case, the study of the image of the sources as seen by the detector should give precise information on both the intensity and the coherence of the magnetic fields crossed by the cosmic rays between source and detection. One will have to deconvolute the effects of the (better known) galactic fields from that of the IGMF, which would be possible with enough statistics from various directions. Here again, the comparison between the data from both hemispheres should be precious for a better understanding of the galactic effects.

See e.g.: Lee et al (1995); Waxman et al (1996); Rachen, Lemoine et al, Medina-Tanco, Lampard et al (1997) in the bibliography related to propagation issues.

New estimates of the radio background radiation in the Universe

All cosmic rays (except neutrinos) interact with the radiation which fills the universe. The 3K Cosmic Microwave Background Radiation (CMBR) is quite well known: it is actually the origin of the GZK cutoff. However, other types of radiations (radio, infrared...) also exist and are much less explored. If there is an important high-energy photon component in the CRs detected by Auger (as predicted by some models) the attenuation in the 100 EeV range should be dominated by the interactions with radio waves. This would be a direct (and unique) way of exploring such a background.

See e.g.: Protheroe & Biermann.

Test of Topological Defect (TD) models

Topological defect formation currently occurs during certain types of phase transitions, even in laboratory systems (e.g. superfluid He³, see Bauerle et al and Ruutu et al -1996). Cosmological theories predict the formation and evolution of several types of TD in the universe: monopoles or monopole-antimonopole pairs, ordinary or superconducting cosmic strings, cosmic necklaces, domain walls etc...

Several of these models are invoked in the litterature as possible explanations of the UHECR - actually, they happen to be the easiest way of explaining them, provided the fluxes are proven to be appropriate, which is the main problem with such theories. Some of them are related to the GUT (decay of superheavy X-particles - see below). Others are based on the possibility that the TD themselves are the observed cosmic rays. Monopoles (Kephart et al - 1996) or vortons (Bonazzola et al - 1997) are candidates from the latter category. The main problem with such models is finding reliable predictions on the interaction of the TD with the earth's atmosphere, and hence giving a specific signature. It is likely that the strongest evidence, if any, on TD theories will come from the models where the TD collapse and produce short-lived X-particles.

See e.g: Sigl et al (1994); Protheroe et al, Kephart et al (1996) in the general Bibliography and the full Topological Defect page. See also GAP-97-069 (Masperi et al). A reference in the field is the book by Vilenkin and Shellard, Cambridge University Press (1994). A recent review article by Berezinsky, Blasi and Vilenkin narrows down the list of models likely to produce detectable fluxes to cosmic necklaces, monopolonia and relic X particles.

Tests of Grand-Unified (GU) theories

The decay of ultra-heavy particles (the so-called GUT X-particles) with masses exceeding 10²¹ eV up to 10²⁵ eV is the basis of the "top-down" mechanisms to produce the UHECR. Such particles can either be long-lived relic objects from the early periods of the universe (long-lived because of some conserved quantum number, in which case they would also be candidates for non-baryonic dark matter WIMPzillas) or particles trapped inside topological defects produced during those early periods and liberated when these latter collapse after a long time. Some scenarios predict that such particles could be concentrated inside the galactic halo. In case the decay occurs within a few tens of Mpc, a very typical signature of these theories will be the presence of UHE gammas in the primary spectrum. The shape of the energy spectrum above the GZK cutoff (a strong break in an otherwise exponentially falling spectrum) is another, quite easy to observe, signature of the top-down models. Finally, if a detectable component in neutrinos exists in the 1 to 10 EeV range or above, this should also be a strong signature of X-particle decay. Auger is probably the most performant detector for validating (or putting strong constraints on) the TD and GUT theories predictions in the cosmic ray sector.

See the Exotica page in the Bibliography, and in particular: Protheroe-Johnson, Sigl (1996); Berezinsky et al (1997); Berezinsky (1998).

Violation of the Lorentz invariance

This hypothesis brings a possible explanation to the problem of propagation (but not to that of acceleration). The basic idea is that a slight violation of the Lorentz invariance which would allow different particles to have different maximum attainable velocities (instead of the universal value of c in vacuum for them all) is not yet ruled out. If this was the case, the threshold energy for the photopion production (responsible for the GZK cutoff) could be relaxed and even cancelled (meaning the reaction would be impossible) for some levels of violation. The test of the model would come from independent experiments on the validity of the Lorentz invariance.

Relation to Gamma Ray Bursts (GRB)

Gamma ray bursts (GRB) are not yet fully understood phenomena producing a very large amount of energy over short periods (seconds to minutes). They are uniformly distributed over the sky and (probably) over cosmological distances. They are frequent (about one per day observed). Some of the GRB models predict the possibility that they may be also the sources of the UHECR, including neutrinos with energies and fluxes detectable with Auger (see Rachen-Meszaros - 1998). Many authors study the signatures expected if this hypothesis is true. Detection of the highest energy cosmic rays correlated in time with GRBs can be envisaged only if the sources are very near (which seems excluded) or the CRs are neutrinos (which seems very unlikely at the highest energies). However, the fact that such sources are point-like in space and time should produce detectable signatures even if the produced cosmic rays are charged hence arrive with huge delays with respect to low energy photons (see e.g. Lemoine et al, astro-ph/9704204 in the Bibliography).

See also: Vietri, Waxman (1995); Milgrom et al, Waxman et al, Vietri (1996); Vietri , Escobar et al (1997); Boettcher et al (1998) etc... in the Bibliography.

GAP note: GAP-97-068 (DuVernois et al).

Supersymmetry

The fact that the UHECRs may be the Lightest Supersymmetric Particle (LSP), e.g. a neutralino, gluino, gravitino etc... is envisaged because those particles may be stable or long-lived (because of the conservation of the R-parity) and produced in the decay of the superheavy X-particles (see above), hence with very large energies (see e.g. Kachelriess). The main problem is that of a specific signature. Hadronic bound states including the gluino (the so-called S⁰ particles) are expected to have interaction properties very similar to protons or photons, with quite subtle differences which seem very difficult to detect. The neutralino (a DM candidate) is expected to have neutralino-nucleon cross-sections close to that of the neutrino. Only resonant production of selectrons from neutralino-electron scattering would have a typical signature as a narrow peak depending on the selectron mass. All in all, and unless large statistics become available, an experimental signature of Supersymmetry seems hopeless, except in some very specific situations.

See e.g. Chung et al, Berezinsky-Kachelriess (1997); Farrar (1998) in the Bibliography.

Particle Physics

The particle physics issues (in the sense of a better understanding of fundamental interactions) in some of the above mentioned fields are obvious: Topological Defects and/or superheavy X-particles, which may give us information on the state of the Universe 10^-35 second after its birth, a time where all the interactions between particles were (maybe) unified into a single one (GUT). Phase transitions thought to have occurred then and there are the phenomena where condensed matter physics, particle physics and cosmology merge.

Since Auger will detect particles interacting with other particles in a range well above any energy that can be reached with a man-made machine, a legitimate question is to know whether we will be able to test models or theories on the interaction mechanisms at such energies. This should be difficult but maybe not hopeless. There are a few directions which can be explored.

The first is the neutrino sector. If the fluxes are at a detectable level (see above) and can be predicted with some precision by other means, we should be able to extract a precious information on the cross sections and check, at least, if they are compatible with an extrapolation of the standard model or if something new happens (such as neutrino-nucleon cross-sections becoming close to hadronic ones at these energies as proposed by some authors).

Another possibility is the information one can expect from the analysis of the penetrating muon tail of the horizontal showers. For such showers, the first interactions happen in diluted atmosphere. Therefore, the pion-muon decays should occur at higher energies than for vertical showers, hence closer to the primary interaction. One can, in such a situation (maybe optimistically) expect to have an almost direct information from the hadronic content of interactions at very high energies. Moreover, HAS are also and to some extent sampled in their horizontal as well as lateral developments by the ground array and although it may be quite delicate to go from very parcellar information to the overall shower properties, this is certainly a field where we expect the analysis of the data to be very exciting and challenging at the same time.

Finally, the subset of hybrid events, where simultaneously the shower tail and the shower profile will be observed, will no doubt help setting severe constraints on the interaction models used for the shower simulation.

See: Frampton et al., Burdman et al (1997); Farrar, Berezinsky (1998) in the Bibliography.