New physics from ultrahigh energy cosmic rays

JETP Letters, 2009

We discuss the GZK horizon of protons and present a method to constrain the injection spectrum of ultrahigh energy cosmic rays (UHECRs) from supposedly identified extragalactic sources. This method can be applied even when only one or two events per source are observed and is based on the analysis of the probability for a given source to populate different energy bins, depending on the actual CR injection spectral index. In particular, we show that for a typical source density of 4 × 10 −5 Mpc −3 , a data set of 100 events above 6 × 10 19 eV allows one in 97% of all cases to distinguish a source spectrum dN/dE ∝ E −1.1 from one with E −2.7 at 95% confidence level.

New Journal of Physics, 2009

Ultrahigh energy cosmic rays that produce giant extensive showers of charged particles and photons when they interact in the Earth's atmosphere provide a unique tool to search for new physics. Of particular interest is the possibility of detecting a very small violation of Lorentz invariance such as may be related to the structure of space-time near the Planck scale of ∼ 10 −35 m. We discuss here the possible signature of Lorentz invariance violation on the spectrum of ultrahigh energy cosmic rays as compared with present observations of giant air showers. We also discuss the possibilities of using more sensitive detection techniques to improve searches for Lorentz invariance violation in the future. Using the latest data from the Pierre Auger Observatory, we derive a best fit to the LIV parameter of 3.0 +1.5 −3.0 × 10 −23 , corresponding to an upper limit of 4.5 × 10 −23 at a proton Lorentz factor of ∼ 2 × 10 11. This result has fundamental implications for quantum gravity models.

We discuss candidates for trans-GZK cosmic rays observed in a variety of detectors. Three types of primaries are represented among the abstracts submitted to this meeting: neutrin os causing a Z-burst, protons arising from the decay of ultra-heavy metastable particles and neutrinos within the framework of low scale string-like models of unification. We attempt to evaluate the relative merits of these schemes. No definite conclusion can be reached at this time. However, we point out that some schemes are more credible/predictive than others. Data to be gathered by the Pierre Auger observatories as well as orbiting detectors (OWL, Airwatch...) should be able to decide between the various schemes.

2013

We discuss the GZK horizon of protons and present a method to constrain the injection spectrum of ultrahigh energy cosmic rays (UHECRs) from supposedly identified extragalactic sources. This method can be applied even when only one or two events per source are observed and is based on the analysis of the probability for a given source to populate different energy bins, depending on the actual CR injection spectral index. In particular, we show that for a typical source density of 4 × 10 −5 Mpc −3, a data set of 100 events above 6 × 10 19 eV allows one in 97 % of all cases to distinguish a source spectrum dN/dE ∝ E −1.1 from one with E −2.7 at 95 % confidence level. PACS: 98.70.Sa Introduction—One of the main obstacles to fast progress in cosmic ray (CR) physics has been the impossibility to identify individual sources. However, there are two pieces of evidence indicating that we are at the dawn of “charged particle astronomy. ” First,

Nature Physical Science, 1971

Astroparticle Physics, 2005

Owing to their isotropy, it is generally believed that ultrahigh energy cosmic rays (UHECRs) are extragalactic in origin. It is then expected that interactions of these cosmic rays with photons of the cosmic background radiation (CBR) should produce a drastic reduction in their flux above and energy of about 5 × 10 19 eV (50 EeV), the so-called "GZK effect". At present, the existence of this effect is uncertain owing to conflicting observational data and small number statistics. We show here that a small amount of Lorentz invariance violation (LIV), which could turn off photomeson interactions of UHECRs with the CBR, could explain the UHECR spectrum as measured by AGASA which shows an excess of UHECRs at energies above 100 EeV. If new results from the Auger array agree with the AGASA spectrum, this may be interpreted as evidence for a small amount of LIV. If, on the other hand, the new results are consistent with the HiRes results favoring a GZK effect, this would place severe constraints on LIV and, by implication, on some Planck scale quantum gravity models. We also discuss the power requirements needed to explain the UHECR spectrum for a range of assumptions, including source evolution and LIV and show that in all cases our results disfavor a γ-ray burst origin for the UHECRs.

2008

This thesis is devoted to the study of phenomenological consequences of theoretical models of Quantum Gravity. In particular, this work is focused on the study of possible violations of Lorentz invariance, which may arise if, owing to quantum gravity effects, the high-energy structure of the spacetime is different from the smooth, continuous one we are used to in our low-energy world. After a brief description of the most widely known models accounting for Lorentz invariance violations, particular focus will be given to astrophysical tests of Lorentz invariance. These are motivated by the fact that some astrophysical objects are able to accelerate particles to extremely high energies, unreachable to terrestrial experiments. This consideration naturally leads us to look at the radiation of the Crab Nebula, one of the most powerful objects in our Galaxy. We first understand how the violation of Lorentz invariance affects the physical processes at the basis of the production of electromagnetic radiation by this object. Then, we compare our prediction for the Lorentz violating spectrum to observational data, exploiting the vast multi-wavelength information on the Crab Nebula radiation. Furthermore, we take advantage of the recent development of new technology to improve on our analysis of the Crab Nebula radiation by extending our research to the effects of Lorentz violation onto hard X-ray polarization. After this investigation we shall move to study the physics of cosmic rays, the most energetic particles ever experienced on Earth. Our interest in this physics is twofold: on the one hand, we want to understand more about their properties and their propagation. To this aim, we develop a new model of propagation for cosmic rays in our Galaxy, exploiting as much as possible of the multi-channel information available at present. On the other hand, according to the multi-channel perspective, we try to understand the consequences of Lorentz symmetry violation on the properties of ultra-highenergy cosmic rays. iii 4 Ultra-high-energy Cosmic Rays and LV 4.

Astronomy & Astrophysics, 2013

The Greisen-Zatsepin-Kuzmin (GZK) effect, i.e. the interaction of ultra-high-energy cosmic ray (UHECR) protons and nuclei with the intergalactic photon background, results in a drastic reduction of the number of sources contributing to the observed flux above ∼60 EeV. We study quantitatively the source statistics as a function of energy for a range of models compatible with the current data, varying source composition, injection spectrum, source density, and luminosity distribution. We also explore various realizations of the source distribution. We find that, in typical cases, the brightest source in the sky contributes more than one-fifth of the total flux above 80 EeV and about one-third of the total flux at 100 EeV. We show that typically between two and five sources contribute more than half of the UHECR flux at 100 EeV. With such low source numbers, the isolation of the few brightest sources in the sky may be possible for experiments collecting sufficient statistics at the highest energies, even in the event of relatively large particle deflections.

arXiv (Cornell University), 1998

Explanations of the origin of ultra-high energy cosmic rays are severely constrained by the Greisen-Zatsepin-Kuz'min effect, which limits their propagation over cosmological distances. We argue that possible departures from strict Lorentz invariance, too small to have been detected otherwise, can affect elementary-particle kinematics so as to suppress or forbid inelastic collisions of cosmic-ray nucleons with background photons. Thereby can the GZK cutoff be relaxed or removed.

Astroparticle Physics, 2002

It has been suggested that cosmological γ-ray bursts (GRBs) can produce the observed flux of cosmic rays at the highest energies. However, recent studies of γ-ray bursts indicate that their redshift distribution likely follows the average star formation rate of the universe and that GRBs were more numerous at high redshifts. As a consequence, we show that photomeson production energy losses suffered by ultrahigh energy cosmic rays coming from GRBs would produce too sharp a spectral energy cutoff to be consistent with the air shower data. Futhermore, we show that cosmolgical GRBs fail to supply the energy input required to account for the cosmic ray flux above 10 19 eV by a factor of 100-1000.

2002

Cosmic rays with energies beyond the Greisen-Zatsepin-Kuzmin 'cutoff' at ∼ 4 × 10 10 GeV pose a conundrum, the solution of which requires either drastic revision of our astrophysical understanding, or new physics beyond the Standard Model. Nucleons of such energies must originate within the local supercluster in order to avoid excessive energy losses through photopion production on the cosmic microwave background. However they do not point back towards possible nearby sources, e.g. the active galaxy Cen A or M87 in the Virgo cluster, so such an astrophysical origin requires intergalactic magnetic fields to be a hundred times stronger than previously believed, in order to isotropise their arrival directions. Alternatively the primaries may be high energy neutrinos, say from distant gamma-ray bursts, which annihilate on the local relic background neutrinos to create "Z-bursts". A related possibility is that the primary neutinos may initiate the observed air showers directly if their interaction cross-sections are boosted to hadronic strength through nonperturbative physics such as TeV-scale quantum gravity. Or the primaries may instead be new strongly interacting neutral particles with a longer mean free path than nucleons, coming perhaps from distant BL-Lac objects or FR-II radio galaxies. Yet another possibility is that Lorentz invariance is violated at high energies thus suppressing the energy loss processes altogether. The idea that has perhaps been studied in most detail is that such cosmic rays originate from the decays of massive relic particles ("wimpzillas") clustered as dark matter in the galactic halo. All these hypotheses will soon be critically tested by the Pierre Auger Observatory, presently under construction in Argentina, and by proposed satellite experiments such as EUSO.

Journal of Physics: Conference Series, 2008

The year 2007 has furnished us with outstanding results about the origin of the most energetic cosmic rays: a flux suppression as expected from the GZK-effect has been observed in the data of the HiRes and Auger experiments and correlations between the positions of nearby AGN and the arrival directions of trans-GZK events have been observed by the Pierre Auger Observatory. The latter finding marks the beginning of ultra high-energy cosmic ray astronomy and is considered a major breakthrough starting to shed first light onto the sources of the most extreme particles in nature. This report summarizes those observations and includes other major advances of the field, mostly presented at the 30 th International Cosmic Ray Conference held in Mérida, Mexico, in July 2007. With increasing statistics becoming available from current and even terminated experiments, systematic differences amongst different experiments and techniques can be studied in detail which is hoped to improve our understanding of experimental techniques and their limitations.

Astro2010 the Astronomy and Astrophysics Decadal Survey, 2009

A fundamental question that can be answered in the next decade is: WHAT IS THE ORIGIN OF THE HIGHEST ENERGY COSMIC PARTICLES? The discovery of the sources of the highest energy cosmic rays will reveal the workings of the most energetic astrophysical environments in the recent universe. Candidate sources range from the birth of compact objects to explosions related to gamma-ray bursts or generated around supermassive black holes in active galactic nuclei. In addition to beginning a new era of high-energy astrophysics, the study of ultra-high energy cosmic rays will constrain the structure of the Galactic and extragalactic magnetic fields. The propagation of these particles from source to Earth also probes the cosmic background radiation and gives insight into particle interactions at orders of magnitude higher energy than can be achieved in terrestrial laboratories. Next generation observatories designed to study the highest energy cosmic rays will have unprecedented sensitivity to ultra-high energy photons and neutrinos, which will further illuminate the workings of the universe at the most extreme energies. For this challenge to be met during the 2010-2020 decade, a significant increase in the integrated exposure to cosmic rays above 6 1019 eV will be necessary. The technical capabilities for answering this open question are at hand and the time is ripe for exploring Charged Particle Astronomy.

Annals of the New York Academy of Sciences, 1991

International Journal of Modern Physics A, 2005

We discuss theoretical issues and experimental data that brought the ultra high energy cosmic rays in the list of Nature's greatest puzzles. After many years of research we still do not know how astrophysical acceleration processes can reach energies exceeding 10 11 GeV. The main alternative top-down mechanism postulates the existence of super massive X-particles that create a particle spectrum extending down to the observed energy through their decay channels. The propagation of nuclei and photons from their sources to us adds to the puzzle as all particles of these energies interact with the ambient photons, mostly of the microwave background. We also describe briefly the main observational results and give some information on the new experiments that are being built and designed now.