This project focuses on the most energetic particles in the Universe, the Ultra-High Energy Cosmic Rays (UHECRs). Efforts to understand their origin have been on-going for decades, and evidence of an anisotropic distribution suggesting a link to nearby active galactic nuclei has been recently revealed by the Pierre Auger Observatory (PAO). However, the measurements of the composition and arrival directions point to somewhat conflicting results. The research will combine future data of the spectrum and angular distribution to unveil the mechanism hurling these particles around the Universe. The project also is concerned with the nature of the cosmic magnetic fields. Charged particle astronomy relies on the small magnetic smearing of point sources. The origin of the large-scale magnetization in the Universe could provide a signature of the Universe at the electroweak phase transition or at the end of infation. The project spans the period in which the PAO will accumulate a data set exceeding the experiments to date by more than an order of magnitude. The PI's research will include the following: 1. Unified analysis of the directions and spectrum. The PI will extend previous analysis to incorporate the different energy spectrum resulting from sources distributed non-uniformly. 2. Cosmic magnetic fields: The compelling evidence for the existence of magnetization in galaxies, clusters and superclusters poses a theoretical problem. One explanation for the origin of these magnetic fields is thought to be the product of sphaleron decays at the electroweak phase transition which results in a non-zero helicity that could be measured using cosmic ray astronomy. The PI will improve calculations of the expected helicity, using lattice simulations, and will devise observables for its detection. The broader impact is as follows: the proposed research will add to our knowledge of the universe in which we live, and of the most energetic particles reaching the Earth. It will allow us to learn more about the extreme conditions at the acceleration sites, and the processes that took place when the excess of matter over antimatter, that eventually gave rise to our planet and the stars and galaxies surrounding us, was generated. Washington University operates a substantial science outreach program to improve teaching and learning in K-12 science and math: the work carried out as part of the project will be presented to school teachers in the context of existing courses that enhance their professional development by bringing them into contact with cutting-edge research. Many of the local school districts that participate in this outreach program have substantial populations of under-represented minorities.
Our present understanding of the composition of the Universe can be summarized as follows: the atoms, light, and all the particles that have ever been observed, or artificially created in man-made accelerators, account for less than one twentieth of the matter-energy of the Universe. Accurate measurements in the past decades have established a simple standard cosmological model, with a dark sector made of dark matter (DM) and dark energy (DE) dominating the dynamics of the Universe. This framework successfully addresses questions that have concerned cosmologists for decades: the size and age of the universe, its composition, the speed at which it is expanding, and how structure is formed. At the same time, a new set of questions constitutes the central challenge in cosmology and particle physics today: what is the nature of the dark sector? The studies funded by this project have addressed this question in a broad sense, and we highlight some of the findings below. The existence of DM is inferred through its gravitational effects, in particular through the analysis of the dynamics of both individual galaxies and clusters of galaxies. Recent surveys indicate that galaxies evolve with time, becoming less compact. One of the findings of this project is that the observed secular evolution can be explained, if DM particles decay with a lifetime larger than 52 Gyr, which is the shortest DM decaying timescale that can be reconciled with observations. We performed computer N-body simulations of the evolution of the galactic DM distribution, and suggested that precise observations of the baryonic gas fractions can provide a robust test of this scenario. Perhaps the most remarkable discovery in the field of cosmic ray astronomy in recent times is the realization that the flux of positrons reaching the Earth increases at high-energies. The results obtained by the PAMELA experiment, and recently confirmed by the AMS-02 collaboration, have been interpreted as the result of annihilation of DM particles in the galactic halo. For this explanation to be viable, however, DM particles should have somewhat unusual properties. In this project, we set out to explore whether these requirements could be met. Using both analytical estimates and statistical Monte Carlo explorations of the parameter space of the model we were able to show that the dark sector generically has to be more complex, with a whole new sector of dark particles, that might be soon unveiled at the Large Hadron Collider. Another area which has benefited from new high quality data is the study of the Cosmic Microwave Background (CMB), which provides a picture of the Universe when it was about one hundred thousand years old (in the history of the Universe, this would correspond to the first two hours in the roughly eighty year long human lifespan). By examining the minute variations in the temperature observed in different parts of the sky, scientists gain information on the composition of the Universe, the mass of the different neutrino species, and even constrain the processes that took place in the first instants after the Big-Bang. The agreement with the standard cosmological model is remarkable, but noticeable deviations exist. Another outcome of this project is that one such anomaly, a dip present in the WMAP satellite data around the first peak at the degree scale, can be traced back to the region near the north ecliptic pole. Although, the addition of the exquisitely precise Planck dataset has confirmed some of the large scale anomalies, our analysis has not yet been extended to include this newly released data. Importantly, if these variations are confirmed by the Planck satellite they cannot be explained within the standard cosmological model, and could provide a signal of new physics. This project stands at the forefront of the ongoing effort of scientists to understand the universe and the laws governing the processes taking place in nature, and to extract the maximum amount of information from future observations at terrestrial accelerators, telescopes and satellites. The findings and outcomes of this project have a broader impact in that they also bridge the gap between the fields of particle physics, and astrophysics and cosmology. The collisions that are taking place at the LHC, for instance, have been occurring in the Earth's atmosphere since the creation of the Solar system. The possibility of a catastrophic event, like the creation of a black hole or a stable strangelet, that could engulf the Earth is practically excluded by this observation. The cosmic ray experiments give, in this way, valuable information to the particle physics community. On the other hand, the laws governing the interactions between particles, that are studied at terrestrial accelerators, also apply to the particles in extensive air showers.
