The High Resolution Fly's Eye (HiRes) experiment has observed the Greisen-Zatsepin-Kuzmin suppression (called the GZK cutoff) with a statistical significance of five standard deviations. HiRes' measurement of the flux of ultrahigh energy cosmic rays shows a sharp suppression at an energy of 6 x 10(19) eV, consistent with the expected cutoff energy. We observe the ankle of the cosmic-ray energy spectrum as well, at an energy of 4 x 10(18) eV. We describe the experiment, data collection, and analysis and estimate the systematic uncertainties. The results are presented and the calculation of the statistical significance of our observation is described.

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We report studies of ultrahigh-energy cosmic-ray composition via analysis of depth of air shower maximum (X(max)), for air shower events collected by the High-Resolution Fly's Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate d/d[log(E)] of 47.9+/-6.0(stat)+/-3.2(syst) g/cm2/decade for energies between 1.6 and 63 EeV, and are consistent with a predominantly protonic composition of cosmic rays when interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction models. These measurements constrain models in which the galactic-to-extragalactic transition is the cause of the energy spectrum ankle at 4x10(18) eV.

We report an experimental confirmation of the power-law relationship between the critical anisotropy parameter and ion number for the linear-to-zigzag phase transition in an ionic crystal. Our experiment uses laser cooled calcium ions confined in a linear radio-frequency trap. Measurements for up to ten ions are in good agreement with theoretical and numeric predictions. Implications on an upper limit to the size of data registers in ion trap quantum computers are discussed.PACS numbers: 32.80.Pj, 03.67. Lx, 52.25.Wz, Ions confined in linear radio-frequency traps, and cooled by laser radiation, will condense into a crystalline state. Such crystals are the most rarefied form of condensed matter known [1]. Besides being of inherent scientific interest for this reason, cold trapped ions have a growing number of applications, notably spectroscopy [2-4], frequency standards [3,5], and quantum computing [6,7]. The existence of different kinds of phase transitions of these crystals has been known for some time [8,9] and has been the subject of various theoretical and numeric studies [1,10,11]. Previous experimental work identified different crystal phases/configurations in a quadrupole ring trap [9]. Here we explicitly investigate the transition between two of these phases: the linear and the zigzag configurations. We report the first experimental confirmation of one of the key theoretical/numeric predictions for the linear-to-zigzag transition, namely, the existence of a power law relating the critical anisotropy parameter to the number of ions in the crystal. Further, we discuss the usefulness of this power-law expression in determining the ultimate size of a quantum logic register realizable using a single ion trap.The potential energy of a crystal of N identical ions of mass M and charge e confined in an effective threedimensional harmonic potential is U͑r 1 , r 2 , . . . , r N ͒ M͑2p͒ 2 2 N

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We have searched for correlations between the pointing directions of ultrahigh energy cosmic rays observed by the High Resolution Fly's Eye experiment and Active Galactic Nuclei (AGN) visible from its northern hemisphere location. No correlations, other than random correlations, have been found. We report our results using search parameters prescribed by the Pierre Auger collaboration. Using these parameters, the Auger collaboration concludes that a positive correlation exists for sources visible to their southern hemisphere location. We also describe results using two methods for determining the chance probability of correlations: one in which a hypothesis is formed from scanning one half of the data and tested on the second half, and another which involves a scan over the entire data set. The most significant correlation found occurred with a chance probability of 24%.