Allan Widom’s research while affiliated with Northeastern University and other places

We discuss materials which owe their stability to external fields. These include: 1) external electric or magnetic fields, and 2) quantum vacuum fluctuations in these fields induced by suitable boundary conditions (the Casimir effect). Instances of the first case include the floating water bridge and ferrofluids in magnetic fields. An example of the second case is taken from biology where the Casimir effect provides an explanation of the formation of stacked aggregations or "rouleaux" by negatively charged red blood cells. We show how the interplay between electrical and Casimir forces can be used to drive self-assembly of nano-structured materials, and could be generalized both as a probe of Casimir forces and as a means of manufacturing nanoscale structures. Interestingly, all the cases discussed involve the generation of the somewhat exotic negative pressures. We note that very little is known about the phase diagrams of most materials in the presence of external fields other than those represented by the macroscopic scalar quantities of pressure and temperature. Many new and unusual states of matter may yet be undiscovered.

To interpret the mean depth of cosmic ray air shower maximum and its dispersion, we parametrize those two observables as functions of the �rst two moments of the lnA distribution. We examine the goodness of this simple method through simulations of test mass distributions. The application of the parameterization to Pierre Auger Observatory data allows one to study the energy dependence of the mean lnA and of its variance under the assumption of selected hadronic interaction models. We discuss possible implications of these dependences in term of interaction models and astrophysical cosmic ray sources.

A thorough search for large-scale anisotropies in the distribution of arrival directions of cosmic rays detected above 10(18) eV at the Pierre Auger Observatory is reported. For the first time, these large-scale anisotropy searches are performed as a function of both the right ascension and the declination and expressed in terms of dipole and quadrupole moments. Within the systematic uncertainties, no significant deviation from isotropy is revealed. Upper limits on dipole and quadrupole amplitudes are derived under the hypothesis that any cosmic ray anisotropy is dominated by such moments in this energy range. These upper limits provide constraints on the production of cosmic rays above 10(18) eV, since they allow us to challenge an origin from stationary galactic sources densely distributed in the galactic disk and emitting predominantly light particles in all directions.

We derive lower bounds on the density of sources of ultra-high energy cosmic rays from the lack of significant clustering in the arrival directions of the highest energy events detected at the Pierre Auger Observatory. The density of uniformly distributed sources of equal intrinsic intensity was found to be larger than ∼ (0.06 − 5) × 10 −4 Mpc −3 at 95% CL, depending on the magnitude of the magnetic deflections. Similar bounds, in the range (0.2 − 7) × 10 −4 Mpc −3 , were obtained for sources following the local matter distribution.

The observation of ultrahigh energy (UHE) neutrinos has become a priority in experimental astroparticle physics. UHE neutrinos can be detected with a variety of techniques. In particular, neutrinos can interact in the atmosphere (downward-going neutrinos) or in the Earth crust (Earth-skimming neutrinos), producing air showers that can be observed with arrays of detectors at the ground. With the Surface Detector Array of the Pierre Auger Observatory we can detect these types of cascades. The distinguishing signature for neutrino events is the presence of very inclined showers produced close to the ground (i.e. after having traversed a large amount of atmosphere). In this work we review the procedure and criteria established to search for UHE neutrinos in the data collected with the ground array of the Pierre Auger Observatory. This includes Earth-skimming as well as downward-going neutrinos. No neutrino candidates have been found, which allows us to place competitive limits to the diffuse flux of UHE neutrinos in the EeV range and above.

A thorough search for large-scale anisotropies in the distribution of arrival directions of cosmic rays detected above 10(18) eV at the Pierre Auger Observatory is presented. This search is performed as a function of both declination and right ascension in several energy ranges above 10(18) eV, and reported in terms of dipolar and quadrupolar coefficients. Within the systematic uncertainties, no significant deviation from isotropy is revealed. Assuming that any cosmic-ray anisotropy is dominated by dipole and quadrupole moments in this energy range, upper limits on their amplitudes are derived. These upper limits allow us to test the origin of cosmic rays above 10(18) eV from stationary Galactic sources densely distributed in the Galactic disk and predominantly emitting light particles in all directions.

We describe the experimental setup and the results of RAuger, a small radio-antenna array, consisting of three fully autonomous and self-triggered radio-detection stations, installed close to the center of the Surface Detector (SD) of the Pierre Auger Observatory in Argentina. The setup has been designed for the detection of the electric field strength of air showers initiated by ultra-high energy cosmic rays, without using an auxiliary trigger from another detection system. Installed in December 2006, RAuger was terminated in May 2010 after 65 registered coincidences with the SD. The sky map in local angular coordinates (i.e., zenith and azimuth angles) of these events reveals a strong azimuthal asymmetry which is in agreement with a mechanism dominated by a geomagnetic emission process. The correlation between the electric field and the energy of the primary cosmic ray is presented for the first time, in an energy range covering two orders of magnitude between 0.1 EeV and 10 EeV. It is demonstrated that this setup is relatively more sensitive to inclined showers, with respect to the SD. In addition to these results, which underline the potential of the radio-detection technique, important information about the general behavior of self-triggering radio-detection systems has been obtained. In particular, we will discuss radio self-triggering under varying local electric-field conditions.

Experiments have been performed to measure magnetoelectric properties of room temperature spirally ordered Sr3Co2Fe24O41 hexaferrite slabs. The measured properties include the magnetic permeability, the magnetization and the strain all as a function of the electric field E and the magnetic intensity H. The material hexaferrite Sr3Co2Fe24O41 exhibits broken symmetries for both time reversal and parity. The product of the two symmetries remains unbroken. This is the central feature of these magnetoelectric materials. A simple physical model is proposed to explain the magnetoelectric effect in these materials.

A substantial neutron flux generated by plasma excitation at the tungsten cathode of an electrolytic cell with alkaline solution is reported. A method based on a CR-39 nuclear track detector coupled to a boron converter was used to detect the neutrons. This method is insensitive to the strong plasma-generated electromagnetic noise that made inconclusive all the previous attempts to identify neutrons in electrolytic plasma environment by means of electric detection techniques.

In the conceptual framework of Quantum ElectroDynamics (QED) it has been proven that liquid water is made up of two phases : 1) a coherent phase where the electron cloud of water molecules oscillates in phase with a trapped electromagnetic field within extended regions, called Coherence Domains (CD); 2) a non coherent phase formed by a gas-like ensemble of molecules filling the interstices among the CD's. The constituent molecules of the coherent phase oscillate between their individual ground state and an excited state where one electron is so loosely bound to be considered quasi-free. Therefore the coherent phase contains a plasma of quasi-free electrons. In the bulk water, as in the case of superfluid liquid Helium, each molecule crosses over continuously between the two phases. On the contrary, close to the surface of a metallic cathode in a chemical cell, the attraction between molecules and wall stabilizes the coherent phases so that the layer of interfacial water is mainly coherent and capable of holding a negative electronic charge. When the chemical cell voltage exceeds a threshold, an i! nterfacial water- cathode metal surface plasma mode is developed. From the collective energies continuously pumped into the plasma, the weak interaction e(-) + p(+) -> n + v(e) may be induced which produces neutrons and neutrinos from Hydrogen atoms. The neutrons may then ultimately induce other nuclear transmutations on the cathode metal surface.