Transverse momentum distributions and their forward-backward correlations in the percolating color string approach

The forward-backward correlations in the p(T) distributions at midrapidity, which present a clear signature of nonlinear effects in particle production, are studied in the model of percolating color strings. Quantitative predictions are given for these correlations at SPS, RHIC, and LHC energies. Interaction of strings also naturally explains the flattening of p(T) distributions and increase of with energy and atomic number for nuclear collisions.     

Correlations between multiplicities and average transverse momentum in the percolating color strings approach

Long range multiplicity-multiplicity, p_T²-multiplicity and p²_T- p²_T correlations are studied in the percolating color string picture under different assumptions of the dynamics of the string interaction. It is found that the strength of these correlations is rather insensitive to these assumptions; nor is it sensitive to the geometry of the fused string clusters that formed, the percolation phase transition in particular. Both multiplicity-multiplicity and p_T²-multiplicity correlations are found to scale and depend only on the string density. p_T²-multiplicity correlations, which are absent in the independent string picture, are found to be of the order of 10% for central heavy ion collisions and can serve as a clear signature of string fusion. In contrast p²_T- p²_T correlations turned out to be inversely proportional to the number of strings and therefore to be very small for realistic collisions.Received: 4 July 2003, Revised: 12 September 2003, Published online: 18 December 2003     

A phenomenological model of percolating magnetic nanostructures

Transport and magnetotransport properties were analysed systematically in percolating magnetic nanostructures such as Ni-rich and films. These granular magnetic films exhibit giant Hall effect. We identified features which are common and unique to these systems. Among the features are the correlation between a -like temperature dependent resistivity and a particle size distribution having a large fraction of small nanometer sized particles, and the power law dependence between the magnetoresistivity and the room temperature resistivity. Assuming the presence of nanometer sized particles in the percolating conduction channels whose contributions are sensitive to temperature and the external magnetic field, we developed a phenomenological model to explain all the common features. Received 4 November 1998     

Longest path in percolating hierarchical lattice

We determine numerically the probability distribution for the longest self-avoiding path lengths connecting two distant points on a diluted hierarchical lattice at the percolation threshold. The evolution of this distribution with the system size is studied and the distribution is observed to approach a universal scale-invariant form under proper rescaling of its argument. The longest path length scales as |p ^max| and our estimate for _max=1.816±0.013 is clearly different from the previously estimated _min=1.531+0.002 for the shortest path lengths on the same hierarchical lattice. This gives support to the multifractal behavior of SAWs on percolating clusters.     

Fundamental Physics in Space: A Quantum-Gravity Perspective

I consider the possibility that space experiments be used to search for quantum properties of spacetime. On the basis of recent quantum-gravity results, I argue that insight on some quantum properties of spacetime can be obtained with experiments planned for the International Space Station, such as AMS and EUSO, and with satellite gamma-ray telescopes, such as GLAST.     

Essay: A New Era in High-Energy Physics

In TeV-scale gravity, scattering of particles with center-of-mass energy of the order of a few TeV can lead to the creation of nonperturbative, extended, higher-dimensional gravitational objects: Branes. Neutral or charged, spinning or spinless, Einsteinian or supersymmetric, low-energy branes could dramatically change our picture of high-energy physics. Will we create branes in future particle colliders, observe them from ultra high energy cosmic rays, and discover them to be dark matter?     

Concepts of Time in Physics: A Synopsis

I summarize the historical development of concepts of time in physics from antiquity to the end of the twentieth century. Editors’ Note: Max Jammer received the American Physical Society/American Institute of Physics Abraham Pais Prize for the History of Physics for 2007, “For his groundbreaking historical studies of fundamental concepts in physics, including his comprehensive account of the development of quantum mechanics.” We publish here his Pais Prize Lecture, which was presented at the APS meeting in Jacksonville, Florida, on April 16, 2007.     

Fundamental Structures in Physics: A Categorical Approach

As already remarked by Eilenberg and MacLane in their seminal paper on categorytheory, preordered classes can be considered as thin categories, that is, categoriesfor which each Hom-set contains at most one element. In this paper I brieflydescribe how this identification not only allows much of the theory of orderstructures to be reformulated in categorical terms, but also permits the applicationof general categorical techniques to specific order-theoretic problems.     

The distribution of percolating concentrations in finite systems

Simulation of finite cubic lattices suggests that even in the limit of large systems, the distribution of percolation thresholds deviates from a Gaussian, thus confirming Ziff's two-dimensional result.     

Two-Boson Exchange Physics: A Brief Review

Current status of the two-boson exchange contributions to the elastic electron–proton scattering, both for parity conserving and parity-violating, is briefly reviewed. How the discrepancy in the extraction of elastic nucleon form factors between unpolarized Rosenbluth and polarization transfer experiments can be understood, in large part, by the two-photon exchange corrections is discussed. We also illustrate how the measurement of the ratio between positron–proton and electron–proton scattering can be used to differentiate different models of two-photon exchange. For the parity-violating electron–proton scattering, the interest is on how the two-boson exchange (TBE), γ Z-exchange in particular, could affect the extraction of the long-sought strangeness form factors. Various calculations all indicate that the magnitudes of effect of TBE on the extraction of strangeness form factors is small, though can be large percentage-wise in certain kinematics.     

High-field vs. low-field magneto-transport in a percolating medium

Recent theoretical and numerical work on high-field magneto-transport in a percolating medium is described and compared to earlier work on weak-field magneto-transport in such systems. While the weak-field behavior is well described by the simple nodes-links picture, which ignores blobs and loops on a scale smaller than the percolation correlation length ξ_p, the strong-field behavior is extremely sensitive to those features. The critical behavior at strong magnetic fields H near the percolation threshold is governed by competition between the usual H=0 fixed point and a new H=∞ fixed point. Which of those fixed points dominates the behavior is determined by the relative sizes of two characteristic lenghts: the percolation correlation length ξ_p and a new, magnetic field dependent length ξ_H.     

Magneto-optical evidence of double exchange in a percolating lattice

Substituting Eu by Ca in ferromagnetic EuB6 leads to a percolation limited magnetic ordering. We present and discuss magneto-optical data of the Eu(1-x)Ca(x)B6 series, based on measurements of the reflectivity R(omega) from the far infrared up to the ultraviolet, as a function of temperature and magnetic field. Via the Kramers-Kronig transformation of R(omega) we extract the complete absorption spectra of samples with different values of x. The change of the spectral weight in the Drude component by increasing the magnetic field agrees with a scenario based on the double-exchange model, and suggests a crossover from a ferromagnetic metal to a ferromagnetic Anderson insulator upon increasing Ca content at low temperatures.