Narrow Results

We suggest a unified description of baryon form factors in terms of a two-component model with an intrinsic q³ structure and a meson cloud, parameterized in terms of vector mesons. We use this model to study the N-Δ form factors. The results are in excellent agreement with data. The N-Δ transition appears to be dominated by coupling to the ρ meson.

As indicated by NMR experimental results, there are two types of electrons in a doped cuprate: the localized Cu3d electrons, which are responsible for magnetism, and the itinerant p-band electrons carrying the charge transports. We show that the uncompensated spins at oxygen sites created by holes can pair into a resonating valence bond (RVB) state via local exchange interactions where the quasi-free local Cu spins play a role of the mediation for spin pairing. The two-component model gives a proper interpretation of the magnetic origin of high T_c superconductivity, especially, in explanation of the cross over from the anti-ferromagnetic to the RVB state.

In the low doping limit, a high T_c cuprate preserves a two band structure. O2p electrons are itinerant, Cu3d electrons are localized. Therefore the two component model is suitable to describe nuclear spin relaxation at copper sites. In addition to the Korringa process, the hyperfine interaction between nuclear spins and local electron spins is considered, which gives rise to the anomalous relaxation rate 1/T₁ = a + bT. The decrease of the susceptibility near T_c, as shown by the Knight shift measurements, can be attributed to the ordering of local spins and the pairing of the uncompensated spins created by holes at the oxygen sites.

We investigate dynamics properties of proton transfer in hydrogen-bonded ferroelectrics with asymmetric double-well potential, in an externally applied field, based on a two-component model. We obtain the equations of motion and their soliton solution. The soliton velocity and conductivity are found; this expression shows that the phase-transition critical index is equal to unity, and this value coincides quite well with the experimental result.

A two-component analysis of spectra to p_t = 12 GeV/c for identified pions and protons from 200 GeV Au–Au collisions is presented. The method is similar to an analysis of the n_ch dependence of p_t spectra from p–p collisions at 200 GeV, but applied to Au–Au centrality dependence. The soft-component reference is a Lévy distribution on transverse mass m_t. The hard-component reference is a Gaussian on transverse rapidity y_t with exponential (p_t power-law) tail. Deviations of data from the reference are described by hard-component ratio r_AA, which generalizes nuclear modification factor R_AA. The analysis suggests that centrality evolution of pion and proton spectra is dominated by changes in parton fragmentation. The structure of r_AA suggests that parton energy loss produces a negative boost Δy_t of a large fraction (but not all) of the minimum-bias fragment distribution, and that lower-energy partons suffer relatively less energy loss, possibly due to color screening. The analysis also suggests that the anomalous p/π ratio may be due to differences in the parton energy-loss process experienced by the two hadron species. This analysis provides no evidence for radial flow.

The relativistic heavy-ion collider (RHIC) was constructed to achieve an asymptotic state of nuclear matter in heavy-ion collisions, a near-ideal gas of deconfined quarks and gluons denoted quark–gluon plasma or QGP. RHIC collisions are indeed very different from the hadronic processes observed at the Bevalac and AGS, but high-energy elementary-collision mechanisms are also non-hadronic. The two-component model (TCM) combines measured properties of elementary collisions with the Glauber eikonal model to provide an alternative asymptotic limit for A–A collisions. RHIC data have been interpreted to indicate formation of a strongly-coupled QGP (sQGP) or "perfect liquid". In this review, I consider the experimental evidence that seems to support such conclusions and alternative evidence that may conflict with those conclusions and suggest different interpretations.

The PYTHIA Monte Carlo (PMC), first introduced more than 30 years ago, remains a popular simulation tool both for analysis of $p$–$p$ collision dynamics and for detector design and calibration. The PMC assumes that almost all hadron production results from parton–parton scatterings described by pQCD and that multiple parton interactions (MPIs) per collision event are a common occurrence. In contrast, a two-component (soft + hard) model (TCM) of high-energy collisions, inferred inductively from a variety of data formats, attributes a majority of final-state hadrons to projectile-nucleon dissociation and a minority to minimum-bias (MB) dijet production (corresponding to measured jet spectra and fragmentation functions (FF)). The observed jet-related hadron yield is precisely proportional to the square of the nonjet yield over an interval corresponding to 100-fold increase in dijet production. The two data descriptions appear to be in conflict. This study presents a detailed comparison of the two models and their relations to a broad array of collision data. The PMC appears to disagree with some data, whereas the TCM provides an accurate and comprehensive data description.