Modern Physics Letters A

The nucleon has been used as a laboratory to investigate its own spin structure and quantum chromodynamics. New experimental data on nucleon spin structure at low to intermediate momentum transfers combined with existing high momentum transfer data offer a comprehensive picture of the transition region from the confinement regime of the theory to its asymptotic freedom regime. Insight for some aspects of the theory is gained by exploring lower moments of spin structure functions and their corresponding sum rules (i.e. the Gerasimov–Drell–Hearn, Bjorken and Burkhardt–Cottingham). These moments are expressed in terms of an operator-product expansion using quark and gluon degrees of freedom at moderately large momentum transfers. The sum rules are verified to good accuracy assuming that no singular behavior of the structure functions is present at very high excitation energies. The higher-twist contributions have been examined through the moments evolution as the momentum transfer varies from higher to lower values. Furthermore, QCD-inspired low-energy effective theories, which explicitly include chiral symmetry breaking, are tested at low momentum transfers. The validity of these theories is further examined as the momentum transfer increases to moderate values. It is found that chiral perturbation calculations agree reasonably well with the first moment of the spin structure function g₁ at momentum transfer of 0.1 GeV² but fail to reproduce the neutron data in the case of the generalized polarizability δ_LT.

If a family symmetry exists for the quarks and leptons, the Higgs sector is expected to be enlarged to be able to support the transformation properties of this symmetry. There are, however, three possible generic ways (at tree level) of hiding this symmetry in the context of the Standard Model with just one Higgs doublet. All three mechanisms have their natural realizations in the unification symmetry E₆ and one in SO(10). An interesting example based on SO(10)×A₄ for the neutrino mass matrix is discussed.

We study a gravity model where a tensionful codimension-one three-brane is embedded in a bulk with infinite transverse length. We find that 4D gravity is induced on the brane already at the classical level if we include higher-curvature (Gauss–Bonnet) terms in the bulk. Consistency conditions appear to require a negative brane tension as well as a negative coupling for the higher-curvature terms.

The influence of spin on a photon's motion in a Schwarzschild and FRW spacetimes is studied. The first-order correction to the geodesic motion is found. It is shown that unlike the worldlines of spinless particles, the photons worldlines do not lie in a plane.

We discuss in this paper the energetics of large gamma-ray superflares observed from Soft-Gamma Repeater sources. The last recorded event has in fact ruled out some models for the energy release. For the first time actual information about a possible associated gravitational wave emission may have been gathered from the LIGO data, even in the case that most of the energy was emitted in gamma-rays. Even upper limits on the amplitude of the latter h at the expected frequency f_c≤2 kHz may be useful to further constrain the remaining mechanisms.

In this paper we have explored the consequences of a model of dark energy with its energy density varying exponentially with the scale factor. We first consider the model with ρ_ϕ∝e^κa, where κ is a constant. This is a kind of generalization of the cosmological constant model with κ=0. We show that such an exponentially varying dark energy density with the scale factor naturally leads to an equivalent phantom field. We also consider a model with ρ_ϕ∝e^κ/a and show that this also naturally leads to an equivalent phantom field.

We have studied a top-Higgs production process in the topcolor assisted technicolor (TC2) model. The studies show that the production cross-section of is at the order of magnitude 10 fb in reasonable parameters space of TC2 model. A few hundred events of top-Higgs can be produced each year at the high energy linear e⁺e^- colliders (LC). The top-Higgs may be directly observed via this process.