Andreas Aste

A direct road to Majorana fields

A concise discussion of spin-1/2 field equations with a special focus on Majorana spinors is presented. The Majorana formalism which describes massive neutral fermions by the help of two-component or four-component spinors is of fundamental importance for the understanding of mathematical aspects of supersymmetric and other extensions of the Standard Model of particle physics, which may play an increasingly important role at the beginning of the LHC era. The interplay between the two-component and the four-component formalism is highlighted in an introductory way. Majorana particles are predicted both by grand unified theories, in which these particles are neutrinos, and by supersymmetric theories, in which they are photinos, gluinos and other states.

Electromagnetic pion and kaon form factors in light-cone resummed perturbative QCD

We analyze the electromagnetic pion and kaon form factor by including radiative and higher-twist effects within the framework of resummed perturbative QCD in the spacelike region. We focus on the transition from the perturbative to nonperturbative behavior in the phenomenological intermediate-energy regime. Using a modified “kT” factorization scheme with transverse degrees of freedom, we evaluate the nonperturbative soft contributions as distinct from the hard contributions, ensuring no double counting via the Ward identity at Q2=0. The soft contributions are obtained via local quark-hadron duality, while the hard contributions rest on the well-known collinear factorization theorem using model wave functions with modified Brodsky-Huang-Lepage–type ansatz and distribution amplitudes derived from light-cone QCD sum rules. Our analysis shows that the perturbative hard part prevails for large Q2 beyond 50–100  GeV2, while for low and moderate momentum transfers below 10–16  GeV2, the soft contributions dominate over the hard part. Thus, we demonstrate the importance of including the soft contributions for explaining the experimental form-factor data.

Electron positron pair production in the external electromagnetic field of colliding relativistic heavy ions

Abstract. The results concerning the

Holographic entropy bound from gravitational Fock space truncation

e^+e^-

Finite field theories and causality

production in peripheral highly relativistic heavy-ion collisions presented in a recent paper by Baltz et al. are rederived in a very straightforward manner. It is shown that the solution of the Dirac equation directly leads to the multiplicity, i.e. to the total number of electron–positron pairs produced by the electromagnetic field of the ions, whereas the calculation of the single pair production probability is much more involved. A critical observation concerns the unsolved problem of seemingly absent Coulomb corrections (Bethe–Maximon corrections) in pair production cross sections. It is shown that neither the inclusion of the vacuum–vacuum amplitude nor the correct interpretation of the solution of the Dirac equation concerning the pair multiplicity is able the explain (from a fundamental point of view) the absence of Coulomb corrections. Therefore the contradiction has to be accounted to the treatment of the high energy limit.

Bound-free pair production cross-section in heavy-ion colliders from the equivalent photon approach

A simplified derivation of Yurtsevers result, which states that the entropy of a truncated bosonic Fock space is given by a holographic bound when the energy of the Fock states is constrained gravitationally, is given for asymptotically flat spacetimes with arbitrary dimension d ≥ 4. For this purpose, a scalar field confined to a spherical volume in d-dimensional spacetime is considered. Imposing an upper bound on the total energy of the corresponding Fock states which ensures that the system is in a stable configuration against gravitational collapse and imposing a cutoff on the maximum energy of the field modes of the order of the Planck energy leads to an entropy bound of holographic type. A simple derivation of the entropy bound is also given for the fermionic case.

Comment on A simple explanation of the nonappearance of physical gluons and quarks

A condensed introduction to the basic concepts of causal perturbation theory is given. Causal perturbation theory is a mathematically rigorous approach to renormalization theory, which makes it possible to put the theoretical setup of perturbative quantum field theory on a sound mathematical basis by avoiding infinities from the outset. It goes back to a seminal work by Henri Epstein and Vladimir Jurko Glaser published in 1973, where a specific causality condition was imposed at every order of perturbation theory in the case of scalar quantum field theory such that divergent integrals could be avoided in actual calculations of loop diagrams. In the meantime, the causal approach has been applied also to a wide range of gauge theories.

Resummation of Mass Terms in Perturbative Massless Quantum Field Theory

Exact calculations of the electron-positron pair production by a single photon in the Coulomb field of a nucleus with simultaneous capture of the electron into the K-shell are discussed for different nuclear charges. Using the equivalent photon method of Weizsacker and Williams, a simple expression for the bound-free production of e+e− pairs by colliding very-high-energy fully stripped heavy ions is derived for nuclei of arbitrary charge.

Dispersive calculation of the massless multi-loop sunrise diagram

In a recent paper by Johan Hansson it is claimed that the nonappearance of quarks and gluons as physical particles is an automatic result of the non-Abelian nature of the color interaction in quantum chromodynamics. It is shown that the arguments given by Hansson are insufficient to support his claim by giving simple counter arguments.

Aspects of the derivative coupling model in four dimensions

The neutral massless scalar quantum field Φ in four-dimensional space-time is considered, which is subject to a simple bilinear self-interaction. Is is well-known from renormalization theory that adding a term of the form