Mikkel B. Johnson

Dense baryon matter calculations with realistic potentials

Abstract The equation of state for dense hyperonic matter is calculated from five potential interaction models. The form of the potential is the same as that of Reid, a sum of Yukawa functions, the coefficients of which are adjusted separately in each partial wave to fit experimental nucleon-nucleon data. The strong short-range central repulsion dominates the equation of state at high density, so special care is taken to make the strengths of the repulsive term consistent with meson theory. A simplified version of the constrained variational technique of Pandharipande is used to calculate the energy versus density. The calculation is performed for neutron matter and a mixture of nucleons and hyperons, M ≦ 1236 MeV. The various interaction models produce a spread of about 30% in the equations of state, which are generally suffer than the equation of state calculated from the Reid potential by Pandharipande. This difference arises because our models have a more realistic repulsion than Reids potential.

Breakdown of QCD factorization at large Feynman x

Recent measurements by the BRAHMS Collaboration of high-p{sub T} hadron production at forward rapidities at the BNL Relativistic Heavy Ion Collider found the relative production rate (d-Au)/(p-p) to be suppressed rather than enhanced. Examining other known reactions (forward production of light hadrons, the Drell-Yan process, heavy flavor production, etc.), one notes that all of these display a similar property, namely, their cross sections in nuclei are suppressed at large x{sub F}. Since this is the region where x{sub 2} is minimal, it is tempting to interpret this as a manifestation of coherence or of a color glass condensate, whereas it is actually a simple consequence of energy conservation and takes place even at low energies. We demonstrate that in all these reactions there is a common suppression mechanism that can be viewed, alternatively, as a consequence of a reduced survival probability for large rapidity gap processes in nuclei, a Sudakov suppression, an enhanced resolution of higher Fock states by nuclei, or an effective energy loss that rises linearly with energy. Our calculations agree with the data.

Neutron star models with realistic high-density equations of state

We calculate neutron star models using four realistic high-density models of the equation of state. We conclude that the maximum mass of a neutron star is unlikely to exceed 2 M/sub sun/. All of the realistic models are consistent with current estimates of the moment of inertia of the Crab pulsar.

A Bargmann-Wightman-Wigner-type quantum field theory

Abstract We show that the ( j , 0) ⊕ (0, j ) representation space associated with massive particles is a concrete realisation of a quantum field theory, envisaged many years ago by Bargmann, Wightman and Wigner, in which bosons and antibosons have opposite relative intrinsic parities. Demonstration of the result requires a careful ab initio study of the ( j , 0) ⊕ (0, j ) representation space for massive particles, introducing a wave equation with well defined transformation properties under C , P and T , and addressing the issue of nonlocality required of such a theory by the work of Lee and Wick.

Broadening of transverse momentum of partons propagating through a medium

Broadening of the transverse momentum of a parton propagating through a medium is treated using the color dipole formalism, which has the advantage of being a well developed phenomenology in deep-inelastic scattering and soft processes. Within this approach, nuclear broadening should be treated as color filtering, i.e. absorption of large-size dipoles leading to diminishing (enlarged) transverse separation (momentum). We also present a more intuitive derivation based on the classic scattering theory of Moli\`ere. This derivation helps to understand the origin of the dipole cross section, part of which comes from attenuation of the quark, while another part is due to multiple interactions of the quark. It also demonstrates that the lowest-order rescattering term provides an A-dependence very different from the generally accepted A^{1/3} behavior. The effect of broadening increases with energy, and we evaluate it using different phenomenological models for the unintegrated gluon density. Although the process is dominated by soft interactions, the phenomenology we use is tested using hadronic cross section data.

Effective two-body interaction in simple nuclear spectra

Abstract The low-lying spectra of some simple nuclei (two nucleons outside closed shells or the equivalent situations) are analyzed in terms of a force with a short range component, taken to be a delta force, and a long range core-mediated component. Formulas are given for the energy splitting among the members of the resulting multiplets. Both the cases of particles sitting in equivalent as well as in non-equivalent orbits are considered. An estimate of the contribution to the residual effective interaction from the different components of the fosy is obtained by means of a least fit to the data.

Open charm tomography of cold nuclear matter

We study the relative contribution of partonic subprocesses to D meson production and D meson-triggered inclusive di-hadrons to lowest order in perturbative QCD. While gluon fusion dominates the creation of large angle DD pairs, charm on light parton scattering determines the yield of single inclusive D mesons. The distinctly different nonperturbative fragmentation of c quarks into D mesons versus the fragmentation of quarks and gluons into light hadrons results in a strong transverse momentum dependence of anticharm content of the away side charm-triggered jet. In p+A reactions, we calculate and resum the coherent nuclear-enhanced power corrections from the final-state partonic scattering in the medium. We find that single and double inclusive open charm production can be suppressed as much as the yield of neutral pions from dynamical high-twist shadowing. Effects of energy loss in p+A collisions are also investigated phenomenologically and may lead to significantly weaker transverse momentum dependence of the nuclear attenuation.

Energy loss of fast quarks in nuclei

We report an analysis of the nuclear dependence of the yield of Drell-Yan (DY) dimuons from the 800 GeV/c proton bombardment of

Nuclear effects in the Drell-Yan process at very high energies

^2H

Probing for Dirac vacuum structure in nuclei

, C, Ca, Fe, and W targets. A light-cone formulation of the DY process is employed in the rest frame of the nucleus. In this frame, for