arXiv subjects

R. Thomas

Publications and source records attributed to R. Thomas.

77 records · Page 5Linked to original sources

arXiv2005

The generic Groebner walk

The Groebner walk is an algorithm for conversion between Groebner bases for different term orders. It is based on the polyhedral geometry of the Groebner fan and involves tracking a line between cones representing the initial and target term order. An important parameter is explicit numerical perturbation of this line. This usually involves both time and space demanding arithmetic of integers much larger than the input numbers. In this paper we show how the explicit line may be replaced by a formal line using Robbiano's characterization of group orders on Q^n. This gives rise to the generic Groebner walk involving only Groebner basis conversion over facets and computations with marked polynomials. The proposed technique is closely related to the lexicographic (symbolic) perturbation method used in optimization and computational geometry. We report on computations with toric ideals, where a version of our algorithm in certain cases computes test sets for hard integer knapsack problems significantly faster than the Buchberger algorithm.

math.AC↗

arXiv2005

Evidence for In-Medium Changes of Four-Quark Condensates

Utilizing the QCD sum rule approach to the behavior of the omega meson in nuclear matter we derive evidence for in-medium changes of particular four-quark condensates from the recent CB-TAPS experiment for the reaction gamma + A -> A' + omega (-> pi0 gamma) with A = Nb and LH2.

hep-ph↗

arXiv2005

Electromagnetic Probes of Strongly Interacting Matter: Probes of Chiral Symmetry Restoration?

The QCD sum rule approach to in-medium modifications of the omega meson in nuclear matter is reviewed with emphasis of its relation to 4-quark condensates and chiral symmetry restoration. Possible implications of the CB-TAPS experiment for the reaction gamma A -> A' omega (-> pi0 gamma) are sketched and the particularly important role of di-electron probes, accessible with HADES, is highlighted. A brief update of a parametrization of the previous dilepton and photon probes from CERES and WA98 of heavy-ion collisions at CERN-SPS energies is presented.

hep-ph↗

arXiv2004

Gluon Emission of Heavy Quarks: Dead Cone Effect

The lowest-order induced soft gluon radiation processes of heavy quarks have been analyzed to quantify the dead cone effect. This effect is most likely expected to suppress significantly the energy loss of charm quarks passing an amorphous color-neutral deconfined medium, as has been concluded from recent experiments at RHIC.

hep-ph↗

arXiv2003

Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many interstellar molecules. In dense clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density, and temperature. On the other hand, observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. However, diffuse cloud models have been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons, the electron fraction, and the cosmic-ray ionisation rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards zeta Persei) where the electron fraction is already known. Taken together, these results allow us to derive the value of the third uncertain model parameter: we find that the cosmic-ray ionisation rate in this sightline is forty times faster than previously assumed. If such a high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.

astro-ph↗