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Frank Daniel Steffen

Publications and source records attributed to Frank Daniel Steffen.

25 records · Page 2Linked to original sources
arXiv2006

Gravitino Dark Matter and Cosmological Constraints

The gravitino is a promising candidate for cold dark matter. We study cosmological constraints on scenarios in which the gravitino is the lightest supersymmetric particle and a charged slepton the next-to-lightest supersymmetric particle (NLSP). We obtain new results for the hadronic nucleosynthesis bounds by computing the 4-body decay of the NLSP slepton into the gravitino, the associated lepton, and a quark-antiquark pair. The bounds from the observed dark matter density are refined by taking into account gravitinos from both late NLSP decays and thermal scattering in the early Universe. We examine the present free-streaming velocity of gravitino dark matter and the limits from observations and simulations of cosmic structures. Assuming that the NLSP sleptons freeze out with a thermal abundance before their decay, we derive new bounds on the slepton and gravitino masses. The implications of the constraints for cosmology and collider phenomenology are discussed and the potential insights from future experiments are outlined. We propose a set of benchmark scenarios with gravitino dark matter and long-lived charged NLSP sleptons and describe prospects for the Large Hadron Collider and the International Linear Collider.

hep-ph↗
arXiv2005

Collider Signatures of Axino and Gravitino Dark Matter

The axino and the gravitino are extremely weakly interacting candidates for the lightest supersymmetric particle (LSP). We demonstrate that either of them could provide the right amount of cold dark matter. Assuming that a charged slepton is the next-to-lightest supersymmetric particle (NLSP), we discuss how NLSP decays into the axino/gravitino LSP can provide evidence for axino/gravitino dark matter at future colliders. We show that these NLSP decays will allow us to estimate the value of the Peccei-Quinn scale and the axino mass if the axino is the LSP. In the case of the gravitino LSP, we illustrate that the gravitino mass can be determined. This is crucial for insights into the mechanism of supersymmetry breaking and can lead to a microscopic measurement of the Planck scale.

hep-ph↗
arXiv2004

Thermal Production of Axinos in the Early Universe

We compute the thermal axino production rate in supersymmetric QCD to leading order in the gauge coupling. Using hard thermal loop resummation and the Braaten-Yuan prescription, we obtain a finite result in a gauge-invariant way, which takes into account Debye screening in the hot quark-gluon-squark-gluino plasma. The relic axino density from thermal reactions in the early Universe is evaluated assuming the axino is the lightest supersymmetric particle and stable due to R-parity conservation. From the comparison with the WMAP results, we find that axinos could provide the dominant part of cold dark matter, for example, for an axino mass of 100 keV and a reheating temperature of 10^6 GeV.

hep-ph↗
arXiv2004

Axinos as Dark Matter in the Universe

The axino is the fermionic superpartner of the axion. Assuming the axino is the lightest supersymmetric particle and stable due to R-parity conservation, we compute the relic axino density from thermal reactions in the early Universe. From the comparison with the WMAP results, we find that thermally produced axinos could provide the dominant part of cold dark matter, for example, for an axino mass of 100 keV and a reheating temperature of 10^6 GeV.

hep-ph↗
arXiv2003

From Static Potentials to High-Energy Scattering

We develop a loop-loop correlation model for a unified description of static color dipole potentials, confining QCD strings, and hadronic high-energy reactions with special emphasis on saturation effects manifesting S-matrix unitarity at ultra-high energies. The model combines perturbative gluon exchange with the non-perturbative stochastic vacuum model which describes color confinement via flux-tube formation of color fields. We compute the chromo-field distributions of static color dipoles in various SU(N_c) representations and find Casimir scaling in agreement with recent lattice QCD results. We investigate the energy stored in the confining string and use low-energy theorems to show consistency with the static quark-antiquark potential. We generalize Meggiolaro's analytic continuation from parton-parton to dipole-dipole scattering and obtain a Euclidean approach to high-energy scattering that allows us in principle to calculate S-matrix elements in lattice QCD. In this approach we compute high-energy dipole-dipole scattering with the Euclidean loop-loop correlation model. Together with a universal energy dependence and reaction-specific wave functions, the result forms the basis for a unified description of proton-proton, pion-proton, kaon-proton, photon-proton, and photon-photon reactions in good agreement with experimental data for cross sections, slope parameters, and structure functions. The obtained impact parameter profiles for proton-proton and longitudinal photon-proton reactions and the impact parameter dependent gluon distribution of the proton xG(x,Q^2,b) show saturation at ultra-high energies in accordance with unitarity constraints.

hep-ph↗
arXiv1999

Bremsstrahlung out of the Quark-Gluon Plasma

A systematic investigation of hard thermal photon spectra from central ultra-relativistic heavy ion collisions is presented with emphasis on the effects of bremsstrahlung processes in the quark-gluon plasma (QGP). Bremsstrahlung photon production in the quark-gluon plasma has recently been considered within the Braaten-Pisarski method in thermal QCD, where rates have been found that exhibit the same order in the coupling constants as those describing the lowest order processes, Compton scattering and q\bar{q}-annihilation. The impact of these bremsstrahlung photon production rates on the thermal photon spectra is studied systematically within a simple, well understood one-fluid hydrodynamical model that describes an only longitudinally expanding fireball (1+1 Bjorken scaling hydrodynamics). It is found that the bremsstrahlung processes enhance the thermal photon yield from the QGP by about one order of magnitude over the complete considered p_{\perp}-range independent of the choice of the model parameters. Experimental upper limits on direct photon production in fixed target 200 A GeV S + Au collisions at the CERN SPS are also considered and used to extract upper limits for the initial temperature of the QGP, where the QGP bremsstrahlung processes are found to make a difference of about 15 to 20 MeV depending on the temperature at which the phase transition is assumed. In comparison with other theoretical studies, the importance of reaction features not described in the simple model are estimated and interesting elements for a future extension of this systematic investigation are identified, which will be of great interest in prospect of the upcoming experiments at the BNL RHIC and the CERN LHC.

nucl-th↗