Frank Daniel Steffen

Thermal gravitino production and collider tests of leptogenesis

Considering gravitino dark matter scenarios, we obtain the full gauge-invariant result for the relic density of thermally produced gravitinos to leading order in the standard model gauge couplings. For the temperatures required by thermal leptogenesis, we find gaugino mass bounds which will be probed at future colliders. We show that a conceivable determination of the gravitino mass will allow for a unique test of the viability of thermal leptogenesis in the laboratory.

Constraints on the reheating temperature in gravitino dark matter scenarios

Abstract Considering gravitino dark matter scenarios, we study constraints on the reheating temperature of inflation. We present the gauge-invariant result for the thermally produced gravitino yield to leading order in the Standard Model gauge couplings. Within the framework of the constrained minimal supersymmetric Standard Model (CMSSM), we find a maximum reheating temperature of about 10 7 GeV taking into account bound-state effects on the primordial 6Li abundance. We show that late-time entropy production can relax this constraint significantly. Only with a substantial entropy release after the decoupling of the lightest Standard Model superpartner, thermal leptogenesis remains a viable explanation of the cosmic baryon asymmetry within the CMSSM.

Gravitino dark matter and cosmological constraints

The gravitino is a promising cold dark matter candidate. 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 four-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.

Dark-matter candidates - Axions, neutralinos, gravitinos, and axinos

The existence of dark matter provides strong evidence for physics beyond the standard model. Extending the standard model with the Peccei–Quinn symmetry and/or supersymmetry, compelling dark-matter candidates appear. For the axion, the neutralino, the gravitino, and the axino, I review primordial production mechanisms, cosmological and astrophysical constraints, experimental searches, and prospects for experimental identification.

Implications of Catalyzed BBN in the CMSSM with Gravitino Dark Matter

Abstract We investigate gravitino dark matter scenarios in which the primordial 6 Li production is catalyzed by bound-state formation of long-lived negatively charged particles X − with 4 He. In the constrained minimal supersymmetric Standard Model (CMSSM) with the stau τ ˜ 1 − as the X − , the observationally inferred bound on the primordial 6 Li abundance allows us to derive a rigid lower limit on the gaugino mass parameter for a standard cosmological history. This limit can have severe implications for supersymmetry searches at the Large Hadron Collider and for the reheating temperature after inflation.

Thermal axion production in the primordial quark-gluon plasma

We calculate the rate for thermal production of axions via scattering of quarks and gluons in the primordial quark-gluon plasma. To obtain a finite result in a gauge-invariant way that is consistent to leading order in the strong gauge coupling, we use systematic field theoretical methods such as hard thermal loop resummation and the Braaten-Yuan prescription. The thermally produced yield, the decoupling temperature, and the density parameter are computed for axions with a mass below 10 meV. In this regime, with a Peccei-Quinn scale above

Thermal relic abundances of long-lived staus

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Probing the reheating temperature at colliders and with primordial nucleosynthesis

, the associated axion population can still be relativistic today and can coexist with the axion cold dark matter condensate.

Axions and saxions from the primordial supersymmetric plasma and extra radiation signatures

Abstract In supersymmetric models with a long-lived stau being the lightest Standard Model superpartner, the stau abundance during primordial nucleosynthesis is tightly constrained. Considering the complete set of stau annihilation channels in the minimal supersymmetric Standard Model (MSSM) with real parameters for scenarios in which sparticle coannihilations are negligible, we calculate the decoupling of the lighter stau from the primordial plasma and identify processes which are capable to deplete the resulting stau abundance significantly. We find particularly efficient stau annihilation at the resonance of the heavy CP-even Higgs boson and for a lighter stau with a sizeable left–right mixing due to enhanced stau–Higgs couplings. Even within the constrained MSSM, we encounter both effects leading to exceptionally small values of the resulting stau abundance. Prospects for collider phenomenology are discussed and possible implications of our findings are addressed with emphasis on gravitino dark matter scenarios.

Dark radiation and dark matter in supersymmetric axion models with high reheating temperature

Abstract Considering gravitino dark matter scenarios with a long-lived charged slepton, we show that collider measurements of the slepton mass and its lifetime can probe not only the gravitino mass but also the post-inflationary reheating temperature T R . In a model independent way, we derive upper limits on T R and discuss them in light of the constraints from the primordial catalysis of 6Li through bound-state effects. In the collider-friendly region of slepton masses below 1 TeV, the obtained conservative estimate of the maximum reheating temperature is about T R = 3 × 10 9 GeV for the limiting case of a small gluino–slepton mass splitting and about T R = 10 8 GeV for the case that is typical for universal soft supersymmetry breaking parameters at the scale of grand unification. We find that a determination of the gluino–slepton mass ratio at the Large Hadron Collider will test the possibility of T R > 10 9 GeV and thereby the viability of thermal leptogenesis with hierarchical heavy right-handed Majorana neutrinos.