arXiv subjects
Publications and source records attributed to Ralf Hempfling.
We have studied the predictions for the LSP decay within the framework of a radiatively broken unified supergravity model without R-parity. Assuming that Higgs/slepton mixing is the only source of R-parity breaking and responsible for the observed neutrino oscillations we obtain predictons for the LSP life-time and branching fractions.
In supersymmetric models without $R$-parity neutrinos naturally become massive and mix with each other. We explore the predictions of a very restricted model with only three free parameters and find that this model naturally yields masses and mixing angles compatible with experimental results from solar and atmospheric neutrino experiments. Furthermore, there is a tiny region in parameter space where the solution to the solar neutrino puzzle is compatible with either the LSND result or the existence of significant hot dark matter neutrinos.
We propose a mechanism for hiding the primordial baryon asymmetry from interactions that could wash it out. It requires the introduction of a baryon number carrying singlet which is in equilibrium in the early universe and shares any existing baryon asymmetry. It decouples from the Standard Model particles before all the interactions required to wash out the asymmetry are in equilibrium ($T \simeq 10$ TeV), and decays after the electroweak phase transition, but before nucleosynthesis. This mechanism can conserve a baryon asymmetry in models (a) with $B-L = 0$, such as many $SU(5)$ GUTs, or (b) with $B-L$ violating interactions in thermal equilibrium, such as SUSY with broken $R$-parity. As a result, cosmological constraints on $R$-parity violating operators are relaxed considerably.
To obtain the most accurate predictions for the Higgs masses in the minimal supersymmetric model (MSSM), one should compute the full set of one-loop radiative corrections, resum the large logarithms to all orders, and add the dominant two-loop effects. A complete computation following this procedure yields a complex set of formulae which must be analyzed numerically. We discuss a very simple approximation scheme which includes the most important terms from each of the three components mentioned above. We estimate that the Higgs masses computed using our scheme lie within 2 GeV of their theoretically predicted values over a very large fraction of MSSM parameter space.
In minimal SUSY GUT models the $R$-parity breaking terms are severely constrained by SU(5) gauge invariance. We consider the particular case where the explicit $R$-parity breaking occurs only via dimension 2 terms of the superpotential. This model possesses only three R-parity breaking parameters. We have studied the predictions of this model for the neutrino masses and mixing angles at the one-loop level within the framework of a radiatively broken unified supergravity model. We find that this model naturally yields masses and mixing angles that can explain the solar and atmospheric neutrino problems. In addition, there are regions in parameter space where the solution to the solar neutrino puzzle is compatible with either the LSND result or the existence of significant hot dark matter neutrinos.
We compute the dominant one-loop radiative corrections to the cross-section for $e^+ e^- \rightarrow V_1 V_2$ $(V_{1/2} = γ, Z, W^\pm$) in the MSSM. We find that the genuine vertex corrections are very small. The oblique corrections are potentially large enough to be tested at LEP-II or NLC. However, the sensitivity is below the one from other high precision electro-weak measurements but can serve as a self-consistency check.
In the standard model (SM) the condition that the Higgs mass parameter vanishes is stable under radiative corrections and yields a theory that can be renormalized using dimensional regularization. Thus, this model allows to predict the Higgs boson mass. However, it is phenomenologically ruled out in its minimal version. Here, we present a phenomenologically viable, minimal extension which only includes an additional SM singlet and a U(1)$_X$ gauge symmetry.
The theoretical expectations for the supersymmetric particle spectrum is reviewed and a brief overview on present constraints on supersymmetric models from collider experiments is presented. Finally, we discuss the discovery potential of future collider experiments.
In grand unified theories (GUT) based on SO(10) all fermions of one generation are embedded in a single representation. As a result, the top quark, the bottom quark, and the $τ$ lepton have a universal Yukawa coupling at the GUT scale. This implies a very large ratio of Higgs vacuum expectation values, $\tanb \simeq m_t/m_b$. We analyze the naturalness of such a scenario quantitatively including all the relevant radiative corrections and find that in minimal unified supergravity models with universal soft supersymmetry breaking parameters at the GUT scale, the necessary amount of fine-tuning needed is excessive. GUT threshold correction to the universal Higgs mass parameter can significantly reduce the fine-tuning required for such large values of $\tanb$. We also point out that the top quark mass can play a crucial role in explaining the hierarchy between the SUSY breaking scale and the electro-weak scale and, hence, the naturalness of large values of $\tanb$.
We study the effects of additional fields on the unification of gauge couplings in supersymmetric models. We find that the effects are quite constrained by the requirement of SU(5) gauge invariance. In general, we find that any extension of the MSSM will form complete SU(5) multiplets or spoil gauge coupling unification.
The unification of gauge coupling constants in the minimal supersymmetric model (MSSM) is unaffected at the one-loop level by the inclusion of additional mass-degenerate SU(5) multiplets. Perturbativity puts an upper limit on the number of additional fields. We analyse the evolution of the gauge coupling constants in all models satisfying these criteria using two-loop $β$ functions and including low energy threshold effects. We find that similarly to the minimal supersymmetric model (MSSM) unification takes place within the theoretical and experimental errors. The dominant proton decay mode is more suppressed in all extended models as opposed to the MSSM due to renormalization group effects. However, the prediction for the bottom to $τ$ mass ratio becomes worse in all models under consideration.
We present the full one-loop correction to the relation between the running MSbar Yukawa coupling, $\overline h_f(μ)$, at some renormalization scale, $μ$, and the pole mass, $m_f$, of a fermion, $f$, in the standard model. Our result complements previous analyses that included just the QCD correction to this relation in the case of quarks. This allows us to convert, without loss of information, the threshold value, $\overline h_f(m_f)$, of $\overline h_f(μ)$, which satisfies a renormalization-group equation and may be driven up to some high-energy scale of new physics, to physical observables at low energy. We investigate the limit of the Higgs boson and/or the top quark being much heavier than all the other particles. We also list simple and yet rather precise approximation formulae, which are convenient for applications.
It is generally assumed that the dynamical generation of the Higgs mass parameter of the superpotential, $μ$, implies the existence of a light singlet at or below the supersymmetry breaking scale, $\msusy$. We present a counter-example in which the singlet field can receive an arbitrarily heavy mass (\eg, of the order of the Planck scale, $M_{\rm P}\approx 10^{19}~\gev$). In this example, a non-zero value of $μ$ is generated through soft supersymmetry breaking parameters and is thus naturally of the order of $\msusy$.
We introduce a new approach for studying unified supergravity models. In this approach all the parameters of the grand unified theory (GUT) are fixed by imposing the corresponding number of low energy observables. This determines the remaining particle spectrum whose dependence on the low energy observables can now be investigated. We also include some SUSY threshold corrections that have previously been neglected. In particular the SUSY threshold corrections to the fermion masses can have a significant impact on the Yukawa coupling unification.
In the minimal supersymmetric model (MSSM), the upper limit of the lightest Higgs boson mass, $\mhl$, depends strongly on the top quark mass, $m_t$. We have computed the dominant two-loop radiative corrections to this upper limit of $\mhl$ in order to eliminate large uncertainties due to QCD and $m_t^6$ corrections. It is shown that the QCD corrections significantly reduce the one-loop corrections. As a result, the SUSY parameter space accessible to LEP experiments is significantly increased.
In the minimal supersymmetric model (MSSM) all Higgs self-coupling parameters are related to gauge couplings at tree-level. Leading-logarithmic radiative corrections to these quantities can be summed using renormalization group techniques. By this procedure we obtain complete leading-log radiative corrections to the Higgs masses, the CP-even Higgs mixing angle, and trilinear Higgs couplings. Additional corrections due to squark mixing can be explicitly incorporated into this formalism. These results incorporate nearly all potentially large corrections. Mass shifts to the neutral CP-even Higgs bosons grow with the fourth power of the top-quark mass and can be significant. The phenomenological consequences of these results are examined.