Unified Supergravity Model Research Articles

Predictive signatures of supersymmetry: Measuring the dark matter mass and gluino mass with early LHC data

We present a focused study of a predictive unified model whose measurable consequences are immediately relevant to early discovery prospects of supersymmetry at the LHC. ATLAS and CMS have released their analysis with $35\text{ }\text{ }{\mathrm{pb}}^{\ensuremath{-}1}$ of data and the model class we discuss is consistent with this data. It is shown that with an increase in luminosity, the LSP dark matter mass and the gluino mass can be inferred from simple observables such as kinematic edges in leptonic channels and peak values in effective mass distributions. Specifically, we consider cases in which the neutralino is of low mass and where the relic density consistent with WMAP observations arises via the exchange of Higgs bosons in unified supergravity models. The magnitudes of the gaugino masses are sharply limited to focused regions of the parameter space, and, in particular, the dark matter mass lies in the range $\ensuremath{\sim}(50--65)\text{ }\text{ }\mathrm{GeV}$ with an upper bound on the gluino mass of 575 GeV, with a typical mass of 450 GeV. We find that all model points in this paradigm are discoverable at the LHC at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$. We determine lower bounds on the entire sparticle spectrum in this model based on existing experimental constraints. In addition, we find the spin-independent cross section for neutralino scattering on nucleons to be generally in the range of ${\ensuremath{\sigma}}_{{\stackrel{\texttildelow{}}{\ensuremath{\chi}}}_{1}^{0}p}^{\mathrm{SI}}={10}^{\ensuremath{-}46\ifmmode\pm\else\textpm\fi{}1}\text{ }\text{ }{\mathrm{cm}}^{2}$ with much higher cross sections also possible. Thus, direct detection experiments such as CDMS and XENON already constrain some of the allowed parameter space of the low mass gaugino models and further data will provide important cross-checks of the model assumptions in the near future.

Phenomenological aspects of heterotic orbifold models at one loop

We provide a detailed study of the phenomenology of orbifold compactifications of the heterotic string within the context of supergravity effective theories. Our investigation focuses on those models where the soft Lagrangian is dominated by loop contributions to the various soft supersymmetry breaking parameters. Such models typically predict non-universal soft masses and are thus significantly different from minimal supergravity and other universal models. We consider the pattern of masses that are governed by these soft terms and investigate the implications of certain indirect constraints on supersymmetric models, such as flavor-changing neutral currents, the anomalous magnetic moment of the muon and the density of thermal relic neutralinos. We also comment on the possible discovery of these models at the LHC. These string-motivated models show a novel behavior that interpolates between the phenomenology of unified supergravity models and models dominated by the superconformal anomaly.

Neutrino masses and mixing angles in SUSY-GUT theories with explicit R-parity breaking

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.

Phenomenology of the minimal supergravitySU(5) model

The minimal grand unified supergravity model is discussed. Requiring radiative breaking of the electroweak gauge symmetry, the unification of b and tau Yukawa couplings, a sufficiently stable nucleon, and not too large a relic density of neutralinos produced in the Big Bang constrains the parameter space significantly. In particular, the soft breaking parameter m_1/2 has to be less than about 130 GeV, and the top quark Yukawa coupling has to be near its (quasi) fixed point. The former condition implies m_gluino < 400 GeV and hence very large production rates for gluino pairs at the LHC, while the latter constraint implies that the lighter stop and sbottom eigenstates are significantly lighter than the other squarks, leading to characteristic signatures for gluino pair events.

Fine-tuning problem in minimal SO(10) supersymmetric grand unified theories.

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 $\tau$ 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 $\tanb$.

Top-down approach to unified supergravity models

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.

Bottom-up approach to unified supergravity models

A new approach is proposed to the phenomenological study of a generic unified supergravity model, which reduces to the minimal supersymmetric standard model. The model is effectively parametrized in terms of five low energy observables. In consequence, it is easy to investigate systematically the parameter space of the model, allowed by the requirement of radiative electroweak symmetry breaking and by the present experimental limits. Radiative corrections due to large Yukawa couplings and particle-sparticle mass splitting are included into the analysis and found to have important effects, in particular on the degree of fine tuning in the model. In this framework there are presented the predictions of the model for various low energy physical observables, and their dependence on the values of the top quark mass and tan β is discussed. Results are also given for the large tan β scenario, tan β ≈ m t / m b . Our approach can be easily extended to non-minimal supergravity models, which do not assume the universality of the soft breaking parameters at the unification scale M X . Such an extension will be particularly useful once the masses of some sparticles are known, allowing for a model independent study of the parameter space at M X .

Single zino production in e+e− annihilation

The leptonic production of a single zino Z˜0, accompanied by a photino γ˜, via e+e−→γ˜Z˜0 is suggested as a clean source to study the properties of the hitherto unexplored supersymmetric neutral fermionic partners of weak gauge bosons. This reaction could provide also basic tests of unified supergravity models already at present e+e− collider energies.