Large Tanβ Research Articles

Radiative generation of non-zero [formula omitted] in MSSM with broken [formula omitted] flavor symmetry

We study the renormalization group effects on neutrino masses and mixing in Minimal Supersymmetric Standard Model (MSSM) by considering a μ–τ symmetric mass matrix at high energy scale giving rise to Tri-Bi-Maximal (TBM) type mixing. We outline a flavor symmetry model based on A4 symmetry giving rise to the desired neutrino mass matrix at high energy scale. We take the three neutrino mass eigenvalues at high energy scale as input parameters and compute the neutrino parameters at low energy by taking into account of renormalization group effects. We observe that the correct output values of neutrino parameters at low energy are obtained only when the input mass eigenvalues are large |m1,2,3|=0.08–0.12 eV with a very mild hierarchy of either inverted or normal type. A large inverted or normal hierarchical pattern of neutrino masses is disfavored within our framework. We also find a preference towards higher values of tanβ, the ratio of vacuum expectation values (vev) of two Higgs doublets in MSSM in order to arrive at the correct low energy output. Such a model predicting large neutrino mass eigenvalues with very mild hierarchy and large tanβ could have tantalizing signatures at oscillation, neutrino-less double beta decay as well as collider experiments.

$B^0\hbox{--}\bar B^0$ MIXING WITH THE CONSTRAINT OF A 125 GeV HIGGS

Considering the constraints from the current experiment results, we analyze [Formula: see text] mixing with a Higgs boson around 125 GeV, and discuss the influence of MA, tanβ to the mhand ΔmBin the Minimal Supersymmetric Standard Model (MSSM). We find these constraints have a strong impact to the parameters of MAand tanβ, they almost exclude the small MAand large tanβ, to have a obvious correction to the mass difference ΔmBwe argue that the MAshould not too large, it is possible to observe the MAboson with a mass 300 ~ 400 GeV in the future.

Search for supersymmetry with jets, missing transverse momentum and at least one hadronically decaying τ lepton in proton–proton collisions at [formula omitted] with the ATLAS detector

A search for production of supersymmetric particles in final states containing jets, missing transverse momentum, and at least one hadronically decaying τ lepton is presented. The data were recorded by the ATLAS experiment in s=7 TeV proton–proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 2.05fb−1 of data. The results are interpreted in the context of gauge mediated supersymmetry breaking models with Mmess=250 TeV, N5=3, μ>0, and Cgrav=1. The production of supersymmetric particles is excluded at 95% C.L. up to a supersymmetry breaking scale Λ=30 TeV, independent of tanβ, and up to Λ=43 TeV for large tanβ.

χ QJ →ℓ + ℓ − within and beyond the Standard Model

We revisit χ QJ →ℓ + ℓ − (with J=0,1,2 and Q=b,c) within the Standard Model (SM). The electromagnetic contributions are given in the color-singlet model with non-vanishing lepton masses at the leading order of v. Numerically, the branching ratios of χ QJ →ℓ + ℓ − predicted within the SM are so small that such decays are barely possible to be detected at future BESIII and SuperB experiments, but may be possible to be observed at the LHC. We investigate χ b0→ℓ + ℓ − in Type-II 2HDM with large tanβ, and χ b2→ℓ + ℓ − in the Randall–Sundrum model, to see their chance to be observed in future experiments.

Constraints from the decay [formula omitted] and LHC limits on Supersymmetry

The pure leptonic decay Bs0→μ+μ− is strongly suppressed in the Standard Model (SM), but can have large enhancements in Supersymmetry, especially at large values of β. New limits on this decay channel from recent LHC data have been used to claim that these limits restrict the SUSY parameter space even more than the direct searches. However, direct searches are hardly dependent on tanβ, while Br(Bs0→μ+μ−) is proportional to tan6β. The relic density constraint requires large tanβ in a large region of the parameter space, which can lead to large values of Bs0→μ+μ−. Nevertheless, the experimental upper limit on Br(Bs0→μ+μ−) is not constraining the parameter space of the CMSSM more than the direct searches and the present Higgs limits, if combined with the relic density. We also observe SUSY parameter regions with negative interferences, where the Bs0→μ+μ− value is up to a factor three below the SM expectation, even at large values of tanβ.

Light neutralino in the MSSM: a playground for dark matter, flavor physics and collider experiments

We investigate the constraints to the light neutralino dark matter scenario in the minimal supersymmetric standard model from available experimental observations such as decays of B and K meson, relic dark matter abundance, and the search for neutralino and Higgs production at colliders. We find that two regions of the MSSM parameter space fulfill all the constraints: a fine-tuned strip with large tanβ where the lightest neutralino can be a slight as 8 GeV, and alow tanβ region providing a neutralino mass larger than 16 GeV. The large tan β strip, which can be compatible with recently reported signals from direct detection experiments, can be fully tested by means of low-energy observables and, in particular, by B s → μμ and Higgs bosons searches at the LHC within the upcoming months.

An improved Standard Model prediction of [formula omitted] and its implications for New Physics

The recently measured B→τν branching ratio allows to test the Standard Model by probing virtual effects of new heavy particles, such as a charged Higgs boson. The accuracy of the test is currently limited by the experimental error on BR(B→τν) and by the uncertainty on the parameters fB and |Vub|. The redundancy of the Unitarity Triangle fit allows to reduce the error on these parameters and thus to perform a more precise test of the Standard Model. Using the current experimental inputs, we obtain BR(B→τν)SM=(0.84±0.11)×10−4, to be compared with BR(B→τν)exp=(1.73±0.34)×10−4. The Standard Model prediction can be modified by New Physics effects in the decay amplitude as well as in the Unitarity Triangle fit. We discuss how to disentangle the two possible contributions in the case of minimal flavour violation at large tanβ and generic loop-mediated New Physics. We also consider two specific models with minimal flavour violation: the Type-II Two Higgs Doublet Model and the Minimal Supersymmetric Standard Model.

SUSY and Higgs signatures implied by cancellations in [formula omitted

Recent re-evaluations of the Standard Model (SM) contribution to Br(b→sγ) hint at a positive correction from new physics. Since a charged Higgs boson exchange always gives a positive contribution to this branching ratio, the constraint points to the possibility of a relatively light charged Higgs. It is found that under the HFAG constraints and with re-evaluated SM results large cancellations between the charged Higgs and the chargino contributions in supersymmetric models occur. Such cancellations then correlate the charged Higgs and the chargino masses often implying both are light. Inclusion of the more recent evaluation of gμ−2 is also considered. The combined constraints imply the existence of several light sparticles. Signatures arising from these light sparticles are investigated and the analysis indicates the possibility of their early discovery at the LHC in a significant part of the parameter space. We also show that for certain restricted regions of the parameter space, such as for very large tanβ under the 1σ HFAG constraints, the signatures from Higgs production supersede those from sparticle production and may become the primary signatures for the discovery of supersymmetry.

Correlation between direct dark matter detection andBr(Bs→μμ)with a large phase ofBs−B¯smixing

We combine the analyses for flavor changing neutral current processes and dark matter solutions in minimal-type supersymmetric grand unified theory models, SO(10) and SU(5), with a large B s -B s mixing phase and large tanβ. For large tanβ, the double-penguin diagram dominates the supersymmetry contribution to the B s -B s mixing amplitude. Also, the Br(B s → μμ) constraint becomes important as it grows as tan 6 β, although it can still be suppressed by a large pseudoscalar Higgs mass m A . We investigate the correlation between B s → μμ and the dark matter direct detection cross section through their dependence on m A . In the minimal-type of SU(5) with type I seesaw, the large mixing in neutrino Dirac couplings results in a large lepton flavor violating decay process τ → μγ, which in turn sets the upper bound on m A . In the SO(10) case, the large mixing can be chosen to be in the Majorana couplings instead, and the constraint from Br(τ → μγ) can be avoided. The heavy Higgs funnel region turns out to be an interesting possibility in both cases and the direct dark matter detection should be possible in the near future in these scenarios.

Lepton flavour violating heavy Higgs decays within the [formula omitted] and their detection at the LHC

Within the νMSSM, a Minimal Supersymmetric neutrino See-saw Model, Lepton Flavour Violating Higgs couplings are strongly enhanced at large tanβ (≳30), which can lead to BR(H0/A0→τμ)≃O(10−4), for MH0/A0≳160 GeV. Enhancements on the production of Higgs bosons, through the gluon fusion mechanism, gg→H0/A0, and the associated production channel gg,qq¯→bb¯H0/A0, whose rates grow with tanβ, as well as the mass degeneracy that occurs between the H0 and A0 states in this regime, also contribute to further the possibilities to detect a heavy Higgs signal into τμ pairs. We show that the separation of τμ Higgs events from the background at the upcoming CERN Large Hadron Collider could be done for Higgs masses up to about 600 GeV for 300 fb−1 of luminosity, for large tanβ values. However, even with as little as 10 fb−1 one can probe H0/A0 masses up to 400 GeV or so, if tanβ=60. Altogether, these processes then provide a new Higgs discovery mode as well as an independent test of flavour physics.

Penguin contribution to the phase of [formula omitted]–[formula omitted] mixing and [formula omitted] in grand unified theories

We investigate the possibility of a large Bs–Bs¯ mixing phase in the context of grand unified theory (GUT) models, e.g., SO(10) and SU(5). In these models, we find that a large phase of Bs mixing is correlated with Br(b→sγ), Br(τ→μγ) and Br(Bs→μμ) for large tanβ. In the case of the SO(10) model, the large phase of Bs mixing is correlated with Br(b→sγ) and Br(Bs→μμ) and we find that a large Bs mixing corresponds to an enhanced Br(Bs→μμ) about to be probed by the Tevatron. In the case of the SU(5) model, the large phase is correlated with Br(τ→μγ) and Br(Bs→μμ). In this case, the Br(τ→μγ) constraint requires a smaller pseudo-scalar Higgs mass which in turn generates a large Br(Bs→μμ) almost at the edge of present experimental constraint. If the present observation of large phase of Bs mixing persists in the upcoming data, using all these branching ratios, we will be able to distinguish these models.

Flavor in supersymmetry with an extendedRsymmetry

We propose a new solution to the supersymmetric flavor problem without flavor-blind mediation. Our proposal is to enforce a continuous or a suitably large discrete R-symmetry on weak scale supersymmetry, so that Majorana gaugino masses, trilinear A terms, and the μ term are forbidden. We find that replacing the minimal supersymmetric standard model with an R-symmetric supersymmetric model allows order one flavor-violating soft masses, even for squarks of order a few hundred GeV. The minimal R-symmetric supersymmetric model contains Dirac gaugino masses and R-symmetric Higgsino masses with no left-right mixing in the squark or slepton sector. Dirac gaugino masses of order a few TeV with vanishing A terms solve most flavor problems, while the R-symmetric Higgs sector becomes important at large tanβ. ∈ K can be accommodated if CP is preserved in the SUSY breaking sector, or if there is a moderate flavor degeneracy, which can arise naturally. ∈'/∈, as well as neutron and electron electric dipole moments, are easily within experimental bounds. The most striking phenomenological distinction of this model is the order one flavor violation in the squark and slepton sector, while the Dirac gaugino masses tend to be significantly heavier than the corresponding squark and slepton masses.

Exclusive Semileptonic Rare Decays B → K(*)l+l− in a Top Quark Two-Higgs-Doublet Model

Employing improved calculations of the decay form factors from light-cone sum rules, we evaluate the invariant mass spectrum, forward-backward asymmetry, and lepton polarizations of the exclusive processes B → K(*)l+l− in the SM and T2HDM. From the recent measurements of their branching ratios, we find that these processes do provide additional bounds on the new parameters in the model considered here. After the inclusion of the new physics contributions, the large enhancement of FBA, which is unobservably small within the SM and of the lepton polarization at large tanβ, may precisely test the SM or reveal new physics in forthcoming accurate experiments.

Unification and fermion mass structure

Grand Unified Theories predict relationships between the GUT-scale quark and lepton masses. Using new data in the context of the MSSM, we update the values and uncertainties of the masses and mixing angles for the three generations at the GUT scale. We also update fits to hierarchical patterns in the GUT-scale Yukawa matrices. The new data shows not all the classic GUT-scale mass relationships remain in quantitative agreement at small to moderate tanβ. However, at large tanβ, these discrepancies can be eliminated by finite, tanβ-enhanced, radiative, threshold corrections if the gluino mass has the opposite sign to the wino mass.

Signature of heavy charged Higgs boson at LHC in the 1 and 3 prong hadronic tau decay channels

We have done a fast simulation analysis of the H± signal at LHC in 1 and 3 prong hadronic τ-jet channels along with a tt background. The τ polarization was effectively used to suppress the background in both channels. Combining this with appropriate cuts on pT of the τ-jet, the missing ET and the azimuthal angle between them reduces the background below the signal level. Consequently, one gets a viable H± signal up to a mass range of 600–700 GeV at moderate to large tanβ.

Neutrino masses, mixing and leptogenesis in a two Higgs doublet model “for the third generation”

We examine neutrino oscillations in a two Higgs doublet model (2HDM) in which the second doublet couples only to the third generation right-handed up-fermions, i.e., to tR and to N3 which is the heaviest right-handed Majorana neutrino. The inherently large tanβ of this model can naturally account for the large top mass and, based on a quark–lepton similarity ansatz, when embedded into a see-saw mechanism it can also account for the observed neutrino masses and mixing angles giving a very small θ13: −0.017≲θ13≲0.021 at 99% CL, and a very restrictive prediction for the atmospheric mixing angle: 42.9°≲θatm≲45.2° at 99% CL. The large value of tanβ also sets the mass scale of the heaviest right-handed Majorana neutrino N3 and triggers successful leptogenesis through a CP-asymmetry in the decays of the N1 (lightest right-handed Majorana) which is tan2β enhanced compared to the CP-asymmetry obtained in models for leptogenesis with one Higgs doublet or in the MSSM. This enhancement allows us to relax the lower bound on MN1 and consequently also the lower bound on the reheating temperature of the early universe.

On the interpretation of [formula omitted] in the CMSSM

We discuss the interpretation of present and possible future experimental constraints on Bs→μ+μ− decay in the context of the constrained minimal extension of the Standard Model (CMSSM) with universal scalar masses. We emphasize the importance of including theoretical and other experimental uncertainties in calculating the likelihood function, which can affect significantly the inferred 95% confidence-level limit on the CMSSM parameters. The principal uncertainties are the Bs meson decay constant, mt and mb. The latter induce uncertainties in the mass of the neutral Higgs boson that dominates the Bs→μ+μ− decay amplitude at large tanβ, reducing the CMSSM region excluded by present and possible future limits from the Fermilab Tevatron collider and the LHC.

New Higgs effects in B-physics in supersymmetry with general flavour mixing

We investigate the effect of general flavour mixing among squarks on the rare decays B¯→Xsγ, B¯s→μ+μ− and B¯s–Bs mixing beyond the leading order in perturbation theory. We include all large tanβ-enhanced corrections whilst also taking into account the effects of general flavour mixing on the uncorrected quark mass matrix and SU(2)L×U(1)Y breaking. For B¯s→μ+μ− and B¯s–Bs mixing we find that, in analogy to B¯→Xsγ, there appears a focusing effect which can reduce the contribution due to the δRR (and the δLL) insertion by up to a factor of two at large tanβ and μ>0. A dependence on δLR and δRL, that otherwise cancels to first order in the mass insertion approximation, is also reintroduced. Taking into account the current experimental bounds on ΔMBs and BR(B¯s→μ+μ−), we find that the insertions δRL and δRR can be significantly constrained compared to bounds obtained from B¯→Xsγ only.

A two-loop contribution to Bs→μ+μ− at large tanβ in the MSSM

We consider a two-loop contribution of Higgs-mediated penguin diagram to B(Bs→μ+μ−) at large tanβ in the MSSM, motivated by a recently proposed two-loop magnetic penguin diagrams by Chen and Geng [hep-ph/0403188]. Typically the two-loop diagram is a αs correction to the one-loop contributions. However the new contribution can dominate one-loop contributions in a region of parameter space where μ, At and Ab are very large and scalar top, scalar bottom and charged Higgs are light. Both constructive and destructive interferences are possible. The total branching ratio can be drastically changed from the one-loop result.

Implications of Bs→μ+μ− in SO(10)-like models

We examine the potentially very promising signal Bs→μ+μ− in supersymmetry with large tanβ in a top-down approach starting from the best fits of an SO(10)-like model studied recently. Our results go beyond minimal flavour violation investigated in previous works. We show that the absolute best fits provide a signal for Bs→μ+μ− at the borderline of the present limits and hence the ongoing search at the Tevatron will start having an impact on the global analysis of this class of SUSY models. We discuss the implications of a measurement of Bs→μ+μ− for restricting the parameter space of gauginos and sfermion masses, and of signals in other channels Bd,s→ℓ+ℓ−. We also discuss correlations of Bs→μ+μ− with the CP-odd Higgs mass, sin(β−α) and b→sγ in SO(10)-like models.