Jaume Guasch

SUSY Les Houches accord: interfacing SUSY spectrum calculators, decay packages, and event generators

An accord specifying a unique set of conventions for supersymmetric extensions of the Standard Model together with generic file structures for 1) supersymmetric model specifications and input parameters, 2) electroweak scale supersymmetric mass and coupling spectra, and 3) decay tables is presented, to provide a universal interface between spectrum calculation programs, decay packages, and high energy physics event generators.

FCNC top quark decays in the MSSM: a door to SUSY physics in high luminosity colliders?

We study the FCNC top quark decays t→ch in the framework of the MSSM, where h≡h0,H0,A0 is any of the supersymmetric neutral Higgs bosons. We include the leading set of SUSY-QCD and SUSY electroweak contributions. While the FCNC top quark decay into the SM Higgs boson has such a negligible rate that it will not be accessible to any presently conceivable accelerator, we find that there is a chance that the potential rates in the MSSM can be measured at the high luminosity colliders round the corner, especially at the LHC and possibly at a future LC, but we deem it difficult at the upgraded Tevatron. In view of the large SUSY-QCD effects that we find in the Higgs channels, and due to some discrepancies in the literature, we have revisited the FCNC top quark decay into gluon, t→cg, in our framework. We confirm that the possibility of sizeable rates does not necessarily require a general pattern of gluino-mediated FCNC interactions affecting both the LH and the RH sfermion sectors – the LH one being sufficient. However, given the present bounds on sparticle masses, the gluon channel turns out to lie just below the expected experimental sensibility, so our general conclusion is that the Higgs channels t→ch (especially the one for the light CP-even Higgs) have the largest potential top quark FCNC rates in the MSSM, namely of order 10−4.

MSSM Higgs boson decays to bottom quark pairs reexamined

We present an update of neutral Higgs boson decays into bottom quark pairs in the minimal supersymmetric extension of the Standard Model. In particular the resummation of potentially large higher-order corrections due to the soft SUSY breaking parameters A_b and mu is extended. The remaining theoretical uncertainties due to unknown higher-order SUSY-QCD corrections are analyzed quantitatively.

Distinguishing Higgs models in H→bb̄/H→τ+τ−

We analyze the ratio of branching ratios R=BR(H->b b-bar)/BR(H->tau^+ tau^-) of Higgs boson decays as a discriminant quantity between supersymmetric and non-supersymmetric models. This ratio receives large renormalization-scheme independent radiative corrections in supersymmetric models at large \tan\beta, which are absent in the Standard Model or Two-Higgs-doublet models. These corrections are insensitive to the supersymmetric mass scale. A detailed analysis in the effective Lagrangian approach shows that, with a measurement of +-21% accuracy, the Large Hadron Collider can discriminate between models if the CP-odd Higgs boson mass is below 900 GeV. An e+e- Linear Collider at 500 GeV center of mass energy can discriminate supersymmetric models up to a CP-odd Higgs mass of ~ 1.8 TeV.

Prospects for heavy supersymmetric charged Higgs boson searches at hadron colliders

We investigate the production of a heavy charged Higgs boson at hadron colliders within the context of the MSSM. A detailed study is performed for all important production modes and basic background processes for the tb signature. In our analysis we include effects of initial and final state showering, hadronization, and principal detector effects. For the signal production rate we include the leading SUSY quantum effects at high tan βmt/mb. Based on the obtained efficiencies for the signal and background we estimate the discovery and exclusion mass limits of the charged Higgs boson at high values of tan β. At the upgraded Tevatron the discovery of a heavy charged Higgs boson (MH±200 GeV) is impossible for the tree-level cross-section values. However, if QCD and SUSY effects happen to reinforce mutually, there are indeed regions of the MSSM parameter space which could provide 3σ evidence and, at best, 5σ charged Higgs boson discovery at the Tevatron for masses MH±300 GeV and MH±250 GeV, respectively, even assuming squark and gluino masses in the (500−1000) GeV range. On the other hand, at the LHC one can discover a H± as heavy as 1 TeV at the canonical confidence level of 5σ; or else exclude its existence at 95% C.L. up to masses ~ 1.5 TeV. Again the presence of SUSY quantum effects can be very important here as they may shift the LHC limits by a few hundred GeV.

Fermionic decays of sfermions: a complete discussion at one-loop order

We present a deflnition of an on-shell renormalization scheme for the sfermion and chargino-neutralino sector of the Minimal Supersymmetric Standard Model (MSSM). Then, apply this renormalization framework to the interaction between charginos/neutra- linos and sfermions. A kind of universal corrections is identifled, which allow to deflne efiective chargino/neutralino coupling matrices. In turn, these interactions generate (uni- versal) non-decoupling terms that grow as the logarithm of the heavy mass. Therefore the full MSSM spectrum must be taken into account in the computation of radiative cor- rections to observables involving these interactions. As an application we analyze the full one-loop electroweak radiative corrections to the partial decay widths i( ~ f ! f´ 0 ) and i( ~ f! f 0 ´ § ) for all sfermion ∞avours and generations. These are combined with the QCD corrections to compute the corrected branching ratios of sfermions. It turns out that the electroweak corrections can have an important impact on the partial decay widths, as well as the branching ratios, in wide regions of the parameter space. The precise value of the corrections is strongly dependent on the correlation between the difierent particle masses.

Full electroweak one-loop radiative corrections to squark decays in the MSSM

Abstract We present results on the full one-loop electroweak radiative corrections to the squark decay partial widths into charginos and neutralinos. We show the renormalization framework, and present numerical results for the third squark family. The corrections can reach values of ∼10%, which are comparable to the radiative corrections from the strong sector of the model. Therefore they should be taken into account for the precise extraction of the SUSY parameters at future colliders.

Higgs boson flavor-changing neutral decays into bottom quarks in supersymmetry

We analyze the maximum branching ratios for the Flavor Changing Neutral Current (FCNC) decays of the neutral Higgs bosons of the Minimal Supersymmetric Standard Model (MSSM) into bottom quarks, h?b ( h = h 0,H 0,A 0 ). We consistently correlate these decays with the radiative B-meson decays (b?s?). A full-fledged combined numerical analysis is performed of these high-energy and low-energy FCNC decay modes in the MSSM parameter space. Our calculation shows that the available data on B?s? severely restricts the allowed values of B(h?b?). While the latter could reach a few percent level in fine-tuned scenarios, the requirement of naturalness reduces these FCNC rates into the modest range B(h?b?) ~ 10?4?10?3 . We find that the bulk of the MSSM contribution to B(h?b?) could originate from the strong supersymmetric sector. The maximum value of the FCNC rates obtained in this paper disagree significantly with recent (over-)estimates existing in the literature. Our results are still encouraging because they show that the FCNC modes h?b can be competitive with other Higgs boson signatures and could play a helpful complementary role to identify the supersymmetric Higgs bosons, particularly the lightest CP-even state in the critical LHC mass region m h 0 90?130?GeV.

Higgs boson flavor-changing neutral decays into top quark in a general two-Higgs-doublet model

Abstract Higgs boson decays mediated by flavor changing neutral currents (FCNC) are very much suppressed in the Standard Model, at the level of 10 −15 for Higgs boson masses of a few hundred GeV. Therefore, any experimental vestige of them would immediately call for new physics. In this paper we consider the FCNC decays of Higgs bosons into a top quark in a general two-Higgs-doublet model (2HDM). The isolated top quark signature, unbalanced by any other heavy particle, should help to identify the potential FCNC events much more than any other final state. We compute the maximum branching ratios and the number of FCNC Higgs boson decay events at the LHC collider at CERN. The most favorable mode for production and subsequent FCNC decay is the lightest CP-even state in the Type II 2HDM, followed by the other CP-even state, if it is not very heavy, whereas the CP-odd mode can never be sufficiently enhanced. Our calculation shows that the branching ratios of the CP-even states may reach 10 −5 , and that several hundred events could be collected in the highest luminosity runs of the LHC. We also point out some strategies to use these FCNC decays as a handle to discriminate between 2HDM and supersymmetric Higgs bosons.

Radiative corrections to pair production of charged Higgs bosons in e+e− collisions

Abstract Charged Higgs particles are a common feature of many extensions of the Standard Model. While its existence with masses up to the TeV scale can be probed at the Tevatron Run II and LHC hadron colliders, a precise determination of its properties would have to wait for a high energy e + e − linear collider. We have computed the complete one-loop electroweak corrections to the cross-section for charged Higgs bosons pair production in e + e − collisions, in the framework of general Two Higgs Doublet Models, as well as the Minimal Supersymmetric Standard Model. A study is presented for typical values of the model parameters, showing the general behaviour of the corrections.