Squark Loops Research Articles

Slepton production as a probe of the squark mass scale

We investigate an important radiative correction to the slepton and chargino production processes. We consider a supersymmetric spectrum with a large splitting between the squark and slepton masses. In this case, in the effective theory below the squark mass threshold, the supersymmetric Slavnov-Taylor identities which enforce the equality of the gauge and gaugino couplings are violated. The gaugino propagators receive potentially large corrections due to virtual quark or squark loop effects. We compute the full one-loop (s)quark corrected slepton production cross sections. The $t$-channel one-loop scattering amplitudes are factorized into an effective chargino-neutralino mass matrix and an effective fermion-sfermion-gaugino coupling. The difference between the effective gaugino coupling and the gauge coupling is proportional to $\mathrm{ln}{(M}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{Q}}{/m}_{\mathcal{l}\ifmmode \tilde{}\else \~{}\fi{}})$ in the large squark mass limit. We find that the one-loop corrected slepton production cross sections can depend on the squark mass strongly, up to $9%\ifmmode\times\else\texttimes\fi{}{\mathrm{log}}_{10}{(M}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{Q}}{/m}_{\mathcal{l}\ifmmode \tilde{}\else \~{}\fi{}})$. We investigate the squark mass sensitivity of the slepton cross-section measurements at a future linear collider. For sneutrino production accessible at $\sqrt{s}=500$ GeV there can be sensitivity to squark masses at or larger than 1 TeV.

The coupling of the lightest SUSY Higgs boson to two photons in the decoupling regime

We analyze the contribution of the SUSY particles to the coupling of the lightest Higgs boson to two photons in supersymmetric theories. We discuss to what extent these contributions can be large enough to allow for a discrimination between the lightest SUSY and the standard Higgs particles in the decoupling limit where all other Higgs bosons are very heavy and no supersymmetric particle has been discovered at future colliders. We find that only chargino and top squark loops can generate a sizeable difference between the standard and the SUSY Higgs-photon couplings. For masses above 250 GeV, the effect of chargino loops on the two-photon width is however smaller than ≈ 10% in the entire SUSY parameter space. Top squarks heavier than 250 GeV can induce deviations larger than 10% only if their couplings to the Higgs boson are large. Since top squark contributions can be sizeable, we derive the two-loop QCD correction to squark loops and show that they are well under control.

QCD Corrections to Supersymmetric Higgs Boson Production: The Role of Squark Loops.

We calculate the two-loop QCD corrections to the production of the neutral supersymmetric Higgs bosons via the gluon fusion mechanism at hadron colliders, including the contributions of squark loops. To a good approximation, these additional contributions lead to the same QCD corrections as in the case where only top and bottom quark loops are taken into account. The QCD corrections are large and increase the Higgs production cross sections significantly.

RENORMALIZATION GROUP IMPROVED RADIATIVE CORRECTIONS TO THE SUPERSYMMETRIC HIGGS BOSON MASSES

The one-loop radiative corrections to the Higgs boson potential in the MSSM, originating from the top quark and squark loops, are summed in the leading log approximation using the renormalization group (RG). The RG improved effective potential is minimized and the corrections to the CP-odd and CP-even Higgs boson masses are calculated. The resulting masses exhibit smoother top mass dependence than those calculated without RG summation. We have also found that for preferable values of the top mass the light Higgs mass does not exceed 100 GeV.

Squark contributions to Higgs boson masses in the next-to-minimal supersymmetric standard model

Within the context of an effective potential formalism we calculate the contribution to Higgs boson masses in the next-to-minimal supersymmetric standard model from squark loops. We then supplement a previously performed renormalisation group analysis of the Higgs sector of this model with these results in order to determine the shift in the bound on the lightest CP-even Higgs boson mass as a result of squark effects. The improved bound on the lightest neutral CP-even Higgs boson mass, including squark contributions, is m h ⩽ 146 (139, 149) GeV for m t = 90 (140, 190) GeV. For m t = 190 GeV squark effects contribute 23 GeV to the bound, with smaller contributions for small m t values.

The supersymmetric charged Higgs boson mass and LEP phenomenology

>We calculate the leading radiative corrections, originating from top and bottom quark and squark loops, to the mass of the charged Higgs boson H± in the minimal supersymmetric standard model, which is given at the tree-level by m2H±=m2w + m2A, where A is the physical CP-odd Higgs boson. We find that these corrections are numerically small, at most a few GeV, for generic values of the parameters, although they can be large in some special regions of parameter space. Our study indicates that generically, if the H± are to be accessible to LEP II (mH±≲90 GeV), then mA≲40 GeV. In this case it is likely that mA<mh, where h is the lighter CP-even Higgs boson, and quite possible that mA<mh/2. After including radiative corrections to the hAA vertex, we find that the h→AA decay could dominate over h→bb or τ+τ−, thereby changing LEP Higgs search strategies.

Could stops lighten the top?

The analysis of the presently available electroweak data including radiative corrections in the standard model suggests that the top quark weighs more than the Z 0. We examine wether squark loops in the minimal supersymmetric model, particularly those involving stops (partners of the top quark), could reduce substantially the preferred range of top quark masses. Given the present lower bounds on squark masses, we find that stop effects can reduce the central value of m t by at most a few GeV, although they do make a very heavy top quark increasingly unlikely.