Margarete Mühlleitner

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.

SDECAY: A Fortran code for the decays of the supersymmetric particles in the MSSM

We present the Fortran code SDECAY, which calculates the decay widths and branching ratios of all the supersymmetric particles in the Minimal Supersymmetric Standard Model, including higher order eects. Besides the usual two-body decays of fermions and gauginos and the three-body decays of charginos, neutralinos and gluinos, we have also implemented the three-body decays of top squarks, and even the four-body decays of the top squark; the important loop-induced decay modes are also included. The QCD corrections to the two-body decays involving strongly interacting particles and the dominant components of the electroweak corrections to all decay modes are implemented.

Effective Lagrangian for a light Higgs-like scalar

A bstractWe reconsider the effective Lagrangian that describes a light Higgs-like boson and better clarify a few issues which were not exhaustively addressed in the previous literature. In particular we highlight the strategy to determine whether the dynamics responsible for the electroweak symmetry breaking is weakly or strongly interacting. We also discuss how the effective Lagrangian can be implemented into automatic tools for the calculation of Higgs decay rates and production cross sections.

NMSSM Higgs Benchmarks Near 125 GeV

The recent LHC indications of a SM-like Higgs boson near 125 GeV are consistent not only with the Standard Model (SM) but also with Supersymmetry (SUSY). However naturalness arguments disfavour the Minimal Supersymmetric Standard Model (MSSM). We consider the Next-to-Minimal Supersymmetric Standard Model (NMSSM) with a SM-like Higgs boson near 125 GeV involving relatively light stops and gluinos below 1 TeV in order to satisfy naturalness requirements. We are careful to ensure that the chosen values of couplings do not become non-perturbative below the grand unification (GUT) scale, although we also examine how these limits may be extended by the addition of extra matter to the NMSSM at the two-loop level. We then propose four sets of benchmark points corresponding to the SM-like Higgs boson being the lightest or the second lightest Higgs state in the NMSSM or the NMSSM-with-extra-matter. With the aid of these benchmark points we discuss how the NMSSM Higgs boson near 125 GeV may be distinguished from the SM Higgs boson in future LHC searches.

Natural NMSSM Higgs Bosons

We study the phenomenology of Higgs bosons close to 126 GeV within the scale invariant unconstrained Next-to-Minimal Supersymmetric Standard Model (NMSSM), focusing on the regions of parameter space favoured by low fine-tuning considerations, namely stop masses of order 400 GeV to 1 TeV and an effective ? parameter between 100–200 GeV, with large (but perturbative) ? and low View the MathML sourcetan?=2–4. We perform scans over the above parameter space, focusing on the observable Higgs cross sections into ??, WW, ZZ, bb, ?? final states, and study the correlations between these observables. We show that the ?? signal strength may be enhanced up to a factor of about two not only due to the effect of singlet–doublet mixing, which occurs more often when the 126 GeV Higgs boson is the next-to-lightest CP-even one, but also due to light stops (and to a lesser extent light chargino and charged Higgs loops). There may be also smaller enhancements in the Higgs decay channels into WW, ZZ, correlated with the ?? enhancement. However there is no such correlation observed involving the Higgs decay channels into bb, ??. The requirement of having perturbative couplings up to the GUT scale favours the interpretation of the 126 GeV Higgs boson as being the second lightest NMSSM CP-even state, which can decay into pairs of lighter neutralinos, CP-even or CP-odd Higgs bosons, leading to characteristic signatures of the NMSSM. In a non-negligible part of the parameter range the increase in the ?? rate is due to the superposition of rates from nearly degenerate Higgs bosons. Resolving these Higgs bosons would rule out the Standard Model, and provide evidence for the NMSSM.

Composite Higgs Boson Pair Production at the LHC

The measurement of the trilinear and quartic Higgs self-couplings is necessary for the reconstruction of the Higgs potential. This way the Higgs mechanism as the origin of electroweak symmetry breaking can be tested. The couplings are accessible in multi-Higgs production processes at the LHC. In this paper we investigate the prospects of measuring the trilinear Higgs coupling in composite Higgs models. In these models, the Higgs boson emerges as a pseudo-Goldstone boson of a strongly interacting sector, and the Higgs potential is generated by loops of the Standard Model (SM) gauge bosons and fermions. The Higgs self-couplings are modified compared to the SM and controlled by the compositeness parameter ξ in addition to the Higgs boson mass. We construct areas of sensitivity to the trilinear Higgs coupling in the relevant parameter space for various final states.

Probing for invisible Higgs decays with global fits

A bstractWe demonstrate by performing a global fit on Higgs signal strength data that large invisible branching ratios (Brinv) for a Standard Model (SM) Higgs particle are currently consistent with the experimental hints of a scalar resonance with mass mh ~ 124 GeV. For this mass, we find Brinv< 0.64 (95% CL) from a global fit to individual channel signal strengths supplied by ATLAS, CMS and the Tevatron collaborations. Novel tests that can be used to improve the prospects of experimentally discovering the existence of a Brinv with future data are proposed. These tests are based on the combination of all visible channel Higgs signal strengths, and allow us to examine the required reduction in experimental and theoretical errors in this data that would allow a more significantly bounded invisible branching ratio to be experimentally supported. We examine in some detail how our conclusions and method are affected when a scalar resonance at this mass scale has couplings deviating from the SM ones.

Aspects of CP violation in the HZZ coupling at the LHC

We examine the CP-conserving (CPC) and CP-violating (CPV) effects of a general HZZ coupling through a study of the process H → ZZ (�) → l + l l ′ + l ′ at the LHC. We construct asymmetries that directly probe these couplings. Further, we present complete analytical formulae for the angular distributions of the decay leptons and for some of the asymmetries. Using these we have been able to identify new observables which can provide enhanced sensitivity to the CPV HZZ coupling. We also explore probing CP violation through shapes of distributions in different kinematic variables, which can be used for Higgs bosons with mH < 2 mZ.

A Note on doubly charged Higgs pair production at hadron colliders

We analyze the next-to-leading order QCD corrections to the production of doubly-charged Higgs particles at hadron colliders in extensions of the SM with Higgs isospin triplets. At both the Tevatron and the LHC, these corrections are found to be moderate in size increasing the cross sections by about 20--30%. The residual theoretical uncertainties are of the order of 10--15% which is sufficient for experimental searches for these particles at the Tevatron and LHC.

Composite Higgs search at the LHC

The Higgs boson production cross-sections and decay rates depend, within the Standard Model (SM), on a single unknown parameter, the Higgs mass. In composite Higgs models where the Higgs boson emerges as a pseudo-Goldstone boson from a strongly-interacting sector, additional parameters control the Higgs properties which then deviate from the SM ones. These deviations modify the LEP and Tevatron exclusion bounds and significantly affect the searches for the Higgs boson at the LHC. In some cases, all the Higgs couplings are reduced, which results in deterioration of the Higgs searches but the deviations of the Higgs couplings can also allow for an enhancement of the gluon-fusion production channel, leading to higher statistical significances. The search in the H → γγ channel can also be substantially improved due to an enhancement of the branching fraction for the decay of the Higgs boson into a pair of photons.