This Report summarizes the results of the activities in 2012 and the first half of 2013 of the LHC Higgs Cross Section Working Group. The main goal of the working group was to present the state of the art of Higgs Physics at the LHC, integrating all new results that have appeared in the last few years. This report follows the first working group report Handbook of LHC Higgs Cross Sections: 1. Inclusive Observables (CERN-2011-002) and the second working group report Handbook of LHC Higgs Cross Sections: 2. Differential Distributions (CERN-2012-002). After the discovery of a Higgs boson at the LHC in mid-2012 this report focuses on refined prediction of Standard Model (SM) Higgs phenomenology around the experimentally observed value of 125-126 GeV, refined predictions for heavy SM-like Higgs bosons as well as predictions in the Minimal Supersymmetric Standard Model and first steps to go beyond these models. The other main focus is on the extraction of the characteristics and properties of the newly discovered particle such as couplings to SM particles, spin and CP-quantum numbers etc.

http://inspirehep.net/record/1241571/files/arXiv:1307.1347.pdf

Handbook of LHC Higgs Cross Sections: 3. Higgs Properties

In models with an extended Higgs sector there exists an alignment limit, in which the lightest CP-even Higgs boson mimics the Standard Model Higgs. The alignment limit is commonly associated with the decoupling limit, where all non-standard scalars are significantly heavier than the Z boson. However, alignment can occur irrespective of the mass scale of the rest of the Higgs sector. In this work we discuss the general conditions that lead to “alignment without decoupling”, therefore allowing for the existence of additional non-standard Higgs bosons at the weak scale. The values of tan β for which this happens are derived in terms of the effective Higgs quartic couplings in general two-Higgs-doublet models as well as in supersymmetric theories, including the MSSM and the NMSSM. Moreover, we study the information encoded in the variations of the SM Higgs-fermion couplings to explore regions in the m
 A
 − tan β parameter space.

/pdf/impersonating-the-standard-model-higgs-boson-alignment-2g91dyt6j4.pdf

Impersonating the Standard Model Higgs boson: Alignment without decoupling

The Majorana nature of neutrinos is strongly motivated from the theoretical and phenomenological point of view. A plethora of neutrino mass models, known collectively as Seesaw models, exist that could generate both a viable neutrino mass spectrum and mixing pattern. They can also lead to rich, new phenomenology, including lepton number non-conservation as well as new particles, that may be observable at collider experiments. It is therefore vital to search for such new phenomena and the mass scale associated with neutrino mass generation at high energy colliders. In this review, we consider a number of representative Seesaw scenarios as phenomenological benchmarks, including the characteristic Type I, II, and III Seesaw mechanisms, their extensions and hybridizations, as well as radiative constructions. We present new and updated predictions for analyses featuring lepton number violation and expected coverage in the theory parameter space at current and future colliders. We emphasize new production and decay channels, their phenomenological relevance and treatment across different facilities in $e^+e^-$, $e^-p$ and $pp$ collisions, as well as the available Monte Carlo tools available for studying Seesaw partners in collider environments.

/pdf/lepton-number-violation-seesaw-models-and-their-collider-4zkld7263o.pdf

Lepton Number Violation: Seesaw Models and Their Collider Tests

There are many indirect and direct experimental indications that the new particle H discovered by the ATLAS and CMS Collaborations has spin zero and (mostly) positive parity, and that its couplings to other particles are correlated with their masses. To a high degree of confidence, it is a Higgs boson, and here we examine the extent to which its couplings resemble those of the single Higgs boson of the Standard Model. Our global analysis of its couplings to fermions and massive bosons determines that they have the same relative sign as in the Standard Model. We also show directly that these couplings are highly consistent with a dependence on particle masses that is linear to within a few %, and scaled by the conventional electroweak symmetry-breaking scale to within 10%. We also give constraints on loop-induced couplings, on the total Higgs decay width, and on possible invisible decays of the Higgs boson under various assumptions.

/pdf/updated-global-analysis-of-higgs-couplings-47iql692uh.pdf

Updated global analysis of Higgs couplings

A vast literature on the theory and phenomenology of two-Higgs-doublet models (2HDM) exists since long. However, the present situation demands a revisit of some 2HDM properties. Now that a 125 GeV scalar resonance has been discovered at the LHC, with its couplings to other particles showing increasing affinity to the Standard Model Higgs-like behaviour, the 2HDM parameter space is more squeezed than ever. We briefly review the different parametrizations of the 2HDM potential and discuss the constraints on the parameter space arising from the unitarity and stability of the potential together with constraints from the oblique electroweak T-parameter. We also differentiate the consequences of imposing a global continuous U(1) symmetry on the potential from a discrete Z
 2 symmetry.

Scalar sector of two-Higgs-doublet models: A minireview

We study the conditions required to make the two-Higgs-doublet-model (2HDM) scalar potential stable up to the Planck scale. The lightest $CP$-even scalar is assumed to have been found at the LHC and the alignment limit is imposed in view of the LHC Higgs data. We find that ensuring stability up to scales $\ensuremath{\gtrsim}1{0}^{10}\text{ }\text{ }\mathrm{GeV}$ necessitates the introduction of a soft breaking parameter in the theory. Even then, some interesting correlations between the nonstandard masses and the soft breaking parameter need to be satisfied. Consequently, a 2HDM becomes completely determined by only two nonstandard parameters, namely, $\mathrm{tan}\ensuremath{\beta}$ and a mass parameter, ${m}_{0}$, with $\mathrm{tan}\ensuremath{\beta}\ensuremath{\gtrsim}3$. These observations make a 2HDM, in the stable alignment limit, more predictive than ever.

Search for a stable alignment limit in two-Higgs-doublet models

We investigate the scalar potential of a general S3-symmetric three-Higgs-doublet model. The outcome of our analysis does not depend on the fermionic sector of the model. We identify a decoupling limit for the scalar spectrum of this scenario. In view of the recent LHC Higgs data, we show our numerical results only in the decoupling limit. Unitarity and stability of the scalar potential demand that many new scalars must be lurking below 1 TeV. We provide numerical predictions for h→γγ and h→Zγ signal strengths, which can be used to falsify the theory.

Analysis of an extended scalar sector with S3 symmetry

We analyze the scalar sector properties of a general class of two-Higgs-doublet models which has a global U(1) symmetry in the quartic terms. We find constraints on the parameters of the potential from the considerations of unitarity of scattering amplitudes, the global stability of the potential and the ρ-parameter. We concentrate on the spectrum of the non-standard scalar masses in the decoupling limit which is preferred by the Higgs data at the LHC. We exhibit charged-Higgs induced contributions to the diphoton decay width of the 125 GeV Higgs boson and its correlation with the corresponding Zγ width.

Scalar sector properties of two-Higgs-doublet models with a global U(1) symmetry

In a truly model-independent approach, we reexamine a minimal extension of the Standard Model (SM) through the introduction of an additional U(1) symmetry leading to a new neutral gauge boson (Z′), allowing its kinetic mixing with the hypercharge gauge boson. An SM neutral scalar is used to spontaneously break this extra symmetry, leading to the mass of the Z′. Except for three right-handed neutrinos, no other fermions are added. We use the current LHC Drell-Yan data to put model-independent constraints in the parameter space of three quantities, namely, MZ′, the Z-Z′ mixing angle (αz), and the extra U(1) effective gauge coupling (gx′), which absorb all model dependence. We impose additional constraints from unitarity and low-energy neutrino-electron scattering. However, limits extracted from direct searches turn out to be most stringent. We obtain MZ′>4.4 TeV and |αz|<0.001 at 95% C.L., when the strength of the additional U(1) gauge coupling is the same as that of the SM SU(2)L.

Dipankar Das

Papers

Scalar sector of two-Higgs-doublet models: A minireview

Search for a stable alignment limit in two-Higgs-doublet models

Analysis of an extended scalar sector with S3 symmetry

Scalar sector properties of two-Higgs-doublet models with a global U(1) symmetry

Reappraisal of constraints on Z′ models from unitarity and direct searches at the LHC