Search

We discuss ways to discriminate at hadron colliders between a quasi-bound toponium state and a pseudoscalar Higgs boson A, as predicted in many extensions of the Standard Model. We apply the discussion to the excess of t tbar threshold events recently observed at the LHC by the CMS collaboration \cite{CMS}, which could be due to either possibility. Working in an effective theory in which only an additional pseudoscalar A boson is present in the spectrum, with a mass close to the 2 m_t threshold and a significant coupling to top quarks, we discuss the interference between A production in the dominant gluon-fusion process gg \to A with subsequent A\to t tbar decays, and the QCD continuum background, gg\to t tbar. While this interference is absent in the case of toponium, it is essential for evaluating the A interpretation of the CMS excess. It is difficult to resolve the peak/dip structure that it generates because of the experimental smearing of the t tbar invariant mass spectrum. However, by comparing the total A production rates for different integration domains of the t tbar invariant mass or, eventually, at different center of mass energies, one may be able to observe its effects. We then discuss additional mechanisms for A production in pp collisions, including loop-induced production in association with the lighter h boson, gg \to hA, and production in association with top-quark pairs, gg/q\bar q \to t tbar. A These mechanisms have small cross sections at the LHC, and their observation will necessitate higher luminosities or collider energies., Comment: 14 pages, 4 figures. We dedicate this paper to the memory of our dear friend Rohini Godbole, with whom we performed a study along similar lines

As a tribute to Steven Weinberg, we summarize the immense impact that he had on the understanding of the mechanism of spontaneous symmetry breaking and on the physics of the Higgs boson. In particular, four landmark contributions to this field are highlighted. A first one is his early work with Goldstone and Salam on spontaneously broken continuous symmetries that paved the way to the Higgs mechanism. A second towering breakthrough is his model of leptons which later became the Standard Model of particle physics and for which he was awarded the Nobel prize with Glashow and Salam. A third seminal work is the so-called Weinberg-Linde lower bound on the Higgs boson mass that was derived from the requirement of the stability of the electroweak vacuum. Finally, we will summarize his important contributions in model-building of new physics with extended Higgs sectors and their possible impact in flavor physics and CP-violation. The historical aspects as well as the contemporary way of viewing these four major topics are summarized and their impact on today Higgs physics, and more generally particle physics, is highlighted., Comment: 14 pages, 10 figures. Prepared for the volume "In memory of Steven Weinberg" to appear in Nuclear Physics B; v2: matches published version

We reanalyze the effective field theory (EFT) approach for the scenario in which the particles that account for the dark matter (DM) in the universe are vector states that interact only through the Standard Model-like Higgs boson. These DM particles are searched for in direct and indirect detection in astrophysical experiments and in invisible Higgs decays at the LHC. The constraints obtained in these two search types are complementary and correlated. In recent years, it has been advocated that the EFT approach is problematic for small DM mass and that it does not capture all the aspects of vector DM; one should thus rather interpret the searches in ultraviolet complete theories that are more realistic. In this note, we show that a more appropriate definition of the EFT with the introduction of an effective New Physics scale parameter, can encompass such issues. We illustrate this by matching the EFT to two examples of ultraviolet completions for it: the U(1) model with a dark photon and a model that was recently adopted by the LHC experiments in which vector-like fermions generate an effective interaction between the Higgs and the DM states at the one-loop level. Additionally, we find that the region of parameter space that is relevant for DM phenomenology is well inside the range of validity of the EFT. It thus provides a general parametrization of the effects of any ultraviolet model in the regime under exploration, making it the ideal framework for model-independent analyses of the vector DM Higgs-portal., Comment: 16 pages, 3 figures

The ATLAS and CMS collaborations at the LHC have recently announced evidence for the rare Higgs boson decay into a $Z$ boson and a photon. We analyze the interference between the process $gg\! \to \! H \! \to \! Z \gamma$ induced by loops of heavy particles, which is by far the dominant contribution to the signal, and the continuum $gg \to Z \gamma$ QCD background process mediated by light quark loops. This interference modifies the event yield, the resonance line-shape and the apparent mass of the Higgs boson. We calculate the radiative corrections to this interference beyond the leading-order approximation in perturbative QCD and find that, while differing numerically from the corresponding effects on the more studied $gg \! \to \! \gamma \gamma$ signal, they are generally rather small. As such, they do not impact significantly the interpretation of the present measurements of the $H \to Z \gamma$ decay mode., Comment: v1: 6 pages, 3 figures; v2: version published in journal

We explore a simple and renormalizable model which incorporates a new stable fermion that accounts for the dark matter in the universe and which, at the same time, provides an interpretation of two recent measurements that deviate from the expectation in the Standard Model: the muon anomalous magnetic moment and the mass of the W-boson recently performed by the CDF collaboration. The model has a fermionic sector that involves a singlet and a doublet fields and in which the lightest state is the DM and interacts mainly through the Higgs portal. Two realizations of such a possibility are considered: one in which the Higgs sector is minimal and another in which it is extended to contain two doublet fields., Comment: 20 pages, 5 figures

We perform a comprehensive study of a model in which the Higgs sector is extended to contain two Higgs doublet fields, with the four types of possibilities to couple to standard fermions, as well as an additional light pseudoscalar Higgs boson which mixes with the one of the two doublets. This 2HD+a model includes also a stable isosinglet massive fermion that has the correct thermal relic abundance to account for the dark matter in the Universe. We summarize the theoretical constraints to which the model is subject and then perform a detailed study of the phenomenological constraints. In particular, we discuss the bounds from the LHC in the search for light and heavy scalar resonances and invisible states and those from high precision measurements in the Higgs, electroweak and flavor sectors, addressing the possibility of explaining the deviation from the standard expectation of the anomalous magnetic moment of the muon and the $W$-boson mass recently observed at Fermilab. We also summarize the astrophysical constraints from direct and indirect detection dark matter experiments. We finally conduct a thorough analysis of the cosmic phase transitions and the gravitational wave spectrum that are implied by the model and identify the parameter space in which the electroweak vacuum is reached after single and multiple phase transitions. We then discuss the prospects for observing the signal of such gravitational waves in near future experiments such as LISA, BBO or DECIGO., Comment: 55 pages, 26 figures and 2 tables. Typos corrected, comments and illustrations added

We study a non-supersymmetric SO(10) Grand Unification Theory with a very high energy intermediate symmetry breaking scale in which not only gauge but also Yukawa coupling unification are enforced via suitable threshold corrections and matching conditions. For gauge unification, we focus on a few symmetry breaking patterns with the intermediate gauge groups ${\rm SU(4)_C \times SU(2)_L \times SU(2)_R}$ (Pati-Salam) and ${\rm SU(3)_C \times SU(2)_L \times SU(2)_R\times U(1)_{B-L}}$ (minimal left-right symmetry) assuming an additional global U(1) Peccei--Quinn symmetry, and having the Standard Model supplemented by a second Higgs doublet field at the electroweak scale. We derive the conditions as well as the approximate analytical solutions for the unification of the gauge coupling constants at the two-loop level and discuss the constraints from proton decay on the resulting high scale. Specializing to the case of the Pati-Salam intermediate breaking pattern, we then impose also the unification of the Yukawa couplings of third generation fermions at the high scale, again at the two-loop level. In the considered context, Yukawa unification implies a relation between the fermion couplings to the 10- and 126-dimensional scalar representations of the SO(10) group. We consider one such possible relation which is obtainable in an ${\rm E_6}$ model where the previous two scalar fields are part of a single multiplet. Taking into account some phenomenological features such as the absence of flavor changing neutral currents at tree-level, we derive constraints on the parameters of the low energy model, in particular on the ratio of the two Higgs doublets vacuum expectation values $\tan\beta$., Comment: 40 pages, 7 figures, Fig. 7 added compared to v1, comments are welcome

We present the program SuSpect3 that calculates the masses and couplings of the Higgs and supersymmetric particles predicted by the Minimal Supersymmetric Standard Model (MSSM). The model is implemented in both its non-constrained version, the MSSM, and its constrained versions, such as the minimal supergravity and the gauge or anomaly mediated supersymmetry breaking models, in which the soft supersymmetry-breaking parameters obey certain universal boundary conditions at the high energy scale. The low energy parameters are then obtained using renormalization group equations and electroweak symmetry breaking, and all the dominant radiative corrections have been consistently implemented. SuSpect3 is a major rewrite, in C++ object oriented programming, of the FORTRAN code SuSpect. It includes all the features of the earlier code in an improved and updated manner, and involves new options such as compressed SUSY scenarios, an MSSM-inflation model and the possibility of using the observed Higgs mass as an input. The main features and the use of the program are explained., Comment: 33 pages

The hMSSM is a special parameterization of the minimal supersymmetric extension of the Standard Model (MSSM) in which the mass of the lightest Higgs boson is automatically set to the LHC measured value, $M_h\!\!=\!\! 125$\,GeV, by adjusting the supersymmetric particle spectrum such that it provides the required amount of radiative corrections to the Higgs boson masses.\ The latter spectrum was in general assumed to be very heavy, as indicated by the present exclusion limits of the LHC, not to affect the phenomenology of the Higgs sector.\ In this work, we investigate the impact on the hMSSM by a light gaugino and higgsino sector, that is allowed by the present LHC data.\ In particular, we discuss the radiative corrections due to charginos and neutralinos to the Higgs boson masses and couplings and show that an hMSSM can still be realized in this context.\ We first describe how this scenario is implemented in the package SuSpect that generates the MSSM Higgs and supersymmetric spectra.\ We then analyze the possible impact of Higgs boson decays into these new states, as well as the reverse cascade channels with Higgs bosons in the final states, for the constraints on the MSSM Higgs sector at the LHC.\ We further explore the cosmological constraints on the hMSSM with a light gaugino--higgsino spectrum.\ We analyze the relic abundance of the lightest neutralino as a candidate of the dark matter in the Universe and the constraints on its mass and couplings by the present and future astroparticle physics experiments., Comment: 57 pages, 21 figures. Version to appear in JHEP

The new measurement of the $W$ boson mass performed by the CDF experiment at the Tevatron shows a significant deviation not only with the expectation in the Standard Model but also with other precision measurements performed at LEP, the Tevatron and the LHC. We nevertheless take this new measurement at face value and interpret it as an effect of new physics. We particularly try to link it with other possible anomalies such as the recent muon $g-2$ and consider a scenario that addresses some shortcomings of the Standard Model. We show that a model with two doublets and a light pseudoscalar Higgs fields, supplemented by a stable isosinglet fermion, can simultaneously explain the possible $M_W$ and $(g-2)_\mu$ anomalies and accounts for the weakly interacting massive particle that could be responsible of the dark matter in the universe., Comment: 6 pages, 4 figures

In the context of a two-Higgs doublet model, supplemented by an additional light pseudoscalar Higgs boson and a stable isosinglet fermion, we consider the possibility of addressing simultaneously the discrepancy from the standard expectation of the anomalous magnetic moment of the muon recently measured at Fermilab and the longstanding problem of the dark matter in the universe which can be accounted for by a thermal weakly interacting massive particle. We show that it is indeed possible, for a range of masses and couplings of the new light pseudoscalar and the fermionic states, to explain at the same time the two features while satisfying all other constraints from astroparticle physics and collider searches, including the constraints from flavor physics., Comment: 14 pages, 4 figures

We discuss the possibility of unifying in a simple and economical manner the Yukawa couplings of third generation fermions in a non-supersymmetric SO(10) model with an intermediate symmetry breaking, focusing on two possible patterns with intermediate Pati-Salam and minimal left-right groups. assume a minimal Yukawa sector at high energy, starting with two Higgs bi-doublets at the intermediate scale which then simply reduce to a two Higgs doublet model at the electroweak scale. We first enforce gauge coupling unification at the two-loop level by including the threshold corrections in the renormalisation group running which are generated by the heavy fields that appear at the intermediate symmetry breaking scale. We then study the running of the Yukawa couplings of the top quark, bottom quark and tau lepton at two-loops in these two breaking schemes, when the appropriate matching conditions are imposed. We find that the unification of the third family Yukawa couplings can be achieved while retaining a viable spectrum, provided that the ratio of the vacuum expectation values of the two Higgs doublet fields is large, $\tan\beta \approx 60$., Comment: 14 pages, 3 figures, published version, comments are welcome

We present an effective field theory of the $\Delta$-resonance as an interacting Weinberg's $(3/2,0)\oplus (0,3/2)$ field in the multi-spinor formalism. We derive its interactions with nucleons $N$, pions $\pi$ and photons $\gamma$, and compute the $\Delta$-resonance cross-sections in pion-nucleon scattering and pion photo-production. The theory contains only the physical spin-3/2 degrees of freedom. Thus, it is intrinsically consistent at the Hamiltonian level and, unlike the commonly used Rarita-Schwinger framework, does not require any additional ad hoc manipulation of couplings or propagators. The symmetries of hadronic physics select a unique operator for each coupling $N\pi\Delta$ and $\gamma\pi\Delta$. The proposed framework can be extended to also describe other higher-spin hadronic resonances., Comment: 8 pages, 2 figures

We present an effective field theory describing the relevant interactions of the Standard Model with an electrically neutral particle that can account for the dark matter in the Universe. The possible mediators of these interactions are assumed to be heavy. The dark matter candidates that we consider have spin 0, 1/2 or 1, belong to an electroweak multiplet with arbitrary isospin and hypercharge and their stability at cosmological scales is guaranteed by imposing a $\mathbb{Z}_2$ symmetry. We present the most general framework for describing the interaction of the dark matter with standard particles, and construct a general non-redundant basis of the gauge-invariant operators up to dimension six. The basis includes multiplets with non-vanishing hypercharge, which can also be viable DM candidates. We give two examples illustrating the phenomenological use of such a general effective framework. First, we consider the case of a scalar singlet, provide convenient semi-analytical expressions for the relevant dark matter observables, use present experimental data to set constraints on the Wilson coefficients of the operators, and show how the interplay of different operators can open new allowed windows in the parameter space of the model. Then we study the case of a lepton isodoublet, which involves co-annihilation processes, and we discuss the impact of the operators on the particle mass splitting and direct detection cross sections. These examples highlight the importance of the contribution of the various non-renormalizable operators, which can even dominate over the gauge interactions in certain cases., Comment: 38 pages, 4 figures, version published in JHEP, typos corrected

The new measurement of the muon's anomalous magnetic moment released by the Muon g-2 experiment at Fermilab sets strong constraints on the properties of many new particles. Using an effective field theory approach to the interactions of higher-spin fields, we evaluate the contribution of an electrically neutral and colour singlet spin-3/2 fermion to $(g-2)_\mu$ and derive the corresponding constraints on its mass and couplings. These constraints are then compared with the ones on spin-1/2 fermions, such as the vector-like leptons that are predicted by various extensions of the Standard Model, the excited leptons which appear in composite models, as well as the charginos and neutralinos of supersymmetric theories. Unlike these new spin-1/2 fermions, the spin-3/2 particles generate only small contributions to the muon anomalous magnetic moment., Comment: References updated, typos fixed and comments added; results unchanged

Using an effective field theory approach for higher-spin fields, we derive the interactions of colour singlet and electrically neutral particles with a spin higher than unity, concentrating on the spin-3/2, spin-2, spin-5/2 and spin-3 cases. We compute the decay rates and production cross sections in the main channels for spin-3/2 and spin-2 states at both electron-positron and hadron colliders, and identify the most promising novel experimental signatures for discovering such particles at the LHC. The discussion is qualitatively extended to the spin-5/2 and spin-3 cases. Higher-spin particles exhibit a rich phenomenology and have signatures that often resemble the ones of supersymmetric and extra-dimensional theories. To enable further studies of higher-spin particles at collider and beyond, we collect the relevant Feynman rules and other technical details., Comment: Published version. 38 pages, 6 figures. Comments welcome

Higgs-portal effective field theories are widely used as benchmarks in order to interpret collider and astroparticle searches for dark matter (DM) particles. To assess the validity of these effective models, it is important to confront them to concrete realizations that are complete in the ultraviolet regime. In this paper, we compare effective Higgs-portal models with scalar, fermionic and vector DM with a series of increasingly complex realistic models, taking into account all existing constraints from collider and astroparticle physics. These complete realizations include the inert doublet with scalar DM, the singlet-doublet model for fermionic DM and models based on spontaneously broken dark SU(2) and SU(3) gauge symmetries for vector boson DM. We also discuss the simpler scenarios in which a new scalar singlet field that mixes with the standard Higgs field is introduced with minimal couplings to isosinglet spin--$0, \frac12$ and 1 DM states. We show that in large regions of the parameter space of these models, the effective Higgs-portal approach provides a consistent limit and thus, can be safely adopted, in particular for the interpretation of searches for invisible Higgs boson decays at the LHC. The phenomenological implications of assuming or not that the DM states generate the correct cosmological relic density are also discussed., Comment: 54 pages, 24 figures

We reanalyze the effective field theory approach for the scenario in which the particles that account for the dark matter (DM) in the universe are vector states that interact only or mainly through the Standard Model-like Higgs boson observed at the LHC. This model-independent and simple approach, with a minimal set of new input parameters, is widely used as a benchmark in DM searches and studies in astroparticle and collider physics. We show that this effective theory could be the limiting case of ultraviolet complete models, taking as an example the one based on a spontaneously broken U(1) gauge symmetry that incorporates a dark gauge boson and an additional scalar that mixes with the standard Higgs boson. Hence, despite the presence of the new degrees of freedom, measurements of the invisible decay branching ratio of the Higgs boson, as performed at colliders such as the CERN LHC, can be interpreted consistently in such an effective framework and can be made complementary to results of DM searches in direct detection experiments., Comment: 16 pages, 5 figures

We review scenarios in which the particles that account for the Dark Matter (DM) in the Universe interact only through their couplings with the Higgs sector of the theory, the so-called Higgs-portal models. In a first step, we use a general and model-independent approach in which the DM particles are singlets with spin $0,\frac12$ or $1$, and assume a minimal Higgs sector with the presence of only the Standard Model (SM) Higgs particle observed at the LHC. In a second step, we discuss non-minimal scenarios in which the spin-$\frac12$ DM particle is accompanied by additional lepton partners and consider several possibilities like sequential, singlet-doublet and vector-like leptons. In a third step, we examine the case in which it is the Higgs sector of the theory which is enlarged either by a singlet scalar or pseudoscalar field, an additional two Higgs doublet field or by both; in this case, the matter content is also extended in several ways. Finally, we investigate the case of supersymmetric extensions of the SM with neutralino DM, focusing on the possibility that the latter couples mainly to the neutral Higgs particles of the model which then serve as the main portals for DM phenomenology. In all these scenarios, we summarize and update the present constraints and future prospects from the collider physics perspective, namely from the determination of the SM Higgs properties at the LHC and the search for its invisible decays into DM, and the search for heavier Higgs bosons and the DM companion particles at high-energy colliders. We then compare these results with the constraints and prospects obtained from the cosmological relic abundance as well as from direct and indirect DM searches in astroparticle physics experiments. The complementarity of collider and astroparticle DM searches is investigated in all the considered models., Comment: 260 pages, 105 figures. The review is open to comments and suggestions

Many extensions of the Standard Model contain (pseudo)scalar bosons with masses in the TeV range. At hadron colliders, such particles would predominantly be produced in gluon fusion and would decay into top quark pair final sates, a signal that interferes with the large QCD background $gg \to t\bar t$. This phenomenon is of interest for searches for by the LHC experiments. Here, we consider the signal and background interference in this process and study it in various benchmark scenarios, including models with extra singlet (pseudo)scalar resonances, two-Higgs doublet models, and the minimal supersymmetric extension of the SM with parameters chosen to obtain the measured light Higgs mass (the hMSSM). We allow for the possible exchanges of beyond the SM vector-like particles as well as scalar quarks. We calculate the possible interference effects including realistic estimates of the attainable detection efficiency and mass resolution. Studies of our benchmark scenarios indicate that searches with an LHC detector could permit the observation of the $t\bar t$ final states or constrain significantly large regions of the parameter spaces of the benchmark scenarios., Comment: 44 pages, 19 figures

The program HDECAY determines the partial decay widths and branching ratios of the Higgs bosons within the Standard Model with three and four generations of fermions, including the case when the Higgs couplings are rescaled, a general two--Higgs doublet model where the Higgs sector is extended and incorporates five physical states and its most studied incarnation, the minimal supersymmetric Standard Model (MSSM). The program addresses all decay channels including the dominant higher-order effects such as radiative corrections and multi-body channels. Since the first launch of the program, more than twenty years ago, important aspects and new ingredients have been incorporated. In this update of the program description, some of the developments are summarized while others are discussed in some detail., Comment: LaTeX, 41 pages

We propose a simple mechanism for copiously producing heavy Higgs bosons with enhanced decay rates to two photons at the LHC, within the context of the Minimal Supersymmetric extension of the Standard Model (MSSM). In the CP-conserving limit of the theory, such a diphoton resonance may be identified with the heavier CP-even $H$ boson, whose gluon-fusion production and decay into two photons are enhanced by loops of the lightest supersymmetric partner of the top quark $\tilde{t}_1$ when its mass $m_{\tilde{t}_1}$ happens to be near the $\tilde{t}^*_1\tilde{t}_1$ threshold, i.e.~for $m_{\tilde{t}_1}\!\simeq \!\frac12 M_H$. The scenario requires a relatively low supersymmetry-breaking scale $M_S\stackrel{<}{{}_\sim} 1$ TeV, but large values of the higgsino mass parameter, $\mu \stackrel{>}{{}_\sim} 1$ TeV, that lead to a strong $H \tilde{t}^*_1 \tilde{t}_1$ coupling. Such parameters can accommodate the observed mass and standard-like couplings of the 125 GeV $h$ boson in the MSSM, while satisfying all other constraints from the LHC and dark matter searches. Additional enhancement to the diphoton rate could be provided by Coulombic QCD corrections and, to a lesser extent, by resonant contributions due to $\tilde{t}_1^* \tilde{t}_1$ bound states. To discuss the characteristic features of such a scenario, we consider as an illustrative example the case of a diphoton resonance with a mass of approximately 750 GeV, for which an excess was observed in the early LHC 13 TeV data and which later turned out to be simply a statistical fluctuation., Comment: 12 pages, 2 figures, title changed and content revised in light of the current LHC results, to appear in Physics Letters B

We consider interference effects in the production via gluon fusion in LHC collisions at 13 TeV and decays into $\gamma \gamma$ and $t {\bar t}$ final states of one or two putative new resonant states $\Phi$, assumed here to be scalar and/or pseudo scalar particles. Although our approach is general, we use for our numerical analysis the example of the putative $750$ GeV state for which a slight excess was observed in the initial LHC $13$ TeV data. We revisit previous calculations of the interferences between the heavy-fermion loop-induced $gg \to \Phi \to \gamma \gamma$ signal and the continuum $gg\to \gamma\gamma$ QCD background, which can alter the production rate as well as modify the line-shape and apparent mass. We find a modest enhancement by $\sim 20$% under favorable circumstances, for a large $\Phi$ width. The effect of interference on the apparent scalar-pseudoscalar mass difference in a two-Higgs-doublet model is found to be also modest. An exploratory study indicates that similar effects are to be expected in the $gg \to \Phi \to Z \gamma$ channel. In this and other models with a large $\Phi$ total width, the dominant $\Phi$ decays are expected to be into $t \bar t$ final states. We therefore also study the effects of interference of the $gg\to \Phi \to t\bar t$ signal with the $gg\to t \bar t$ continuum QCD background and show that in the presence of standard fermions only in the $gg\to \Phi$ loops, it is destructive causing a dip in the $t \bar t$ mass distribution. Including additional vector-like quarks leads to a different picture as peaks followed by dips can then occur. We use the absence of such effects in ATLAS and CMS data to constrain models of the production and decays of the $\Phi$ state(s)., Comment: 31 pages, v2 contains additional comments

The data collected by the LHC collaborations at an energy of 13 TeV indicates the presence of an excess in the diphoton spectrum that would correspond to a resonance of a 750 GeV mass. The apparently large production cross section is nevertheless very difficult to explain in minimal models. We consider the possibility that the resonance is a pseudoscalar boson $A$ with a two--photon decay mediated by a charged and uncolored fermion having a mass at the $\frac12 M_A$ threshold and a very small decay width, $\ll 1$ MeV; one can then generate a large enhancement of the $A\gamma\gamma$ amplitude which explains the excess without invoking a large multiplicity of particles propagating in the loop, large electric charges and/or very strong Yukawa couplings. The implications of such a threshold enhancement are discussed in two explicit scenarios: i) the Minimal Supersymmetric Standard Model in which the $A$ state is produced via the top quark mediated gluon fusion process and decays into photons predominantly through loops of charginos with masses close to $\frac12 M_A$ and ii) a two Higgs doublet model in which $A$ is again produced by gluon fusion but decays into photons through loops of vector--like charged heavy leptons. We also comment on a minimal scenario in which the $A$ state couples only to photons through a heavy lepton loop and is both produced and decays through this coupling. In all these scenarios, while the mass of the charged fermion has to be adjusted to be extremely close to half of the $A$ resonance mass, the small total widths are naturally obtained if only suppressed three-body decay channels occur. Finally, the implications of some of these scenarios for dark matter are discussed., Comment: 15 pages, 4 figures, version submitted to journal with typos corrected

If the recent indications of a possible state $\Phi$ with mass $\sim 750$ GeV decaying into two photons reported by ATLAS and CMS in LHC collisions at 13 TeV were to become confirmed, the prospects for future collider physics at the LHC and beyond would be affected radically, as we explore in this paper. Even minimal scenarios for the $\Phi$ resonance and its $\gamma \gamma$ decays require additional particles with masses $\gtrsim \frac12 m_\Phi$. We consider here two benchmark scenarios that exemplify the range of possibilities: one in which $\Phi$ is a singlet scalar or pseudoscalar boson whose production and $\gamma \gamma$ decays are due to loops of coloured and charged fermions, and another benchmark scenario in which $\Phi$ is a superposition of (nearly) degenerate CP-even and CP-odd Higgs bosons in a (possibly supersymmetric) two-Higgs doublet model also with additional fermions to account for the $\gamma \gamma$ decay rate. We explore the implications of these benchmark scenarios for the production of $\Phi$ and its new partners at colliders in future runs of the LHC and beyond, at higher-energy $pp$ colliders and at $e^+ e^-$ and $\gamma \gamma$ colliders, with emphasis on the bosonic partners expected in the doublet scenario and the fermionic partners expected in both scenarios., Comment: 52 pages, 24 figures, v2 corrects two plots and some typos, and contains a new section on production in electron-positron collisions as well as additional references

A Higgs-like resonance with a mass of approximately 750 GeV has recently been observed at the LHC in its diphoton decay. If this state is not simply a statistical fluctuation which will disappear with more data, it will have important implications not only for particle physics but also for cosmology. In this note, we analyze the implications of such a resonance for the dark matter (DM). Assuming a spin 1/2 DM particle, we first verify that indeed the correct relic density can be obtain for a wide range of the particle mass and weak scale coupling that are compatible with present data. We then show that the combination of near future direct and indirect detection experiments will allow to probe the CP-nature of the mediator resonance, i.e. check whether it is a scalar or a pseudoscalar like particle., Comment: 9 pages, 5 figures, typos corrected, text improved, reference added and analysis extended by one more scenario; conclusions unchanged

An evidence for a diphoton resonance at a mass of 750 GeV has been observed in the data collected at the LHC run at a center of mass energy of 13 TeV. We explore several interpretations of this signal in terms of Higgs-like resonances in a two-Higgs doublet model and its supersymmetric incarnation, in which the heavier CP-even and CP-odd states present in the model are produced in gluon fusion and decay into two photons through top quark loops. We show that one cannot accommodate the observed signal in the minimal versions of these models and that an additional particle content is necessary. We then consider the possibility that vector-like quarks or leptons strongly enhance the heavy Higgs couplings to photons and eventually gluons, without altering those of the already observed 125 GeV state., Comment: 13 pages, 2 figures. v3: Discussion in section 3 extended and inconsistency in the pseudoscalar coupling corrected; conclusions unchanged

We propose an interpretation of the diboson excess recently observed by the ATLAS and CMS collaborations in terms of Kaluza-Klein excitations of electroweak gauge bosons stemming from a realization of a warped extra-dimensional model that is protected by a custodial symmetry. Besides accounting for the LHC diboson data, this scenario also leads to an explanation of the anomalies that have been observed in the measurements of the forward-backward asymmetries for bottom quarks at LEP and top quarks at the Tevatron., Comment: 10 pages, 2 figures

We summarise the prospects for Higgs boson physics at future proton-proton colliders with centre of mass (c.m.) energies up to 100 TeV. We first provide the production cross sections for the Higgs boson of the Standard Model from 13 TeV to 100 TeV, in the main production mechanisms and in subleading but important ones such as double Higgs production, triple production and associated production with two gauge bosons or with a single top quark. We then discuss the production of Higgs particles in beyond the Standard Model scenarios, starting with the one in the continuum of a pair of scalar, fermionic and vector dark matter particles in Higgs-portal models in various channels with virtual Higgs exchange. The cross sections for the production of the heavier CP-even and CP-odd neutral Higgs states and the charged Higgs states in two-Higgs doublet models, with a specific study of the case of the Minimal Supersymmetric Standard Model, are then given. The sensitivity of a 100 TeV proton machine to probe the new Higgs states is discussed and compared to that of the LHC with a c.m. energy of 14 TeV and at high luminosity., Comment: 61 pages, 16 figures, 6 tables; review article. v2: numbers and figures updated, aknowledgments modified, references added and typos corrected. Matches the published version

Vector-like quarks (VLQs) that are partners of the heavy top and bottom quarks are predicted in many extensions of the Standard Model (SM). We explore the possibility that these states could explain not only the longstanding anomaly in the forward-backward asymmetry in $b$-quark production at LEP, $A_{\rm FB}^b $, but also the more recent $\sim 2\sigma$ deviation of the cross section for the associated Higgs production with top quark pairs at the LHC, $\sigma(pp\to t\bar t H)$. Introducing three illustrative models for VLQs with different representations under the SM gauge group, we show that the two anomalies can be resolved while satisfying all other theoretical and experimental constraints. In this case, the three different models predict VLQ states in the $1-2$ TeV mass range that can be soon probed at the LHC. In a second step, we discuss the sensitivity on the VLQ masses and couplings that could be obtained by means of a percent level accuracy in the measurement of ratios of partial Higgs decay widths, in particular $\Gamma(H \! \to\! \gamma\gamma)/\Gamma(H \! \to\! ZZ^*)$ and $\Gamma(H \! \to \! b\bar b)/\Gamma(H \! \to \! WW^*)$. We show that top and bottom VL partners with masses up to $\sim 5$ TeV and exotic VLQs with masses in the $10$ TeV range can be probed at the high-luminosity LHC., Comment: 27 pages, 6 figures; v2: added references

With the upgrade of the LHC, the couplings of the observed Higgs particle to fermions and gauge bosons will be measured with a much higher experimental accuracy than current measurements, but will still be limited by an order 10% theoretical uncertainty. In this paper, we re-emphasize the fact that the ratio of Higgs signal rates into two photons and four leptons, $D_{\gamma \gamma}= \sigma(pp\to H \to \gamma \gamma)/\sigma(pp\to H \to ZZ^* \to 4\ell^\pm)$ can be made free of these ambiguities. Its measurement would be limited only by the statistical and systematic errors, which can in principle be reduced to the percent level at a high-luminosity LHC. This decay ratio would then provide a powerful probe of new physics effects in addition to high precision electroweak observables or the muon g-2. As an example, we show that the Higgs couplings to top quarks and vector bosons can be constrained at the percent level and that new Higgs or supersymmetric particles that contribute to the H$\gamma\gamma$ loop can be probed up to masses in the multi-TeV range and possibly larger than those accessible directly., Comment: 12 pages. V2: a few clarifications have been made and some references added

The implications of the discovery of a scalar Higgs boson at the LHC with a mass of approximately 125 GeV are summarised in the context of the Standard Model of particle physics with its unique scalar boson and of its most celebrated new physics extension, the minimal supersymmetric Standard Model or MSSM, in which the Higgs sector is extended to contain three neutral and two charged scalar bosons. Discussed are the implications from the measured mass, the production and decay rates of the observed particle and, in the MSSM, from the constraints in the search for the heavier Higgs states. The perspectives for Higgs and new physics searches at the next LHC upgrades as well as at future hadron and lepton colliders are then briefly summarized., Comment: 18 pages, 15 figures. Proceedings of the Corfu Summer Institute 2014 "School and Workshops on Elementary Particle Physics and Gravity", 3-21 September 2014, Corfu, Greece

The implications of the discovery of the Higgs boson at the LHC with a mass of approximately 125 GeV are summarised in the context of the minimal supersymmetric extension of the Standard Model, the MSSM. Discussed are the implications from the measured mass and production/decay rates of the observed particle and from the constraints in the search for the heavier Higgs states at the LHC., Comment: 21 pages, 27 figures. Review to appear in a special issue of EPJC and extended version of talks given at various recent conferences

One of the main implications of the LHC discovery of a Higgs boson with a mass $M_h \approx 126$ GeV is that the scale of supersymmetry-breaking in the Minimal Supersymmetric Standard Model (MSSM) might be rather high, $M_S \gg M_Z$. In this paper, we consider the high $M_S$ regime and study the spectrum of the extended Higgs sector of the MSSM, including the LHC constraints on the mass and the rates of the observed light $h$ state. In particular, we show that in a simplified model that approximates the important radiative corrections, the unknown scale $M_S$ (and some other leading SUSY parameters) can be traded against the measured value of $M_h$. One would be then essentially left with only two free parameters to describe the Higgs sector, tan$\beta$ and the pseudoscalar Higgs mass $M_A$, even at higher orders. The main phenomenological consequence of these high $M_S$ values is to reopen the low tan$\beta$ region, tan$\beta \lsim 3-5$, which was for a long time buried under the LEP constraint on the lightest $h$ mass when a low SUSY scale was assumed. We show that, in this case, the heavier MSSM neutral $H/A$ and charged $H^\pm$ states can be searched for in a variety of interesting final states such as decays into gauge and lighter Higgs bosons (in pairs on in mixed states) and decays into heavy top quarks. Examples of sensitivity on the $[tan\beta, M_A]$ parameter space at the LHC in these channels are given., Comment: 36 pages, 11 figures, 1 table, pdflatex; v2: quality of figures improved, typos corrected and references added

Using the full set of the LHC Higgs data from the runs at 7 and 8 TeV center of mass energies that have been released by the ATLAS and CMS collaborations, we determine the couplings of the Higgs particle to fermions and gauge bosons as well as its parity or CP composition. We consider ratios of production cross sections times decay branching fractions in which the theoretical (and some experimental) uncertainties as well as as some ambiguities from new physics cancel out. A fit of both the signal strengths in the various search channels that have been conducted, H -> Z Z, W W, gamma gamma, tau tau and b b, and their ratios shows that the observed ~126 GeV particle has couplings to fermions and gauge bosons that are Standard Model-like already at the 68% confidence level (CL). From the signal strengths in which the theoretical uncertainty is taken to be a bias, the particle is shown to be at most 68% CP-odd at the 99%CL and the possibility that it is a pure pseudoscalar state is excluded at the 4 sigma level when including both the experimental and theoretical uncertainties. The signal strengths also measure the invisible Higgs decay width which, with the same type of uncertainty analysis, is shown to be Gamma_H^inv / Gamma_H^SM < 0.52 at the 68%CL., Comment: 15 pages, 13 figures

Now that the Higgs particle has been observed by the ATLAS and CMS experiments at the LHC, the next endeavour would be to probe its fundamental properties and to measure its couplings to fermions and gauge bosons with the highest possible accuracy. However, the measurements will be limited by significant theoretical uncertainties that affect the production cross section in the main production channels as well as by experimental systematical errors. Following earlier work, we propose in this paper to consider ratios of Higgs production cross sections times decay branching ratios in which most of the theoretical uncertainties and some systematical errors, such as the ones due to the luminosity measurement and the Higgs decay branching fractions, cancel out. The couplings of the Higgs particle could be then probed in a way that will be mostly limited by the statistical accuracy achievable at the LHC and accuracies at the percent level are foreseen for some of the ratios at the end of the LHC run. At the theoretical level, these ratios are also interesting as they do not involve the ambiguities that affect the Higgs total decay width in new physics scenarios. To illustrate how these ratios can be used to determine the Higgs couplings, we perform a rough analysis of the recent ATLAS and CMS data which shows that there is presently no significant deviation from the Standard Model expectation., Comment: 15 pages, 2 tables and 2 figures. V2: typos corrected and small comments added

The long awaited discovery of a new light scalar at the LHC opens up a new era of studies of the Higgs sector in the SM and its extensions. In this paper we discuss the consequences of the observation of a light Higgs boson with the mass and rates reported by the ATLAS and CMS collaborations on the parameter space of the phenomenological MSSM, including also the so far unsuccessful LHC searches for the heavier Higgs bosons and supersymmetric particle partners in missing transverse momentum as well as the constraints from B physics and dark matter. We explore the various regimes of the MSSM Higgs sector depending on the parameters MA and tan beta and show that only two of them are still allowed by all present experimental constraints: the decoupling regime where there is only one light and standard--like Higgs boson and the supersymmetric regime in which there are light supersymmetric particle partners affecting the decay properties of the Higgs boson, in particular its di-photon and invisible decays., Comment: 21 pages, 9 figures v2 - Discussion of the impact of LHC data extended, scan statistics increased, a few figures added and typos corrected

We consider the process in which a Higgs particle is produced in association with jets and show that monojet searches at the LHC already provide interesting constraints on the invisible decays of a 125 GeV Higgs boson. Using the existing monojet searches performed by CMS and ATLAS, we show the 95% confidence level limit on the invisible Higgs decay rate is of the order of the total Higgs production rate in the Standard Model. This limit could be significantly improved when more data at higher center of mass energies are collected, provided systematic errors on the Standard Model contribution to the monojet background can be reduced. We also compare these direct constraints on the invisible rate with indirect ones based on measuring the Higgs rates in visible channels. In the context of Higgs portal models of dark matter, we then discuss how the LHC limits on the invisible Higgs branching fraction impose strong constraints on the dark matter scattering cross section on nucleons probed in direct detection experiments., Comment: 6 pages, 3 figures; v2: references added; v3: monojet and Higgs data updated, version published in EPJC

The search for the effects of heavy fermions in the extension of the Standard Model with a fourth generation is part of the experimental program of the Tevatron and LHC experiments. Besides being directly produced, these states affect drastically the production and decay properties of the Higgs boson. In this note, we first reemphasize the known fact that in the case of a light and long-lived fourth neutrino, the present collider searches do not permit to exclude a Higgs boson with a mass below the WW threshold. In a second step, we show that the recent results from the ATLAS and CMS collaborations which observe an excess in the $\gamma \gamma$ and $4\ell^\pm$ search channels corresponding to a Higgs boson with a mass $M_H \approx 125$ GeV, cannot rule out the fourth generation possibility if the $H \to \gamma \gamma$ decay rate is evaluated when naively implementing the leading ${\cal O}(G_F m_{f'}^2)$ electroweak corrections. Including the exact next-to-leading order electroweak corrections leads to a strong suppression of the $H \to \gamma \gamma$ rate and makes this channel unobservable with present data. Finally, we point out that the observation by the Tevatron collaborations of a $\gsim 2\sigma$ excess in the mass range $M_H = 115$-135 GeV in the channel $q\bar q \to WH \to Wb\bar b$ can definitely not be accommodated by the fourth generation fermion scenario. All in all, if the excesses observed at the LHC and the Tevatron are indeed due to a Higgs boson, they unambiguously exclude the perturbative fermionic fourth generation case. In passing, we also point out that the Tevatron excess definitely rules out the fermiophobic Higgs scenario as well as scenarios in which the Higgs couplings to gauge bosons and bottom quarks are significantly reduced., Comment: 8 pages, 3 figures; Discussion of the recent Higgs results in a Note added

I review the theoretical aspects of the physics of Higgs bosons, focusing on the elements that are relevant for the production and detection at present hadron colliders. After briefly summarizing the basics of electroweak symmetry breaking in the Standard Model, I discuss Higgs production at the LHC and at the Tevatron, with some focus on the main production mechanism, the gluon-gluon fusion process, and summarize the main Higgs decay modes and the experimental detection channels. I then briefly survey the case of the minimal supersymmetric extension of the Standard Model. In a last section, I review the prospects for determining the fundamental properties of the Higgs particles once they have been experimentally observed., Comment: 21 pages, 15 figures. Talk given at the XXV International Symposium on Lepton Photon Interactions at High Energies (Lepton Photon 11), 22-27 August 2011, Mumbai, India

The search for the a Standard Model Higgs boson at the LHC is reaching a critical stage as the possible mass range for the particle has become extremely narrow and some signal at a mass of about 125 GeV is starting to emerge. We study the implications of these LHC Higgs searches for Higgs portal models of dark matter in a rather model independent way. Their impact on the cosmological relic density and on the direct detection rates are studied in the context of generic scalar, vector and fermionic thermal dark matter particles. Assuming a sufficiently small invisible Higgs decay branching ratio, we find that current data, in particular from the XENON experiment, essentially exclude fermionic dark matter as well as light, i.e. with masses below 50 GeV, scalar and vector dark matter particles. Possible observation of these particles at the planned upgrade of the XENON experiment as well in collider searches is discussed., Comment: 6 pages, 5 figures; references added, typos corrected, discussion and figure for LHC modified

In this note, we respond to the comments and criticisms made by the representatives of the CDF and D0 collaborations on our recent papers in which we point out that the theoretical uncertainties in the Higgs production cross section have been largely un- derestimated and, if properly taken into account, will significantly loosen the Tevatron Higgs exclusion bounds. We show that our approach to the theoretical uncertainties is reasonable and fully justified. In particular, we show that our procedure is not very different from that adopted by the LHC experiments and if the latter is used in the Tevatron case, one obtains much larger uncertainties that those assumed by the CDF and D0 collaborations. Furthermore, we provide additional details on our statistical analysis of the CDF and D0 exclusion limit and show that it is conceptually correct., Comment: 17 pages, 3 figures. Extended version of talks given at several winter conferences by the authors

We discuss the implications of the recent constraints on the Higgs sector of the Minimal Supersymmetric extension of the Standard Model obtained by the ATLAS and CMS collaborations at the lHC with $\sqrt s=7$ TeV and 36 pb$^{-1}$ of data. The main production and detection channel that is relevant in these analyses is the gluon--gluon and bottom quark fusion mechanisms leading to neutral Higgs bosons which subsequently decay into tau lepton pairs, $gg, b\bar b \to Higgs \to \tau^+\tau^-$. In this note, we show that: i) the exclusion limits are in fact more general than indicated by the ATLAS and CMS analyses and are essentially independent of the scenario for the supersymmetric particle spectrum; ii) when the exclusion limits are applied to the lowest theory prediction for the Higgs production cross section times branching ratio, when all theoretical uncertainties are taken into account, the bounds are somewhat less stringent; iii) the exclusion limits from the $pp \to Higgs \to \tau^+ \tau^-$ process are so strong that only a modest improvement would be possible when other MSSM Higgs detection channels are considered, even with femtobarn level accumulated data. Finally and most important, we point out that the prospects for the search for the Standard Model Higgs boson in the inclusive $gg \to H \to \tau^+\tau^-$ channel, that is not currently considered by the ATLAS and CMS collaborations, turn out to be very promising and with a few inverse femtobarn data it might provide a convincing discovery signal in the difficult 115--135 GeV mass range for the standard Higgs boson., Comment: 12 pages, 5 figures

We analyze the production of the neutral Higgs particles of the Minimal Supersymmetric extension of the Standard Model at the Fermilab Tevatron collider. We consider the two main production and detection channels: gluon--gluon and bottom quark fusion leading to Higgs bosons which subsequently decay into tau leptons, $gg, b\bar b \to {\rm Higgs} \to \tau^+\tau^-$. We update the production cross sections and the decay branching ratios and obtain production rates that are significantly smaller at high masses than the ones used by the CDF and D0 experiments in their search. We then evaluate the various theoretical uncertainties that affect these rates, uncertainties that have not been been considered in the experimental analyses and which turn out to be rather large. Including these two effects will significantly loosen the constraints obtained on the supersymmetric Higgs sector at the Tevatron., Comment: 10 pages, 2 tables, 3 figures. Version to appear in Physics Letter B

We analyze the production of Higgs particles at the early stage of the CERN large Hadron Collider with a 7 TeV center of mass energy (lHC). We first consider the case of the Standard Model Higgs boson that is mainly produced in the gluon-gluon fusion channel and to be detected in its decays into electroweak gauge bosons, $gg\to H \to WW,ZZ,\gamma\gamma$. The production cross sections at $\sqrt s=7$ TeV and the decay branching ratios, including all relevant higher order QCD and electroweak corrections, are evaluated. An emphasis is put on the various theoretical uncertainties that affect the production rates: the significant uncertainties from scale variation and from the parametrization of the parton distribution functions as well as the uncertainties which arise due to the use of an effective field theory in the calculation of the next-to-next-to-leading order corrections. The parametric uncertainties stemming from the values of the strong coupling constant and the heavy quark masses in the Higgs decay branching ratios, which turn out to be non-negligible, are also discussed. The implications for different center of mass energies of the proton collider, $\sqrt s=8$-10 TeV as well as for the design energy $\sqrt s= 14$ TeV, are briefly summarized. We then discuss the production of the neutral Higgs particles of the Minimal Supersymmetric extension of the Standard Model in the two main channels: gluon-gluon and bottom quark fusion leading to Higgs bosons which subsequently decay into tau lepton or $b$-quark pairs, $gg, b\bar b \to {\rm Higgs} \to \tau^+\tau^-,b\bar b$. The Higgs production cross sections at the $\lhc$ and the decay branching ratios are analyzed. The associated theoretical uncertainties are found to be rather large and will have a significant impact on the parameter space of the model that can be probed., Comment: 49 pages, 19 figures, 9 tables. Addendum included compared to published version with updated results at a c.m. energy of 8 TeV

In a recent paper, we updated the theoretical predictions for the production cross sections of the Standard Model Higgs boson at the Tevatron and estimated the various uncertainties affecting these predictions. We found that there is a large theoretical uncertainty, of order 40%, on the cross section for the main production channel, gluon-gluon fusion into a Higgs boson. Since then, a note from the Higgs working groups of the CDF and D0 collaborations criticizing our modeling of the $gg\to H$ cross section has appeared. In this addendum, we answer to this criticism point by point and, in particular, perform an analysis of $\sigma(gg\to H)$ for a central value of the renormalization and factorization scales $\mu_0=\frac12 M_H$ for which higher order corrections beyond next-to-next-to-leading order (that we discarded in our previous analysis) are implicitly included. Our results show that the new Tevatron exclusion bound on the Higgs boson mass, $M_{H}!=!158$-175 GeV at the 95% confidence level, is still largely debatable., Comment: 10 pages, 6 figures, 1 table. Addendum to http://arxiv.org/abs/1003.4266

The CDF and D0 experiments have reported on the measurement of the forward-backward asymmetry of top quark pair production at the Tevatron and the result is that it is more than 2 standard deviations above the predicted value in the Standard Model. This has to be added to the longstanding anomaly in the forward-backward asymmetry for bottom quark production at LEP which is 3 standard deviations different from the Standard Model value. The discrepancy in the bottom asymmetry can be accounted for by the contributions of Kaluza-Klein excitations of electroweak gauge bosons at LEP in warped extra dimensional models in which the fermions are localized differently along the extra dimension so that the gauge interactions of heavy third generation fermions are naturally different from that of light fermions. In this paper, we show that it is more difficult to elaborate a model generating a significant top asymmetry in a similar way -- through exchanges of Kaluza-Klein gluons at the Tevatron -- due to the indirect constraints originating from precision electroweak data., Comment: 4 pages, 1 figure, published version

One of the main motivations for low energy supersymmetric theories is their ability to address the hierarchy and naturalness problems in the Higgs sector of the Standard Model. In these theories, at least two doublets of scalar fields are required to break the electroweak symmetry and to generate the masses of the elementary particles, resulting in a rather rich Higgs spectrum. The search for the Higgs bosons of Supersymmetry and the determination of their basic properties is one of the major goals of high--energy colliders and, in particular, the LHC which will soon start operation. We review the salient features of the Higgs sector of the Minimal Supersymmetric Standard Model and of some of its extensions and summarize the prospects for probing them at the LHC and at the future ILC., Comment: 50 pages. To be published in "Supersymmetry on the Eve of the LHC" a special volume of European Physical Journal C in memory of Julius Wess. v2: references and comments added, some typos corrected

This article reviews the physics case for the ILC. Baseline running at 500 GeV as well as possible upgrades and options are discussed. The opportunities on Standard Model physics, Higgs physics, Supersymmetry and alternative theories beyond the Standard Model are described., Comment: The full author list is inside the paper

We consider the Randall-Sundrum extra dimensional model with fields propagating in the bulk based on an extended electroweak gauge symmetry with specific fermion charges and localizations that allow to explain the LEP anomaly of the forward--backward asymmetry for $b$--quarks, $A_{FB}^b$. We study the manifestations of the strongly--interacting and electroweak gauge boson Kaluza--Klein excitations $\vkk$ at the LHC, with masses of the order of a few TeV, which dominantly decays into top and bottom quark pairs. We first analyze the two--body tree--level production processes $pp \to t\bar t$ and $b\bar b$ in which the Kaluza--Klein (KK) excitations of gauge bosons are exchanged. We find that the additional channels can lead to a significant excess of events with respect to the Standard Model prediction; characteristic top quark polarization and angular asymmetries are quantitatively studied and turn out to probe the chiral structure of couplings to excited states. We then analyze higher order production processes for the gauge boson excitations which have too weak or no couplings to light quarks and, in particular, the loop induced process $gg \to \vkk \to t\bar t$ and $b\bar b$ in which the anomalous $gg \vkk $ four--dimensional vertex has to be regulated. The RS effects in this process, as well as in the four--body reactions $pp \to t\bar t b\bar b$, $t\bar t t\bar t$, $b\bar b b\bar b$ and in the related three--body reactions $gb \to b t \bar t$, $b b\bar b$, in which the $\vkk$ excitations are mainly radiated off the eavy quarks, are shown to be potentially difficult to test at LHC, due to small hase space and low parton density for $\mkk \gsim 3$ TeV., Comment: 31 pages, 9 figures, usages axodraw.sty (included)