Search

We present a 5D Z2-symmetric IR-UV-IR model with a warped KK-parity under which the bulk fields have towers of either even or odd KK-modes. We show that this Z2-symmetric geometry is equivalent to two times the UV-IR geometry (Randall-Sundrum model) provided each bulk field is subject to Neumann (or mixed) and Dirichlet boundary conditions at the UV-brane for even and odd fields, respectively. The 5D Standard Model (SM) bosonic sector is considered, such that in the 4D low-energy effective theory the Z2-even zero-modes correspond to the SM degrees of freedom, whereas the Z2-odd zero modes serve as a dark sector. In the zero-mode scalar sector, the even scalar mimics the SM Higgs boson, while the odd scalar (dark-Higgs) is stable and serves as a dark matter candidate. Implications for this dark matter are discussed; it is found that the dark-Higgs can provide only a small fraction of the observed dark matter abundance.

We explore the possibilities in two-Higgs-doublet models (2HDMs) of Type I and Type II for Higgs states with mass below about 60 GeV, i.e. less than half of the ~125 GeV mass of the observed SM-like Higgs boson. We identify the latter as either the lighter or the heavier CP-even state, h or H, and employ scans of the 2HDM parameter space taking into account all relevant theoretical and experimental constraints, including the most up-to-date Higgs signal strength measurements. We find that, in both Type I and Type II models, such light Higgs states are phenomenologically viable and can lead to interesting signatures. Part of the relevant parameter space may be testable with the existing 8 TeV LHC data, e.g. by looking for direct production of the light state via gg-fusion or bb-associated-production using its \tau^+ \tau^- and \mu^+ \mu^- decays at low invariant mass., Comment: 19 pages, 15 figures, references and a short comment added

I summarize the basic theory and selected phenomenology for the Higgs boson(s) of the standard model, the minimal supersymmetric model and some extensions thereof, including the next-to-minimal supersymmetric model.

We derive the effective potential for the Standard Model Higgs-boson sector interacting with Kaluza-Klein excitations of the graviton ($h_\mu^{\nu n}$) and the radion ($\phi$) and show that {\it only} the Standard Model vacuum solution of $\partial V(h)/\partial h =0$ (h is the Higgs field) is allowed. We then consider the consequences of the curvature-scalar mixing xi R \Hhat^\dagger \Hhat$ (where $\Hhat$ is a Higgs doublet field on the visible brane), which causes the physical mass eigenstates h and $\phi$ to be mixtures of the original Higgs and radion fields. First, we discuss the theoretical constraints on the allowed parameter space. Next, we give precise procedures for computing the h and $\phi$ couplings given the {\it physical} eigenstate masses, mh and $\mphi$, xi and the new physics scales of the model. We show that LEP/LEP2 data implies that not both the h and $\phi$ can be light. In the allowed region of parameter space, we examine numerically the couplings and branching ratios of the h and $\phi$ for several cases with $\mh=120\gev$ and $\mphi\leq 300\gev$. The resulting prospects for detection of the h and $\phi$ at the LHC, a future LC and a $\gam\gam$ collider are reviewed. For moderate $|\xi|$, both the anomalous $h\to gg$ coupling and (when $\mh>2\mphi$) the non-standard decay channel $h \to \phi\phi$ can substantially impact h discovery. Presence of the latter is a direct signature for non-zero xi. We find that $BR(h \to \phi\phi)$ as large as $30 \div 40 %$ is possible when $|\xi|$ is large. Conversely, if $\mphi>2\mh$ then $BR(\phi\to hh)$ is generally large. The feasibility of experimentally measuring the anomalous gg and $\gam\gam$ couplings of the h and $\phi$ is examined., Comment: 62 page, 30 figures. Because some figures had to be reduced in resolution, they will only accurately appear in magnified ghostview. For full abstract see paper

We summarize some of the unique physics capabilities of a e- e- collider and its companion facilities for e- γ and γγ collisions.

We extend the two-Higgs doublet models of Type I and Type II by adding a real gauge-singlet scalar S dark matter candidate (2HDMS models). We impose theoretical constraints deriving from perturbativity, stability, unitarity and correct electroweak symmetry breaking and require that the lightest CP-even Higgs, h, fit the LHC data for the $\sim 125.5$ GeV state at the $68\%$~C.L. after including existing constraints from LEP and B physics and LHC limits on the heavier Higgs bosons. We find that these models are easily consistent with the LUX and SuperCDMS limits on dark-matter-Nucleon scattering and the observed $\Omega h^2$ for S masses above about 55 GeV. At lower $m_S$, the situation is more delicate. For points with $m_S$ in the 6-25 GeV range corresponding to the CDMS~II and CRESST-II positive signal ranges, the dark-matter-Nucleon cross sections predicted by the Type I and Type II models more or less automatically fall within the $95\%-99\%$~C.L. signal region boundaries. Were it not for the LUX and SuperCDMS limits, which exclude all (almost all) such points in the case of Type I (Type II), this would be a success for the 2HDMS models. In fact, in the case of Type II there are a few points with 5.5 GeV $\leq m_S \leq$ 6.2 GeV that survive the LUX and SuperCDMS limits and fall within the CDMS~II 99\% C.L. signal region. Possibilities for dark matter to be isospin-violating in this 2HDMS context are also examined., Comment: 30 pages, 20 figures, 6 tables

We study the use of polarization asymmetries at a muon collider to determine the CP-even and CP-odd couplings of a Higgs boson to $\mu^+mu^-$. We determine achievable accuracy as a function of beam polarization and luminosity. The appropriate techniques for dealing with the polarization precession are outlined. Strategies especially appropriate for a two-Higgs-doublet model (including the MSSM) are given. Our general conclusion is that polarization will be very useful, especially if the proton source is such that full luminosity in the storage ring can be retained even after imposing cuts on the originally accepted muons necessary for $P\gsim 0.4$ for each beam., Comment: 33 pages, 10 figures, equations.sty

Already in the simplest two-Higgs-doublet model with CP violation in the Higgs sector, the $3\times3$ mixing matrix for the neutral Higgs bosons can substantially modify their couplings, thereby endangering the ``classical'' Higgs search strategies. However, there are sum rules relating Yukawa and Higgs-Z couplings which ensure that the ZZ, $b\anti b$ and $t\anti t$ couplings of a given neutral 2HDM Higgs boson cannot all be simultaneously suppressed. This result implies that any single Higgs boson will be detectable at an e^+e^- collider if the Z+Higgs, $b\anti b+$Higgs {\it and} $t\anti t+$Higgs production channels are all kinematically accessible {\it and} if the integrated luminosity is sufficient. We explore, as a function of Higgs mass, the luminosity required to guarantee Higgs boson detection, and find that for moderate $\tan\beta$ values the needed luminosity is unlikely to be available for all possible mixing scenarios. The additional difficulties for the case when the two-doublet Higgs sector is extended by adding one more singlet are summarized. Implications of the sum rules for Higgs discovery at the Tevatron and LHC are briefly discussed., Comment: 16 pages, 4 postscript figures, references added in replacement

Assuming perturbativity up to a high energy scale $\ensuremath{\sim}{10}^{16}\mathrm{GeV}$, we demonstrate that a future ${e}^{+}{e}^{\ensuremath{-}}$ linear collider operating at $\sqrt{s}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}500\mathrm{GeV}$ with $\ensuremath{\int}L\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}500{\mathrm{fb}}^{\ensuremath{-}1}$ per year (such as the recently proposed TESLA facility) will detect a Higgs boson signal regardless of the complexity of the Higgs sector and of how the Higgs bosons decay.

We demonstrate that the relative size of the CP-even and CP-odd couplings of a Higgs boson to $t\anti t$ can be determined at the LHC at a level that is very useful in discriminating between models, provided that a Higgs signal can be observed at a reasonable $S/\sqrt B$ level in the $t\anti t\h$ (with $\h\to\gamma\gamma$ or $b\anti b)$ channels. In particular, the CP-even nature of a SM-like Higgs boson's coupling to $t\anti t$ can be confirmed with very substantial statistical significance using the $t\anti t \h$ ($\h\to \gamma\gamma$) production/decay mode. The key to achieving good discrimination between models is to make full use of the difference in the final state distributions for different ratios of the CP-even to CP-odd couplings. For our analysis, we employ the very convenient and simple optimal analysis procedures derived previously., Comment: 12 pages

We demonstrate that supersymmetric decays, as typified by the predictions of several grand unified theory (GUT)-scale boundary condition choices, do not prevent detection of Z{sup {star}}{r_arrow}H{sup 0}A{sup 0},H{sup +}H{sup {minus}}, at a 1{minus}4TeV e{sup +}e{sup {minus}} or {mu}{sup +}{mu}{sup {minus}} collider operating at anticipated luminosity. For much of the parameter space the relative branching ratios for various supersymmetric (SUSY) and non-SUSY decays can be measured with sufficient accuracy that different GUT-scale boundary condition choices can be distinguished from one another at a very high confidence level. {copyright} {ital 1997} {ital The American Physical Society}

The optimal procedure for extracting the coefficients of different components of a cross section which takes the form of unknown coefficients times functions of known kinematical form is developed. When applied to e + e → tt+Higgs production at √ s = 1TeV and integrated luminosity of 200 fb 1 , we find that the tt →Higgs CP-even and CP-odd couplings and, to a lesser extent, the ZZ →Higgs (CP-even) coupling can be extracted with reasonable errors, assuming the Higgs sector parameter choices yield a significant production rate. Indeed, the composition of a mixed-CP Higgs eigenstate can be determined with sufficient accuracy that a SM-like CPeven Higgs boson can be distinguished from a purely CP-odd Higgs boson at a high level of statistical significance, and vice versa.

We develop diagnostic tools that would provide incontrovertible evidence for the presence of more than one Higgs boson near 125 GeV in the LHC data., Comment: 4 pages, 2 figures

We examine the level of enhancement that can be achieved in the ZZ and \gamma\gamma channels for a two-Higgs-doublet model Higgs boson (either the light h or the heavy H) with mass near 125 GeV after imposing all constraints from LEP data, B physics, precision electroweak data, vacuum stability, unitarity and perturbativity. The latter constraints restrict substantially the possibilities for enhancing the gg -> h -> \gamma\gamma or gg -> H -> \gamma\gamma signal relative to that for the SM Higgs, hSM. Further, we find that a significant enhancement of the gg -> h -> \gamma\gamma or gg -> H -> \gamma\gamma signal in Type II models is possible only if the gg -> h -> ZZ or gg -> H -> ZZ mode is even more enhanced, a situation disfavored by current data. In contrast, in the Type I model one can achieve enhanced rates in the \gamma\gamma final state for the h while having the ZZ mode at or below the SM rate - the largest [gg -> h -> \gamma\gamma]/[gg -> hSM -> \gamma\gamma] ratio found is of order ~1.3 when the two Higgs doublet vacuum expectation ratio is tan\beta = 4 or 20 and the charged Higgs boson has its minimal LEP-allowed value of m_{H^\pm} = 90 GeV., Comment: 15 pages, 9 figures

We examine the extent to which it is possible to realize the NMSSM "ideal Higgs" models espoused in several papers by Gunion et al in the context of partially universal GUT scale boundary conditions. To this end we use the powerful methodology of nested sampling. We pay particular attention to whether ideal-Higgs-like points not only pass LEP constraints but are also acceptable in terms of the numerous constraints now available, including those from the Tevatron and $B$-factory data, $(g-2)_\mu$ and the relic density $\Omega h^2$. In general for this particular methodology and range of parameters chosen, very few points corresponding to said previous studies were found, and those that were found were at best $2\sigma$ away from the preferred relic density value. Instead, there exist a class of points, which combine a mostly singlet-like Higgs with a mostly singlino-like neutralino coannihilating with the lightest stau, that are able to effectively pass all implemented constraints in the region $80, Comment: 22 pages, 15 figures

We consider the production of the Higgs boson(s) of both the standard model and the minimal supersymmetric model via the collision of two polarized backscattered laser photons at an [ital e][sup +][ital e[minus]] linear collider. The anticipated photon-photon luminosity should be sufficient to allow detection of any one of these Higgs bosons over a significant range of Higgs boson masses. In particular, the Higgs boson mass reach in the [gamma][gamma] collider mode may be somewhat larger than that of the corresponding [ital e][sup +][ital e[minus]] linear collider. Moreover, [gamma][gamma][r arrow][ital H] will provide a measure of the [gamma][gamma][ital H] coupling which arises at one loop, and therefore is potentially sensitive to all charged particles which gain mass through the Higgs mechanism.

For some choices of soft SUSY-breaking parameters, the LSP is a stable neutralino [Formula: see text], the NLSP is a chargino [Formula: see text] almost degenerate in mass with the LSP [Formula: see text], and all other sparticles are relatively heavy. We discuss the potential of hadron colliders, particularly the LHC, for studying such models.

We outline the motivations for having a light Higgs boson with fairly SM-like couplings to WW, ZZ but with unusual decays that elude LEP limits and that will also make its detection at the LHC quite challenging. The Next-to-Minimal Supersymmetric Model provides a very appealing model of this type.

Recent BaBar limits on $\br(\Upsilon(3S)\to \gam a\to \gam \tau^+\tau^-)$ and $\br(\Upsilon(3S)\to \gam a\to \gam \mu^+\mu^-)$ provide increased constraints on the $a b\anti b$ coupling of a CP-odd Higgs boson, $a$, with $m_a7.5\gev$ scenarios, which are theoretically the most favored, survive. We also outline the impact of the new ALEPH LEP results in the $\epem\to Z+4\tau$ channel. For $\tan\beta\geq 3$, only NMSSM ideal Higgs scenarios with $m_{h_1}\gsim 98\gev$ and $m_{a_1}$ close to $2m_B $ satisfy the ALEPH limits. For $\tan\beta\lsim 2$, the ALEPH limits are easily satisfied for the most theoretically preferred NMSSM scenarios, which are those with $m_{a_1}$ close to $2m_B$ and $m_{h_1}\sim 90\gev-100\gev$., Comment: 24 pages, 25 figures, paper updated to incorporate final ALEPH limits in Z+4\tau channel.

We classify the variety of low-energy supersymmetric signatures that can be probed at future colliders. We focus on phenomena associated with the minimal supersymmetric extension of the Standard Model. The structure of the supersymmetry-breaking introduces additional model assumptions. The approaches considered here are supergravity-mediated and gauge-mediated supersymmetry-breaking. Alternative phenomenologies arising in non-minimal and/or R-parity-violating approaches are also briefly examined.

The {ital CP}-odd Higgs boson {ital A}{sup 0} of a nonminimal Higgs model possesses no tree-level couplings to vector-boson pairs. As a result, many of the techniques employed to search for the standard model Higgs boson at the Superconducting Super Collider (SSC) and CERN Large Hadron Collider (LHC) are not likely to be useful in searches for the {ital A}{sup 0}. This paper focuses on the phenomenology of {ital A}{sup 0} production and decay in the minimal supersymmetric model. We evaluate a comprehensive set of branching ratios for {ital A}{sup 0} (which includes all two-body tree-level and one-loop decays into nonsupersymmetric final states), including the effects of leading-log radiative corrections. The one-loop decay {ital A}{sup 0}{r arrow}{ital ZZ} leads to the gold-plated'' signature {ital ZZ}{r arrow}{ital l}{sup +}{ital l{minus}}{ital l+}{ital l{minus}} and provides an observable signature for {ital A}{sup 0} production at the SSC and LHC only over a very narrow region of parameter space. More promising signatures for the {ital A}{sup 0} are also discussed.

We define a particularly useful asymmetry in t→W+b decays, which is sensitive to non-standard sources of CP violation. Prospects for observation of the predicted asymmetry at a hadron collider are estimated. As an illustration, the asymmetry arising from loop diagrams involving the neutral Higgs bosons of a two-doublet Higgs model with CP-violating neutral sector is computed.

The next-to-minimal supersymmetric model with a light doubletlike CP-odd Higgs boson and small tan{beta} can satisfy all experimental limits on Higgs bosons even with light superpartners. In these scenarios, the two lightest CP-even Higgs bosons, h{sub 1} and h{sub 2}, and the charged Higgs boson, h{sup +}, can all be light enough to be produced at CERN LEP and yet have decays that have not been looked for or are poorly constrained by existing collider experiments. The channel h{sub 1}{yields}a{sub 1}a{sub 1} with a{sub 1}{yields}{tau}{sup +}{tau}{sup -} or 2j is still awaiting LEP constraints for m{sub h{sub 1}}>86 or 82 GeV, respectively. LEP data may also contain e{sup +}e{sup -}{yields}h{sub 2}a{sub 1} events where h{sub 2}{yields}Za{sub 1} is the dominant decay, a channel that was never examined. Decays of the charged Higgs bosons are often dominated by H{sup {+-}}{yields}W{sup {+-}}{sup (}*{sup )}a{sub 1} with a{sub 1}{yields}gg, cc, and {tau}{sup +}{tau}{sup -}. This is a channel that has so far been ignored in the search for t{yields}h{sup +}b decays at the Tevatron. A specialized analysis might reveal a signal. The light a{sub 1} might be within the reach of B factories via {upsilon}{yields}{gamma}a{sub 1} decays. We study typical mass ranges and more » branching ratios of Higgs bosons in this scenario and compare these scenarios where the a{sub 1} has a large doublet component to the more general scenarios with arbitrary singlet component for the a{sub 1}. « less

We show that the existing CDF $L=630 pb^{-1}$ Tevatron data on $pp\to ��^+��^- + X$ places substantial limits on a light CP-odd Higgs boson $a$ with $m_a2m_��$ for which $BR(a\to ��^+��^-)$ is relatively small. Extrapolation of this existing CDF analysis to $L=10 fb^{-1}$ suggests that Tevatron limits on the $a b\anti b$ coupling strength in the region $m_a>8\gev$ could be comparable to or better than limits from Upsilon decays in the $m_a, 25 pages, 12 figures, updated LHC estimates and table based on ATLAS-related input now included (v4 and v5). In addition, comments regarding final ALEPH limits in the Z+4\tau channel are included in v5.

We compute the magnitude of flavor changing neutral current (FCNC) decays of the t quark and of the Z boson expected in a two-Higgs doublet model. The charged Higgs boson couplings are constrained to be consistent with errors with KLKS mixing, B0d− B 0 d mixing, and the observed ratio of b→u/b→c decays. We find that large corrections to the predictions of the standard model are possible, but that even the maximum possible levels for FCNC decays are so small as to make their detection very difficult.

I specify the characteristics of a Higgs boson that would be “ideal” in the light of current data and theoretical attractiveness. I then review why it is that the Higgs bosons of the Standard Model and the Minimal Supersymmetric Model cannot be ideal whereas the lightest Higgs boson of the Next to Minimal Supersymmetric Model can be ideal. Experimental consequences for Higgs and supersymmetry discovery are then reviewed. I then examine the alternatives to the NMSSM in which the MSSM is extended via an extra U(1) symmetry.

We consider constraints imposed on the top quark and on the charged Higgs boson of a two-doublet model by experimental results for K 0 − K 0 mixing, B d − B d mixing and the ratio of b → u to b → c decays.

Completely natural electroweak symmetry breaking is easily achieved in supersymmetric models if there is a SM-like Higgs boson, h, with m{sub h} 0.7, and if m{sub a}

Dominant decay of a SM-like Higgs boson into particles beyond those contained in the minimal supersymmetric standard model has been identified as a natural scenario to avoid fine tuning in electroweak symmetry breaking while satisfying all LEP limits. In the simplest such extension, the next-to-minimal supersymmetric model, the lightest CP-even Higgs boson can decay into two pseudoscalars. In the scenario with least fine tuning the lightest CP-even Higgs boson has mass of order 100 GeV. In order to escape LEP limits it must decay to a pair of the lightest CP-odd Higgs bosons with Br(h → aa) > .7 and ma < 2mb (so that a → � + � − or light quarks and gluons). The mass of the lightest CP-odd Higgs boson is controlled by the soft-trilinear couplings, A�(mZ) and A�(mZ). We identify the region of parameter space where this situation occurs and discuss how natural this scenario is. It turns out that in order to achieve ma < 2mb with A�(mZ), A�(mZ) of order the typical radiative corrections, the required tuning of trilinear couplings need not be larger than 5-10 %. Further, the necessity for this tuning can be eliminated in specific SUSY breaking scenarios. Quite interestingly, Br(h → aa) is typically above 70 % in this region of parameter space and thus an appropriately large value requires no additional tuning.

Naturalness of electroweak symmetry breaking in the next-to-minimal supersymmetric model points to scenarios in which the lightest CP-even Higgs boson has SM-like ZZh coupling and mass m_h ~ 100 GeV and decays partly via h -> b \bar b but dominantly into two CP-odd Higgs bosons, h -> aa, where m_a < 2m_b so that a -> tau+ tau- (or light quarks and gluons) decays are dominant. These preferred scenarios correlate well with the observed excess of events at LEP for m_h ~ 100 GeV., 4 pages, 2 figures; to appear in Proceedings of SUSY06, the 14th International Conference on Supersymmetry and the Unification of Fundamental Interactions, UC Irvine, California, 12-17 June 2006

The simplest supersymmetric model that solves the mu problem and in which the GUT-scale parameters need not be finely tuned in order to predict the correct value of the Z boson mass at low scales is the Next-to-Minimal Supersymmetric Standard Model (NMSSM). However, in order that fine tuning be absent, the lightest CP-even Higgs boson h should have mass ~100 GeV and SM couplings to gauge bosons and fermions. The only way that this can be consistent with LEP limits is if h decays primarily via h->aa->4 tau or 4j but not 4b, where a is the lighter of the two pseudo-scalar Higgses that are present in the NMSSM. Interestingly, m_a < 2 m_b is natural in the NMSSM with m_a > 2 m_tau somewhat preferred. Thus, h -> 4 tau becomes a key mode of interest. Meanwhile, all other Higgs bosons of the NMSSM are typically quite heavy. Detection of any of the NMSSM Higgs bosons at the LHC in this preferred scenario will be very challenging using conventional channels. In this paper, we demonstrate that the h -> aa -> 4 tau decay chain should be visible if the Higgs is produced in the process pp -> p+h+p with the final state protons being measured using suitably installed forward detectors. Moreover, we show that the mass of both the h and the a can be determined on an event-by-event basis., 23 pages

We examine the LEP limits for the Zh{yields}Z+b's final state and find that the excess of observed events for m{sub h}{approx}100 GeV correlates well with there being an m{sub h}{approx}100 GeV Higgs boson with SM-like ZZh coupling that decays partly via h{yields}bb+{tau}{sup +}{tau}{sup -} [with B(h{yields}bb){approx}0.08] but dominantly via h{yields}aa [with B(h{yields}aa){approx}0.9], where m{sub a}

Neutralino dark matter is generally assumed to be relatively heavy, with a mass near the electroweak scale. This does not necessarily need to be the case, however. In the next-to-minimal supersymmetric standard model (NMSSM) and other supersymmetric models with an extended Higgs sector, a very light CP-odd Higgs boson can naturally arise making it possible for a very light neutralino to annihilate efficiently enough to avoid being overproduced in the early Universe. In this article, we explore the characteristics of a supersymmetric model needed to include a very light neutralino, 100 MeV

The most general Higgs potential of the two-Higgs-doublet model (2HDM) contains three squared-mass parameters and seven quartic self-coupling parameters. Among these, one squared-mass parameter and three quartic coupling parameters are potentially complex. The Higgs potential explicitly violates CP symmetry if and only if no choice of basis exists in the two-dimensional Higgs ``flavor'' space in which all the Higgs potential parameters are real. We exhibit four independent potentially complex invariant (basis-independent) combinations of mass and coupling parameters and show that the reality of all four invariants provides the necessary and sufficient conditions for an explicitly CP-conserving 2HDM scalar potential. Additional potentially complex invariants can be constructed that depend on the Higgs field vacuum expectation values (vevs). We demonstrate how these can be used together with the vev-independent invariants to distinguish between explicit and spontaneous CP-violation in the Higgs sector., Comment: 46 pages, minor typographical errors corrected, accepted for publication in Phys. Rev. D

We review the theory of Higgs bosons, with emphasis on the Higgs scalars of the Standard Model and its non-supersymmetric and supersymmetric extensions. After surveying the expected knowledge of Higgs boson physics after the Tevatron and LHC experimental programs, we examine in detail expectations for precision Higgs measurements at a future e + e − linear collider (LC). A comprehensive phenomenological profile can be assembled from LC Higgs studies (both in e + e − and γγ collisions). The Giga-Z option can provide important constraints and consistency checks for the theory of electroweak symmetry breaking.

We demonstrate that the NMSSM can have small fine tuning and modest light stop mass while still evading all experimental constraints. For small tan(beta) [large tan(beta)], the relevant scenarios are such that there is always (often) a SM-like Higgs boson that decays to two lighter -- possibly much lighter -- pseudoscalar Higgses., Comment: 4 pages, 4 figures, revised to conform with PRL published version

We compute the cross section for e+e- --> nu nu-bar A, where A is the CP-odd scalar, in the general CP-conserving type-II two-Higgs-doublet model. We sum the contributions from the ``t-channel'' e+e- --> nu nu-bar W W --> nu nu-bar A graphs and ``s-channel'' e+e- --> Z A --> nu nu-bar A graphs, including their interference. Higgs-triangle graphs and all box diagrams are included. For many parameter choices, especially those in the decoupling region of parameter space (light h and m_A, m_H, m_H^+ > 2 m_Z), the Higgs-triangle and box diagrams are found to be of minor importance, the main contributing loops being the top and bottom quark triangle diagrams. The predicted cross section is rather small for tan beta > 2 and/or m_A > 2 m_t. However, we also show that if parameters are chosen corresponding to large Higgs self-couplings then the Higgs-triangle graphs can greatly enhance the cross section. We also demonstrate that the SUSY-loop corrections to the b b-bar A coupling could be such as to greatly enhance this coupling, resulting in an enhanced nu nu-bar A cross section. Complete cross section expressions are given in the Appendices.

Generalizations of the Randall-Sundrum model containing a bulk scalar field $\Phi$ interacting with the curvature $R$ through the general coupling $R f(\Phi)$ are considered. We derive the general form of the effective 4D potential for the spin-zero fields and show that in the mass matrix the radion mixes with the Kaluza-Klein modes of the bulk scalar fluctuations. We demonstrate that it is possible to choose a non-trivial background form $\Phi_0(y)$ (where $y$ is the extra dimension coordinate) for the bulk scalar field such that the exact Randall-Sundrum metric is preserved (i.e. such that there is no back-reaction). We compute the mass matrix for the radion and the KK modes of the excitations of the bulk scalar relative to the background configuration $\Phi_0(y)$ and find that the resulting mass matrix implies a non-zero value for the mass of the radion (identified as the state with the lowest eigenvalue of the scalar mass matrix). We find that this mass is suppressed relative to the Planck scale by the standard warp factor needed to explain the hierarchy puzzle, implying that a mass $\sim 1\tev$ is a natural order of magnitude for the radion mass. The general considerations are illustrated in the case of a model containing an $R\Phi^2$ interaction term., Comment: 22 pages, 3 figures