Search

Old and new ideas regarding Higgs physics are reviewed. We first summarize the quadratic divergence / hierarchy problem which strongly suggests that the SM Higgs sector will be supplemented by new physics at high scales. We next consider means for delaying the hierarchy problem of the SM Higgs sector to unexpectedly high scales. We then outline the properties of the most ideal Higgs boson. The main advantages of a supersymmetric solution to the high scale problems are summarized and the reasons for preferring the next-to-minimal supersymmetric model over the minimal supersymmetric model in order to achieve an ideal Higgs are emphasized. This leads us to the strongly motivated scenario in which there is a Higgs $h$ with SM-like $WW,ZZ$ couplings and $\mh\sim 100\gev$ that decays via $h\to aa$ with $m_a2m_\tau$ is preferred, implying $a\to \tau^+ \tau^-$. The means for detecting an $h\to aa\to 4\tau$ signal are then discussed. Some final cautionary and concluding remarks are given., Comment: 15 pages, 9 figures

This review summarizes the motivations for and phenomenological consequences of nonstandard Higgs boson decays, with emphasis on final states containing a pair of non-Standard-Model particles that subsequently decay to Standard Model particles. Typically these non-Standard-Model particles are part of a ``hidden'' sector, for example a pair of neutral Higgs bosons or a pair of unstable neutralinos. We emphasize that such decays allow for a Higgs substantially below the Standard Model Higgs LEP limit of 114 GeV. This in turn means that the ``fine-tuning'' problems of many Beyond the Standard Model (BSM) theories, in particular supersymmetric models, can be eliminated while achieving excellent consistency with precision electroweak data which favor a Higgs boson with mass below $100\gev$ and standard $WW$, $ZZ$, and top couplings., 53 pages, review intended for Annual Reviews of Nuclear Science. Final corrections and references now included

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}

We study scenarios in the minimal and next-to minimal supersymmetric models in which the lightest CP-even Higgs boson can have mass below the 114 GeV standard model LEP limit by virtue of reduced ZZ coupling due to substantial mixing among the Higgs bosons. We pay particular attention to the size of corrections from superpartners needed for these scenarios to be viable and point to boundary conditions at large scales which lead to these scenarios while at the same time keeping electroweak fine tuning modest in size. We find that naturalness of electroweak symmetry breaking in the mixed-Higgs scenarios of both models points to the same region of soft supersymmetry breaking terms, namely those leading to large mixing in the stop sector at the electroweak scale, especially if we also require that the lightest CP-even Higgs explains the Higgs-like LEP events at about 98 GeV., 14 pages, 19 figures

We describe a kinematic method which is capable of determining the overall mass scale in SUSY-like events at a hadron collider with two missing (dark matter) particles. We focus on the kinematic topology in which a pair of identical particles is produced with each decaying to two leptons and an invisible particle (schematically, $pp\to YY+jets$ followed by each $Y$ decaying via $Y\to \ell X\to \ell\ell'N$ where $N$ is invisible). This topology arises in many SUSY processes such as squark and gluino production and decay, not to mention $t\anti t$ di-lepton decays. In the example where the final state leptons are all muons, our errors on the masses of the particles $Y$, $X$ and $N$ in the decay chain range from 4 GeV for 2000 events after cuts to 13 GeV for 400 events after cuts. Errors for mass differences are much smaller. Our ability to determine masses comes from considering all the kinematic information in the event, including the missing momentum, in conjunction with the quadratic constraints that arise from the $Y$, $X$ and $N$ mass-shell conditions. Realistic missing momentum and lepton momenta uncertainties are included in the analysis., 41 pages, 14 figures, various clarifications and expanded discussion included in revised version that conforms to the version to be published

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 show that perturbative unitarity for $W_LW_L$ scattering places significant constraints on the Randall-Sundrum theory with two 3-branes, with matter confined to the TeV brane. The exchange of massive 4D Kaluza-Klein gravitons leads to amplitudes growing linearly with the CM energy squared. Summing over KK gravitons up to a scale $\lbar$ and testing unitarity at $\sqrt s=\lbar$, one finds that unitarity is violated for $\lbar$ below the 'naive dimensional analysis' scale, $\Lambda_{NDA}$. We evaluate $\lbar$ as a function of the curvature ratio $m_0/M_{Planck}$ for the pure gravity theory. We then demonstrate that unitarity need not be violated at $\lbar$ in the presence of a heavy Higgs boson. In fact, much larger Higgs masses are consistent with unitarity than if no KK gravitons are present. Observation of the mass and width (or cross section) of one or more KK gravitons at the LHC will directly determine $m_0/M_{Planck}$ and the scale $\hat \Lambda$ specifying the couplings of matter to the KK gravitons. With this information in hand and a measurement of the Higgs boson mass, one can determine the precise scale $\lbar$ below which unitarity will remain valid., Comment: 14 pages, 8 figures, major revisions made

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

18 páginas, 6 figuras, 12 tablas.-- et al., High-precision analyses of supersymmetry parameters aim at reconstructing the fundamental supersymmetric theory and its breaking mechanism. A well defined theoretical framework is needed when higher-order corrections are included. We propose such a scheme, Supersymmetry Parameter Analysis SPA, based on a consistent set of conventions and input parameters. A repository for computer programs is provided which connect parameters in different schemes and relate the Lagrangian parameters to physical observables at LHC and high energy e + e - linear collider experiments, i.e., masses, mixings, decay widths and production cross sections for supersymmetric particles. In addition, programs for calculating high-precision low energy observables, the density of cold dark matter (CDM) in the universe as well as the cross sections for CDM search experiments are included. The SPA scheme still requires extended efforts on both the theoretical and experimental side before data can be evaluated in the future at the level of the desired precision. We take here an initial step of testing the SPA scheme by applying the techniques involved to a specific supersymmetry reference point.

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 identify scenarios not ruled out by LEP data in which NMSSM Higgs detection at the LHC will be particularly challenging. We first review the `no-lose' theorem for Higgs discovery at the LHC that applies if Higgs bosons do not decay to other Higgs bosons - namely, with L=300 fb^-1, there is always one or more `standard' Higgs detection channel with at least a 5 sigma signal. However, we provide examples of no-Higgs-to-Higgs cases for which all the standard signals are no larger than 7 sigma implying that if the available L is smaller or the simulations performed by ATLAS and CMS turn out to be overly optimistic, all standard Higgs signals could fall below 5 sigma even in the no-Higgs-to-Higgs part of NMSSM parameter space. In the vast bulk of NMSSM parameter space, there will be Higgs-to-Higgs decays. We show that when such decays are present it is possible for all the standard detection channels to have very small significance. In most such cases, the only strongly produced Higgs boson is one with fairly SM-like couplings that decays to two lighter Higgs bosons (either a pair of the lightest CP-even Higgs bosons, or, in the largest part of parameter space, a pair of the lightest CP-odd Higgs bosons). A number of representative bench-mark scenarios of this type are delineated in detail and implications for Higgs discovery at various colliders are discussed., 31 pages, 5 figures

The Fortran code NMHDECAY computes the masses, couplings and decay widths of all Higgs bosons of the NMSSM in terms of its parameters at the electroweak (SUSY breaking) scale: the Yukawa couplings lambda and kappa, the soft trilinear terms A_lambda and A_kappa, and tan(beta) and mu_eff = lambda*. The computation of the spectrum includes leading two loop terms, electroweak corrections and propagator corrections. The computation of the decay widths is carried out as in HDECAY, but (for the moment) without three body decays. Each point in parameter space is checked against negative Higgs bosons searches at LEP, including unconventional channels relevant for the NMSSM. One version of the program uses generalized SLHA conventions for input and output., Comment: Typos corrected, references added, radiative corrections written out explicitely in new appendix C

We derive the Lagrangian and Feynman rules up to bilinear scalar fields for the mixed Higgs-radion eigenstates interacting with Standard Model particles confined to a 3-brane in Randall-Sundrum warped geometry. We use the results to compute precision electroweak observables and compare theory predictions with experiment. We characterize the interesting regions of parameter space that simultaneously enable a very heavy Higgs mass and a very heavy radion mass, both masses being well above the putative Higgs boson mass limit in the Standard Model derived from the constraints of precision electroweak observables. For parameters consistent with the precision constraints the Higgs boson physical eigenstate is typically detectable, but its properties may be difficult to study at the Large Hadron Collider. In contrast, masses and couplings are allowed for the physical radion eigenstate that make it unobservable at the LHC. A Linear Collider will significantly improve our ability to study the Higgs eigenstate, and will typically allow detection of the radion eigenstate if it is within the machine's kinematical reach., 14 pages, 5 figures; revisions: typo correction for Feynman rules and 1 reference added

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

We examine the prospects for discovering and elucidating the weakly-coupled Higgs sector at future collider experiments. The Higgs search consists of three phases: (i) discovery of a Higgs candidate, (ii) verification of the Higgs interpretation of the signal, and (iii) precision measurements of Higgs sector properties. The discovery of one Higgs boson with Standard Model properties is not sufficient to expose the underlying structure of the electroweak symmetry breaking dynamics. It is critical to search for evidence for a non-minimal Higgs sector and/or new physics associated with electroweak symmetry breaking dynamics. An improvement in precision electroweak data at future colliders can play a useful role in confirming the theoretical interpretation of the Higgs search results. We examine the prospects for discovering and elucidating the weakly-coupled Higgs sector at future collider experiments. The Higgs search consists of three phases: (i) discovery of a Higgs candidate, (ii) verification of the Higgs interpretation of the signal, and (iii) precision measurements of Higgs sector properties. The discovery of one Higgs boson with Standard Model properties is not sufficient to expose the underlying structure of the electroweak symmetry breaking dynamics. It is critical to search for evidence for a non-minimal Higgs sector and/or new physics associated with electroweak symmetry breaking dynamics. An improvement in precision electroweak data at future colliders can play a useful role in confirming the theoretical interpretation of the Higgs search results.

Models with 3-branes in extra dimensions typically imply the existence of a radion, phi that can mix with the Higgs, h, thereby modifying the Higgs properties and the prospects for its detectability at the LHC. The presence of the phi will extend the scope of the LHC searches. Detection of both the phi and the h might be possible. In this paper, we study the complementarity of the observation of gg -> h, with h -> gamma gamma or h -> ZZ -> 4 leptons, and gg -> phi -> ZZ -> 4 leptons at the LHC in the context of the Randall-Sundrum model. The potential for determining the nature of the detected scalar(s) at the LHC and at an e+e- linear collider is discussed, both separately and in combination., 11 pages, 5 figures

We present new results from studies in progress on physics at a two-photon collider. We report on the sensitivity to top squark parameters of MSSM Higgs boson production in two-photon collisions; Higgs boson decay to two photons; radion production in models of warped extra dimensions; chargino pair production; sensitivity to the trilinear Higgs boson coupling; charged Higgs boson pair production; and we discuss the backgrounds produced by resolved photon-photon interactions.

A CP-even neutral Higgs boson with Standard-Model-like couplings may be the lightest scalar of a two-Higgs-doublet model. We study the decoupling limit of the most general CP-conserving two-Higgs-doublet model, where the mass of the lightest Higgs scalar is significantly smaller than the masses of the other Higgs bosons of the model. In this case, the properties of the lightest Higgs boson are nearly indistinguishable from those of the Standard Model Higgs boson. The first non-trivial corrections to Higgs couplings in the approach to the decoupling limit are also evaluated. The importance of detecting such deviations in precision Higgs measurements at future colliders is emphasized. We also clarify the case in which a neutral Higgs boson can possess Standard-Model-like couplings in a regime where the decoupling limit does not apply. The two-Higgs-doublet sector of the minimal supersymmetric model illustrates many of the above features., 54 pages, 2 tables, revtex4 format, some new material added (including elegant forms for the three-Higgs and four-Higgs couplings) and typographical errors fixed

Report of the "Beyond the Standard Model" working group for the Workshop `Physics at TeV Colliders', Les Houches, France, 21 May - 1 June 2001. It consists of 18 separate parts: 1. Preface; 2. Theoretical Discussion; 3. Numerical Calculation of the mSUGRA and Higgs Spectrum; 4. Theoretical Uncertainties in Sparticle Mass Predictions; 5. High Mass Supersymmetry with High Energy Hadron Colliders; 6. SUSY with Heavy Scalars at LHC; 7. Inclusive Study of MSSM in CMS; 8. Establishing a No-Lose Theorem for NMSSM Higgs Boson Discovery at the LHC; 9. Effects of Supersymmetric Phases on Higgs Production in Association with Squark Pairs in the Minimal Supersymmetric Standard Model; 10. Study of the Lepton Flavour Violating Decays of Charged Fermions in SUSY GUTs; 11. Interactions of the Goldstino Supermultiplet with Standard Model Fields; 12. Attempts at Explaining the NuTeV Observation of Di-Muon Events; 13. Kaluza-Klein States of the Standard Model Gauge Bosons: Constraints From High Energy Experiments; 14. Kaluza-Klein Excitations of Gauge Bosons in the ATLAS Detector; 15. Search for the Randall Sundrum Radion Using the ATLAS Detector; 16. Radion Mixing Effects on the Properties of the Standard Model Higgs Boson; 17. Probing Universal Extra Dimensions at Present and Future Colliders; 18. Black Hole Production at Future Colliders., 153 pages, latex, many figures, uses cernrep and axodraw, proceedings of the Workshop `Physics at TeV Colliders', Les Houches, France, 21 May - 1 June 2001

We investigate the potential for detecting and studying Higgs bosons in {gamma}{gamma} and e{gamma} collisions at future linear colliders with energies below a TeV. Our study incorporates realistic {gamma}{gamma}spectra based on available laser technology, and NLC and CLIC acceleration techniques. Results include detector simulations. We study the cases of: (a) a SM-like Higgs boson based on a devoted low energy machine with {radical}s{sub ee} {le} 200 GeV; (b) the heavy MSSM Higgs bosons; and (c) charged Higgs bosons in e{gamma} collisions.

The ``Snowmass Points and Slopes'' (SPS) are a set of benchmark points and parameter lines in the MSSM parameter space corresponding to different scenarios in the search for Supersymmetry at present and future experiments. This set of benchmarks was agreed upon at the 2001 ``Snowmass Workshop on the Future of Particle Physics'' as a consensus based on different existing proposals., 12 pages, 3 figures

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs Factories and compact high energy lepton colliders. The status and timescale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons., Comment: 112 pages, Large report

For some choices of soft supersymmetry-breaking parameters, the lighest supersymmetric particle is a stable neutralino ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{1}^{0}$ that is almost degenerate in mass with the lightest chargino ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{1}^{\ifmmode\pm\else\textpm\fi{}}$ $(\ensuremath{\Delta}{m}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}\ensuremath{\equiv}{m}_{{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{1}^{\ifmmode\pm\else\textpm\fi{}}}\ensuremath{-}{m}_{{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{1}^{0}}\ensuremath{\sim}{m}_{\ensuremath{\pi}}\ensuremath{-}\mathrm{few}\mathrm{}\mathrm{GeV}),$ and all other sparticles are relatively heavy. We discuss the potential of a $\sqrt{s}\ensuremath{\sim}600\mathrm{GeV}$ ${e}^{+}{e}^{\ensuremath{-}}$ collider for studying such models.

We examine the potential for detecting and studying Higgs bosons at a photon-photon collider facility associated with a future linear collider. Our study incorporates realistic $\gam\gam$ luminosity spectra based on the most probable available laser technology. Results include detector simulations. We study the cases of: a) a SM-like Higgs boson; b) the heavy MSSM Higgs bosons; c) a Higgs boson with no $WW/ZZ$ couplings from a general two Higgs doublet model., Comment: 52 pages, 26 figures, revised version with new appendix

The manner in which the parameters of a two-Higgs-doublet model can be chosen so that no Higgs boson is discovered at a $\sqrt{s}\leq 800\gev$ $\epem$ collider, while maintaining consistency with current precision electroweak measurements, is described. The importance of a Giga-$Z$ factory and higher collider energies for such a scenario is emphasized., Comment: 6 pages, 3 figures

For some choices of soft SUSY–breaking parameters, the LSP is a stable neutralino χ10, the NLSP is a chargino χ1± almost degenerate in mass with the LSP (Δmχ1≡mχ1±−mχ10∼mπ–few GeV), and all other sparticles are relatively heavy. We discuss the potential of the NLC for studying such models.

We consider a CP-conserving two-Higgs-doublet type II model with a light scalar or pseudoscalar neutral Higgs boson ($\h=\hl$ or $\h=\ha$) that has no $ZZ/WW$ coupling and, thus, cannot be detected in $\epem\to Z\h$ (Higgs-strahlung) or $\nu\anti\nu \h$ (via $WW$ fusion). Despite sum rules which ensure that the light $\h$ must have significant $t\anti t$ or $b\anti b$ coupling, for a wedge of moderate $\tanb$, that becomes increasingly large as $\mh$ increases, the $\h$ can also escape discovery in both $b\anti b \h$ and $t\anti t \h$ production at a $\rts=500-800\gev$ $\epem$ collider (for expected luminosities). If the other Higgs bosons happen to be too heavy to be produced, then no Higgs boson would be detected. We demonstrate that, despite such high masses for the other Higgs bosons, only the low-$\tanb$ portion of the no-discovery wedges in $[\mh,\tanb]$ parameter space can be excluded due to failure to fit precision electroweak observables. In the $\tanb\gsim 1$ regions of the no-discovery wedges, we find that the 2HDM fit to precision electroweak observables has small $\Delta\chi^2$ relative to the best minimal one-doublet SM fit., Comment: 14 pages, 4 figures. 1st two figures have been reformatted to improve readability

For some choices of soft SUSY-breaking parameters, the LSP is a stable neutralino $\cnone$, the NLSP is a chargino $\cpmone$ almost degenerate in mass with the LSP ($\dmchi\equiv\mcpmone-\mcnone\sim \mpi-$few GeV), and all other sparticles are relatively heavy. In this case, detection of sparticles in the usual, mSUGRA-motivated signals will be difficult, since the decay products in $\cpmone\to\cnone...$ will be very soft, and alternative signals must be considered. Here, we study the viability of signatures at the Tevatron based on highly-ionizing charged tracks, disappearing charged tracks, large impact parameters, missing transverse energy and a jet or a photon, and determine the mass reach of such signatures assuming that only the $\cpmone$ and $\cnone$ are light. We also consider the jet+$\etmiss$ and $\gamma+\etmiss$ signatures assuming that the gluino is also light with $\mgl\sim \mcpmone$. We find that the mass reach is critically dependent upon $\dmchi$ and $\mgl-\mcpmone$. If $\dmchi$ is sufficiently big that $c\tau(\cpmone)\lsim$few cm and $\mgl$ is large, there is a significant possibility that the limits on $\mcpmone$ based on LEP2 data cannot be extended at the Tevatron. If $c\tau(\cpmone)>$few cm, relatively background-free signals exist that will give a clear signal of $\cpmone$ production (for some range of $\mcpmone$) even if $\mgl$ is very large., Comment: 44 pages, equations.sty, submitted to Phys. Rev. D

We discuss a general two-Higgs-doublet model with CP violation in the Higgs sector. In general, the three neutral Higgs fields of the model all mix and the resulting physical Higgs bosons have no definite CP properties. We derive a new sum rule relating Yukawa and Higgs–Z couplings which implies that a neutral Higgs boson cannot escape detection at an e + e − collider if it is kinematically accessible in Z+Higgs, bb+Higgs and tt+Higgs production, irrespective of the mixing angles and the masses of the other neutral Higgs bosons. We also discuss modifications of the sum rules and their phenomenological consequences in the case when the two-doublet Higgs sector is extended by adding one or more singlets.

I discuss the exciting prospects for exploring a wide range of new physics at a low-energy muon collider., Comment: 21 pages, full postscript file is also available via anonymous ftp at ftp://ucdhep.ucdavis.edu/gunion/fnalmumuplenary.ps

We consider the possibility that the lightest supersymmetric particle is a heavy gluino. After discussing models in which this is the case, we demonstrate that the gluino-LSP could evade cosmological and other constraints by virtue of having a very small relic density. We then consider how neutral and charged hadrons containing a gluino will behave in a detector, demonstrating that there is generally substantial apparent missing momentum associated with a produced gluino-LSP. We next investigate limits on a (heavy) gluino-LSP deriving from LEP, LEP2 and RunI Tevatron experimental searches for excess events in the jets plus missing momentum channel and for stable heavily-ionizing charged particles. The range of gluino mass that can be excluded depends upon the path length of the gluino in the detector, the amount of energy it deposits in each hadronic collision, and the probability for the gluino to fragment to a pseudo-stable charged hadron after a given hadronic collision. We explore how the range of excluded gluino mass depends upon these ingredients, concluding that for non-extreme cases the range $3\gev\lsim\mgl\lsim 130-150\gev$ can be excluded at 95% CL based on currently available OPAL and CDF analyses. We find that RunII at the Tevatron can extend the excluded region (or discover the gluino) up to $\mgl\sim 160-180\gev$. For completeness, we also analyze the case where the gluino is the NLSP (as possible in gauge-mediated supersymmetry breaking) decaying via gluino -> g + gravitino. We find that the Tevatron RunI data excludes $\mgl\leq 240$ GeV. Finally, we discuss application of the procedures developed for the heavy gluino-LSP to searches for other stable strongly interacting particles, such as a stable heavy quark., 70 pages, latex, 43 figures, submitted to Phys. Rev. D. Revisions: a) event-by-event Monte Carlo employed for all gluino -> charged fragmentation probabilities; b) CDF limits corrected to conform to the exact algorithm employed in CDF analysis --- net result is that limits are improved at low mass -> no OPAL/CDF gap for any charged fragmentation probability; c) sensitivity to gluino path length and energy deposit explored; d) limits in case where gluino -> gluon + gravitino determined

A review of muon colliders is presented. Basic features of the accelerator and detector are outlined, and the very exciting physics prospects are reviewed., Comment: 18 pages, full postscript file is available via anonymous ftp at ftp://ucdhep.ucdavis.edu/gunion/bsmv.ps

I give an overview of the motivations for and theory/phenomenology of supersymmetry., Comment: 24 pages, to appear in ``Future High Energy Colliders'', proceedings of the ITP Symposium, U.C. Santa Barbara, October 21-25, 1996, AIP Press. Also presented at the Aspen Winter Conference on High Energy Physics, January 1997, Aspen, CO. Full postscript file, is available via anonymous ftp at ftp://ucdhep.ucdavis.edu/gunion/itp96.ps

Minijet production in $\sqrt{s} = 5.5$ TeV Pb+Pb collisions at the LHC is expected to produce a gluon-dominated plasma with large initial temperatures and energy densities. We discuss the implications of the high initial temperatures on the screening mass and on the relative suppression of members of the $\Upsilon$ family. We demonstrate that $p_T$-dependence of the $\Upsilon'/\Upsilon$ ratio is only expected if a quark-gluon plasma with significant energy density is present, and that the precise dependence of this ratio will strongly constrain the QGP models. We also propose that the $p_T$ spectra of the $\Upsilon$ resonances themselves will provide valuable information by comparing these spectra to that of the $Z$ boson, which should not be influenced by plasma production., Comment: 46 pages, full postscript file also available via anonymous ftp at ftp://ucdhep.ucdavis.edu/gunion/upsilon.ps

We demonstrate that certain weighted moments of $t\overline{t}h$ production can provide a determination of the CP nature of a light neutral Higgs boson produced and detected in this mode at the CERN Large Hadron Collider.

Techniques and strategies for discovering and measuring the properties of Higgs bosons via $s$-channel production at a $\mu^+\mu^-$ collider, and the associated requirements for the machine and detector, are discussed in detail. The unique feature of $s$-channel production is that, with good energy resolution, the mass, total width and partial widths of a Higgs boson can be directly measured with remarkable accuracy in most cases. For the expected machine parameters and luminosity the Standard Model (SM) Higgs boson $\hsm$, with mass $\lsim 2\mw$, the light $\hl$ of the minimal supersymmetric Standard Model (MSSM), and the heavier MSSM Higgs bosons (the CP-odd $\ha$ and the CP-even $\hh$) can all be studied in the $s$-channel, with the heavier states accessible up to the maximal $\sqrt s$ over a large fraction of the MSSM parameter space. In addition, it may be possible to discover the $\ha$ and $\hh$ by running the collider at full energy and observing excess events in the bremsstrahlung tail at lower energy. The integrated luminosity, beam resolution and machine/detector features required to distinguish between the $\hsm$ and $\hl$ are delineated., Comment: 87 pages, uuencoded tex file and postscript figure files submitted, full postscript file, including embedded figures, is available via anonymous ftp at ftp://ucdhep.ucdavis.edu/gunion/higgs_mupmum.ps

We demonstrate that decay angle correlations in $\taum\taup$ and $t\anti t$ decay modes could allow a determination of whether or not a neutral Higgs boson is a CP eigenstate. Sensitivity of the correlations is illustrated in the case of the $\epem\rta Z \hn$ and $\mupmum\rta \hn$ production processes for a two-doublet Higgs model with CP-violating neutral sector. A very useful technique for minimizing `depolarization-factor' suppressions of the correlations in the $t\anti t$ mode is introduced., Comment: 13 pages, phyzzx.tex macro, full postscript file including embedded figure available at ftp://ucdhep.ucdavis.edu/gunion/cpasym.ps