Search Results (136 total)

We discuss kinematic methods for determining the masses of the particles in events at a hadron collider in which a pair of identical particles is produced with each decaying via a series of on shell intermediate beyond-the-standard model (BSM) particles to visible standard model (SM) particles and an invisible particle (schematically, pp→ZZ+jets with Z→Aa→Bba→Ccba→…→cba…+N where a,b,c,… are visible SM particles or groups of SM particles, A,B,C,… are on shell BSM particles, and N is invisible). This topology arises in many models including supersymmetry (SUSY) processes such as squark and gluino pair production and decay. We present the detailed procedure for the case of Z→3  visible particles+N and demonstrate that the masses obtained from the kinematic procedure are independent of the model by comparing SUSY to universal extra dimensions.

The next-to-minimal supersymmetric model with a light doubletlike CP-odd Higgs boson and small tan⁡β can satisfy all experimental limits on Higgs bosons even with light superpartners. In these scenarios, the two lightest CP-even Higgs bosons, h₁ and h₂, and the charged Higgs boson, h⁺, 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 h₁→a₁a₁ with a₁→τ⁺τ^- or 2j is still awaiting LEP constraints for m_h1>86 or 82 GeV, respectively. LEP data may also contain e⁺e^-→h₂a₁ events where h₂→Za₁ is the dominant decay, a channel that was never examined. Decays of the charged Higgs bosons are often dominated by H^±→W^±(⋆)a₁ with a₁→gg, cc̅ , and τ⁺τ^-. This is a channel that has so far been ignored in the search for t→h⁺b decays at the Tevatron. A specialized analysis might reveal a signal. The light a₁ might be within the reach of B factories via Υ→γa₁ decays. We study typical mass ranges and branching ratios of Higgs bosons in this scenario and compare these scenarios where the a₁ has a large doublet component to the more general scenarios with arbitrary singlet component for the a₁.

Many beyond the standard model theories include a stable dark matter candidate that yields missing or invisible energy in collider detectors. If observed at the CERN Large Hadron Collider, we must determine if its mass and other properties (and those of its partners) predict the correct dark matter relic density. We give a new procedure for determining its mass with small error.

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 as in the decoupled scenarios with mass of the CP even Higgs boson above 114 GeV, 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 ∼98  GeV.

We present an extended study of how the next-to-minimal supersymmetric model easily avoids fine-tuning in electroweak symmetry breaking for a SM-like light Higgs with mass in the vicinity of 100 GeV, as beautifully consistent with precision electroweak data, while escaping LEP constraints due to the dominance of h→aa decays with m_a<2m_b so that a→τ⁺τ^- or jets. The residual ∼10% branching ratio for h→bb̅ explains perfectly the well-known LEP excess at m_h∼100  GeV. Details of model parameter correlations and requirements are discussed as a function tan⁡β. Comparisons of fine-tuning in the NMSSM to that in the MSSM are presented. We also discuss fine-tuning associated with scenarios in which the a is essentially pure singlet, has mass m_a>30  GeV, and decays primarily to γγ leading to an h→aa→4γ Higgs signal.

Completely natural electroweak symmetry breaking is easily achieved in supersymmetric models if there is a SM-like Higgs boson, h, with m_h≲100  GeV. In the minimal supersymmetric model, such an h decays mainly to bb̅ and is ruled out by LEP constraints. However, if the MSSM Higgs sector is expanded so that h decays mainly to still lighter Higgs bosons, e.g. h→aa, with Br(h→aa)>0.7, and if m_a<2m_b, then the LEP constraints are satisfied even if m_h≲100 GeV. In this paper, we show that in the next-to-minimal supersymmetric model the above h and a properties (for the lightest CP-even and CP-odd Higgs bosons, respectively) imply a lower bound on Br(Υ→γa) that dedicated runs at present (and future) B factories can explore.

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 of such an extension, the next-to-minimal supersymmetric model, the lightest CP-even Higgs boson can decay into two pseudoscalars. In the scenario with the least fine-tuning the lightest CP-even Higgs boson has a 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 m_a<2m_b (so that a→τ⁺τ^- or light quarks and gluons). The mass of the lightest CP-odd Higgs boson is controlled by the soft-trilinear couplings, A_λ(m_Z) and A_κ(m_Z). 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 m_a<2m_b with A_λ(m_Z), A_κ(m_Z) of order the typical radiative corrections, the required tuning of trilinear couplings needs 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.

We examine the LEP limits for the Zh→Z+b’s final state and find that the excess of observed events for m_h∼100  GeV correlates well with there being an m_h∼100  GeV Higgs boson with SM-like ZZh coupling that decays partly via h→bb̅ +τ⁺τ^- [with B(h→bb̅ )∼0.08] but dominantly via h→aa [with B(h→aa)∼0.9], where m_a<2m_b so that a→τ⁺τ^- (or light quarks and gluons) decays are dominant. This type of scenario is precisely that predicted in the Next-to-Minimal Supersymmetric Model for parameter choices yielding the lowest possible fine-tuning.

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<m_χ˜1⁰<20  GeV, using the NMSSM as a prototype. We discuss the most important constraints from Upsilon decays, b→sγ, B_s→μ⁺μ^- and the magnetic moment of the muon, and find that a light bino or singlino neutralino is allowed, and can be generated with the appropriate relic density. It has previously been shown that the positive detection of dark matter claimed by the DAMA collaboration can be reconciled with other direct dark matter experiments such as CDMS II if the dark matter particle is rather light, between about 6 and 9 GeV. A singlino or binolike neutralino could easily fall within this range of masses within the NMSSM. Additionally, models with sub-GeV neutralinos may be capable of generating the 511 keV gamma-ray emission observed from the galactic bulge by the INTEGRAL/SPI experiment. We also point out measurements which can be performed immediately at CLEO, BABAR, and Belle using existing data to discover or significantly constrain this scenario.

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.

We demonstrate that the next to minimal supersymmetric model can have small fine-tuning and modest top-squark mass while still evading all experimental constraints. For small tan⁡β (large tan⁡β), the relevant scenarios are such that there is always (often) a standard-model-like Higgs boson that decays to two lighter—possibly much lighter—Higgs pseudoscalars.

We compute the cross section for e⁺e^-→νν̅ A⁰ in the general CP-conserving type-II two-Higgs-doublet model. We sum the contributions from the “t-channel” e⁺e^-→νν̅ WW→νν̅ A⁰ graphs and “s-channel” e⁺e^-→ZA⁰→νν̅ A⁰ 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 h⁰ and m_A⁰,m_H⁰,m_H^±>2m_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 β>2 and/or m_A⁰>2m_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 supersymmetry-loop corrections to the bb̅ A⁰ coupling could be such as to greatly enhance this coupling, resulting in an enhanced νν̅ A⁰ cross section. Complete cross-section expressions are given in the Appendixes.

Generalizations of the Randall-Sundrum model containing a bulk scalar field Φ interacting with the curvature R through the general coupling Rf(Φ) 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 (KK) modes of the bulk scalar fluctuations. We demonstrate that it is possible to choose a nontrivial background form Φ₀(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 Φ₀(y). We find that for any (consistent) Φ₀(y) the expected mass for the radionlike eigenstate is suppressed relative to the Planck scale by the standard warp factor needed to explain the hierarchy puzzle, implying that ∼1 TeV is a natural order of magnitude for this mass. The general considerations are illustrated in the case of a model containing an RΦ² interaction term.

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 time scale 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.

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 nontrivial corrections to Higgs boson couplings in the approach to the decoupling limit are also evaluated. The importance of detecting such deviations in precision Higgs boson 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.

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 γγ luminosity spectra based on the most probable available laser technology. Results include detector simulations. We study the cases of (a) a standard-model-like Higgs boson, (b) the heavy minimal supersymmetric standard model Higgs bosons, and (c) a Higgs boson with no WW/ZZ couplings from a general two Higgs doublet model.

For some choices of soft supersymmetry-breaking parameters, the lighest supersymmetric particle is a stable neutralino χ̃₁⁰ that is almost degenerate in mass with the lightest chargino χ̃₁^± (Δm_χ̃≡m_χ̃1^±-m_χ̃1⁰∼m_π-few GeV), and all other sparticles are relatively heavy. We discuss the potential of a sqrt[s]∼600 GeV e⁺e^- collider for studying such models.

For some choices of soft supersymmetry (SUSY)-breaking parameters, the lightest supersymmetric particle (LSP) is a stable neutralino χ̃₁⁰, the NLSP is a chargino χ̃₁^± almost degenerate in mass with the LSP (Δm_χ̃1≡m_χ̃1^±-m_χ̃1⁰∼m_π-few GeV), and all other sparticles are relatively heavy. In this case, detection of sparticles in the usual, supergravity (MSUGRA)-motivated signals will be difficult, since the decay products in χ̃₁^±→χ̃₁⁰… will be very soft, and alternative signals must be considered. Here, we study the viability of signatures at the Fermilab 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 χ̃₁^± and χ̃₁⁰ are light. We also consider the jet+E_T and γ+E_T signatures assuming that the gluino is also light with m_g̃∼m_χ̃1^±. We find that the mass reach is critically dependent upon Δm_χ̃1 and m_g̃-m_χ̃1^±. If Δm_χ̃1 is sufficiently big that cτ(χ̃₁^±)≲few cm and m_g̃ is large, there is a significant possibility that the limits on m_χ̃1^± based on CERN LEP2 data cannot be extended at the Fermilab Tevatron. If cτ(χ̃₁^±)>few cm, relatively background-free signals exist that will give a clear signal of χ̃₁^± production (for some range of m_χ̃1^±) even if m_g̃ is very large.

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 boson, bb̅ +Higgs boson and tt̅ +Higgs boson 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. A brief discussion of the implications of the sum rules for Higgs boson discovery at the Fermilab Tevatron and CERN LHC is given.

We explore an alternative to the usual procedure of scanning for determining the properties of a narrow s-channel resonance. By varying the beam energy resolution while sitting on the resonance peak, the width and branching ratios of the resonance can be determined. The statistical accuracy achieved is superior to that of the usual scan procedure in the case of a light standard-model-like Higgs boson with M_H>130 GeV or for the lightest pseudogoldstone boson of a strong electroweak breaking model if M_P⁰>150 GeV.

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3–4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from a heavy-Z target and proceeding through the phase rotation and decay (π→μν_μ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96 Workshop [R. B. Palmer, A. Sessler, and A. Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics (Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].

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 g̃-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 g̃-LSP. We next investigate limits on the g̃-LSP deriving from CERN, LEP, LEP2 and run I Fermilab Tevatron experimental searches for excess events in the jets plus missing momentum channel and for stable heavily ionizing charged particles. The range of m_g̃ that can be excluded depends upon the path length of the g̃ in the detector, the amount of energy it deposits in each hadronic collision, and the probability for the g̃ to fragment to a pseudo-stable charged hadron after a given hadronic collision. We explore how the range of excluded m_g̃ depends upon these ingredients, concluding that for non-extreme cases the range 3 GeV≲m_g̃≲130–150 GeV can be excluded at 95% C.L. based on currently available OPAL and CDF analyses. We find that run II at the Tevatron can extend the excluded region (or discover the g̃) up to m_g̃∼160–180 GeV. For completeness, we also analyze the case where the g̃ is the NLSP (as possible in gauge-mediated supersymmetry breaking) decaying via g̃→g+gravitino. We find that the Tevatron run I data exclude m_g̃<~240 GeV. Finally, we discuss application of the procedures developed for the heavy g̃-LSP to searches for other stable strongly interacting particles, such as a stable heavy quark.

Assuming perturbativity up to a high energy scale ∼10¹⁶ GeV, we demonstrate that a future e⁺e^- linear collider operating at sqrt[s]  =  500 GeV with ∫L  =  500 fb^-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.