Showing papers on "Elementary particle published in 2001"

Extra families, Higgs spectrum, and oblique corrections

Abstract: The standard model accommodates, but does not explain, three families of leptons and quarks, while various extensions suggest extra matter families. The oblique corrections from extra chiral families with relatively light (weak-scale) masses, ${M}_{f}\ensuremath{\sim}〈H〉,$ are analyzed and used to constrain the number of extra families and their spectrum. The analysis is motivated, in part, by recent $N=2$ supersymmetry constructions, but is performed in a model-independent way. It is shown that the correlations among the contributions to the three oblique parameters, rather than the contribution to a particular one, provide the most significant bound. Nevertheless, a single extra chiral family with a constrained spectrum is found to be consistent with precision data without requiring any other new physics source. Models with three additional families may also be accommodated but only by invoking additional new physics, most notably, a two-Higgs-doublet extension. The interplay between the spectra of the extra fermions and the Higgs boson(s) is analyzed in the case of either one or two Higgs doublets, and its implications are explored. In particular, the precision bound on the standard model-like Higgs boson mass is shown to be significantly relaxed in the presence of an extra relatively light chiral family.

Coupled fields in external background with application to nonthermal production of gravitinos

Abstract: We provide the formalism for the quantization of systems of coupled bosonic and fermionic fields in a time dependent classical background. The occupation numbers of the particle eigenstates can be clearly defined and computed, through a generalization of the standard procedure valid for a single field in which Bogolyubov coefficients are employed. We apply our formalism to the problem of non-thermal gravitino production in a two-fields model where supersymmetry is broken gravitationally in the vacuum. Our explicit calculations show that this production is strongly suppressed in the model considered, due to the weak coupling between the sector which drives inflation and the one responsible for supersymmetry breakdown.

Muon Anomalous Magnetic Moment, Two-Higgs-Doublet Model, and Supersymmetry

Abstract: The recent measurement of the muon anomalous magnetic moment ${a}_{\ensuremath{\mu}}$ shows a $26\ensuremath{\sigma}$ deviation from the standard model value We show that it puts strong constraints on the parameter space of the two-Higgs-doublet model (2HDM) II The dominant contribution of the Higgs bosons comes at the two-loop level, and in order to explain the data it favors a pseudoscalar A with a light mass range and a large $\mathrm{tan}\ensuremath{\beta}$ At 95% CL, the upper limit for ${m}_{A}$ is 29 (55) (85) GeV for $\mathrm{tan}\ensuremath{\beta}=30$ (45) (60), and $\mathrm{tan}\ensuremath{\beta}$ is bounded below at 17 This is in sharp contrast to the conclusion one draws from considering one-loop Higgs contributions alone Finally, we also discuss the role of the Higgs contributions in the minimal supersymmetric standard model

h → μ τ at Hadron Colliders

Abstract: We study the observability for a lepton flavor-changing decay of a Higgs boson $h\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau}$ at hadron colliders. Flavor-changing couplings of a Higgs boson exist at tree level in models with multiple Higgs doublets. The $h\ensuremath{\mu}\ensuremath{\tau}$ coupling is particularly motivated by the favorable interpretation of ${\ensuremath{ u}}_{\ensuremath{\mu}}\ensuremath{-}{\ensuremath{ u}}_{\ensuremath{\tau}}$ oscillation. We find that at the Tevatron run II the unique $\ensuremath{\mu}\ensuremath{\tau}$ signature could serve as the Higgs discovery channel, surpassing expectations for Higgs boson searches in the SM and in a large parameter region of the MSSM. The sensitivity will be greatly improved at the LHC, beyond the coverage at a muon collider Higgs factory.

Potential of photon collider in resolving SM like scenarios

Abstract: After operations at the LHC and e+e− Linear Colliders, it may be found that a Standard-Model-like (SM-like) scenario is realized. In this scenario, no new particle will be discovered, except a single Higgs boson having partial widths or coupling constants squared with fundamental particles close, within anticipated experimental uncertainty, to those of the SM. Experiments at a Photon Collider can resolve whether the SM model or e.g. the Two-Higgs-Doublet Model is realized in Nature. For the SM-like version of the 2HDM (II), we study the loop couplings of the Higgs boson with γγ and Zγ, and also with gluons. The deviation of the two-photon width from its SM value is generally higher than the expected inaccuracy in the measurement of Γγγ at a Photon Collider. The result is sensitive to the parameters of the Higgs self interaction.

Higgs boson decays in the minimal supersymmetric standard model with radiative Higgs sector CP violation

Abstract: We re-evaluate the decays of the Higgs bosons in the minimal supersymmetric standard model (MSSM) where the tree-level CP invariance of the Higgs potential is explicitly broken by the loop effects of the third-generation squarks with CP-violating soft-breaking Yukawa interactions. This study is based on the mass matrix of the neutral Higgs bosons that is valid for arbitrary values of all the relevant MSSM parameters. It extends the previous work considerably by including neutral Higgs-boson decays into virtual gauge bosons and those into top-squark pairs, by implementing squark-loop contributions to the two-gluon decay channel, and by incorporating the decays of the charged Higgs boson. The constraints from the electron electric dipole moment on the CP phases are also discussed. We find that the branching fractions of both the neutral and charged Higgs-boson decays and their total decay widths depend strongly on the CP phases of the top (and bottom) squark sectors through the loop-induced neutral Higgs boson mixing as well as the direct couplings of the neutral Higgs bosons to top squark pairs.

Implications of the Higgs Boson Searches on Different Soft SUSY-breaking Scenarios

Abstract: We investigate the Higgs boson sector of the Minimal Supersymmetric Standard Model (MSSM) in the framework of the three most prominent soft SUSY-breaking scenarios, mSUGRA, MGMSB and mAMSB. For each scenario, we determine the parameters at the electroweak scale from the set of input variables at higher energy scales (depending on the specific scenario) and evaluate the Higgs boson properties. The latter are based on results obtained within the Feynman-diagrammatic approach by taking into account the complete one-loop and the dominant two-loop contributions. The maximum value of the mass of the lightest neutral CP-even MSSM Higgs boson, m_h, is determined in the three scenarios, and the behavior of the Higgs couplings to fermions and gauge bosons is investigated. Restrictions on tan beta and on the set of higher-energy scale parameters are derived from the lower limits arising from the Higgs search at LEP2. We furthermore discuss the regions of parameter space in the three scenarios compatible with interpreting the excess observed at LEP2 as a Higgs signal, with m_h = (115.0 + 1.3 - 0.9) GeV. The case where the events observed at LEP2 could originate from the production of the heavier neutral CP-even Higgs boson is also considered. The implications of a possible Higgs signal at 115 GeV for SUSY searches at future colliders are briefly discussed for each of the three scenarios.

Establishing a No-Lose Theorem for NMSSM Higgs Boson Discovery at the LHC

Abstract: We scan the parameter space of the NMSSM for the observability of at least one Higgs boson at the LHC with $300\fbi$ integrated luminosity, taking the present LEP2 constraints into account. We restrict the scan to those regions of parameter space for which Higgs boson decays to other Higgs bosons and/or supersymmetric particles are kinematically forbidden. We find that if $WW$-fusion detection modes for a light Higgs boson are not taken into account, then there are still significant regions in the scanned portion of the NMSSM parameter space where no Higgs boson can be observed at the $5\sigma$ level, despite the recent improvements in ATLAS and CMS procedures and techniques and even if we combine all non-fusion discovery channels. However, if the $WW$-fusion detection modes are included using the current theoretical study estimates, then we find that for all scanned points at least one of the NMSSM Higgs bosons will be detected. If the estimated $300\fbi$ significances for ATLAS and CMS are combined, one can also achieve $5\sigma$ signals after combining just the non-$WW$-fusion channels signals. We present the parameters of several particularly difficult points, and discuss the complementary roles played by different modes. We conclude that the LHC will discover at least one NMSSM Higgs boson unless there are large branching ratios for decays to SUSY particles and/or to other Higgs bosons.

Can the Higgs sector contribute significantly to the muon anomalous magnetic moment

Abstract: A light CP-even Higgs boson (h) with a mass of about 10 GeV could explain the recent BNL measurement of the muon anomalous magnetic moment, in the framework of a general CP-conserving two-Higgs-doublet extension of the Standard Model with no tree-level flavor-changing neutral Higgs couplings. However, the allowed Higgs mass window is quite small and the corresponding model parameters are very constrained. The Higgs sector can contribute significantly to the observed BNL result for g-2 without violating known experimental constraints only if the hZZ coupling (approximately) vanishes and m_h lies between the Upsilon mass and 2m_B.

Charged Higgs boson production from supersymmetric particle cascade decays at the CERN LHC

Abstract: We analyze the cascade decays of the scalar quarks and gluinos of the Minimal Supersymmetric extension of the Standard Model, which are abundantly produced at the Large Hadron Collider, into heavier charginos and neutralinos which then decay into the lighter ones and charged Higgs particles, and show that they can have substantial branching fractions. The production rates of these Higgs bosons can be much larger than those from the direct production mechanisms, in particular for intermediate values of the parameter $\tan \beta$, and could therefore allow for the detection of these particles. We also discuss charged Higgs boson production from direct two-body top and bottom squark decays as well as from two- and three-body gluino decays.

Search for neutral supersymmetric Higgs bosons in pp̄ collisions at √s = 1.8 TeV

Abstract: We present the results of a search for neutral Higgs bosons produced in association with b quarks in pp-->bb(phi)-->bbb final states with 91+/-7 pb(-1) of pp collisions at sqrt[s] = 1.8 TeV recorded by the Collider Detector at Fermilab. We find no evidence of such a signal and the data are interpreted in the context of the neutral Higgs sector of the minimal supersymmetric extension of the standard model. With basic parameter choices for the supersymmetric scale and the stop-quark mixing, we derive 95% C.L. lower mass limits for neutral Higgs bosons for tan(beta) values in excess of 35.

Higgs mass bounds in a Triplet Model

Abstract: We perform a global fit to high energy precision electroweak data in a Higgs model containing the usual isospin doublet plus a real isospin triplet. The analysis is performed in terms of the oblique parameters S, T and U and we show that the mass of the lightest Higgs boson can be as large as 2 TeV.

Phenomenological study of hadron interaction models

Abstract: We present a phenomenological study of three models with different effective degrees of freedom: a Goldstone Boson Exchange (GBE) model which is based on quark-meson couplings, the quark delocalization, color screening model (QDCSM) which is based on quark-gluon couplings with delocalized quark wavefunctions, and the Fujiwara-Nijmegen (FN) mixed model which includes both quark-meson and quark-gluon couplings. We find that for roughly two-thirds of 64 states consisting of pairs of octet and decuplet baryons, the three models predict similar effective baryon-baryon interactions. This suggests that the three very different models, based on different effective degrees of freedom, are nonetheless all compatible with respect to baryon spectra and baryon-baryon interactions. We also discuss the differences between the three models and their separate characteristics.

Low-energy limits of theories with two supersymmetries

Abstract: Given its non-renormalization properties, low-energy supersymmetry provides an attractive framework for extending the standard model and for resolving the hierarchy problem. Models with softly broken $N=1$ supersymmetry have been extensively studied and are phenomenologically successful. However, it could be that an extended $N=2$ supersymmetry survives to low energies, as suggested by various constructions. We examine the phenomenological viability and implications of such a scenario. We show that consistent chiral fermion mass generation emerges in $N=2$ theories, which are vectorial, as a result of supersymmetry breaking at low energies. A rich mirror quark and lepton spectrum near the weak scale with model-dependent decay modes is predicted. A ${Z}_{2}$ mirror parity is shown to play an important role in determining the phenomenology of the models. It leads, if conserved, to a new stable particle, the LMP. Consistency of the $N=2$ framework and its unique spectrum with electroweak precision data is considered, and the discovery potential in the next generation of hadron collider experiments is stressed. Mirror quark pair production provides the most promising discovery channel. Higgs boson searches are also discussed and it is shown that there is no upper bound on the prediction for the Higgs boson mass in the framework of low-energy supersymmetry breaking, in general, and in the $N=2$ framework, in particular. Possible $N=2$ realizations of flavor symmetries and of neutrino masses are also discussed.

Measuring supersymmetric particle masses at the LHC in scenarios with Baryon-number R-parity violating couplings

Abstract: The measurement of sparticle masses in the Minimal Supersymmetric Standard Model at the LHC is analysed, in the scenario where the lightest neutralino, the 01, decays into three quarks. Such decays, occurring through the baryon-number violating coupling λ''ijk, pose a severe challenge to the capability of the LHC detectors since the final state has no missing energy signature and a high jet multiplicity. We focus on the case λ''212≠0 which is the most difficult experimentally. The proposed method is valid over a wide range of SUGRA parameter space with λ''212 ~ 10−5−0.1. Simulations are performed of the ATLAS detector at the Large Hadron Collider. Using the 01 from the decay chain L→02q→Rq→01q, we show that the 01 and 02 masses can be measured by 3-jet and 3-jet + lepton pair invariant mass combinations. At the SUGRA point m0 = 100 GeV, m1/2 = 300 GeV, A0 = 300 GeV, tan β = 10, 0$>μ > 0 and with λ''212 = 0.005, we achieve statistical (systematic) errors of 3 (3), 3 (3), 0.3 (4) and 5 (12) GeV, respectively for the masses of the 01, 02, R and L, with an integrated luminosity of 30 fb−1.

Sphalerons in two Higgs doublet theories

Abstract: We undertake a comprehensive investigation of the properties of the sphaleron in electroweak theories with two Higgs doublets. We do this in as model-independent a way as possible: by exploring the physical parameter space described by the masses and mixing angles of the Higgs particles. If there is a large split in the masses of the neutral Higgs particles, there can be several sphaleron solutions, distinguished by their properties under parity and the behavior of the Higgs field at the origin. In general, these solutions appear in parity conjugate pairs and are not spherically symmetric, although the departure from spherical symmetry is small. Including CP violation in the Higgs potential can change the energy of the sphaleron by up to 14% for a given set of Higgs boson masses, with significant implications for the baryogenesis bound on the mass of the lightest Higgs boson.

OMS-bar scheme of UV renormalization in the presence of unstable fundamental particles

Abstract: A generalization of the on-mass-shell scheme of UV renormalization (the OMS-bar scheme) to the case of presence of unstable fundamental particles (like W and Z bosons) is proposed. Its basic ingredients are as follows: (i) the renormalized mass coincides with a real part of the position of the complex pole of the corresponding propagator, (ii) the imaginary part of the on-shell self-energy coincides with the imaginary part of the complex pole position. The latter property implies the gauge-invariance of the imaginary part of the on-shell self-energy in the OMS-bar scheme and its connection with the lifetime of an unstable particle. Starting with the three-loops this connection becomes nontrivial.

A muon collider as a Higgs factory

Abstract: Evidence from the LEP experiments for a Higgs boson at a mass of 115 GeV has recently been presented at CERN. The first muon collider would be the best accelerator at which to study such a Higgs boson. The cross section for direct Higgs production is much larger than the associated production with a Z, which is an advantage for the muon collider. The storage ring would be 350 meters in circumference with a luminosity of 2 /spl times/ 10/sup 31/ cm/sup 2/ s/sup -1/ and an energy spread of 0.0001 [/spl Delta/ E/E]. Feasibility studies for muon storage rings as neutrino factories have been undertaken. In addition, further R&D has been done on emittance exchange needed for longitudinal cooling at a high luminosity muon collider. Using the results of these R&D efforts, a new study to develop a muon collider Higgs factory is underway. The current status of this study will be presented.

2001 Report on the Next Linear Collider

Abstract: Recent studies in elementary particle physics have made the need for an e{sup +}e{sup -} linear collider able to reach energies of 500 GeV and above with high luminosity more compelling than ever [1]. Observations and measurements completed in the last five years at the SLC (SLAC), LEP (CERN), and the Tevatron (FNAL) can be explained only by the existence of at least one particle or interaction that has not yet been directly observed in experiment. The Higgs boson of the Standard Model could be that particle. The data point strongly to a mass for the Higgs boson that is just beyond the reach of existing colliders. This brings great urgency and excitement to the potential for discovery at the upgraded Tevatron early in this decade, and almost assures that later experiments at the LHC will find new physics. But the next generation of experiments to be mounted by the world-wide particle physics community must not only find this new physics, they must find out what it is. These experiments must also define the next important threshold in energy. The need is to understand physics at the TeV energy scale as well as the physics at the 100-GeV energy scalemore » is now understood. This will require both the LHC and a companion linear electron-positron collider.« less

Standard-Model-like scenarios in the 2HDM and Photon Collider potential

Abstract: After operations at the LHC and e+e− Linear Colliders it may be found that a Standard-Model-like scenario is realized. In this scenario no new particle will be discovered, except a single Higgs boson having partial widths or coupling constants with fundamental particles, whose squares are close, within anticipated experimental uncertainty, to those of the SM. Experiments at a Photon Collider can resolve whether the SM or e.g. the Two-Higgs-Doublet Model is realized in Nature. For the SM-like realizations of the 2HDM (II) we study the loop couplings of the Higgs boson with γγ and Zγ, and also with gluons. The deviation of the two-photon width from its SM value is generally higher than the expected inaccuracy in the measurement of Γγγ at a Photon Collider. The deviation is sensitive to the parameters of the Higgs self interaction.

Tau physics: theoretical perspective

Abstract: The leptonic decays of the τ lepton provide relevant tests on the structure of the weak currents and the universality of their couplings to the gauge bosons. The hadronic τ decay modes constitute an ideal tool for studying low-energy effects of the strong interaction in very clean conditions. Accurate determinations of the QCD coupling and the strange quark mass have been obtained with τ decay data. New physics phenomena, such as a non-zero mντ or violations of conservation laws can also be searched for with τ decays.

Implications of the Muon Anomalous Magnetic Moment for Direct Detection of Neutralino Dark Matter

Abstract: We investigate the implications of a recent measurement of the muon anomalous magnetic moment for the direct detection of neutralino dark matter in three different SUSY models: mSUGRA, a model with non-universal Higgs mass, and an SO(10) GUT model. We consider two cases for the value of ∆aµ ,2 7× 10 −10 <∆ a µ < 59 × 10 −10 (corresponding to a range of 1σ about the experimental value) and0 <∆ a µ < 11 × 10 −10 (corresponding to a range more than 2σ below the experimental value). In the mSUGRA model, the counting ratio may be above the sensitivity of future experiments in the case that parameters are within a 1σ bound of ∆aµ. However, Ωχ tends to be large compared to the currently accepted value Ω =0 .3. For models with non-universal scalar masses, the possibility of having a consistent Ωχ and a large counting ratio exists in the region of parameter space where the Higgsino mass µ is smaller than the mSUGRA prediction. In particular, in the SO(10) model, the LSP dark matter detection rate may be enhanced by almost one order of magnitude compared to that in mSUGRA andthe mod el with non-universal Higgs mass, for cosmologically acceptable Ωχh 2 . The highest detection rate of LSP dark matter occurs in the region where the LSP constitutes a subdominant part of the local halo DM. The implications of the SUSY mass parameter measurement subject to the cosmological constraint are also discussed.