arXiv subjects
Publications and source records attributed to T. Han.
We demonstrate that, at a 500 GeV e+e- collider, inclusion of the $ZZ$-fusion process for production of a light standard-model-like Higgs boson can substantially increase the precision with which the $ZZh$ coupling can be determined (using the model-independent recoil mass technique) as compared to employing only $Zh$ associated production.
We consider states in the adjoint representation of the Standard Model gauge group as messengers for mediation of supersymmetry (SUSY) breaking. These new messengers can shift the gauge coupling unification to the string scale at O(5x10^{17} GeV) if their masses are at O(10^{14} GeV). The predicted SUSY mass spectrum at the electroweak scale is significantly different from those in other gauge-mediated or supergravity models, resulting in robust mass relations. The gravitino mass is predicted to be about 1-10 GeV. The heavy messenger sector could provide a superheavy dark matter candidate.
We analyze the prospects at a muon collider for measuring chargino masses in the $μ^+μ^-\to \tildeχ^+\tildeχ^-$ process in the threshold region. We find that for the lighter chargino of a mass $100-200$ GeV, a measurement better than $50-300$ MeV should be possible with 50 fb$^{-1}$ integrated luminosity. The accuracy obtained here is better than with other techniques or at other facilities. The muon sneutrino mass, which enters through the $\tildeν_μ$ exchange diagram, can also be simultaneously measured to a few GeV if it is not too heavy.
We study new physics effects to the quartic gauge boson couplings formulated by the electroweak chiral Lagrangian. Five next-to-leading order operators characterize the anomalous quartic gauge interactions which involve pure Goldstone boson dynamics for the electroweak symmetry breaking. We estimate the typical size of these couplings in different strongly-interacting models and examine the sensitivity to directly probing them via the WWZ/ZZZ triple gauge boson production at the high energy linear colliders. The important roles of polarized e- and e+ beams are stressed. We then compare the results with those from the W-pair production of the WW-fusion processes, and analyze the interplay of these two production mechanisms for an improved probe of the quartic gauge boson interactions.
In supersymmetric models with R-parity violating interactions, sneutrinos may be produced as $s$-channel resonances at $μ^+ μ^-$ colliders. We demonstrate that, for R-parity violating couplings as low as $10^{-4}$, sneutrinos can be discovered and their couplings measured to high accuracy. The excellent beam energy resolution of muon colliders is found to be especially useful for studying such resonances in certain cases.
The processes $pp/ pbar p \to Z γ+ X \to \ell^+ \ell^- γ+ X$ ($\ell=e, μ$) and $pp/ pbarp \to Z γ+ X \to \barννγ+ X$ are calculated to ${\cal O}(α_s)$ for general $ZZγ$ and $Zγγ$ couplings. The impact of ${\cal O}(α_s)$ QCD corrections on the observability of $ZZγ$ and $Zγγ$ couplings in $Zγ$ production at the Tevatron and the Large Hadron Collider (LHC) is discussed.
One of the options for an accelerator beyond the LHC is a hadron collider with higher energy. Work is going on to explore accelerator technologies that would make such a machine feasible. This workshop concentrated on the physics and detector issues associated with a hadron collider with an energy in the center of mass of the order of 100 to 200 TeV.
We explore the potential of electron-photon colliders to measure fundamental supersymmetry parameters via the processes $eγ\to \tilde e \tildeχ^0$ (selectron-neutralino) and $eγ\to \tildeν\tildeχ^-$ (sneutrino-chargino). Given the $\tildeχ^0$ and $\tildeχ^-$ masses from $e^+e^-$ and hadron collider studies, cross section ratios $σ(γ_-)/σ(γ_+)$ for opposite photon helicities determine the $\tildeν_L$, $\tilde e_L$ and $\tilde e_R$ masses, independent of the sparticle branching fractions. The difference $m_{\tildeν_L}^2-m_{\tilde e_L}^2$ measures $M_W^2\cos2β$ in a model-independent way. The $\tilde e_L$ and $\tilde e_R$ masses test the universality of soft supersymmetry breaking scalar masses. The cross section normalizations provide information about the gaugino mixing parameters.
In models where supersymmetry breaking is communicated into the visible sector via gauge interactions the lightest supersymmetric particle is typically the gravitino which is too light to account for cold dark matter. We point out that the lightest messenger sneutrinos with mass in the range of one to three TeV may serve as cold dark matter over most of the parameter space due to one-loop electroweak radiative corrections. However, in the minimal model this mass range has been excluded by the direct dark matter searches. We propose a solution to this problem by introducing terms that explicitly violate the messenger number. This results in low detection rate for both direct and indirect searches and allows messenger sneutrinos to be a valid dark matter candidate in a wide region of SUSY parameter space.
We summarize the potential of muon colliders to probe fundamental physics. $W^+W^-$, \bar tt, and $Zh$ threshold measurements could determine masses to precisions $ΔM_W = 6$ MeV, $Δm_t = 70$ MeV, and $Δm_h = 45$ MeV, to test electroweak radiative corrections. With $s$-channel Higgs production, unique to a muon collider, the Higgs mass could be pinpointed ($Δm_h < 1$ MeV) and its width measured. The other Higgs bosons of supersymmetry can be produced and studied by three methods. If instead the $WW$ sector turns out to be strongly interacting, a 4 TeV muon collider is ideally suited to its study.
The electroweak process $p\bar p \to \ell^\pmνγγ$ is calculated at tree level, including finite W width effects. In order to obtain a gauge invariant amplitude, the imaginary parts of $WWγ$ triangle graphs and $WWγγ$ box diagrams have to be included, in addition to resumming the imaginary contributions to the W polarization. We demonstrate the existence of a radiation amplitude zero in $p\bar p \to W^\pm γγ\to \ell^\pmνγγ$, and discuss how it may be observed in correlations of the $γγ$ and lepton rapidities at the Fermilab Tevatron.
Precise determinations of the masses of the $W$ boson and of the top quark could stringently test the radiative structure of the Standard Model (SM) or provide evidence for new physics. We analyze the excellent prospects at a muon collider for measuring $M_W$ and $m_t$ in the $W^+W^-$ and $t\bar t$ threshold regions. With an integrated luminosity of 10 (100) fb$^{-1}$, the $W$-boson mass could be measured to a precision of 20 (6) MeV, and the top-quark mass to a precision of 200 (70) MeV, provided that theoretical and experimental systematics are understood. A measurement of $Δm_t=200$ MeV for fixed $M_W$ would constrain a 100 GeV SM Higgs mass within about $\pm 2$ GeV, while $ΔM_W=6$ MeV for fixed $m_t$ would constrain $m_h$ to about $\pm 10$ GeV.
We demonstrate that a measurement of the Bjorken process $e^+e^-, μ^+μ^-\to ZH$ in the threshold region can yield a precise determination of the Higgs boson mass. With an integrated luminosity of $100 fb^{-1}$, it is possible to measure the Higgs mass to within 60 MeV (100 MeV) for m_H=100 GeV (150 GeV).
The measurement of anomalous gauge boson self couplings is reviewed for a variety of present and planned accelerators. Sensitivities are compared for these accelerators using models based on the effective Lagrangian approach. The sensitivities described here are for measurement of generic parameters kappa_v, lambda_v, etc., defined in the text. Pre-LHC measurements will not probe these couplings to precision better than O(1/10). The LHC should be sensitive to better than O(1/100), while a future NLC should achieve sensitivity of O(1/1000) to O(1/10000) for center of mass energies ranging from 0.5 to 1.5 TeV.
The process $p \bar p \rightarrow μ^- \bar νγγ$ is calculated including finite lepton mass effects. Implications for precision measurements of the $W$ mass at the Tevatron are discussed.
We discuss the excellent prospects for a detailed study of a strongly interacting electroweak sector at a muon collider with c.m. energy $\rts\sim 4\tev$. For expected luminosity of $L=200-1000\fbi$ per year, $\mup\mum$ and $\mup\mup$ (or $\mum\mum$) collisions can be used to study longitudinal $\wp\wm$ and $\wp\wp$ (or $\wm\wm$) scattering with considerable precision. In particular, detailed measurements of the distribution in the $VV$ pair masses ($V=\wpm,Z$) will be possible. The shape and magnitude of these distributions will provide a powerful tool for determining the nature of strong gauge boson interactions. Event rates will be large enough that projection techniques can be used to directly isolate final states with different polarizations of the $V$'s and verify that the strong interaction cross section excess is mainly in the longitudinal-longitudinal mode.
We examine the experimental observability of the decay mode $t \rightarrow c g$ at the Fermilab Tevatron via the flavor-changing neutral current vertex $t \bar c g$. We find that with the existing data, one should be able to probe the $t \bar c g$ coupling to a value smaller than indirect limits previously obtained from $b\rightarrow s γ$ and the measured branching fraction for $t\rightarrow bW$, reaching $BF(t \rightarrow cg) \sim$ 15\% - 28\%. A data sample of 1~fb$^{-1}$ (10~fb$^{-1}$) at $\sqrt{s}=2$~TeV may probe the $t \rightarrow c g$ branching fraction to a level of 2\% (0.5\%).
It is proposed that, in a strongly-interacting electroweak sector, besides the Goldstone bosons, the coexistence of a scalar state ($H$) and vector resonances such as $A_1$ [$I^G(J^P)=1^-(1^+$)], $V$ [$1^+(1^-)$] and $ω_H^{}$ [$0^-(1^-)$] is required by the proper Regge behavior of the forward scattering amplitudes. This is a consequence of the following well-motivated assumptions: (a). Adler-Weisberger-type sum rules and the superconvergence relations for scattering amplitudes hold in this strongly interacting sector; (b). the sum rules at $t=0$ are saturated by a minimal set of low-lying states with appropriate quantum numbers. It therefore suggests that a complete description should include all these resonances. These states may lead to distinctive experimental signatures at future colliders.