arXiv subjects

Tao Han

Publications and source records attributed to Tao Han.

At least 325 records · Page 18Linked to original sources

arXiv2002

TeV String State Excitation via High Energy Cosmic Neutrinos

We construct the open-string scattering amplitudes for neutrino-gluon collisions and evaluate the high energy neutrino-nucleon scattering cross section via string state excitations in the TeV string-scale scenario. We find that the neutrino-gluon scattering is the dominant contribution, 5-10 times larger than neutrino-quark processes, though black hole production may be larger than the string contribution at higher energies. We illustrate the observability of the string signal at the Auger Observatory and the IceCube neutrino telescope for a string scale about 1 TeV.

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arXiv2002

τ- μFlavor Violation as a Probe of the Scale of New Physics

Motivated by the recent strong experimental evidence of large ν_mu-ν_tau neutrino mixing, we explore current bounds on the analogous mixing in the charged lepton sector. We present a general formalism for dimension-6 fermionic effective operators involving tau-mu mixing with typical Lorentz structure (\bar{mu} Γtau)(\bar{q}^a Γ{q}^b), and discuss their relationship to the standard model gauge symmetry and the underlying flavor dynamics. We derive the low-energy constraints on the new physics scale associated with each operator, mostly from current experimental bounds on rare decay processes of tau, hadrons or heavy quarks. For operators involving at least one light quark (u,d,s), these constraints typically give a bound on the new physics scale of a few TeV or higher. Those operators with two heavy quarks turn out to be more weakly constrained at the present, giving bounds of a few hundred GeV. A few scalar and pseudo-scalar operators are free from all current experimental constraints.

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arXiv2002

Black hole evaporation with separated fermions

In models with a low quantum gravity scale, a well-motivated reason to expect quark and lepton fields are localized but physically separated is to avoid proton decay. This could happen in a ``fat-brane'' or in an additional, orthogonal 1/TeV sized dimension in which the gauge and Higgs fields live throughout. Black holes with masses of order the quantum gravity scale are therefore expected to evaporate non-universally, preferentially radiating directly into quarks or leptons but not both. Should black holes be copiously produced at a future hadron collider, we find the ratio of final state jets to charged leptons to photons is 113:8:1, which differs from previous analyses that assumed all standard model fields live at the same point in the extra dimensional space.

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arXiv2002

Electroweak Symmetry Breaking by Strong Dynamics and the Collider Phenomenology

We discuss the possible signatures in the electroweak symmetry breaking sector by new strong dynamics at future hadron colliders such as the Tevatron upgrade, the LHC and VLHC, and $e^+e^-$ linear colliders. Examples include a heavy Higgs-like scalar resonance, a heavy Technicolor-like vector resonance and pseudo-Goldstone states, non-resonance signatures via enhanced gauge-boson scattering and fermion compositeness.

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arXiv2002

Detecting Microscopic Black Holes with Neutrino Telescopes

If spacetime has more than four dimensions, ultra-high energy cosmic rays may create microscopic black holes. Black holes created by cosmic neutrinos in the Earth will evaporate, and the resulting hadronic showers, muons, and taus may be detected in neutrino telescopes below the Earth's surface. We simulate such events in detail and consider black hole cross sections with and without an exponential suppression factor. We find observable rates in both cases: for conservative cosmogenic neutrino fluxes, several black hole events per year are observable at the IceCube detector; for fluxes at the Waxman-Bahcall bound, tens of events per year are possible. We also present zenith angle and energy distributions for all three channels. The ability of neutrino telescopes to differentiate hadrons, muons, and possibly taus, and to measure these distributions provides a unique opportunity to identify black holes, to experimentally constrain the form of black hole production cross sections, and to study Hawking evaporation.

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arXiv2002

$h \to μ^+μ^-$ via gluon fusion at the LHC

We study the observability of the $h\to μ^+μ^-$ decay in the Standard Model and the MSSM at the LHC. The observation of the $hμμ$ coupling is important to determine whether the Higgs particle that generates mass for the weak bosons is also responsible for mass generation of the second generation of fermions. We find that the signal via the gluon fusion channel is comparable to that from the weak-boson fusion. By combining these two channels, observing $h\to μ^+μ^-$ is feasible at the LHC with a delivered luminosity of $300 fb^{-1}$ at $3σ$ statistical significance for 110 GeV $ 8$ and $M_A<130$.

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arXiv2001

Search for t->ch at e+e- Linear Colliders

We study the rare top-quark decay t->ch, where h is a generic Higgs boson, at a linear collider. If kinematically accessible, all models contain this decay at some level due to quark flavor mixing. Some models, such as Model III of the two-Higgs doublet model, have a tree-level top-charm-Higgs coupling, and the branching ratio is close to 0.5%. Others, such as the MSSM, have a coupling induced at one-loop, and can have a branching ratio in the range of 10^{-5} - 5x10^{-4}. We find that a linear collider of root(s) = 500 GeV and a luminosity of 500 fb^{-1} will begin to be sensitive to this range of the coupling.

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arXiv2001

Radion effects on unitarity in gauge-boson scattering

The scalar field associated with fluctuations in the positions of the two branes, the ``radion'', plays an important role determining the cosmology and collider phenomenology of the Randall-Sundrum solution to the hierarchy problem. It is now well known that the radion mass is of order the weak scale, and that its couplings to standard model fields are order 1/TeV to the trace of the energy momentum tensor. We calculate longitudinal vector boson scattering amplitudes to explore the constraints on the radion mass and its coupling from perturbative unitarity. The scattering cross section can indeed become non-perturbative at energies prior to reaching the TeV brane cutoff scale, but only when some curvature-Higgs mixing on the TeV brane is present. We show that the coefficient of the curvature-Higgs mixing operator must be less than about 3 for the 4-d effective theory to respect perturbative unitarity up to the TeV brane cutoff scale. Mass bounds on the Higgs boson and the radion are also discussed.

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arXiv2000

$h\to μτ$ at Hadron Colliders

We study the observability for a lepton flavor-changing decay of a Higgs boson $h\to μτ$ at hadron colliders. Flavor-changing couplings of a Higgs boson exist at tree level in models with multiple Higgs doublets. The $hμτ$ coupling is particularly motivated by the favorable intepretation of $ν_μ-ν_τ$ oscillation. We find that at the Tevatron Run II the unique $μτ$ 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.

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arXiv2000

Oblique Parameter Constraints on Large Extra Dimensions

We consider the Kaluza-Klein scenario in which gravity propagates in the $4+n$ dimensional bulk of spacetime and the Standard Model particles are confined to a 3-brane. We calculate the gauge boson self-energy corrections arising from the exchange of virtual gravitons and present our results in the $STU$-formalism. We find that the new physics contributions to $S$, $T$ and $U$ decouple in the limit that the string scale $M_S$ goes to infinity. The oblique parameters constrain the lower limit on $M_S$. Taking the quantum gravity cutoff to be $M_S$, $S$-parameter constraints impose $M_S>1.55$ TeV for $n=2$ at the 1$σ$ level. $T$-parameter constraints impose $M_S>1.25 (0.75)$ TeV for $n=3 (6)$.

hep-ph↗

arXiv2000

On Kaluza-Klein States from Large Extra Dimensions

We consider the novel Kaluza-Klein (KK) scenario where gravity propagates in the $4+n$ dimensional bulk of spacetime, while gauge and matter fields are confined to the 3+1 dimensional world-volume of a brane configuration. For simplicity we assume compactification of the extra $n$ dimensions on a torus with a common scale $R$, and identify the massive KK states in the four-dimensional spacetime. For a given KK level $\vec{n}$ there are one spin-2 state, $(n-1)$ spin-1 states and $n(n-1)/2$ spin-0 states, all mass-degenerate. We construct the effective interactions between these KK states and ordinary matter fields (fermions, gauge bosons and scalars). We find that the spin-1 states decouple and that the spin-0 states only couple through the dilaton mode. We then derive the interacting Lagrangian for the KK states and Standard Model fields, and present the complete Feynman rules. We discuss some low energy phenomenology for these new interactions for the case when 1/R is small compared to the electroweak scale, and the ultraviolet cutoff of the effective KK theory is on the order of 1 TeV.

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arXiv1999

Strong interactions of weak bosons

We discuss the parameterization for electroweak gauge boson interactions without a light Higgs boson. We present the constraints on the anomalous gauge-boson couplings from the current experiments. We emphasize that the four-point couplings involving the longitudinal weak bosons are genuine to the underlying strong dynamics responsible for the electroweak symmetry breaking. We study the sensitivity to the four-point couplings and the possible heavy resonant states in this sector at future TeV $e^+e^-$ linear colliders.

hep-ph↗

arXiv1999

A gauge-mediated supersymmetry breaking model with an extra singlet Higgs field

We study in some detail the next-to-minimal supersymmetric standard model with gauge mediation of supersymmetry breaking. We find that it is feasible to spontaneously generate values of the Higgs mass parameters $μ$ and $B_μ$ consistent with radiative electroweak symmetry breaking. The model has a phenomenologically viable particle spectrum. Messenger sneutrinos with mass in the range 6 to 25 TeV can serve as cold dark matter. It is also possible to evade the cosmological domain wall problem in this scenario.

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arXiv1998

Exploiting $h \to W^W^$ Decays at the Upgraded Fermilab Tevatron

We study the observability of a Standard Model-like Higgs boson at an upgraded Fermilab Tevatron via the mode $h \to W^*W^*$. We concentrate on the main channel $gg \to h \to W^*W^* \to l νl ν$. We also find the mode $q\bar q'\to W h \to W W^*W^* \to l^\pm νl^\pm νjj$ useful. We perform detector level simulations by making use of a Monte Carlo program SHW. Optimized searching strategy and kinematical cuts are developed. We find that with a c.m. energy of 2 TeV and an integrated luminosity of 30 fb$^{-1}$ the signal should be observable at a 3$σ$ level or better for the mass range of 145 GeV < m_h < 180 GeV. For 95% confidence level exclusion, the mass reach is 135 GeV < m_h <190 GeV. We also present results of studying these channels with a model-independent parameterization. Further improvement is possible by including other channels. We conclude that the upgraded Fermilab Tevatron will have the potential to significantly advance our knowledge of Higgs boson physics.

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arXiv1998

Extending the Higgs Boson Reach at the Upgraded Fermilab Tevatron

We study the observability for a Standard Model-like Higgs boson at an upgraded Tevatron via the modes $p \bar p \to gg \to h \to \ww \to \ljj$ and $\ll$. We find that with c. m. energy of 2 TeV and an integrated luminosity of 30 fb$^{-1}$ the signal may be observable for the mass range of $135 {\gev} \lsim \mh \lsim 180$ GeV at a $3-5σ$ statistical level. We conclude that the upgraded Tevatron may have the potential to detect a SM-like Higgs boson in the mass range from the LEP2 reach to 180 GeV.

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arXiv1998

Top-Charm Associated Production in High Energy $e^+e^-$ Collisions

The possibility of exploring the flavor changing neutral current $tcZ/tcγ$ couplings in the production vertex for the reaction $\epem\to t\bar c + \bar tc$ is examined. Using a model independent parameterization for the effective Lagrangian to describe the most general three-point interactions, production cross sections are found to be relatively small at LEP II, but potentially sizeable at higher energy \epem colliders. The kinematic characteristics of the signal are studied and a set of cuts are devised for clean separation of the signal from background. The resulting sensitivity to anomalous flavor changing couplings at LEP II with an integrated luminosity of $4\times 500$ pb$^{-1}$ is found to be comparable to their present indirect constraints from loop processes, while at higher energy colliders with $0.5-1$ TeV center-of-mass energy and 50-200 fb$^{-1}$ luminosity, one expects to reach a sensitivity at or below the percentage level.

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arXiv1998

Discovery limits for Techni-Omega production in $eγ$ Collisions

In a strongly-interacting electroweak sector with an isosinglet vector state, such as the techni-omega, $ω_T$, the direct $ ω_T Z γ$ coupling implies that an $ω_T$ can be produced by $Z γ$ fusion in $e γ$ collisions. This is a unique feature for high energy $e^+e^-$ or $e^-e^-$ colliders operating in an $eγ$ mode. We consider the processes $e^- γ\to e^- Zγ$ and $e^- γ\to e^- W^+ W^- Z$, both of which proceed via an intermediate $ω_T$. We find that at a 1.5 TeV $e^+e^-$ linear collider operating in an $eγ$ mode with an integrated luminosity of 200 fb$^{-1}$, we can discover an $ω_T$ for a broad range of masses and widths.

hep-ph↗

arXiv1998

Discovery Limits for Techni-Omega Production in $eγ$ Collisions

In a strongly-interacting electroweak sector with an isosinglet vector state, such as the techni-omega, $ω_T$, the direct $ω_T Z γ$ coupling implies that an $ω_T$ can be produced by $Z γ$ fusion in $eγ$ collisions. This is a unique feature for high energy $e^+e^-$ or $e^-e^-$ colliders operating in an $eγ$ mode. We consider the processes $e^- γ\to e^- Zγ$ and $e^- γ\to e^- W^+ W^-Z$, both of which proceed via an intermediate $ω_T$. We find that at a 1.5 TeV $e^+e^-$ linear collider operating in an $eγ$ mode with an integrated luminosity of 200 fb$^{-1}$, an $ω_T$ can be discovered for a broad range of masses and widths.

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