arXiv subjects

C. -P. Yuan

Publications and source records attributed to C. -P. Yuan.

At least 235 records · Page 13Linked to original sources

arXiv1997

Probing the Electroweak Symmetry Breaking Sector with the Top Quark

A study on the effective anomalous interactions, up to dimension 5, of the top quark with the electroweak gauge bosons is made in the non-linear Chiral Lagrangian approach. Bounds on the anomalous dimension four terms are obtained from their contribution to low energy data. Also, the potential contribution to the production of top quarks at hadron colliders (the Tevatron and the LHC) and the electron Linear Collider from both dimension 4 and 5 operators is analyzed.

hep-ph↗

arXiv1997

Global Analysis for Probing Electroweak Symmetry Breaking Mechanism at High Energy Colliders

We review our recent global analysis for probing the electroweak symmetry breaking (EWSB) mechanism by the universal effective Lagrangian description. After summarizing and commenting upon the current bounds for the EWSB parameters, we focus on testing them at the forthcoming high energy colliders such as the CERN LHC and the future linear colliders. We develop a precise electroweak power counting method (a la Weinberg) and formulate the longitudinal-Goldstone boson equivalence as a necessary criterion for sensitively probing the EWSB sector. Armed with these, we systematically estimate and classify the contributions of {all} next-to-leading order (NLO) bosonic operators to various scattering processes at the high energy colliders. The experimental signatures at these colliders are also analyzed. Furthermore, the {complementarity} of different scattering processes via different colliders for a complete probe of all these NLO effective operators is demonstrated.

hep-ph↗

arXiv1997

Soft gluon effects on lepton pairs at hadron colliders

With a large integrated luminosity expected at the Tevatron, a next-to-leading order (NLO) calculation is no longer sufficient to describe the data which yield the precision measurement of $M_W$, etc. Thus, we extend the Collins-Soper-Sterman resummation formalism, for on-shell vector boson production, to correctly include the effects of the polarization and the width of the vector boson to the distributions of the decay leptons. We show how to test the rich dynamics of the QCD multiple soft gluon radiation, for example, by measuring the ratio $R_{CSS} \equiv \frac{σ(Q_T>Q_T^{\min})}{σ_{Total}}$. ($Q_T$ is the transverse momentum of the vector boson.) We conclude that both the total rates and the distributions of the lepton charge asymmetry predicted by the resummed and the NLO calculations are different when kinematic cuts are applied.

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arXiv1997

Testing Multiple Gluon Dynamics at the Tevatron

We propose the measurement of the ratio $R_{CSS}(Q_T^{\min}) \equiv \frac{σ(Q_T>Q_T^{\min})}{σ_{Total}}$ to study the effects of the multiple soft gluon radiation, predicted by QCD, on the transverse momentum ($Q_T$) distribution of the weak gauge bosons $W^{\pm }$ and $Z^0$ produced at the Tevatron. We compare the prediction of the extended Collins-Soper-Sterman resummation formalism with the next-to-leading and next-to-next-to-leading order calculations. We show that both the rich dynamics of the QCD multiple soft gluon radiation and the non-perturbative sector of QCD can be tested by measuring $R_{CSS}$.

hep-ph↗

arXiv1997

Effects of QCD Resummation on $W^+h$ and $t\bar b$ Production at the Tevatron

The resummation of multiple soft gluon emission affects the production rate and kinematic distributions of $W^+h$ (where h is a Higgs boson) and $t \bar b$ pairs at the Tevatron with $\sqrt{s}=2$ TeV. Using the Collins-Soper-Sterman resummation formalism, the production rate is enhanced over the next-to-leading-order (NLO) prediction by 2-3% for the $W^+h$ process, for Higgs boson masses between 80-120 GeV, and over 3% for the $t\bar b$ process for $m_t=175$ GeV. After resummation, the $t\bar b$ rate changes by 12-13% when $m_t$ is varied by $\pm 5$ GeV. Various kinematic distributions are presented for the individual final state particles and for the pair. The explicit radiation of hard gluons in NLO QCD is included also for the $t\bar b$ final state.

hep-ph↗

arXiv1997

Supersymmetric Electroweak Parity Nonconservation in Top Quark Pair Production at the Fermilab Tevatron

We evaluate the supersymmetric (SUSY) electroweak corrections to the effect of parity nonconservation in $q {\bar q} \to t {\bar t}$ production at the Fermilab Tevatron predicted by the Minimal Supersymmetric Model (MSSM). We find that the parity nonconserving asymmetry from the SUSY electroweak and SUSY Yukawa loop corrections predicted by the minimal supergravity (mSUGRA) model and the MSSM models with scenarios motivated by current data is about one percent. It will be challenging to observe such a small asymmetry at the Tevatron with 10 fb^{-1} of luminosity. It could however be observable if both the top- and bottom-squarks are light and $\tan β$ is smaller than 1, though theses parameters are not favored by mSUGRA.

hep-ph↗

arXiv1997

Probing Flavor Changing Neutral Higgs Couplings at LHC

Uniquely heavy with mass at the weak scale, the top quark may have large flavor changing couplings to Higgs bosons that are as yet unexplored. We show that such couplings can be directly probed at the LHC through the parton subprocess $c(\bar c)g \to t (\bar t)A^0$, where the pseudoscalar $A^0$ subsequently decays into $t \bar c$ or ${\bar t} c$, giving rise to the intriguing final state of like sign top quark pairs. After demanding $\ell^\pm\ell^\pm$, missing energy and two $b$-jets, the major background turns out to be $q\bar q' \to Wt \bar t$, which can be partially suppressed by jet counting. The signal can then manifest itself in the asymmetry of numbers of $\ell^+\ell^+$ and $\ell^-\ell^-$ events. To further improve the signal over background, efficient $t$ vs. $\bar t$ tagging methods should be developed.

hep-ph↗

arXiv1997

Top Quark Interactions and the Search for New Physics

We give a complete set of 19 independent effective dimension 5 interactions involving the top and bottom quarks with the electroweak gauge bosons within the context of the chiral Lagrangian. The anomalous operators contribute to the leading energy power E^3 of the V_L V_L -> t\bar t or t\bar b amplitudes. We also show the top quark production rates at the LHC and the LC via V_L V_L fusion processes. If no anomalous production rate is found, then these coefficients can be bound (based on the naive dimensional analysis) to be of order 10^{-2} or 10^{-1}. This is about an order of magnitude more stringent than the bounds for the next-to-leading order bosonic operators commonly studied in V_L V_L -> V_L V_L scatterings. The effect on a CP-odd observable is also briefly discussed.

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arXiv1996

Probing Electroweak Symmetry Breaking Mechanism at the LHC: A Guideline from Power Counting Analysis

We formulate the equivalence theorem as a theoretical criterion for sensitively probing the electroweak symmetry breaking mechanism, and develop a precise power counting method for the chiral Lagrangian formulated electroweak theories. Armed with these, we perform a systematic analysis on the sensitivities of the scattering processes $W^\pm W^\pm \rightarrow W^\pm W^\pm$ and $q\bar{q}'\rightarrow W^\pm Z$ for testing all possible effective bosonic operators in the chiral Lagrangian formulated electroweak theories at the CERN Large Hadron Collider (LHC). The analysis shows that these two kinds of processes are "complementary" in probing the electroweak symmetry breaking sector.

hep-ph↗

arXiv1996

Probing the Electroweak Symmetry Breaking Sector with the Top Quark

A study on effective anomalous interactions up to dimension 5 of the top quark with the electroweak gauge bosons is made in the non-linear Chiral Lagrangian approach. Bounds on the anomalous dimension four terms are obtained from their contribution to low energy data. Also, the potential contribution to the production of top quarks at hadron colliders (the Tevatron and the LHC) and the electron Linear Collider from both dimension 4 and 5 operators is analysed.

hep-ph↗

arXiv1996

Estimating The Sensitivity Of The LHC To Electroweak Symmetry Breaking: Longitudinal-Goldstone Boson Equivalence As A Criterion

Based upon our recent study, we reveal the profound physical content of the longitudinal-Goldstone boson equivalence theorem as being able to discriminate physical processes which are sensitive/insensitive to probing the electroweak symmetry breaking (EWSB) sector. We then develop a precise electroweak power counting rule (a generalization from Weinberg's counting method) to `separately' count the power dependences on the energy E and all relevant mass scales. With these, we analyze the complete set of the bosonic operators in the electroweak chiral Lagrangian and systematically estimate and classify the sensitivities for testing all these effective operators at the CERN LHC via the weak-boson fusions and quark-anti-quark annihilations. These two kinds of processes are shown to be `complementary' in probing the EWSB mechanism.

hep-ph↗

arXiv1996

Global Power Counting Analysis On Probing Electroweak Symmetry Breaking Mechanism At High Energy Colliders

We develop a precise power counting rule (a generalization of Weinberg's counting method for the nonlinear sigma model) for the electroweak theories formulated by chiral Lagrangians. Then we estimate the contributions of ``all'' next-to-leading order (NLO) bosonic operators to the amplitudes of the relevant scattering processes which can be measured at high energy colliders, such as the LHC and future Linear Colliders. Based upon these results, we globally classify the sensitivities of testing all NLO bosonic operators for probing the electroweak symmetry breaking mechanism at high energy colliders.

hep-ph↗

arXiv1996

Effects of QCD Resummation on Distributions of Top--Antitop Quark Pairs Produced at the Tevatron

We study the kinematic distributions of top--antitop quark pairs produced at the Tevatron, including the effects of initial state and final state multiple soft gluon emission, using the Collins--Soper--Sterman resummation formalism. The resummed results are compared with those predicted by the showering event generator PYTHIA for various distributions involving the top--antitop quark pair and the individual top quark or antiquark. The comparison between the experimental and predicted distributions will be a strong test of our understanding and application of perturbative QCD. Our results indicate that the showering event generators do not produce enough radiation. We reweight the PYTHIA distributions to agree with our resummed calculation, then use the reweighted events to better estimate the true hadronic activity in top--antitop quark pair production at hadron colliders.

hep-ph↗

arXiv1996

Physics of Top Quark at the Tevatron

We discuss the physics of the top quark at the Fermilab Tevatron. By the year 2000, many properties of the top quark can be measured at the Tevatron.

hep-ph↗

arXiv1996

Detecting a Light Gravitino at Linear Collider to Probe the SUSY Breaking Scale

If supersymmetry is dynamically broken at a low scale ($M_{susy}$), within a few orders of magnitude of the weak scale, then the lightest supersymmetric partner is the gravitino and the next to lightest supersymmetric partner is a neutralino $χ^0_1$ with mass $m_{χ^0_1}$, which can decay into a photon ($γ$) plus a gravitino ($\widetilde{G}$). We study the detection of $e^{-}e^{+}\rightarrow χ^0_1 χ^0_1 \rightarrow γ\widetilde{G}γ\widetilde{G}$ at the proposed Linear Collider, and find the range of the parameters $M_{susy}$ and $m_{χ^0_1}$ that can be accessible with a right-hand polarized electron beam at $\sqrt{S}=500$\,GeV, with 50\,${\rm fb}^{-1}$ integrated luminosity. We also discuss briefly the accessible range for current electron and hadron colliders.

hep-ph↗

arXiv1995

Detecting a Light Stop from Top Decays at the Tevatron

We study the possibility of discovering or excluding a light top squark (stop) based on top quark decays in the t-tbar events produced at the Fermilab Tevatron. In particular, we consider the Minimal Supersymmetric Standard Model with the sparticle spectrum m_{chi^+_1}+m_b, M_W+m_{chi^0_1}+m_b > m_{stop} > m_{chi^0_1}+m_c, where chi^0_1 is the lightest neutralino and chi^+_1 is the lightest chargino, so that t -> stop chi^0_1 and stop -> c chi^0_1. All other sparticle masses are assumed to be heavier than m_t. Such a spectrum seeks to explain the experimental values of alpha_s(M_Z^2), R_b and A_{LR} obtained from LEP/SLC data. We find that the prospect to observe a light stop via this channel at the Tevatron is very promising.

hep-ph↗

arXiv1995

Probing New Physics from the Single-Top Production

We show that the production rate of the single-top quark event from the $W$-gluon fusion process is sensitive to new physics which strongly modifies the interactions of the top quark. The measurement of its production rate gives the partial decay width of the top quark which combined with the branching ratio measurement from the $t \bar t$ pair event determines the lifetime of the top quark.

hep-ph↗

arXiv1995

Top Quark Physics at the Tevatron

We discuss physics of the top quark at the Fermilab Tevatron. By the year 2000, many properties of the top quark can be measured at the Tevatron.

hep-ph↗