arXiv subjects

C. -P. Yuan

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

At least 217 records · Page 12Linked to original sources

arXiv1999

Semi-Inclusive Hadron Production at HERA: the Effect of QCD Gluon Resummation

We present a formalism that improves the applicability of perturbative QCD in the current region of semi-inclusive deep inelastic scattering. The formalism is based on all-order resummation of large logarithms arising in the perturbative treatment of hadron multiplicities and energy flows in this region. It is shown that the current region of semi-inclusive DIS is similar to the region of small transverse momenta in vector boson production at hadron colliders. We use this resummation formalism to describe transverse energy flows and charged particle multiplicity measured at the electron-proton collider HERA. We find good agreement between our theoretical results and experimental data for the transverse energy flows.

hep-ph↗

arXiv1999

New Physics in the Third Family and its Effect on Low Energy Data

We investigate, in detail, a model in which the third family fermions are subjected to an SU(2) dynamics different from the first two families. Constrained by the precision Z-pole data, the heavy gauge boson mass is bounded from below to be about 1.7 TeV at the $2σ$ level. The flavor-changing neutral current (FCNC) in the lepton sector can be significant in $τ\leftrightarrow e$ and $τ\leftrightarrow μ$ transitions. In the latter case, the ratio $Br(τ\to μ\bar{ν_μ} ν_τ)/ Br(τ\to e \bar{ν_e} ν_τ)$ and $Br(τ\to μμμ)$ can constrain the model better than LEP/SLC data in some region of the parameter space. Furthermore, FCNCs are unavoidable in the quark sector. Significant effects to the $B^0-\bar{B^0}$ mixing and the rare decays of the K and B mesons, such as $K^\pm \to π^\pm ν{\bar ν}$, $b \to s ν{\bar ν}, B_s \to τ^+τ^-, μ^+μ^-$ and $B_{s,d} \to μ^\pm τ^\mp$, are expected.

hep-ph↗

arXiv1999

New Fits for the Non-Perturbative Parameters in the CSS Resummation Formalism

We update the non-perturbative function of the Collins-Soper- Sterman resummation formalism in hadron collisions. Two functional forms in impact parameter space are considered, one with a pure Gaussian form with two parameters and the other with an additional linear term. The results for the two parameter fit are found to be g1=0.24+0.08-0.07 GeV^2, g2=0.34+0.07-0.08 GeV^2. The results for the three parameter fit are g1=0.15+004-0.03 GeV^2, g2=0.48+0.07-0.05 GeV^2, and g3=-0.58+0.26-0.20 GeV^-1. We discuss the potential for the full Tevatron Run I Z boson data for further testing of the universality of the non-perturbative function.

hep-ph↗

arXiv1999

New Method for Detecting Charged (Pseudo-)Scalars at Colliders

We propose a new method for detecting a charged (pseudo-)scalar at colliders, based upon the observation that its Yukawa coupling to charm and bottom quarks can be large due to a significant mixing of the top and charm quarks. After analyzing the typical flavor mixing allowed by low energy data in the topcolor and the generic two-Higgs doublet models, we study the physics potential of the Tevatron, LHC, and linear colliders for probing such an s-channel charged resonance via the single-top (as well as Wh^0) production. We show that studying its detection at colliders can also provide information on the dynamics of flavor-changing neutral current phenomena.

hep-ph↗

arXiv1999

Collider Tests of Compact Space Dimensions Using Weak Gauge Bosons

We present collider tests of the recent proposal for weak-scale quantum gravity due to new large compact space dimensions in which only the graviton ($\G$) propagates. We show that the existing high precision LEP-I $Z$-pole data can impose non-trivial constraints on the scale of the new dimensions, via the decay mode $Z\to f\bar{f}+\G$ ($f=q,\ell$). These bounds are comparable to those obtained at high energy colliders and provide the first sensitive probe of the scalar graviton. We also study $W(Z)+\G$ production and the anomalous $WW(ZZ)$ signal from virtual $\G$-states at the Fermilab Tevatron, and compare them with the LEP-I bound and those from LEP-II and future linear colliders.

hep-ph↗

arXiv1999

Analysis of tbW and ttZ couplings from CLEO and LEP/SLC data

We update the constraints on anomalous dimension four $\tbw$ and $\ttz$ couplings by using CLEO $b \ra s γ$ and LEP/SLC precision $Z$-pole data. It is found that the data imposes very stringent bounds on them. Moreover, the $2 σ$ pull from SM predictions of $A_{LR}$(hadrons), $A_b$ and $A_{FB}$(b) have little chance of being explained by the strongly constrained anomalous couplings.

hep-ph↗

arXiv1998

Probing Higgs Bosons with Large Bottom Yukawa Coupling at Hadron Colliders

The small mass of the bottom quark, relative to its weak isospin partner, the top quark, makes the bottom an effective probe of new physics in Higgs and top sectors. We study the Higgs boson production associated with bottom quarks, ppbar/pp to phi+b+bbar to 4b, at the Fermilab Tevatron and the CERN LHC. We find that strong and model-independent constraints on the size of the phi-b-bbar coupling can be obtained for a wide range of Higgs boson masses. Their implications for the composite Higgs models with strong dynamics associated with the third family quarks (such as the top-condensate/topcolor models with naturally large bottom Yukawa couplings), and for the supersymmetric models with large tan(beta), are analyzed. We conclude that the Tevatron and the LHC can put stringent bounds on these models, if the phi+b+bbar signal is not found.

hep-ph↗

arXiv1998

Low Energy Data and a Model of Flavor Mixing

We consider a model in which the third family fermions are subjected to an SU(2) interaction different from the first two family fermions. Constrained by the Z-pole data, the heavy gauge boson mass is bounded from below to be about 1.7 TeV at the $2σ$ level. In this model, the flavor mixing between $τ$ and $μ$ can be so large that $Br(τ\to μ\bar{ν_μ} ν_τ)/ Br(τ\to e \bar{ν_e} ν_τ)$ and $Br(τ\to μμμ)$ provide a better constraint than the LEP/SLC data in some region of parameter space. Furthermore, flavor-changing neutral currents are unavoidable in the quark sector of the model. Significant effects to the $B^0-\bar{B^0}$ mixing and the rare decays of the $K$ and $B$ mesons, such as $K^\pm \to π^\pm ν{\bar ν}, b \to s ν{\bar ν}, B_s \to τ^+τ^-, μ^+μ^-$ and $B_{s,d} \to μ^\pm τ^\mp$, are expected.

hep-ph↗

arXiv1998

Top Quark and Electroweak Symmetry Breaking Mechanism

After a brief comment on the role of top quark in models of electroweak symmetry breaking (EWSB), I shall discuss what we know about top quark interaction and how to improve that knowledge. Since bottom quark is the weak isospin partner of the top quark, its interaction with a scalar boson may also distinguish models of EWSB. We show that Tevatron can provide useful information through the associated production of a scalar with a bottom quark pair.

hep-ph↗

arXiv1998

Gluon Resummation in Vector Boson Production and Decay

After a brief review on the technique of resumming the large logarithmic terms $α^n_s \ln^m(Q^2/Q^2_T)$ due to the effects of multiple gluon emission predicted by the QCD theory, I discuss its application in the production of $W^\pm$, $Z$ and $γγ$ in hadron collision.

hep-ph↗

arXiv1998

Higgs Boson with Large Bottom Yukawa Coupling at Tevatron and LHC

We study the discovery reach of the Tevatron and the LHC for detecting a Higgs boson (h), predicted in composite models of the electroweak symmetry breaking or in supersymmetric theories, with an enhanced b-quark Yukawa coupling via p \bar{p} / p p \to b \bbar h (\to b \bbar) + X. Our analysis shows that studying this process at the Tevatron Run II or the LHC can provide strong constraints on these models.

hep-ph↗

arXiv1998

Signatures of the light gluino in the top quark production

If a light gluino, with a mass of the order of GeV, exists in the minimal supersymmetric extension of the Standard Model, then it can contribute to the production rate of the top quark pairs at hadron colliders via the process gluino + gluino --> t + tbar. Because the top quark is heavy, the masses of the superpartners of the left-handed and right-handed top quarks can be very different such that a parity-violating observable can be induced at the tree level. We discuss the phenomenology of this parity violating asymmetry at the CERN Large Hadron Collider.

hep-ph↗

arXiv1998

Quartic Gauge Boson Couplings at Linear Colliders ---Interplay of WWZ/ZZZ Production and WW-Fusion--

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.

hep-ph↗

arXiv1997

Proposals for Studying TeV $WW \to WW$ Interactions Experimentally

We discuss how to experimentally study the electroweak symmetry breaking (EWSB) sector by observing $WW \to WW$ interactions in the TeV region. We discuss some general features of the event structure in the signal and the background events. Various techniques to enhance the signal-to-background ratio are also presented. We show how to detect longitudinal W-bosons either in the central rapidity region of the detector or in the beam pipe direction. Finally, we globally classify the sensitivities of the Large Hadron Collider and the Linear Colliders to probing the next-to-leading order bosonic operators of the electroweak chiral lagrangian used to parametrize models of strongly coupled EWSB sector.

hep-ph↗

arXiv1997

Photon Pair Production with Soft Gluon Resummation in Hadronic Interactions

The production rate and kinematic distributions of isolated photon pairs produced in hadron interactions are studied. The effects of the initial-state multiple soft-gluon emission to the scattering subprocesses q-qbar, qg, and gg to gamma gamma X are resummed with the Collins-Soper-Sterman soft gluon resummation formalism. The effects of fragmentation photons from qg to gamma q, followed by q to gamma X, are also studied. The results are compared with data from the Fermilab Tevatron collider. A prediction of the production rate and kinematic distributions of the diphoton pair in proton-nucleon reactions is also presented.

hep-ph↗

arXiv1997

Strongly Interacting Vector Bosons at TeV e+-e- Linear Colliders

In the absence of light Higgs bosons, the W and Z bosons become strongly interacting particles at energies of about 1 TeV. If the longitudinal W,Z components are generated by Goldstone modes associated with spontaneous symmetry breaking in a new strong interaction theory, the quasi-elastic W,Z scattering amplitudes can be predicted as a systematic chiral expansion in the energy. We study the potential of TeV e+e- and e-e- linear colliders in investigating these scattering processes. We estimate the accuracy with which the coefficients of the chiral expansion can be measured in a multi-parameter analysis. The measurements will provide us with a quantitative test of the dynamics underlying the W,Z interactions.

hep-ph↗

arXiv1997

Probing Electroweak Symmetry Breaking via WW Interactions at High Energy Colliders

We classify the sensitivities of the future high energy hadron and electron colliders, such as the LHC and the future Linear Colliders, to probing all the next-to-leading order (NLO) bosonic operators for studying the electroweak symmetry breaking (EWSB) mechanism. We first develop a power counting rule (a generalization of Weinberg's counting method for the nonlinear sigma model) for the electroweak theories formulated by the chiral Lagrangian. Then, we formulate the Longitudinal-Goldstone Boson Equivalence Theorem as a physical criterion to discriminate the scattering processes that are not sensitive to the EWSB sector. The "complementarity" of different scattering processes via different colliders for a complete probe of all these NLO effective operators is demonstrated.

hep-ph↗

arXiv1997

Comment on the $W + 1$ jet to $W + 0$ jets ratio

To offer a possible resolution to the apparent discrepancy between the experimental and the theoretical values of the $W + 1$ jet to $W + 0$ jets ratio reported by the \D0 group, we examine the effects of the multiple soft gluon radiation on the $W$ boson production at the Tevatron. Based on the calculation of the $W$ boson transverse momentum ($Q_T$) distribution in the Collins-Soper-Sterman resummation formalism, we conclude that the effect of the soft gluon radiation is important in the region of $Q_T < 50$ GeV, and it can be better tested by a more inclusive observable $R_{CSS}(Q_T^{\min})\equiv \frac{σ(Q_T>Q_T^{\min})}{σ_{Total}}$.

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