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C. --P. Yuan

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

At least 19 recordsLinked to original sources
arXiv2026

A Fast Method for Correlated Updates of Proton PDFs and the Strong Coupling $α_s$

We present an extended version of the \texttt{ePump} framework that enables the simultaneous profiling of proton parton distribution functions (PDFs) and the strong coupling $α_s$ using new experimental data. By promoting $α_s$ to a fit parameter within the Hessian updating formalism, the method performs coherent updates of $\{\text{PDFs},α_s\}$ while preserving parameter correlations and the full covariance structure. Validation studies based on CTEQ-TEA analyses with collider data demonstrate that the upgraded \texttt{ePump} accurately reproduces the shifts in PDFs, the preferred $α_s(m_Z)$, and the associated uncertainty reductions obtained in full global fits, including those inferred from Lagrange--Multiplier scans; small deviations arise only for data sets whose $χ^2$ profiles exhibit nonlinear behavior. Applications to representative collider measurements illustrate the impact on the gluon distribution and on precision observables such as the Higgs boson production cross section via gluon fusion. This enhanced framework provides a fast and reliable tool for assessing the effects of new data on the global QCD parameter space, offering near-global-fit accuracy at a fraction of the computational cost.

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arXiv2021

Reduction of the electroweak correlation in the PDF updating by using the forward-backward asymmetry of Drell-Yan process

We propose a new observable for the measurement of the forward-backward asymmetry $(A_{FB})$ in Drell-Yan lepton production. At hadron colliders, the $A_{FB}$ distribution is sensitive to both the electroweak (EW) fundamental parameter $\sin^2 θ_{W}$, the weak mixing angle, and the parton distribution functions (PDFs). Hence, the determination of $\sin^2 θ_{W}$ and the updating of PDFs by directly using the same $A_{FB}$ spectrum are strongly correlated. This correlation would introduce large bias or uncertainty into both precise measurements of EW and PDF sectors. In this article, we show that the sensitivity of $A_{FB}$ on $\sin^2 θ_{W}$ is dominated by its average value around the $Z$ pole region, while the shape (or gradient) of the $A_{FB}$ spectrum is insensitive to $\sin^2 θ_{W}$ and contains important information on the PDF modeling. Accordingly, a new observable related to the gradient of the spectrum is introduced, and demonstrated to be able to significantly reduce the potential bias on the determination of $\sin^2 θ_{W}$ when updating the PDFs using the same $A_{FB}$ data.

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arXiv2021

Connected and Disconnected Sea Partons from CT18 Parametrization of PDFs

The separation of the connected and disconnected sea partons, which were uncovered in the Euclidean path-integral formulation of the hadronic tensor, is accommodated with the CT18 parametrization of the global analysis of the parton distribution functions (PDFs). This is achieved with the help of the distinct small $x$ behaviors of these two sea parton components and the constraint from the lattice calculation of the ratio of the strange momentum fraction to that of the ${\bar u}$ or ${\bar d}$ in the disconnected insertion. This allows lattice calculations of separate flavors in both the connected and disconnected insertions to be directly compared with the global analysis results term by term.

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arXiv2020

An exploratory study of the impact of CMS double-differential top distributions on the gluon parton distribution function

LHC $t\bar{t}$ data have the potential to provide constraints on the gluon distribution, especially at high $x$, with both ATLAS and CMS performing differential measurements. Recently, CMS has measured double-differential $t\bar{t}$ distributions at 8 TeV. In this paper we examine the impact of this data set on the gluon distribution. To that end we develop novel, double-differential NNLO predictions for that data. No significant impact is found when the CMS data is added to the CT14HERA2 global PDF fit, due to the larger impact of the inclusive jet data from both the Tevatron and the LHC. If the jet data are removed from the fit, then an impact is observed. If the CMS data is scaled by a larger weight, representing the greater statistical power of the jet data, a roughly equal impact on the gluon distribution is observed for the $t\bar{t}$ as for the inclusive jet data. For data samples with higher integrated luminosity at 13 TeV, a more significant impact of the double-differential $t\bar{t}$ data may be observed.

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arXiv2005

Uncertainties of the Inclusive Higgs Production Cross Section at the Tevatron and the LHC

We study uncertainties of the predicted inclusive Higgs production cross section due to the uncertainties of parton distribution functions (PDF). Particular attention is given to bbH Yukawa coupling enhanced production mechanisms in beyond SM scenarios, such as MSSM. The PDF uncertainties are determined by the robust Lagrange Multiplier method within the CTEQ global analysis framework. We show that PDF uncertainties dominate over theoretical uncertainties of the perturbative calculation (usually estimated by the scale dependence of the calculated cross sections), except for low Higgs masses at LHC. Thus for the proper interpretation of any Higgs signal, and for better understanding of the underlying electroweak symmetry breaking mechanism, it is important to gain better control of the uncertainties of the PDFs.

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arXiv2004

Testing Supersymmetry in $A H^\pm$ Associated Production

In the Minimal Supersymmetric Standard Model, the masses of the charged Higgs boson ($H^\pm$) and the CP-odd scalar ($A$) are related by $M_{H^+}^2=M_A^2+m_W^2$ at the Born level. Because the coupling of $W^-$-$A$-$H^+$ is fixed by gauge interaction, the Born level production rate of $q \bar q' \to W^{\pm \ast} \to A H^\pm$ depends only on one supersymmetry parameter -- the mass ($M_A^{}$) of $A$. We illustrate how to test the mass relation between $A$ and $H^+$ in case that the signal is found at the LHC. If the signal is not found, the product of the decay branching ratios of $A$ and $H^\pm$ predicted by the MSSM is bounded from above as a function of $M_A$.

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arXiv1998

Supersymmetric QCD Parity Nonconservation in Top Quark Pairs at the Tevatron

In the supersymmetry (SUSY) models, because of the mass difference between the left- and right-top squarks, the supersymmetric QCD interactions can generate parity violating effects in the production of $ t \bar t$ pairs. We show that SUSY QCD radiative corrections to the parity violating asymmetry in the production rates of the left- and right-handed top quarks via the $q \bar q \ra t \bar t$ process can reach about 3% at the Fermilab Tevatron with $\sqrt{s}=2$ TeV. This could be observable with an integrated luminosity larger than $2 fb^{-1}$. We also show that in these models the SUSY QCD radiative corrections to the $t \bar t$ production rate are small unless gluinos are very light, of the order of 1 GeV.

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arXiv1997

The Phenomenology of Single Top Quark Production at the Fermilab Tevatron

Single top quark production at the Fermilab Tevatron Run II (a p \bar{p} collider with \sqrt{S} = 2 TeV) proceeds dominantly via two sub-processes, a t-channel W-gluon fusion process, and an s-channel W^* process. We show that these two sub-processes have different sensitivities to new physics effects in the top quark's electro-weak interactions. The W^* process is sensitive to new heavy charged resonances, such as a W' boson, while the W-gluon fusion process is more sensitive to modifications to the top's interaction, including flavor-changing neutral currents involving the top quark. We examine the implications of these results on our ability to measure V_{tb} with confidence, and propose a quantity R = σ_{Wg} / σ_{W^*}, which may be studied in order to characterize the confidence one may place upon a given measurement of V_{tb} from single top production.

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arXiv1997

Anomalous W^+ W^- t {\bar t} Couplings at the e^+ e^- Linear Collider

We study production of t {\bar t} via W^+ W^- fusion, including the relevant backgrounds at the proposed Linear Collider (an e^+ e^- collider with \sqrt{S} = 1.5 TeV) in the context of a Higssless Standard Model (SM), i.e. a nonlinear SU(2)_L \times U(1)_Y chiral Lagrangian, including dimension five W^+ W^- t {\bar t} interactions. Deviation from the SM total cross section can be used to constrain the coefficients of these operators to an order of 2 \times 10^{-1} (divided by the cut-off scale Λ= 3.1 TeV) with a 95% C.L.. However, there are three ways in which this sensitivity can be improved by a factor of two. First, by studying the deviation of the t {\bar t} kinematics from what is predicted by the SM; second, by polarizing the collider electron beam; and third, by studying the polarization of the produced top quarks. In this way, we show that it is also possible to attempt to dis-entangle the contributions from different anomalous operators, isolating the form of new physics contributions.

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arXiv1996

Anomalous t-c-g coupling: The connection between single top production and top decay

Continuing earlier work, we examine the constraint on an anomalous t-c-g coupling from top quark decay. We find that from current CDF measurements of the branching ratio $t \rightarrow W b$, the minimum scale at which new physics can strongly modify the t-c-g coupling is \Ltcg $\geq$ about 950 GeV. At the upgraded Tevatron, single top production can constrain \Ltcg $\geq$ 4.5 TeV. The connection between t-c production and the decay $t \rightarrow c g$ is examined, showing how constraints on one lead to a constraint on the other.

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arXiv1996

A Model of Strong Flavor Dynamics for the Top Quark

We discuss a model in which the third generation fermions undergo a different $SU(2)$ weak interaction from the first two generation fermions. In general, a flavor changing neutral current interaction is expected. Constrained by the precision low energy data, the mass ($M_{Z'}=M_{W'}$) of the heavy gauge bosons is bounded from below to be about $1.1$ TeV for $α_s=0.125$ and about $1.3$ TeV for $α_s=0.115$, at the $3σ$ level. This model favors a larger $R_b$ and a smaller $R_c$ as compared with the Standard Model, but it does not explain the $R_c$ data. If one takes the $R_b$ data seriously, then $M_{Z'}$ is bounded, at the $3σ$ level, to be $462$ GeV $< M_{Z^{\prime}} \cosϕ< 1481\,\, {\rm{GeV}}$, where $\cosϕ$ is the mixing angle between the two $SU(2)$'s in the model. Effects predicted for high energy experiments at the Tevatron, LEP140, LEP-II, LHC, and future linear colliders are also discussed.

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arXiv1995

LHC Analysis of the Strongly Interacting $WW$ System: Gold-Plated Modes

We study the gold-plated purely leptonic signal and background rates at the LHC for the $ZZ,$ $W^{+}W^-,$ $W^\pm Z$ and $W^\pm W^\pm$ final states associated with strongly interacting electroweak symmetry breaking. We work at an energy of $\sqrt s = 14$ TeV, and develop a combination of back-to-back leptonic, central-jet-vetoing and forwaring cuts that suppresses the Standard-Model backgrounds. We find that the LHC with an annual luminosity of 100 fb$^{-1}$ will achieve a reasonably good sensitivity to the physics of strongly interacting electroweak symmetry breaking.

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arXiv1995

Studying Anomalous $WWγ$ and $WWZ$ Couplings with Polarized $p\bar{p}$ Collisions

We calculate tree-level cross-sections for $W^{+}γ$ and $W^{+}W^{-}$ production in proton-antiproton collisions, with one $W$ decaying to leptons, with anomalous electroweak triple-boson coupling parameters $Δκ$ and $λ$. We compare the unpolarized cross-sections to those for a polarized proton beam, to study how a polarized proton beam would improve experimental tests of anomalous couplings.

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arXiv1995

Effects of QCD Resummation on Distributions of Leptons from the Decay of Electroweak Vector Bosons

We study the distributions of leptons from the decay of electroweak vector bosons produced in hadron collisions. The effects of the initial state multiple soft-gluon emission, using the Collins--Soper resummation formalism, are included. The resummed results are compared with the next-to-leading-order results for the distributions of the transverse momentum, rapidity asymmetry, and azimuthal angle of the decay leptons.

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arXiv1995

Effects of a heavy top quark on low energy data using the electroweak chiral Lagrangian

We study large top quark mass effects on low energy physics in the chiral Lagrangian formulated electroweak theories. We show that these radiative corrections can be easily obtained from a set of Feynman diagrams involving only the scalar Goldstone bosons and the fermions when the contributions of the order $g$ (the weak coupling) are ignored. Using CERN LEP and SLAC Linear Collider (SLC) data we constrain on the nonstandard couplings, which might originate from the spontaneous symmetry-breaking sector, of the top quark to the electroweak gauge bosons.

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arXiv1994

The Nonperturbative Regime in QCD Resummation for Gauge Boson Production at Hadron Colliders

We study the nonperturbative functions in the Collins--Soper--Sterman resummation formalism by examing Drell--Yan data in both fixed target and collider experiments and then predict the transverse momentum distributions of the $W^\pm$ and $Z^0$ bosons at the Tevatron. Our results differ from that in the literature and agree better with published CDF data. Using statistical arguments, we find a $1\,$fb${}^{-1}$ luminosity at the Fermilab Tevatron should be able to provide useful constraints on the nonperturbative functions.

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arXiv1994

Probing the Couplings of the Top Quark to Gauge Bosons

We parameterize the non-universal couplings of t-t-Z and t-b-W in the electroweak chiral lagrangian approach, and examine the constraints on these parameters from the LEP data. We also study how the SLC, Tevatron, LHC and NLC can improve the measurement of these couplings. Different symmetry breaking scenarios imply different correlations among these couplings, whose measurement will then provide a means to probe the electroweak symmetry breaking sector.

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