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C. -P. Yuan

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

At least 145 records · Page 8Linked to original sources
arXiv2008

Lone Higgs at the LHC

We address the possible scenario that the Large Hadron Collider (LHC) discovers only a Higgs boson after 10 fb^{-1} of operation, and attempt to identify this Higgs boson as that of the Standard Model (SM), the minimal universal extra dimension model (MUED), the littlest Higgs model with T-parity (LHT), or the minimal supersymmetric Standard Model (MSSM), using only the measurement of the product of gluon-fusion production cross section and the di-photon branching ratio. In MUED, by decoupling any new physics sufficiently to evade the discovery reach at the LHC, the deviation of the signal from the SM is not statistically significant. However, in LHT and MSSM, it is possible to have a significant deviation in the signal that is consistent with this "lone Higgs scenario", and, in the case of a very large suppression, we can distinguish MSSM and LHT before the discovery of any new resonances. Starting with the lone Higgs scenario and the deviation in this measurement from the Standard Model prediction (whether or not statistically significant), we offer tests that may discriminate the models and search strategies of discovering new physics signatures with increasing integrated luminosity.

hep-ph↗
arXiv2008

Implications of CTEQ global analysis for collider observables

The latest CTEQ6.6 parton distributions, obtained by global analysis of hard scattering data in the framework of general-mass perturbative QCD, are employed to study theoretical predictions and their uncertainties for significant processes at the Fermilab Tevatron and CERN Large Hadron Collider. The previously observed increase in predicted cross sections for the standard-candle W and Z boson production processes in the general-mass scheme (compared to those in the zero-mass scheme) is further investigated and quantified. A novel method to constrain PDF uncertainties in LHC observables, by effectively exploiting PDF-induced correlations with benchmark standard model cross sections, is presented. Using this method, we show that the top-antitop pair cross section can potentially serve as a standard candle observable for the LHC processes dominated by initial-state gluon scattering. Among other benefits, precise measurements of $t\bar{t}$ cross sections would reduce PDF uncertainties in predictions for single-top quark and Higgs boson production in the standard model and minimal supersymmetric standard model.

hep-ph↗
arXiv2008

General Analysis of Single Top Production and W Helicity in Top Decay

We provide a framework for the analysis of the W boson helicity in the decay of the top quark that is based on a general effective tbW coupling. Four independent coupling coefficients can be uniquely determined by the fractions of longitudinal and transverse W boson polarizations as well as the single top production rates for the t-channel and the s-channel processes. The knowledge of these coefficients can be used to discriminate models of electroweak symmetry breaking.

hep-ph↗
arXiv2007

Multipartite Dark Matter

Dark matter (comprising a quarter of the Universe) is usually assumed to be due to one and only one weakly interacting particle which is neutral and absolutely stable. We consider the possibility that there are several coexisting dark-matter particles, and explore in some detail the generic case where there are two. We discuss how the second dark-matter particle may relax the severe constraints on the parameter space of the Minimal Supersymmetric Standard Model, as well as other verifiable predictions in both direct and indirect search experiments.

hep-ph↗
arXiv2007

Search for New Physics via Single Top Production at the LHC

We consider single-top production as a probe for new physics effects at the Large Hadron Collider (LHC). We argue that for natural theories a small deviation from the Standard Model tree-level couplings in this reaction can be parameterized by 3 higher dimension operators. Precision measurement of these effective couplings in the single-top events, via studying their interference effects with the SM contributions, can discriminate several new physics models. In particular, combining the production rate of three single-top production modes will provide a severe test of the Little Higgs model with T-parity. We find that at the LHC, a 5% accuracy in the measurement of the single-top cross sections would probe the new physics scale up to about $3 {\rm TeV}$.

hep-ph↗
arXiv2007

Gluon-gluon contributions to the production of continuum diphoton pairs at hadron colliders

We compute the contributions to continuum photon pair production at hadron colliders from processes initiated by gluon-gluon and gluon-quark scattering into two photons through a four-leg virtual quark loop. Complete two-loop cross sections in perturbative quantum chromodynamics are combined with contributions from soft parton radiation resummed to all orders in the strong coupling strength. The structure of the resummed cross section is examined in detail, including a new type of unintegrated parton distribution function affecting azimuthal angle distributions of photons in the pair's rest frame. As a result of this analysis, we predict diphoton transverse momentum distributions in gluon-gluon scattering in wide ranges of kinematic parameters at the Fermilab Tevatron collider and the CERN Large Hadron Collider.

hep-ph↗
arXiv2007

Calculation of prompt diphoton production cross sections at Tevatron and LHC energies

A fully differential calculation in perturbative quantum chromodynamics is presented for the production of massive photon pairs at hadron colliders. All next-to-leading order perturbative contributions from quark-antiquark, gluon-(anti)quark, and gluon-gluon subprocesses are included, as well as all-orders resummation of initial-state gluon radiation valid at next-to-next-to-leading logarithmic accuracy. The region of phase space is specified in which the calculation is most reliable. Good agreement is demonstrated with data from the Fermilab Tevatron, and predictions are made for more detailed tests with CDF and DO data. Predictions are shown for distributions of diphoton pairs produced at the energy of the Large Hadron Collider (LHC). Distributions of the diphoton pairs from the decay of a Higgs boson are contrasted with those produced from QCD processes at the LHC, showing that enhanced sensitivity to the signal can be obtained with judicious selection of events.

hep-ph↗
arXiv2007

Global QCD Analysis and Collider Phenomenology--CTEQ

An overview is given of recent progress on a variety of fronts in the global QCD analysis of the parton structure of the nucleon and its implication for collider phenomenology, carried out by various subgroups of the CTEQ collaboration.

hep-ph↗
arXiv2007

Heavy Quark Mass Effects in PQCD and Heavy Flavor Parton Distributions

The systematic treatment of heavy quark mass effects in DIS in current CTEQ global analysis is summarized. Applications of this treatment to the comparison between theory and experimental data on DIS charm production are described. The possibility of intrinsic charm in the nucleon is studied. The issue of determining the charm mass in global analysis is discussed.

hep-ph↗
arXiv2007

The Strange Parton Distribution of the Nucleon: Global Analysis and Applications

The strangeness degrees of freedom in the parton structure of the nucleon are explored in the global analysis framework, using the new CTEQ6.5 implementation of the general mass perturbative QCD formalism of Collins. We systematically determine the constraining power of available hard scattering experimental data on the magnitude and shape of the strange quark and anti-quark parton distributions. We find that current data favor a distinct shape of the strange sea compared to the isoscalar non-strange sea. A new reference parton distribution set, CTEQ6.5S0, and representative sets spanning the allowed ranges of magnitude and shape of the strange distributions, are presented. Some applications to physical processes of current interest in hadron collider phenomenology are discussed.

hep-ph↗
arXiv2007

Heavy Quark Mass Effects in Deep Inelastic Scattering and Global QCD Analysis

A new implementation of the general PQCD formalism of Collins, including heavy quark mass effects, is described. Important features that contribute to the accuracy and efficiency of the calculation of both neutral current (NC) and charged current (CC) processess are explicitly discussed. This new implementation is applied to the global analysis of the full HERA I data sets on NC and CC cross sections, with correlated systematic errors, in conjunction with the usual fixed-target and hadron collider data sets. By using a variety of parametrizations to explore the parton parameter space, robust new parton distribution function (PDF) sets (CTEQ6.5) are obtained. The new quark distributions are consistently higher in the region x ~ 10^{-3} than previous ones, with important implications on hadron collider phenomenology, especially at the LHC. The uncertainties of the parton distributions are reassessed and are compared to the previous ones. A new set of CTEQ6.5 eigenvector PDFs that encapsulates these uncertainties is also presented.

hep-ph↗
arXiv2007

Phenomenology of Littlest Higgs Model with T-parity: including effects of T-odd fermions

We study the collider phenomenology of a Littlest Higgs model with T-parity. We first stress the important role of the T-odd SU(2) doublet fermions (introduced to make the model T-parity invariant) in high energy scattering processes, such as $q\bar{q}\to W_H^+ W_H^-$ where $W_H^\pm$ are the T-odd partners of $W$-bosons. Because the mass of the T-odd SU(2) doublet fermions cannot be too heavy to be consistent with low energy data, they can be copiously produced at the CERN Large Hadron Collider (LHC). Therefore, we study the collider phenomenology of the model with emphasis on the contributions of the T-odd fermion to the production of the heavy T-parity partners (either bosons or fermions) of the usual particles at the LHC. The production cross sections and the decay branching ratios of the new heavy particles are classified and various experimental signatures are discussed.

hep-ph↗
arXiv2006

Impact of Single-Top Measurement to Littlest Higgs Model with T-Parity

We show that a precise measurement of the single-top production cross section at the Tevatron and the LHC can strongly constrain the model parameters of the Littlest Higgs model with T-parity. A reduction in the single-top production rate from the Standard Model prediction implies new physics phenomena generated by the heavy T-parity partners of the top quark. We show that the degree of polarization of the top quark produced from the decay of its heavy T-odd partner ($T_{-}$) can be utilized to determine the new physics energy scale, and the mass of $T_{-}$ can be measured from the missing transverse momentum distribution in the $T_{-}\bar{T}_{-}$ event.

hep-ph↗
arXiv2006

Higgs Boson Production and Decay in Little Higgs Models with T-parity

We study Higgs boson production and decay in a certain class of Little Higgs models with T-parity in which some T-parity partners of the Standard Model (SM) fermions gain their masses through Yukawa-type couplings. We find that the Higgs boson production cross section of a 120 GeV Higgs boson at the CERN LHC via gg fusion process at one-loop level could be reduced by about 45%, 35% and 20%, as compared to its SM prediction, for a relatively low new particle mass scale f = 600, 700 and 1000 GeV, respectively. On the other hand, the weak boson fusion cross section is close to the SM value. Furthermore, the Higgs boson decay branching ratio into di-photon mode can be enhanced by about 35% in small Higgs mass region in certain case, for the total decay width of Higgs boson in the Little Higgs model is always smaller than that in the SM.

hep-ph↗
arXiv2006

Light MSSM Higgs boson scenario and its test at hadron colliders

We show that in the Minimal Supersymmetric Standard Model, the possibility for the lightest CP-even Higgs boson to be lighter than $Z$ boson (as low as about 60 GeV) is, contrary to the usual belief, not yet excluded by LEP2 data or any other existing experimental data. The characteristic of the light Higgs boson scenario (LHS) is that the $ZZh$ coupling and the decay branching ratio ${\rm Br}(h/A\to b\bar{b})$ are simultaneously suppressed as a result of generic supersymmetric loop corrections. Consequently, the $W^\pm H^\mp h$ coupling has to be large due to the sum rule of Higgs couplings to weak gauge bosons. In addition to discussing the potential of the Tevatron and $B$-factories to test the LHS, we show that the associate neutral and charged Higgs boson production process, $pp\to H^\pm h (A)$, can completely probe LHS at the CERN Large Hadron Collider.

hep-ph↗
arXiv2006

Probing the CP nature of Higgs boson through e^-e^+ --> e^-e^+ phi

We study the production of Higgs boson (phi) via the e^{-}e^{+}--> e^{-}e^{+} phi process at a future International Linear Collider (ILC) with sqrt{S}=1 TeV. At this energy the ZZ-fusion rate, as predicted by the Standard Model, dominates over the associated ZH production rate. Here, we consider a class of theory models based on the effective phi-VV (with V=Z, gamma) couplings generated by dimension-6 operators. We also show how to use the kinematical distributions of the final state leptons to discriminate the CP-even and CP-odd Higgs bosons produced at the ILC, when the phi-gamma-gamma coupling dominates the Higgs boson production rate.

hep-ph↗
arXiv2006

All-orders resummation for diphoton production at hadron colliders

We present a QCD calculation of the transverse momentum distribution of photon pairs produced at hadron colliders, including all-orders soft-gluon resummation valid at next-to-next-to-leading logarithmic accuracy. We specify the region of phase space in which the calculation is most reliable, compare our results with data from the Fermilab Tevatron, and make predictions for the Large Hadron Collider. The uncertainty of predictions for production of diphotons from fragmentation of final-state quarks is examined.

hep-ph↗