arXiv subjects
Publications and source records attributed to C. -P. Yuan.
A precise determination of the $W$ boson mass at the CERN LHC requires a theoretical calculation in which the effect of the initial-state multiple soft-gluon emission and the final-state photonic correction are simultaneously included. Here, we present such a calculation and discuss its prediction on a few most relevant distributions of the decay leptons.
Analysis of small-x semi-inclusive DIS hadroproduction suggests that multiple parton radiation leads to a broadening of transverse momentum distributions beyond that predicted by a straightforward calculation in the Collins-Soper-Sterman formalism. This effect can be modeled by a modification of the resummed form factor in the small-x region. We discuss the impact of such a modification on the production of electroweak bosons at hadron-hadron colliders. We show that if substantial small-x broadening is observed in forward Z boson production in the Tevatron Run-2, it would strongly affect predicted transverse momentum distributions for W, Z, and Higgs boson production at the Large Hadron Collider.
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 examine the sensitivity of the LHC to this signal event in the $A(\ra b {\bar b})H^+(\ra τ^+ ν_τ)$ and $A(\ra b {\bar b})H^+(\ra t {\bar b})$ decay channels. We illustrate how to test the mass relation between $A$ and $H^+$ in case that the signal is found. 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 $M_A$.
A precise determination of the W boson mass at the Fermilab Tevatron requires a theoretical calculation in which the effects of the initial-state multiple soft-gluon emission and the final-state photonic correction are simultaneously included . Here, we present such a calculation and discuss its prediction on the transverse mass distribution of the W boson and the transverse momentum distribution of its decay charged lepton, which are the most relevant observables for measuring the W boson mass at hadron colliders.
We study single charged Higgs boson production in photon-photon collision as a probe of the new dynamics of Higgs interactions. This is particularly important when the mass ($M_{H^\pm}$) of charged Higgs bosons ($H^{\pm}$) is relatively heavy and above the kinematic limit of the pair production ($M_{H^\pm} > \sqrt{s}/2$). We analyze the cross sections of single charged Higgs boson production from the photon-photon fusion processes, $γγ\to τ\bar νH^+$ and $γγ\to b \bar c H^+$, as motivated by the minimal supersymmetric standard model and the dynamical Topcolor model. We find that the cross sections at such a $γγ$ collider can be sufficiently large even for $M_{H^\pm} > \sqrt{s}/2$, and is typically one to two orders of magnitude higher than that at its parent $e^-e^+$ collider. We further demonstrate that the polarized photon beams can provide an important means to determine the chirality structure of Higgs Yukawa interactions with the fermions.
We propose a sensitive way to test the anomalous HVV couplings (V=W(+/-),Z(0)) of the Higgs boson (H), which can arise from either the dimension-3 effective operator in a nonlinearly realized Higgs sector or the dimension-6 effective operators in a linearly realized Higgs sector, via studying the VV scattering processes at the CERN LHC. The gold-plated pure leptonic decay modes of the final state weak bosons in the processes pp->VVjj are studied. For comparison, we also analyze the constraints from the precision electroweak data, the expected precision of the measurements of the Higgs boson production rate, decay width and branching ratios at the Tevatron run-2 and the CERN LHC, and the requirement of unitarity of the S-matrix. We show that, with an integrated luminosity of 300 inverse fb and sufficient kinematical cuts for suppressing the backgrounds, studying the process pp->w(+)+W(+)+j+j->l(+)+nu+l(+)+nu+j+j can probe the anomalous HWW couplings at a few tens of percent level for the nonlinearly realized Higgs sector, and at the level of 0.01-0.08 inverse TeV for the linearly realized effective Lagrangian.
Accurate measurement of spin-dependent parton distributions in production of electroweak bosons with polarized proton beams at the Relativistic Heavy Ion Collider depends on good understanding of QCD radiation at small transverse momenta $q_T$ of vector bosons. We present a theoretical formalism for small-$q_T$ resummation of the cross sections for production of virtual photons, W, and Z bosons, with the subsequent decay of these bosons into lepton pairs, for arbitrary longitudinal polarizations of the proton beams.
We examine the effect of the Z-boson transverse momentum distribution measured at the Run-1 of the Tevatron on the nonperturbative function of the Collins-Soper-Sterman (CSS) formalism, which resums large logarithmic terms from multiple soft gluon emission in hadron collisions. The inclusion of the Tevatron Run-1 Z-boson data strongly favors a Gaussian form of the CSS nonperturbative function, when combined with the other low energy Drell-Yan data in a global fit.
In order to measure spin-dependent parton distributions in production of weak bosons using the non-hermetic detectors of the Relativistic Heavy Ion Collider, an accurate model for distributions of charged leptons from the W boson decay is needed. We discuss the predictions for production and decay of the W bosons based on a calculation for resummation of large logarithmic contributions originating from multiple soft gluon radiation. We compare the predictions for the spin-dependent resummed cross sections with the corresponding next-to-leading order results. We show that the lepton-level asymmetries can be reliably predicted by the resummation calculation and directly compared to the experimental data. A program for the numerical analysis of such cross sections in $γ^{*}, W^{\pm},$ and $Z^{0}$ boson production is also presented.
One-loop corrections to the self-coupling constant of the lightest CP-even Higgs boson are studied in the two Higgs doublet model. After renormalization is performed, quartic contributions of heavy particle's mass can appear in the effective coupling. We find that these non-decoupling effects can yield ${\cal O}(100)$ % deviations from the Standard Model prediction, even when all the other couplings of the lightest Higgs boson to gauge bosons and fermions are in good agreement with the Standard Model.
Interaction of Higgs scalar (H) with weak gauge bosons (V=W,Z) is the {\it key} to understand electroweak symmetry breaking (EWSB) mechanism. New physics effects in the HVV interactions, as predicted by models of compositeness, supersymmetry and extra dimensions, can be formulated as anomalous couplings via a generic effective Lagrangian. We first show that the existing electroweak precision data already impose nontrivial indirect constraints on the anomalous HVV couplings. Then, we systematically study VV --> VV scatterings in the TeV region, via Gold-plated pure leptonic decay modes of the weak bosons. We demonstrate that, even for a light Higgs boson in the mass range 115GeV < m_H < 300GeV, this process can directly probe the anomalous HVV interactions at the LHC with an integrated luminosity of 300fb^{-1}, which further supports the ``No-Lose'' theorem for the LHC to uncover the EWSB mechanism. Comparisons with the constraints from measuring the cross section of VH associate production and the Higgs boson decay width are also given.
Differential distributions for heavy quark production depend on the heavy quark mass and other momentum scales, which can yield additional large logarithms and inhibit accurate predictions. Logarithms involving the heavy quark mass can be summed in heavy quark parton distribution functions in the ACOT factorization scheme. A second class of logarithms involving the heavy-quark transverse momentum can be summed using an extension of Collins-Soper-Sterman (CSS) formalism. We perform a systematic summation of logarithms of both types, thereby obtaining an accurate description of heavy-quark differential distributions at all energies. Our method essentially combines the ACOT and CSS approaches. As an example, we present angular distributions for bottom quarks produced in parity-conserving events at large momentum transfers Q at the ep collider HERA.
To measure spin-dependent parton distribution functions in the production of W bosons at the Relativistic Heavy Ion Collider, an accurate model for distributions of charged leptons from the W boson decay is needed. We present single-spin lepton-level cross sections of order $α_S$ for this process, as well as resummed cross sections, which include multiple parton radiation effects. We also present a program RhicBos for the numerical analysis of single-spin and double-spin cross sections in the Drell-Yan process, W and Z boson production.
We discuss quantum corrections of new physics to the triple coupling of the lightest CP-even Higgs boson in the Two Higgs Doublet Model (THDM) and also in the Minimal Supersymmetric Standard Model (MSSM). In the THDM, quartic contributions of the mass of heavy particles in the loop can appear, which are not absorbed by renormalization of the Higgs boson mass. Such non-decoupling effects on the self-coupling can give corrections of ${\cal O}(100%)$, even when all measured Higgs couplings with gauge bosons and fermions are consistent with the Standard Model prediction. In the MSSM, the loop-corrections decouple in such a scenario.
In a previous work we have shown that a general two-Higgs-doublet model (THDM) with a very light CP-odd scalar can be compatible with electroweak precision data, such as the rho parameter, BR(b --> s gamma), R_b, A_b, BR(Upsilon --> A gamma), BR(eta --> A gamma), and (g-2) of muon. Prompted by the recent significant change in the theoretical status of the latter observable, we comment on the consequences for this model and update the allowed parameter region. It is found that the presence of a very light scalar with a mass of 0.2 GeV is still compatible with the new theoretical prediction of the muon anomalous magnetic moment.
When the charged Higgs boson is too heavy to be produced in pairs, the predominant production mechanism at Linear Colliders is via the single charged Higgs boson production processes, such as $e^-e^+ \to b \bar c H^+, τ\bar νH^+$ and $γγ\to b \bar c H^+, τ\bar νH^+$. We show that the yield of a heavy charged Higgs boson at a $γγ$ collider is typically one or two orders of magnitude larger than that at an $e^-e^+$ collider. Furthermore, a polarized $γγ$ collider can determine the chirality of the Yukawa couplings of fermions with charged Higgs boson via single charged Higgs boson production, and thus discriminate models of new physics.
When the energy of the heavy quark is comparable with its mass, it is natural to attribute this heavy quark to the hard part of the reaction. At large energies, this approach is impractical due to large logarithms from intensive QCD radiation affecting both inclusive and differential observables. We present a formalism for all-order summation of such logarithms and reliable description of heavy-quark distributions at all energies. As an illustration, we calculate angular distributions of B-mesons produced in neutral-current events at large momentum transfers at the ep collider HERA.
This Report documents the results obtained by the Working Group on Quantum ChromoDynamics and the Standard Model for the Workshop ``Physics at TeV Colliders'', Les Houches, France, 21 May - 1 June 2001. The account of uncertainties in Parton Distribution Functions is reviewed. Progresses in the description of multiparton final states at Next-to-Leading Order and the extension of calculations for precision QCD observables beyond this order are summarized. Various issues concerning the relevance of resummation for observables at TeV colliders is examined. Improvements to algorithms of jet reconstruction are discussed and predictions for diphoton and photon pi-zero production at the LHC are made for kinematic variables of interest regarding searches for a Higgs boson decaying into two photons. Finally, several improvements implemented in Monte-Carlo event generators are documented.