arXiv subjects

Hua-Sheng Shao

Publications and source records attributed to Hua-Sheng Shao.

89 records · Page 5Linked to original sources

arXiv2015

Double-quarkonium production at a fixed-target experiment at the LHC (AFTER@LHC)

We present predictions for double-quarkonium production in the kinematical region relevant for the proposed fixed-target experiment using the LHC beams (dubbed as AFTER@LHC). These include all spin-triplet S -wave charmonium and bottomonium pairs, i.e. Psi(n_1S) + Psi(n_2S), Psi(n_1S) + Upsilon(m_1S) and Upsilon(m_1S) + Upsilon(m_2S ) with n_1,n_2 = 1,2 and m_1,m_2 = 1,2,3. We calculate the contributions from double-parton scatterings and single-parton scatterings. With an integrated luminosity of 20 fb-1 to be collected at AFTER@LHC, we find that the yields for double-charmonium production are large enough for differential distribution measurements. We discuss some differential distributions for J/Psi + J/Psi production, which can help to study the physics of double-parton and single-parton scatterings in a new energy range and which might also be sensitive to double intrinsic c-bar(c) coalescence at large negative Feynman x.

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arXiv2015

Measuring the Top Yukawa Coupling at 100 TeV

We propose a measurement of the top Yukawa coupling at a 100 TeV hadron collider, based on boosted Higgs and top decays. We find that the top Yukawa coupling can be measured to 1%, with excellent handles for reducing systematic and theoretical uncertainties, both from side bands and from $t\bar{t}H/t\bar{t}Z$ ratios.

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arXiv2015

HELAC-Onia 2.0: an upgraded matrix-element and event generator for heavy quarkonium physics

We present an upgraded version (denoted as version 2.0) of the program HELAC-Onia for the automated computation of heavy-quarkonium helicity amplitudes within non-relativistic QCD framework. The new code has been designed to include many new and useful features for practical phenomenological simulations. It is designed for job submissions under cluster enviroment for parallel computations via Python scripts. We have interfaced HELAC-Onia to the parton shower Monte Carlo programs Pythia 8 and QEDPS to take into account the parton-shower effects. Moreover, the decay module guarantees that the program can perform the spin-entangled (cascade-)decay of heavy quarkonium after its generation. We have also implemented a reweighting method to automatically estimate the uncertainties from renormalization and/or factorization scales as well as parton-distribution functions to weighted or unweighted events. A futher update is the possiblity to generate one-dimensional or two-dimensional plots encoded in the analysis files on the fly. Some dedicated examples are given at the end of the writeup.

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arXiv2015

Automated next-to-leading order predictions for new physics at the LHC: the case of colored scalar pair production

We present for the first time the full automation of collider predictions matched with parton showers at the next-to-leading accuracy in QCD within non-trivial extensions of the Standard Model. The sole inputs required from the user are the model Lagrangian and the process of interest. As an application of the above, we explore scenarios beyond the Standard Model where new colored scalar particles can be pair produced in hadron collisions. Using simplified models to describe the new field interactions with the Standard Model, we present precision predictions for the LHC within the MadGraph5 aMC@NLO framework.

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arXiv2015

$η_c$ production at LHC and indications on the understanding of $J/ψ$ production

We present a complete evaluation for the prompt $η_c$ production at the LHC at next-to-leading order in $α_s$ in nonrelativistic QCD. By assuming heavy quark spin symmetry, the recently observed $η_c$ production data by LHCb results in a very strong constraint on the upper bound of the color-octet long distance matrix element $1S0$ of $J/ψ$. We find this upper bound is consistent with our previous study of the $J/ψ$ yield and polarization and can give good descriptions for the measurements, but inconsistent with some other theoretical estimates. This may provide important information for understanding the nonrelativistic QCD factorization formulism.

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arXiv2014

Probing Heavy Quarkonium Production Mechnism: $χ_c$ polarization

The necessicity of the color-octet mechanism in describing heavy quarkonium production is a longstanding puzzle. Compared to the yields of heavy quarkonium, its polarizations should be a sensitive observable to pin down the color-octet contributions. In this talk, I will focus on the $χ_c$ polarization in hadroproduction processes, which may provide a unique test for the color-octet mechanism in nonrelativistic QCD.

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arXiv2014

Spin correlations in polarizations of P-wave charmonia $χ_{cJ}$ and impact on $J/ψ$ polarization

Based on a general form of the effective vertex functions for the decays of P-wave charmonia $\chicj$, angular distribution formulas for the subsequent decays $\chicj\rightarrow \jpsi γ$ and $\jpsi \to μ^+μ^-$ are derived. The formulas are the same as those obtained in a different approach in the literature. Our formulas are expressed in a more general form, including parity violation effects and the full angular dependence of $\jpsi$ and muon in the cascade decay $\chicj\to\jpsiγ\toμ^+μ^-γ$. The $\chicj$ polarization observables are expressed in terms of rational functions of the spin density matrix elements of $\chicj$ production. Generalized rotation-invariant relations for arbitrary integer-spin particles are also derived and their expressions in terms of observable angular distribution parameters are given in the $χ_{c1}$ and $χ_{c2}$. To complement our previous direct-$\jpsi$ polarization result, we also discuss the impact on the observable prompt-$\jpsi$ polarization. As an illustrative application of our angular distribution formulas, we present the angular distributions in terms of the tree-level spin density matrix elements of $χ_{c1}$ and $χ_{c2}$ production in several different frames at the Large Hadron Collider. Moreover, a reweighting method is also proposed to determine the entire set of the production spin density matrix elements of the $χ_{c2}$, some of which disappear or are suppressed for vanishing higher-order multipole effects making the complete extraction difficult experimentally.

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arXiv2014

Polarizations of $χ_{c1}$ and $χ_{c2}$ in prompt production at the LHC

Prompt $χ_c$ production at hadron colliders may provide a unique test for the color-octet mechanism in nonrelativistic QCD. We present an analysis for the polarization observables of $χ_{c1}$ and $χ_{c2}$ at next-to-leading order in $α_S$, and propose to measure them at the LHC, which is expected to be important for testing the validity of NRQCD.

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arXiv2014

Initial state radiation effects in inclusive $J/ψ$ production at B factories

Based on Monte Carlo techniques, we analyze the initial state radiation (ISR) effects in prompt $\jpsi$ inclusive production at B factories. ISR enhances cross section $σ(e^-e^+\to\jpsi+gg+X)$ by about $15-25\%$, which is almost the same size as the QCD and relativistic correction. Moreover, ISR slightly changes $σ(e^-e^+\to\jpsi+c\bar{c}+X)$. The $\jpsi$ momentum spectrum in $e^-e^+\to\jpsi+gg+X$ and in $e^-e^+\to\jpsi+c\bar{c}+X$ is softer after the photon showering from the initial $e^{\pm}$ beam radiation. After combining the QCD,relativistic, and ISR corrections,a more precise theoretical result is obtained. The new result provides a more stringent constraint of the color-octet contribution to $σ(e^-e^+\to\jpsi+X_{\rm{non-}c\bar{c}})$.

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arXiv2013

Production of J/psi+eta(c) vs. J/psi+J/psi at the LHC: Impact of Real alpha_s^5 corrections

We proceed for the first time to the evaluation of the Born cross section for J/psi+eta(c) production, namely via g+g -> J/psi+eta(c)+g, and show that it has a harder P_T spectrum than the J/psi-pair yield at Born level. If one stuck to a comparison at Born level, one would conclude that J/psi+eta(c) production would surpass that of J/psi+J/psi at large P_T. This is nonetheless not the case since J/psi-pair production, as for single J/psi, receives leading-P_T contributions at higher orders in alpha_s. We also present the first evaluation of these leading-P_T next-to-leading order contributions. These are indeed significant for increasing P_T and are of essential relevance for comparison with forthcoming data. We also compute kinematic correlations relevant for double-parton-scattering studies. Finally, we evaluate the polarisation of a J/psi accompanied either by a eta(c) or a J/psi and another light parton. These results may be of great help to understand the polarisation of quarkonia produced at high energies.

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arXiv2013

HELAC-Onia: an automatic matrix element generator for heavy quarkonium physics

By the virtues of the Dyson-Schwinger equations, we upgrade the published code \mtt{HELAC} to be capable to calculate the heavy quarkonium helicity amplitudes in the framework of NRQCD factorization, which we dub \mtt{HELAC-Onia}. We rewrote the original \mtt{HELAC} to make the new program be able to calculate helicity amplitudes of multi P-wave quarkonium states production at hadron colliders and electron-positron colliders by including new P-wave off-shell currents. Therefore, besides the high efficiencies in computation of multi-leg processes within the Standard Model, \mtt{HELAC-Onia} is also sufficiently numerical stable in dealing with P-wave quarkonia (e.g. $h_{c,b},χ_{c,b}$) and P-wave color-octet intermediate states. To the best of our knowledge, it is a first general-purpose automatic quarkonium matrix elements generator based on recursion relations on the market.

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arXiv2012

Feynman Rules for the Rational Part of One-loop QCD Corrections in the MSSM

The complete set of Feynman rules for the rational part R of QCD corrections in the MSSM are calculated at the one-loop level, which can be very useful in the next-to-leading order calculations in supersymmetric models. Our results are expressed in the 't Hooft-Veltman regularization scheme and in the Four Dimensional Helicity scheme with non-anticommutating and anticommutating $γ_5$ strategies.

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arXiv2012

J/psi polarization at hadron colliders in nonrelativistic QCD

With nonrelativistic QCD factorization, we present a full next-to-leading order computation of the polarization observable for $J/psi$ production at hadron colliders including all important Fock states, i.e. $^3S_1[1,8]$, $^1S_0[8]$, and $^3P_J[8]$. We find the $^3P_J[8]$ channel contributes a positive longitudinal component and a negative transverse component. So the $J/psi$ polarization puzzle may be understood as the transverse components canceling between $^3S_1[8]$ and $^3P_J[8]$ channels, which results in mainly the unpolarized (even slightly longitudinally polarized) $J/psi$. This may give a possible solution to the long-standing $J/psi$ polarization puzzle. Predictions for $J/psi$ polarization at the LHC are also presented.

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arXiv2012

Reduction Schemes in Cutoff Regularization and Higgs Decay into Two Photons

We present a new systematic method to evaluate one-loop tensor integrals in conventional ultraviolet cutoff regularization. By deriving a new recursive relation that describes the momentum translation variance of ultraviolet integrals, we implement this relation in the Passarino-Veltman reduction method. With this method, we recalculated the Higgs boson decay into two photons process at one-loop level in the Standard Model. We reanalyze this process carefully and clarify some issues arisen recently in cutoff regularization.

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arXiv2011

Dijet Invariant Mass Distribution in Top Quark Hadronic Decay with QCD Corrections

The dijet invariant mass distributions from the hadronic decay of unpolarized top quark ($t\to b W^+$ followed by $W^+\to u \bar{d}$) are calculated, including the next-to-leading order QCD radiative corrections. We treat the top decay in the complex mass scheme due to the existence of the intermediate state W boson. Our analytical expressions are also available in different dimensional regularization schemes and $γ_5$ strategies. Finally, in order to construct the jets, we use different jet algorithms to compare their influences on our results. The obtained dijet mass distributions from the top quark decay are useful to distinguish these dijets from those produced via other sources and to clarify the issue about the recent CDF Collaborations' $Wjj$ anomaly.

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arXiv2011

Feynman Rules for the Rational Part of the Standard Model One-loop Amplitudes in the 't Hooft-Veltman $γ_5$ Scheme

We study Feynman rules for the rational part $R$ of the Standard Model amplitudes at one-loop level in the 't Hooft-Veltman $γ_5$ scheme. Comparing our results for quantum chromodynamics and electroweak 1-loop amplitudes with that obtained based on the Kreimer-Korner-Schilcher (KKS) $γ_5$ scheme, we find the latter result can be recovered when our $γ_5$ scheme becomes identical (by setting $g5s=1$ in our expressions) with the KKS scheme. As an independent check, we also calculate Feynman rules obtained in the KKS scheme, finding our results in complete agreement with formulae presented in the literature. Our results, which are studied in two different $γ_5$ schemes, may be useful for clarifying the $γ_5$ problem in dimensional regularization. They are helpful to eliminate or find ambiguities arising from different dimensional regularization schemes.

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arXiv2010

Leptonic Decay of Upsilon, a Possible Signature of New Physics

We calculate the inclusive decay width of $Υ\to l^+ l^-$. Then we get the ratio $R_{τμ}=Γ[Υ\to τ^+τ^-]/ Γ[Υ\to μ^+μ^-]$ to ${\cal{O}}(α)$ and ${\cal{O}}(α_s^2)$ within the Standard Model(SM). Comparing with the recent Babar's data $R_{τμ}=1.005\pm0.013\pm 0.022$, we find that SM prediction $R_{τμ}$ is not consistent with the experimental data in the error bar. The discrepancy is about $1.25σ$. So leptonic decay of $Υ$ may be a possible signature of New Physics(NP). We present a better approach to test the Standard Model, $R_{τμ}(E_{soft})=.Γ[Υ\to τ^+τ^-+X]/ Γ[Υ\to μ^+μ^-+X]|_{E_X<E_{soft}}$. After resumming the large logarithms, we get $R_{τμ}(E_{soft})$ at the precision level of 0.1%. It can be compared with experimental data more precise. We also consider the impact of $R_{τμ}(E_{soft})$ and $R_{τμ}$ from light Higgs $h$ and pseudoscalar Higgs $A_0$.

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