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Jun Gao

Publications and source records attributed to Jun Gao.

377 records · Page 21Linked to original sources
arXiv2011

Model independent analysis of top quark forward-backward asymmetry at the Tevatron up to $\mathcal{O}(\as^2/Λ^2)$

We present the complete calculations of the forward-backward asymmetry ($A_{\rm FB}$) and the total cross section of top quark pair production induced by dimension-six four quark operators at the Tevatron up to $\mathcal{O}(\as^2/Λ^2)$. Our results show that next-to-leading order (NLO) QCD corrections can change $A_{\rm FB}$ and the total cross section by about 10%. Moreover, NLO QCD corrections reduce the dependence of $A_{\rm FB}$ and total cross section on the renormalization and factorization scales significantly. We also evaluate the total cross section and the charge asymmetry ($A_{\rm C}$) induced by these operators at the Large Hadron Collider (LHC) up to $\mathcal{O}(\as^2/Λ^2)$, for the parameter space allowed by the Tevatron data. We find that the value of $A_{\rm C}$ induced by these operators is much larger than SM prediction, and LHC has potential to discover these NP effects when the measurement precision increases.

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arXiv2011

Search for anomalous top quark production at the early LHC

We present a detailed study of the anomalous top quark production with subsequent decay at the LHC induced by model-independent flavor-changing neutral-current couplings, incorporating the complete next-to-leading order QCD effects. Our results show that, taking into account the current limits from the Tevatron, the LHC with $\sqrt{s}=7$ TeV may discover the anomalous coupling at 5$σ$ level for a very low integrated luminosity of 61 pb$^{-1}$. The discovery potentials for the anomalous couplings at the LHC are examined in detail. We also discuss the possibility of using the charge ratio to distinguish the $tug$ and $tcg$ couplings.

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arXiv2011

One-loop Helicity Amplitudes for Top Quark Pair Production in Randall-Sundrum Model

In this paper, we show how to calculate analytically the one-loop helicity amplitudes for the process $q\bar{q} rightarrow t\bar{t}$ induced by KK gluon, using the spinor-helicity formalism. A minimal set of Feynman rules which are uniquely fixed by gauge invariance and the color representation of the KK gluon are derived and used in the calculation. Our results can be applied to a variety of models containing a massive color octet vector boson.

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arXiv2011

Next-to-leading order QCD corrections to $tZ$ associated production via the flavor-changing neutral-current couplings at hadron colliders

We present the complete next-to-leading order (NLO) QCD corrections to $tZ$ associated production induced by the model-independent $tqg$ and $tqZ$ flavor-changing neutral-current couplings at hadron colliders, respectively. Our results show that, for the $tuZ$ coupling the NLO QCD corrections can enhance the total cross sections by about 60% and 42%, and for the $tcZ$ coupling by about 51% and 43% at the Tevatron and LHC, respectively. The NLO corrections, for the $tug$ couplings, can enhance the total cross sections by about 27%, and by about 42% for the $tcg$ coupling at the LHC. We also consider the mixing effects between the $tqg$ and $tqZ$ couplings for this process, which can either be large or small depending on the values of the anomalous couplings. Besides, the NLO corrections reduce the dependence of the total cross sections on the renormalization or factorization scale significantly, which lead to increased confidence on the theoretical predictions. And we also evaluate the NLO corrections to several important kinematic distributions.

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arXiv2011

Next-to-leading order QCD predictions for $A^{0}γ$ associated production at the CERN Large Hadron Collider

We calculate the complete next-to-leading-order (NLO) QCD corrections (including SUSY QCD corrections) to the inclusive total cross sections of the associated production processes $pp\rightarrow A^{0}γ+X$ in the minimal supersymmetric standard model (MSSM) at the CERN Large Hadron Collider (LHC). Our results show that the enhancement of the total cross sections from the NLO QCD corrections can reach $15%\sim20%$ for 200 GeV$<m_{A}<500$ GeV and $\tanβ=50$. The scale dependence of the total cross section is improved by the NLO corrections, which is less than 5%. We also show the Monte Carlo simulation results for the $τ^{+}τ^{-}+γ$ signature including the complete NLO QCD effects, and find an observable signature above the standard model (SM) background for a normal luminosity of 100 fb$^{-1}$ at the LHC.

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arXiv2011

Next-to-leading QCD effect to the quark compositeness search at the LHC

Recently, the CMS and ATLAS Collaborations at the CERN Large Hadron Collider (LHC) have set exclusion limits on the quark compositeness scale by comparing their data to the leading order and the scaled next-to-leading order (NLO) QCD calculations, respectively. In this Letter, we present the exact NLO QCD corrections to the dijet production induced by the quark contact interactions. We show that as compared to the exact calculation, the scaled NLO QCD prediction adopted by the ATLAS Collaboration has overestimated the new physics effect on some direct observables by more than 30% and renders a higher limit on the quark compositeness scale. The destructive contribution from the exact NLO correction will also lower the compositeness scale limit set by the CMS Collaboration.

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arXiv2011

Next-to-leading order QCD corrections to the top quark associated with $γ$ production via model-independent flavor-changing neutral-current couplings at hadron colliders

We present the complete next-to-leading order (NLO) QCD corrections to the top quark associated with $γ$ production induced by model-independent $tqγ$ and $tqg$ flavor-changing neutral-current (FCNC) couplings at hadron colliders, respectively. We also consider the mixing effects between the $tqγ$ and $tqg$ FCNC couplings for this process. Our results show that, for the $tqγ$ couplings, the NLO QCD corrections can enhance the total cross sections by about 50% and 40% at the Tevatron and LHC, respectively. Including the contributions from the $tqγ$, $tqg$ FCNC couplings and their mixing effects, the NLO QCD corrections can enhance the total cross sections by about 50% for the $tuγ$ and $tug$ FCNC couplings, and by about the 80% for the $tcγ$ and $tcg$ FCNC couplings at the LHC, respectively. Moreover, the NLO corrections reduce the dependence of the total cross section on the renormalization and factorization scale significantly. We also evaluate the NLO corrections for several important kinematic distributions.

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arXiv2010

Next-to-leading order QCD corrections to a heavy resonance production and decay into top quark pair at the LHC

We present a complete next-to-leading order (NLO) QCD calculation to a heavy resonance production and decay into a top quark pair at the LHC, where the resonance could be either a Randall-Sundrum (RS) Kaluza-Klein (KK) graviton $G$ or an extra gauge boson $Z'$. The complete NLO QCD corrections can enhance the total cross sections by about $80\%- 100\%$ and $20\%- 40\%$ for the $G$ and the $Z'$, respectively, depending on the resonance mass. We also explore in detail the NLO corrections to the polar angle distributions of the top quark, and our results show that the shapes of the NLO distributions can be different from the leading order (LO) ones for the KK graviton. Moreover, we study the NLO corrections to the spin correlations of the top quark pair production via the above process, and find that the corrections are small.

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arXiv2010

Next-to-leading order QCD corrections to the top quark decay via the Flavor-Changing Neutral-Current operators with mixing effects

In this paper detailed calculations of the complete $\mathcal{O}(α_s)$ corrections to top quark decay widths $Γ(t\to q+V)$ are presented ($V=g,γ,Z$). Besides describing in detail the calculations in our previous paper (arXiv:0810.3889), we also include the mixing effects of the Flavor-Changing Neutral-Current (FCNC) operators for $t\to q+γ$ and $t\to q+Z$, which were not considered in our previous paper. The results for $t\to q+g$ are the same as in our previous paper. But the mixing effects can either be large or small, and increase or decrease the branching ratios for $t\to q+γ$ and $t\to q+Z$, depending on the values of the anomalous couplings ($κ^{g,γ,Z}_{\mathrm{tq}}/Λ$, $f^{g,γ,Z}_{\mathrm{tq}}$ and $h^{g,γ,Z}_{\mathrm{tq}}$).

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arXiv2010

Next-to-leading order QCD predictions for graviton and photon associated production in the Large Extra Dimensions model at the LHC

We present the calculations of the complete next-to-leading order(NLO) QCD corrections to the inclusive total cross sections for the Kaluza-Klein(KK) graviton and photon associated production process $pp \to γG_{KK} + X$ in the large extra dimensions(LED) model at the LHC. We show that the NLO QCD corrections in general enhance the total cross sections and reduce the dependence of the total cross sections on the factorization and renormalization scales. When jet veto is considered, the NLO corrections reduce the total cross sections. We also calculate some important differential cross sections for this process at NLO: the missing transverse momentum distribution, the transverse momentum distribution and the pseudorapidity distribution of photon.

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arXiv2009

Next-to-leading order QCD corrections to the single top quark production via model-independent t-q-g flavor-changing neutral-current couplings at hadron colliders

We present the calculations of the complete next-to-leading order (NLO) QCD effects on the single top productions induced by model-independent $tqg$ flavor-changing neutral-current couplings at hadron colliders. Our results show that, for the $tcg$ coupling the NLO QCD corrections can enhance the total cross sections by about 60% and 30%, and for the $tug$ coupling by about 50% and 20% at the Tevatron and LHC, respectively, which means that the NLO corrections can increase the experimental sensitivity to the FCNC couplings by about 10%$-$30%. Moreover, the NLO corrections reduce the dependence of the total cross sections on the renormalization or factorization scale significantly, which lead to increased confidence on the theoretical predictions. Besides, we also evaluate the NLO corrections to several important kinematic distributions, and find that for most of them the NLO corrections are almost the same and do not change the shape of the distributions.

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arXiv2009

Signature of Large Extra Dimensions from Z boson pair production at the CERN Large Hadron Collider

We study the Z boson pair production mediated by the Kaluza-Klein (KK) graviton in large extra dimensions (LED) at the CERN Large Hadron Collider (LHC). We use the partial wave unitarity to discuss the constraints on the process energy scale in order to give a self-consistent calculation. We find that the LED contributions can enhance the Z boson pair production cross sections significantly when the fundamental scale $M_S$ of the large extra dimensions is up to several TeV. We also show that the kinematic distributions of the LED signals are greatly different from the standard model ones and the LHC can probe the $M_S$ values up to $4.3\sim 5.6$ TeV at $3σ$ level depending on the number of the extra dimensions.

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arXiv2009

Next-to-leading order QCD corrections to the top quark decay via model-independent FCNC couplings

D0 and CDF collaborations at the Fermilab Tevatron have searched for non-standard-model single top-quark production and set upper limits on the anomalous top quark flavor-changing neutral current (FCNC) couplings $κ^g_{tc}/Λ$ and $κ^g_{tu}/Λ$ using the measurement of total cross section calculated at the next-to-leading order (NLO) in QCD. In this Letter, we report on the effect of anomalous FCNC couplings to various decay branching ratios of the top quark, calculated at the NLO. This result is not only mandatory for a consistent treatment of both the top quark production and decay via FCNC couplings by D0 and CDF at the Tevatron but is also important for the study of ATLAS and CMS sensitivity to these anomalous couplings at the CERN LHC. We find that the NLO corrections to the partial decay widths of the three decay channels $ t\to q + g$, $ t\to q + γ$ and $ t\to q + Z$ are at the order of 10% in magnitude and modify their branching ratios by about 20%, 0.4% and 2%, respectively, as compared to their leading order predictions.

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arXiv2008

Same-sign top pair production in an extra-dimension model of flavor at the CERN Large Hadron Collider

We study the same-sign top pair production mediated by the first Kluza-Klein (KK) excitation of the gluon in the Randall-Sundrum (RS) model with flavor violation at the Large Hadron Collider (LHC), in which the nonuniversal couplings between fermions and KK gauge bosons will lead to observable tree level flavor-changing neutral current (FCNC) effects. We find that the same-sign top quarks produced in our case have property of high energy and high transverse momentum, and lead to an observable signal in the same-sign dilepton channel even when the mass of the KK gluon reach up to 3 TeV. We further investigate the potential of the LHC to probe the flavor violating parameters and find that the LHC can probe their values down to 0.06.

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arXiv2007

Next-to-leading order QCD effects in associated charged Higgs and W boson production in the MSSM at the CERN Large Hadron Collider

We present the calculations of the next-to-leading order (NLO) QCD corrections to the inclusive total cross sections for the associated production of the $W^{\pm}H^{\mp}$ through $b\bar{b}$ annihilation in the Minimal Supersymmetric Standard Model at the CERN Large Hadron Collider. The NLO QCD corrections can either enhance or reduce the total cross sections, but they generally efficiently reduce the dependence of the total cross sections on the renormalization/factorization scale. The magnitude of the NLO QCD corrections is about 10% in most of the parameter space and can reach 15% in some parameter regions. We also show the Monte Carlo simulation results for the $2j+τ_{jet}+\not{p}_{T}$ signature from the $W^{\pm}$ and the $H^{\mp}$ decays including the NLO QCD effects, and find an observable signal at a $5σ$ level in some parameter region of the minimal supergravity model.

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