arXiv subjects

Kuang-Ta Chao

Publications and source records attributed to Kuang-Ta Chao.

At least 37 records · Page 2Linked to original sources

arXiv2013

Search for $C=+$ charmonium states in $e^+e^-\to γ+~X$ at BEPCII/BESIII

We extend our original study in Ref. [1] on the production of $C=+$ charmonium states $X=η_c(1S/2S)$ and $χ_{cJ}(1P/2P)$ in $e^+e^-\toγ~+~X$ at B factories to the BEPCII/BESIII energy region with $\sqrt{s}=4.0\mbox{-}5.0$ GeV. In the framework of nonrelativistic QCD factorization, the cross sections are estimated to be as large as $0.1\mbox{-}0.9$ pb. The results could be used to search for the missing $2P$ charmonium states or to estimate the continuum backgrounds in the resonance region.

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arXiv2013

QCD and Relativistic $O(α_{s}v^2)$ Corrections to Hadronic Decays of Spin-Singlet Heavy Quarkonia $h_c, h_b$ and $η_b$

We calculate the annihilation decay widths of spin-singlet heavy quarkonia $h_c, h_b$ and $η_b$} into light hadrons with both QCD and relativistic corrections at order $O(α_{s}v^{2})$ in nonrelativistic QCD. With appropriate estimates for the long-distance matrix elements by using the potential model and operator evolution method, we find that our predictions of these decay widths are consistent with recent experimental measurements. We also find that the $O(α_{s}v^{2})$ corrections are small for $b\bar{b}$ states but substantial for $c\bar{c}$ states. In particular, the negative contribution of $O(α_{s}v^{2})$ correction to the $h_{c}$ decay can lower the decay width, as compared with previous predictions without the $O(α_{s}v^{2})$ correction, and thus result in a good agreement with the recent BESIII measurement.

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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

$Υ(1S)$ prompt production at the Tevatron and LHC in nonrelativistic QCD

With nonrelativistic QCD factorization, we calculate the $Υ(1S)$ prompt production at hadron colliders at next-to-leading order in $α_s$. In addition to the color-singlet contribution, color-octet channels (especially the P-wave channel) up to $O(v^4)$ are all considered. Aside from direct production, the feed-down contributions from higher excited S-wave and P-wave $b\bar b$ states to $Υ(1S)$ production are also included. We use the potential model estimates as input for color-singlet long-distance matrix elements (LDMEs). While for color-octet contributions, we find they can be approximately described by three LDMEs: $\mo{}{3}{S}{1}{8}$, $\mo{}{1}{S}{0}{8}$ and $\mo{}{3}{P}{0}{8}$. By fitting the Tevatron data we can determine some linear combinations of these LDMEs, and then use them to predict $Υ(1S)$ production at the LHC. Our predictions are consistent with the new experimental data of CMS and LHCb.

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arXiv2012

B-meson Semi-inclusive Decay to $2^{-+}$ Charmonium in NRQCD and X(3872)

The semi-inclusive B-meson decay into spin-singlet D-wave $2^{-+}$ charmonium, $B\to η_{c2}+X$, is studied in nonrelativistic QCD (NRQCD). Both color-singlet and color-octet contributions are calculated at next-to-leading order (NLO) in the strong coupling constant $α_s$. The non-perturbative long-distance matrix elements are evaluated using operator evolution equations. It is found that the color-singlet $^1D_2$ contribution is tiny, while the color-octet channels make dominant contributions. The estimated branching ratio $B(B\to η_{c2}+X)$ is about $0.41\,\times10^{-4}$ in the Naive Dimensional Regularization (NDR) scheme and $1.24\,\times10^{-4}$ in the t'Hooft-Veltman (HV) scheme, with renormalization scale $μ=m_b=4.8$\,GeV. The scheme-sensitivity of these numerical results is due to cancelation between ${}^1S_0^{[8]}$ and ${}^1P_1^{[8]}$ contributions. The $μ$-dependence curves of NLO branching ratios in both schemes are also shown, with $μ$ varying from $\frac{m_b}{2}$ to $2m_b$ and the NRQCD factorization or renormalization scale $μ_Λ$ taken to be $2m_c$. Comparison of the estimated branching ratio of $B\to η_{c2}+X$ with the observed branching ratio of $B \to X(3872)+K$ may lead to the conclusion that X(3872) is unlikely to be the $2^{-+}$ charmonium state $η_{c2}$.

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arXiv2012

Search for $χ_{c_J}(2P)$ from Higher Charmonim E1 Transitions and X,Y,Z States

We calculate the E1 transition widths of higher vector charmonium states into the spin-triplet 2P states in three typical potential models, and discuss the possibility to detect these 2P states via these E1 transitions. We attempt to clarify the nature of some recently observed X,Y,Z states by comparing them with these 2P charmonium states in these E1 transitions. In particular, the calculated branching ratios of $ψ(4040),ψ(4160)\to χ^{'}_{c_J}γ$ (J=0,1,2) are found to be in the range of $10^{-4}-10^{-3}$, and sensitive to the 3S-2D mixing of $ψ(4040)$ and $ψ(4160)$. The mixing angle may be constrained by measuring $ψ(4040),ψ(4160)\to Z(3930)γ$, if Z(3930) is identified with the $χ^{'}_{c_2}$ state, and then be used in measuring $χ^{'}_{c_{0,1}}$ states. These processes can be studied experimentally at $e^+e^-$ colliders such as BEPCII/BESIII and CESR/CLEO.

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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

A complete NLO calculation of the $J/ψ$ and $ψ'$ production at hadron colliders

A complete next-to-leading order (NLO) calculation in $α_s$ for the $\jpsi$ and $\psip$ prompt production at the Tevatron, LHC, and RHIC in nonrelativistic QCD is presented. We argue that the next-to-next-to-leading order (NNLO) color-singlet contribution may not be so important as to resolve the large discrepancy between theory and experiment in $\jpsi$ large $p_T$ production cross sections. Therefore, a complete NLO calculation, including both color-singlet and color-octet contribution, is necessary and essential to give a good description for $\jpsi$ and $\psip$ production. We also study the methods to fit the long-distance matrix elements using either two linear combined matrix elements or three matrix elements, and find these two methods can give consistent results. Compared with the measurements at the LHC and RHIC for prompt $\jpsi$ and $\psip$ production, our predictions are found to agree with all data. In particular, the recently released large $p_T$ data (up to 60-70 GeV) at the LHC are in good agreement with our predictions. Our results imply that the universality of color-octet matrix elements holds approximately in charmonium hadroproduction, when one uses fixed order perturbative calculation to describe data (the data in small $p_T$ region are not included). Our work may provide a new test for the universality of color-octet matrix elements, and the color-octet mechanism in general.

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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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arXiv2011

QCD corrections to e^+ e^- to J/ψ(ψ(2S))+χ_{cJ} (J=0,1,2) at B Factories

We analytically calculate the cross sections of double charmonium production in $e^+ e^- \to J/ψ(ψ(2S))χ_{cJ}$ (J=0,1,2) at next-to-leading order (NLO) in $α_s$ in nonrelativistic QCD, and confirm factorization of these processes. In contrast to $χ_{c0}$ production, for which the NLO correction is large and positive, the NLO corrections for $χ_{c1,2}$ production can be negative, resulting in decreased $K$ factors of 0.91 and 0.78 for J=1 and 2 respectively when $μ= 2 m_{c}$. Consequently, the NLO QCD corrections markedly enlarge the difference between cross sections of $χ_{c0}$ and $χ_{c1,2}$. This may explain why $e^+ e^- \to J/ψ(ψ(2S))χ_{c0}$ but not $e^+ e^- \to J/ψ(ψ(2S))χ_{c1,2}$ is observed experimentally. Moreover, for $J/ψ(ψ(2S))χ_{c1,2}$, the NLO QCD corrections substantially reduce the $μ$ dependence and lead to predictions with small theoretical uncertainties.

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arXiv2011

$χ_{cJ}$ production associated with a $c\bar c$ pair at hadron colliders

$χ_{cJ}$ (J=0,1,2) production associated with a charm quark pair in hadron collisions is calculated in the framework of nonrelativistic QCD at the Tevatron and LHC. It is found that the color-singlet contribution is small and the color-octet contribution may be dominant in the large $p_T$ region. The differential cross section of $p\bar p\toχ_c+c\bar{c}$ is at least one order of magnitude smaller than the next-to-leading order result of $χ_{cJ}$ inclusive production $p\bar p\toχ_c+X$, therefore $χ_{cJ}$ production in $p\bar p\toχ_c+c\bar{c}$ may have negligible influence on the ratio $R_{χ_c}=\frac{σ_{χ_{c2}}}{σ_{χ_{c1}}}$ measured by CDF at the Tevatron. The feeddown contribution from $χ_{cJ}+c+\bar{c}$ to $J/ψ+c+\bar{c}$ is found to be large compared with $J/ψ$ direct production and may have important influence on the measurement of $J/ψ+c+\bar{c}$. The validity of fragmentation approximation for the process is also discussed.

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arXiv2011

$O(α_s v^2)$ Corrections to Hadronic and Electromagnetic Decays of $^1S_0$ Heavy Quarkonium

We study $O(α_sv^2)$ corrections to decays of $^1S_0$ heavy quarkonium into light hadrons and two photons within the framework of nonrelativistic QCD, and find these $O(α_sv^2)$ corrections to have significant contributions especially for the decay into light hadrons. With these new results, experimental measurements of the hadronic width and the $γγ$ width of $η_c$ can be described more consistently. By fitting experimental data, we find the long-distance matrix elements of $η_c$ to be $|\mathcal{R}_{η_c}(0)|^2=0.834^{+0.281}_{-0.197}\ \textrm{GeV}^3$ and $< \bm{v}^2 >_{η_c}=0.232^{+0.121}_{-0.098}$. Moreover, $η_c(2S)$ is also discussed and the $γγ$ decay width is predicted to be $3.34^{+2.06}_{-2.10}\ \textrm{KeV}$.

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arXiv2011

QCD radiative corrections to $χ_{cJ}$ production at hadron colliders

To clarify the outstanding problem in charmonium production that existing theories cannot explain the observed cross sections of $χ_{cJ}$ (J=0,1,2) and ratio $R_{χ_c}=σ_{χ_{c2}}/σ_{χ_{c1}}\approx 0.75$ (in contrast to the spin counting value 5/3) at the Tevatron, we study the complete next-to-leading order radiative corrections in nonrelativistic QCD, and find next-to-leading order contributions of $^3P_J^{[1]}$ are more important than leading order at high $p_T$, and $^3P_1^{[1]}$ decreases slower than $^3P_2^{[1]}$, implying a natural explanation for the $R_{χ_c}$ puzzle. By fitting $R_{χ_c}$, the predicted cross sections of $χ_{cJ}$ are found to agree with data. The result indicates color-octet contribution is crucially needed, thus providing a unique test for heavy quarkonium production mechanisms. Feed-down contributions of $χ_{cJ}$ to prompt $J/ψ$ production are estimated to be substantial, about $30-40%$ at $p_T=20$ GeV. Production of $χ_{cJ}$ (J=0,1,2) at the LHC is also predicted.

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arXiv2011

J/psi (psi') production at the Tevatron and LHC at O(α_s^4v^4) in nonrelativistic QCD

We present a complete evaluation for $\jpsi(\psip)$ prompt production at the Tevatron and LHC at next-to-leading order in nonrelativistic QCD, including color-singlet, color-octet, and higher charmonia feeddown contributions. The short-distance coefficients of $\pj$ at next-to-leading order are found to be larger than leading order by more than an order of magnitude but with a minus sign at high transverse momentum $p_T$. Two new linear combinations of color-octet matrix elements are obtained from the CDF data, and used to predict $\jpsi$ production at the LHC, which agrees with the CMS data. The possibility of $\sa$ dominance and the $\jpsi$ polarization puzzle are also discussed.

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arXiv2010

NRQCD Predictions of D-Wave Quarkonia $^3D_{J}(J=1,2,3)$ Decay into Light Hadrons at Order $α_{s}^{3}$

In this paper, in the framework of NRQCD we study the light hadron (LH) decays of the spin-triplet (S=1) D-wave heavy quarkonia. The short distance coefficients of all Fock states in the $^3D_J(J=1,2,3)$ quarkonia including D-wave color-singlet, P-wave color-octet and S-wave color-singlet and color-octet are calculated perturbatively at $α_{s}^3$ order. The operator evolution equations of the four-fermion operators are also derived and are used to estimate the numerical values of the long distance matrix elements. We find that for the $c\bar{c}$ system, the LH decay widths of $ψ(1^3D_J)$ predicted by NRQCD is about $2\sim3$ times larger than the phenomenological potential model results, while for the $b\bar{b}$ system the two theoretical estimations of $Γ(Υ(1^3D_J)\to LH)$ are in coincidence with each other. Our predictions for $ψ(1^3D_J)$ LH decay widths are $Γ(ψ(1^3D_J)\to LH)=(0.43,0.05,0.17)$MeV for J=1,2,3; and for $Υ(1^3D_J)$, $Γ(Υ(1^3D_J)\to LH)=(6.91,0.75,2.75)$KeV for J=1,2,3.

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arXiv2010

Relativistic correction to $e^{+}e^{-}\to J/ψ+gg$ at $B$ factories and constraint on color-octet matrix elements

We calculate the relativistic correction to $J/ψ$ production in the color-singlet process $e^{+}e^{-}\to J/ψ+gg$ at B-factories. We employ the non-relativistic QCD (NRQCD) factorization approach, where the short-distance coefficients are calculated perturbatively and the long-distance matrix elements are extracted from the decays of $J/ψ$ into $e^{+}e^{-}$ and light hadrons. We find that the $O(v^2)$ relativistic correction can enhance the cross section by a factor of 20-30%, comparable to the enhancement due to the $O(α_s)$ radiative correction obtained earlier. Combining the relativistic correction with the QCD radiative correction, we find that the color-singlet contribution to $e^{+}e^{-}\to J/ψ+gg$ can saturate the latest observed cross section $σ(e^{+}e^{-}\to J/ψ+X_{\mathrm{non-c\bar{c}}})=0.43 \pm0.09\pm0.09$ pb by Belle, thus leaving little room to the color-octet contributions. This gives a very stringent constraint on the color-octet contribution, and may imply that the values of color-octet matrix elements are much smaller than expected earlier by using the naive velocity scaling rules or extracted from fitting experimental data with the leading-order calculations.

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arXiv2009

Search for $C=+$ charmonium and bottomonium states in $e^+e^-\to γ+ X$ at B factories

We study the production of $C=+$ charmonium states $X$ in $e^+e^-\to γ+ X$ at B factories with $X=η_c(nS)$ (n=1,2,3), $χ_{cJ}(mP)$ (m=1,2), and $^1D_2(1D)$. In the S and P wave case, contributions of tree-QED with one-loop QCD corrections are calculated within the framework of nonrelativistic QCD(NRQCD) and in the D-wave case only the tree-QED contribution are considered. We find that in most cases the QCD corrections are negative and moderate, in contrast to the case of double charmonium production $e^+e^-\to J/ψ+ X$, where QCD corrections are positive and large in most cases. We also find that the production cross sections of some of these states in $e^+e^-\to γ+ X$ are larger than that in $e^+e^-\to J/ψ+ X$ by an order of magnitude even after the negative QCD corrections are included. So we argue that search for the X(3872), X(3940), Y(3940), and X(4160) in $e^+e^-\to γ+ X$ at B factories may be helpful to clarify the nature of these states. For completeness, the production of bottomonium states in $e^+e^-$ annihilation is also discussed.

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