arXiv subjects

Zhi-Guo He

Publications and source records attributed to Zhi-Guo He.

At least 37 records · Page 2Linked to original sources

arXiv2015

$η_c$ production at the LHC challenges nonrelativistic-QCD factorization

We analyze the first measurement of $η_c$ production, performed by the LHCb Collaboration, in the nonrelativistic-QCD (NRQCD) factorization framework at next-to-leading order (NLO) in the strong-coupling constant $α_s$ and the relative velocity $v$ of the bound quarks including the feeddown from $h_c$ mesons. Converting the long-distance matrix elements (LDMEs) extracted by various groups from $J/ψ$ yield and polarization data to the $η_c$ case using heavy-quark spin symmetry, we find that the resulting NLO NRQCD predictions greatly overshoot the LHCb data, while the color-singlet model provides an excellent description.

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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

NLO NRQCD disfavors the interpretation of X(3872) as $χ_{c1}(2P)$

We study $χ_{c1}(2P)$ inclusive hadroproduction at next-to-leading order (NLO), both in $α_s$ and $v^2$, within the factorization formalism of nonrelativistic quantum chromodynamics (NRQCD), including the color-singlet $^3P_1^{[1]}$ and color-octet $^3S_1^{[8]}$ $c\bar c$ Fock states as well as the mixing of the latter with the $^3D_1^{[8]}$ state. Assuming the recently discovered X(3872) hadron to be the $J^{PC}=1^{++}$ charmonium state $χ_{c1}(2P)$, we perform a fit to the cross sections measured by the CDF, CMS, and LHCb Collaborations. We either obtain an unacceptably high value of $χ^2$, a value of $|R_{2P}^\prime(0)|$ incompatible with well-established potential models, or an intolerable violation of the NRQCD velocity rules. We thus conclude that NLO NRQCD is inconsistent with the hypothesis $X(3872)\equivχ_{c1}(2P)$.

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arXiv2013

The QED contribution to $J/ψ$ plus light hadrons production at B-factories

To understand the direct $J/ψ+X_{\mathrm{non-c\bar{c}}}$ production mechanism in $e^{+}e^{-}$ annihilation, in this work, we propose to measure the inclusive $J/ψ$ plus light hadrons (LH) production at B-factories and present a detailed study on its QED production due to $ψ(2S)$ feed-down, where the $ψ(2S)$ are produced in $e^{+}e^{-}\to ψ(2S)+γ$ and $e^{+}e^{-}\to ψ(2S)+f\bar{f}, f=lepton,light quark$, and QED contribution to direct $J/ψ+q\bar{q}$ production with $q=u,d,s$ quark. We find that the QED contribution is huge in the whole phase space region, but can be reduced largely and is in the same order as the QCD contribution when a suitable cut on the angel $θ_{J/ψ}$ between $J/ψ$ and the $e^{+}e^{-}$ beam is made. In this way, the cross section of $J/ψ+LH$ QCD production %which was predicted theoretical at next-to-leading order QCD together with relativistic correction, can be obtained by subtracting the QED contribution from the experimental measurement on inclusive $J/ψ$ plus light hadrons. To help to remove the QED background, we also calculate the angular and momentum distribution of $J/ψ$ in the QED contribution.

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arXiv2012

Investigating the heavy quarkonium radiative transitions with the effective Lagrangian method

In this work, we study the radiative decay of heavy quarkonium states by using the effective Lagrangian approach. Firstly, we construct the spin-breaking terms in the effective Lagrangian for the $nP\leftrightarrow mS$ transitions and determine the some of the coupling constants by fitting the experimental data. Our results show that in $χ_{cJ}$, $ψ(2S)$, $Υ(2S)$, and $Υ(3S)$ radiative decays, the spin-breaking effect is so small that can be ignored. Secondly, we investigate the radiative decay widths of the $c\bar{c}(1D)$ states and find the if $ψ(3770)$ is a pure $^3D_1$ state its radiative decay into $χ_{cJ}+γ$ roughly preserve the heavy-quark spin symmetry, while if it is a $S-D$ mixing state with mixing angel $12^{\circ}$ the heavy quark-spin symmetry in its radiative decay and in the radiative decay of $ψ(3686)$ will be largely violated. In the end, we show that combining the radiative decay and the light hadron decay of $P$-wave $χ_{bJ}(1,2P)$ can provide another way to extract the information of the color-octet matrix element in the context of non-relativistic QCD (NRQCD) effective theory, and our result is consistent with potential NRQCD hypothesis.

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arXiv2011

The discrete contribution to $ψ^{\prime}\to J/ψ+γγ$

The decay mode $ψ(2S)\to J/ψ+γγ$ is proposed in order to experimentally identify the effects of the coupling of charmonium states to the continuum $D\bar D$ states. To have a better understanding of such a two-photon decay process, in this work we restrict ourselves to investigate the contribution of the discrete part, in which the photons are mainly produced via the intermediate states $χ_{cJ}(nP)$. Besides calculating the resonance contributions of $χ_{cJ}(1P)\; (J=0,1,2)$, we also take into account the contributions of the higher excited states $χ_{cJ}(2P)$ and the interference effect among the 1P and 2P states. We find that the contribution of the 2P states and the interference terms to the total decay width is very tiny. However, for specific regions of the Dalitz plot, off the resonance peaks, we find that these contributions are sizable and should also be accounted for. We also provide the photon spectrum and study the polarization of $J/ψ$.

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arXiv2010

Color-singlet J/psi production at alpha_s^6 in Upsilon decay

To clarify the conflict between the theoretical predictions and experimental measurements of the inclusive J/psi production in Upsilon decay, We consider the alpha_s^6 order color-singlet contributions of processes Upsilon --> J/psi+gg and Upsilon --> J/psi+gggg. Both the branching ratio and J/psi momentum spectrum are calculated, and the branching ratio 4.7 x 10^{-4} is larger than the leading-order contribution (alpha_s^5, Upsilon --> J/psi+c\bar{c}g). Together with the QCD and QED leading-order contributions considered in our previous work, the color-singlet prediction of the branching ratio for the direct J/psi production is Br(Upsilon--> J/psi_{direct}+X)=0.90^{+0.49}_{-0.31} x 10^{-4}, which is still about 3.8 times less than the CLEO measurement. We also obtain a preliminary color-singlet prediction of R_{cc}=sigma(Upsilon --> J/psi+c\bar{c}+X)/ sigma(Upsilon --> J/psi+X) and find the value 0.39^{+0.21}_{-0.20} is much larger than the color-octet predictions, and suggest to measure this quality in future experimental analysis.

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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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arXiv2009

Inclusive J/ψProduction In ΥDecay Via Color-singlet Mechanism

We reconsider the tree level color-singlet contribution for the inclusive J/ψproduction in Υdecay with the α_{s}^{5} order QCD process Υ\to J/ψ+c\bar{c}+g and α^{2}α_s^{2} order QED processes Υ\toγ^{\ast}\to J/ψ+c\bar{c} and Υ\to J/ψ+gg. It is found that the contribution of the QED process is compatible with that of the QCD process, and the numerical results for the QCD process alone is an order of magnitude smaller than the previous theoretical predictions, and our theoretical prediction in total is about an order of magnitude smaller than the recent CLEO measurement on the branching fraction B(Υ\to J/ψ+X). It indicates that the J/ψproduction mechanism in Υdecay is not well understood, and further theoretical work and experimental analysis are still necessary.

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arXiv2009

Inclusive decays of $η_{b}$ into $S$- and $P$-wave charmonium states

Inclusive S- and P-wave charmonium productions in the bottomonium ground state $η_b$ decay are calculated at the leading order in the strong coupling constant $α_s$ and quarkonium internal relative velocity $v$ in the framework of the NRQCD factorization approach. We find the contribution of $η_b \to χ_{c_J}+ g g$ followed by $χ_{c_J} \to J/ψ+ γ$ is also very important to the inclusive $J/ψ$ production in the $η_b$ decays, which maybe helpful to the investigation of the color-octet mechanism in the inclusive $J/ψ$ production in the $η_b$ decays in the forthcoming LHCb and SuperB. As a complementary work, we also study the inclusive production of $η_c$, and $χ_{cJ}$ in the $η_b$ decays, which may help us understand the X(3940) and X(3872) states.

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arXiv2009

QED contribution to the production of J/ψ+c\bar{c}+X at the Tevatron and LHC

We calculate α^{2}α_{s}^{2} order QED contribution to J/ψproduction in pp(\bar{p})\to J/ψ+c\bar{c} color-singlet process at the Tevatron and LHC in the framework of non-relativistic QCD. The contribution of the interference between the α^{2}α_{s}^{2} QED and α_{s}^{4} QCD is also taken into account. The J/ψproduction associated with a charm quark pair could be a measurable signal at hadron collider experiment. Our calculations show that by including the QED contribution, the p_{t} distribution is enhanced by a factor of 1.5 (1.9) at the Tevatron (LHC) at p_{t}=50 (100) GeV. In addition, the polarization of J/ψturns from unpolarized in all region to increasingly transverse when p_{t} becomes larger.

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arXiv2009

J/ψProduction Via Photon Fragmentation at Hadron Colliders

The transverse momentum ($p_t$) distributions of production and polarization for $J/ψ$ measured by CDF Collaboration are still challenging our understanding of the heavy quarkonium production mechanism even with recent significant theoretical progresses on the next-to-leading-order (NLO) QCD calculation. We suggest a new mechanism for $J/ψ$ production at hadron collider, $pp(\bar p)\to γ^{\ast}(J/ψ)+X$ with $J/ψ$ from a virtual photon $γ^{\ast}$ fragmentation. Our calculations show it's $p_t$ distribution at NLO will be larger than that of the conventional $J/ψ$ production from the color-singlet mechanism at NLO when $p_{t}>26$ (35) GeV at the Tevatron (LHC) and reach about 6 (10) times of the conventional one when $p_t=50$ (100) GeV at the Tevatron (LHC), in spite of a suppression factor $(α/α_s)^2$ that is associated with the QED and QCD coupling constants. In addition, it also has large impact on the $p_{t}$ distribution of $J/ψ$ polarization in large $p_{t}$ region. Therefore, it is a important mechanism for $J/ψ$ production at large $p_t$ region especially for the LHC.

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arXiv2009

Predictions of Light Hadronic Decays of Heavy Quarkonium 1D_2 States in NRQCD

The inclusive light hadronic decays of 1D_2 heavy quarkonia are studied within the framework of NRQCD at the leading order in $v$ and up to the order of $α_s^3$. With one-loop QCD corrections, the infrared divergences and Coulomb singularities in the decay amplitudes are proved to be absorbed by the renormalization of the matrix elements of corresponding NRQCD operators, and the infrared finite short-distance coefficients are obtained through the matching calculations. By taking the factorization scale to be 2m_Q, the light hadronic decay widths are estimated to be about 274, 4.7, and 8.8 KeV for the $η_{c2}, η_{b2}$, and $η_{b2}'$ respectively. Based on the above estimates, and using the E1 transition width and dipion transition width for the $η_{c2}$ estimated elsewhere, we get the total width of $η_{c2}$ to be about 660-810 KeV, and the branching ratio of the E1 transition $η_{c2}\toγh_c$ to be about (44-54)%, which will be useful in searching for this missing charmonium state through, e.g., the process $η_{c2}\toγh_c$ followed by $h_c\toγη_c$.

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arXiv2008

QCD prediction for the non-$D\bar D$ annihilation decay of $ψ(3770)$

To clarify the marked difference between BES and CLEO measurements on the non-$D\bar D$ decays of the $ψ(3770)$, a $1^{3}D_{1}$-dominated charmonium, we calculate the annihilation decay of $ψ(3770)$ in NRQCD. By introducing the color-octet contributions, the results are free from infrared divergences. The color-octet matrix elements are estimated by solving the evolution equations. The S-D mixing effect is found to be very small. With $m_{c}=1.5\pm0.1\textrm{GeV}$ our result is $Γ(ψ(3770)\to \textrm{light hadrons})=467^{-187}_{+338}\textrm{KeV}$. For $m_c=1.4$ GeV, together with the observed hadronic transitions and E1 transitions, the non-$D\bar D$ decay branching ratio of $ψ(3770)$ could reach about 5%. Our results do not favor either of the results of BES and CLEO collaborations, and further experimental tests are urged.

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arXiv2008

Search for excited charmonium states in $e^+e^-$ Annihilation at $\sqrt{s}=10.6$ GeV

We suggest searching for excited charmonium states in $e^+e^-$ annihilation via double charmonium production at $\sqrt{s}=10.6$ GeV with $B$ factories, based on a more complete leading order calculation including both QCD and QED contributions for various processes. In particular, for the C=+ states, the $χ_{c0}(nP)$ (n=2,3) and $η_c(mS)$ (m=3,4) may have appreciable potentials to be observed; while for the C=- states, the $η_ch_c$ production and especially the $χ_{c1}h_c$ production might provide opportunities for observing the $h_c$ with higher statistics in the future. A brief discussion for the X(3940) observed in the double charmonium production is included.

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arXiv2007

Relativistic corrections to $J/ψ$ exclusive and inclusive double charm production at B factories

In order to clarify the puzzling problems in double charm production, relativistic corrections at order $v^{2}$ to the processes $e^{+}e^{-}\to J/ψ+η_{c}$ and $e^{+}e^{-}\to J/ψ+c\bar{c}$ at B factories are studied in non-relativistic quantum chromodynamics. The short-distance parts of production cross sections are calculated perturbatively, while the long-distance matrix elements are estimated from $J/ψ$ and $η_c$ decays up to errors of order $v^4$. Our results show that the relativistic correction to the exclusive process $e^{+}e^{-}\to J/ψ+η_{c}$ is significant, which, when combined together with the next-to-leading order $α_{s}$ corrections, could resolve the large discrepancy between theory and experiment; whereas for the inclusive process $e^{+}e^{-}\to J/ψ+c\bar{c}$ the relativistic correction is tiny and negligible. The physical reason for the above difference between exclusive and inclusive processes largely lies in the fact that in the exclusive process the relative momentum between quarks in charmonium substantially reduces the virtuality of the gluon that converts into a charm quark pair, but this is not the case for the inclusive process, in which the charm quark fragmentation $c\to J/ψ+c$ is significant, and QCD radiative corrections can be more essential.

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