arXiv subjects

Xu-Chang Zheng

Publications and source records attributed to Xu-Chang Zheng.

At least 37 records · Page 2Linked to original sources

arXiv2022

Excited doubly heavy baryons production via top-quark decays

Within the framework of NRQCD, we calculate the production of excited doubly heavy baryons $Ξ_{bQ}$ through the semi-inclusive production process $t\rightarrow \langle bQ\rangle[n]\rightarrow Ξ_{bQ}+ \bar {Q} + W^+ $, where $Q= b$ or $c$ quark. The intermediate diquark state $\langle bQ\rangle[n]$ is in the excited $P$-wave state, including $[^1P_1]$ and $[^3P_J]$ ($J=$0, 1 or 2) in both color antitriplet state $\mathbf{\mathbf{\overline 3}}$ and color sixtuplet state $\mathbf{6}$, that is, $\langle bc\rangle[^{1}P_{1}]_{\mathbf{\overline 3}/ \mathbf{6}}$, $\langle bc\rangle[^{3}P_{J}]_{\mathbf{\overline 3}/ \mathbf{6}}$, $\langle bb\rangle[^{1}P_{1}]_{\mathbf{\overline 3}}$, and $\langle bb\rangle[^{3}P_{J}]_{\mathbf{6}}$. We find that the contributions from the P-wave states are about one order lower than the S-wave contributions, and this conclusion is consistent with others. We also analyze the invariant mass and angle differential distributions, and the theoretical uncertainty from the mass parameters and the renormalization scale. Finally, we can expect that about $1.14 \times10^{3-5}$ events of excited $Ξ_{bc}$ and $2.47 \times10^{1-3}$ events of excited $Ξ_{bb}$ can be produced per year at the LHC or HL-LHC with $\mathcal{L}$ =$10^{34-36}~\rm{cm}^{-2}~\rm{s}^{-1}$.

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arXiv2022

Photoproduction of the $B_c$ meson at the future $e^+e^-$ colliders

In the present paper, we study the photoproduction of various $S$-wave $B_c$ states, i.e. $B_c(1{}^1S_0)$, $B_c^*(1{}^1S_0)$, $B_c(2{}^1S_0)$ and $B^*_c(2{}^3S_1)$, at future $e^+e^-$ colliders within the framework of non-relativistic QCD. Two sources of the initial photons can be realized in the production, i.e., the LBS photon and the WWA photon. In addition to the direct photoproduction via the sub-process, $γ+ γ\rightarrow B_c + b + \bar{c}$, we also calculate the resolved photoproduction via the sub-processes, $γ+ g \rightarrow B_c + b + \bar{c}$ and $q + \bar{q} \rightarrow B_c + b + \bar{c}$ with $q=u,d,s,g$, respectively. Numerical results indicate that the contributions from the single resolved photoproduction are significant and even dominant at certain collision configuration. At the future high-energy and high-luminosity $e^+e^-$ colliders, the $B_c$ meson generated via the photoproduction mechanism is promisingly observable and can be well studied.

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arXiv2022

Next-to-leading order QCD corrections to $Z\to η_Q+Q+\bar{Q}$

It has been found that at a high luminosity $e^+ e^-$ collider, sizable $η_c+c\bar{c}X$ and $η_b+b\bar{b}X$ events can be produced when it works around the $Z$ peak. In this paper, we calculate the decay widths of $Z \to η_c+c+\bar{c}+X$ and $Z \to η_b+b+\bar{b}+X$ up to next-to-leading order (NLO) accuracy. We find that the NLO corrections are significant in these two processes. After including the NLO corrections, the decay widths of $Z \to η_c+c+\bar{c}+X$ and $Z \to η_b+b+\bar{b}+X$ are enhanced by about $37\%$ and $28\%$ for the case of $μ_R=2m_c$ and $μ_R=2m_b$, respectively. The differential decay widths $dΓ/ds_1$ and $dΓ/dz$ for these two decay processes are also analyzed.

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arXiv2022

BEEC2.0: An Upgraded Version for the Production of Heavy Quarkonium at Electron-positron Collider

The event generator BEEC [Z. Yang, X.G. Wu and X.Y. Wang, Comput. Phys. Commun. 184, 2848 (2013)] was devoted to the simulation of heavy quarkonium production at an unpolarized electron-positron collider. We upgraded it here by adding the generation of quarkonium with polarized electron and positron beams. In addition, the production of color-singlet 2S-wave states were included. Several future electron-positron colliders with high luminosity have been under discussion in the past decade. Especially, their possibility of producing polarized beams is important for providing more insights into the underlying physics. This upgraded version offers a useful tool for the feasibility study on quarkonium from the experimental side.

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arXiv2022

Inclusive $J/ψ$ photoproduction at the ILC within the framework of non-relativistic QCD

Based on the non-relativistic quantum chromodynamics factorization framework, we study the inclusive $J/ψ$ photoproduction at the future high energy $e^+e^-$ collider, International Linear Collider(ILC), where the initial photons come from the back-scattering of laser and electron (positron). The intermediate states, $c\bar{c}$$({}^3\!S_1^{[\textbf{1}]}$,${}^1\!S_0^{[\textbf{8}]}$,${}^3\!S_1^{[\textbf{8}]}$,${}^3\!P_{0,1,2}^{[\textbf{8}]})$, and resolved photoproduction processes are considered. Numerical results show that the cross section of $J/ψ$ photoproduction at the ILC could be very large, and the single resolved process via $c\bar{c}({}^1\!S_0^{[\textbf{8}]})$ intermediate state dominates primarily the production. We also present various kinematic distributions for $J/ψ$ photoproduction. Combining the high luminosity of the collision, $J/ψ$ photoproduction at the ILC shall provide a good platform to test the NRQCD factorization.

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arXiv2022

Fragmentation functions for gluon into $B_c$ or $B_c^*$ meson

In the paper, we calculate the fragmentation functions for $g \to B_c$ and $g \to B_c^*$. The ultraviolet divergences in the calculation are removed through the renormalization of the operator definition of the fragmentation functions under the modified minimal subtraction scheme. We then obtain the fragmentation functions $D_{g \to B_c}(z,μ_F)$ and $D_{g \to B_c^*}(z,μ_F)$, which are presented as figures and fitting functions. The obtained fragmentation functions are complementary to the previous work on the next-to-leading order fragmentation functions for $\bar{b}\to B_c(B_c^*)$ and $c\to B_c(B_c^*)$.

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arXiv2022

Production of the $B_c$ meson at the CEPC

In this paper, we make a detailed study on the production of the $B_c$, $B_c^*$, $B_c(2\,^1S_0)$, and $B^*_c(2\,^3S_1)$ mesons via the three planned running modes ($Z$, $W$, and $H$) at the future Circular Electron-Positron Collider (CEPC). The fragmentation-function and full nonrelativistic QCD approaches are adopted to calculate the production cross sections. Considering the excited states shall decay to the ground $B_c$ state (e.g. $1^1S_0$-state) with almost $100\%$ probability, our numerical results show that up to next-to-leading order QCD corrections, there are about $1.4\times 10^8$ $B_c$ events to be accumulated via the $Z$ mode ($\sqrt{s}=m_{_Z}$) of the CEPC, but only about $1.6\times 10^4$ and $1.1\times 10^4$ $B_c$ events to be accumulated via the $W$ ($\sqrt{s}=160\,{\rm GeV}$) and $H$ ($\sqrt{s}=240\,{\rm GeV}$) modes, respectively. Since the $Z$ mode is the best mode among the three planned modes of the CEPC for studying the production of the $B_c$, $B_c^*$, $B_c(2\,^1S_0)$, and $B^*_c(2\,^3S_1)$ mesons and the differential distributions of these mesons may be measured precisely at this mode, we further present the differential cross sections $dσ/(dz\,d{\rm cos}θ)$ and $dσ/dz$ via the $Z$ mode of the CEPC.

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arXiv2022

Excited doubly heavy baryon production via $W^+$ boson decays

In this paper, decay widths of the doubly heavy baryons ($Ξ_{cc} ~\text{and} ~Ξ_{bc}$) production are theoritically calculated in the whole phase space through $W^+ \to Ξ_{cc}+ \bar{c}+\bar{s}$ and $W^+ \to Ξ_{bc}+ \bar{b}+\bar{s}$, within the framework of nonrelativistic QCD (NRQCD). Differential widths $dΓ/ds_{12}$, $dΓ/ds_{23}$, $dΓ/dcosθ_{12}$, and $dΓ/dcosθ_{13}$ are also given. In addition to the ordinary S wave contributions for the baryons, we specifically calculate P wave contributions as a comparison, namely the high excited states of the intermediate diquark, including $[^1P_1]$ and $[^3P_J]$ (with $J=0,1,2$) in both color anti-triplet state $\overline{\mathbf{3}}$ and color sextuplet state $\mathbf{6}$. It shows that the contribution from P wave is about one order lower than S wave. According to the results, we can expect plentiful events produced at the LHC, i.e., $3.69\times10^5$ $Ξ_{cc}$ events and $4.91\times10^4$ $Ξ_{bc}$ events per year.

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arXiv2021

Inclusive production of heavy quarkonium $η_Q$ via $Z$ boson decays within the framework of nonrelativistic QCD

In the paper, the inclusive production of heavy quarkonium $η_Q$ ($Q=b$ or $c$) via $Z$ boson decays within the framework of nonrelativistic QCD effective theory are studied. The contributions from the leading color-singlet and color-octet Fock states are considered. Total and differential decay widths for the inclusive decays $Z \to η_Q+X$ are presented. It is found that the decays $Z\to η_Q +X$ are dominated by the $^3S_1^{[8]}$ component, so the decays can be inversely adopted to determine the values of the long-distance matrix elements $\langle {\cal O}^{η_{c}}(^{3}S_{1}^{[8]})\rangle$ and $\langle {\cal O}^{η_{b}}(^{3}S_{1}^{[8]})\rangle$, respectively. Our numerical results show that at an $e^+e^-$ collider running at the $Z$ pole with a high luminosity around $10^{35}{\rm cm}^{-2}{\rm s}^{-1}$ (a super $Z$ factory), there are about $4.5\times 10^7$ $η_c$ meson events and $6.1\times 10^5$ $η_b$ meson events to be produced per operation year, and the inclusive decays may be used for clarifying some problems on the heavy quarkonium $η_Q$ and nonrelativistic QCD.

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arXiv2021

Generalized Crewther relation and a novel demonstration of the scheme independence of commensurate scale relations up to all orders

In the paper, we make a detailed study on the generalized Crewther Relation (GCR) between the Adler function ($D$) and the Gross-Llewellyn Smith sum rules coefficient ($C^{\rm GLS}$) by using the newly suggested single-scale approach of the principle of maximum conformality (PMC). The resultant GCR is scheme-independent, whose residual scale dependence due to unknown higher-order terms are highly suppressed. Thus a precise test of QCD theory without renormalization scheme and scale ambiguities can be achieved by comparing with the data. Moreover, a demonstration of the scheme independence of commensurate scale relation up to all orders has been presented. And as the first time, the Pade approximation approach has been adopted for estimating the unknown $5_{\rm th}$-loop contributions from the known four-loop perturbative series.

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arXiv2021

NLO fragmentation functions for a quark into a spin-singlet quarkonium: Same flavor case

In the paper, we calculate the fragmentation functions for $c \to η_c$ and $b \to η_b$ up to next-to-leading-order (NLO) QCD accuracy. The ultraviolet divergences in the real corrections are removed through operator renormalization under the modified minimal subtraction scheme. We then obtain the fragmentation functions $D_{c \to η_c}(z,μ_F)$ and $D_{b \to η_b}(z,μ_F)$ up to NLO QCD accuracy, which are presented as figures and fitting functions. The numerical results show that the NLO corrections are significant. The sensitives of the fragmentation functions to the renormalization scale and the factorization scale are analyzed explicitly.

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arXiv2021

The $P$-wave charmonium annihilation into two photons $χ_{c0, c2}\rightarrow γγ$ with high-order QCD corrections

In this paper, we present a new analysis on the $P$-wave charmonium annihilation into two photons up to next-to-next-to-leading order (NNLO) QCD corrections by using the principle of maximum conformality (PMC). The conventional perturbative QCD prediction shows strong scale dependence and deviates largely from the BESIII measurements. After applying the PMC, we obtain a more precise scale-invariant pQCD prediction, which also agrees with the BESIII measurements within errors, i.e. $R={Γ_{γγ}(χ_{c2})} /{Γ_{γγ}(χ_{c0})}=0.246\pm0.013$, where the error is for $Δα_s(M_τ)=\pm0.016$. By further considering the color-octet contributions, even the central value can be in agreement with the data. This shows the importance of a correct scale-setting approach. We also give a prediction for the ratio involving $χ_{b0, b2} \toγγ$, which could be tested in future Belle II experiment.

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arXiv2021

The Gross-Llewellyn Smith sum rule up to ${\cal O}(α_s^4)$-order QCD corrections

In the paper, we analyze the properties of Gross-Llewellyn Smith (GLS) sum rule by using the $\mathcal{O}(α_s^4)$-order QCD corrections with the help of principle of maximum conformality (PMC). By using the PMC single-scale approach, we obtain an accurate renormalization scale-and-scheme independent fixed-order pQCD contribution for GLS sum rule, e.g. $S^{\rm GLS}(Q_0^2=3{\rm GeV}^2)|_{\rm PMC}=2.559^{+0.023}_{-0.024}$, where the error is squared average of those from $Δα_s(M_Z)$, the predicted $\mathcal{O}(α_s^5)$-order terms predicted by using the Padé approximation approach. After applying the PMC, a more convergent pQCD series has been obtained, and the contributions from the unknown higher-order terms are highly suppressed. In combination with the nonperturbative high-twist contribution, our final prediction of GLS sum rule agrees well with the experimental data given by the CCFR collaboration.

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arXiv2021

$Z$-boson hadronic decay width up to $\mathcal{O}(α_s^4)$-order QCD corrections using the single-scale approach of the principle of maximum conformality

In the paper, we study the properties of the $Z$-boson hadronic decay width by using the $\mathcal{O}(α_s^4)$-order quantum chromodynamics (QCD) corrections with the help of the principle of maximum conformality (PMC). By using the PMC single-scale approach, we obtain an accurate renormalization scale-and-scheme independent perturbative QCD (pQCD) correction for the $Z$-boson hadronic decay width, which is independent to any choice of renormalization scale. After applying the PMC, a more convergent pQCD series has been obtained; and the contributions from the unknown $\mathcal{O}(α_s^5)$-order terms are highly suppressed, e.g. conservatively, we have $ΔΓ_{\rm Z}^{\rm had}|^{{\cal O}(α_s^5)}_{\rm PMC}\simeq \pm 0.004$ MeV. In combination with the known electro-weak (EW) corrections, QED corrections, EW-QCD mixed corrections, and QED-QCD mixed corrections, our final prediction of the hadronic $Z$ decay width is $Γ_{\rm Z}^{\rm had}=1744.439^{+1.390}_{-1.433}$ MeV, which agrees with the PDG global fit of experimental measurements, $1744.4\pm 2.0$ MeV.

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arXiv2021

Fragmentation functions for a quark into a spin-singlet quarkonium: Different flavor case

In the paper, we calculate the fragmentation functions for a quark to fragment into a spin-singlet quarkonium, where the flavor of the initial quark is different from that of the constituent quark in the quarkonium. The ultraviolet divergences in the phase space integral are removed through the operator renormalization under the modified minimal subtraction scheme. The fragmentation function $D_{q \to η_Q}(z,μ_F)$ is expressed as a two-dimensional integral. Numerical results for the fragmentation functions of a light quark or a bottom quark to fragment into the $η_c$ are presented. As an application of those fragmentation functions, we study the processes $Z \to η_c+q\bar{q}g(q=u,d,s)$ and $Z \to η_c+b\bar{b}g$ under the fragmentation and the direct nonrelativistic QCD approaches.

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arXiv2020

Determination of the top-quark $\overline{\rm MS}$ running mass via its perturbative relation to the on-shell mass with the help of principle of maximum conformality

In the paper, we study the properties of the top-quark $\overline{\rm MS}$ running mass computed from its on-shell mass by using both the four-loop $\overline{\rm MS}$-on-shell relation and the principle of maximum conformality (PMC) scale-setting approach. The PMC adopts the renormalization group equation to set the correct magnitude of the strong running coupling of the perturbative series, its prediction avoids the conventional renormalization scale ambiguity, and thus a more precise pQCD prediction can be achieved. After applying the PMC to the four-loop $\overline{\rm MS}$-on-shell relation and taking the top-quark on-shell mass $M_t=172.9\pm0.4$ GeV as an input, we obtain the renormalization scale-invariant $\overline{\rm MS}$ running mass at the scale $m_t$, e.g., $m_t(m_t)\simeq 162.6\pm 0.4$ GeV, in which the error is the squared average of those from $Δα_s(M_Z)$, $ΔM_t$, and the approximate error from the uncalculated five-loop terms predicted by using the Padé approximation approach.

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arXiv2020

Gauge dependence of the perturbative QCD predictions under the momentum space subtraction scheme

The momentum space subtraction (MOM) scheme is one of the most frequently used renormalization schemes in perturbative QCD (pQCD) theory. In the paper, we make a detailed discussion on the gauge dependence of the pQCD prediction under the MOM scheme. Conventionally, there is renormalization scale ambiguity for the fixed-order pQCD predictions, which assigns an arbitrary range and an arbitrary error for the fixed-order pQCD prediction. The principle of maximum conformality (PMC) adopts the renormalization group equation to determine the magnitude of the coupling constant and hence determines an effective momentum flow of the process, which is independent to the choice of renormalization scale. There is thus no renormalization scale ambiguity in PMC predictions. To concentrate our attention on the MOM gauge dependence, we first apply the PMC to deal with the pQCD series. We adopt the Higgs boson decay width, $Γ(H\to gg)$, up to five-loop QCD contributions as an example to show how the gauge dependence behaves before and after applying the PMC. It is found that the Higgs decay width $Γ(H\to gg)$ depends very weakly on the choices of the MOM schemes, being consistent with the renormalization group invariance. It is found that the gauge dependence of $Γ(H\to gg)$ under the $\rm{MOMgg}$ scheme is less than $\pm1\%$, which is the smallest gauge dependence among all the mentioned MOM schemes.

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arXiv2020

Next-to-leading order QCD corrections to the production of $B_c$ and $B_c^*$ through $W^+$-boson decays

In this paper, we calculate the total decay widths for the $W^+$-boson decays, $W^+ \to B_c+b+\bar{s}+X$ and $W^+ \to B^*_c+b+\bar{s}+X$, up to next-to-leading order (NLO) accuracy within the framework of the nonrelativistic QCD theory. Both the fixed-order and the fragmentation approaches are adopted to do the calculation. Differential decay widths $dΓ/dz$ and $dΓ/ds_1$ are also given. We find that the NLO corrections are significant in those two $W^+$ decay channels. Our numerical results show that at the LHC, there are about $7.03\times 10^4$ $B_c$-meson events and $5.10\times 10^4$ $B^*_c$-meson events to be produced via the $W^+$-boson decays per operation year.

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