arXiv subjects

Yu-Jie Zhang

Publications and source records attributed to Yu-Jie Zhang.

62 records · Page 4Linked to original sources

arXiv2009

QCD corrections to $\bm{e^+ e^- \to J/ψg g}$ at B Factories

In heavy quarkonium production, the measured ratio $R_{c \bar c}=σ[J/ψ+ c\bar c + X]/σ[J/ψ+X]$ at B factories is much larger than existing theoretical predictions. To clarify this discrepancy, in nonrelativistic QCD (NRQCD) we find the next-to-leading-order (NLO) QCD correction to $e^+ e^- \to J/ψ+gg$ can enhance the cross section by about 20%. Together with the calculated NLO result for $e^+ e^- \to J/ψ+c\bar c$, we show that the NLO corrections can significantly improve the fit to the ratio $R_{c \bar c}$. The effects of leading logarithm resummation near the end point on the $J/ψ$ momentum distribution and total cross section are also considered. Comparison of the calculated cross section for $e^+ e^- \to J/ψ+gg$ with observed cross section for $e^+ e^- \to J/ψ+non(c\bar c)$ is expected to provide unique information on the issue of color-octet contributions.

hep-ph↗

arXiv2009

Pair Production of Heavy Quarkonium and $B_c(^*)$ Mesons at Hadron Colliders

We investigate the pair production of S-wave heavy quarkonium at the LHC in the color-singlet mechanism (CSM) and estimate the contribution from the gluon fragmentation process in the color-octet mechanism (COM) for comparison. With the matrix elements extracted previously in the leading order calculations, the numerical results show that the production rates are quite large for the pair production processes at the LHC. The $p_t$ distribution of double $J/ψ$ production in the CSM is dominant over that in the COM when $p_t$ is smaller than about 8GeV. For the production of double $Υ$, the contribution of the COM is always larger than that in the CSM. The large differences in the theoretical predictions between the CSM and COM for the $p_t$ distributions in the large $p_t$ region are useful in clarifying the effects of COM on the quarkonium production. We also investigate the pair production of S-wave $B_c$ and $B_c^*$ mesons, and the measurement of these processes is useful to test the CSM and extract the LDMEs for the $B_c$ and $B_c^*$ mesons.

hep-ph↗

arXiv2008

$Υ$ decay to two-charm quark jets as a Probe of the Color Octet Mechanism

We calculate the decay rate of bottomonium to two-charm quark jets $Υ\to c \bar c$ at the tree level and one-loop level including color-singlet and color-octet $b \bar b$ annihilations. We find that the short distance coefficient of the color-octet piece is much larger than the color-singlet piece, and that the QCD correction will change the endpoint behavior of the charm quark jet. The color-singlet piece is strongly affected by the one-loop QCD correction. In contrast, the QCD correction to the color-octet piece is weak. Once the experiment can measure the branching ratio and energy distribution of the two-charm quark jets in the $Υ$ decay, the result can be used to test the color octet mechanism or give a strong constraint on the color-octet matrix elements.

hep-ph↗

arXiv2008

Factorization and NLO QCD correction in e^+ e^- \to J/ψ(ψ(2S))+ χ_{c0} at B Factories}

In nonrelativistic QCD (NRQCD), we study $e^+ e^- \to J/ψ(ψ(2S)) +χ_{c0}$ at $B$ factories, where the P-wave state $χ_{c0}$ is associated with an S-wave state $J/ψ$ or $ψ(2S)$. In contrast to the failure of factorization in most cases involving P-wave states, e.g. in B decays, we find that factorization holds in this process at next to leading order (NLO) in $α_s$ and leading order (LO) in $v$, where the associated S-wave state plays a crucial rule in canceling the infrared (IR) divergences. We also give some general analyses for factorization in various double charmonium production. The NLO corrections in $e^+ e^- \to J/ψ(ψ(2S)) +χ_{c0}$ at $\sqrt{s}=10.6$ GeV are found to substantially enhance the cross sections by a factor of about 2.8; hence crucially reduce the large discrepancy between theory and experiment. With $m_c=1.5{\rm GeV}$ and $μ=2m_c$, the NLO cross sections are estimated to be $17.9(11.3)$ fb for $e^+ e^- \to J/ψ(ψ(2S))+ χ_{c0}$, which reach the lower bounds of experiment.

hep-ph↗

arXiv2007

Double charm production e^+ e^- \to J/ψ+ c \bar{c} at B factories with next-to-leading order QCD corrections

The inclusive $J/ψ$ production in $e^+ e^- \to J/ψc\bar c$ at B factories is one of the most challenging open problems in heavy quarkonium physics. The observed cross section of this double charm production process is larger than existing leading order (LO) QCD predictions by a factor of 5. In the nonrelativistic QCD (NRQCD) factorization formalism, we calculate the next-to-leading order (NLO) QCD virtual and real corrections to this process, and find that these corrections can substantially enhance the cross section with a $K$ factor of about 1.8. We further take into account the feeddown contributions from higher charmonium states (mainly the $ψ(2S)$ as well as $χ_{cJ}$) and the two-photon contributions, and find that the discrepancy between theory and experiment can be largely removed.

hep-ph↗

arXiv2007

Radiative decays of bottomonia into charmonia and light mesons

In the framework of nonrelativistic QCD, we study the radiative decays of bottomonia into charmonia, including $Υ\to χ_{cJ}γ$, $Υ\to η_cγ$, $η_b\to J/ψγ$, and $χ_{bJ}\to J/ψγ$. We give predictions for their branching ratios with numerical calculations. E.g., we predict the branching ratio for $η_b\to J/ψγ$ is about $1\times 10^{-7}$. As a phenomenological model study, we further extend our calculation to the radiative decays of bottomonia into light mesons by assuming the $f_2(1270)$, $f_2'(1525)$ and other light mesons to be described by nonrelativistic $q\bar q ~(q=u,d,s)$ bound states with constituent quark masses. The calculated branching ratios for $Υ\to f_2(1270)γ$ and $Υ\to f_2'(1525)γ$ are roughly consistent with the CLEO data. Comparisons with radiative decays of charmonium into light mesons such as $J/ψ\to f_2(1270)γ$ are also given. In all calculations the QED contributions are taken into account and found to be significant in some processes.

hep-ph↗

arXiv2007

$Υ$ radiative decays to light quark jets and color octet mechanism

We study radiative decays of $Υ$ to light quark jets in nonrelativistic QCD by taking both the color singlet and color octet $b\bar b$ operators into consideration. The cut for quark jet energy and cut for the angle between two quark jets are introduced. The sensitivity to the soft and collinear singularities in the loop integrals are greatly reduced by these cuts. With the jet energy cut of about 1 GeV, and the jet angle cut of about $36^\circ$, the branching ratio for $Υ\toγq\bar q$ is found to be $8.2\times 10^{-4}$ from color singlet contributions. The color octet contributions could be much larger than that of color singlet, depending on the estimate of the color octet matrix elements. This process may provide a new test for the color octet mechanism in nonrelativistic QCD.

hep-ph↗

arXiv2007

Radiative decays of charmonium into light mesons

We apply perturbative QCD to the radiative decays of charmonia $J/ψ$ and $χ_{cJ}$ into light mesons. We perform a complete numerical calculation for the quark-gluon loop diagrams involved in these processes. The calculated $J/ψ$ decay branching ratios to P-wave mesons $f_2(1270)$ and $f_1(1285)$ fit the data well, while that of $f_0(980)$ (if treated as an $s\bar s$ meson) is predicted to be $1.6\times 10^{-4}$, which implies that $f_0(1710)$ can not be the $s\bar s$ or $(u\bar u+d\bar d)/\sqrt{2}$ meson. Decays of P-wave charmonia $χ_{cJ}\to ρ(ω, ϕ)γ$ (J=0,1,2) are also studied, and the branching ratio of $χ_{c1}\to ργ$ is predicted to be $1.4\times 10^{-5}$, which may be tested by CLEO-c and BESIII with future experiments.

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