arXiv subjects

Yang Ma

Publications and source records attributed to Yang Ma.

62 records · Page 4Linked to original sources

arXiv2015

Setting the Renormalization Scale in pQCD: Comparisons of the Principle of Maximum Conformality with the Sequential Extended Brodsky-Lepage-Mackenzie Approach

A key problem in making precise perturbative QCD (pQCD) predictions is how to set the renormalization scale of the running coupling unambiguously at each finite order. The elimination of the uncertainty in setting the renormalization scale in pQCD will greatly increase the precision of collider tests of the Standard Model and the sensitivity to new phenomena. Renormalization group invariance requires that predictions for observables must also be independent on the choice of the renormalization scheme. The well-known Brodsky-Lepage-Mackenzie (BLM) approach cannot be easily extended beyond next-to-next-to-leading order of pQCD. Several suggestions have been proposed to extend the BLM approach to all-orders. In this paper we discuss two distinct methods. One is based on the "Principle of Maximum Conformality" (PMC), which provides a systematic all-orders method to eliminate the scale- and scheme- ambiguities of pQCD. The PMC extends the BLM procedure to all orders using renormalization group methods; as an outcome, it significantly improves the pQCD convergence by eliminating renormalon divergences. An alternative method is the "sequential extended BLM" (seBLM) approach, which has been primarily designed to improve the convergence of pQCD series. The seBLM, as originally proposed, introduces auxiliary fields and follows the pattern of the $β_0$-expansion to fix the renormalization scale. However, the seBLM requires a recomputation of pQCD amplitudes including the auxiliary fields; due to the limited availability of calculations using these auxiliary fields, the seBLM has only been applied to a few processes at low-orders......

hep-ph↗

arXiv2015

General Properties on Applying the Principle of Minimum Sensitivity to High-order Perturbative QCD Predictions

As one of the key components of perturbative QCD theory, it is helpful to find a systematic and reliable way to set the renormalization scale for a high-energy process. The conventional treatment is to take a typical momentum as the renormalization scale, which assigns an arbitrary range and an arbitrary systematic error to pQCD predictions, leading to the well-known renormalization scheme and scale ambiguities. As a practical solution for such scale setting problem, the "Principle of Minimum Sensitivity" (PMS), has been proposed in the literature. The PMS suggests to determine an optimal scale for the pQCD approximant of an observable by requiring its slope over the scheme and scale changes to vanish. In the paper, we present a detailed discussion on general properties of PMS by utilizing three quantities $R_{e^+e^-}$, $R_τ$ and $Γ(H\rightarrow b\bar{b})$ up to four-loop QCD corrections. After applying the PMS, the accuracy of pQCD prediction, the pQCD convergence, the pQCD predictive power and etc., have been discussed. Furthermore, we compare PMS with another fundamental scale setting approach, i.e. the Principle of Maximum Conformality (PMC)... Our results show that PMS does provide a practical way to set the effective scale for high-energy process, and the PMS prediction agrees with the PMC one by including enough high-order QCD corrections, both of which shall be more accurate than the prediction under the conventional scale setting. However, the PMS pQCD convergence is an accidental, which usually fails to achieve a correct prediction of unknown high-order contributions with next-to-leading order QCD correction only, i.e. it is always far from the "true" values predicted by including more high-order contributions.

hep-ph↗

arXiv2015

$B\to ρ$ transition form factors within the QCD light-cone sum rules and the $ρ$-meson leading-twist distribution amplitude

The QCD light-cone sum rules (LCSR) provides an effective way for dealing with the heavy-to-light transition form factors (TFFs), whose non-perturbative dynamics are parameterized into the light-meson's light-cone distribution amplitudes (LCDAs) with various twist structures. By taking the chiral correlator as the starting point, we calculate the LCSRs for the $B\toρ$ TFFs up to twist-4 accuracy. As for the TFFs at the large recoil region, we observe that the twist-2 transverse DA $ϕ_{2;ρ}^\bot$ provides the dominant contribution, while the contributions from the remaining twist-3 and twist-4 terms are $δ^2$-suppressed. Thus, our present improved LCSRs provides a good platform for testing the $ϕ_{2;ρ}^\bot$ behavior. For the purpose, we suggest a convenient WH-model for the $ρ$-meson leading-twist wavefunction, in which the parameter $B_{2;ρ}^\bot\sim a^\bot_2$ dominantly controls its longitudinal distribution. Typically, its DA $ϕ_{2;ρ}^\bot$ is CZ-like as $B_{2;ρ}^\bot\simeq-0.20$, which changes to be asymptotic-like as $B_{2;ρ}^\bot\simeq0.00$. By varying $B_{2;ρ}^\bot\in[-0.20,0.20]$, we present a detailed comparison of the LCSR estimation for the $B\toρ$ TFFs with those of pQCD and Lattice QCD predictions. Furthermore, by using the extrapolated TFFs, we estimate the CKM-matrix element $|V_{\rm ub}|$ with the help of two $B\toρ$ semi-leptonic decays. The predicted value for $|V_{\rm ub}|$ increases with the increment of $B_{2;ρ}^\bot$, i.e. we have $\Vub=(2.91\pm0.19)\times 10^{-3}$ for $B_{2;ρ}^\bot=-0.20$ and $\Vub=(3.11\pm0.19)\times 10^{-3}$ for $B_{2;ρ}^\bot=0.00$. If using the BABAR prediction as a criteria, we observe that $B^{\perp}_{2;ρ}\in[-0.2,0.10]$, which indicates that the $ρ$-meson DA $ϕ_{2;ρ}^\bot$ prefers a single-peak behavior rather than a double-humped behavior.

hep-ph↗

arXiv2014

Photoproduction of doubly heavy baryon at the ILC

In the present paper, we make a detailed study on the doubly heavy baryon photoproduction in the future $e^+e^-$ International Linear Collider (ILC). The baryons $Ξ_{cc}$, $Ξ_{bc}$, and $Ξ_{bb}$ are produced via the channel $γγ\to Ξ_{QQ'} +\bar{Q'} +\bar{Q}$, where $Q$ and $Q'$ stand for heavy $c$ or $b$ quark, respectively. As for the $Ξ_{QQ'}$-baryon production, it shall first generate a $(QQ')[n]$-diquark and then form the final baryon via fragmentation, where $[n]$ stands for the color- and spin- configurations for the $(QQ')$-diquark states. According to the non-relativistic QCD theory, four diquark configurations shall provide sizable contributions to the baryon production, e.g., $[n]$ equals $[^3S_1]_{\bar{\textbf{3}}}$, $[^1S_0]_{\textbf{6}}$, $[^3S_1]_{\textbf{6}}$, or $[^1S_0]_{\bar{\textbf{3}}}$, respectively. We adopt the improved helicity amplitude approach for the hard scattering amplitude to improve the calculation efficiency. Total and differential cross sections of those channels, as well as the theoretical uncertainties, are presented. We show that sizable amounts of baryon events can be generated at the ILC, i.e., about $2.0\times 10^{6}$ $Ξ_{cc}$, $2.2\times 10^{5}$ $Ξ_{bc}$, as well as $3.0\times 10^{3}\;Ξ_{bb}$ events are to be generated in one operation year for $\sqrt{S}=500$ GeV and ${\cal L}\simeq 10^{36}$cm$^{-2}$s$^{-1}$.

hep-ph↗

arXiv2014

The $ρ$-meson longitudinal leading-twist distribution amplitude

In the present paper, we suggest a convenient model for the vector $ρ$-meson longitudinal leading-twist distribution amplitude $ϕ_{2;ρ}^\|$, whose distribution is controlled by a single parameter $B^\|_{2;ρ}$. By choosing proper chiral current in the correlator, we obtain new light-cone sum rules (LCSR) for the $B\toρ$ TFFs $A_1$, $A_2$ and $V$, in which the $δ^1$-order $ϕ_{2;ρ}^\|$ provides dominant contributions. Then we make a detailed discussion on the $ϕ_{2;ρ}^\|$ properties via those $B\toρ$ TFFs. A proper choice of $B^\|_{2;ρ}$ can make all the TFFs agree with the lattice QCD predictions. A prediction of $|V_{\rm ub}|$ has also been presented by using the extrapolated TFFs, which indicates that a larger $B^{\|}_{2;ρ}$ leads to a larger $|V_{\rm ub}|$. To compare with the BABAR data on $|V_{\rm ub}|$, the longitudinal leading-twist DA $ϕ_{2;ρ}^\|$ prefers a doubly-humped behavior.

hep-ph↗

arXiv2014

The QCD improved electroweak parameter $ρ$

In the present paper, we make a detailed analysis for the QCD corrections to the electroweak $ρ$ parameter by applying the principle of maximum conformality (PMC). As a comparison, we show that under the conventional scale setting, we have $Δρ|_{\rm N^3LO} = \left(8.257^{+0.045}_{-0.012}\right) \times10^{-3}$ by varying the scale $μ_{r}\in[M_{t}/2$, $2M_{t}]$. By defining a ratio, $ΔR=Δρ/3X_t-1$, which shows the relative importance of the QCD corrections, it is found that its scale error is $\sim \pm9 \%$ at the two-loop level, which changes to $\sim\pm4\%$ at the three-loop level and $\sim \pm 2.5\%$ at the four-loop level, respectively. These facts well explain why the conventional scale uncertainty constitutes an important error for estimating the $ρ$ parameter. On the other hand, by applying the PMC scale setting, the four-loop estimation $Δρ|_{\rm N^3LO}$ shall be almost fixed to $8.228\times10^{-3}$, which indicates that the conventional scale error has been eliminated. We observe the pQCD convergence for the $ρ$ parameter has also been greatly improved due to the elimination of the divergent renormalon terms. As applications of the present QCD improved $ρ$ parameter, we show the shifts of the $W$-boson mass and the effective leptonic weak-mixing angle due to $Δρ$ can be reduced to $δM_{W}|_{\rm N^3LO} =0.7$ MeV and $δ\sin^2θ_{\rm eff}|_{\rm N^3LO}=-0.4\times10^{-5}$.

hep-ph↗

arXiv2014

$V_{cb}$ from the semileptonic decay $B\to D \ell \barν_{\ell}$ and the properties of the $D$ meson distribution amplitude

The improved QCD light-cone sum rule (LCSR) provides an effective way to deal with the heavy-to-light transition form factors (TFFs). Firstly, we adopt the improved LCSR approach to deal with the $B\to D$ TFF $f^{+}(q^2)$ up to twist-4 accuracy. Due to the elimination of the most uncertain twist-3 contribution and the large suppression of the twist-4 contribution, the obtained LCSR shall provide us a good platform for testing the $D$-meson leading-twist DA. For the purpose, we suggest a new model for the $D$-meson leading-twist DA ($ϕ_{3D}$), whose longitudinal behavior is dominantly determined by a parameter $B$. Moreover, we find its second Gegenbauer moment $a^D_2\sim B$. Varying $B$ within certain region, one can conveniently mimic the $D$-meson DA behavior suggested in the literature. Inversely, by comparing the estimations with the experimental data on the $D$-meson involved processes, one can get a possible range for the parameter $B$ and a determined behavior for the $D$-meson DA. Secondly, we discuss the $B\to D$ TFF at the maximum recoil region and present a detailed comparison of it with the pQCD estimation and the experimental measurements. Thirdly, by applying the LCSR on $f^{+}(q^2)$, we study the CKM matrix element $\Vcb$ together with its uncertainties by adopting two types of processes, i.e. the $B^0/\bar{B}^0$-type and the $B^{\pm}$-type. It is noted that a smaller $B \precsim 0.20$ shows a better agreement with the experimental value on $\Vcb$. For example, for the case of $B=0.00$, we obtain $|V_{cb}|(B^0/\bar{B}^0-{\rm type})=(41.28 {^{+5.68}_{-4.82}} {^{+1.13}_{-1.16}}) \times 10^{-3}$ and $|V_{cb}|(B^{\pm}-{\rm type})=(40.44 {^{+5.56}_{-4.72}} {^{+0.98}_{-1.00}}) \times 10^{-3}$, whose first (second) uncertainty comes from the squared average of the mentioned theoretical (experimental) uncertainties.

hep-ph↗

arXiv2014

Bottomonium production associated with a photon at a high luminosity $e^+e^-$ collider with next-to-leading order QCD corrections

We make a detailed discussion on the one-loop QCD correction to the bottomonium production associated with a photon, i.e. via the channel $e^{+}e^{-} \toγ^*/Z^0 \to |H_{b\bar{b}}\rangle + γ$, where $|H_{b\bar{b}}\rangle$ stands for the color-singlet bottomonium state as $η_b$, $Υ$, $h_b$ or $χ_{bJ}$ ($J$=0, 1 or 2), respectively. At the super $Z$ factory with the collision energy $E_{cm} \sim m_Z$, by summing up the cross sections for all bottomonium states, we obtain a large one-loop QCD correction, i.e. $|R|\sim 30\%$. This ensures the necessity and importance of the one-loop QCD corrections for the present processes. Further more, for the $η_b$, $h_b$ and $χ_{bJ}$ production, their cross sections are dominated by the $s$-channel diagrams and are enhanced by the $Z^0$ boson resonance effect when $E_{cm}\sim m_Z$. While, for the $Υ$ production, such resonance effect shall be smeared by a large $t(u)$-channel contribution that dominant over the $s$-channel one. Theoretical uncertainties caused by slight change of $E_{cm}$, the $b$-quark mass, the renormalization scale and etc. have been presented. At the super $Z$ factory with a high luminosity up to ${\cal L}=10^{36}{\rm cm}^{-2}{\rm s}^{-1}$, the bottomonium plus one photon events are sizable, especially for $η_b$ and $Υ$, which have large signal significance. Summing up all bottomonium states, we shall totally have $\sim 3.8\times10^{5}$ bottomonium events in one operation year. So, the super $Z$ factory shall provide a good platform for studying the bottomonium properties.

hep-ph↗