arXiv subjects

Yu Jia

Publications and source records attributed to Yu Jia.

At least 127 records · Page 7Linked to original sources

arXiv2013

$O(α_s)$ corrections to $J/ψ+χ_{cJ}$ production at $B$ factories

We investigate the ${\cal O}(α_s)$ corrections to $e^+e^-\to J/ψ(ψ')+χ_{cJ}$ ($J=0,1,2$) in the NRQCD factorization approach. These next-to-leading order (NLO) corrections are calculated at the level of helicity amplitude. We have made a detailed analysis for both polarized and unpolarized cross sections, and compared our predictions with the measurements at the $B$ factories. We also derive the asymptotic expressions for each of the NLO helicity amplitudes, and confirm the earlier speculation that at NLO in $α_s$, the double logarithm of type $\ln^2 (s/m_c^2)$ appearing in the NRQCD short-distance coefficient is always associated with the helicity-suppressed channels.

hep-ph↗

arXiv2013

${\mathcal O}(α_s)$ corrections to $e^+e^-\rightarrow J/ψ+η_{c2}(χ_{c1}^\prime)$ at $B$ factories

We investigate the ${\cal O}(α_s)$ correction to $e^+e^-\to J/ψ+η_{c2}$ in the NRQCD factorization approach. A detailed comparative study between $e^+e^-\to J/ψ+η_{c2}$ and $e^+e^-\to J/ψ+χ^\prime_{c1}$ at $B$ factory energy is also carried out. After incorporating the ${\cal O}(α_s)$ correction, we predict the cross section for the former process to be around 0.3 fb, while that of the latter about 6 times greater. The outgoing $J/ψ$ is found to be dominantly transversely-polarized in the former process, while longitudinally-polarized in the latter. These features may provide valuable guidance for the future experiment to examine the ${}^3P_1$ or ${}^1D_2$ charmonium option of the X(3872) meson through the exclusive double-charmonium production processes. The observation potential of $e^+e^-\to J/ψ+χ^\prime_{c1}$ looks bright for the current data sample of the \textsc{Belle} experiment, provided that the $χ^\prime_{c1}$ is indeed the narrow X(3872) state. In the appendix, we also identify the coefficients of the double logarithms of form $\ln^2(s/m_c^2)$ associated with all the relevant next-to-leading order Feynman diagrams, for the helicity-suppressed double-charmonium production channels $e^+ e^- \to J/ψ+ η_{c2}(η_c,χ_{c0,1,2})$.

hep-ph↗

arXiv2012

Exclusive decay of $Υ$ into $J/ψ+χ_{c0,1,2}$

We study the $Υ$ exclusive decay into double charmonium, specifically, the $S$-wave charmonium $ J/ψ$ plus the $P$-wave charmonium $χ_{c0,1,2}$ in the NRQCD factorization framework. Three distinct decay mechanisms, i.e., the strong, electromagnetic and radiative decay channels are included and their interference effects are investigated. The decay processes $Υ(1S,2S,3S)\to J/ψ+χ_{c1,0}$ are predicted to have the branching fractions of order $10^{-6}$, which should be observed in the prospective Super $B$ factory.

hep-ph↗

arXiv2012

Inclusive $h_c$ production at $B$ factories

Within the nonrelativistic QCD (NRQCD) factorization framework, we investigate the inclusive production of the $h_c$ meson associated with either light hadrons or charmed hadrons at $B$ factory energy $\sqrt{s}=10.58$ GeV. Both the leading color-singlet and color-octet channels are included. For the $h_c$ production associated with light hadrons, the total production rate is dominated by the color-octet channel, thus the future measurement of this process may impose useful constraint on the value of the color-octet matrix element $<{\cal O}^{h_c}_8({}^1S_0)>$; for the $h_c$ production associated with charmed hadrons, the total production rate is about one order of magnitude smaller, and dominated by the color-singlet channel.

hep-ph↗

arXiv2012

$O(α_s v^2)$ correction to $e^+e^-\to J/ψ+η_c$ at $B$ factories

We investigate the ${\mathcal O}(α_s v^2)$ correction to the $e^+e^-\to J/ψ+η_c$ process in the nonrelativistic QCD (NRQCD) factorization approach. Within some reasonable choices of the relative order-$v^2$ NRQCD matrix elements, we find that including this new ingredient of correction only mildly enhances the existing NRQCD predictions. We have also deduced the asymptotic expressions for the ${\mathcal O}(α_s v^2)$ short-distance coefficients, and reconfirm the early speculation that at next-to-leading order in $α_s$, the double logarithm of type $\ln^2 (s/m_c^2)$ appearing in various NRQCD short-distance coefficients is always associated with the helicity-suppressed channels.

hep-ph↗

arXiv2012

New results on the baryon decay Lambda_b -> Lambda_c ell nu in Heavy Quark Effective Theory

The baryon differential spectrum of the baryon decay $Λ_b \to Λ_c \ell \barν_\ell$ will be measured in detail at LHCb. We obtain new results on the form factors in the heavy quark expansion of Heavy Quark Effective Theory that can be useful in the interpretation of the data. We formulate a sum rule for the elastic subleading form factor $A(w)$ at order $1/m_Q$, that originates from the Lagrangian perturbation $\mathcal{L}_{kin}$. In the sum rule appear only the intermediate states $(j^P, J^P) = (0^+, {1 \over 2}^+)$, entering also in the $1/{m_Q^2}$ correction to the axial form factor $G_1(w)$, that contributes to the differential rate at zero recoil $w = 1$. This result, together with another sum rule in the forward direction for $|G_1(1)|^2$, allows us to obtain a lower bound for the correction at zero recoil $- δ_{1/{m_Q^2}}^{(G_1)}$ in terms of the derivative $A'_1(1)$ and the slope $ρ^2_Λ$ and curvature $σ^2_Λ$ of the elastic Isgur-Wise function $ξ_Λ(w)$. Another theoretical implication is that $A'(1)$ must vanish for some relation between $ρ^2_Λ$ and $σ^2_Λ$, as well as for $ρ^2_Λ\to 0$, establishing a non-trivial correlation between the leading IW function $ξ_Λ(w)$ and the subleading one $A(w)$. A phenomenological estimation of these two functions allows to obtain a lower bound on $- δ_{1/{m_Q^2}}^{(G_1)}$.

hep-ph↗

arXiv2011

Bridging light-cone and NRQCD approaches: asymptotic behavior of $B_c$ electromagnetic form factor

This work aims at illustrating that, for a class of leading-twist hard exclusive reactions involving two heavy quarkonia, the light-cone approach, when equipped with the strategy of refactorization of the light-cone distribution amplitude of quarkonium, can be employed to elegantly reproduce the corresponding predictions made in the nonrelativistic QCD (NRQCD) factorization approach, order by order in perturbative expansion. Taking the electromagnetic form factor of the $B_c$ meson at large momentum transfer, $Q^2$, as a concrete example, we compare the results obtained from both NRQCD-based and light-cone-based calculations through the next-to-leading order (NLO) in $α_s$, while at the leading order (LO) in both velocity and $1/Q^2$ expansion, and explicitly confirm their mutual agreement. As a byproduct, we apply our NLO result to explore certain features about the asymptotic behavior of the heavy-light meson form factor. We also address the major theoretical obstacles that prevent us from establishing an analogous equivalence between these two approaches for the double charmonium production process of phenomenological interest, $e^+e^-\to J/ψ+η_c$.

hep-ph↗

arXiv2011

$O(α_s v^2)$ correction to pseudoscalar quarkonium decay to two photons

We investigate the $O(α_s v^2)$ correction to the process of pseudoscalar quarkonium decay to two photons in nonrelativistic QCD (NRQCD) factorization framework. The short-distance coefficient associated with the relative-order $v^2$ NRQCD matrix element is determined to next-to-leading order in $α_s$ through the perturbative matching procedure. Some technical subtleties encountered in calculating the ${O(α_s)$ QCD amplitude are thoroughly addressed.

hep-ph↗

arXiv2010

Is the $J^P=2^-$ assignment for the $X(3872)$ compatible with the radiative transition data?

The very recent analysis by BaBar Collaboration indicates that the $X(3872)$ may favor the quantum number $J^{PC}=2^{-+}$ rather than the previously assumed $1^{++}$. By pretending the $η_{c2}(1D)$ charmonium to be the $X(3872)$, we study the parity-even radiative transition processes $η_{c2}(1D)\to J/ψ(ψ')+γ$ within several phenomenological potential models. We take the ${}^3D_1$ admixture in $ψ'$ into account, and consider the contributions from the magnetic dipole ($M1$), electric quadrupole ($E2$), and magnetic octupole ($M3$) amplitudes. It is found that the ratio of the branching fractions of these two channels, as well as the absolute branching fraction of $η_{c2}\to ψ'γ$, are in stark contradiction with the existing \textsc{BaBar} measurements. This may indicate that the $2^{-+}$ assignment for the $X(3872)$ is highly problematic.

hep-ph↗

arXiv2010

Color-singlet relativistic correction to inclusive $J/ψ$ production associated with light hadrons at $B$ factories

We study the first-order relativistic correction to the associated production of $J/ψ$ with light hadrons at $B$ factory experiments at $\sqrt{s}=10.58$ GeV, in the context of NRQCD factorization. We employ a strategy for NRQCD expansion that slightly deviates from the orthodox doctrine, in that the matching coefficients are not truly of ``short-distance" nature, but explicitly depend upon physical kinematic variables rather than partonic ones. Our matching method, with validity guaranteed by the Gremm-Kapustin relation, is particularly suited for the inclusive quarkonium production and decay processes with involved kinematics, exemplified by the process $e^+e^-\to J/ψ+gg$ considered in this work. Despite some intrinsic ambiguity affiliated with the order-$v^2$ NRQCD matrix element, if we choose its value as what has been extracted from a recent Cornell-potential-model-based analysis, including the relative order-$v^2$ effect is found to increase the lowest-order prediction for the integrated $J/ψ$ cross section by about 30\%, and exert a modest impact on $J/ψ$ energy, angular and polarization distributions except near the very upper end of the $J/ψ$ energy. The order-$v^2$ contribution to the energy spectrum becomes logarithmically divergent at the maximum of $J/ψ$ energy. A consistent analysis may require that these large end-point logarithms be resummed to all orders in $α_s$.

hep-ph↗

arXiv2010

"Hard-scattering" approach to very hindered magnetic-dipole transitions in quarkonium

For a class of hindered magnetic dipole ($M1$) transition processes, such as $Υ(3S)\to η_b+γ$ (the discovery channel of the $η_b$ meson), the emitted photon is rather energetic so that the traditional approaches based on multipole expansion may be invalidated. We propose that a "hard-scattering" picture, somewhat analogous to the pion electromagnetic form factor at large momentum transfer, may be more plausible to describe such types of transition processes. We work out a simple factorization formula at lowest order in the strong coupling constant, which involves convolution of the Schrödinger wave functions of quarkonia with a perturbatively calculable part induced by exchange of one semihard gluon between quark and antiquark. This formula, without any freely adjustable parameters, is found to agree with the measured rate of $Υ(3S)\to η_b+γ$ rather well, and can also reasonably account for other recently measured hindered $M1$ transition rates. The branching fractions of $Υ(4S)\to η_b^{(\prime)}+γ$ are also predicted.

hep-ph↗

arXiv2010

$γ^{*}ρ^0\toπ^0$ Transition Form Factor in Extended AdS/QCD Models

The $γ^{*}ρ^0\toπ^0$ transition form factor is extracted from recent result for the $γ^* γ^* π^0$ form factor obtained in the extended hard-wall AdS/QCD model with a Chern-Simons term. In the large momentum region, the form factor exhibits a $1/Q^4$ behavior, in accordance with the perturbative QCD analysis, and also with the Light-Cone Sum Rule (LCSR) result if the pion wave function exhibits the same endpoint behavior as the asymptotic one. The appearance of this power behavior from the AdS side and the LCSR approach seem to be rather similar: both of them come from the {"}soft" contributions. Comparing the expressions for the form factor in both sides, one can obtain the duality relation $z\propto \sqrt{u(1-u)}$, which is compatible with one of the most important relations of the Light-Front holography advocated by Brodsky and de Teramond. In the moderate $Q^2$ region, the comparison of the numerical results from both approaches also supports a asymptotic-like pion wave function, in accordance with previous studies for the $γ^* γ^* π^0$ form factor. The form factor at zero momentum transfer gives the $γ^{*}ρ^0π^0$ coupling constant, from which one can determine the partial width for the $ρ^0(ω)\to π^0 γ$ decay. We also calculate the form factor in the time-like region, and study the corresponding Dalitz decays $ρ^0(ω)\to π^0 e^+e^-, π^0μ^+μ^-$. Although all these results are obtained in the chiral limit, numerical calculations with finite quark masses show that the corrections are extremely small. Some of these calculations are repeated in the Hirn-Sanz model and similar results are obtained.

hep-ph↗

arXiv2009

Observation prospects of leptonic and Dalitz decays of pseudoscalar quarkonia

Two types of pseudoscalar quarkonium electromagnetic decay processes, i.e. decay to a lepton pair, and to a lepton pair plus a photon (Dalitz decay), are analyzed at the leading order in NRQCD expansion. The former type of processes, highly suppressed in the Standard Model, have been hoped to act as the sensitive probes of the possible new physics. The latter type of processes generally possess much greater decay rates than the former, owing to several conspiring factors. The recently launched BES-III program, with $10^8$ $η_c$ samples to be anticipated in the coming years, may be able to observe the Dalitz decays $η_c\to e^+e^-γ$ and $η_c\to μ^+μ^-γ$, which have branching ratios of order $10^{-6}$. When the radiated photon becomes very soft, the Dalitz decay events will be experimentally tagged as the exclusive lepton pair events. It is found that, those quasi-two-body events that arise from $η_c\to e^+ e^-γ$ with photon energy less than the minimum sensitivity of the electromagnetic calorimeter, can vastly outnumber the literal $η_c\to e^+ e^-$ events, however this amplification is still not dramatic enough for the BES-III experiment to establish these events. Consequently, the expectation of looking for new physics signature in the $η_c\to l^+l^-$ channel is obscured, unless the contamination from $η_c \to l^+l^-γ$ has been taken into account carefully.

hep-ph↗

arXiv2009

Refactorizing NRQCD short-distance coefficients in exclusive quarkonium production

In a typical exclusive quarkonium production process, when the center-of-mass energy, $\sqrt{s}$, is much greater than the heavy quark mass $m$, large kinematic logarithms of $s/m^2$ will unavoidably arise at each order of perturbative expansion in the short-distance coefficients of the nonrelativistic QCD (NRQCD) factorization formalism, which may potentially harm the perturbative expansion. This symptom reflects that the hard regime in NRQCD factorization is too coarse and should be further factorized. We suggest that this regime can be further separated into "hard" and "collinear" degrees of freedom, so that the familiar light-cone approach can be employed to reproduce the NRQCD matching coefficients at the zeroth order of $m^2/s$ and order by order in $α_s$. Taking two simple processes, exclusive $η_b+γ$ production in $e^+ e^-$ annihilation and Higgs boson radiative decay into $Υ$, as examples, we illustrate how the leading logarithms of $s/m^2$ in the NRQCD matching coefficients are identified and summed to all orders in $α_s$ with the aid of Brodsky-Lepage evolution equation.

hep-ph↗

arXiv2008

Exclusive $eta_b$ decay to double $J/ψ$ at next-to-leading order in $alpha_s$

Within the nonrelativistic QCD (NRQCD) factorization framework, we calculate the exclusive decay process η_b \to J/ψJ/ψto next-to-leading order in the strong coupling constant, while at leading order in charm quark relative velocity. It is found that this new contribution to the amplitude is comparable in magnitude with the previously calculated relativistic correction piece, but differs by a phase about 90^\circ. Including this new contribution will increase the previous prediction to B(η_b \to J/ψJ/ψ) substantially, thus brightening the discovery potential of this clean hadronic decay channel of η_b in the forthcoming LHC experiment.

hep-ph↗

arXiv2008

Which hadronic decay modes are good for $η_b$ searching: double $J/ψ$ or something else?

It has been controversial whether $η_b$ can be discovered in Tevatron Run 2 through the decay $η_b\to J/ψJ/ψ$ followed by $J/ψ\to μ^+μ^-$. We clear this controversy by an explicit calculation which predicts ${\rm Br}[η_b\to J/ψJ/ψ]$ to be of order $10^{-8}$. It is concluded that observing eta_b through this decay mode in Tevatron Run 2 is rather unrealistic. The eta_b may be observed in the forthcoming LHC experiments through the 4-lepton channel, if the background events can be significantly reduced by imposing some kinematical cuts. By some rough but plausible considerations, we find that the analogous decay processes eta_b to VV, D^*\bar{D}^* also have very suppressed branching ratios, nevertheless it may be worth looking for η_b at LHC and Super B factory through the decay modes η_b \to K_S K^{\pm}π^{\mp}, D^*\bar{D}.

hep-ph↗

arXiv2008

One Special Identity between the complete elliptic integrals of the first and the third kind

I prove an identity between the first kind and the third kind complete elliptic integrals with the following form: $$Π({(1+x) (1-3 x)\over (1-x) (1+3 x)}, {(1+x)^3(1-3 x)\over (1-x)^3 (1+3x)})- {1+ 3 x \over 6 x} K ({(1+x)^3(1-3x)\over (1-x)^3 (1+3x)}) = 0, (0< x < 1); =-{π\over 12} {(x-1)^{3/2}\sqrt{1+3 x}\over x} (x<0 or x>1).$$ This relation can be applied to eliminate the complete elliptic integral of the third kind from the analytic solutions of the imaginary part of two-loop sunset diagrams in the equal mass case. The validity of this relation in the complex domain is also briefly discussed.

math-ph↗

arXiv2007

Exclusive Double Charmonium Production from $Υ$ Decay

The exclusive decay of $Υ$ to a vector plus pseudoscalar charmonium is studied in perturbative QCD. The corresponding branching ratios are predicted to be of order $10^{-6}$ for first three $Υ$ resonances, and we expect these decay modes should be discovered in the prospective high-luminosity $e^+e^-$ facilities such as super $B$ experiment. As a manifestation of the short-distance loop contribution, the relative phases among strong, electromagnetic and radiative decay amplitudes can be deduced. It is particularly interesting to find that the relative phase between strong and electromagnetic amplitudes is nearly orthogonal. The resonance-continuum interference effect for double charmonium production near various $Υ$ resonances in $e^+e^-$ annihilation is addressed.

hep-ph↗