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Hsiang-nan Li

Publications and source records attributed to Hsiang-nan Li.

At least 163 records · Page 9Linked to original sources
arXiv1998

Unification of the $k_T$ and threshold resummations

We derive a resummation formula for a $k_T$-dependent parton distribution function at threshold, where $k_T$ is a parton transverse momentum. The derivation requires infrared cutoffs for both longitudinal and transverse loop momenta as evaluating soft gluon emissions in the Collins-Soper resummation framework. This unified resummation exhibits suppression at large $b$, $b$ being the conjugate variable of $k_T$, which is similar to the $k_T$ resummation, and exhibits enhancement at small $b$, similar to the threshold resummation.

hep-ph↗
arXiv1998

The $Λ_b\to pl{\barν}$ decay in perturbative QCD

We develop perturbative QCD factorization theorems for the exclusive semileptonic heavy baryon decay $Λ_b \to pl{\barν}$, in which the relevant hadronic form factor is expressed as the convolution of a hard subamplitude with the $Λ_b$ baryon wave function and the proton wave function. The specific evolution scale for the proton wave function, determined from the best fit to the data of the proton form factor, is adopted. The Sudakov resummation for a heavy-light system (the $Λ_b$ baryon) and the quark-level decay diagrams with at least one hard gluon attaching the $b$ quark, which were ignored in the literature, are incorporated. It turns out that these additional ingredients are important: the neglect of the former and the latter reduces the results of the form factor by factors 1.5 and 3, respectively. We present the predictions of the form factor and of the proton energy spectrum for the various choices of the parameter involved in the $Λ_b$ baryon wave function.

hep-ph↗
arXiv1998

Perturbative QCD analysis of exclusive $B$ meson decays

We review the perturbative QCD formalism for exclusive heavy meson decays, concentrating on the three-scale factorization theorem for nonleptonic processes. The formalism is then extended to the radiative decay $B\to K^*γ$, which occurs through penguin diagrams. It is observed that the contributions from the operators other than the penguin one $b\to sγ$ are not negligible. From the best fit to the experimental data of the branching ratio ${\cal B}(B\to K^*γ)$, we extract the $B$ meson wave function, which possesses a sharp peak in the region with a small momentum fraction.

hep-ph↗
arXiv1998

Exclusive Processes at Intermediate Energy, Quark-Hadron Duality and the Transition to Perturbative QCD

Experiments at CEBAF will scan the intermediate-energy region of the QCD dynamics for the nucleon form factors and for Compton Scattering. These experiments will definitely clarify the role of resummed perturbation theory and of quark-hadron duality (QCD sum rules) in this regime. With this perspective in mind, we review the factorization theorem of perturbative QCD for exclusive processes at intermediate energy scales, which embodies the transverse degrees of freedom of a parton and the Sudakov resummation of the corresponding large logarithms. We concentrate on the pion and proton electromagnetic form factors and on pion Compton scattering. New ingredients, such as the evolution of the pion wave function and the complete two-loop expression of the Sudakov factor, are included. The sensitivity of our predictions to the infrared cutoff for the Sudakov evolution is discussed. We also elaborate on QCD sum rule methods for Compton Scattering, which provide an alternative description of this process. We show that, by comparing the local duality analysis to resummed perturbation theory, it is possible to describe the transition of exclusive processes to perturbative QCD.

hep-ph↗
arXiv1998

Unified derivation of evolution equations

We derive the evolution equations of parton distribution functions appropriate in different kinematic regions in a unified and simple way using the resummation technique. They include the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equation for large momentum transfer $Q$, the Balitskii-Fadin-Kuraev-Lipatov equation for a small Bjorken variable $x$, and the Ciafaloni-Catani-Fiorani-Marchesini equation which embodies the above two equations. The relation among these equations is explored, and possible applications of our approach are proposed.

hep-ph↗
arXiv1998

The perturbative proton form factor reexamined

We recalculate the proton Dirac form factor based on the perturbative QCD factorization theorem which includes Sudakov suppression. The evolution scale of the proton wave functions and the infrared cutoffs for the Sudakov resummation are carefully chosen, such that the soft divergences from large coupling constants are diminished and perturbative QCD predictions are stablized. We find that the King-Sachrajda model for the proton wave function leads to results which are in better agreement with experimental data compared to the Chernyak-Zhitnitsky wave function.

hep-ph↗
arXiv1998

Small-x evolution with Q dependence and unitarity

We propose a modified Balitsky-Fadin-Kuraev-Lipatov equation from the viewpoint of the resummation technique, which contains an intrinsic dependence on momentum transfer Q, and satisfies the unitarity bound. The idea is to relax the strong rapidity ordering and to restrict phase space for real gluon emissions in the evaluation of the BFKL kernel. It is shown that the power-law rise of the gluon distribution function with the small Bjorken variable x turns into a logarithmic rise, and that the predictions for the proton structure function $F_2(x,Q^2)$ are consistent with the HERA data.

hep-ph↗
arXiv1998

PQCD study of the $B_{SL}$ and $n_c$ controversy in inclusive $B$ decays

We calculate the semileptonic branching ratio and the charm counting of inclusive $B$ meson decays using the perturbative QCD formalism. For the nonleptonic decays, we employ the modified factorization theorem, in which Wilson coefficients evolve with the characteristic scales of the decay modes. It is found that the decay rate of the single-charm mode $b\to c{\bar u}d$ is enhanced, and a lower $B_{SL}$ is obtained without increasing $n_c$. We predict a larger $b\to c τ\barν$ branching ratio compared to that from the conventional heavy-quark-effective-theory based operator product expansion. Our result of the $B$ meson lifetime is also consistent with the data.

hep-ph↗
arXiv1998

Gauge invariance of the resummation approach to evolution equations

We show that the Collins-Soper-Sterman resummation approach to the derivation of the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equation is gauge invariant. The special gauge-dependent parton distribution function employed in the resummation technique is expressed as the convolution of an infrared finite function with the standard distribution function. By means of this convolution relation, we explain how the technique works in summing large logarithmic corrections, and how the gauge invariance of the special distribution function is restored.

hep-ph↗
arXiv1998

Origin of the $k_T$ smearing in direct photon production

We show that the Sudakov factor from the resummation of double logarithms $\ln(s/k_T^2)$ contained in the distribution functions is responsible for the $k_T$ smearing mechanism employed in the next-to-leading-order QCD ($αα_s^2$) calculations of direct photon production. $s$ is the center-of-mass energy, and $k_T$ the transverse momentum carried by a parton in a colliding hadron. This factor exhibits the appropriate $s$-dependent Gaussian width in $k_T$, such that our predictions are in good agreement with experimental data.

hep-ph↗
arXiv1997

Perturbative pion form factor reexamined

We recalculate the pion electromagnetic form factor based on the perturbative QCD formalism that includes the Sudakov resummation. We take into account the evolution of the pion wave function in $b$, which represents the transverse extent of the pion. An infrared enhancement is observed when $1/b$ approaches $Λ_{QCD}$. We propose to freeze the evolution of the wave function at some scale above $Λ_{QCD}$, such that perturbative contributions are stablized. Our predictions are consistent with experimental data, and insensitive to the variation of relevant parameters.

hep-ph↗
arXiv1997

A modified BFKL equation with Q depedence

We propose a modified Balitsky-Fadin-Kuraev-Lipatov (BFKL) equation for the summation of large $\ln(1/x)$, $x$ being the Bjorken variable, which contains an extra dependence on momentum transfer $Q$ compared to the conventional BFKL equation. This $Q$ dependence comes from the phase space constraint for soft real gluon emissions in the derivation of the BFKL kernel. The modified equation gives a slower rise of the gluon distribution function with $1/x$ for smaller $Q$, such that the predictions for the structure functions $F_{2,L}(x,Q^2)$ involved in deep inelastic scattering match the HERA data. These results are similar to those obtained from the Ciafaloni-Catani-Fiorani-Marchesini equation, which embodies both the $\ln Q$ and $\ln(1/x)$ summations. The reason that the conventional BFKL equation can not be applied to the low $Q$ (soft pomeron dominant) region is supplied.

hep-ph↗
arXiv1997

A modified BFKL equation with unitarity

We propose a modified Balitskii-Fadin-Kuraev-Lipatov equation from the viewpoint of the resummation technique, which satisfies the unitarity bound. The idea is to relax the strong rapidity ordering and to restrict phase space for real gluon emissions in the evaluation of the BFKL kernel. It is found that the gluon distribution function rises as a power of the Bjorken variable $x$, and then saturates at $x\to 0$. We estimate that the saturation begins to occur for $x< 10^{-4}$.

hep-ph↗
arXiv1997

Nonfactorizable soft gluons in nonleptonic heavy meson decays

We include nonfactorizable soft gluon corrections into the perturbative QCD formalism for exclusive nonleptonic heavy meson decays, which combines factorization theorems and effective field theory. These corrections are classified according to their color structures, and exponentiated separately to complete the Sudakov resummation up to next-to-leading logarithms. The nonfactorizable contributions in nonleptonic decays are clearly identified in our formalism, and found to be positive for bottom decays and negative for charm decays. Our analysis confirms that the large-$N_c$ approximaton is applicable to charm decays, but not to bottom decays, consistent with the phenomenological implications of experimental data. The comparision of our predictions with those from QCD sum rules is also made.

hep-ph↗
arXiv1997

The resummation approach to evolution equations

We derive the evolution equations of parton distribution functions appropriate in different kinematic regions in a unified and simple way using the resummation technique. They include the Gribov-Lipatov-Altarelli-Parisi equation for large momentum transfer $Q$, the Balitskii-Fadin-Kuraev-Lipatov equation for a small Bjorken variable $x$, and the Ciafaloni-Catani-Fiorani-Marchesini equation which unifies the above two equations. We propose a modified Balitskii-Fadin-Kuraev-Lipatov equation from the view point of the resummation. This modified version possesses an intrinsic $Q$ dependence and its predictions for the structure function $F_2(x,Q^2)$ are consistent with the HERA data.

hep-ph↗
arXiv1997

Factorization theorems, effective field theory, and nonleptonic heavy meson decays

The nonleptonic heavy meson decays $B\to D^{(*)}π(ρ), J/ψK^{(*)}$ and $D\to K^{(*)}π$ are studied based on the three-scale perturbative QCD factorization theorem developed recently. In this formalism the Bauer-Stech-Wirbel parameters a_1 and a_2 are treated as the Wilson coefficients, whose evolution from the W boson mass down to the characteristic scale of the decay processes is determined by effective field theory. The evolution from the characteristic scale to a lower hadronic scale is formulated by the Sudakov resummation. The scale-setting ambiguity, which exists in the conventional approach to nonleptonic heavy meson decays, is moderated. Nonfactorizable and nonspectator contributions are taken into account as part of the hard decay subamplitudes. Our formalism is applicable to both bottom and charm decays, and predictions, including those for the ratios R and R_L associated with the $B\to J/ψK^{(*)}$ decays, are consistent with experimental data.

hep-ph↗