arXiv subjects

Hsiang-nan Li

Publications and source records attributed to Hsiang-nan Li.

At least 127 records · Page 7Linked to original sources

arXiv2004

Two-parton twist-3 factorization in perturbative QCD

We prove collinear factorization theorem for the process $πγ^*\toπ$ at the twist-3 level in the covariant gauge by means of the Ward identity, concentrating on the two-parton case. It is shown that soft divergences cancel and collinear divergences are grouped into the pseudo-scalar and pseudo-tensor two-parton twist-3 pion distribution amplitudes. The delicate summation of a complete set of diagrams for achieving factorization in momentum, spin, and color spaces is emphasized. The proof is then extended to the exclusive semileptonic decay $B\toπl\barν$, assuming the hard scale to be of $O(\sqrt{\barΛM_B})$, where $\barΛ$ is a hadronic scale and $M_B$ the $B$ meson mass. We explain the distinction between the factorization of collinear divergences for a pion distribution amplitude and of soft divergences for a $B$ meson distribution amplitude. The gauge invariance and universality of the two-parton twist-3 pion distribution amplitudes are confirmed. The proof presented here can accommodate the leading twist-2 case. We then compare our proof with that performed in the framework of soft-collinear effective theory.

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arXiv2004

Weak phases from topological-amplitude parametrization

We propose a parametrization for two-body nonleptonic $B$ meson decays, in which the various topologies of amplitudes are counted in terms of powers of the Wolfenstein parameter $λ\sim 0.22$. The weak phases and the amplitudes are determined by comparing this parametrization with available measurements. It is possible to obtain the phase $ϕ_3$ from the $B\to Kπ$ data up to theoretical uncertainty of $O(λ^3)\sim 1%$. The recently measured $B_d^0\toπ^0π^0$ branching ratio implies a large color-suppressed or penguin amplitude, and that the extraction of the phase $ϕ_2$ from the $B\toππ$ data may suffer theoretical uncertainty more than the expected one of $O(λ^2)\sim 5%$.

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arXiv2003

Search of $D^{*}_{sJ}$ mesons in $B$ meson decays

We propose that the search of the $B\to D^{*}_{sJ}M$ decays, $M=D$, $π$ and $K$, can discriminate the different theoretical postulations for the nature of the recently observed $D^{*}_{sJ}$ mesons. The ratio of the branching ratios $B(B\to D^{*}_{sJ}M)/B(B\to D^{(*)}_{s}M)\approx 1$ (0.1) supports that the $D^{*}_{sJ}$ mesons are quark-antiquark (multi-quark) bound states. The Belle measurement of the $B\to D^{*}_{sJ}D$ branching ratios seems to indicate an unconventional picture.

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arXiv2003

k_T factorization of exclusive B decays

I compare collinear and $k_T$ factorization theorems in perturbative QCD, and discuss their application to exclusive $B$ meson decays. Especially, I concentrate on the recently measured time-dependent CP asymmetry of the $B^0_d\toπ^+π^-$ modes, from which the unitarity angle can be extracted.

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arXiv2003

Three-body nonleptonic B decays in perturbative QCD

We develop perturbative QCD formalism for three-body nonleptonic $B$ meson decays. Leading contributions are identified by defining power counting rules for various topologies of amplitudes. The analysis is simplified into the one for two-body decays by introducing two-meson distribution amplitudes. This formalism predicts both nonresonant and resonant contributions, and can be generalized to baryonic decays.

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arXiv2003

QCD aspects of exclusive B meson decays

I review recent progress on understanding QCD dynamics involved in exclusive $B$ meson decays. Different frameworks, including light-cone sum rules, QCD factorization, perturbative QCD, soft-collinear effective theory, light-front QCD, are discussed. Results from lattice QCD are quoted for comparison. I point out the important issues in the above QCD methods, which require further investigation.

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arXiv2002

$B \to D^{(*)}$ form factors in perturbative QCD

We calculate the $B\to D^{(*)}$ form factors in the heavy-quark and large-recoil limits in the perturbative QCD framework based on $k_T$ factorization theorem, assuming the hierachy $M_B\gg M_{D^{(*)}}\gg \barΛ$, with the $B$ meson mass $M_B$, the $D^{(*)}$ meson mass $M_{D^{(*)}}$, and the heavy meson and heavy quark mass difference $\barΛ$. The qualitative behavior of the light-cone $D^{(*)}$ meson wave function and the associated Sudakov resummation are derived. The leading-power contributions to the $B\to D^{(*)}$ form factors, characterized by the scale $\barΛ\sqrt{M_B/M_{D^{(*)}}}$, respect the heavy-quark symmetry. The next-to-leading-power corrections in $1/M_B$ and $1/M_{D^{(*)}}$, characterized by a scale larger than $\sqrt{\barΛM_B}$, are estimated to be less than 20%. The $D^{(*)}$ meson wave function is determined from the fit to the observed $B\to D^{(*)} lν$ decay spectrum, which can be employed to make predictions for nonleptonic decays, such as $B\to D^{(*)}π(ρ)$.

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arXiv2002

Threshold resummation for nonleptonic B meson decays

We investigate the double logarithmic corrections $α_s\ln^2 x$, x being a parton momentum fraction, in two-body nonleptonic B meson decays in collinear factorization theorem of perturbative QCD (PQCD). It is found that these corrections are universal for factorizable amplitudes, {\it i.e.}, at the leading power, and that their threshold resummation smears the end-point singularities from $x\to 0$. The double logarithmic corrections, depending on the topologies of nonfactorizable amplitudes at the subleading power, are negligible due to the color-transparency argument and to the overlap of three meson distribution amplitudes. We show that the PQCD approach to two-body nonleptonic B meson decays respects the factorization assumption in the heavy-quark limit.

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arXiv2002

$k_T$ factorization of exclusive processes

We prove $k_T$ factorization theorem in perturbative QCD (PQCD) for exclusive processes by considering $πγ^*\to γ(π)$ and $B\toγ(π) l\barν$. The relevant form factors are expressed as the convolution of hard amplitudes with two-parton meson wave functions in the impact parameter $b$ space, $b$ being conjugate to the parton transverse momenta $k_T$. The point is that on-shell valence partons carry longitudinal momenta initially, and acquire $k_T$ through collinear gluon exchanges. The $b$-dependent two-parton wave functions with an appropriate path for the Wilson links are gauge-invariant. The hard amplitudes, defined as the difference between the parton-level diagrams of on-shell external particles and their collinear approximation, are also gauge-invariant. We compare the predictions for two-body nonleptonic $B$ meson decays derived from $k_T$ factorization (the PQCD approach) and from collinear factorization (the QCD factorization approach).

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arXiv2002

PQCD approach to exclusive B decays

I review the recent progress on the perturbative QCD approach to exclusive $B$ meson decays, discussing the comparison of collinear and $k_T$ factorizations, the CP asymmetry in the $B^0\toπ^+π^-$ decay, penguin enhancement, the branching ratio of the $B^0\to D^0π^0$ decay, and three-body nonleptonic decays.

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arXiv2002

Threshold resummation for exclusive B meson decays

We argue that double logarithmic corrections $α_s\ln^2 x$ need to be resumed in perturbative QCD factorization theorem for exclusive $B$ meson decays, when the end-point region with a momentum fraction $x\to 0$ is important. These double logarithms, being of the collinear origin, are absorbed into a quark jet function, which is defined by a matrix element of a quark field attached by a Wilson line. The factorization of the jet function from the decay $B\toγl\barν$ is proved to all orders. Threshold resummation for the jet function leads to a universal, {\it i.e.}, process-independent, Sudakov factor, whose qualitative behavior is analyzed and found to smear the end-point singularities in heavy-to-light transition form factors.

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arXiv2002

Perturbative QCD analysis of $B \to ϕK^* $ decays

We study the first observed charmless $B\to VV$ modes, the $B\toϕK^*$ decays, in perturbative QCD formalism. The obtained branching ratios $B(B\toϕK^*)\sim 15 \times 10^{-6}$ are larger than $\sim 9\times 10^{-6}$ from QCD factorization. The comparison of the predicted magnitudes and phases of the different helicity amplitudes, and branching ratios with experimental data can test the power counting rules, the evaluation of annihilation contributions, and the mechanism of dynamical penguin enhancement in perturbative QCD, respectively.

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arXiv2002

$1/m_b$ source in inclusive b-hadron decays

We show that threshold resummation of the end-point logarithms in inclusive $b$-hadron decays leads to a Sudakov factor, which introduces $1/m_b$ power corrections through infrared renormalons. These power corrections are strong enough to account for the observed semileptonic branching ratio, charm yield, and lifetime ratios by breaking the local quark-hadron duality.

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arXiv2001

Λ_b \to ΛJ/ψdecay in perturbative QCD

We calculate the amplitudes involved in the heavy baryon nonleptonic decay $Λ_b \rar ΛJ/ψ$ using perturbative QCD factorization theorem, which are expressed as convolutions of hard $b$ quark decay amplitudes with the $Λ_b$ baryon, $Λ$ baryon and $J/ψ$ meson distribution amplitudes. It is found that nonfactorizable contributions dominate over factorizable ones. Because of soft cancellation in pairs of nonfactorizable diagrams, the $Λ_b \rar ΛJ/ψ$ decay is characterized by a large scale, such that perturbative QCD is applicable. Employing the distribution amplitudes determined in our previous works and from QCD sum rules, we derive the branching ratio $B(Λ_b \rar ΛJ/ψ)=(1.7 \sim 5.3)\times 10^{-4}$ in agreement with data. We predict an asymmetry parameter $α= -0.17 \sim -0.14$ associated with the anisotropic angular distribution of the $Λ$ baryons produced in polarized $Λ_b$ baryon decays.

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arXiv2001

PQCD factorizaiton of two-body B decays

I review the known approaches to two-body nonleptonic $B$ meson decays, including factorization assumption, modified factorization assumption, QCD factorization, and perturbative QCD factorization. Important phenomenological aspects of these approaches are emphasized.

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arXiv2001

Perturbative QCD analysis of $B \to ϕK$ decays and Power counting

We investigate exclusive nonleptonic $B$ meson decays $B\toϕK$ in perturbative QCD formalism. It is shown that the end-point (logarithmic and linear) singularities in decay amplitudes do not exist, after $k_T$ and threshold resummations are included. Power counting for emission and annihilation topologies of diagrams, including both factorizable and nonfactorizable ones, is discussed with Sudakov effects taken into account. Our predictions for the branching ratios $B(B\toϕK)\sim 10 \times 10^{-6}$ are larger than those ($\sim 4 \times 10^{-6}$) from the factorization approach because of dynamical enhancement of penguin contributions. Whether this enhancement is essential for penguin-dominated modes can be justified by experimental data.

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arXiv2001

Leading-power contributions to $B\toπ,ρ$ transition form factors

We calculate the $B\toπ,ρ$ transition form factors in the framework of perturbative QCD to leading power of $1/M_B$, $M_B$ being the $B$ meson mass. We explain the basic principle by discussing the pion electromagnetic form factor. It is shown that the logarithmic and linear singularities occurring at small momentum fractions of light meson distribution amplitudes do not exist in a self-consistent perturbative analysis, which includes $k_\perp$ and threshold resummations.

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arXiv2001

Perturbative QCD factorization of $πγ^*\to γ(π)$ and $B\to γ(π)l\bar ν$

We prove factorization theorem for the processes $πγ^*\toγ$ and $πγ^*\toπ$ to leading twist in the covariant gauge by means of the Ward identity. Soft divergences cancel and collinear divergences are grouped into a pion wave function defined by a nonlocal matrix element. The gauge invariance and universality of the pion wave function are confirmed. The proof is then extended to the exclusive $B$ meson decays $B\toγl\barν$ and $B\toπl\barν$ in the heavy quark limit. It is shown that a light-cone $B$ meson wave function, though absorbing soft dynamics, can be defined in an appropriate frame. Factorization of the $B\toπl\barν$ decay in $k_T$ space, $k_T$ being parton transverse momenta, is briefly discussed. We comment on the extraction of the leading-twist pion wave function from experimental data.

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