arXiv subjects

Yu-Qi Chen

Publications and source records attributed to Yu-Qi Chen.

35 records · Page 2Linked to original sources

arXiv2001

Semileptonic Decays of $B_c$ Meson to a P-Wave Charmonium State $χ_c$ or $h_c$

The semileptonic decays of meson $B_c$ to a P-wave charmonium state $χ_c(^3P_J)$ or $h_c(^1P_1)$ are computed. The results show that the decays are sizable so they are accessible in Tevatron and in LHC, especially, with the detectors LHCB and BTeV in the foreseeable future, and of them, the one to the $^1P_1$ charmonium state potentially offers us a novel window to see the unconfirmed $h_c$ particle. In addition, it is pointed out that since the two charmonium radiative decays $χ_c(^3P_{1,2}) \to J/ψ+γ$ have sizable branching ratios, the cascade decays of the concerned decays and the charmonium radiative decays may affect the result of the observing the $B_c$ meson through the semileptonic decays $B_{c}\to {J/ψ}+{l}+ν_{l}$ substantially.

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arXiv1999

Renormalons in Electromagnetic Annihilation Decays of Quarkonium

We study the large-order asymptotic behavior of the perturbation series for short-distance coefficients in the NRQCD factorization formulas for the decays J/psi --> e^+e^- and eta_c --> gamma gamma. The short-distance coefficients of the leading matrix elements are calculated to all orders in the large-N_f limit. We find that there is a universal Borel resummable renormalon associated with the cancellation of the Coulomb singularity in the short-distance coefficients. We verify that the ambiguities in the short-distance coefficients from the first infrared renormalon are canceled by ambiguities in the nonperturbative NRQCD matrix elements that contribute through relative order v^2. Our results are used to estimate the coefficients of higher order radiative corrections in the decay rates for J/psi --> e^+ e^-, eta_c --> gamma gamma, and J/ψ--> gamma gamma gamma.

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arXiv1999

Renormalon Ambiguities in NRQCD Operator Matrix Elements

We analyze the renormalon ambiguities that appear in factorization formulas in QCD. Our analysis contains a simple argument that the ambiguities in the short-distance coefficients and operator matrix elements are artifacts of dimensional-regularization factorization schemes and are absent in cutoff schemes. We also present a method for computing the renormalon ambiguities in operator matrix elements and apply it to a computation of the ambiguities in the matrix elements that appear in the NRQCD factorization formulas for the annihilation decays of S-wave quarkonia. Our results, combined with those of Braaten and Chen for the short-distance coefficients, provide an explicit demonstration that the ambiguities cancel in the physical decay rates. In addition, we analyze the renormalon ambiguities in the Gremm-Kapustin relation and in various definitions of the heavy-quark mass.

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arXiv1998

Solving the rho - pi puzzle by higher charmonium Fock states

A new explanation for the longstanding puzzle of the tiny branching fraction of psi' --> rho pi relative to that for J/psi --> rho pi was proposed. In the case of J/psi, we argue that this decay is dominated by a higher Fock state in which the c cbar pair is in a color-octet ^3S_1 state and annihilates via the process c cbar --> q qbar. In the case of the psi', we argue that the probability for the c cbar c pair in this higher Fock state to be close enough to annihilate is suppressed by a dynamical effect related to the small energy gap between the mass of the psi' and the D Dbar threshold.

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arXiv1998

An Explanation for the rho-pi Puzzle of J/psi and psi' Decays

We propose a new explanation for the longstanding puzzle of the tiny branching fraction of psi' -> rho pi relative to that for J/psi -> rho pi. In the case of J/psi, we argue that this decay is dominated by a higher Fock state in which the ccbar pair is in a color-octet ^3S_1 state and via annihilate process ccbar -> qqbar. In the case of the psi', we argue that the probability for the ccbar pair in this higher Fock state to be close enough to annihilate is suppressed by a dynamical effect related to the small energy gap between the mass of the psi' and the DDbar threshold.

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arXiv1997

Dimensional Regularization in Quarkonium Calculations

Dimensional regularization is incompatible with the standard covariant projection methods that are used to calculate the short-distance coefficients in inclusive heavy quarkonium production and annihilation rates. A new method is developed that allows dimensional regularization to be used consistently to regularize the infrared and ultraviolet divergences that arise in these perturbative calculations. We illustrate the method by calculating the leading color-octet terms and the leading color-singlet terms in the gluon fragmentation functions for arbitrary quarkonium states. We resolve a discrepancy between two previous calculations of the gluon fragmentation functions for the spin-triplet P-wave quarkonium states.

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arXiv1997

Calculation of P-Wave Charmonium Decay Rates Using Dimensional Regularization

Contributions to the decay rates of P-wave charmonium states that are proportional to $n_f α_s^3$, where $n_f$ is the number of flavors of light quarks, are calculated in the framework of nonrelativistic QCD using the threshold expansion method. Dimensional regularization is used to regularize the infrared divergences that arise from the emission of a soft gluon. Our results are consistent with the original calculations of Barbieri et al.

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arXiv1996

Production of the B_c Meson

The production of the B_c and B_c^* mesons was studied in the framework of the factorization formalism and perturbative QCD. Predicted results implied that an observable number of B_c and B_c^* events can be produced at LEP I and at Tevatron. The fragmentation approximation describes the production of the B_c and B_c^* mesons very well in high energy e^+e^- collisions. In hadronic collisions, it is valid when and only when the transverse momentum P_T of the produced B_c and B_c^* is much larger than the mass of the B_c meson.

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arXiv1996

Helicity Decomposition for Inclusive $J/ψ$ Production

We present a general method for calculating inclusive cross sections for the production of heavy quarkonium states with definite polarization within the NRQCD factorization approach. Cross sections for polarized production can involve additional matrix elements that do not contribute to cross sections for unpolarized production. They can also include interference terms between parton processes that produce $Q \bar Q$ pairs with different total angular momentum. The interference terms cancel upon summing over polarizations. Our method can be generalized to $N$ dimensions and is therefore compatible with the use of dimensional regularization to calculate radiative corrections. We illustrate the method by applying it to the production of $J/ψ$ via the parton processes $q \bar q \to c \bar c$ and $g g \to c \bar c$.

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arXiv1996

Comparative Study of the Hadronic Production of $B_c$ Mesons

A comparative study of the full $α_s^4$ perturbative QCD calculation of the hadronic production of the $B_c$ and $B_c^*$ mesons and the fragmentation approximation is presented. We examined the various subprocesses in detail and the distribution $C(z)$, where $z$ is twice the fraction of the $B_c$ or $B_c^*$ meson energy in the center of mass of the subprocess, which proved insightful and revealed the importance of certain non-fragmentation contributions. We concluded that the condition for the applicability of the fragmentation approximation in hadronic collisions is that the transverse momentum $P_T$ of the produced $B_c$ and $B_c^*$ be much larger than the mass of the $B_c$ meson; i.e., $P_T\gg M_{B_c}$. Numerical results for the cross sections at the Fermilab Tevatron are also presented using updated parton distribution functions with various kinematic cuts.

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arXiv1995

Signature for Color-Octet Production of $J/ψ$ in $e^+ e^-$ Annihilation

The inclusive cross section for the production of $J/ψ$ in $e^+ e^-$ annihilation is studied using a recently developed factorization formalism. In addition to the conventional color-singlet contribution, we include contributions from $c \bar c$ pairs that are produced at short distances in color-octet states. The color-octet contributions are suppressed by the smaller probability for the color-octet $c \bar c$ pair to evolve into a state that includes a $ψ$, but they may dominate near the upper endpoint of the $ψ$ energy spectrum. The signature for the color-octet contribution is a dramatic change in the angular distribution of the $ψ$ near the endpoint.

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arXiv1995

On Hadronic Production of the $B_c$ Meson

Two of the approaches to the hadronic productions of the double heavy mesons $B_c$ and $B_c^*$ are investigated. Comparison in various aspects on the results obtained by the approaches is made and shown in figures and a table. Some trial understanding of the approaches themselves and the achieved results is presented. The results may be used as some references for discovering the mesons at Tevatron and LHC.

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arXiv1995

The Hyperfine Spin Splittings In Heavy Quarkonia

The hyperfine spin splittings in heavy quarkonia are studied using the recently developed renormalization group improved spin-spin potential which is independent of the scale parameter $μ$. The calculated energy difference between the $J/ψ$ and the $η_c$ fits the experimental data well, while the predicted energy difference $ΔM_p$ between the center of the gravity of $1^3P_{0,1,2}$ states and the $1^1P_1$ state of charmonium has the correct sign but is somewhat larger than the experimental data. This is not surprising since there are several other contributions to $ΔM_p$, which we discuss, that are of comparable size ($\sim 1$ MeV) that should be included, before precise agreement with the data can be expected. The mass differences of the $ψ'-η_c'$, $Υ(1S)-η_b$, $Υ(2S)-η_b'$, and $B_c^*-B_c$ are also predicted.

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arXiv1994

On The Spin-Dependent Potential Between Heavy Quark And Antiquark

A new formula for the heavy quark-antiquark spin dependent potential is given by using the techniques developed in the heavy quark effective theory. The leading logarithmic quark mass terms emerging from the loop contributions are explicitly extracted and summed up. There is no renormalization scale ambiguity in this new formula. The spin-dependent potential in the new formula is expressed in terms of three independent color-electric and color-magnetic field correlation functions, and it includes both the Eichten-Feinberg's formula and the one-loop QCD result as special cases.

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arXiv1994

Remark on Charm Quark Fragmentation to $D^{**}$ Mesons

The observed $D^{**}$ mesons have $c\bar q$ flavor quantum numbers and spin-parity of the light degrees of freedom $s_\ell^{π_{\ell}} = 3/2^+$. In the $m_c \rightarrow \infty$ limit the spin of the charm quark is conserved and the $c \rightarrow D^{**}$ fragmentation process is characterized by the probability for the charm quark to fragment to a $D^{**}$ meson with a given helicity for the light degrees of freedom. We consider the calculated $b \rightarrow B_c^{**}$ fragmentation functions in the limit $m_c/m_b \rightarrow 0$ as a qualitative model for the $c \rightarrow D^{**}$ fragmentation functions. We find that in this model charm quark fragmentation to $s_\ell^{π_{\ell}} = 3/2^+$ light degrees of freedom with helicities $\pm 1/2$ is favored over fragmentation to $s_\ell^{π_{\ell}} = 3/2^+$ light degrees of freedom with helicities $\pm 3/2$.

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arXiv1994

The Instantaneous Approximation to the Transition Matrix Elements between Two Bound States

Under the framework of the Bethe-Salpeter (B.S.) wave functions and the Mandelstam formalism as well, to make ``instantaneous approximation'' to a transition matrix element (a current operator sandwiched between two bound-states of double heavy quarks) is described. By taking the typical concerned decays as examples, such as $B_c$ meson decaying to $J/ψ+ (\bar l ν)$, the advantages of the approach and its limitations are illustrated. Finally, potential applications to various processes for possible double heavy flavoured systems, such as those of $(Q' \bar Q)$ and $(Q' Q)$ ($m_Q, m_{Q'}\gg Λ_{QCD}$), are discussed.

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arXiv1993

General relations of heavy quark-antiquark potentials induced by reparameterization invariance

A set of general relations between the spin-independent and spin-dependent potentials of heavy quark and anti-quark interactions are derived from reparameterization invariance in the Heavy Quark Effective Theory. It covers the Gromes relation and includes some new interesting relations which are useful in understanding the spin-independent and spin-dependent relativistic corrections to the leading order nonrelativistic potential.

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