B-meson Semi-inclusive Decay to $2^{-+}$ Charmonium in NRQCD and X(3872)

TL;DR

This paper calculates the decay rate of B-mesons into a specific type of charmonium state using NRQCD, highlighting the dominance of color-octet contributions and implications for the nature of X(3872).

Contribution

It provides the first NLO calculation of B-meson decay into $2^{-+}$ charmonium, including color-octet effects and scheme-dependent branching ratio estimates.

Findings

Color-octet channels dominate the decay process.

Estimated branching ratio is around 0.41 to 1.24 times 10^{-4}.

X(3872) unlikely to be the $2^{-+}$ charmonium state.

Abstract

The semi-inclusive B-meson decay into spin-singlet D-wave $2^{-+}$ charmonium, $B\to \eta_{c2}+X$, is studied in nonrelativistic QCD (NRQCD). Both color-singlet and color-octet contributions are calculated at next-to-leading order (NLO) in the strong coupling constant $\alpha_s$. The non-perturbative long-distance matrix elements are evaluated using operator evolution equations. It is found that the color-singlet $^1D_2$ contribution is tiny, while the color-octet channels make dominant contributions. The estimated branching ratio $B(B\to \eta_{c2}+X)$ is about $0.41\,\times10^{-4}$ in the Naive Dimensional Regularization (NDR) scheme and $1.24\,\times10^{-4}$ in the t'Hooft-Veltman (HV) scheme, with renormalization scale $\mu=m_b=4.8$\,GeV. The scheme-sensitivity of these numerical results is due to cancelation between ${}^1S_0^{[8]}$ and ${}^1P_1^{[8]}$ contributions. The $\mu$-dependence curves of NLO branching ratios in both schemes are also shown, with $\mu$ varying from $\frac{m_b}{2}$ to $2m_b$ and the NRQCD factorization or renormalization scale $\mu_{\Lambda}$ taken to be $2m_c$. Comparison of the estimated branching ratio of $B\to \eta_{c2}+X$ with the observed branching ratio of $B \to X(3872)+K$ may lead to the conclusion that X(3872) is unlikely to be the $2^{-+}$ charmonium state $\eta_{c2}$.