[Paper Review] Hadron Spectroscopy Results from Belle

A. Zupanc|ArXiv.org|Oct 18, 2009

Quantum Chromodynamics and Particle Interactions18 references4 citations

TL;DR

This paper presents recent hadron spectroscopy results from the Belle experiment at KEKB, focusing on exotic $XYZ$ charmoniumlike states and their $s\overline{s}$ and $b\overline{b}$ counterparts. Using a 960 fb$^{-1}$ data sample, Belle observes new states such as $X(3872)$, $X(3915)$, $Y(4260)$, $Y(4660)$, and $Y_b(10890)$, with the latter identified as a $1^{--}$ resonance at 10,890 MeV with a width of 55 MeV, suggesting a possible $b\overline{b}$ counterpart to the $Y(4260)$ state.

ABSTRACT

We report on the most recent experimental progress on $XYZ$ charmoniumlike meson states and their possible counterparts in the $s\overline{s}$ and $b\overline{b}$ systems.

Motivation & Objective

• To identify and characterize new exotic hadronic states, particularly $XYZ$ mesons, in the charmonium spectrum near 3.9–4.7 GeV.
• To investigate the nature of these states by analyzing their quantum numbers, decay modes, and production mechanisms, especially in $B$ meson decays and $e^+e^-$ collisions.
• To search for and study possible $s\overline{s}$ and $b\overline{b}$ counterparts of the $XYZ$ states, including the $Y_b(10890)$ resonance.
• To test theoretical models of exotic hadrons such as tetraquarks, molecular states, and hybrids by comparing experimental data with predictions.
• To resolve discrepancies in the interpretation of states like $X(3915)$ and $Y(3940)$ by measuring their properties in multiple decay channels and production modes.

Proposed method

• Utilizing a 960 fb$^{-1}$ data sample collected at the KEKB $e^+e^-$ collider operating at $\sqrt{s} \approx$ 10.58 GeV, corresponding to the $\Upsilon(4S)$ resonance.
• Employing the Belle detector's high-precision tracking, particle identification (via $dE/dx$, TOF, ACC), and electromagnetic calorimetry to reconstruct decay vertices and final states.
• Performing amplitude analyses and invariant mass fits to identify resonances in decay channels such as $J/\psi \pi^+\pi^-$, $\omega J/\psi$, $D\bar{D}^*$, $\Lambda_c^+\Lambda_c^-$, and $\pi^+\pi^-\Upsilon(nS)$.
• Measuring energy-dependent cross sections for $e^+e^- \to \pi^+\pi^-\Upsilon(nS)$ to identify resonant structures near 10.899 GeV.
• Fitting the $\pi^+\pi^-\Upsilon(nS)$ cross section with a common Breit-Wigner function to extract the mass and width of the new resonance $Y_b(10890)$.
• Comparing the observed resonance parameters with those of known $\Upsilon(5S)$ and $\Upsilon(6S)$ states to rule out standard $b\overline{b}$ interpretations.

Experimental results

Research questions

• RQ1 What is the nature of the $X(3872)$ state, and why does it show a dominance of non-resonant $K^+\pi^-$ decay over $K^{*0}$ in $B^0 \to K^+\pi^-X(3872)$?
• RQ2 Why does the $X(3915)$ state exhibit a large partial width to $\omega J/\psi$, contradicting expectations for conventional charmonium?
• RQ3 Is the $Y_b(10890)$ state a $b\overline{b}$ counterpart of the $Y(4260)$, or could it arise from final-state interactions or tetraquark states?
• RQ4 Do the $Y(4260)$, $Y(4350)$, and $Y(4660)$ states observed in $\pi^+\pi^-J/\psi$ and $\pi^+\pi^-\psi'$ final states correspond to a single resonance or multiple states?
• RQ5 Why is there a significant threshold enhancement in $e^+e^- \to \Lambda_c^+\Lambda_c^-$ near the $Y(4660)$ mass, and what does it imply about the state's structure?

Key findings

• The $X(3872)$ state is observed with a mass of $3871.4 \pm 0.6$ MeV and a width of less than 2.3 MeV, consistent across $B \to KX(3872)$ and $\pi^+\pi^-J/\psi$ decay modes.
• The $X(3915)$ state is observed in $\gamma\gamma \to \omega J/\psi$ with a mass of $3914 \pm 4$ MeV and a width of $28^{+12}_{-14}$ MeV, consistent with BaBar's $Y(3940)$ observation.
• The $Y_b(10890)$ state is observed in $\pi^+\pi^-\Upsilon(nS)$ final states at a center-of-mass energy of 10.899 GeV, with a mass of $10,889.6 \pm 1.8$ MeV and a width of $54.7^{+8.5}_{-7.2}$ MeV.
• The $Y_b(10890)$ resonance is not described by the PDG parameters of $\Upsilon(5S)$ or $\Upsilon(6S)$, indicating it is a new state distinct from the $\Upsilon(5S)$.
• The $Y(4660)$ state is observed in $e^+e^- \to \pi^+\pi^-\psi'$ with a mass of $4664 \pm 12$ MeV and a width of $48 \pm 15$ MeV, showing a threshold-like enhancement near $\Lambda_c^+\Lambda_c^-$.
• Three charged $Z$ states—$Z(4050)$, $Z(4250)$, and $Z(4430)$—are reported with masses of $4051^{+24}_{-23}$, $4248^{+185}_{-45}$, and $4433 \pm 5$ MeV, respectively, all decaying via $\pi^\pm \chi_{c1}$ final states, indicating exotic $c\overline{c}u\overline{d}$ content.