Observation of two charged bottomonium-like resonances in Y(5S) decays

Abstract: We report the observation of two narrow structures in the mass spectra of the pi+-Y(nS) (n=1,2,3) and pi+-hb(mP)(m$ (m=1,2) pairs that are produced in association with a single charged pion in Y(5S) decays. The measured masses and widths of the two structures averaged over the five final states are M_1=(10607.2+-2.0) MeV/c2, Gamma_1=(18.4+-2.4) MeV and M_2=(10652.2+-1.5) MeV/c2, Gamma_2=(11.5+-2.2) MeV. The results are obtained with a 121.4 1/fb data sample collected with the Belle detector in the vicinity of the Y(5S) resonance at the KEKB asymmetric-energy e+e- collider.

• The paper reports the observation of two charged bottomonium-like resonances, Z_b(10610) and Z_b(10650), in Y(10860) decays with distinct mass and width measurements.
• It employs precise amplitude analysis using unbinned maximum likelihood fits on Belle detector data to reconstruct complex three-body decay channels.
• The findings shed light on exotic meson structures and suggest a potential molecular nature for these states near open beauty thresholds.

Observation of Two Charged Bottomonium-like Resonances in Y(10860) Decays

The study of heavy quarkonium has yielded significant insights into the various interactions and decay modes of these systems. In the context of such research, this paper presents the observation of two charged bottomonium-like resonances, specifically the $Z_b(10610)$ and $Z_b(10650)$, which occur in the decay processes originating from the $\Upsilon(10860)$ state. Utilizing a data sample of 121.4 fb$^{-1}$ collected with the Belle detector at the KEKB asymmetric $e^+e^-$ collider, this study provides a detailed analysis of these resonances observed in the mass spectra of associated pion pairs.

Summary of Results

The observed structures are narrow and show distinct resonant behavior in the mass spectra of the $\pi^\pm$ pairs. The extracted masses and widths for the resonances, averaged over five final states, are as follows:

• $Z_b(10610)$: Mass = $10607.2 \pm 2.0$ MeV, Width = $18.4 \pm 2.4$ MeV
• $Z_b(10650)$: Mass = $10652.2 \pm 1.5$ MeV, Width = $11.5 \pm 2.2$ MeV

The observation was consistent across multiple decay channels involving the $h_b(mP)$ and $\Upsilon(nS)$ states, highlighting the robustness of these findings.

Methodology

The Belle detector was used to capture the decay events with its comprehensive instrumentation, spanning from silicon vertex detectors to electromagnetic calorimeters. The analysis conducted a precise reconstruction of $\Upsilon(10860)$ events, focusing on the three-body decays to $\Upsilon(nS)$ and $h_b(mP)$ states. Amplitude analysis involved unbinned maximum likelihood fits to Dalitz distributions of the decay products.

A key part of the analysis involved the parameterization of the decay amplitudes using a model incorporating amplitudes for the resonances $Z_b(10610)$ and $Z_b(10650)$, S-wave Breit-Wigner functions, and taking advantage of isospin symmetry to symmetrize amplitude expressions. The systematic uncertainties were rigorously evaluated, accounting for model dependencies and efficiencies.

Implications and Future Research

The discovery of these resonances extends the understanding of bottomonium interactions, particularly in the context of non-conventional mesons that include more complex structures, such as tetraquarks or molecular states. The proximity of the measured masses to the threshold for open beauty channels $B^*\overline{B}$ and $B^* \overline{B^*}$ suggests a potential molecular nature, which has been a subject of theoretical propositions and discussion based on results like this.

Future research could explore further the internal structure and properties of these resonances, utilizing both experimental techniques and theoretical models to explore their quark compositions and potential decay channels. Additionally, further analysis in different collider settings or using higher statistics could help refine the mass and width measurements and sharpen the theoretical predictions about these states' nature and their role in the broader spectrum of particle interactions.