[Paper Review] Study of χ_{bJ}(1P) Properties in the Radiative Υ(2S) Decays

A. Abdesselam, I. Adachi+447 more|arXiv (Cornell University)|Jun 3, 2016

Quantum Chromodynamics and Particle Interactions3 citations

TL;DR

This study measures properties of $\chi_{bJ}(1P)$ states produced via radiative decays of $\Upsilon(2S)$ using data from the Belle experiment. It reports the first observation of $\chi_{b0}(1P)$, $\chi_{b1}(1P)$, and $\chi_{b2}(1P)$ in 27, 28, and 30 hadronic decay modes, respectively, and sets a 90% confidence-level upper limit of $\Gamma_{\text{total}} < 2.4\,\text{MeV}$ on the $\chi_{b0}(1P)$ width due to insufficient statistical significance.

ABSTRACT

We report a study of radiative decays of χ_{bJ}(1P)(J=0,1,2) mesons into 74 hadronic final states comprising charged and neutral pions, kaons, protons; out of these, 41 modes are observed with at least 5 standard deviation significance. Our measurements not only improve the previous measurements by the CLEO Collaboration but also lead to first observations in many new modes. The large sample allows us to probe the total decay width of the χ_{b0}(1P). In the absence of a statistically significant result, a 90% confidence-level upper limit is set on the width at Γ_{total}&lt; 2.4 MeV. Our results are based on 24.7 fb^{-1} of e+e- collision data recorded by the Belle detector at the Υ(2S) resonance, corresponding to (157.8\pm3.6) imes10^6 Υ(2S) decays.

Motivation & Objective

• To measure the properties of $\chi_{bJ}(1P)$ states produced in radiative decays of $\Upsilon(2S)$.
• To search for $\chi_{bJ}(1P)$ states in various hadronic final states, particularly $\chi_{b0}(1P)$, which had not been previously observed in such decays.
• To determine the total width of the $\chi_{b0}(1P)$ state, given the absence of a statistically significant signal.
• To improve precision over prior measurements, especially those from the CLEO Collaboration.

Proposed method

• A maximum likelihood fit is performed on the $\Delta M$ distribution (mass difference between $\Upsilon(2S)$ and the final state) across 41 decay modes to extract the $\chi_{bJ}(1P)$ signal.
• The fit uses a combination of signal components for $\chi_{b0}(1P)$, $\chi_{b1}(1P)$, and $\chi_{b2}(1P)$, with background contributions modeled separately.
• Systematic uncertainties are evaluated by varying PDF parameters and testing linearity of the fit response to width hypotheses.
• A 90% confidence-level upper limit on the $\chi_{b0}(1P)$ width is derived by integrating the likelihood function over width values, with systematic uncertainties convolved via a Gaussian function.
• Pseudo-experiments are generated to test linearity and estimate bias in width estimation.
• The analysis combines data from the Belle experiment at the KEK $e^+e^-$ collider, focusing on $\Upsilon(2S) \to \gamma \chi_{bJ}(1P)$ decays followed by hadronic decays of $\chi_{bJ}(1P)$.

Experimental results

Research questions

• RQ1 What is the total width of the $\chi_{b0}(1P)$ state, given the lack of a significant signal in the data?
• RQ2 Can $\chi_{b0}(1P)$ be observed in hadronic decay modes, and if so, in how many distinct final states?
• RQ3 How do the measured properties of $\chi_{bJ}(1P)$ states compare to previous measurements, particularly those from the CLEO Collaboration?
• RQ4 What is the significance of the $\chi_{b1}(1P)$ and $\chi_{b2}(1P)$ signals in the combined analysis across multiple decay modes?
• RQ5 What systematic effects influence the width measurement, and how are they quantified?

Key findings

• The $\chi_{b0}(1P)$ width is measured as $1.3 \pm 0.9\,\text{MeV}$, but this result is not statistically significant.
• A 90% confidence-level upper limit on the total width of $\chi_{b0}(1P)$ is set at $\Gamma_{\text{total}} < 2.4\,\text{MeV}$.
• This is the first observation of $\chi_{b0}(1P)$ in 27 hadronic decay modes, $\chi_{b1}(1P)$ in 28 modes, and $\chi_{b2}(1P)$ in 30 modes.
• The measurement precision for $\chi_{bJ}(1P)$ properties exceeds that of previous CLEO measurements.
• Systematic uncertainties in the width measurement are estimated at $\pm 0.2\,\text{MeV}$ from PDF parameter variation and $\pm 0.1\,\text{MeV}$ from the assumption of negligible $\chi_{b2}(1P)$ width.
• A linearity test using pseudo-experiments confirms that bias in width estimation is negligible, with an additional uncertainty of $\pm 0.1\,\text{MeV}$ assigned.