[Paper Review] Comment on "e+ e- annhilation into J/psi J/psi"

Belle Collaboration, Ken Abe|ArXiv.org|Jun 5, 2003

Algorithms and Data Compression3 citations

TL;DR

This paper investigates whether the observed $e^+e^-$ annihilation into $J/\psi\eta_c$ could be contaminated by $J/\psi J/\psi$ events, which might inflate the measured cross-section. Using 101.8 fb$^{-1}$ of Belle data, the authors perform a recoil mass analysis and set a stringent upper limit of 0.008 pb at 90% CL for $J/\psi J/\psi$ production, ruling out this explanation for the discrepancy with non-relativistic QCD predictions.

ABSTRACT

Using a data sample of 101.8 fb collected by the Belle detector at the KEKB asymmetric energy e+ e- collider, we confirm our published observation of e+ e- -> J/psi eta_c and find no evidence for the process e+ e- -> J/psi J/psi. The latter process was suggested as means to explain the large discrepancy between the published Belle result and theoretical predictions.

Motivation & Objective

• To test whether the observed $e^+e^- \to J/\psi\eta_c$ signal could be explained by misidentified $J/\psi J/\psi$ events.
• To evaluate the contribution of $J/\psi J/\psi$ production via two virtual photons, which was proposed as a possible explanation for the discrepancy between Belle's measurement and NRQCD predictions.
• To confirm the $J/\psi\eta_c$ signal using a larger data sample and rule out systematic biases from recoil mass scale shifts.
• To set a conservative upper limit on the $J/\psi J/\psi$ cross-section using detection efficiency assumptions that minimize sensitivity.

Proposed method

• Recoil mass spectrum analysis of $e^+e^- \to J/\psi X$ events using 101.8 fb$^{-1}$ of Belle data.
• Monte Carlo simulations used to model signal line shapes for charmonium states, including $\eta_c$, $\chi_{c0}$, $\chi_{c1}$, $\chi_{c2}$, $\psi(2S)$, and $\eta_c(2S)$.
• Background parametrized by a second-order polynomial function in the recoil mass region below the open charm threshold ($M_{\text{recoil}} < 3.7$ GeV/$c^2$).
• Mass positions for $\eta_c$, $\chi_{c0}$, and $\eta_c(2S)$ treated as free parameters; others fixed at nominal values.
• Fit performed with $J/\psi$, $\chi_{c1}$, $\chi_{c2}$, and $\psi(2S)$ yields constrained to zero or their 90% CL upper limits to set conservative limits.
• Systematic checks using $\psi(2S) \to J/\psi \pi^+\pi^-$ decays to calibrate and verify the recoil mass scale, finding shifts < 3 MeV/$c^2$.

Experimental results

Research questions

• RQ1 Could the observed $e^+e^- \to J/\psi\eta_c$ signal be significantly contaminated by $J/\psi J/\psi$ events?
• RQ2 Is the $J/\psi J/\psi$ production rate large enough to explain the discrepancy between Belle's measurement and NRQCD predictions?
• RQ3 Does a momentum scale bias in the recoil mass reconstruction affect the $\eta_c$ peak position or width?
• RQ4 What is the upper limit on the cross-section for $e^+e^- \to J/\psi J/\psi$ at 90% confidence level?
• RQ5 Can the $J/\psi\eta_c$ signal be confirmed independently with a larger data sample and improved systematic control?

Key findings

• The $\eta_c$ signal is confirmed with a yield of $175 \pm 23$ events and a mass of $2.972 \pm 0.007$ GeV/$c^2$.
• The $\chi_{c0}$ state is observed with a yield of $61 \pm 21$ events and a mass of $3.409 \pm 0.010$ GeV/$c^2$.
• The $\eta_c(2S)$ state is observed with a yield of $107 \pm 24$ events and a mass of $3.630 \pm 0.008$ GeV/$c^2$.
• The $J/\psi$ yield is measured as $-9 \pm 17$, consistent with zero, indicating no significant $J/\psi J/\psi$ contribution.
• The upper limit for $\sigma(e^+e^- \to J/\psi J/\psi) \times \mathcal{B}(J/\psi \to \text{>2 charged})$ is set at less than 0.008 pb at 90% confidence level.
• No significant recoil mass scale bias is found, with shifts below 3 MeV/$c^2$, validating the mass reconstruction accuracy.