Average of R(J/ψ)

Average of R(J/ψ) for May 2026

Last update on 18.05.2026

This web page contains the average of the ratio of Branching Fractions R(J/ψ)=BF(B_c→ψτν_τ)/BF(B_c→ψ μ ν_μ).
In the present average we consider the following inputs:

CMS Collaboration: Combination of R(J/ψ) measurement using using the τ→μν_μν_τ decay mode Phys. Rev. D 111 (2025) 051102 [arXiv:2408.00678]
and the three-prong hadronic decay channels τ→3π(π⁰)ν_τ [arXiv:2510.21559], combination included in [arXiv:2510.21559], Submitted to Physical Review D.
LHCb Collaboration: Phys. Rev. Lett. 120, 121801 (2018) [arXiv:1711.05623]
Run 1 (2011-2012, 3fb^-1) R(J/ψ) measurement using τ→μν_μν_τ decays.
LHCb Collaboration: [CERN Seminar 19/05/2026]
Run 2 (2016-2018, 5.4fb^-1) R(J/ψ) measurement using τ→μν_μν_τ decays.

In the average we assume 100% correlation for the systematics uncertainties due to B_c→& J/ψ hadronic form factors reported by the CMS combination and the latest LHCb result (while the previous result used older theory predictions). Systematic uncertainties due to B_c→ψ(2S) form factors and BF(τ→μν_μν_τ) are assumed 100% correlated between the two LHCb measurements, while partial correlation is assumed for the systematics assigned for the modelling of background from particles incorrectly identified as muons, and feed-down from B_c→ψ(2S) l ν_l and B_c→χ_c l ν. The other uncertainties are considered uncorrelated between the various measurements. The average is obtained using the the CoCo averaging tool.

Experiment	R(J/ψ)	Inputs	Remarks
CMS τ_μ+τ_3h	0.49 ± 0.11 ± 0.24	input	[arXiv:2510.21559] Submitted to Physical Review D.
LHCb Run 1 τ_μ	0.71 ± 0.17 ± 0.18	input	Phys. Rev. Lett. 131, 111802 [arXiv:2302.02886]
LHCb Run 2 τ_μ	0.51 ± 0.12 ± 0.08	input	[CERN Seminar 19/05/2026]
Average logfile.txt	0.54 ± 0.12	chi2/dof = 0.547/2 = 0.274 (CL = 0.760528)	R(J/ψ) plot .pdf

Compatibility with SM predictions

SM predictions for R(J/ψ) based on following Lattice calculations:

R(D) =0.2597 ± 0.0027, J. Harrison (HPQCD Collaboration), Phys. Rev. D 112, 034503, arXiv:2503.15090 [hep-lat]

The experimental average of R(J/ψ) exceeds the SM prediction given above by 2.4σ.

Acknowledgments

We thank Veronika Georgieva Chobanova for the development of the CoCo averaging tool and the support provided.